UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

Filed by the Registrant  ☒

Filed by a Party other than the Registrant  ☐

Check the appropriate box:

HORIZON THERAPEUTICS PUBLIC LIMITED COMPANY

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

HORIZON THERAPEUTICS PLC

70 St. Stephen’s Green

Dublin 2, D02 E2X4, Ireland

TRANSACTION PROPOSED—YOUR VOTE IS VERY IMPORTANT

Dear Fellow Shareholder:

You are cordially invited to attend two special meetings of the shareholders of Horizon Therapeutics plc (“Horizon”). The first, the special Irish High Court-ordered meeting, is to be held on February 24, 2023 at 10:30 a.m. (Irish time), at Horizon’s registered office at 70 St. Stephen’s Green, Dublin 2, D02 E2X4, Ireland (the “Scheme Meeting”), and the second, the extraordinary general meeting of shareholders, is to be held on February 24, 2023 at 10:45 a.m. (Irish time), at Horizon’s registered office at 70 St. Stephen’s Green, Dublin 2, D02 E2X4, Ireland (the “EGM”), or, if the Scheme Meeting has not concluded by 10:45 a.m. (Irish time), as soon as possible after the conclusion of the Scheme Meeting (or any adjournment thereof). The Scheme Meeting and the EGM are referred to collectively as the “special meetings.”

As previously announced, on December 11, 2022, Horizon entered into a Transaction Agreement (the “Transaction Agreement”), by and among Horizon, Amgen Inc., a Delaware corporation (“Amgen”), and Pillartree Limited, a private limited company incorporated under the laws of Ireland and a wholly owned subsidiary of Amgen (“Acquirer Sub”). Under the terms of the Transaction Agreement, Acquirer Sub will acquire Horizon (the “Transaction”) pursuant to a scheme of arrangement under Chapter 1 of Part 9 of the Irish Companies Act 2014 (the “Companies Act”), the terms of which are set out in full in “Part 3—The Scheme of Arrangement” (the “Scheme”) of the accompanying proxy statement. As a result of the Scheme, Horizon will become a wholly owned subsidiary of Amgen.

As consideration for the Transaction, holders of ordinary shares of Horizon, nominal value $0.0001 per share (each a “Horizon Share” and such holders, the “Horizon Shareholders”), will be entitled to receive at the effective time of the Scheme (the “effective time”), $116.50 in cash for each Horizon Share held by them as of the effective time (the “Scheme Consideration”).

Horizon equity awards will be treated as set forth in the Transaction Agreement, such that at the effective time:

You are being asked to vote on a proposal to approve the Scheme at both special meetings as well as an additional proposal being presented at the EGM to amend Horizon’s Articles of Association that Horizon Shareholders must approve in order to properly implement the Scheme and upon which the Transaction is conditioned. At the EGM, Horizon Shareholders are also being asked to approve, on a non-binding, advisory basis, certain specified compensatory arrangements between Horizon and its named executive officers. The Scheme is also subject to sanction by the Irish High Court. More information about the Transaction and the proposals is contained in the accompanying proxy statement. You are urged to read the accompanying proxy statement, including the annexes and the documents incorporated by reference therein, carefully and in their entirety.

Your proxy is being solicited by (and on behalf of) the members of the Horizon Board entitled to solicit your proxy under the Irish Takeover Rules.

After careful consideration, the members of the Horizon Board entitled to make a recommendation in this matter under the Irish Takeover Rules have unanimously determined that the Transaction Agreement and the transactions contemplated by the Transaction Agreement, including the Scheme, are fair to and in the best interests of Horizon and the Horizon Shareholders and that the terms of the Scheme are fair and reasonable. Additional information regarding interests in the transaction of the directors entitled to make such recommendation may be found in the section of the accompanying proxy statement entitled “The Transaction—Interests of Certain Persons in the Transaction,” and each subsequent reference to the Horizon Board is intended to refer to the members so entitled. The Horizon Board recommends unanimously that you vote “FOR” all proposals. In considering the recommendation of the Horizon Board, you should be aware that certain directors and executive officers of Horizon have interests in the proposed transaction that are in addition to, or different from, any interests they might have as shareholders. See the section of the accompanying proxy statement entitled “The Transaction—Interests of Certain Persons in the Transaction” for more information. Your vote is very important. Please vote as soon as possible, whether or not you plan to attend the special meetings, by following the instructions in the accompanying proxy statement.

If you have any questions or need assistance voting your Horizon Shares, please contact Alliance Advisors, Horizon’s proxy solicitor in connection with the Scheme Meeting and the EGM:

Alliance^™ Advisors

200 Broadacres Drive, 3rd Floor

Bloomfield, NY 07003

+1 (973) 873-7700

www.allianceadvisors.com

On behalf of the Horizon Board, thank you for your consideration and continued support.

Sincerely,

Timothy P. Walbert

Chairman, President and Chief Executive Officer

Neither the U.S. Securities and Exchange Commission (the “SEC”) nor any state securities commission has approved or disapproved of the Transaction, passed upon the merits or fairness of the Transaction Agreement or the transactions contemplated by it or if the information contained in the accompanying proxy statement is accurate or adequate. Any representation to the contrary is a criminal offense.

The accompanying proxy statement is dated January 23, 2023, and is first being mailed to Horizon Shareholders on or about January 23, 2023.

ADDITIONAL INFORMATION

The accompanying proxy statement incorporates by reference important business and financial information about Horizon from documents that are not included in or delivered with the accompanying proxy statement. Horizon files annual, quarterly and current reports, proxy statements and other information with the SEC. The SEC maintains an Internet site that contains reports, proxy statements and other information regarding issuers that file electronically with the SEC. The SEC’s Internet site can be found at http://www.sec.gov. Horizon also maintains an Internet site that contains the reports, proxy statements and other information that it files with the SEC. Horizon’s Internet site can be found at www.horizontherapeutics.com. Information found on, or accessible through, Horizon’s website is not a part of and is not incorporated into, the accompanying proxy statement. This information incorporated into the accompanying proxy statement from other documents is also available to you without charge upon your written or oral request. The documents incorporated by reference will not be provided to you unless they are requested by you. You can obtain the documents incorporated by reference into the accompanying proxy statement by requesting them in writing or by telephone from Horizon at the following address or telephone number:

Horizon Therapeutics plc

70 St. Stephen’s Green

Dublin 2, D02 E2X4, Ireland

Attn: Company Secretary

+353 1 7722 100 (Ireland)

In addition, if you have questions about the Transaction or the special meetings, or if you need to obtain copies of the accompanying proxy statement, proxy cards or other documents incorporated by reference into the accompanying proxy statement, you may contact Horizon’s proxy solicitor for the special meetings using the contact information listed below. You will not be charged for any of the documents you request.

Alliance^™ Advisors

200 Broadacres Drive, 3rd Floor

Bloomfield, NY 07003

+1 (973) 873-7700

www.allianceadvisors.com

If you would like to request documents, please do so by 5:00 p.m. (Eastern time in the U.S.) on February 17, 2023 in order to receive them before the special meetings.

For a more detailed description of the information incorporated by reference into the accompanying proxy statement and how you may obtain it, see the section of the accompanying proxy statement entitled “Where You Can Find More Information.”

THE ACCOMPANYING PROXY STATEMENT DOES NOT CONSTITUTE THE SOLICITATION OF A PROXY IN ANY JURISDICTION TO OR FROM ANY PERSON TO WHOM OR FROM WHOM IT IS UNLAWFUL TO MAKE SUCH PROXY SOLICITATION IN THAT JURISDICTION. YOU SHOULD RELY ONLY ON THE INFORMATION CONTAINED OR INCORPORATED BY REFERENCE INTO THE ACCOMPANYING PROXY STATEMENT TO VOTE YOUR HORIZON SHARES AT THE SCHEME

MEETING AND EGM. WE HAVE NOT AUTHORIZED ANYONE TO PROVIDE YOU WITH INFORMATION THAT IS DIFFERENT FROM WHAT IS CONTAINED IN THE ACCOMPANYING PROXY STATEMENT. THE ACCOMPANYING PROXY STATEMENT IS DATED JANUARY 23, 2023. YOU SHOULD NOT ASSUME THAT THE INFORMATION CONTAINED IN THE ACCOMPANYING PROXY STATEMENT IS ACCURATE AS OF ANY DATE OTHER THAN THAT DATE, AND THE MAILING OF THE ACCOMPANYING PROXY STATEMENT TO HORIZON SHAREHOLDERS DOES NOT CREATE ANY IMPLICATION TO THE CONTRARY.

HORIZON THERAPEUTICS PLC

70 St. Stephen’s Green

Dublin 2, D02 E2X4, Ireland

NOTICE OF SCHEME MEETING OF SHAREHOLDERS

THE HIGH COURT, 2023 No. 5 COS

IN THE MATTER OF HORIZON THERAPEUTICS PLC

– and –

IN THE MATTER OF THE COMPANIES ACT 2014

NOTICE IS HEREBY GIVEN that by an order dated January 23, 2023 made in the above matter, the Irish High Court has (in accordance with Section 450 of the Irish Companies Act 2014 (the “Companies Act”)) directed a meeting (the “Scheme Meeting”) to be convened of the holders of the Scheme Shares (as defined in the proposed scheme of arrangement that is included in the document of which this Notice forms a part (the “Scheme” or the “Scheme of Arrangement”)) in the capital of Horizon Therapeutics plc (“Horizon”) for the purpose of considering and, if thought fit, approving a resolution to approve (with or without modification) a scheme of arrangement pursuant to Chapter 1 of Part 9 of the Companies Act proposed to be made between Horizon and the holders of the Scheme Shares and that such meeting will be held at Horizon’s registered office at 70 St. Stephen’s Green, Dublin 2, D02 E2X4, Ireland on February 24, 2023, commencing at 10:30 a.m. (Irish time), at which place and time all holders of the said shares are invited to attend such meeting; such resolution being in the following terms:

“That the Scheme in its original form or with or subject to any modification(s), addition(s) or condition(s) approved or imposed by the Irish High Court be agreed to.”

To be passed, the resolution to approve the Scheme requires the approval at the Scheme Meeting (or any adjournment of such meeting) of the Scheme by a majority in number of Scheme Shareholders (as defined in the Scheme of Arrangement) representing at least three-fourths (75%) in value of the Scheme Shares, voted at such meeting, either in person or by proxy. The quorum for the Scheme Meeting shall be at least one or more Scheme Shareholders present (in person or by proxy) holding not less than a majority of the issued and outstanding Scheme Shares entitled to vote at the Scheme Meeting.

A copy of the Scheme and a copy of the explanatory statement required to be furnished pursuant to Section 452 of the Companies Act are incorporated into the document of which this Notice forms part. Capitalized terms used in this Notice have the meanings given to them in the document of which this Notice forms part (save as otherwise defined in this Notice).

By the said order, the Irish High Court has designated Timothy P. Walbert, Chairman, President and Chief Executive Officer of Horizon, or, failing him, any other director of Horizon as the board of directors of Horizon may determine, to act as Chairman of the Scheme Meeting and has directed the Chairman to report the result thereof to the Irish High Court.

Subject to, amongst other items, the approval of the resolution to approve the Scheme proposed at the meeting convened by this Notice and resolutions 1 and 2 to be proposed at the extraordinary general meeting of Horizon

convened for February 24, 2023, the prior satisfaction of the other Conditions (as defined in the Scheme) to the completion of the Scheme (other than those Conditions which by their nature cannot be satisfied prior to the hearing by the Irish High Court of the application to sanction the Scheme) and the availability of the Irish High Court, the said application is anticipated to be heard in March 2023.

The Scheme will be subject to the subsequent sanction of the Irish High Court.

Matheson LLP

Irish Legal Counsel for Horizon

70 Sir John Rogerson’s Quay

Dublin 2

D02 R296, Ireland

Dated: January 23, 2023

Statement of Procedures

Availability of documents and information in connection with the Scheme Meeting

Entitlement to attend and vote

Appointment of proxies

Further information for participants holding interests in “street name”

Deadlines for receipt by Horizon of proxy voting instructions

Voting rights

YOUR VOTE IS IMPORTANT

IT IS IMPORTANT THAT AS MANY VOTES AS POSSIBLE ARE CAST AT THE SCHEME MEETING (WHETHER IN PERSON OR BY PROXY) SO THAT THE IRISH HIGH COURT CAN BE SATISFIED THAT THERE IS A FAIR AND REASONABLE REPRESENTATION OF HORIZON SHAREHOLDER OPINION. TO ENSURE YOUR REPRESENTATION AT THE SCHEME MEETING, YOU ARE REQUESTED TO COMPLETE, SIGN AND DATE THE ENCLOSED FORM OF PROXY AS PROMPTLY AS POSSIBLE AND RETURN IT IN THE POSTAGE PREPAID ENVELOPE ENCLOSED FOR THAT PURPOSE OR TO PROVIDE A PROXY AND VOTE BY INTERNET OR TELEPHONE IN THE MANNER PROVIDED ABOVE. IF YOU ATTEND THE SCHEME MEETING, YOU MAY VOTE IN PERSON EVEN IF YOU HAVE RETURNED A COMPLETED FORM OF PROXY.

HORIZON THERAPEUTICS PLC

70 St. Stephen’s Green

Dublin 2, D02 E2X4, Ireland

NOTICE OF EXTRAORDINARY GENERAL MEETING

OF

HORIZON THERAPEUTICS PLC

NOTICE IS HEREBY GIVEN that an EXTRAORDINARY GENERAL MEETING (“EGM”) of Horizon Therapeutics plc (“Horizon”) will be held at Horizon’s registered office at 70 St. Stephen’s Green, Dublin 2, D02 E2X4, Ireland on February 24, 2023, commencing at 10:45 a.m. (Irish time) or, if later, as soon thereafter as the special Irish High Court-ordered meeting (the “Scheme Meeting”) shall have been concluded or adjourned, for the purpose of considering and, if thought fit, passing the following resolutions of which resolutions 1, 3 and 4 will be proposed as ordinary resolutions and resolution 2 as a special resolution (collectively, the “Horizon Proposals”).

The Horizon Proposals may be voted on in such order as is determined by the Chairman of the EGM:

That, subject to the approval by the requisite majorities at the Scheme Meeting, the scheme of arrangement that is included in the document of which this Notice forms part (the “Scheme” or the “Scheme of Arrangement”) (a copy of which has been produced to the meeting and, for the purposes of identification, signed by the Chairman thereof) in its original form or with or subject to any modification(s), addition(s) or condition(s) approved or imposed by the High Court be approved and the directors of Horizon be authorized to take all such action as they consider necessary or appropriate for carrying the Scheme of Arrangement into effect.

That, subject to the Scheme becoming effective, the Articles of Association of Horizon be amended by adding the following new Article 191:

“191. Scheme of Arrangement

That, on a non-binding, advisory basis, specified compensatory arrangements between Horizon and its named executive officers relating to the Transaction (as more particularly described in the section of the accompanying proxy statement entitled “The Transaction—Interests of Certain Persons in the Transaction”) be approved.

That, any motion by the Chairman to adjourn the EGM, or any adjournments thereof, to another time and place if necessary or appropriate to solicit additional proxies if there are insufficient votes at the time of the EGM to approve the Scheme, or the proposed amendment of the Articles of Association of Horizon, be approved.

By order of the Board

David Caraher

Company Secretary

Horizon Therapeutics plc

70 St. Stephen’s Green

Dublin 2

D02 E2X4, Ireland

Dated: January 23, 2023

Statement of Procedures

Availability of documents and information in connection with the EGM

Entitlement to attend and vote

Appointment of proxies

Further information for participants holding interests in “street name”

Deadlines for receipt by Horizon of proxy voting instructions

Voting rights and the total number of issued shares

YOUR VOTE IS IMPORTANT

IT IS IMPORTANT THAT AS MANY VOTES AS POSSIBLE ARE CAST AT THE EGM (WHETHER IN PERSON OR BY PROXY). TO ENSURE YOUR REPRESENTATION AT THE EGM, YOU ARE REQUESTED TO COMPLETE, SIGN AND DATE THE ENCLOSED FORM OF PROXY AS PROMPTLY AS POSSIBLE AND RETURN IT IN THE POSTAGE PREPAID ENVELOPE ENCLOSED FOR THAT PURPOSE OR TO PROVIDE A PROXY AND VOTE BY INTERNET OR TELEPHONE IN THE MANNER PROVIDED ABOVE. IF YOU ATTEND THE EGM, YOU MAY VOTE IN PERSON EVEN IF YOU HAVE RETURNED A COMPLETED FORM OF PROXY.

TABLE OF CONTENTS

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This proxy statement and the documents incorporated by reference herein contain “forward-looking statements”—that is, statements related to future, not past, events that reflect Horizon’s current expectations regarding its future growth, results of operations, business strategy and plans, financial condition, cash flows, performance, development plans and timelines, business prospects and opportunities, as well as assumptions made by, and information currently available to, its management. Forward-looking statements include any statement that does not directly relate to a current or historical fact. Forward-looking statements generally can be identified by words such as “believe,” “may,” “could,” “will,” “estimate,” “continue,” “anticipate,” “intend,” “seek,” “plan,” “expect,” “should,” “would” or similar expressions. These statements are based on current expectations and assumptions that are subject to risks and uncertainties inherent in Horizon’s business, which could cause Horizon’s actual results to differ materially from those indicated in the forward-looking statements. Factors that could cause actual results to differ materially from those indicated in the forward-looking statements include, without limitation:

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Although Horizon believes that the expectations reflected in its forward-looking statements are reasonable, Horizon cannot guarantee future results, events, levels of activity, performance or achievement. Horizon undertakes no obligation to publicly update or revise any forward-looking statements, whether as a result of new information, future events or otherwise, unless required by law.

The above list of factors is not exhaustive. You should carefully consider the factors and the other risks and uncertainties that affect the businesses described in Horizon’s most recent Annual Report on Form 10-K and other documents filed from time to time with the SEC or incorporated herein by reference.

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PART 1—THE TRANSACTION AND THE SPECIAL MEETINGS

THE SPECIAL MEETINGS OF HORIZON’S SHAREHOLDERS

Overview

This proxy statement is being provided to Horizon Shareholders as part of a solicitation of proxies by the Horizon Board for use at the special meetings of Horizon Shareholders referred to below and at any adjournments of such meetings. This proxy statement is being furnished to Horizon Shareholders on or about January 23, 2023 and provides Horizon Shareholders with information they need to be able to vote or instruct their vote to be cast at the special meetings.

Date, Time and Place of the Special Meetings

The Scheme Meeting has been scheduled for February 24, 2023 at 10:30 a.m. (Irish time), at Horizon’s registered office at 70 St. Stephen’s Green, Dublin 2, D02 E2X4, Ireland. Horizon has also convened the EGM for February 24, 2023 at 10:45 a.m. (Irish time), at Horizon’s registered office at 70 St. Stephen’s Green, Dublin 2, D02 E2X4, Ireland, or, if the Scheme Meeting has not concluded by 10:45 a.m. (Irish time), as soon as possible after the conclusion of the Scheme Meeting (or any adjournment thereof).

Attendance

Attendance at the Scheme Meeting and the EGM is limited to Horizon Shareholders at the Voting Record Time. Please indicate on the proxy cards if you plan to attend the special meetings. If your shares are held through a bank, broker or other nominee and you would like to attend, please write to Horizon Therapeutics plc, 70 St. Stephen’s Green, Dublin 2, D02 E2X4, Ireland, Attention: Company Secretary, or bring to the applicable meeting a statement or a letter from the bank, broker or other nominee confirming your beneficial ownership of your Horizon Shares as of the Voting Record Time for the meetings. Any beneficial holder who plans to vote at either meeting must obtain a legal proxy from his or her bank, broker or other nominee and should contact such bank, broker or other nominee for instructions on how to obtain a legal proxy. Each Horizon Shareholder may be asked to provide a valid picture identification, such as a driver’s license or passport and proof of ownership as of the Voting Record Time. The use of cell phones, smartphones, pagers, recording and photographic equipment will not be permitted in the meeting room(s).

Proposals

Scheme Meeting: Horizon Shareholders (excluding Amgen or any of its affiliates, to the extent they hold Horizon Shares) are being asked to consider and vote on a proposal at the Scheme Meeting to approve the Scheme.

