阿尔特米姆公司美股招股说明书(2025-11-06版)

Up to $200,000,000 Common Stock We have entered into an equity distribution agreement, dated November 6, 2025, or the Equity DistributionAgreement, with Leerink Partners LLC, or the sales agent, relating to shares of our common stock, parvalue $0.0001 per share, offered by this prospectus supplement. In accordance with the terms of the EquityDistribution Agreement, we may offer and sell shares of our common stock having an aggregate offeringprice of up to $200.0 million from time to time through the sales agent,pursuant to this prospectussupplement. Our common stock is currently listed on The Nasdaq Global Market under the symbol “ALT.” OnNovember 5, 2025, the last reported sale price of our common stock was $3.75 per share. Sales of our common stock, if any, under this prospectus supplement will be made in sales deemed to be“at the market offerings” as defined in Rule 415(a)(4) promulgated under the Securities Act of 1933, asamended, or the Securities Act. The sales agent is not required to sell any specific amount of securities, butwill act as our sales agent using commercially reasonable efforts consistent with their normal trading andsales practices, on mutually agreed terms between the sales agent and us. There is no arrangement forfunds to be received in any escrow, trust or similar arrangement. The sales agent will be entitled to compensation at a commission rate of up to 3.0% of the aggregate grosssales sold thereunder. In connection with its sale of common stock on our behalf, the sales agent will bedeemed to be an “underwriter” within the meaning of the Securities Act, and the compensation of the salesagent will be deemed to be underwriting commissions or discounts. Investing in our common stock involves a high degree of risk. You should review carefully the risksand uncertainties described under the heading “Risk Factors” on pageS-5of this prospectussupplement, and under similar headings in the documents that are incorporated by reference intothis prospectus supplement and the accompanying prospectus. Neither the Securities and Exchange Commission nor any state securities commission has approvedor disapproved of these securities, or passed upon the adequacy or accuracy of this prospectussupplement and the accompanying prospectus. Any representation to the contrary is a criminaloffense. Leerink Partners The date of this prospectus supplement is November 6, 2025. TABLE OF CONTENTS PROSPECTUS SUPPLEMENT ABOUT THIS PROSPECTUS SUPPLEMENTS-1PROSPECTUS SUPPLEMENT SUMMARYS-3THE OFFERINGS-4RISK FACTORSS-5SPECIAL STATEMENT REGARDING FORWARD-LOOKING STATEMENTSS-7USE OF PROCEEDSS-9DILUTIONS-10PLAN OF DISTRIBUTIONS-11LEGAL MATTERSS-12EXPERTSS-13WHERE YOU CAN FIND MORE INFORMATIONS-14INCORPORATION OF DOCUMENTS BY REFERENCES-15PROSPECTUSABOUT THIS PROSPECTUS1RISK FACTORS2CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS3OUR COMPANY4USE OF PROCEEDS6SECURITIES WE MAY OFFER7DESCRIPTION OF CAPITAL STOCK8DESCRIPTION OF DEBT SECURITIES12DESCRIPTION OF WARRANTS18DESCRIPTION OF UNITS19PLAN OF DISTRIBUTION22LEGAL MATTERS25EXPERTS25WHERE YOU CAN FIND MORE INFORMATION26INCORPORATION BY REFERENCE27 ABOUT THIS PROSPECTUS SUPPLEMENT This document is part of a “shelf” registration statement on Form S-3 that we filed with the SEC andis in two parts. The first part, the accompanying prospectus dated February 27, 2025, including thedocuments incorporated by reference into it, provides more general information. Generally, when we referto this “prospectus,” we are referring to both parts of this document combined. The second part is thisprospectus supplement, which describes the specific terms of this offering of shares of common stock andalso adds to and updates information contained in the accompanying prospectus and the documentsincorporated by reference into this prospectus supplement and the accompanying prospectus. This prospectus supplement relates only to an offering of up to $200.0 million of shares of ourcommon stock through the sales agent. These sales, if any, will be made pursuant to the terms of the EquityDistribution Agreement, a copy of which is filed as an exhibit to the registration statement of which thisprospectus supplement forms a part and is incorporated by reference into this prospectus supplement. Before you invest, you should carefully read this prospectus supplement, the accompanyingprospectus, all information incorporated by reference herein and the additional information describedunder “Where You Can Find More Information” and “Incorporation of Documents by Reference”. Thesedocuments contain information you should consider when making your investment decision. To the extentthat any statement that we make in this prospectus supplement is inconsistent with statements made in theaccompanying prospectus or in any documents incorporated by reference, the statements made in thisprospectus supplement will be deemed to modify or supersede those made in such documents; however, ifany sta