标记临床研究美股招股说明书(2025-10-03版)

$50,000,000 Depositary Shares Each Representing 1/1000th of a Share of 8.375% Series B Cumulative Perpetual Preferred Stock(Liquidation Preference Equivalent to $25.00 Per Depositary Share) We have entered into a sales agreement (the “Sales Agreement”) with H.C. Wainwright& Co., LLC (“Wainwright” or the “Sales Agent”) on October 3, 2025, relating to the sale of depositaryshares (the “depositary shares” or “Series B Depositary Shares”), each representing a 1/1000th fractional interest in a share of 8.375% Series B Cumulative Perpetual Preferred Stock (the“Series B Preferred Stock”). In accordance with the terms of the Sales Agreement, pursuant to this prospectus supplement and the accompanying prospectus, from time to time we may offerand sell Series B Depositary Shares having an aggregate gross sales price of up to $50,000,000 through or to Wainwright, acting as sales agent or principal. As a holder of the depositaryshares, you will be entitled to all proportional rights, preferences and privileges of the Series B Preferred Stock represented thereby, including dividend, voting, redemption and liquidationrights and preferences. The proportionate liquidation preference of each depositary share is $25.00. Sales of the depositary shares, if any, under this prospectus supplement and the accompanying prospectus may be made in transactions that are deemed to be “at the market offerings” asdefined in Rule 415 under the Securities Act of 1933, as amended (the “Securities Act”). The Agent is not required to sell any specific number or dollar amount of the depositary shares, butthe Agent will make all sales using commercially reasonable efforts consistent with its normal trading and sales practices on mutually agreed terms between the Agent and us. See “Plan ofDistribution” for more information. The depositary shares to which this prospectus supplement and the accompanying prospectus relate will be offered and sold through or to the Agent over a period of time and from time totime. Under the Sales Agreement, the Agent will be entitled to compensation of up to 3.0% of the gross proceeds of all depositary shares sold through or to it, as sales agent or principal. Inconnection with the sale of the depositary shares, the Agent will be deemed to be an “underwriter” within the meaning of the Securities Act, and the compensation of the Agent will be deemedto be underwriting commissions or discounts. There is no arrangement to place proceeds of the offering in escrow, trust or similar arrangement. See “Plan of Distribution” for moreinformation. Our depositary shares are trading on the Nasdaq Global Market (“Nasdaq”) under the symbol “XOMAO.” On October1, 2025, the last reported sale price of our depositary shares on theNasdaq Global Market was $25.62 per share. Investing in the depositary shares and our preferred stock involves a high degree of risk. Please read the information contained in andincorporated by reference under the heading “Risk Factors” beginning on page S-10 of this prospectus supplement and the accompanyingprospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus supplement and theaccompanying prospectus. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplementand the accompanying prospectus are truthful or complete. Any representation to the contrary is a criminal offense. H.C. Wainwright& Co. The date of this prospectus supplement is October 3, 2025. Table of Contents TABLE OF CONTENTS Prospectus Supplement About This Prospectus SupplementProspectus Supplement SummaryRisk FactorsSpecial Note Regarding Forward-Looking StatementsUse of ProceedsDescription of Series B Preferred Stock and the Depositary SharesBook-Entry ProceduresU.S. Federal Income Tax ConsiderationsPlan of DistributionLegal MattersExpertsWhere You Can Find More InformationIncorporation of Certain Information by Reference Prospectus ABOUT THIS PROSPECTUSSPECIAL NOTE REGARDING FORWARD LOOKING STATEMENTSTHE COMPANYRISK FACTORSUSE OF PROCEEDSDESCRIPTION OF SECURITIESWE MAY OFFER Table of Contents ABOUT THIS PROSPECTUS SUPPLEMENT This document is in two parts. The first part is this prospectus supplement, which describes the terms of this offering of our depositary shares and alsoadds to, updates and changes information contained in the accompanying prospectus and the documents incorporated by reference into theaccompanying prospectus. The second part is the accompanying prospectus, which provides more general information. Generally, when we refer to thisprospectus, we are referring to both parts of this document combined. To the extent there is a conflict between the information contained in thisprospectus supplement, on the one hand, and the information contained in the accompanying prospectus or in any document incorporated by referencethat was filed