ARCH Venture Fund XII (SPTX) adds shares and converts preferred in Seaport

Rhea-AI Filing Summary

ARCH Venture Fund XII, L.P., a 10% owner of Seaport Therapeutics, Inc., increased its indirect position through an open-market purchase and preferred stock conversions. On May 4, 2026, ARCH XII bought 1,100,000 shares of common stock at $18.00 per share in an open-market transaction.

On the same date, ARCH XII also converted 2,681,265 shares of Series B Preferred Stock and 2,513,686 shares of Series A-2 Preferred Stock into common stock, eliminating these preferred positions. Following these transactions, ARCH XII indirectly held 6,294,951 shares of Seaport Therapeutics common stock through its fund structure.

Insights

Equity ownership and insider-transactions analyst

Venture fund increased Seaport stake via open-market buying and IPO-related conversions.

ARCH Venture Fund XII, L.P. executed a sizeable open-market purchase of 1,100,000 common shares at $18.00 on May 4, 2026, alongside converting its Series A-2 and Series B Preferred Stock into common upon the company’s initial public offering.

The Form 4 shows net-buy activity with no sales, and preferred holdings dropping to zero after conversion. Indirect ownership stands at 6,294,951 common shares held through the ARCH XII fund structure, with individuals associated with ARCH disclaiming beneficial ownership beyond any pecuniary interest.

This pattern reflects a transition from preferred to common equity typical around an IPO plus additional open-market buying. The ultimate impact on Seaport’s trading dynamics depends on broader float and future disclosures about any subsequent transactions.

Insider Trade Summary

Net Buyer: 1,100,000 shares ($19,800,000)

Net Buy 5 txns

Insider ARCH Venture Partners XII, LLC, ARCH Venture Partners XII, L.P., ARCH Venture Fund XII, L.P., CRANDELL KEITH, NELSEN ROBERT, Burow Kristina, GILLIS STEVEN

Role null | null | null | null | null | null | null

Bought 1,100,000 shs ($19.80M)

Type	Security	Shares	Price	Value
Conversion	Series A-2 Preferred Stock	2,513,686	$0.00	--
Conversion	Series B Preferred Stock	2,681,265	$0.00	--
Conversion	Common Stock	2,513,686	$0.00	--
Conversion	Common Stock	2,681,265	$0.00	--
Purchase	Common Stock	1,100,000	$18.00	$19.80M

Holdings After Transaction: Series A-2 Preferred Stock — 0 shares (Indirect, By ARCH Venture Fund XII, L.P.); Series B Preferred Stock — 0 shares (Indirect, By ARCH Venture Fund XII, L.P.); Common Stock — 2,513,686 shares (Indirect, By ARCH Venture Fund XII, L.P.)

Footnotes (1)

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. 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.

Key Figures

Open-market purchase: 1,100,000 shares at $18.00 Post-transaction holdings: 6,294,951 shares Series B Preferred converted: 2,681,265 shares +2 more

5 metrics

Open-market purchase 1,100,000 shares at $18.00 Common stock bought on May 4, 2026

Post-transaction holdings 6,294,951 shares Common shares indirectly held after transactions

Series B Preferred converted 2,681,265 shares Converted into common stock

Series A-2 Preferred converted 2,513,686 shares Converted into common stock

Conversion ratio 1 for 3.1407 Preferred Stock to common stock basis

Key Terms

open-market purchase, derivative conversion, Preferred Stock, initial public offering, +1 more

5 terms

open-market purchase financial

"transaction_action: "open-market purchase""

An open-market purchase is when an investor or a company buys shares on a public stock exchange at the going market price, rather than through a private deal. It matters to investors because these purchases change how many shares are available, can push the stock price up or signal confidence from large buyers, and often affect per-share metrics like earnings—think of it like someone buying lots of apples off a grocery shelf, reducing supply and potentially raising the price.

derivative conversion financial

"transaction_action: "derivative conversion""

Preferred Stock financial

"Each share of Series A-2 and Series B Preferred Stock (collectively, the "Preferred Stock")"

Preferred stock is a type of ownership in a company that typically offers investors higher and more consistent dividend payments than common stock. Unlike regular shares, preferred stock usually doesn’t come with voting rights but provides a priority claim on the company’s assets and profits, making it a more stable and predictable investment option. This makes preferred stock attractive to those seeking steady income with lower risk.

initial public offering financial

"automatically converted upon the closing of the Issuer's initial public offering"

An initial public offering (IPO) is when a private company first sells its shares to the public and becomes a stock-listed company. It matters because it allows the company to raise money from a wide range of investors, helping it grow, while giving early shareholders a way to sell some of their ownership.

beneficially own financial

"may be deemed to beneficially own the shares held by ARCH XII"

Beneficially own means having the economic rights and risks of a security—such as the right to receive dividends, sell the shares, or profit from price changes—whether or not your name appears on the official share register. Think of it like renting a car: you use it and reap the benefits even if the title lists someone else. Investors care because beneficial ownership determines who truly controls value, must be disclosed under securities rules, and can signal potential influence or trading activity that affects a stock’s price.

