方舟生物医药美股招股说明书(2025-11-05版)

Common Stock We have entered into a sales agreement (the “Sales Agreement”) with TD Securities (USA) LLC (“TD Cowen”), datedOctober 3, 2025, relating to the sale of shares of our common stock, $0.001 par value per share, offered by this prospectus. Inaccordance with the terms of the Sales Agreement, under this prospectus, we may offer and sell shares of our common stock having anaggregate offering price of up to $200,000,000 from time to time through TD Cowen, acting as our agent or principal. Sales of shares of our common stock, if any, under this prospectus will be made in negotiated transactions, including blocktrades or block sales, or by any method permitted by law deemed to be an “at the market offering” as defined in Rule 415(a)(4) underthe Securities Act of 1933, as amended, (the “Securities Act”), including, without limitation, sales made through The Nasdaq GlobalMarket (“Nasdaq”), or any other existing trading market for our common stock or by any other method permitted by law. TD Cowen isnot required to sell any specific number or dollar amount of shares of our common stock, but will act as our sales agent usingcommercially reasonable efforts consistent with its normal trading and sales practices. There is no arrangement for funds to be received in any escrow, trust or similar arrangement. TD Cowen will be entitled to compensation at a commission rate of up to 3.0% of the gross sales price per share sold underthe Sales Agreement. See “Plan of Distribution” beginning on page 9 for additional information regarding the compensation to be paidto TD Cowen. In connection with the sale of shares of our common stock on our behalf, TD Cowen will be deemed to be an “underwriter”within the meaning of the Securities Act and the compensation of TD Cowen will be deemed to be underwriting commissions ordiscounts. We have also agreed to provide indemnification and contribution to TD Cowen with respect to certain liabilities, includingliabilities under the Securities Act. Shares of our common stock trade on the Nasdaq Global Market under the symbol “ORKA.” On October 2, 2025, the lastsale price of our common stock as reported on the Nasdaq Global Market was $19.18 per share. Investing in our common stock involves a high degree of risk. See “Risk Factors” beginning on page 4 of thisprospectus and in the documents incorporated by reference into this prospectus. We are a “smaller reporting company” as defined by Rule 12b-2 of the Exchange Act and are subject to reducedpublic company reporting requirements. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved ofthe securities being offered by this prospectus, or determined if this prospectus is truthful or complete. Any representation tothe contrary is a criminal offense. TD Cowen The date of this prospectus is November 3, 2025. Table of Contents PageAbout This ProspectusiiSpecial Note Regarding Forward-Looking StatementsiiiProspectus Summary1Risk Factors4Use Of Proceeds6Dilution7Dividend Policy8Plan of Distribution9Legal Matters10Experts10Where You Can Find More Information10Incorporation of Certain Documents by Reference11 ABOUT THIS PROSPECTUS This prospectus is part of a registration statement on Form S-3 that we filed with the Securities and Exchange Commission(the “SEC”) using a “shelf” registration process. Under this prospectus, we may offer and sell shares of our common stock having anaggregate offering price of up to $200,000,000 from time to time through TD Cowen acting as our agent or principal at prices and onterms to be determined by market conditions at the time of the offering. The $200,000,000 of shares of our common stock that may besold under this prospectus are included in the $500,000,000 of shares of common stock that may be sold under the registrationstatement. We provide information to you about this offering of shares of our common stock in two separate documents that are boundtogether: (1) this prospectus, which describes the specific details regarding this offering; and (2) the accompanying base prospectus,which provides general information, some of which may not apply to this offering. Generally, when we refer to this “prospectus,” weare referring to both documents combined. Before buying any of the common stock that we are offering, we urge you to carefully read both this prospectus together withall of the information incorporated by reference herein, as well as the additional information described under the headings “Where YouCan Find More Information” and “Incorporation of Certain Documents by Reference.” These documents contain importantinformation that you should consider when making your investment decision. To the extent there is a conflict between the information contained in this prospectus, on the one hand, and the informationcontained in any document incorporated by reference in this prospectus, on the other hand, you should rely on the information in th