Amylyx Pharmaceuticals Inc美股招股说明书(2025-09-10版)

17,500,000 Shares of common stock We are offering 17,500,000 shares of our common stock, par value $0.0001 per share. Our common stock is listed on the Nasdaq Global Select Market, or Nasdaq, under the symbol “AMLX.” On September9, 2025, the closing price of our commonstock, as reported on Nasdaq, was $10.51 per share. Investing in our securities involves significant risks that are described in the “RISKFACTORS” section beginning on pageS-6ofthis prospectus supplement and page 9 of the accompanying prospectus, and in the other documents that are incorporated by referenceherein. You should read the entire prospectus supplement and the accompanying prospectus, including any information incorporatedby reference herein or therein, carefully, before you make your investment decision. We are a smaller reporting company as defined under Rule 405 of the Securities Act of 1933, as amended, or the Securities Act, and, as such, are subject to certainreduced public company reporting requirements. See “Prospectus Supplement Summary—Implications of Being a Smaller Reporting Company” on pageS-3of thisprospectus supplement. (1)We have agreed to reimburse the underwriters for certain expenses. See “Underwriting” beginning on pageS-21of this prospectus supplement for additionalinformation regarding the compensation payable to the underwriters. We have granted the underwriters an option for a period of 30 days to purchase up to an additional 2,625,000 shares of our common stock at the public offeringprice less the underwriting discounts and commissions. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined ifthis prospectus supplement is truthful or complete. Any representation to the contrary is a criminal offense. The underwriters expect to deliver the shares of common stock against payment on or about September11, 2025. Lead Bookrunning Managers GuggenheimSecurities LeerinkPartners Lead Manager H.C. Wainwright& Co. Prospectus Supplement dated September 9, 2025 Table of Contents Table of Contents Prospectus Supplement ABOUT THIS PROSPECTUS SUPPLEMENTPROSPECTUS SUPPLEMENT SUMMARYTHE OFFERINGRISK FACTORSSPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTSUSE OF PROCEEDSDIVIDEND POLICYDILUTIONCERTAIN U.S. FEDERAL INCOME TAX CONSIDERATIONSUNDERWRITINGLEGAL MATTERSEXPERTSWHERE YOU CAN FIND MORE INFORMATIONINCORPORATION OF CERTAIN INFORMATION BY REFERENCE Prospectus ABOUT THIS PROSPECTUSSPECIAL NOTE ON FORWARD-LOOKING STATEMENTSPROSPECTUS SUMMARYSECURITIES WE MAY OFFERWHERE YOU CAN FIND MORE INFORMATIONINCORPORATION BY REFERENCERISK FACTORSUSE OF PROCEEDSTAXATIONDESCRIPTION OF CAPITAL STOCKDESCRIPTION OF DEBT SECURITIESDESCRIPTION OF WARRANTSDESCRIPTION OF UNITSPLAN OF DISTRIBUTIONLEGAL MATTERSEXPERTS Table of Contents ABOUT THIS PROSPECTUS SUPPLEMENT This document has two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of common stock.The second part is the accompanying prospectus, which provides more general information, some of which may not apply to this offering. Theinformation included or incorporated by reference in this prospectus supplement also adds to, updates and changes information contained orincorporated by reference in the accompanying prospectus. If information included or incorporated by reference in this prospectus supplement isinconsistent with the accompanying prospectus or the information incorporated by reference therein, then this prospectus supplement or the informationincorporated by reference in this prospectus supplement will apply and will supersede the information in the accompanying prospectus and thedocuments incorporated by reference therein. This prospectus supplement is part of a registration statement on FormS-3(FileNo.333-285525)that we filed with the Securities and ExchangeCommission, or the SEC, using a “shelf” registration process. Under the shelf registration process, we may from time to time offer and sell anycombination of the securities described in the accompanying prospectus, of which this offering is a part. You should rely only on the information contained or incorporated by reference in this prospectus supplement, the accompanyingprospectus and any free writing prospectus prepared by us or on our behalf. We have not, and the underwriters have not, authorized any otherperson to provide you with information different from that contained in this prospectus supplement and the accompanying prospectus orincorporated by reference in this prospectus supplement and the accompanying prospectus. If anyone provides you with different orinconsistent information, you should not rely on it. We are not, and the underwriters are not, making an offer to sell or soliciting an offer to buythese securities under any circumstance in any jurisdiction where the offer or solicitation is not permitted. You should assume that theinformation