Rule 17 (a) Announcement – Horizon Therapeutics plc

Horizon Therapeutics plc (NASDAQ: HZNP):

RECOMMENDED CASH OFFER

for

HORIZON THERAPEUTICS PLC

by

Pillartree Limited, a newly formed private limited company wholly owned by Amgen Inc.

to be implemented by way of a scheme of arrangement under Chapter 1 of Part 9 of the Irish Companies Act 2014

Announcement relating to convening of Scheme Meeting and Extraordinary General Meeting

On December 12, 2022, the board of directors of Horizon Therapeutics plc, a public limited company incorporated in Ireland (the “Company” or "Horizon”) and the board of directors of Amgen Inc., a Delaware corporation (“Amgen”), announced that they had reached agreement on the terms of a cash offer for the Company by Pillartree Limited, a newly formed private limited company wholly owned by Amgen (“Acquirer Sub”), pursuant to which Acquirer Sub will acquire the entire issued and to be issued ordinary share capital of the Company (the “Proposed Transaction”). As outlined in that announcement, the Proposed Transaction is to be implemented by way of a scheme of arrangement under Chapter 1 of Part 9 of the Irish Companies Act of 2014 (the “Scheme”).

The Company announces that earlier today the Irish High Court made an order directing that a scheme meeting of Horizon shareholders to consider and vote on the Scheme (the “Scheme Meeting”) be convened. Details in relation to the date, time and place of the Scheme Meeting and related extraordinary general meeting (the “EGM”) are as set out below.

The Scheme Meeting will be held on February 24, 2023 commencing at 10:30 a.m. (Irish time) and the EGM will be held on February 24, 2023 commencing at 10:45 a.m. (Irish time), or, if the Scheme Meeting has not concluded by 10:45 a.m. (Irish time), as soon as possible after the conclusion or adjournment of the Scheme Meeting. Both the Scheme Meeting and the EGM will be held at Horizon's registered office at 70 St. Stephen's Green, Dublin 2, D02 E2X4, Ireland.

A proxy statement, which constitutes a scheme circular, relating to the Proposed Transaction, setting out, amongst other things, the full terms and conditions of the Proposed Transaction, information required under Section 452 of the Irish Companies Act 2014, notices convening the Scheme Meeting and the EGM and details of the actions to be taken by Horizon shareholders in relation to the Scheme will be mailed to Horizon shareholders later today. A further announcement will be made today once this has taken place.