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

Rhea-AI Filing Summary

On 01 Aug 2025, Prime Medicine, Inc. (PRME) received a Form 4 from its largest outside shareholder group, ARCH Venture Partners. Affiliate ARCH Venture Fund XII, L.P. executed an open-market purchase of 3,030,300 common shares at $3.30 (Transaction Code P). The buy lifts that fund’s direct position to 6,230,300 shares. Two related funds—ARCH Venture Fund X, L.P. and ARCH Venture Fund X Overage, L.P.—each continue to hold 6,128,297 shares. Taken together, the ARCH funds now report indirect beneficial ownership of more than 18 million PRME shares, maintaining their status as a 10% owner.

No derivative securities were involved and no sales were reported. The filing is part of a joint submission; a separate Form 4 covers shares held personally by ARCH co-founder Robert Nelsen. The sizable purchase at a single-digit share price signals continued confidence and long-term support from a strategic life-science investor.

Positive

• 3,030,300 shares purchased at $3.30 by a 10% owner represents a sizeable capital commitment (~$10 M) at market prices.
• Aggregate insider stake now exceeds 18 M shares, signalling strong sponsor conviction and tighter public float.

Negative

• None.

Insights

Securities Analyst

TL;DR: Large insider buy (3.0 M shares) at $3.30 by 10% owner boosts stake, generally bullish signal, no sales or derivatives reported.

The ARCH group’s $10 M+ purchase materially increases insider ownership and removes any overhang fears from potential sales. Because ARCH already surpassed the 10 % threshold, the additional 3 M shares deepen alignment with minority shareholders and could constrain float. While the filing lacks financial metrics, historical studies show sizeable open-market buys by venture sponsors often precede positive corporate actions (financing, partnerships, clinical milestones). Given PRME’s small-cap nature, the move is positively impactful for sentiment and could tighten supply at current valuation.

Portfolio Manager

TL;DR: Venture backer doubles down, adding 3 M shares; ownership concentration rises, signalling conviction but limiting liquidity.

From a portfolio standpoint, the transaction converts roughly $10 M cash into equity exposure at a depressed price, implying ARCH views the risk-reward as attractive ahead of catalysts. Higher insider concentration reduces free float, which can amplify volatility—in bulls’ favor if upcoming data are positive. However, concentrated ownership also heightens single-holder exit risk down the line. Overall, the trade skews net-positive for near-term perception, with moderate liquidity considerations.

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
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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 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
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 XII, 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 XII, 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 XII, 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)

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 XII, L.P., By: ARCH Venture Partners XII, L.P., its General Partner, By: ARCH Venture Partners XII, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact	08/01/2025
	/s/ ARCH Venture Partners XII, L.P., By: ARCH Venture Partners XII, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact	08/01/2025
	/s/ ARCH Venture Fund XII, LLC, By: Mark McDonnell, attorney-in-fact	08/01/2025
	** Signature of Reporting Person	Date
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FAQ

How many Prime Medicine (PRME) shares did ARCH Venture Partners buy?

ARCH Venture Fund XII, L.P. purchased 3,030,300 common shares on 01 Aug 2025.

What was the purchase price disclosed in the Form 4?

The shares were acquired at $3.30 per share in an open-market transaction.

What is ARCH Venture Partners’ total ownership after the transaction?

Directly, ARCH XII now holds 6,230,300 shares; together with related funds, the group beneficially owns ≈18 million shares.

Did the Form 4 report any stock sales or derivative exercises?

No. The filing shows only purchases; no sales or derivative securities were reported.

Why is ARCH classified as a 10% owner of PRME?

Because the combined holdings of the ARCH funds exceed 10 % of PRME’s outstanding shares, triggering Section 16 reporting.