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Form 4 Seaport Therapeutics, For: May 04 Filed by: ARCH Venture Partners XII, LLC

May 4, 2026 6:03 PM
FORM 4 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
or Section 30(h) of the Investment Company Act of 1940
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Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).
1. Name and Address of Reporting Person *
ARCH Venture Partners XII, LLC
(Last) (First) (Middle)
8755 W. HIGGINS ROAD, SUITE 1025
(Street)
CHICAGO IL 60631
(City) (State) (Zip)
2. Issuer Name and Ticker or Trading Symbol
Seaport Therapeutics, Inc. [ SPTX ] 		5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
Director X 10% Owner
Officer (give title below) Other (specify below)
3. Date of Earliest Transaction (Month/Day/Year)
05/04/2026
4. If Amendment, Date of Original Filed (Month/Day/Year)
 6. Individual or Joint/Group Filing (Check Applicable Line)
Form filed by One Reporting Person
X Form filed by More than One Reporting Person
Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 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 05/04/2026 C 2,513,686 A (1) 2,513,686 I By ARCH Venture Fund XII, L.P. (2)
Common Stock 05/04/2026 C 2,681,265 A (1) 5,194,951 I By ARCH Venture Fund XII, L.P. (2)
Common Stock 05/04/2026 P 1,100,000 A $ 18 6,294,951 I By ARCH Venture Fund XII, L.P. (2)
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 (Month/Day/Year) 3A. Deemed Execution Date, if any (Month/Day/Year) 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 (Month/Day/Year) 7. Title and Amount of Securities Underlying Derivative Security (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: 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-2 Preferred Stock (1) 05/04/2026 C 2,513,686 (1) (1) Common Stock 2,513,686 $ 0 0 I By ARCH Venture Fund XII, L.P. (2)
Series B Preferred Stock (1) 05/04/2026 C 2,681,265 (1) (1) Common Stock 2,681,265 $ 0 0 I By ARCH Venture Fund XII, L.P. (2)
Explanation of Responses:
1. Each share of Series A-2 and Series B Preferred Stock (collectively, the "Preferred Stock") was convertible into Common Stock of the Issuer on a one-for-3.1407 basis at the option of the holder, and automatically converted upon the closing of the Issuer's initial public offering without payment of further consideration. The Preferred Stock had no expiration date.
2. Shares held directly by ARCH Venture Fund XII, L.P. ("ARCH XII"). ARCH Venture Partners XII, L.P. ("AVP XII LP") is the sole general partner of ARCH XII. ARCH Venture Partners XII, LLC ("AVP XII LLC") is the sole general partner of AVP XII LP. Keith Crandell, Kristina Burow, Steven Gillis and Robert Nelsen are members of the investment committee of AVP XII LLC (the "AVP XII LLC Committee Members"). Each of AVP XII LP and AVP XII LLC may be deemed to beneficially own the shares held by ARCH XII, and each of the AVP XII LLC Committee Members may be deemed to share the power to direct the disposition and vote of the shares held by ARCH XII. Each of AVP XII LP, AVP XII LLC and the AVP XII LLC Committee Members disclaims beneficial ownership except to the extent of their pecuniary interest therein, if any.
ARCH Venture Fund XII, L.P. By: ARCH Venture Partners XII, L.P., its General Partner By: ARCH Venture Partners XII, LLC, its General Partner By: /s/ Mark McDonnell, as Attorney-in-Fact 05/04/2026
** Signature of Reporting Person Date
ARCH Venture Partners XII, L.P. By: ARCH Venture Partners XII, LLC, its General Partner By: /s/ Mark McDonnell, as Attorney-in-Fact 05/04/2026
** Signature of Reporting Person Date
ARCH Venture Partners XII, LLC By: /s/ Mark McDonnell, as Attorney-in-Fact 05/04/2026
** Signature of Reporting Person Date
/s/ Keith Crandell, By: Mark McDonnell, attorney-in-fact 05/04/2026
** Signature of Reporting Person Date
/s/ Kristina Burow, By: Mark McDonnell, attorney-in-fact 05/04/2026
** Signature of Reporting Person Date
/s/ Steven Gillis, By: Mark McDonnell, attorney-in-fact 05/04/2026
** Signature of Reporting Person Date
/s/ Robert Nelsen, By: Mark McDonnell, attorney-in-fact 05/04/2026
** Signature of Reporting Person Date
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).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
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