EGM: Horizon Shareholders (including Amgen and any of its affiliates, to the extent they hold Horizon Shares) are also being asked to consider and vote on a proposal at the EGM to approve the Scheme, in addition to certain other proposals as set forth in the resolutions described below.

The Transaction is conditioned on the approval of the Scheme Approval Resolution and the Articles of Association Amendment Resolution.

1

The Transaction is not conditioned on the approval of the remaining resolutions.

Record Date; Outstanding Ordinary Shares; Ordinary Shares Entitled to Vote

Only holders of Horizon Shares as of the Voting Record Time, which is 5:00 p.m. (Eastern time in the U.S.) on January 19, 2023, will be entitled to notice of, and to vote at the special meetings or any adjournments thereof. At the Voting Record Time, there were 228,391,421 Horizon Shares outstanding and there were 21 registered members whose names appeared in the Register of Members. Each outstanding Horizon Share (other than in the case of the Scheme Meeting only, any Horizon Shares held by Amgen, Acquirer Sub or any subsidiary of Amgen) is entitled to one vote on each proposal and any other matter properly coming before the special meetings.

Quorum

One or more holders of Horizon Shares, present in person or by proxy, entitling them to exercise a majority of the voting power of Horizon at the relevant meeting will constitute a quorum for each of the special meetings. Horizon does hold certain ordinary shares in treasury; such shares will not be included in the calculation of the number of ordinary shares present at the special meetings for the purposes of determining a quorum. Horizon’s inspector of election intends to treat as “present” for these purposes shareholders who have submitted properly executed or transmitted proxies that are marked “abstain.” The inspector will also treat as “present” shares held in “street name” by brokers that are voted on at least one proposal to come before the meeting.

Ordinary Share Ownership and Voting by Horizon’s Directors and Officers

As of the Voting Record Time, the full Horizon Board and executive officers had the right to vote approximately 1,780,034 of the then-outstanding Horizon Shares at the special meetings. The Horizon Board and executive officers who are shareholders of Horizon intend to vote “FOR” the proposal to approve the Scheme at the Scheme Meeting, “FOR” the Scheme Approval Resolution at the EGM, “FOR” the Articles of Association Amendment Resolution at the EGM, “FOR” the Compensation Resolution at the EGM and “FOR” the Adjournment Resolution at the EGM.

Vote Required; Recommendation of the Horizon Board

Scheme Meeting

Proposal to approve the Scheme of Arrangement: Horizon Shareholders (excluding Amgen, Acquirer Sub and any subsidiaries of Amgen, to the extent they hold Horizon Shares) are being asked to vote on a proposal to approve the Scheme at both the Scheme Meeting and the EGM. The vote required for such proposal is different at each of the meetings, however. As set out in full under the section of the proxy statement entitled “Part 2—Explanatory Statement—Consents and Special Meetings,” in order for the resolution at the Scheme Meeting to pass, it requires the approval at the Scheme Meeting by a majority in number of the Scheme Shareholders present and voting, either in person or by proxy, representing at least three-fourths (75%) in value of the Scheme Shares, voted at such meeting, either in person or by proxy.

Because the vote required to approve the proposal at the Scheme Meeting is based on votes properly cast at the meeting, and because abstentions and broker non-votes are not considered votes properly cast, abstentions and broker non-votes, along with failures to vote, will have no effect on such proposal.

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The Transaction is conditioned on the approval of the Scheme at the Scheme Meeting.

The Horizon Board recommends that Horizon Shareholders vote “FOR” the proposal to approve the Scheme at the Scheme Meeting.

In considering the recommendation of the Horizon Board, you should be aware that certain directors and executive officers of Horizon have interests in the Transaction that are in addition to, or different from, any interests they might have as shareholders. See the section of this proxy statement entitled “The Transaction—Interests of Certain Persons in the Transaction.”

EGM

Set forth below is a table summarizing certain information with respect to the resolutions to be voted upon at the EGM:

At the EGM, the requisite approval of each of the resolutions depends on whether it is (i) an “ordinary resolution” (the Scheme Approval Resolution, the Compensation Resolution and the Adjournment Resolution), which requires the approval of at least a majority of the votes cast by the holders of Horizon Shares present and voting in person or by proxy, or a “special resolution” (the Articles of Association Amendment Resolution), which requires the approval of at least seventy-five percent (75%) of the votes cast by the holders of Horizon Shares present and voting, in person or by proxy.

As the votes required to approve each of the resolutions are based on votes properly cast at the EGM, and because abstentions and broker non-votes are not considered votes properly cast, abstentions and broker non-votes, along with failures to vote, will have no effect on the resolutions.

The Horizon Board recommends that Horizon Shareholders vote “FOR” the proposals to approve each of the resolutions to be voted upon at the EGM.

3

In considering the recommendations of the Horizon Board described above, you should be aware that certain directors and executive officers of Horizon have interests in the Transaction that are in addition to, or different from, any interests they might have as shareholders. See the section of this proxy statement entitled “The Transaction—Interests of Certain Persons in the Transaction.”

Voting Your Ordinary Shares

Horizon Shareholders may vote by proxy or in person at each special meeting. Horizon recommends that you submit your proxy even if you plan to attend either or both of the special meetings. If you vote by proxy, you may change your vote, among other ways, if you attend and vote at the applicable special meeting. Completion and return of a form of proxy will not preclude a Horizon Shareholder from attending and voting in person at the Scheme Meeting or the EGM, or any adjournments thereof, if that Horizon Shareholder wishes to do so. Any alteration to the form of proxy must be initialed by the person who signs it.

If you own shares in your own name, you are considered, with respect to those shares, the “shareholder of record.” If your shares are held in a stock brokerage account or by a bank or other nominee, you are considered the beneficial owner of shares held in “street name.”

If you are a Horizon Shareholder of record you may use the enclosed proxy cards to tell the persons named as proxies how to vote your shares. If you are a Horizon Shareholder of record, the shares listed on your proxy cards will include, if applicable, shares held in a book-entry account at Computershare Shareowner Services LLC, Horizon’s transfer agent.

If you properly complete, sign and date your proxy cards, your shares will be voted in accordance with your instructions. The named proxies will vote all shares at the meeting for which proxies have been properly submitted and not revoked. If you sign and return your proxy cards appointing the Chairman as your proxy but do not mark your cards to tell the proxy how to vote on a voting item, your shares will be voted in respect of such voting item at the discretion of the Chairman.

Horizon Shareholders may also vote over the Internet at www.proxyvote.com or by telephone at 1-800-690-6903 anytime up to 11:59 p.m. (Eastern time in the U.S.) on February 23, 2023. Voting instructions are printed on the proxy cards or voting instruction form you received. Either method of submitting a proxy will enable your shares to be represented and voted at the special meetings. You may also hand your applicable completed and signed proxy card to the Chairman of the applicable special meeting before the start of the applicable special meeting.

Voting Ordinary Shares Held in Street Name

If your shares are held in an account through a bank, broker or other nominee, you must likewise instruct the bank, broker or other nominee how to vote your shares by following the instructions that the bank, broker or other nominee provides to you along with this proxy statement. Your bank, broker or other nominee, as applicable, may have an earlier deadline by which you must provide instructions to it as to how to vote your shares, so you should read carefully the materials provided to you by your bank, broker or other nominee.

If you do not provide a signed voting instruction form to your bank, broker or other nominee, your shares will not be voted on any proposal on which the bank, broker or other nominee does not have discretionary authority to vote. This is referred to in this proxy statement and in general as a broker non-vote. In these cases, the bank, broker or other nominee will not be able to vote your shares on those matters for which specific authorization is required. Brokers do not have discretionary authority to vote on any of the proposals at either special meeting.

Accordingly, if you fail to provide a signed voting instruction form to your bank, broker or other nominee, your shares held through such bank, broker or other nominee will not be voted at either special meeting.

4

Revoking Your Proxy

You may change your vote at any time before your proxy is voted at the Scheme Meeting or at any time before your proxy is voted at the EGM. If you are a registered shareholder (i.e., a shareholder who holds shares directly with Horizon’s transfer agent in his, her or its own name), you may revoke your proxy before the applicable special meeting by:

Horizon Therapeutics plc

70 St. Stephen’s Green

Dublin 2, D02 E2X4, Ireland

Attention: Company Secretary

If you are a beneficial shareholder (i.e., your shares are held in the name of a bank, broker or other holder of record, sometimes referred to as holding shares in “street name”), follow the voting instructions provided to you by your bank, broker or other holder of record to determine how you may change your vote or revoke your proxy.

Simply attending the applicable special meeting, however, will not revoke your proxy or change your voting instructions; you must vote by ballot at the applicable meeting to change your vote.

Costs of Solicitation

Horizon will bear the cost of soliciting proxies from Horizon Shareholders.

Horizon will solicit proxies by mail. In addition, the directors, officers and employees of Horizon may solicit proxies from Horizon Shareholders by telephone, electronic communication, or in person, but will not receive any additional compensation for their services. Horizon will make arrangements with brokerage houses and other custodians, nominees and fiduciaries for forwarding proxy solicitation material to the beneficial owners of Horizon Shares held of record by those persons and will reimburse them for their reasonable out-of-pocket expenses incurred in forwarding such proxy solicitation materials.

Horizon has engaged Alliance Advisors, a proxy solicitation firm, to solicit proxies in connection with the special meetings at a cost of approximately $30,000, plus expenses.

Other Business

Horizon is not aware of any other business to be acted upon at the special meetings. If, however, other matters are properly brought before the special meetings, the proxies will have discretion to vote or act on those matters according to their best judgment. Where the Chairman of the special meetings acts as proxy on behalf of Horizon Shareholders, he intends to vote the shares as the Horizon Board may recommend.

Adjournment

Any adjournment of the Scheme Meeting will result in an adjournment, as applicable, of the EGM.

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Assistance

If you need assistance in completing your proxy cards or have questions regarding Horizon’s special meetings, please contact Alliance Advisors, the proxy solicitation agent for Horizon, by mail at 200 Broadacres Drive, 3^rd Floor, Bloomfield, NY 07003 or by telephone at +1 (973) 873-7700.

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THE TRANSACTION

The Transaction

On December 11, 2022, Horizon entered into the Transaction Agreement by and among Horizon, Amgen and Acquirer Sub. Under the terms of the Transaction Agreement, Acquirer Sub will acquire Horizon pursuant to a scheme of arrangement under Chapter 1 of Part 9 of the Companies Act. As a result of the Scheme, Horizon will become a wholly owned subsidiary of Amgen.

At the effective time, Horizon Shareholders shall be entitled to receive $116.50 in cash in exchange for each Horizon Share held by such Horizon Shareholders. Horizon’s equity awards shall be treated as set forth in the Transaction Agreement, such that at the effective time:

Amgen reserves the right, subject to the prior written approval of the Irish Takeover Panel (if required), to effect the Transaction by way of a takeover offer, as an alternative to the Scheme, in the circumstances described in and subject to the terms of the Transaction Agreement. In such event, such takeover offer will be implemented on terms and conditions that are at least as favorable to Horizon Shareholders and Horizon equity award holders as those which would apply in relation to the Scheme (except for an acceptance condition set at eighty percent (80%) of the nominal value of the Horizon Shares to which such offer relates and which are not already beneficially owned by Amgen) as those which would apply in relation to the Scheme, among other requirements.

Background of the Transaction

Horizon is a biotechnology company focused on the discovery, development and commercialization of medicines that address critical needs for people impacted by rare, autoimmune and severe inflammatory diseases. The Horizon Board, together with Horizon’s senior management, and with the assistance of its financial and legal advisors, regularly reviews Horizon’s strategic and financial prospects on a standalone basis in light of

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developments in Horizon’s business, the sectors in which it operates and the economy and financial markets generally.

On October 15, 2022, Horizon received an unsolicited non-binding proposal from Sanofi to acquire the entire issued and to be issued ordinary share capital of Horizon for $90 per Horizon Share in cash (the “October 15 Proposal”). The closing price of a Horizon Share was $62.51 on the Nasdaq Global Select Market on October 14, 2022.

On October 16, 2022, the Horizon Board, together with Horizon’s senior management, and with the assistance of its outside financial and legal advisors, Morgan Stanley, J.P. Morgan, Cooley LLP and Matheson LLP, met to discuss the October 15 Proposal. During the meeting, Matheson reviewed the Horizon Board’s fiduciary duties under Irish law in the context of the proposed transaction. Representatives of Morgan Stanley and J.P Morgan reviewed recent market data and performance metrics regarding Horizon and provided an overview of Sanofi and other large multinational pharmaceutical companies. Representatives of Morgan Stanley reviewed a preliminary financial analysis for Horizon based on Horizon’s risk-adjusted long range plan, which had been reviewed by the Horizon Board at its regular board meeting in July 2022. Horizon had previously retained Morgan Stanley and J.P. Morgan for advice on takeover defense and activism. The Horizon Board also discussed Horizon’s strategic alternatives, including continuing to execute on its long-term strategy. Following discussion, the Horizon Board determined that the October 15 Proposal substantially undervalued Horizon and rejected the proposal. Following the meeting, the rejection was conveyed to Sanofi.

On October 19, 2022, Sanofi submitted a revised unsolicited non-binding proposal to acquire the entire issued and to be issued ordinary share capital of Horizon for $95 per Horizon Share in cash (the “October 19 Proposal”) and requested access to due diligence.

On October 21, 2022, the Horizon Board, together with Horizon’s senior management, Cooley and Matheson, met to discuss further fiduciary duty considerations relating to evaluation of the October 19 Proposal, including Horizon’s strategic alternative of remaining an independent public company and executing on its long range strategy.

On October 26, 2022, the Horizon Board held a regular board meeting in Dublin, Ireland. During the board meeting, the Horizon Board, together with Horizon’s senior management, and with the assistance of its outside financial and legal advisors, Morgan Stanley, J.P. Morgan, Cooley and Matheson, reviewed the October 19 Proposal. Senior management presented updates to Horizon’s risk-adjusted long range plan to reflect developments since July 2022, as well as key clinical data readouts expected in 2023 and potential near term performance opportunities and risks. Representatives of Morgan Stanley reviewed a preliminary financial analysis for Horizon based on Horizon’s updated risk-adjusted long range plan and key sensitivities in the valuation model. Representatives of Morgan Stanley and J.P. Morgan then discussed potential responses to Sanofi, including potential price guidance, and discussed conducting a market check, including the potential strategic parties with the capability of acquiring Horizon for cash, and the possible timing of any market check given the constraints of the Irish Takeover Rules. The Horizon Board discussed the risks of a market check given the risk of leaks and the disclosure obligations under the Irish Takeover Rules and agreed that conducting a market check at this time would be premature. After the financial advisors left the meeting, the Horizon Board reviewed Horizon’s strategic alternatives and determined that the October 19 Proposal significantly undervalued Horizon and did not justify allowing Sanofi access to any due diligence and rejected the proposal. The Horizon Board further determined to convey to Sanofi that the Horizon Board was not prepared to authorize a transaction at a double digit discount to Horizon’s 52-week high, which at that time was $120.54 per Horizon Share. Following the meeting, the proposal rejection and price guidance were provided to Sanofi.

On November 2, 2022, Horizon reported its third quarter 2022 financial results, increased its full-year 2022 net sales and adjusted EBITDA guidance and increased its TEPEZZA^® and KRYSTEXXA^® peak annual net sales expectations.

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The closing price of a Horizon Share was $72.93 on the Nasdaq Global Select Market on November 3, 2022.

On November 12, 2022, Sanofi submitted a revised unsolicited non-binding proposal to acquire the entire issued and to be issued ordinary share capital of Horizon for $105 per Horizon Share in cash (the “November 12 Proposal”). The November 12 Proposal indicated that Sanofi would not submit any further offers unless it received access to non-public information providing insight into potential incremental value.

On November 13, 2022, the Horizon Board, together with Horizon’s senior management, and with the assistance of its outside financial and legal advisors, Morgan Stanley, J.P. Morgan, Cooley and Matheson, met to discuss the November 12 Proposal. Horizon’s financial advisors reviewed the November 12 Proposal and the preliminary financial analyses of Horizon based on the risk-adjusted long-range plan. The Horizon Board determined to respond to Sanofi that although the November 12 Proposal did not meet Horizon’s guidance related to the 52-week high, Horizon was willing to provide limited due diligence on non-public, value-related items to enable Sanofi to submit a further enhanced offer. Horizon’s financial advisors and senior management further recommended that Horizon seek approval from the Irish Takeover Panel to reach out to Amgen, Janssen Global Services, LLC (“JNJ”) and one other global pharmaceutical company, such company being referred to herein as Company A, to determine whether they would be interested in receiving a confidential management presentation and considering an all cash acquisition of Horizon on an expedited timetable. The companies recommended for outreach were believed to have interest in a potential acquisition of Horizon and would have the financial means to complete an all cash acquisition of this size. The Horizon Board discussed the risks associated with the proposed market check and determined that the best way to maximize shareholder value would be to approach the specified other potential bidders and authorized Horizon’s senior management and its financial advisors to contact Amgen, JNJ and Company A, subject to obtaining the relevant consents from the Irish Takeover Panel. After the financial advisors left the meeting, the Horizon Board also discussed the proposed fee structure for Morgan Stanley and J.P. Morgan as financial advisors and reviewed the preliminary conflicts disclosure that each of the financial advisors submitted. Following the review, the Horizon Board authorized retaining Morgan Stanley as its financial advisor for purposes of Rule 3 of the Irish Takeover Rules and rendering a fairness opinion to the Horizon Board, and J.P. Morgan as a financial advisor and approved the proposed fees for both advisors.

After obtaining the consents from the Irish Takeover Panel, on November 14 and November 15, Horizon’s financial advisors contacted Amgen, JNJ and Company A, and conveyed that Horizon had received an unsolicited proposal to be acquired on an expeditious timetable and that Horizon was willing to allow initial diligence via a confidential management presentation. The financial advisors further communicated the guidance that the Horizon Board would not support a transaction at a double digit discount to Horizon’s previous 52-week high. All three parties expressed an interest in receiving a management presentation and engaging in due diligence to determine whether they would submit a proposal.

Horizon entered into confidentiality agreements with all four parties, each of which contained a standstill provision that permitted confidential discussions with Horizon and an automatic termination of the standstill if Horizon entered into an agreement to be acquired by a third party. Horizon and Amgen entered into the confidentiality agreement on November 18, 2022.

On November 17, 2022, Horizon gave management presentations to each of Sanofi and Company A and on November 18, 2022, Horizon gave management presentations to each of JNJ and Amgen. Following the management presentations, Horizon held due diligence calls with Sanofi, JNJ and Amgen to respond to specific value-related inquiries.

On November 19, 2022, Sanofi submitted a revised non-binding proposal at $106 per Horizon Share in cash and requested that full diligence commence by November 21st and that a transaction be announced by December 5, 2022. Later in the day, the financial advisors informed each of Amgen, JNJ and Company A that

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Horizon had received a higher proposal from the initial bidder and that Horizon needed to receive an indication of value by November 25, 2022. Company A indicated that it had decided not to continue in the process. JNJ indicated its interest in continuing in the process but that it would not be able to meet the November 25th timing and Amgen indicated that it had a strong interest in engaging and would be able to meet the timing for submitting a proposal.

On November 25, 2022, Robert A. Bradway, Chairman and Chief Executive Officer of Amgen, called Tim Walbert, Chairman, President and Chief Executive Officer of Horizon, to discuss Amgen’s interest in acquiring Horizon, the criticality of key Horizon management in a go-forward combined company and plans to submit an initial non-binding proposal to Horizon via its financial advisor, PJT Partners LP (“PJT Partners”). Following the call, Amgen submitted in writing, to Horizon’s financial advisors, a non-binding proposal at $105 per Horizon Share (the “Initial Amgen Proposal”).

On November 25, 2022, Sanofi reiterated its non-binding proposal at $106 per Horizon Share in cash as a best and final non-binding proposal and indicated that the proposal, if not accepted by November 26, 2022, should be considered withdrawn and discussions terminated (the “November 25 Proposal”).

