Summit Therapeutics Inc美国首次公开募股招股说明书（2026年6月10日版本）

Preliminary prospectus supplement(To the prospectus dated June9, 2026) $500,000,000 Common stock We are offering $500.0 million of our common stock. Our common stock is listed on The Nasdaq Global Market under the symbol “SMMT.” The last reported sale price of ourcommon stock on The Nasdaq Global Market on June8, 2026 was $14.11 per share. At an assumed public offering price of Public offering price (1)See “Underwriting” for additional information regarding underwriting compensation. We have granted the underwriters an option for a period of 30 days from the date of this prospectus supplement to purchase upto $75,000,000 of additional shares of our common stock from us at the public offering price less the underwriting discounts and Investing in our common stock involves significant risks. See “Risk factors” on page S-6 of this prospectus supplementand in the documents incorporated by reference in this prospectus supplement concerning factors you should considerbefore investing in our securities. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved ofthese securities or passed upon the adequacy or accuracy of this prospectus supplement or the accompanying prospectus. The underwriters expect to deliver the shares of common stock to purchasers on or about June GoldmanSachs&Co.LLC J.P.Morgan Citigroup The information in this preliminary prospectus supplement is not complete and may be changed. This preliminary prospectus supplement and theaccompanying prospectus are not an offer to sell these securities and we are not soliciting an offer to buy these securities in any jurisdiction where theoffer or sale is not permitted.June, 2026 Table of Contents Table of contentsProspectus supplementABOUT THIS PROSPECTUS SUPPLEMENTPROSPECTUS SUPPLEMENT SUMMARYTHE OFFERINGRISK FACTORSSPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTSUSE OF PROCEEDSDILUTIONMATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES TO NON-U.S. HOLDERSUNDERWRITINGLEGAL MATTERSEXPERTSWHERE YOU CAN FIND MORE INFORMATION; INCORPORATION BY REFERENCEProspectusABOUT THIS PROSPECTUSCAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTSPROSPECTUS SUMMARYRISK FACTORSUSE OF PROCEEDSDESCRIPTION OF CAPITAL STOCKDESCRIPTION OF DEBT SECURITIESDESCRIPTION OF DEPOSITARY SHARESDESCRIPTION OF WARRANTSDESCRIPTION OF SUBSCRIPTION RIGHTSDESCRIPTION OF UNITSDIVIDEND POLICYSELLING SECURITYHOLDERSPLAN OF DISTRIBUTIONLEGAL MATTERSEXPERTSWHERE YOU CAN FIND MORE INFORMATIONINCORPORATION OF CERTAIN INFORMATION BY REFERENCE Table of Contents About this prospectus supplement This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of common stock and alsoadds to and updates information contained in the accompanying prospectus and the documents incorporated by reference into this prospectussupplement and the accompanying prospectus. The second part is the accompanying prospectus dated June9, 2026, included in our registrationstatement on Form S-3 (File No.333-296642), along with the documents incorporated by reference therein, which provides more general information,some of which may not apply to this offering. Generally, when we refer to this prospectus, we are referring to both parts of this document combined. Tothe extent there is a conflict between the information contained in this prospectus supplement, on the one hand, and the information contained in the This prospectus supplement and the accompanying prospectus are part of a “shelf” registration statement that we filed with the SEC. Under this shelfregistration process, we may offer from time to time various securities, of which this offering of shares of our common stock is a part. Such registrationstatement also includes exhibits that provide more detail on the matters discussed in this prospectus supplement and the accompanying prospectus. Youshould read this prospectus supplement, the accompanying prospectus, including the information incorporated by reference herein and therein, the We have not, and the underwriters have not, authorized anyone to provide you with information, or to make any representation, different from thatcontained or incorporated by reference in this prospectus supplement and the accompanying prospectus, along with the information contained in anyfree writing prospectus that we have authorized for use in connection with this offering. We and the underwriters take no responsibility for, and canprovide no assurance as to the reliability of, any other information that others may give you. You should assume that the information appearing in this We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that isincorporated by reference in this prospectus supplement or the accompanying prospectus were made solely for the benefit of the parties to suchagreement, including, in some case