Definium Therapeutics Inc美股招股说明书(2026-06-24版)

20,588,236 Common shares Definium Therapeutics, Inc. We are offering 20,588,236 of our common shares, without par value, at a public offering price of$34.00 per share. Our common shares are listed on the Nasdaq Global Select Market (“Nasdaq”) under the symbol“DFTX”. The last reported sale price of our common shares on Nasdaq on June23, 2026 was$36.18 per share. Investing in our securities involves a high degree of risk. See “Risk Factors” beginning onpageS-9of this prospectus supplement, page 8 of the accompanying prospectus andunder similar headings in the documents incorporated by reference into this prospectussupplement and the accompanying prospectus. We are an “emerging growth company” under applicable Securities and Exchange Commissionrules and are subject to reduced public company reporting requirements. See “ProspectusSupplement Summary—Implications of Being an Emerging Growth Company.” Neither the Securities and Exchange Commission nor any state or other securitiescommission has approved or disapproved of these securities or determined if thisprospectus supplement or the accompanying prospectus is truthful or complete. Anyrepresentation to the contrary is a criminal offense. We have granted the underwriters an option to purchase up to an additional 3,088,235 commonshares from us at the public offering price, less the underwriting discounts and commissionspayable by us. This option is exercisable, in whole or in part, for a period of 30days following thedate of this prospectus supplement. If the underwriters exercise the option in full, the totalunderwriting discounts and commissions payable by us will be $46,287,501, and the totalproceeds to us, before expenses, will be $758,712,513. Delivery of the common shares is expected to be made on or about June 25, 2026. BofASecurities Evercore ISIOppenheimer & Co. StifelLifeSci Capital Prospectus Supplement dated June 23, 2026 Table of contents About this prospectus supplementS-iiProspectus supplement summaryS-1The offeringS-7Risk factorsS-9Special note regarding forward-looking statementsS-12Use of proceedsS-15Dividend policyS-16DilutionS-17Description of the securities we are offeringS-19Certain Canadian federal income tax considerationsS-20Certain U.S. federal income tax considerationsS-22UnderwritingS-28Legal mattersS-34ExpertsS-34Where you can find more informationS-34Incorporation of certain information by referenceS-35Enforceability of civil liabilitiesS-36 About this prospectusiiSpecial note regarding forward-looking statements1Market, industry and other data3The company4Risk factors8Use of proceeds9Selling securityholders10Plan of distribution11General description of our securities14Description of our common shares15Description of our warrants16Description of our debt securities18Description of our units23Where you can find more information24Incorporation by reference24Legal matters24Experts25 About this prospectus supplement This prospectus supplement and the accompanying prospectus are part of a “shelf” registrationstatement on Form S-3 that went effective upon filing with the Securities and ExchangeCommission (the “SEC”) on June28, 2024. Under this shelf registration process, we may sell anycombination of the securities described in our base prospectus included in the shelf registrationstatement in one or more offerings. This prospectus supplement relates to the offering of our common shares. Before buying any ofsuch common shares, we urge you to carefully read this prospectus supplement, together withthe accompanying prospectus and the information incorporated by reference as described underthe headings “Where You Can Find More Information” and “Incorporation of Certain Informationby Reference” in this prospectus supplement. These documents contain important informationthat you should consider when making your investment decision. This prospectus supplement describes the terms of this offering of common shares, and alsoadds to, and updates, information contained in the documents incorporated by reference into thisprospectus supplement. To the extent there is a conflict between the information contained in thisprospectus supplement, on the one hand, and the information contained in any documentincorporated by reference into this prospectus supplement that was filed with the SEC before thedate of this prospectus supplement, on the other hand, you should rely on the information in thisprospectus supplement. If any statement in one of these documents is inconsistent with astatement in another document having a later date (for example, a document incorporated byreference into this prospectus supplement) the statement in the document having the later datemodifies or supersedes the earlier statement. You should rely only on the information contained in or incorporated by reference in thisprospectus supplement, the accompanying prospectus and in any free writing prospectus that wehave authorized for use in connection with this offe