On November 26, 2022, the Horizon Board, together with Horizon’s senior management, and with the assistance of its outside financial and legal advisors, Morgan Stanley, J.P. Morgan, Cooley and Matheson, met to discuss the November 25 Proposal and the Initial Amgen Proposal. The financial advisors reviewed the process status, potential next steps and expected timing. The financial advisors also explained that an announcement in mid-December would be advantageous from an acquirer financing perspective as it would provide any bridge financing sources with time before year-end to syndicate loans. Following discussion, the Horizon Board authorized Horizon’s senior management and advisors to allow Sanofi and Amgen to access a data room for confirmatory due diligence, share a draft transaction agreement and draft announcement under Rule 2.7 of the Irish Takeover Rules (a “Rule 2.7 Announcement”) with Sanofi and Amgen, share the targeted transaction announcement date of December 12, 2022 with both parties, and inform Sanofi of the receipt of a competitive bid from another party. The Horizon Board further authorized allowing JNJ to continue to engage in limited due diligence relating to sources of value but informing JNJ that two other parties were being given data room access and that they were falling behind. After the financial advisors left the meeting, the Horizon Board also reviewed updated conflict disclosures from J.P. Morgan relating to Amgen and JNJ.

Following the Horizon Board meeting, on November 26, 2022, Horizon’s financial advisors reached out to Amgen, Sanofi and JNJ to convey the messages authorized by the Horizon Board. On the same day, Cooley sent a draft transaction agreement to Amgen’s outside counsel, Sullivan & Cromwell LLP, and to Sanofi’s outside counsel. Further, on November 27, 2022, Horizon provided access to the electronic data room to both Amgen and Sanofi and their respective advisors.

Throughout the period from November 27, 2022 until December 9, 2022, Horizon responded to due diligence requests from both Amgen and Sanofi, providing access to requested information and documents in the electronic data room accessible to both parties or through clean teams pursuant to separate clean team agreements for certain competitively sensitive information, and held due diligence calls with each of Amgen and Sanofi as requested, covering various areas including corporate organization and structure, tax matters, commercial operations, litigation and legal compliance, executive compensation and employee benefits, intellectual property matters, regulatory matters, cybersecurity and data privacy and general corporate matters.

On November 29, 2022, in response to a report that the Wall Street Journal was about to publish an article that Horizon was in discussions to be sold, Horizon released an announcement of a possible offer under Rule 2.4 of the Irish Takeover Rules, confirming that it was in highly preliminary discussions with Amgen, Sanofi and JNJ. Following the Rule 2.4 announcement, Horizon did not receive any acquisition proposals from any third parties who were not part of the process.

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On November 30, 2022, Cooley sent Sullivan & Cromwell and William Fry LLP, Amgen’s Irish counsel, and Sanofi’s counsel a draft Rule 2.7 Announcement.

On December 2, 2022, Sanofi and Amgen each released a statement as required by Rule 2.12 of the Irish Takeover Rules confirming that any offer, if made, would be in cash.

On December 2, 2022, Sanofi’s counsel submitted a draft mark-up of the transaction agreement and the draft Rule 2.7 Announcement. Later in the day on December 2, 2022, Sullivan & Cromwell submitted a draft mark-up of the transaction agreement.

During the period from December 2, 2022 until December 9, 2022, Amgen’s and Sanofi’s counsel each negotiated a transaction agreement, disclosure schedules and Rule 2.7 Announcement, with Cooley and Matheson. The negotiations on the transaction agreement focused primarily on the structure of the scheme of arrangement, conditions to the acquisition, including the required antitrust and foreign investment clearances and related antitrust efforts covenants, remedy requirements and termination fees, and the operating covenants imposed on Horizon during the period prior to closing.

During this period, Amgen and Sanofi also provided copies of bridge credit agreements being negotiated by each party and Cooley and Matheson provided comments on the respective bridge credit agreements to Amgen and Sanofi’s counsel.

On December 3, 2022, the Horizon Board, together with Horizon’s senior management, and with the assistance of its outside advisors, Morgan Stanley, J.P. Morgan, Cooley and Matheson, met to discuss the status of the process and Horizon’s risk-adjusted long range plan. Senior management reviewed the key assumptions in Horizon’s risk adjusted long range plan and the material changes to the plan since the July board meeting. Following discussion, the Horizon Board approved the use of the risk-adjusted long range plan projections by Morgan Stanley in connection with providing financial analyses and a fairness opinion. The Horizon Board also reviewed the updated conflict disclosure of Morgan Stanley relating to Amgen and JNJ.

Subsequently, JNJ conveyed to Horizon’s financial advisors that it had decided not to submit a proposal and issued a statement on December 3, 2022 under Rule 2.8 of the Irish Takeover Rules that it did not intend to make an offer for Horizon.

On December 5, 2022, at the direction of the Horizon Board, representatives of Morgan Stanley and J.P. Morgan sent Amgen and Sanofi a process letter, requesting that final, fully-financed, irrevocable and binding written offers, with fully negotiated transaction agreements, be submitted by December 9, 2022. The process letter also indicated that the final written offer should confirm completion of due diligence.    

On December 6, 2022, Mr. Bradway called Mr. Walbert to convey Amgen’s vision for Horizon if Amgen were the winning bidder and to reiterate Amgen’s interest in retaining key employee talent at Horizon. The parties did not discuss any transaction terms and Mr. Bradway did not identify specific employees for retention or discuss any retention terms.

On December 9, 2022, Sanofi’s counsel submitted a draft transaction agreement and Rule 2.7 Announcement to Cooley. Sanofi later submitted a revised proposal of $109 per Horizon Share in cash.

Later in the day on December 9, 2022, Sullivan & Cromwell submitted a draft transaction agreement on behalf of Amgen to Cooley. At the direction of Amgen, PJT Partners subsequently sent to Morgan Stanley and J.P. Morgan Amgen’s revised proposal of $112.75 per Horizon Share in cash, plus potential cash dividends of $2.75 per share to be paid by Horizon to its shareholders of record on June 30, 2023 (assuming that the transaction had not closed before the dividend date) and $2.75 per share to be paid by Horizon to its shareholders of record on September 30, 2023 (assuming that the transaction had not closed before the dividend date).

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On December 9 and December 10, 2022, representatives of Morgan Stanley and J.P Morgan discussed the bid submissions with Horizon’s senior management and legal advisors and then, at the direction of and on behalf of Horizon, provided to each of Amgen and Sanofi feedback on their respective bid submissions. Following the feedback, on December 10, Sanofi submitted a revised proposal of $110.50 per Horizon Share in cash and Amgen submitted a revised proposal of $115.50 per Horizon Share in cash plus the two potential dividends of $2.75 per share to be paid by Horizon if the transaction had not closed prior to the dividend dates.

Following receipt of the December 10 bid submissions, representatives of Morgan Stanley and J.P. Morgan discussed the bid submissions with Horizon’s senior management and legal advisors and then, at the direction of and on behalf of Horizon, provided to each of Amgen and Sanofi feedback on their respective bid submission and requested each of Amgen and Sanofi to submit best and final proposals later in the day and before the Horizon Board meeting. With respect to Amgen’s bid, based on a belief that neither dividend was likely to become payable because the transaction was likely to be closed prior to June 30, 2023 (and a belief that the dividend feature would add unnecessary deal complexity), Mr. Walbert directed Morgan Stanley and J.P. Morgan, on behalf of Horizon, to communicate to Amgen a preference for a higher upfront cash offer without the dividends.

Later in the day on December 10, 2022, representatives of Sanofi contacted representatives of Morgan Stanley and J.P. Morgan to confirm that Sanofi’s December 10 proposal of $110.50 per Horizon Share in cash was Sanofi’s best and final proposal. Also later in the day on December 10, 2022, at the direction of Amgen, representatives of PJT Partners contacted representatives of Morgan Stanley and J.P. Morgan to communicate a willingness to consider a higher upfront cash offer without the dividends, and Mr. Walbert directed Morgan Stanley and J.P. Morgan, on behalf of Horizon, to propose to Amgen a transaction price of $118.25 per Horizon Share without the proposed dividends. Horizon’s proposal was later confirmed by Mr. Walbert in a telephone call with Mr. Bradway. Amgen subsequently proposed $116 per Horizon Share without the proposed dividends. Mr. Walbert, then directed Morgan Stanley and J.P. Morgan, on behalf of Horizon, to counter with a final price of $116.50 per Horizon Share, which Amgen subsequently agreed to pay.

Later in the day, the Horizon Board met in Dublin, Ireland, with Horizon’s senior management and financial and legal advisors, Morgan Stanley, J.P. Morgan, Cooley and Matheson, to consider the proposed terms of the best and final proposals from each of Amgen and Sanofi. Matheson reviewed the Horizon Board’s fiduciary duties under Irish law in the context of considering the proposals and the director responsibilities for information contained in the Rule 2.7 Announcement under the Irish Takeover Rules. Cooley and Sean Clayton, Horizon’s executive vice president and general counsel, reviewed the key legal terms of the respective transaction agreements negotiated with Amgen and Sanofi. Following discussions, the Horizon Board determined that the final proposal from Amgen was superior to the proposal from Sanofi and the near final drafts of the transaction agreements submitted by each party were substantially comparable. Morgan Stanley then rendered its oral opinion, subsequently confirmed in writing, that, as of December 10, 2022, and based upon and subject to the assumptions made, procedures followed, matters considered and qualifications and limitations on the scope of review undertaken by Morgan Stanley as set forth in the written opinion, the $116.50 per Horizon Share in cash to be received by holders of the Horizon Shares was fair from a financial point of view to such holders of Horizon Shares. Following discussions, the Horizon Board unanimously determined to accept the terms of the offer received from Amgen of $116.50 per Horizon Share, resolving that such offer was in the best interests of Horizon and Horizon Shareholders, and thereby approved the transaction and the execution and delivery of the Transaction Agreement and resolved to recommend to Horizon Shareholders that they approve the transaction.

On December 11, 2022, Sanofi issued a statement under Rule 2.8 of the Irish Takeover Rules stating that Sanofi no longer intended to make an offer for Horizon because the transaction price expectations did not meet its value criteria.

Following the meeting of the Horizon Board, Amgen and Horizon finalized the transaction agreement, the disclosure schedules to the transaction agreement and the terms of the Rule 2.7 Announcement. Amgen also finalized the terms of its bridge financing documents with its lenders. The Transaction Agreement was executed on December 11, 2022 and the Bridge Financing documents were executed on December 12, 2022.

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On the morning of December 12, 2022, Amgen and Horizon announced the execution of the Transaction Agreement and issued the Rule 2.7 Announcement.

Recommendation of the Horizon Board and Horizon’s Reasons for the Transaction

At its meeting on December 10, 2022, the Horizon Board unanimously approved the Transaction Agreement and determined that the Transaction Agreement and the transactions contemplated by the Transaction Agreement, including the Scheme, are in the best interests of Horizon and its shareholders and that the terms of the Scheme are fair and reasonable.

The Horizon Board unanimously recommends that Horizon shareholders vote:

Horizon’s Reasons for the Transaction

The Horizon Board carefully considered the Transaction Agreement and the Transaction, consulted with senior management and outside legal and financial advisors at various times, and took into account the reasons enumerated below (which are not in any relative order of importance).

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The Horizon Board also considered a variety of risks and other potentially negative reasons in determining whether to approve the Transaction Agreement and the Transactions, including the following:

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In light of these various reasons and having weighed the risks, uncertainties, restrictions and potentially negative reasons associated with the Scheme and Transaction with the potential benefits of the Scheme and the Transaction, the Horizon Board unanimously determined to accept the terms of the offer received from Amgen of $116.50 per Horizon Share, resolved that such offer was in the best interests of Horizon and its shareholders, and thereby approved the Transaction and the execution and delivery of the Transaction Agreement and resolved to recommend to the Horizon Shareholders that they approve the Transaction.

The foregoing discussion of the Horizon Board’s reasons for its recommendation that Horizon Shareholders approve the Transaction is not meant to be exhaustive, but addresses the material information and reasons considered by the Horizon Board in connection with its recommendation. In view of the wide variety of factors considered in connection with the evaluation of the Transaction and the complexity of these matters, the Horizon Board did not find it practicable to, and did not, quantify or otherwise assign relative weights to the specific reasons considered in reaching its determination and recommendation. Rather, Horizon’s directors made their determinations and recommendations based on the totality of the information presented to them, and the judgments of individual members of the Horizon Board may have been influenced to a greater or lesser degree by

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different reasons. In arriving at their respective recommendations, the members of the Horizon Board also considered the interests of Horizon’s executive officers and directors as described under “The Transaction—Interests of Certain Persons in the Transaction.”

Opinion of Morgan Stanley & Co. LLC

Horizon retained Morgan Stanley to act as its financial advisor in connection with a potential sale of Horizon and to provide financial advice and assistance and, upon the request of Horizon, to render a financial opinion in each case in connection therewith. Horizon selected Morgan Stanley to act as its financial advisor based on Morgan Stanley’s qualifications, expertise and reputation and its knowledge of and involvement in recent transactions in Horizon’s industry. On December 10, 2022, Morgan Stanley rendered its oral opinion, which was subsequently confirmed in writing, to the Horizon Board to the effect that, as of that date, and based upon and subject to the assumptions made, procedures followed, matters considered and qualifications and limitations on the scope of the review undertaken by Morgan Stanley as set forth in Morgan Stanley’s written opinion, the consideration to be received by the holders of Horizon Shares (other than shares held in treasury or owned by Amgen, Acquirer Sub or any subsidiaries of Amgen) was fair, from a financial point of view, to such holders of Horizon Shares.

The full text of the written opinion of Morgan Stanley delivered to the Horizon Board, dated as of December 10, 2022, which sets forth, among other things, the assumptions made, procedures followed, matters considered and qualifications and limitations on the scope of the review undertaken by Morgan Stanley in rendering its opinion, is attached to this proxy statement as Annex C and incorporated by reference into this proxy statement in its entirety. The summary of the opinion of Morgan Stanley in this proxy statement is qualified in its entirety by reference to the full text of the opinion. Horizon Shareholders are urged to, and should, read Morgan Stanley’s opinion carefully and in its entirety. Morgan Stanley’s opinion was directed to the Horizon Board and addressed only the fairness, from a financial point of view, of the consideration to be received by Horizon Shareholders (other than shares held in treasury or owned by Amgen, Acquirer Sub or any subsidiaries of Amgen). Morgan Stanley did not express any view on, and the opinion did not address, any other term or aspect of the Transaction Agreement or the Transaction. Morgan Stanley’s opinion did not address the relative merits of the Transaction as compared to business or financial strategies that might be available to Horizon, or the underlying business decision of Horizon to enter into the Transaction Agreement or proceed with any other transaction contemplated by the Transaction Agreement. Morgan Stanley’s opinion does not constitute an opinion or recommendation as to how the shareholders of Horizon should vote at the special meetings to be held in connection with the Transaction. The summary of Morgan Stanley’s opinion set forth in this proxy statement is qualified in its entirety by reference to the full text of Morgan Stanley’s opinion.

For purposes of rendering its opinion, Morgan Stanley, among other things:

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In arriving at its opinion, Morgan Stanley assumed and relied upon, without independent verification, the accuracy and completeness of the information that was publicly available or supplied or otherwise made available to Morgan Stanley by Horizon, which formed a substantial basis for Morgan Stanley’s opinion. With respect to the financial projections, Morgan Stanley assumed that they had been reasonably prepared on bases reflecting the best currently available estimates and judgments of the management of Horizon of the future financial performance of Horizon. In addition, Morgan Stanley assumed that the Transaction will be consummated in accordance with the terms set forth in the Transaction Agreement without any waiver, amendment or delay of any terms or conditions, including, among other things, that Amgen will obtain financing in accordance with the terms set forth in their draft commitment letters and that the definitive Transaction Agreement will not differ in any material respect from the draft thereof furnished to Morgan Stanley on December 10, 2022. Morgan Stanley assumed that in connection with the receipt of all the necessary governmental, regulatory or other approvals and consents required for the Transaction, no delays, limitations, conditions or restrictions will be imposed that would have a material adverse effect on the contemplated benefits expected to be derived in the Transaction. Morgan Stanley is not a legal, tax or regulatory advisor. Morgan Stanley is a financial advisor only and relied upon, without independent verification, the assessment of Horizon and its legal, tax, or regulatory advisors with respect to legal, tax or regulatory matters. Morgan Stanley expressed no opinion with respect to the fairness of the amount or nature of the compensation to any of Horizon’s officers, directors or employees or any class of such persons, relative to the consideration to be received by the holders of Horizon Shares in the Transaction. Morgan Stanley did not make any independent valuation or appraisal of the assets or liabilities of Horizon, nor was it furnished with any such valuations or appraisals. Morgan Stanley’s opinion is necessarily based on financial, economic, market and other conditions as in effect on, and the information made available to Morgan Stanley as of the date of such opinion. Events occurring after such date may affect Morgan Stanley’s opinion and the assumptions used in preparing it, and Morgan Stanley did not assume any obligation to update, revise or reaffirm its opinion.

Summary of Financial Analyses

The following is a brief summary of the material analyses performed by Morgan Stanley in connection with its oral opinion and the preparation of its written opinion letter, dated as of December 10, 2022, to the Horizon Board. The following summary is not a complete description of Morgan Stanley’s opinion or the financial analyses performed and factors considered by Morgan Stanley in connection with its opinion, nor does the order of analyses described represent the relative importance or weight given to those analyses.

In performing the financial analyses summarized below and in arriving at its opinion, Morgan Stanley utilized and was directed by the Horizon Board to rely upon, among other matters, the Horizon Projections, which are more fully described in the section of this proxy statement titled “—Horizon Unaudited Prospective Financial Information.” In accordance with direction from the Horizon Board, Morgan Stanley utilized the Horizon Projections in its financial analyses described below. Morgan Stanley utilized in each of its analyses described below the number of outstanding Horizon Shares on a fully diluted basis, as of December 8, 2022, utilizing the treasury stock method and including the dilutive impact resulting from additional equity grants anticipated to be issued in January 2023 and assuming maximum payout for PSUs, each as provided by Horizon management.

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Discounted Cash Flow Analysis

Morgan Stanley performed a discounted cash flow analysis for Horizon, which is designed to provide an implied value of a company by calculating the present value of the estimated future cash flows and terminal value of such company. Morgan Stanley calculated a range of implied fully diluted equity values per share of Horizon Shares, based on estimates of future unlevered free cash flows projected to be generated by Horizon for fiscal years 2023 through 2032 based on the Horizon Projections. These unlevered free cash flows were calculated by Morgan Stanley as net operating income after tax (1) less (a) capital expenditures and (b) tax-effected share-based compensation expense and (2) plus or minus changes in net working capital, which such estimated unlevered free cash flows, in each case, were reviewed and approved by the management of Horizon for Morgan Stanley’s use. Terminal value was calculated using perpetual growth rates ranging from negative 4% to negative 1%, with such rates selected upon the application of Morgan Stanley’s professional judgment and experience, given the nature of Horizon and its business, the industries in which it operates and the risk of generic competition. The unlevered free cash flows and terminal value were discounted to present values as of December 31, 2022, using a midyear convention, at discount rates ranging from 8.9% to 10.2%, which discount rates were selected upon the application of Morgan Stanley’s professional judgment and experience, to reflect an estimate of Horizon’s weighted average cost of capital determined by the application of the capital asset pricing model. The resulting range of implied aggregate values were then adjusted by the expected net debt of approximately $290 million of Horizon as of December 31, 2022, as provided by the management of Horizon, to derive a range of implied equity values. Morgan Stanley then divided the implied equity values by the number of fully diluted shares, as of December 8, 2022, as provided by the management of Horizon.

This analysis resulted in a range of implied equity values per Horizon Share of approximately $101 to $133. Morgan Stanley compared this range to the price per Horizon Share of $78.76 as of November 29, 2022, the last closing price of Horizon Shares prior to Horizon’s issuance of an announcement of a possible offer under Rule 2.4 of the Irish Takeover Rules (the “Last Unaffected Price”), and to the Scheme Consideration.

Sum-of-the-Parts Discounted Cash Flow Analysis

Morgan Stanley performed a sum-of-the-parts discounted cash flow analysis of Horizon, which is designed to provide an implied value of a company by calculating the present value of the estimated future cash flows and terminal values of separate product lines or businesses of a company. Morgan Stanley calculated estimates of future unlevered free cash flows projected to be generated by Horizon’s existing products and product candidates for fiscal years 2023 through 2037 based on the Horizon Projections, which was reviewed and approved by the management of Horizon for Morgan Stanley’s use.

