Statement of Changes in Beneficial Ownership (Form 4)

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b). Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES 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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1. Name and Address of Reporting Person * ARCH Venture Partners XII, LLC 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) (Last) (First) (Middle) 8755 W. HIGGINS ROAD, SUITE 1025 3. Date of Earliest Transaction (Month/Day/Year) 05/04/2026 (Street) CHICAGO, IL 60631 4. If Amendment, Date 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 (City) (State) (Zip) 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)

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

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
ARCH Venture Partners XII, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
ARCH Venture Partners XII, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
ARCH Venture Fund XII, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
CRANDELL KEITH 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
NELSEN ROBERT 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
Burow Kristina 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
GILLIS STEVEN 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X

Signatures

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

Explanation of Responses:

*	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).
(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.

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