X0609 4 2026-05-04 0002042347 Seaport Therapeutics, Inc. SPTX 0001979548 ARCH Venture Partners XII, LLC false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 false false true false 0001979765 ARCH Venture Partners XII, L.P. false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 false false true false 0001906837 ARCH Venture Fund XII, L.P. false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 false false true false 0001219039 CRANDELL KEITH false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 false false true false 0001219042 NELSEN ROBERT false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 false false true false 0001569248 Burow Kristina false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 false false true false 0001229592 GILLIS STEVEN false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 false false true false false Common Stock 2026-05-04 4 C false 2513686 A 2513686 I By ARCH Venture Fund XII, L.P. Common Stock 2026-05-04 4 C false 2681265 A 5194951 I By ARCH Venture Fund XII, L.P. Common Stock 2026-05-04 4 P false 1100000 18 A 6294951 I By ARCH Venture Fund XII, L.P. Series A-2 Preferred Stock 2026-05-04 4 C false 2513686 0 D Common Stock 2513686 0 I By ARCH Venture Fund XII, L.P. Series B Preferred Stock 2026-05-04 4 C false 2681265 0 D Common Stock 2681265 0 I By ARCH Venture Fund XII, L.P. 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. 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 2026-05-04 ARCH Venture Partners XII, L.P. By: ARCH Venture Partners XII, LLC, its General Partner By: /s/ Mark McDonnell, as Attorney-in-Fact 2026-05-04 ARCH Venture Partners XII, LLC By: /s/ Mark McDonnell, as Attorney-in-Fact 2026-05-04 /s/ Keith Crandell, By: Mark McDonnell, attorney-in-fact 2026-05-04 /s/ Kristina Burow, By: Mark McDonnell, attorney-in-fact 2026-05-04 /s/ Steven Gillis, By: Mark McDonnell, attorney-in-fact 2026-05-04 /s/ Robert Nelsen, By: Mark McDonnell, attorney-in-fact 2026-05-04