In connection with this analysis, Morgan Stanley performed separate discounted cash flow analyses with respect to the following medicines and medicine candidates (and indications specified by the Horizon Projections):

The analyses assumed no contribution from Horizon’s inflammation medicines, given Horizon’s recent wind-down of its inflammation business activities.

For purposes of this analysis, Morgan Stanley utilized the Horizon Projections, which include probability of success risk adjusted estimates of the net revenue generated from TEPEZZA, KRYSTEXXA, UPLIZNA, Rare

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Disease BU and Development Candidates, and management’s allocation of related operating expenses, in each case for fiscal year 2023 through fiscal year 2037.

Morgan Stanley calculated terminal values for the foregoing items by applying perpetuity growth rates of between negative 15% to negative 100% to each existing medicine or medicine candidate, which Morgan Stanley estimated based on its professional judgment and experience, given the nature of Horizon and its business, the industries in which it operates and the risk of generic competition. The cash flows and terminal values were then discounted to present value as of December 31, 2022, using a midyear convention, at discount rates ranging from 8.9% to 10.2%. This range of discount rates was based on Morgan Stanley’s analysis of Horizon’s weighted average cost of capital determined by the application of the capital asset pricing model. The foregoing analyses reflect estimated research and development overhead costs as 40% of the total unallocated research and development costs and exclude the remaining 60% from the calculations, based on management guidance. The foregoing analyses assume (i) allocation of cost of goods sold based on Horizon management estimates, (ii) allocation of selling, general and administrative expenses and research and development costs based on Horizon management allocations and (iii) cash flow items allocated as a percentage of net sales based on Horizon management guidance. From the foregoing analyses, Morgan Stanley derived a range of illustrative aggregate values for each medicine and medicine candidate. Unallocated selling, general and administrative expenses and research and development overhead costs were then subtracted from the implied range to arrive at a burdened implied total aggregate medicine value. The resulting range of implied aggregate values were then adjusted by the expected net debt of approximately $290 million of Horizon as of December 31, 2022, as provided by the management of Horizon. Morgan Stanley then divided the implied equity values by the number of fully diluted shares, as of December 8, 2022, as provided by the management of Horizon.

This analysis resulted in a range of implied equity values per Horizon Share of approximately $102 to $113. Morgan Stanley compared this range to the Last Unaffected Price and to the Scheme Consideration.

Other Information

Morgan Stanley observed additional factors that were not considered part of Morgan Stanley’s financial analyses with respect to its opinion, but which were noted as reference data for the Horizon Board, including the following information described in the sections of this proxy statement titled “—Premium Paid Analysis,” “—Historic Trading Range,” and “—Analyst Price Targets.”

Precedent Transactions – Premia Paid Analysis

For reference only, and not as a component of its fairness analysis, Morgan Stanley considered, based on publicly available transaction information, transactions with global public biopharmaceutical targets larger than $10 billion in aggregate value, paid in all-cash consideration, since 2014 and announced on or before December 8, 2022. For these transactions, Morgan Stanley noted (i) the percentage premia paid over unaffected stock price and (ii) percentage premia paid over the volume-weighted average stock price over a thirty-day period.

Based on its analysis of the premia for these transactions and based upon the application of its professional judgment and experience, (i) Morgan Stanley selected a range of premia to unaffected stock price of 39%-64% based on 25th and 75th percentiles and applied such range to the Last Unaffected Price, which resulted in an implied price range per share of $109 to $130 and (ii) Morgan Stanley selected a range of premia of the unaffected price per share relative to the thirty-day volume-weighted average stock price of 45% to 85% based on 25th and 75th percentiles and applied such range to the Last Unaffected Price, which resulted in an implied price range per share of $102-$131. Morgan Stanley compared these ranges to the Last Unaffected Price, and to the Scheme Consideration.

No company or transaction utilized in the premia paid analysis is identical to Horizon or the Transaction. In evaluating the precedent transactions, Morgan Stanley made judgments and assumptions with regard to industry

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performance, general business, market and financial conditions and other factors beyond the control of Horizon, such as the impact of competition on the business of Horizon or the industry generally, industry growth and the absence of any adverse material change in the financial condition of Horizon or the industry or in the financial markets in general, which could affect the public trading value of the companies and the aggregate value and equity value of the transactions to which they are being compared. Mathematical analysis (such as determining the mean and median) is not in itself a meaningful method of using precedent transaction data.

Historical Trading Range

For reference only, and not as a component of its fairness analysis, Morgan Stanley reviewed the historic trading range of Horizon Shares for the 52-week period ending November 29, 2022, the last trading day prior to Horizon’s issuance of an announcement of a possible offer under Rule 2.4 of the Irish Takeover Rules and noted low and high per share closing prices of $57.84 on September 1, 2022, and $117.49 on April 21, 2022.

Analyst Price Targets

For reference only, and not as a component of its fairness analysis, Morgan Stanley reviewed and analyzed future public market trading price targets for Horizon Shares that were prepared and published by equity research analysts on or before November 29, 2022, the last trading day prior to Horizon’s issuance of an announcement of a possible offer under Rule 2.4 of the Irish Takeover Rules. These one-year forward targets reflected each analyst’s estimate of the future public market trading price of Horizon Shares. The range of undiscounted analyst price targets was $74.00 to $146.00 per Horizon Share as of November 29, 2022. Morgan Stanley also noted that the mean and median of equity analyst undiscounted price targets for Horizon Shares was $105 and $98, respectively.

In order to better compare the equity analysts’ stock price targets with the Scheme Consideration, based on its professional judgment and experience, Morgan Stanley discounted each analyst’s price target to present value by applying, for a one-year discount period, an illustrative discount rate of 10.1%, which was selected by Morgan Stanley based on Horizon’s assumed mid-point cost of equity of 10.1%. This analysis resulted in a discounted analyst price target range of $67 to $133 per Horizon Share, rounded to the nearest $1 per share.

The public market trading price targets published by equity research analysts do not necessarily reflect current market trading prices for Horizon Shares and these estimates are subject to uncertainties, including the future financial performance of Horizon and future financial market conditions.

General

In connection with the review of the Transaction by the Horizon Board, Morgan Stanley performed a variety of financial and comparative analyses for purposes of rendering its opinion. The preparation of a financial opinion is a complex process and is not necessarily susceptible to a partial analysis or summary description. In arriving at its opinion, Morgan Stanley considered the results of all of its analyses as a whole and did not attribute any particular weight to any analysis or factor it considered. Morgan Stanley believes that selecting any portion of its analyses, without considering all analyses as a whole, would create an incomplete view of the process underlying its analyses and opinion. In addition, Morgan Stanley may have given various analyses and factors more or less weight than other analyses and factors and may have deemed various assumptions more or less probable than other assumptions. As a result, the ranges of valuations resulting from any particular analysis described above should not be taken to be Morgan Stanley’s view of the actual value of Horizon.

In performing its analyses, Morgan Stanley made numerous assumptions with regard to industry performance, general business, regulatory, economic, market and financial conditions and other matters, which are beyond the control of Horizon. Any estimates contained in Morgan Stanley’s analyses are not necessarily indicative of future results or actual values, which may be significantly more or less favorable than those suggested by such estimates.

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Morgan Stanley conducted the analyses described above solely as part of its analysis of the fairness, from a financial point of view, of the consideration to be received by the holders of Horizon Shares (other than shares held in treasury or owned by Amgen, Acquirer Sub or any subsidiaries of Amgen) and in connection with the delivery of its opinion to the Horizon Board. These analyses do not purport to be appraisals or to reflect the prices at which Horizon Shares might actually trade.

The consideration to be received by the holders of Horizon Shares was determined through arm’s-length negotiations between Horizon and Amgen and was approved by the Horizon Board. Morgan Stanley acted as financial advisor to Horizon during these negotiations but did not recommend any specific consideration to Horizon or the Horizon Board, or that any specific consideration constituted the only appropriate consideration for the Transaction. Morgan Stanley’s opinion was not intended to, and does not, constitute an opinion or a recommendation as to how holders of Horizon Shares should vote at the special meetings to be held in connection with the Transaction.

Morgan Stanley’s opinion and its presentation to the Horizon Board was one of many factors taken into consideration by the Horizon Board in deciding to approve the execution, delivery and performance by Horizon of the Transaction Agreement and the Transaction. Consequently, the analyses as described above should not be viewed as determinative of the opinion of the Horizon Board with respect to the consideration or of whether the Horizon Board would have been willing to agree to different consideration. Morgan Stanley’s opinion was approved by a committee of Morgan Stanley investment banking and other professionals in accordance with its customary practice.

Horizon retained Morgan Stanley based upon Morgan Stanley’s qualifications, experience and expertise. Morgan Stanley is a global financial services firm engaged in the securities, investment management and individual wealth management businesses. Its securities business is engaged in securities underwriting, trading and brokerage activities, foreign exchange, commodities and derivatives trading, prime brokerage, as well as providing investment banking, financing and financial advisory services. Morgan Stanley, its affiliates, directors and officers may at any time invest on a principal basis or manage funds that invest, hold long or short positions, finance positions, and may trade or otherwise structure and effect transactions, for their own account or the accounts of its customers, in debt or equity securities or loans of Amgen, Horizon, or any other company, or any currency or commodity, that may be involved in the Transaction, or any related derivative instrument.

Under the terms of its engagement letter, Morgan Stanley provided Horizon financial advisory services and the Horizon Board with a fairness opinion, described in this section and attached to this proxy statement as Annex C, in connection with the Transaction, and Horizon has agreed to pay Morgan Stanley a fee of $80 million for its services and for the fairness opinion, all of which is contingent upon the consummation of the Transaction. Horizon has also agreed to reimburse Morgan Stanley for its documented, out-of-pocket expenses, including documented, out-of-pocket fees of outside counsel and other professional advisers, incurred in connection with its engagement. In addition, Horizon has agreed to indemnify Morgan Stanley and its affiliates, its and their respective officers, directors, employees and agents and each other person, if any, controlling Morgan Stanley or any of its affiliates against certain losses, claims, damages, liabilities and expenses related to, arising out of or in connection with Morgan Stanley’s engagement.

During the two (2) years preceding the date of delivery of Morgan Stanley’s written opinion, Morgan Stanley or its affiliates provided financing services to Amgen and the majority-controlled affiliates of Amgen and have received fees of approximately $10-$15 million in connection with such services, and financial advisory and financing services to Horizon and its majority-controlled affiliates and have received fees of approximately $30-$35 million in connection with such services (excluding compensation for asset management services, share buybacks and derivative transactions). Morgan Stanley may also seek to provide financial advisory and financing services to Amgen, Horizon and their respective affiliates in the future and would expect to receive fees for the rendering of these services.

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Horizon Unaudited Prospective Financial Information

On an annual basis, Horizon prepares a long-range business and financial plan. Horizon’s current long-range business and financial plan was reviewed by the Horizon Board at its meeting in July 2022 in connection with its with its long-range and strategic planning process.

The long-range plan included certain unaudited prospective financial information concerning Horizon on a standalone basis for the years 2023 through 2037, adjusted to reflect Horizon management’s estimate of the probability of technical and regulatory success for Horizon’s development candidates and certain other matters. These unaudited risk-adjusted projections are referred to as the “Horizon Projections.” In connection with the consideration of a possible transaction, Horizon management updated the Horizon Projections to reflect developments between the July 2022 board meeting and December 3, 2022, and reviewed the Horizon Projections with the Horizon Board, and the Horizon Board authorized Morgan Stanley to use the Horizon Projections for purposes of its financial analysis and fairness opinion.

Other than annual financial guidance provided to investors, which is generally updated each quarter, Horizon does not as a matter of course make public long-term forecasts or projections as to future performance, revenues, earnings or other results, due to, among other reasons, the inherent difficulty of accurately predicting financial performance for future periods and the uncertainty of the underlying assumptions and estimates. However, the Horizon Projections are being included in this proxy statement to give Horizon Shareholders access to certain non-public information provided to the Horizon Board and its financial advisors. The inclusion of the financial projections by Horizon should not be regarded as an indication that the Horizon Board, Horizon, Morgan Stanley, or any other recipient of this information considered, or now considers, it to be an assurance of the achievement of future results or an accurate prediction of future results, and they should not be relied on as such.

In addition, the Horizon Projections were not prepared with a view toward public disclosure or with a view toward compliance with the published guidelines established by the SEC or the American Institute of Certified Public Accountants for preparation or presentation of prospective financial information, or GAAP, but, in the view of Horizon management were prepared on a reasonable basis, reflected the best available estimates and judgments at the time of preparation, and presented as of the time of preparation, to the best of management’s knowledge and belief, the expected course of action and the expected future financial performance of Horizon and estimated outcomes of other future events. However, this information is not fact and should not be relied upon as being necessarily indicative of future results, and readers of this proxy statement are cautioned not to place undue reliance on the Horizon Projections.

The Horizon Projections reflect estimates and assumptions made by Horizon senior management with respect to the peak sales of existing medicines, the length of exclusivity of medicines, the timing of entry of competitive products and the timing of generic competition, product launch years, peak sales and patent and regulatory exclusivity for identified development candidates, general business, economic, competitive, regulatory and other market and financial conditions and other future events, all of which are difficult to predict and many of which are beyond Horizon’s control. In particular, the Horizon Projections, while presented with numerical specificity, necessarily were based on numerous variables and assumptions that are inherently uncertain. Because the Horizon Projections cover multiple years, by their nature, they become subject to greater uncertainty with each successive year and are unlikely to anticipate each and every circumstance that could have an effect on Horizon’s business and its results of operations. The Horizon Projections were developed solely using the information available to Horizon senior management at the time they were created and reflect assumptions as to certain business decisions that are subject to change. Important factors that may affect actual results or that may result in the Horizon Projections not being achieved include the risk factors described in Horizon’s annual report on Form 10-K for the fiscal year ended December 31, 2021, and subsequent quarterly reports on Form 10-Q and current reports on Form 8-K. The Horizon Projections also reflect assumptions as to certain business decisions that are subject to change. Modeling and forecasting the future in the biopharmaceutical industry, in particular, is a highly speculative endeavor.

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The Horizon Projections contain certain non-GAAP financial measures that Horizon believes are helpful in understanding its past financial performance and future results. Horizon’s senior management regularly uses a variety of financial measures that are not in accordance with GAAP for forecasting, budgeting and measuring financial performance. The non-GAAP financial measures are not meant to be considered in isolation or as a substitute for comparable GAAP measures. While Horizon believes that these non-GAAP financial measures provide meaningful information to help investors understand Horizon’s operating results and to analyze Horizon’s financial and business trends on a period-to-period basis, there are limitations associated with the use of these non-GAAP financial measures.

None of Horizon, Amgen, the combined group after the Transaction (the “Combined Group”) or their respective affiliates, advisors, officers, directors or other representatives can provide any assurance that actual results will not differ from the Horizon Projections and none of them undertakes any obligation to update, or otherwise revise or reconcile, the Horizon Projections to reflect circumstances existing after the date the Horizon Projections were prepared or to reflect the occurrence of future events, even in the event that any or all of the assumptions underlying the Horizon Projections are shown to be in error. Except as required by applicable securities laws, Horizon does not intend to make publicly available any update or other revision to the Horizon Projections, even in the event that any or all assumptions are shown to be in error. None of Horizon or its affiliates, advisors, officers, directors or representatives has made or makes any representation to any Horizon Shareholder or other person regarding Horizon’s ultimate performance compared to the information contained in the Horizon Projections or that forecasted results will be achieved. Horizon has made no representation to Amgen, in the Transaction Agreement or otherwise, concerning the Horizon Projections. The Horizon Projections have been prepared by, and are the responsibility of, Horizon management. In addition, PricewaterhouseCoopers LLP has neither audited, reviewed, examined, compiled nor applied agreed-upon procedures with respect to the Horizon Projections and, accordingly, PricewaterhouseCoopers LLP does not express an opinion or any other form of assurance with respect thereto. The PricewaterhouseCoopers LLP report included in Horizon’s Annual Report on Form 10-K for the year ended December 31, 2022 that is incorporated by reference into this proxy statement relates to Horizon’s previously issued financial statements. It does not extend to the Horizon Projections and should not be read to do so.

The following tables present a summary of the Horizon Projections:

The Irish Takeover Panel considers the forward-looking financial information for Horizon for each of the calendar years from 2023 to 2037, as set out above and used by Morgan Stanley in connection with its financial analyses for the purpose of preparing its fairness opinion, to be profit forecasts within the meaning of Rule 28 of the Irish Takeover Rules. However, following consideration of the exceptional circumstances of the matter the Irish Takeover Panel decided to waive (i) the requirement under Rule 28.1(a) of the Irish Takeover Rules to have the forecast for the 2023 calendar reported on by an independent accountant and Morgan Stanley, or any other

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financial advisor, and (ii) the requirement under Rule 28.2(a) for the Horizon Board to provide certain confirmations concerning the forecasts for the calendar years from 2024 to 2037, because the relevant forward-looking information is included in this proxy statement only because it is required by the SEC.

Financing

Horizon is required to use its reasonable best efforts, and to cause each of its subsidiaries and its and their respective officers, employees and advisors and other representatives, to use their respective reasonable best efforts, to provide to Amgen such assistance as may be reasonably requested by Amgen that is customary in connection with, among other things, (1) the arranging, obtaining and syndication of Amgen’s financing, (2) certain exchange offers, offers to purchase and/or consent solicitations with respect to certain of Horizon’s outstanding debt securities, and (3) the redemption of Horizon’s outstanding debt securities and repayment of the obligation outstanding under Horizon’s credit agreement, in each case subject to certain exceptions and qualifications.

The consummation of the Transaction is not conditioned upon the consummation of, or the receipt by Amgen of proceeds from, any debt financing.

Interests of Certain Persons in the Transaction

In considering the recommendation of the Horizon Board with respect to the Transaction Agreement, you should be aware that some of Horizon’s directors and executive officers have interests in the Transaction that are in addition to, or different from, any interests of Horizon Shareholders generally. These interests are described in more detail below, and, with respect to the named executive officers of Horizon, are quantified in the table below. The Horizon Board was aware of these interests and considered them when it adopted the Transaction Agreement and approved the Transaction.

Treatment of Equity Awards

Each Option that is outstanding as of immediately prior to the effective time (whether or not vested) shall be canceled and converted into the right to receive cash, without interest, in an amount equal to (a) the total number of Horizon Shares subject to such Option immediately prior to the effective time, multiplied by (b) the excess of (i) $116.50 over (ii) the exercise price payable per Horizon Share under such Option.

Each Horizon RSU that is outstanding as of immediately prior to the effective time (whether or not vested) shall (a) if granted to a non-employee member of the Horizon Board or held by a person who, as of the Completion is a former service-provider of Horizon, be canceled and converted into the right to receive cash, without interest, in an amount equal to (i) the total number of Horizon Shares subject to such Horizon RSU immediately prior to the effective time multiplied by (ii) $116.50 and (b) if not granted to an individual described in clause (a) above, be canceled and converted into an Amgen RSU denominated in shares of Amgen common stock. The number of shares of Amgen common stock subject to each such Amgen RSU shall be equal to the product (rounded down to the nearest whole number) of (a) the total number of shares subject to such Horizon RSU immediately prior to the effective time multiplied by (b) (i) $116.50 divided by (ii) the volume weighted average of the per share closing price of Amgen common stock on the Nasdaq Global Select Market for five trading days ending on the second business day prior to the Completion. Following the effective time, each Amgen RSU shall continue to be governed by the same terms and conditions (including vesting terms) as were applicable to such Horizon RSU immediately prior to the effective time.

Each Horizon PSU that is outstanding as of immediately prior to the effective time (whether or not vested) shall be canceled and converted into the right to receive cash, without interest, in an amount equal to (i) the total number of Horizon Shares issuable upon settlement of such PSU as determined, in accordance with the terms of such PSU, by the compensation committee of the Horizon Board prior to the effective time multiplied by (ii) $116.50.