Form 4 Insider Transactions: What Each Field Means →

SEC Form 4

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

OMB APPROVAL

OMB Number:			3235-0287
Estimated average burden
hours per response:			0.5

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 for the purchase or sale of equity securities of the issuer that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

1. Name and Address of Reporting Person^*

ARCH Venture Partners XII, LLC

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO ILLINOIS 60631

(City) (State) (Zip)

UNITED STATES

(Country)

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)

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
05/04/2026

6. Individual or Joint/Group Filing (Check Applicable Line)

	Form filed by One Reporting Person
X	Form filed by More than One Reporting Person

4. If Amendment, Date of Original Filed (Month/Day/Year)

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)

1. Name and Address of Reporting Person^*

ARCH Venture Partners XII, LLC

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO ILLINOIS 60631

(City) (State) (Zip)

UNITED STATES

(Country)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

ARCH Venture Partners XII, L.P.

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO ILLINOIS 60631

(City) (State) (Zip)

UNITED STATES

(Country)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

ARCH Venture Fund XII, L.P.

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO ILLINOIS 60631

(City) (State) (Zip)

UNITED STATES

(Country)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

CRANDELL KEITH

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO ILLINOIS 60631

(City) (State) (Zip)

UNITED STATES

(Country)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

NELSEN ROBERT

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO ILLINOIS 60631

(City) (State) (Zip)

UNITED STATES

(Country)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

Burow Kristina

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO ILLINOIS 60631

(City) (State) (Zip)

UNITED STATES

(Country)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

GILLIS STEVEN

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO ILLINOIS 60631

(City) (State) (Zip)

UNITED STATES

(Country)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

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
	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
	ARCH Venture Partners XII, LLC By: /s/ Mark McDonnell, as Attorney-in-Fact	05/04/2026
	/s/ Keith Crandell, By: Mark McDonnell, attorney-in-fact	05/04/2026
	/s/ Kristina Burow, By: Mark McDonnell, attorney-in-fact	05/04/2026
	/s/ Steven Gillis, By: Mark McDonnell, attorney-in-fact	05/04/2026
	/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).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.
* Form 4: SEC 1474 (03-26)

FAQ

What did ARCH Venture Fund XII do in this Seaport Therapeutics (SPTX) Form 4?

ARCH Venture Fund XII, L.P. increased its indirect stake in Seaport Therapeutics by buying 1,100,000 common shares at $18.00 and converting its Series A-2 and Series B Preferred Stock into common shares, resulting in 6,294,951 common shares held indirectly after the transactions.

How many Seaport Therapeutics (SPTX) shares did ARCH XII buy on the open market?

ARCH Venture Fund XII, L.P. executed an open-market purchase of 1,100,000 Seaport Therapeutics common shares at $18.00 per share on May 4, 2026. This purchase was reported as an indirect ownership position held by ARCH XII, without any corresponding sales reported in the same filing.

Which preferred stock classes did ARCH XII convert into Seaport Therapeutics common stock?

ARCH XII converted 2,681,265 shares of Series B Preferred Stock and 2,513,686 shares of Series A-2 Preferred Stock into Seaport Therapeutics common stock. These conversions eliminated its preferred positions and increased its common stock holdings as part of the company’s initial public offering process.

How many Seaport Therapeutics (SPTX) common shares does ARCH XII hold after these transactions?

After the reported transactions, ARCH Venture Fund XII, L.P. indirectly holds 6,294,951 shares of Seaport Therapeutics common stock. This figure reflects the combined effect of the 1,100,000-share open-market purchase and the conversions of Series A-2 and Series B Preferred Stock into common shares.

Were the Seaport Therapeutics (SPTX) transactions direct or indirect for ARCH XII and related parties?

All reported positions are indirect and held by ARCH Venture Fund XII, L.P. The general partners and investment committee members may be deemed to beneficially own these shares but disclaim beneficial ownership except to the extent of any pecuniary interest, according to the filing’s footnote disclosures.

How were Seaport’s preferred shares converted according to the Form 4 footnotes?

Each share of Seaport’s Series A-2 and Series B Preferred Stock was convertible into common stock on a one-for-3.1407 basis. The preferred stock automatically converted into common upon the closing of Seaport’s initial public offering, without additional consideration and with no expiration date on the preferred shares.