Sec Form 4 Filing - GORDON CARL L @ Theseus Pharmaceuticals, Inc. - 2021-10-12

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person GORDON CARL L 2. Issuer Name and Ticker or Trading Symbol Theseus Pharmaceuticals, Inc. [ THRX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O THESEUS PHARMACEUTICALS, INC., 245 MAIN STREET 3. Date of Earliest Transaction (MM/DD/YY) 10/12/2021 (Street) CAMBRIDGE, MA02142 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 10/12/2021 C( 1 ) 15,777,018 A 15,777,018 I By OrbiMed Private Investments VII, LP ( 3 ) Common Stock 10/12/2021 C( 1 ) 436,650 A 436,650 I By OrbiMed Genesis Master Fund, L.P. ( 4 ) Common Stock 10/12/2021 P( 5 ) 562,500 A $ 16 16,339,518 I By OrbiMed Private Investments VII, LP ( 3 ) ( 5 ) Common Stock 10/12/2021 P( 5 ) 62,500 A $ 16 499,150 I By OrbiMed Genesis Master Fund, L.P. ( 4 ) ( 5 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Series A Convertible Preferred Stock ( 2 ) 10/12/2021 C 14,903,718 ( 2 ) ( 2 ) Common Stock 14,903,718 ( 2 ) 0 I By OrbiMed Private Investments VII, LP ( 3 ) Series B Convertible Preferred Stock ( 2 ) 10/12/2021 C 873,300 ( 2 ) ( 2 ) Common Stock 873,300 ( 2 ) 0 I By OrbiMed Private Investments VII, LP ( 3 ) Series B Convertible Preferred Stock ( 2 ) 10/12/2021 C 436,650 ( 2 ) ( 2 ) Common Stock 436,650 ( 2 ) 0 I By OrbiMed Genesis Master Fund, L.P. ( 4 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other GORDON CARL L C/O THESEUS PHARMACEUTICALS, INC. 245 MAIN STREET CAMBRIDGE, MA02142 X X Signatures /s/ Bradford Dahms - Attorney-in-Fact 10/14/2021 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The total represents shares received upon conversion of shares of Series A Convertible Preferred Stock and/or Series B Convertible Preferred Stock. ( 2 )The reported security converted into the issuer's Common Stock on a one-for-one basis automatically immediately prior to the closing of the issuer's initial public offering without payment of further consideration. The shares have no expiration date. ( 3 )The shares are held of record by OrbiMed Private Investments VII, LP ("OPI VII"). OrbiMed Capital GP VII LLC ("GP VII") is the general partner of OPI VII. OrbiMed Advisors LLC ("OrbiMed Advisors"), a registered investment adviser under the Investment Advisers Act of 1940, as amended, is the managing member of GP VII. By virtue of such relationship, OrbiMed Advisors and GP VII may be deemed to have voting power and investment power over the securities held by OPI VII and as a result, may be deemed to have beneficial ownership over such securities. OrbiMed Advisors exercises voting and investment power through a management committee comprised of the Reporting Person, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the shares held by OPI VII. ( 4 )The shares are held of record by OrbiMed Genesis Master Fund, L.P. ("OrbiMed Genesis"). OrbiMed Genesis GP LLC ("Genesis GP") is the general partner of OrbiMed Genesis. OrbiMed Advisors is the managing member of Genesis GP. By virtue of such relationship, OrbiMed Advisors and Genesis GP may be deemed to have voting power and investment power over the securities held by Genesis and as a result, may be deemed to have beneficial ownership over such securities. OrbiMed Advisors exercises voting and investment power through a management committee comprised of the Reporting Person, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the shares held by OrbiMed Genesis. ( 5 )Represents shares purchased in the issuer's initial public offering.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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