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The following table sets forth (i) the number of Horizon Shares underlying the Horizon Options, Horizon RSUs, and Horizon PSUs, whether vested or unvested, held by each Horizon executive officer and director as of January 13, 2023, the latest practicable date to determine these numbers before the filing of this proxy statement and (ii) the value of such equity awards as of such date, determined in each case by multiplying (a) the number of Horizon Shares subject to the Horizon equity award (assuming, in the case of Horizon PSUs, performance at target levels) by (b) $116.50, less the applicable exercise price for Options. For information regarding the beneficial ownership of Horizon Shares held by each of Horizon’s directors and named executive officers and all of Horizon’s directors and executive officers as a group, see the section of this proxy statement entitled “Security Ownership of Management and Certain Beneficial Owners.” Each of Horizon’s directors and executive officers shall be entitled to receive, for each Horizon Share he or she holds, the same consideration in the same manner as other Horizon Shareholders shall receive for a Horizon Share in connection with the Transaction. In addition, for information regarding the treatment in connection with the Transaction of the purchase rights granted to Horizon’s executive officers under the Horizon 2020 Employee Share Purchase Plan (the “ESPP”), see the section of this proxy statement entitled “—Treatment of Purchase Rights Under the Employee Share Purchase Plan.”

Treatment of Purchase Rights Under the Employee Share Purchase Plan

The ESPP allows Horizon’s regular full-time employees, including Horizon’s executive officers, to purchase up to $25,000 per year of Horizon Shares at the lower of: (i) 85% of the fair market value on the

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applicable offering date (as defined in the ESPP) or (ii) 85% of the fair market value on the applicable purchase date (as defined in the ESPP), with the objective of allowing employees to profit when the value of Horizon Shares increases over time.

The most recent purchase date under the current ESPP offering occurred on December 1, 2022, and the next purchase date under the current ESPP offering is scheduled to occur on June 1, 2023. Under the terms of the Transaction Agreement, (i) following the date of the Transaction Agreement, participation has been and will be limited to those employees who participated in the ESPP immediately prior to the date of the Transaction Agreement, (ii) no participant has been or will be allowed to increase the percentage of his or her payroll deduction election from the percentage in effect on the date of the Transaction Agreement, (iii) no new ESPP offering will be authorized or commence on or after the date of the Transaction Agreement, (iv) the last day and final purchase date of the current ESPP offering will be the earlier of (x) the last business day prior to the effective time or (y) June 1, 2023 and (v) the ESPP shall terminate as of the effective time, contingent upon the occurrence of the effective time.

Severance Entitlements

Each Horizon executive officer is a party to an executive employment agreement with Horizon or a related corporation, which provides for the severance benefits described below if Horizon terminates an executive officer’s employment without “cause,” or if the executive officer terminates his or her employment for “good reason” (such terms as defined in the employment agreement and each such termination, a “qualifying termination”). In the event of a qualifying termination within three (3) months prior to or eighteen (18) months following a change in control (including the Transaction), the executive is entitled to receive: (i) a severance payment equal to the executive’s base salary then in effect, payable in substantially equal installments over eighteen (18) months (or thirty-six (36) months in the case of Mr. Walbert); (ii) a lump sum payment equal to a severance multiple of the executive’s target annual bonus for the year of termination; (iii) in the event the executive officer timely elects continued coverage, reimbursement of premiums to continue benefits coverage under the Consolidated Omnibus Reconciliation Act of 1985, as amended (COBRA), for up to eighteen (18) months (or thirty-six (36) months in the case of Mr. Walbert) and (iv) full acceleration of time-based vesting equity awards. The severance multiples are three (3) for Mr. Walbert and one and one half (1.5) for the other executive officers.

As provided under their employment agreements, in the event of a termination of employment due to death or disability, each executive officer is entitled to receive a pro-rata bonus for the performance period in which the termination occurs, based on actual performance through the date of termination, and payable in a single lump sum within thirty (30) days after termination.

All severance benefits (other than due to death or complete disability) provided to Horizon’s executive officers pursuant to their employment agreements are contingent upon the executive’s execution of a standard release of claims in Horizon’s favor.

Retention Bonuses

In December 2022, Horizon granted retention bonus awards to certain employees, including two executive officers: Elizabeth H.Z. Thompson, Horizon’s Executive Vice President, Research and Development, and Sean Clayton, Horizon’s Executive Vice President, General Counsel. The aggregate amount of such retention bonus awards granted to all employees (including those awarded to Dr. Thompson and Mr. Clayton) was $20 million. Such retention bonus awards are payable in two installments: (i) fifty percent (50%) upon the Completion Date and (ii) the remaining fifty percent (50%) on the six (6) month anniversary of the Completion Date, subject in each case to continued employment through such date (provided that certain employees, including Dr. Thompson and Mr. Clayton, instead received payment of their bonus awards in December 2022, subject to a claw-back right that lapses in accordance with the same conditions). In the event of a recipient employee’s (i) termination without

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“cause,” (ii) termination due to death or disability or (iii) resignation for “good reason” (each as defined in the agreements governing such retention bonuses) at any time while a portion of the retention bonus award or the claw-back right, as applicable, is outstanding, such outstanding portion of the retention bonus award shall vest and be paid, or the remaining claw-back right will lapse, as applicable, in connection with such termination or resignation.

Short-term Acceleration of Certain Payments

In December 2022, Horizon took the following steps to eliminate or mitigate the potential impact of Section 280G of the Code on past or future payments to certain executive officers: accelerating into December 2022 (a) the vesting and settlement of RSU awards and/or PSU awards that would otherwise have vested and settled in January 2023 and/or (b) the payment of annual bonus amounts in respect of fiscal year 2022.

Make-Whole Payments

Horizon intends to enter into tax reimbursement agreements with certain employees, including certain executive officers (excluding the Chief Executive Officer or other named executive officers), pursuant to which Horizon would agree to make tax reimbursement payments to such employees to the extent such employees are subject, in connection with the transactions, to an excise tax imposed by Section 4999 of the Code in an amount that generally shall place them in the same after-tax position that they would have been in if no excise tax had applied and no tax reimbursement payment had been made. Under the terms of the Transaction Agreement, the maximum potential tax reimbursement payments to all affected employees shall not exceed $30 million in the aggregate. The amount of any such tax reimbursement payment and the employees who receive tax reimbursement agreements shall be based on a number of factors, including the aggregate reimbursement limit described above, and is uncertain as of the date of this proxy statement.

Employee Benefits Matters

As further described in the section of this proxy statement entitled “The Transaction Agreement—Employee Matters,” the Transaction Agreement generally requires Amgen to provide certain compensation that is no less favorable than immediately prior to the effective time and benefits that are in the aggregate no less favorable than similarly-situated Amgen employees both for one (1) year following the effective time to all Horizon employees, including executive officers, who remain employed following the effective time.

Director and Officer Indemnification Insurance

Pursuant to the terms of the Transaction Agreement, Horizon’s directors and officers shall be entitled to certain ongoing indemnification and coverage under directors’ and officers’ liability and fiduciary liability insurance policies following the Transaction. Such indemnification and insurance coverage is further described in “—The Transaction Agreement—Directors’ and Officers’ Indemnification and Insurance.”

Quantification of Payments and Benefits to Horizon’s Named Executive Officers

The table below sets forth the information required by Item 402(t) of Regulation S-K regarding the amount of payments and benefits that each of Horizon’s named executive officers may receive in connection with the transaction. While Brian K. Beeler, Horizon’s former Executive Vice President and General Counsel, was a named executive officer for Horizon’s most recently completed fiscal year, he terminated employment prior to the date on which the Transaction Agreement was executed and is not entitled to any payments or benefits that are based on or otherwise relate to the Transaction. In addition, Barry J. Moze, Horizon’s former Executive Vice President and Chief Administrative Officer, and Paul W. Hoelscher, Horizon’s former Executive Vice President and Chief Financial Officer, were each named executive officers for Horizon’s most recently completed fiscal year. Although Mr. Moze and Mr. Hoelscher each terminated employment prior to the date on which the

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Transaction Agreement was executed, each still holds outstanding Horizon share awards and are entitled to certain equity vesting acceleration that is based upon or otherwise relates to the Transaction.

Assumptions

Unless otherwise noted, the estimates set forth in the table below assume the following:

The amounts set forth in the table below are based on multiple assumptions that may or may not actually occur or be accurate on the relevant date, including assumptions above and described in footnotes to the table. The amounts below do not reflect certain compensation actions that may occur before the effective time. The actual amounts payable to Horizon’s named executive officers, if any, will depend on whether the named executive officer incurs a qualifying termination, the date of termination of the named executive officer’s employment (if applicable), the closing date of the Transaction, the value of shares of Amgen common stock on the termination date, the manner of termination and the terms of the plans or agreements in effect at such time.

Golden Parachute Compensation

The cash payments to each other named executive officer consist of (a) base salary continuation for a period of eighteen (18) months following the named executive officer’s termination and (b) a lump sum cash payment equal to 1.5 times the named executive officer’s target annual bonus amount as of the date of termination

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The cash severance payments are “double-trigger” benefits that would be payable upon a termination without cause or resignation with good reason that occurs, in each case, during the three (3) months prior to or eighteen (18) months following the effective time. For more information, please see the section of this proxy statement entitled “—Interests of Certain Persons in the Transaction—Severance Entitlements.”

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Amgen’s Reasons for the Transaction

Amgen believes there is a compelling strategic and financial rationale for undertaking the Transaction, which is expected to deliver the following benefits:

The Transaction adds first-in-class and best-in-class innovative medicines that fit well with Amgen’s portfolio and strategic vision

Amgen’s global scale and 20-year history in commercializing inflammation therapies can accelerate growth of Horizon’s portfolio

Horizon’s platform strategies in R&D and manufacturing can be strengthened by Amgen’s 40-year history in biologics, development and manufacturing

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Strong financial profile with non-GAAP EPS accretion from 2024 and significant cash flow generation enabling continued attractive shareholder payouts and investment in innovation

Regulatory Approvals Required

Antitrust and Foreign Investment Law Matters

Under the HSR Act and the rules promulgated thereunder, certain transactions may not be completed until certain information and materials have been furnished to DOJ and the FTC and the applicable HSR Act waiting period requirements have been satisfied. The waiting period under the HSR Act applicable to the Transaction is thirty (30) calendar days, unless the waiting period is terminated earlier or extended. If the DOJ or FTC issues a request for additional information and documentary materials (a “Second Request”), the parties must observe a second thirty (30)-day waiting period, which would begin to run only after both parties have substantially complied with such Second Request, unless the waiting period is terminated. The parties may also agree with the DOJ or FTC to not consummate the Transaction for a specified period of time. If any waiting period expires on a Saturday, Sunday or federal holiday, then the period is extended until 11:59 p.m., Eastern time, in the U.S. on the next day that is not a Saturday, Sunday or federal holiday.

The Transaction is subject to the provisions of the HSR Act and therefore cannot be completed until each of Amgen and Horizon file a notification and report form with the DOJ and the FTC under the HSR Act and the applicable waiting period (and any extension thereof) has expired or been terminated. Amgen and Horizon each filed a notification and report form with respect to the Transaction with the DOJ and the FTC under the HSR Act on December 29, 2022. The waiting period with respect to the notification and report forms filed under the HSR Act is scheduled to expire at 11:59 p.m., Eastern time, on January 30, 2023, unless extended or earlier terminated.

The Transaction is also conditioned on the receipt of approvals or clearances under the (a) applicable Antitrust Laws (as defined in the Transaction Agreement) of each of Germany and Austria and (b) applicable Foreign Investment Laws (as defined in the Transaction Agreement) of France, Germany, Denmark and Italy. The German merger control filing was submitted on January 13, 2023 and its review period is due to expire at the latest one month from submission, i.e., on February 13, 2023, unless extended or earlier terminated. The Austrian merger control filing was submitted on January 16, 2023 and its review period is due to expire at the latest four weeks from submission, i.e., on February 13, 2023, unless extended or earlier terminated.

The Danish Investment Screenings Act allows for the submission of a pre-screening notification under the Danish FDI regime. In line with this option, on December 23, 2022, Amgen submitted a notification for a pre-screening of the Transaction to the DBA. On January 2, 2023, the DBA informed the parties that based on

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the pre-screening notification, they were unable to confirm whether the Transaction falls within the applicable FDI framework and requested a formal filing for approval to be submitted. Thus, in line with the Danish FDI regime contained in the Investment Screening Act and the accompanying executive orders, Amgen submitted the formal filing to the DBA on January 11, 2023, and received confirmation that the filing is complete on January 12, 2023. Under the Danish FDI regime, the DBA reviews whether foreign direct investments and certain special economic agreements pose a threat to the national security or public order in Denmark. According to the applicable regulation, the DBA has sixty (60) working days to review the FDI filing. The review period may be extended to ninety (90) business days if further review is required or if the parties submit mitigation measures before the end of the period. The deadline of ninety (90) business days is not binding on the DBA. If the DBA has not issued a decision at the end of the review period, the Transaction will not automatically be considered approved.

In line with the Italian regulation on the exercise of the special powers of the state on strategic assets and activities (i.e., Italian Law Decree No. 21 of March 15, 2012, converted with amendments into Law no. 56 of May 11, 2012, as subsequently amended and integrated), on January 3, 2023, Amgen submitted a pre-filing (prenotifica) pursuant to Article 2-quater of Law Decree No. 21/2012 and Article 7 of Presidential Decree No. 133/2022 to the Italian Presidency of the Council of Ministers. The pre-filing is a tool introduced in the Italian FDI regulation, in order to seek a confirmation that the FDI regulation does not concretely apply to the notified transaction, or else a declaration that, even if it applies, there are clearly no risks of prejudice to the relevant public interests and the transaction can be cleared. According to the applicable regulation, the Presidency of the Council of Ministers has up to thirty (30) calendar days to conclude the screening activated with the pre-filing. If the Presidency of the Council of Ministers confirms that the Transaction falls outside the scope of the mandatory screening mechanism (or that, in any event, it clearly does not pose any risks of prejudice to the relevant public interests) the Transaction can be consummated. If the authority does not reply in the aforementioned deadline, or if it informs Amgen that the Transaction is within the scope of the mandatory screening mechanism, a formal filing for approval must be submitted, and approval must be obtained before the Transaction can be consummated.

In line with the German FDI regime contained in the German Foreign Trade Act and the German Foreign Trade Ordinance, on January 4, 2023, Amgen submitted a filing for clearance to the BMWK. Under the German FDI regime, the BMWK reviews whether the Transaction is likely to endanger the public order or security of the Federal Republic of Germany or of another member state of the European Union or in connection with certain projects or programs within the interest of the European Union. Upon receipt of a filing, the BMWK can clear the Transaction pursuant to Section 58a para. 1 of the German Foreign Trade Ordinance (in case of mandatory filings) or, alternatively, issue a clearance certificate pursuant to Section 58 para. 1 of the German Foreign Trade Ordinance (in case of voluntary filings). In both cases, clearance is deemed to have been granted within two (2) months from receipt of the filing if the BMWK has not cleared the Transaction or initiated formal review proceedings. If the BMWK initiates formal review proceedings, it has four (4) months after having received the complete set of information required for the formal review proceedings, to decide pursuant to Section 59 of the German Foreign Trade Ordinance whether to clear the Transaction, to prohibit the Transaction or to issue orders to ensure the public order and security of the Federal Republic of Germany.

In accordance with French Foreign Investment Regulations, on January 16, 2023, Amgen submitted a filing for clearance to the Minister. Under the French FDI regime, the Minister reviews whether the Transaction falls within the list of sectors specifically listed by the French Foreign Investment Regulations and, if so, whether it may raise concerns with regard to the protection of national interests. The Minister has 30 working days from receipt of a complete filing (Phase 1) to confirm the investment is not covered by the French FDI regime, approve the Transaction unconditionally, or launch an additional examination (Phase 2) to determine if conditions are required to safeguard national interests. The French Foreign Investment Regulations give the Minister an additional 45 working days to complete the second phase of examination. If the Minister fails to issue a decision within the applicable review period the Transaction would be deemed rejected.

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The Transaction is further conditioned on there not having been issued by a court of competent jurisdiction, and remain in effect, an order that prevents the consummation of the Transaction, and the absence of any law or order that directly or indirectly prohibits the consummation of the Transaction or imposes a Burdensome Condition (as defined in the Transaction Agreement).

At any time before or after the consummation of the Transaction, notwithstanding the termination or expiration of the waiting period under the HSR Act, the DOJ, FTC, state attorneys general or non-U.S. authorities could take such action under the Antitrust Laws as it deems necessary or desirable in the public interest, including seeking to preliminarily or permanently enjoin the Completion, seeking divestiture of substantial assets of one or both of the parties, requiring the parties to license or hold separate assets or terminate existing relationships and contractual rights, or requiring the parties to agree to other remedies. Private parties may also seek to take legal action under the antitrust laws under certain circumstances, including by seeking to intervene in the regulatory process, to litigate to enjoin or overturn regulatory approvals or to bring private actions to enjoin the Transaction. Any of these actions could impede or preclude obtaining regulatory approvals or consummating the Transaction. There can be no assurances that a challenge to the Transaction will not be made or that, if a challenge is made, Horizon will prevail.

One or more governmental bodies could take such action under Foreign Investment Laws as it deems necessary or desirable in order to mitigate any identified national security or public order concerns, including seeking divestiture of assets or operations, the safeguarding of intellectual property rights, the protection of sensitive information from unauthorized access, requiring the parties to continue to provide strategic capabilities or requiring the parties to agree to other remedies.

Amgen and Horizon have agreed to use their respective reasonable best efforts to obtain the regulatory approvals required to consummate the Transaction, subject to certain limitations as set forth in the Transaction Agreement. Although Horizon expects that all required antitrust and foreign investment clearances and approvals will be obtained, there can be no assurances that these regulatory clearances and approvals will be timely obtained, obtained at all, or that the granting of these regulatory clearances and approvals will not involve the imposition of additional conditions, restrictions, qualifications, requirements or limitations on the Completion, including the requirement to divest assets, license or hold separate assets, or terminate existing relationships and contractual rights, or agree to other remedies, or require changes to the terms of the Transaction Agreement. These conditions or changes could result in the conditions to the Transaction not being satisfied.

Irish High Court Approvals

The Scheme of Arrangement requires the approval of the Irish High Court, which involves an application by Horizon to the Irish High Court to sanction the Scheme.

Payment of Consideration

Settlement of the Scheme Consideration to which any Horizon Shareholder is entitled will be paid to Horizon Shareholders of record within fourteen (14) days of the Completion. For further information regarding the settlement of consideration, see the section of this proxy statement entitled “Part 2—Explanatory Statement—Settlement, Delisting and Dealings.”

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NO DISSENTERS’ RIGHTS

Under Irish law, holders of Horizon Shares do not have appraisal or dissenters’ rights with respect to the Transaction or any of the other transactions described in this proxy statement.

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MATERIAL TAX CONSEQUENCES OF THE PROPOSED TRANSACTION

Irish Tax Considerations

This is a summary of the principal Irish tax considerations for certain beneficial owners of Horizon Shares who receive cash under the Scheme based on existing Irish taxation laws and our understanding of the practices of the Irish Revenue Commissioners as of the date hereof. Legislative, administrative or judicial changes may modify the tax consequences described in this summary. Furthermore, we can provide no assurances that the tax consequences contained in this summary will not be challenged by the Irish Revenue Commissioners or will be sustained by an Irish court if they were to be challenged. This summary deals with Horizon Shareholders who beneficially own their Horizon Shares as an investment. Particular rules not discussed below may apply to certain classes of taxpayers holding Horizon Shares, such as dealers in securities, collective investment schemes, insurance companies, trusts etc. If you are in doubt as to your tax position or are subject to tax in a jurisdiction other than Ireland, you should consult an appropriate professional adviser without delay.

The summary does not constitute tax or legal advice and the comments below are of a general nature only. Holders of Horizon Shares should consult their professional advisers on the tax implications of the Scheme under the laws of their country of residence, citizenship or domicile.

Taxation of Chargeable Gains

The current rate of tax on chargeable gains in Ireland is thirty-three percent (33%).

Non-resident shareholders

Horizon Shareholders that are neither resident nor ordinarily resident in Ireland for Irish tax purposes should not be liable for Irish capital gains tax, referred to as “Irish CGT,” on the disposal of their Horizon Shares pursuant to the Scheme unless such shares were used in or for the purposes of a trade carried on by the shareholder in Ireland through a branch or agency, or were used or held or acquired for use by or for the purposes of the branch or agency.

A Horizon Shareholder who is an individual and who is temporarily non-resident in Ireland may, under Irish anti-avoidance legislation, still be liable to Irish CGT on any chargeable gain realized on the disposal of their Horizon Shares during the period in which the individual is a non-resident.

