[Form 4] Prime Medicine, Inc. Insider Trading Activity

Rhea-AI Filing Summary

Prime Medicine (PRME) – Form 4 insider transaction filed 08/01/2025. ARCH Venture entities, collectively 10% owners, disclosed a purchase of 3,030,300 common shares at $3.30 per share (approx. $10.0 million). The buyer of record is ARCH Venture Fund XII, L.P.

Post-transaction indirect holdings stand at:

• ARCH Venture Fund X, L.P.: 6,128,297 shares (unchanged)
• ARCH Venture Fund X Overage, L.P.: 6,128,297 shares (unchanged)
• ARCH Venture Fund XII, L.P.: 6,230,300 shares (up from zero disclosed in this filing)

Total indirect ownership across the reporting group rises to roughly 18.5 million shares. The filing is one of two joint reports; Robert Nelsen is filing separately for his direct holdings.

Key take-away: A sizable open-market purchase by a long-term venture backer at a depressed price signals increased confidence in PRME’s prospects and tightens insider alignment with public shareholders.

Insights

Securities Analyst

TL;DR Large venture fund bought 3.0 M PRME shares at $3.30, lifting insider stake to 18.5 M; bullish governance signal.

The ARCH Venture complex, already a 10%+ holder, added 3,030,300 shares via Fund XII for about $10 million. No dispositions were reported. Given PRME’s 52-week low near $3, the purchase suggests insiders see deep value and are willing to add liquidity. The acquisition raises group ownership to ~18.5 M shares, likely >15% of outstanding shares (exact float not in filing). Such concentrated ownership can both support the share price and influence strategic direction. Impact classified as positive because fresh capital came at market—no dilutive structure—and reinforces long-term commitment.

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

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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).

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 X, 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
Prime Medicine, Inc. [ PRME ]

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)
08/01/2025

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								6,128,297	I	By ARCH Venture Fund X, L.P.(1)(3)
Common Stock								6,128,297	I	By ARCH Venture Fund X Overage, L.P.(2)(3)
Common Stock	08/01/2025		P		3,030,300	A	$3.3	6,230,300	I	By ARCH Venture Fund XII, L.P.(4)

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

1. Name and Address of Reporting Person^*

ARCH Venture Partners X, LLC

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO IL 60631

(City) (State) (Zip)

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 X, L.P.

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO IL 60631

(City) (State) (Zip)

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 X Overage, L.P.

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO IL 60631

(City) (State) (Zip)

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 X, L.P.

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO IL 60631

(City) (State) (Zip)

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 X Overage, L.P.

(Last) (First) (Middle)

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO IL 60631

(City) (State) (Zip)

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)

C/O ARCH VENTURE PARTNERS IX, LLC

8755 W. HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO IL 60631

(City) (State) (Zip)

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 IL 60631

(City) (State) (Zip)

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)

C/O ARCH VENTURE PARTNERS

8755 WEST HIGGINS ROAD, SUITE 1025

(Street)

CHICAGO IL 60631

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

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

Explanation of Responses:

1. Represents shares held directly by ARCH Venture Fund X, L.P. ("ARCH X"). ARCH Venture Partners X, L.P. ("AVP X LP") is the sole general partner of ARCH X.

2. Represents shares held directly by ARCH Venture Fund X Overage, L.P. ("ARCH X Overage"). ARCH Venture Partners X Overage, L.P. ("AVP X Overage LP") is the sole general partner of ARCH X Overage.

3. ARCH Venture Partners X, LLC ("AVP X LLC") is the sole general partner of each of AVP X LP and AVP X Overage LP. Keith Crandell, Kristina Burow and Steven Gillis are members of the investment committee of AVP X LLC (the "AVP X Committee Members"). AVP X LP and AVP X Overage LP may be deemed to beneficially own the shares held by ARCH X and ARCH X Overage, respectively, AVP X LLC may be deemed to beneficially own the shares held by ARCH X and ARCH X Overage, and each of the AVP X Committee Members may be deemed to share the power to direct the disposition and vote of the shares held by ARCH X and ARCH X Overage. AVP X LP, AVP X Overage LP, AVP X LLC, and the AVP X Committee Members each disclaim beneficial ownership except to the extent of any pecuniary interest therein, and this report shall not be deemed an admission that such shares are beneficially owned by them for Section 16 or any other purpose.

4. Represents shares held directly by ARCH Venture Fund XII, L.P. ("ARCH XII"). ARCH Venture Partners XII, L.P. ("AVP XII LP") is the general partner of ARCH XII. ARCH Venture Partners XII, LLC ("AVP XII LLC") is the general partner of AVP XII LP. Keith Crandell, Kristina Burow and Steven Gillis 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.

Remarks:

This Form 4 is one of two reports relating to the same transaction being filed jointly by ARCH X, AVP X LP, ARCH X Overage, AVP X Overage LP, AVP X LLC, ARCH XII, AVP XII LP, AVP XII LLC, Keith Crandell, Kristina Burow and Steven Gillis (collectively, the "Reporting Persons"). Robert Nelsen has direct ownership of Common Stock and is filing his own Form 4 separately.

	/s/ ARCH Venture Fund X, L.P., By: ARCH Venture Partners X, L.P., its General Partner, By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact	08/01/2025
	/s/ ARCH Venture Fund X Overage, L.P., By: ARCH Venture Partners X Overage, L.P., its General Partner, By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact	08/01/2025
	/s/ ARCH Venture Partners X, L.P., By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact	08/01/2025
	/s/ ARCH Venture Partners X Overage, L.P., By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact	08/01/2025
	/s/ ARCH Venture Partners X, LLC, By: Mark McDonnell, attorney-in-fact	08/01/2025
	/s/ Keith Crandell, By: Mark McDonnell, attorney-in-fact	08/01/2025
	/s/ Kristina Burow, By: Mark McDonnell, attorney-in-fact	08/01/2025
	/s/ Steven Gillis, By: Mark McDonnell, attorney-in-fact	08/01/2025
	** Signature of Reporting Person	Date
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