Irish resident shareholders

Horizon Shareholders that are resident or ordinarily resident in Ireland for Irish tax purposes or that have used their Horizon Shares in or for the purposes of a trade carried on by the shareholder in Ireland through a branch or agency, or whose Horizon Shares were used or held or acquired for use by or for the purposes of such a branch or agency (each an “Irish Holder”) will, subject to the availability of any exemptions and reliefs, generally be within the charge to Irish CGT on the disposal of their Horizon Shares pursuant to the Scheme.

For the purpose of Irish CGT, an Irish Holder should be treated as having made a disposal of their Horizon Shares for consideration of an amount equal to the cash received pursuant to the Scheme. This may, subject to the Irish Holder’s individual circumstances and any available exemption or relief, give rise to a chargeable gain (or allowable loss) for the purposes of Irish CGT.

Any gain or loss will be calculated by reference to the difference between the amount of cash received and the Irish Holder’s Irish CGT base cost in their Horizon Shares plus incidental acquisition and disposal expenses.

Where proceeds are received in a currency other than Euro, they must be translated into Euro amounts using the foreign exchange rate on the date of the disposal to calculate the amount of the chargeable gain or allowable loss. Similarly, acquisition or disposal costs denominated in a currency other than Euro must be translated at the date of acquisition or disposal into Euro amounts.

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The amount of Irish CGT, if any, payable as a consequence of the disposal of the Horizon Shares by an Irish Holder pursuant to the Scheme will depend on his or her own personal tax position. In the case of individuals, an annual exemption from Irish CGT applies (EUR (€) 1,270 for 2022). This annual exemption is not available to companies. Irish Holders are required, under Ireland’s self-assessment system, to file a tax return reporting any chargeable gains arising to them in a particular year.

Irish Holders that realize a loss on the disposal of their Horizon Shares will generally be entitled to offset such capital losses against chargeable gains realized from other sources in determining their liability to Irish CGT. Capital losses which remain unrelieved in a year may generally be carried forward and applied against chargeable gains realized in future years.

Stamp duty

No Irish stamp duty should be payable by a Horizon Shareholder in relation to the disposal of their Horizon Shares for cash pursuant to the Scheme.

Material U.S. Federal Income Tax Considerations

The following is a general discussion of the material U.S. federal income tax consequences of the Scheme to U.S. holders (as defined below) that receive cash in exchange for their Horizon Shares pursuant to the Scheme. This discussion is limited to U.S. holders who hold their Horizon Shares as “capital assets” within the meaning of Section 1221 of the U.S. Internal Revenue Code of 1986, as amended (referred to hereinafter as the “Code”) (generally, property held for investment). This discussion is based on current provisions of the Code, the Treasury Regulations promulgated thereunder, judicial interpretations thereof and administrative rulings and published positions of the Internal Revenue Service (the “IRS”), each as of the date hereof, and all of which are subject to change or differing interpretations, possibly with retroactive effect. Any such change or interpretation could affect the accuracy of the statements and conclusions set forth herein.

This discussion is for general information only and does not purport to address all aspects of U.S. federal income taxation that may be relevant to particular holders of Horizon Shares in light of their particular facts and circumstances and does not apply to holders of Horizon Shares that are subject to special rules under the U.S. federal income tax laws (including, for example, banks or other financial institutions, insurance companies, regulated investment companies, real estate investment trusts, mutual funds, dealers in securities or currencies, traders in securities that elect to apply a mark-to-market method of accounting, tax-exempt entities, entities or arrangements treated as partnerships for U.S. federal income tax purposes or other flow-through entities (and investors therein), subchapter S corporations, retirement plans, individual retirement accounts or other tax-deferred accounts, U.S. holders having a “functional currency” other than the U.S. dollar, holders who hold Horizon Shares as part of a straddle, constructive sale, conversion transaction or other integrated or risk reduction transaction, holders required to accelerate the recognition of any item of gross income as a result of such income being recognized on an applicable financial statement, holders that actually or constructively hold five percent (5%) or more of the combined voting power of Horizon or of the total value of Horizon’s shares, holders that directly, indirectly or constructively own shares of Amgen common stock and holders who acquired their Horizon Shares through the exercise of an Option or otherwise as compensation or through a retirement plan). This discussion does not address any considerations under state, local or non-U.S. tax laws, any considerations under U.S. federal tax laws other than those pertaining to income tax (including estate and gift tax consequences), or considerations in respect of the Medicare contribution tax on net investment income, the alternative minimum tax or the Foreign Account Tax Compliance Act of 2010 (including the Treasury Regulations promulgated thereunder and any intergovernmental agreements entered in connection therewith and any laws, regulations or practices adopted in connection with any such agreement). Furthermore, this discussion does not address any tax consequences to holders who are not U.S. holders.

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For purposes of this discussion, the term “U.S. holder” means a beneficial owner of Horizon Shares that, for U.S. federal income tax purposes, is:

If a partnership (including any entity or arrangement treated as a partnership for U.S. federal income tax purposes) holds Horizon Shares, the tax treatment of a person treated as a partner in such partnership generally will depend on the status of the partner, the activities of the partnership and certain determinations made at the partnership level. Such partnerships and any person that for U.S. federal income tax purposes is treated as a partner in a partnership holding Horizon Shares should consult their tax advisors regarding the tax consequences of the Scheme to them.

All holders of Horizon Shares should consult their own tax advisors to determine the particular tax consequences to them of the Scheme, including the applicability and effect of any U.S. federal, state, local, non-U.S. and other tax laws.

The receipt of cash in exchange for Horizon Shares pursuant to the Scheme is intended to be a taxable transaction for U.S. federal income tax purposes. A U.S. holder generally will recognize gain or loss for U.S. federal income tax purposes in an amount equal to the difference, if any, between (i) the sum of the cash received pursuant to the Scheme and (ii) such U.S. holder’s adjusted tax basis in the Horizon Shares surrendered in exchange therefor. Such gain or loss generally will be capital gain or loss, and will be long-term capital gain or loss if the U.S. holder’s holding period for such shares exceeds one (1) year as of the effective date of the Scheme. Long-term capital gains for certain noncorporate U.S. holders, including individuals, are generally eligible for a preferential rate of federal income taxation. The deductibility of capital losses is subject to limitations. If a U.S. holder acquired different blocks of Horizon Shares at different times or at different prices, such U.S. holder must determine its tax basis, holding period and gain or loss separately with respect to each block of Horizon Shares.

Passive Foreign Investment Company Considerations

A non-U.S. corporation, such as Horizon, will be classified as a “passive foreign investment company” (referred to as a “PFIC”) for any taxable year if, after the application of certain “look-through” rules, (a) at least seventy-five percent (75%) of its gross income is “passive income” as that term is defined in the relevant provisions of the Code (e.g., dividends, interest, rents, royalties, or gains on the disposition of certain investment property) or (b) at least fifty percent (50%) of the average quarterly value of its assets consists of assets that produce, or are held for the production of, “passive income.” Horizon is not expected to be classified as a PFIC for U.S. federal income tax purposes, but this conclusion is a factual determination made annually and, thus, is subject to change.

If Horizon were classified as a PFIC for any taxable year during which a U.S. holder held Horizon Shares, such classification could result in adverse tax consequences to such U.S. holder, and different U.S. federal income tax consequences from those described above may apply to the receipt of cash by such U.S. holder in exchange for Horizon Shares. These consequences may include having gains realized on the receipt of cash in exchange for Horizon Shares treated as ordinary income rather than capital gain and being subject to interest

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charges on such gains. U.S. holders should consult their own tax advisors regarding the potential application of the PFIC rules to their disposition of Horizon Shares in connection with the Scheme.

Information Reporting and Backup Withholding

Payments of cash to a U.S. holder pursuant to the Scheme may be subject to information reporting and backup withholding (currently at a rate of twenty-four percent (24%)), unless such U.S. holder provides proof of an applicable exemption or furnishes its taxpayer identification number and otherwise complies with all applicable requirements of the backup withholding rules. Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules will be allowed as a refund or credit against a U.S. holder’s U.S. federal income tax liability, if any, provided that certain required information is timely furnished to the IRS.

The preceding discussion is intended only as a general summary of material U.S. federal income tax consequences of the Scheme. It is not a complete analysis or discussion of all potential tax effects that may be important to a particular holder. All holders of Horizon Shares should consult their own tax advisors as to the specific tax consequences of the Transaction to them, including tax reporting requirements, and the applicability and effect of any federal, state, local and non-U.S. tax laws.

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DELISTING AND DEREGISTRATION OF HORIZON SHARES

Following the consummation of the Transaction, Horizon Shares will be delisted from the Nasdaq Global Select Market and deregistered under the Exchange Act and will cease to be publicly traded. As a result, Horizon would no longer file periodic or other reports with the SEC.

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PARTIES TO THE TRANSACTION

Amgen

Amgen is a highly focused biotechnology company committed to unlocking the potential of biology for patients suffering from serious illnesses by discovering, developing, manufacturing and delivering innovative human therapeutics. This approach begins by using tools like advanced human genetics to unravel the complexities of disease and understand the fundamentals of human biology. Amgen focuses on areas of high unmet medical need and leverages its expertise to strive for solutions that improve health outcomes and dramatically improve people’s lives. A biotechnology pioneer since 1980, Amgen has grown to be one of the world’s leading independent biotechnology companies. Amgen is one of the 30 companies that comprise the Dow Jones Industrial Average and is also part of the Nasdaq-100 index. Amgen is incorporated in Delaware and its common stock is listed on the Nasdaq Global Select Market under the ticker “AMGN.”

Amgen’s principal executive offices are located at One Amgen Center Drive, Thousand Oaks, California, 91320-1799, USA, its telephone number is + 1 (805) 447-1000 and its website is www.amgen.com. Information on Amgen’s website is not incorporated by reference into or otherwise part of this proxy statement.

Horizon

Horizon is a global biotechnology company headquartered in Dublin, Ireland and focused on the discovery, development and commercialization of medicines that address critical needs for people impacted by rare, autoimmune and severe inflammatory diseases. Horizon has 12 marketed medicines and a pipeline with more than 20 development programs. Horizon has offices or a presence across four continents and more than 2,000 employees. Horizon is a public company registered in Ireland whose ordinary shares are listed on the Nasdaq Global Select Market under the ticker “HZNP.”

Horizon’s principal executive offices are located at 70 St. Stephen’s Green, Dublin 2, D02 E2X4, Ireland, and its telephone number is + 353 1 772 2100 and its website is www.horizontherapeutics.com. Information on Horizon’s website is not incorporated by reference into or otherwise part of this proxy statement.

Acquirer Sub

Acquirer Sub is a private limited company incorporated in Ireland established solely for the purpose of effecting the Transaction and is a newly formed wholly owned subsidiary of Amgen. To date, Acquirer Sub has not conducted any activities other than those incidental to its formation and the execution of the Transaction Agreement and the other documents relating to the Transaction.

Acquirer Sub’s registered office is located at 2 Grand Canal Square, Dublin 2, D02 A342, Ireland, and its telephone number is +353 1 639 5000.

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THE TRANSACTION AGREEMENT

The following is a summary of certain material terms of the Transaction Agreement and the conditions appendix and is qualified in its entirety by reference to (i) the complete text of the Transaction Agreement, which is incorporated into this proxy statement by reference and attached as Annex A to this proxy statement, and (ii) the complete text of the conditions appendix, which is incorporated into this proxy statement by reference and attached as Annex B to this proxy statement. This summary is not intended to provide you with any other factual information about Horizon or Amgen. You are urged to read carefully this entire proxy statement, including the Annexes and the documents incorporated by reference. You should also review “Where You Can Find More Information.”

Structure of the Transaction

Pursuant to the Transaction, Acquirer Sub will acquire the entire issued ordinary share capital of Horizon, in accordance with the terms of the Transaction Agreement, by way of the Scheme. Upon the closing of the Transaction, Horizon will be a wholly owned subsidiary of Amgen.

Closing of the Transaction

The closing of the Transaction will take place remotely at 9:00 a.m. New York City time, on a date to be selected by Amgen in consultation with Horizon as promptly as reasonably practicable following, but not later than the third business day after the satisfaction or, in the sole discretion of the applicable party, waiver (where applicable) of conditions to the closing of the Transaction (other than those conditions that by their nature are to be satisfied at the closing, but subject to the satisfaction or waiver of such conditions at the closing date). For a description of the conditions to the closing of the Transaction, see the section of this proxy statement entitled “—Conditions to the Closing of the Transaction.”

Consideration to Horizon Shareholders

At the effective time, Horizon Shareholders will be entitled to receive the Scheme Consideration. Within fourteen (14) days following the effective date, in respect of each Horizon Share subject to the Scheme of Arrangement, Amgen will pay, or cause to be paid, in respect of each holder of Horizon Shares at the record time (as specified in the Scheme of Arrangement), the Scheme Consideration in accordance with the terms and conditions of the Scheme of Arrangement.

Treatment of Horizon Options, Horizon RSUs and Horizon PSUs

Horizon equity awards will be treated as set forth in the Transaction Agreement, such that at the effective time:

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Representations and Warranties

Horizon, Amgen and Acquirer Sub made customary representations and warranties set forth in the Transaction Agreement on behalf of themselves and their respective subsidiaries that are subject, in some cases, to specified exceptions and qualifications contained in the Transaction Agreement or in the disclosure schedules to the Transaction Agreement. The representations and warranties made by Horizon, Amgen and Acquirer Sub are also subject to and qualified by certain information included in filings Horizon and Amgen have made with the SEC.

The Transaction Agreement contains representations and warranties of Horizon regarding, among other things:

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In addition, Amgen and Acquirer Sub have made representations and warranties, among other things, regarding:

Under the Transaction Agreement, the parties agreed that except for the representations and warranties contained in the Transaction Agreement, none of Horizon, Amgen or Acquirer Sub make any other representation or warranty.

Many of the representations and warranties made by each of Horizon, Amgen and Acquirer Sub are qualified by a material adverse effect standard. For the purpose of the Transaction Agreement, a “Material Adverse Effect” with respect to Amgen and Acquirer Sub means any effect, change, event or occurrence that would, individually or in the aggregate, prevent, materially delay or materially impair the ability of Amgen or Acquirer Sub to consummate the Transaction. For the purpose of the Transaction Agreement, a “Material

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Adverse Effect” with respect to Horizon means: an event, change, effect, development or occurrence that, individually or together with any other event, change, effect, development or occurrence, (a) would prevent, materially delay or materially impair the ability of Horizon to consummate the Transaction or (b) has had or would reasonably be expected to have a material adverse effect on the condition (financial or otherwise), business, assets, liabilities or results of operations of Horizon and its subsidiaries, taken as a whole, however, solely for the purposes of clause (b), excluding any event, change, development or occurrence to the extent resulting from or arising out of:

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However, the exceptions laid out in the first and seventh bullets will not prevent or otherwise affect a determination that the underlying cause of any such decline or failure (if not otherwise expressly excluded under any of the exceptions provided by the second, third, fourth, fifth, sixth, eighth, ninth and tenth bullets) is a Horizon Material Adverse Effect.

THE DESCRIPTION OF THE TRANSACTION AGREEMENT IN THIS PROXY STATEMENT HAS BEEN INCLUDED TO PROVIDE YOU WITH INFORMATION REGARDING ITS TERMS. THE TRANSACTION AGREEMENT CONTAINS REPRESENTATIONS AND WARRANTIES MADE BY AND TO THE PARTIES AS OF SPECIFIC DATES. THE STATEMENTS EMBODIED IN THOSE REPRESENTATIONS AND WARRANTIES WERE MADE FOR PURPOSES OF THE CONTRACT BETWEEN THE PARTIES AND ARE SUBJECT TO QUALIFICATIONS AND LIMITATIONS AGREED BY THE PARTIES IN CONNECTION WITH NEGOTIATING THE TERMS OF THE TRANSACTION AGREEMENT AND IN SOME CASES WERE QUALIFIED BY CONFIDENTIAL DISCLOSURES MADE BY THE PARTIES, WHICH DISCLOSURES ARE NOT REFLECTED IN THE TRANSACTION AGREEMENT. IN ADDITION, CERTAIN REPRESENTATIONS AND WARRANTIES WERE MADE AS OF A SPECIFIED DATE OR MAY HAVE BEEN USED FOR THE PURPOSE OF ALLOCATING RISK BETWEEN THE PARTIES RATHER THAN ESTABLISHING MATTERS AS FACTS.

Shareholders Meetings and Recommendations

Horizon has agreed to (i) convene or seek an order of the Irish High Court convening a meeting of its shareholders to approve the Scheme of Arrangement in accordance with the requirements of the Companies Act (the Scheme Meeting) and (ii) convene the EGM, in order to approve the resolutions required to effect the Scheme, subject to the specified exception described in “—Conditions to the Closing of the Transaction—Termination.” Additionally, the Horizon Board has, subject to the specified exceptions described in “—Third-Party Acquisition Proposals” recommended that Horizon Shareholders vote to approve the Scheme of Arrangement at the Scheme Meeting and vote to approve the resolutions required to effect the Scheme at the EGM.

Third-Party Acquisition Proposals

Horizon has agreed in the Transaction Agreement that it will not and it will cause its subsidiaries and its and their respective directors, officers and employees not to, and it will use its reasonable best efforts to cause its and its subsidiaries’ other representatives not to, directly or indirectly:

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However, if at any time prior to the receipt of approval of the Transaction by Horizon Shareholders at the special meetings (and in no event after that time), the Horizon Board receives a written Horizon Alternative Proposal made after the date of the Transaction Agreement which has not resulted from a breach in any material respect of Horizon’s non-solicitation obligations described in this section, the Horizon Board, directly or indirectly through its representatives, may:

Horizon must notify Amgen promptly (but in any event within thirty-six (36) hours) if any Horizon Alternative Proposal or any indication, proposal or inquiry by a third party that would reasonably be expected to lead to a Horizon Alternative Proposal, is received by Horizon. Each such notice must be provided in writing and must identify the third party making, and, to the extent applicable, the material terms and conditions (including price) of, any such Horizon Alternative Proposal, indication, proposal or inquiry. Following such initial notice,

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Horizon must keep Amgen reasonably informed, on a reasonably current basis, of any material changes in the status and details of any such Horizon Alternative Proposal, indication, proposal or inquiry by a third-party and will promptly (but in no event later than thirty-six (36) hours after receipt) provide to Amgen copies of all written proposals, offers or draft agreements relating to a Horizon Alternative Proposal. Neither Horizon nor any of its subsidiaries will enter into any agreement with any person which prohibits Horizon from providing any information to Amgen in accordance with, or otherwise complying with, this section.

Subject to Amgen’s right to negotiate with Horizon to amend the Transaction Agreement in light of any Horizon Superior Proposal, prior to the approval of the Transaction by Horizon Shareholders at the special meetings (and in no event after that time), the Horizon Board may (A) in response to the receipt of a written Horizon Alternative Proposal received after the date of the Transaction Agreement that did not result from a material breach of Horizon’s obligations set forth in this section, or in response to the occurrence of a Horizon Intervening Event (as defined below), make a Horizon Change of Recommendation, or (B) in response to the receipt of a written Horizon Alternative Proposal received after the date of the Transaction Agreement that did not result from a material breach of Horizon’s obligations set forth in this section, effect a Horizon Change of Recommendation and terminate the Transaction Agreement in order to substantially concurrently enter into a definitive agreement providing for a Horizon Superior Proposal if, in each case of clause (A) and clause (B), (x) in the case of such an action taken in connection with a Horizon Alternative Proposal, the Horizon Alternative Proposal has not been withdrawn and the Horizon Board determines in good faith, after consultation with outside legal counsel and its financial advisor that such Horizon Alternative Proposal constitutes a Horizon Superior Proposal, and (y) the Horizon Board determines in good faith, after consultation with its financial advisor and outside legal counsel, that the failure to take such action would be inconsistent with its directors’ fiduciary duties under applicable law.

The Horizon Board and Horizon, as applicable, may not take any of the actions described in the previous paragraph unless prior to taking such action (i) Horizon has notified Amgen, in writing at least four (4) business days before taking such action, that Horizon intends to take such action, which notice attaches, in the case of a Horizon Change of Recommendation in response to a Horizon Superior Proposal or the termination of the Transaction Agreement (A) substantially concurrently with entry into a definitive agreement providing for a Horizon Superior Proposal or (B) to effect a Superior Proposal Termination, the most current version of each proposed contract providing for or related to such Horizon Superior Proposal (including any contract relating to financing or expense reimbursement) and the identity of the third party(ies) making the Horizon Superior Proposal or, in the case of a Horizon Intervening Event, a reasonably detailed description of the facts relating to such Horizon Intervening Event, (ii) if requested by Amgen, during such four (4) business day period, Horizon and its representatives will discuss and negotiate in good faith with Amgen (to the extent that Amgen desires to so discuss or negotiate) regarding any proposal by Amgen to amend the terms of the Transaction Agreement in response to such Horizon Superior Proposal or other potential Horizon Change of Recommendation and (iii) after such four (4) business day period, the Horizon Board determines in good faith, after consultation with its financial advisor and outside legal counsel and taking into account any proposal by Amgen to amend the terms of the Transaction Agreement, that the failure to take such action would be inconsistent with its fiduciary duties under applicable laws and, in the case of any such action in connection with a Horizon Alternative Proposal, such Horizon Alternative Proposal continues to constitute a Horizon Superior Proposal (in the event of any amendment to the financial terms or other material terms of any such Horizon Superior Proposal, a new written notification from Horizon consistent with that described in clause (i) above is required, and a new notice period under clause (i) will commence, during which notice period Horizon will be required to comply with the requirements described in this paragraph anew, except that such new notice period will be for three (3) business days (as opposed to four (4) business days)). After delivery of such written notice described in this paragraph, Horizon will promptly inform Amgen of all material developments affecting the material terms of any such Horizon Superior Proposal.

The Transaction Agreement provides that a “Horizon Alternative Proposal” means: any bona fide proposal or offer (including non-binding proposals or offers) from any person or group, other than Amgen and its

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controlled affiliates or any of its concert parties, relating to any (i) direct or indirect acquisition (whether in a single transaction or a series of related transactions) of assets of Horizon or any of its subsidiaries (including equity securities of its subsidiaries) equal to twenty percent (20%) or more of the consolidated assets of Horizon, or to which twenty percent (20%) or more of the revenues or earnings of Horizon on a consolidated basis are attributable for the most recent fiscal year for which audited financial statements are then available, (ii) direct or indirect acquisition (including by scheme of arrangement or takeover offer) or issuance (whether in a single transaction or a series of related transactions) of twenty percent (20%) or more of any class of equity or voting securities of Horizon, (iii) scheme of arrangement, tender offer, takeover offer or exchange offer that, if consummated, would result in a person or group beneficially owning twenty percent (20%) or more of any class of equity or voting securities of Horizon or (iv) scheme of arrangement, merger, consolidation, share exchange, business combination, joint venture, reorganization, recapitalization or similar transaction involving Horizon or any of its subsidiaries, under which a person or group or, in the case of clause (B) below, the shareholders or equityholders of any person or group would, directly or indirectly, (A) acquire assets equal to twenty percent (20%) or more of the consolidated assets of Horizon, or to which twenty percent (20%) or more of the revenues or earnings of Horizon on a consolidated basis are attributable for the most recent fiscal year for which audited financial statements are then available or (B) immediately after giving effect to such transactions, beneficially own twenty percent (20%) or more of any class of equity or voting securities of Horizon or the surviving or resulting person (including any parent person) in such transaction.

The Transaction Agreement provides that a “Horizon Superior Proposal” means: any bona fide, written Horizon Alternative Proposal (with all references to “twenty percent (20%)” in the definition of Horizon Alternative Proposal being deemed to be references to “fifty percent (50%)”) on terms that the Horizon Board determines in good faith, after consultation with its financial advisor and outside legal counsel, (a) is more favorable to the Horizon Shareholders (in their capacity as such) from a financial point of view than the Transaction and (b) taking into account all the financial, legal, regulatory and other terms and conditions of the Horizon Alternative Proposal that the Horizon Board considers to be appropriate (including the expected timing of consummation, any governmental or other approval requirements, and the availability of necessary financing).

The Transaction Agreement provides that a “Horizon Intervening Event” means any material event, fact, change, effect, development or occurrence arising or occurring after the date of the Transaction Agreement and prior to the approval of the Transaction by Horizon Shareholders at the special meetings that (i) was not known or reasonably foreseeable, or the material consequences of which were not known or reasonably foreseeable, in each case to the Horizon Board as of or prior to the date of the Transaction Agreement and (ii) does not relate to (a) any Horizon Alternative Proposal or consequence thereof, (b) Amgen or any of its subsidiaries or (c) any change in the market price or trading volume of Horizon Shares or the fact that Horizon meets or exceeds any internal or analysts’ expectations or projections of the results of operations of Horizon and its subsidiaries, (it being understood that the underlying causes of such change or fact will not be excluded by this clause (c)).

The obligations of the parties under the Transaction Agreement are subject in all respects to the parties’ obligations under the Irish Takeover Rules.

Efforts to Consummate

Subject to the terms and conditions of the Transaction Agreement, each of Horizon and Amgen agreed to, and agreed to cause its subsidiaries to, use their respective reasonable best efforts to take, or cause to be taken, (i) all actions and to do, or cause to be done, all things necessary, proper or advisable, to the extent permitted by applicable laws, to achieve satisfaction of the closing conditions and to consummate the Transaction and the other transactions and (ii) all actions, to file, or cause to be filed, all documents and to do, or cause to be done, and to assist and cooperate with the other parties in doing, all things, necessary, proper or advisable under applicable antitrust laws to consummate and make effective the Transaction as soon as reasonably practicable, including: (A) the obtaining of all necessary clearances from applicable governmental entities and expirations or terminations of waiting periods from governmental entities and the making of all necessary registrations and

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filings and the taking of all steps as may be necessary to obtain any such clearances from the applicable governmental entities or expiration or termination of a waiting period by or from, or to avoid an action or proceeding by, any governmental entity in connection with any antitrust law, (B) the obtaining of all necessary clearances from third parties and (C) the execution and delivery of any additional instruments necessary to consummate the Transaction.

In furtherance and not in limitation of the foregoing, the parties to the Transaction Agreement agree to use “reasonable best efforts” to, and cause their affiliates to use their reasonable best efforts to, promptly take all actions and steps requested or required by any governmental entity as a condition to granting any clearances, and to cause the prompt expiration or termination of any applicable waiting period and to resolve objections, if any, of the FTC or the DOJ or other governmental entities of any non-U.S. jurisdiction where filing, notification, expiration of a waiting period or consent or approval is a condition to the closing of the Transaction as agreed by the parties (each a “Required Non-U.S. Jurisdiction”), so as to obtain such clearances under the HSR Act or other antitrust laws, and to avoid the commencement of a lawsuit by the FTC, the DOJ or other governmental entities under antitrust laws, and to avoid the entry of, or to effect the dissolution of, any order in any legal proceeding which would otherwise have the effect of preventing the closing of the Transaction or delaying the closing of the Transaction beyond June 12, 2023, as may be subsequently extended to September 12, 2023 and December 12, 2023, pursuant to the Transaction Agreement (the “End Date”), including (i) negotiating, committing to and effecting, by consent decree, hold separate order or otherwise, the sale, lease, license, divestiture or disposition of any assets, rights, product lines or businesses of Horizon or any of its subsidiaries, (ii) terminating existing relationships, contractual rights or obligations of Horizon or any of its subsidiaries, (iii) terminating any venture or other arrangement of Horizon or any of its subsidiaries, (iv) creating any relationship, contractual rights or obligations of Horizon or any of its subsidiaries, (v) effectuating any other change or restructuring of Horizon or any of its subsidiaries and (vi) otherwise taking or committing to take any actions with respect to the businesses, product lines or assets of Horizon or any of its subsidiaries (the actions referred to in clauses (i) through (vi), collectively, “Remedy Actions”); provided, however, that, notwithstanding anything to the contrary in the Transaction Agreement (including the “reasonable best efforts standard” described above), neither Amgen nor any of its affiliates will be required to proffer, consent to or agree to or effect any Remedy Action (x) with respect to any assets, categories of assets or portions of any business of Horizon or any of its subsidiaries if, in each case, any such Remedy Action would, individually or in the aggregate, reasonably be expected to be material to the business, assets or financial condition of Horizon and its subsidiaries, taken as a whole or (y) with respect to the items listed on the applicable section of the disclosure schedules to the Transaction Agreement or (z) for the avoidance of doubt, with respect to any assets, categories of assets or portions of any business of Amgen or its affiliates; provided, further, that Horizon will only be permitted to take or commit to take such Remedy Action with the prior written consent of Amgen; provided, further, that Amgen may only require Horizon to take or commit to take any such Remedy Action if such Remedy Action is binding on Horizon only in the event the closing of the Transaction occurs.

Subject to the terms and conditions of the Transaction Agreement, each of the parties to the Transaction Agreement will (and will cause their respective affiliates, if applicable, to) (i) promptly, but in no event later than fifteen (15) business days after the date of the Transaction Agreement (or such later date as may be agreed in writing among the parties), make an appropriate filing of all notification and report forms as required by the HSR Act with respect to the transactions contemplated by the Transaction Agreement and (ii) as promptly as reasonably practicable after the date of the Transaction Agreement, but in no event later than January 16, 2023 (or such later date as may be agreed in writing among the parties), make all other filings, notifications or other consents as may be required to be made or obtained by such party under foreign antitrust laws or foreign investment laws in the Required Non-U.S. Jurisdictions.

Amgen must consult in advance with Horizon, and take into account in good faith the views of Horizon, regarding the strategy for all matters with any governmental entity; provided, that, in the event of a disagreement regarding such strategy, the determination of Amgen will be final so long as consistent with its obligations under the Transaction Agreement. The parties agree that neither Horizon nor Amgen will commit to or agree with any

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governmental entity to stay, toll or extend any applicable waiting period under the HSR Act, or pull and refile under the HSR Act, or other applicable antitrust laws or foreign investment laws, without the prior written consent of the other party (such consent not to be unreasonably withheld, conditioned or delayed). With respect to any request, inquiry, investigation, action or legal proceeding under applicable antitrust laws, each of Horizon and Amgen agree to use its reasonable best efforts to, among other things, (i) cooperate in all respects and consult with each other in connection with any filing or submission in connection with any investigation or other inquiry, including allowing the other party to have a reasonable opportunity to review in advance and comment on drafts of filings and submissions, in each case, as required under applicable antitrust laws, (ii) give the other parties prompt notice of the making or commencement of any request, inquiry, investigation or legal proceeding brought by a governmental entity with respect to the transactions contemplated by the Transaction Agreement under applicable antitrust laws, (iii) promptly keep the other parties informed as to the status of any such requests, (iv) promptly inform the other parties of any communication to or from the FTC, the DOJ or any other governmental entity in connection with any such request, (v) promptly furnish to the other party copies of documents, communications or materials provided to or received from any governmental entity in connection with any such request, (vi) consult in advance and cooperate with the other parties and consider in good faith the views of the other parties in connection with any substantive communication or materials to be made or submitted in connection with any such request and (vii) provide advance notice of and permit the authorized representatives of the other party to be present at each meeting or conference relating to such request and to have access to and be consulted in advance in connection with any argument, opinion or proposal to be made or submitted to any governmental entity in connection with such request. Each party must supply, as promptly as practicable, such information, documentation, other material or testimony that may be reasonably requested by any governmental entity. Amgen and Acquirer Sub must pay all filing fees under the HSR Act and for any filings required under Required Non-U.S. Jurisdictions, but Horizon will bear its own costs for the preparation of any such filings.

In the event that the latest date on which the Irish High Court or the Irish Takeover Panel would permit the closing to occur is prior to the End Date, the parties to the Transaction Agreement will use their respective reasonable best efforts to obtain the consent of the Irish High Court or the Irish Takeover Panel, as applicable, to an extension of such latest date (but not beyond the End Date). If (i) the Irish High Court or the Irish Takeover Panel requires the lapsing of the Scheme of Arrangement prior to the End Date or (ii) the condition set out in paragraph 1 of the conditions to the Scheme of Arrangement and the Transaction fails to be satisfied, the parties to the Transaction Agreement will (unless and until the Transaction Agreement is terminated pursuant to and in accordance with “—Termination”) take all reasonable actions required in order to re-initiate the Scheme of Arrangement process as promptly as practicable (it being understood that no such lapsing described in clause (i) or clause (ii) will, in and of itself, result in a termination of, or otherwise affect any rights or obligations of any party under, the Transaction Agreement).

Financing

Until the earlier to occur of the closing of the Transaction and the termination of the Transaction Agreement, Horizon will use its reasonable best efforts, and will cause each of its subsidiaries to use its reasonable best efforts, and will use its reasonable best efforts to cause its and their respective officers, employees and advisors and other representatives, including legal and accounting advisors, to use their reasonable best efforts, to provide to Amgen and its subsidiaries such assistance as may be reasonably requested by Amgen in writing that is customary in connection with the offering, arranging, obtaining, syndication, consummation, issuance or sale of the debt financing provided by that certain Bridge Credit Agreement dated December 12, 2022 and any other third party debt financing that is necessary, or that is otherwise incurred or intended to be incurred by Amgen or any of its subsidiaries, to refinance or refund any existing indebtedness for borrowed money of Horizon, Amgen or any of their respective subsidiaries in each case in connection with the Transaction (the “Financing”), including using reasonable best efforts with respect to (i) participating in and assisting with the due diligence, syndication or other marketing of the Financing, (ii) timely furnishing Amgen and its financing sources with existing historical financial and other customary information, (iii) providing to

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Amgen’s legal counsel and its independent auditors customary documents and information in connection with their delivery of any customary negative assurance opinions and customary comfort letters relating to the Financing, (iv) obtaining the consents of Horizon’s independent auditors to use their audit reports on the audited historical financial statements, (v) obtaining Horizon’s independent auditors’ customary assistance with the accounting due diligence activities of the financing sources, (vi) providing customary authorization letters to prospective lenders in connection with a syndicated bank financing, (vii) assisting Amgen and its subsidiaries in obtaining or updating corporate and facility credit ratings, (viii) assisting Amgen and its subsidiaries in the negotiation of and cooperating with the preparation of any material agreement in connection with the Financing, (ix) cooperating with internal and external counsel of Amgen in connection with providing customary back-up certificates and factual information regarding any legal opinion, (x) providing customary payoff letters and other documents reasonably relating to the repayment or refinancing of any indebtedness for borrowed money of Horizon or any of its subsidiaries, (xi) procuring consents to the reasonable use of all of Horizon’s and its subsidiaries’ logos in connection with the Financing and (xii) providing any documentation required by “know your customer” and anti-money laundering rules and regulations.

In addition, in connection with any marketing efforts of the Financing by Amgen and its subsidiaries, Amgen may reasonably request Horizon to file a Current Report on Form 8-K pursuant to the Exchange Act that contains material non-public information with respect to Horizon and its subsidiaries, which Amgen, in consultation with the financing sources and upon the advice of outside counsel, reasonably determines is necessary to include in a registration statement, customary offering memorandum or other offering document for the Financing (each an “Offering Document”) in order to (i) correct any untrue statement of a material fact or an omission of a material fact necessary in order to make the statements therein not misleading or (ii) to cause such Offering Document to comply with the requirements of the Securities Act. Horizon will consider such request promptly, and if Horizon approves of such request (such approval not to be unreasonably withheld), then Horizon will promptly file such Current Report on Form 8-K in a form reasonably satisfactory to Amgen. Horizon will not be obligated, however, to effect any such filing of a Current Report on Form 8-K if in the good faith judgment of the Horizon Board it would be detrimental to Horizon and its shareholders for such Current Report on Form 8-K to be filed at such time or in the near future, in which case Horizon will not be obligated to file such Current Report on Form 8-K.

If requested by Amgen, Horizon will use its reasonable best efforts to assist Amgen or one or more of its subsidiaries in (i) commencing any of the following (as requested by Amgen): (A) one or more offers to purchase any or all of the outstanding debt issued under that certain Horizon indenture dated July 16, 2019, as supplemented (the “Indenture”) for cash (the “Offers to Purchase”); or (B) one or more offers to exchange any or all of the outstanding debt issued under the Indenture for securities issued by Amgen or any of its affiliates (the “Offers to Exchange”); and (ii) soliciting the consent of the holders of debt issued under the Indenture regarding certain proposed amendments to the Indenture (the “Consent Solicitations” and, together with the Offers to Purchase and Offers to Exchange, if any, the “Horizon Note Offers and Consent Solicitations”); provided that the closing (or effectiveness) of any such transaction will not be consummated until the closing of the Transaction and any such transaction will be funded using consideration provided by Amgen. Subject to the receipt of the requisite holder consents, in connection with any or all of the Consent Solicitations, Horizon will execute a supplemental indenture to the Indenture in accordance with the terms thereof amending the terms and provisions of the Indenture as described in the applicable Horizon Note Offers and Consent Solicitations in a form as reasonably requested by Amgen. Subject to the paragraph above, until the earlier of the closing of the Transaction and the termination of the Transaction Agreement, Horizon will use its reasonable best efforts, and will cause each of its subsidiaries to use its reasonable best efforts, and will use its reasonable best efforts to cause its and their respective Representatives to use their reasonable best efforts, to provide all reasonable and customary cooperation as may be reasonably requested by Amgen in writing to assist Amgen in connection with any Horizon Note Offers and Consent Solicitations. The consummation of any or all of the Horizon Note Offers and Consent Solicitations will not be a condition to closing of the Transaction.

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Horizon will, not later than 11:00 a.m., New York City time three (3) business days prior to the expected closing of the Transaction and otherwise in accordance with the terms of that certain Horizon Credit Agreement dated on May 7, 2015, as subsequently amended (the “Horizon Credit Agreement”) and on a date determined by Horizon in consultation with Amgen, execute and deliver to the Administrative Agent (as defined in the Horizon Credit Agreement) the requisite prepayment notices to repay, in full, all loans outstanding under the Horizon Credit Agreement, contingent upon the consummation of the Transaction. Horizon and its subsidiaries must use reasonable best efforts to cooperate with any back-stop, “rollover” or termination of any existing letters of credit under the Horizon Credit Agreement or otherwise, will take all actions reasonably requested by Amgen to cause the release and discharge of all related liens and security interests under the Horizon Credit Agreement, and will take such other actions as Amgen may reasonably request in connection with such repayment, redemption or prepayment. Horizon will deliver to Amgen executed payoff letters on the closing date.

Prior to the expected closing date, Horizon will use its reasonable best efforts to cooperate with Amgen, if Amgen will so request, to effect a redemption of the 5.500% Senior Notes of Horizon due 2027 issued under the Indenture (the “Horizon Notes”) in accordance with their terms (the “Redemption”); provided that the consummation of any such Redemption will be conditional upon the closing of the Transaction. Such cooperation by Horizon will include: (i) delivery of a notice of redemption, (ii) concurrently with the deposit by Amgen with the paying agent under the Horizon Notes of an amount sufficient to satisfy and discharge the Horizon Notes, delivery of a notice of satisfaction and discharge and (iii) use of its best efforts to take all other actions necessary to facilitate the Redemption.

Conduct of Business Pending the Closing Date

From the date of the Transaction Agreement until the closing or termination of the Transaction Agreement, except as may be prohibited or required by law, applicable local COVID-19 measures, the Transaction Agreement or as set forth in the disclosure schedules to the Transaction Agreement, without Amgen’s prior written consent (such consent not to be unreasonably withheld, conditioned or delayed), Horizon has agreed to use commercially reasonable efforts, and will cause each of its subsidiaries to use commercially reasonable efforts, to conduct its business in the ordinary course of business and to preserve intact Horizon and its subsidiaries’ business organization and relationships with significant customers, suppliers, licensors, licensees and other third parties and keep available the services of its present officers and employees (without any obligations to put in place any retention program).

Horizon has also agreed that, except as may be prohibited or required by law or the Transaction Agreement or as set forth in the disclosure schedules to the Transaction Agreement, without Amgen’s prior written consent (such consent not to be unreasonably withheld, conditioned or delayed), Horizon will not, and will not permit any of its subsidiaries to, among other things, undertake the following actions:

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Directors’ and Officers’ Indemnification and Insurance

For a period of not less than six (6) years after the effective date, Amgen will maintain in effect the provisions for indemnification, advancement of expenses and limitations of liability in the organizational documents of Horizon and its respective subsidiaries in favor of the current or former directors or officers of Horizon or any of their respective subsidiaries no less favorable than currently in force and, for a period of six (6) years from the effective date, will not amend, repeal or modify such provisions in any manner that would adversely affect the rights of any individuals who are entitled to such rights (unless such change is required by law and then only to the extent the change is required).

For a period of not less than six (6) years from the effective date, Amgen will cause Horizon or any applicable subsidiary to indemnify and hold harmless to the fullest extent permitted under the applicable laws and pursuant to existing arrangements and the organizational documents of Horizon and its subsidiaries each person, who is at the date of the Transaction Agreement, was previously, or during the period from the date of the Transaction Agreement through the date of the effective time, serving as a director or officer of Horizon or any of its subsidiaries, or at the request or for the benefit of Horizon or any of its subsidiaries as a director, trustee or officer of any other entity or any benefit plan maintained by Horizon or any of its subsidiaries (each referred to as an “indemnified party”), against any threatened, asserted, pending or completed legal proceeding, whether instituted by any governmental entity or any other person, arising out of or pertaining to acts or omissions occurring at or prior to the effective time that relate to any such indemnified party’s duties or service (A) as a director or officer of Horizon or the applicable subsidiary thereof at or prior to the effective time (including with respect to any acts, facts, events or omissions occurring in connection with the approval of the Transaction

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Agreement, the Scheme, the Transaction and the consummation of the other transactions contemplated thereby (including the Scheme), including the consideration and approval thereof and the process undertaken in connection therewith) or (B) as a director, trustee or officer of any other entity or any benefit plan maintained by Horizon or any of its subsidiaries at or prior to the effective time (any such action referred to as a “director and officer claim”) and any losses, claims, damages, liabilities, claim expenses, judgments, fines, penalties and amounts paid in settlement (including all interest, assessments and other charges paid or payable in connection with or in respect of any thereof) relating to or resulting from a director and officer claim. For the same period, Amgen will also cause Horizon or any applicable subsidiary to promptly advance to such indemnified party any claim expenses incurred in defending, serving as a witness with respect to or otherwise participating with respect to any director and officer claim in advance of the final disposition of such director and officer claim, including payment on behalf of or advancement to the indemnified party of any claim expenses incurred by the indemnified party in connection with enforcing any rights with respect to such indemnification or advancement, in each case without the requirement of any bond or other security, but subject to the indemnifying party’s receipt of a written undertaking by or on behalf of such indemnified party.

Prior to the effective time, at Horizon’s option and expense, Horizon may purchase (and pay in full the aggregate premium for) a six (6) year prepaid “tail” insurance policy of at least the same coverage and amounts and containing terms and conditions that are no less favorable to the directors and officers of Horizon or any of its subsidiaries than the current coverage with a claims period of six (6) years from the effective time for director and officer claims arising from facts, acts, events or omissions that occurred on or prior to the effective time. However, the aggregate premium may not exceed three hundred percent (300%) of the annual amount currently paid by Horizon and its subsidiaries for such insurance (the “maximum rate”). If Horizon fails to obtain such tail insurance policy prior to the effective time, Amgen will obtain such policy. If such policy cannot be obtained or can be obtained only by paying a premium in excess of the maximum rate, Amgen is only required to obtain as much coverage as can be obtained by paying a premium equal to the maximum rate.

Employee Matters

From the effective time through the first anniversary of the effective time (the “Continuation Period”), Amgen will provide, or will cause Acquirer Sub to provide (i) each continuing Horizon employee with a base salary or hourly rate no less favorable than as provided immediately prior to the effective time, (ii) each continuing Horizon employee with a target annual or quarterly cash bonus, incentive compensation (excluding any special, retention or one-time award opportunities), and commissions opportunities (as applicable) that are no less favorable than that in effect immediately prior to the effective time, and (iii) for continuing Horizon employees as a group, employee benefits that are no less favorable in the aggregate than those provided to similarly-situated employees of Amgen and its subsidiaries. For purposes of clause (iii), any equity, defined benefit pension plans, nonqualified deferred compensation plans, retiree health or welfare benefits, post-termination health or welfare benefits and retention or change in control payments or awards will not be taken into account.

Amgen will cause Acquirer Sub to provide each continuing Horizon employee who experiences a termination of employment during the Continuation Period with severance, termination and similar benefits no less favorable than those which such employee would have been entitled to as of immediately prior to the effective time.

Amgen or any affiliate providing benefits will recognize continuing Horizon employees’ length of employment with Horizon and its subsidiaries for purposes of determining eligibility, vesting and level of benefits under their benefit plans providing benefits to continuing Horizon employees to the same extent and for the same purpose as such service was recognized under the corresponding Horizon benefit plans in which the continuing Horizon employee participated immediately prior to the effective time, except that length of employment will not be recognized (i) by any Amgen defined benefit pension plan, (ii) by any Amgen retiree medical plan or post-termination health or welfare benefit plan, (iii) by any Amgen benefit plan that is frozen or

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for which participation is limited to a grandfathered population or (iv) for any cash- or equity-based compensation arrangements, subject to limited exceptions. Service will not be credited if it would result in a duplication of benefits or compensation for the same period of service. Service will only be credited to the extent such service is credited for similarly situated Amgen employees.

Each continuing Horizon employee will be immediately eligible to participate, without any waiting time, in any and all Amgen benefit plans to the extent coverage under such Amgen benefit plan is replacing comparable coverage under an employee plan in which such continuing Horizon employee had already satisfied any such waiting period and participated immediately before the effective time (such plans, collectively, the “Old Plans”) and for purposes of each Amgen benefit plan providing medical, dental, pharmaceutical or vision benefits to any continuing Horizon employee, Amgen will use its reasonable best efforts to (1) cause all pre-existing condition exclusions and actively-at-work requirements of such Amgen benefit plan to be waived for such employee and his or her covered dependents, unless and to the extent the individual, immediately prior to entry in the Amgen benefit plans, was subject to such conditions under the comparable Old Plans and (2) to the extent not prohibited by the terms, or by any third-party administrator, of any new benefit plan that is a fully insured medical, dental, pharmaceutical or vision benefit plan of Amgen or any affiliate, credit each continuing Horizon employee with all deductible payments, co-payments and other out-of-pocket expenses incurred by such continuing Horizon employee and his or her covered dependents under the medical, dental, pharmaceutical or vision benefit plans of Horizon prior to the closing of the Transaction during the plan year in which the closing of the Transaction occurs for the purpose of determining the extent to which such continuing Horizon employee has satisfied the deductible, co-payments or maximum out-of-pocket requirements applicable to such continuing Horizon employee and his or her covered dependents for such plan year under any new benefit plan that is a medical, dental, pharmaceutical or vision benefit plan of Amgen or its affiliates, as if such amounts had been paid in accordance with such plan (to the extent such credit would have been given under comparable Old Plans prior to the closing of the Transaction).

Prior to the effective time, if requested by Amgen in writing, Horizon will cause any United States Tax-qualified defined contribution plan (collectively, the “Horizon 401(k) Plan”) to be terminated effective immediately prior to the effective time. Upon the distribution of the assets in the accounts under the Horizon 401(k) Plan to the participants, Amgen will permit the continuing Horizon employees who are then actively employed by Amgen or its subsidiaries to make rollover contributions of “eligible rollover distributions” (within the meaning of Section 401(a)(31) of the Code), in the form of cash, from the Horizon 401(k) Plan to the applicable tax-qualified defined contribution plans of Amgen or its subsidiaries.

As promptly as reasonably practicable following the date of the execution of the Transaction Agreement, Horizon will take all actions necessary or required under the ESPP and applicable laws to (i) limit participation in the ESPP to those employees who participated in the ESPP immediately prior to the execution and delivery of the Transaction Agreement, (ii) prevent participants from increasing their payroll deductions or purchase elections from those in effect immediately prior to the execution of the Transaction Agreement, (iii) ensure that, except for any offering period in existence under the ESPP on the date of the Transaction Agreement, no offering period will be authorized or commenced on or after the date of the Transaction Agreement, and no existing offering period will be extended and (iv) if the Completion will occur prior to the end of any existing offering period, cause the rights of participants with respect to any such offering period (and purchase period thereunder) to be determined by treating the last business day prior to the effective time as the last day of such offering period and purchase period. Horizon will terminate the Horizon ESPP in its entirety effective as of the effective time, contingent upon the effective time.

Conditions to the Closing of the Transaction

The Scheme and the closing of the Transaction are conditioned on the satisfaction (or waiver, as applicable) of among other things:

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Amgen reserves the right, subject to the prior written approval of the Irish Takeover Panel (if required), to effect the Transaction by way of a takeover offer, as an alternative to the Scheme, in the circumstances described in and subject to the terms of the Transaction Agreement. In such event, such takeover offer will be implemented on terms and conditions that are at least as favorable to the Horizon Shareholders and Horizon equity award holders (except for an acceptance condition set at eighty percent (80%) of the nominal value of the Horizon Shares to which such offer relates and which are not already beneficially owned by Amgen) as those which would apply in relation to the Scheme, among other requirements.

The Transaction is also conditioned on the Scheme becoming effective and unconditional by not later than the End Date. In addition, the Scheme will lapse unless it is effective on or prior to the End Date (or such later date as Amgen and Horizon may, subject to receiving the consent of the Irish Takeover Panel and the Irish High Court, in each case if required, agree).

Survival of Representations and Warranties

None of the representations and warranties of the Transaction Agreement will survive the consummation of the Transaction or the termination of the Transaction Agreement.

Termination of the Transaction Agreement

The Transaction Agreement may be terminated at any time prior to the effective time in any of the following ways:

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Expenses

Except as otherwise provided in the Transaction Agreement, all costs and expenses incurred in connection with the Transaction will be paid by the party incurring such cost or expense, except the following: (i) the Irish Takeover Panel’s document review fees will be paid by Amgen, (ii) the costs associated with the filing, printing, publication and posting of the Rule 2.7 Announcement will be borne by Amgen, (iii) the costs associated with the filing, printing, publication and proposing of the Scheme Document, proxy statement and any other materials required to be proposed to Horizon Shareholders pursuant to SEC rules, the Companies Act or the Irish Takeover Rules will be borne by Horizon, (iv) the filing fees incurred in connection with notifications with any governmental entities under any antitrust laws, will be borne by Amgen and (v) the cost incurred in connection with soliciting proxies in connection with the Scheme Meeting and the EGM will be borne by Horizon.

Reverse Termination Payment and Reimbursement Payment

The Transaction Agreement provides that, in the event of a termination of the Transaction Agreement as described in this section (each a “Specified Termination”), Amgen will pay Horizon (or an entity designated by Horizon) $974,415,054 (the “Reverse Termination Payment”) in immediately available funds within seven (7) business days thereafter; provided that Horizon will not be entitled to receive the Reverse Termination Payment if Horizon’s breach of the Transaction Agreement is the primary cause of such Specified Termination. The Reverse Termination Payment is owed when there is a valid termination of the Transaction Agreement if:

The Transaction Agreement also provides that, in the event of a termination of the Transaction Agreement as described in this section, Horizon will reimburse Amgen for an amount equal to the documented, specific, quantifiable third party costs and expenses incurred, directly or indirectly, by Amgen or its subsidiaries, or on their behalf, for the purposes of, in preparation for or in connection with the Transaction, including third party costs and expenses incurred in connection with exploratory work carried out in contemplation of and in connection with the Transaction, legal, financial and commercial due diligence, the arrangement of financing and the engagement of third party representatives to assist in the process, subject to a cap of $278,404,301 (being equal to one percent (1%) of the aggregate value of the total consideration payable with respect to the Horizon Shares in connection with the Transaction) (the “Reimbursement Payment”), in immediately available funds within seven (7) business days following Horizon’s receipt of invoices or written documentation supporting Amgen’s request for a Reimbursement Payment. The reimbursement payment is owed if one of the following events occurs:

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The parties to the Transaction Agreement acknowledged and agreed that in no event will Amgen or Horizon be required to pay, as applicable, the Reverse Termination Payment or the Reimbursement Payment on more than one occasion, whether or not the Reverse Termination Payment or the Reimbursement Payment may be payable pursuant to more than one provision of the Transaction Agreement at the same or at different times and upon the occurrence of different events.

Amendment and Waiver

The Transaction Agreement may not be amended except by an instrument in writing signed by each of the parties, except that following approval of the Scheme by the Horizon Shareholders there will be no further amendment which by applicable law would require further approval by the Horizon Shareholders without such further approval nor will there be any amendment or change not permitted under applicable law. No delay or omission by either party to the Transaction Agreement in exercising any right, power or remedy provided by law or under the Transaction Agreement will operate as a waiver. Furthermore, certain provisions of the Transaction Agreement may not be amended without the prior written consent of sources of financing for the Transaction.

Specific Performance; Third-Party Beneficiaries

The parties agreed in the Transaction Agreement that irreparable harm would occur and that the parties would not have any adequate remedy at law for any breach of any of the provisions of the Transaction Agreement or in the event that any provisions of the Transaction Agreement were not performed in accordance with their specific terms. Except where the Transaction Agreement is validly terminated, as described above, each party is entitled to seek an injunction or injunctions to prevent breaches or threatened breaches of the Transaction Agreement and to specifically enforce the terms and provisions of the Transaction Agreement, without proof of actual damages, and each party agreed to waive any requirement for the securing or posting of any bond in connection with such remedy.

The Transaction Agreement is not intended to confer upon any person other than Horizon and Amgen any rights or remedies with the exception of the rights of the specified directors, officers and employees to certain indemnification and insurance and certain rights provided to the financing sources of Amgen in the Transaction Agreement.

Legal Proceedings Relating to the Transaction

In January 2023, two individual lawsuits (captioned Stein v. Horizon Therapeutics plc, et al., No. 23-cv-00058 (S.D.N.Y. filed January 4, 2023) and O’Dell v. Horizon Therapeutics plc, et al., No. 23-cv-00082

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(S.D.N.Y. filed January 5, 2023)) challenging the Transaction were filed in U.S. federal court. Additionally, in January 2023, Horizon received four demands from purported Horizon Shareholders challenging the Transaction. The complaints and demands generally allege that certain disclosures in the Preliminary Proxy Statement (filed on December 30, 2022) were false or misleading, and assert claims against Horizon and the Horizon Board for violations of Sections 14(a) and 20(a) of the Exchange Act. The plaintiffs seek, among other things, an injunction against the consummation of the Transaction and an award of costs and expenses, including a reasonable allowance for attorneys’ and experts’ fees.

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FINANCING RELATING TO THE TRANSACTION

The Scheme Consideration will be funded by Amgen from (i) its own cash resources, (ii) borrowings under its existing and new credit facilities, including as described herein, (iii) proceeds from the sale of debt securities or (iv) any combination of the foregoing.

PJT Partners LP, lead financial advisor to Amgen, is satisfied that sufficient resources are available to satisfy in full the Scheme Consideration payable to Horizon Shareholders under the terms of the Transaction.

Bridge Credit Agreement

On December 12, 2022, Amgen entered into the Bridge Credit Agreement. Under the Bridge Credit Agreement, the lenders party thereto committed to provide Amgen with an unsecured bridge credit facility in an aggregate principal amount of $28.5 billion (the “Bridge Credit Facility”). The commitments are intended to be available to finance the Scheme Consideration, fees and expenses related thereto and the repayment of certain existing indebtedness of Horizon. The commitments under the Bridge Credit Agreement were reduced by $4.0 billion upon the execution of the Term Loan Credit Agreement described below.

Term Loan Credit Agreement

On December 22, 2022, Amgen entered into the Term Loan Credit Agreement. Under the Term Loan Credit Agreement, the lenders party thereto have committed to provide Amgen with an unsecured term loan financing consisting of (i) a $2.0 billion 18-month term loan tranche (the “18-Month Tranche”) and (ii) a $2.0 billion three (3)-year term loan tranche (the “3-Year Tranche” and together with the 18-Month Tranche, the “Term Loan Credit Facility” and together with the Bridge Credit Facility, the “Credit Facilities”), in an aggregate principal amount of $4.0 billion. The Term Loan Credit Facility is available to finance the payment of the Scheme Consideration, fees and expenses related thereto and the repayment of certain existing indebtedness of Horizon.

Summary of Terms of the Credit Agreements

The Bridge Credit Facility will mature on the date that is 364 days after the date on which the first advance is made under the Bridge Credit Facility. The Term Loan Credit Facility will mature on (i) in the case of advances made under the 18-Month Tranche, on the date that is eighteen (18) months following the funding date of such advances and (ii) in the case of advances made under the 3-Year Tranche, the date that is three (3) years following the funding date of such advances.

Advances under the Bridge Credit Agreement will bear interest at an annual rate of, at Amgen’s option, either (i) term SOFR plus 0.10%, plus between 0.875% and 2.125%, depending on the rating of its senior long-term unsecured debt and the number of days elapsed between the initial funding day and the day of determination or (ii) the highest of (A) Citibank’s base commercial lending rate, (B) the overnight federal funds rate plus 1/2 of 1.00% and (C) one-month adjusted term SOFR plus 1.00%, plus between 0.000% and 1.125%, depending on the rating of Amgen’s senior long-term unsecured debt and the number of days elapsed between the initial funding day and the day of determination. Amgen is also required to pay, commencing on the 90th day after the effective date, a ticking fee of 0.125% per annum that will accrue on the aggregate undrawn commitments under the Bridge Credit Agreement.

Advances under the Term Loan Credit Agreement will bear interest at an annual rate of, at Amgen’s option, either (i) term SOFR plus 0.10%, plus between 0.875% and 1.375%, depending on the rating of its senior long-term unsecured debt or (ii) the highest of (A) Citibank’s base commercial lending rate, (B) the overnight federal funds rate plus 1/2 of 1.00% and (C) one-month adjusted term SOFR plus 1.00%, plus between 0.000% and 0.375%, depending on the rating of Amgen’s senior long-term unsecured debt. Amgen is also required to pay, commencing March 12, 2023, a ticking fee of 0.125% per annum that will accrue on the aggregate undrawn commitments under the Term Loan Credit Agreement.

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The availability of advances under each of the Credit Agreements is subject to the satisfaction of certain conditions set forth in each Credit Agreement. The commitments under each Credit Agreement automatically terminate on the earlier of (a) the date on which the Transaction has been completed and the other purposes of the respective Credit Facility have been achieved without the making of any advances under the relevant Credit Facility and (b) the time after certain mandatory cancellation events occur, including the abandonment of the Transaction.

The commitments under the Bridge Credit Facility were reduced by $4.0 billion upon the execution of the Term Loan Credit Agreement and will be further reduced by the net cash proceeds received by Amgen or its subsidiaries in connection with debt and equity issuances and non-ordinary course asset dispositions, with certain exceptions specified in the Bridge Credit Agreement. The Credit Agreements contain customary affirmative and negative covenants, including limitations on mergers, consolidations and sales of assets, limitations on liens and sales and leasebacks, limitations on transactions with affiliates, limitations on subsidiary indebtedness and limitations on material changes to the nature of the business of Amgen and its subsidiaries. In addition, each of the Credit Agreements requires maintenance of a minimum consolidated interest coverage ratio of EBITDA to total interest expense, on a consolidated basis.

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SECURITY OWNERSHIP OF MANAGEMENT AND CERTAIN BENEFICIAL OWNERS

Stock Ownership of Certain Beneficial Owners

The following table sets forth the name, address (where required) and beneficial ownership of each person (including any “group” as defined in Section 13(d)(3) of the Exchange Act) known by us to be the beneficial owner of more than five percent (5%) of Horizon Shares, based on filings made under Section 13 of the Exchange Act as of January 19, 2023 and based on 228,391,421 Horizon Shares outstanding as of that date.

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Stock Ownership of Directors and Executive Officers

The following table sets forth, as of January 19, 2023, based on 228,391,421 ordinary shares outstanding as of that date, the beneficial ownership of Horizon ordinary shares (including rights to acquire shares within sixty (60) days of that date) by (i) each Horizon director, (ii) each Horizon named executive officer and (iii) all current directors and executive officers (including named executive officers) as a group. No shares have been pledged as security by any of the Horizon directors or executive officers named below.

Unless otherwise indicated in the footnotes to this table and pursuant to applicable community property laws, Horizon believes the persons named in this table have sole voting and investment power with respect to all ordinary shares reflected in this table. The business address of each of Horizon’s directors and named executive officers is 70 St. Stephen’s Green, Dublin 2, D02 E2X4, Ireland.