Definium Therapeutics legal chief sells 14,051 shares

The chief legal officer’s shares were sold to cover withholding taxes under a Rule 10b5-1 plan adopted March 14, 2024.

Rhea-AI Filing Summary

Definium Therapeutics, Inc. Chief Legal Officer Mark Sullivan sold 14,051 Common Shares on September 25, 2026, at a weighted average price of $36.57 per share. The shares were sold to satisfy withholding tax obligations connected with settlement of vested restricted stock units, through sell-to-cover elections under a Rule 10b5-1 plan adopted on March 14, 2024. Prices across the transactions ranged from $36.24 to $36.98. Sullivan reported direct holdings of 494,228 Common Shares after the sale.

Insider Trade Summary 10b5-1

Net Seller: 14,051 shares

Net Sell 1 txn

Insider Sullivan Mark

Role Chief Legal Officer

Sold 14,051 shs ($514K)

Type	Security	Shares	Price	Value
Sale	Common Shares F1, F2	14,051	$36.57	$514K

Holdings After Transaction: Common Shares — 494,228 shares (Direct)

Footnotes (2)

1. F1. Represents the number of Common Shares sold to satisfy withholding tax obligations in connection with the settlement of vested restricted stock units, pursuant to sell-to-cover elections under a Rule 10b5-1 plan adopted on March 14, 2024.
2. F2. The reported price is a weighted average sales price. These shares were sold in multiple transactions on September 25, 2026 at prices ranging from $36.24 to $36.98. The Reporting Person undertakes to provide the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.

Key Figures

Common Shares sold: 14,051 shares Weighted average sales price: $36.57 per share Transaction price range: $36.24–$36.98 per share +1 more

4 metrics

Common Shares sold 14,051 shares September 25, 2026

Weighted average sales price $36.57 per share September 25, 2026

Transaction price range $36.24–$36.98 per share Prices across multiple transactions on September 25, 2026

Direct Common Shares held after sale 494,228 shares Following the September 25, 2026 transaction

Key Terms

sell-to-cover elections, Rule 10b5-1 plan, restricted stock units, weighted average sales price

4 terms

sell-to-cover elections financial

"pursuant to sell-to-cover elections under a Rule 10b5-1 plan"

Rule 10b5-1 plan regulatory

"under a Rule 10b5-1 plan adopted on March 14, 2024"

A Rule 10b5-1 plan is a prearranged, written schedule that lets corporate insiders buy or sell company stock at set times or amounts, even if they later learn material nonpublic information. Think of it like setting an automatic thermostat for trades: it creates a clear record that trades were planned in advance, reducing the risk of insider-trading accusations and helping investors trust that insider transactions are routine rather than based on secret information.

restricted stock units financial

"settlement of vested restricted stock units"

Restricted stock units are a type of company reward where employees are promised shares of stock, but they only fully own these shares after meeting certain conditions, like staying with the company for a set time. They matter because they can become valuable assets and are often used to motivate employees to help the company succeed.

weighted average sales price financial

"The reported price is a weighted average sales price."

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

How many DFTX shares did Chief Legal Officer Mark Sullivan sell, and at what price?

Mark Sullivan sold 14,051 Common Shares on September 25, 2026, at a weighted average price of $36.57 per share. Prices across the transactions ranged from $36.24 to $36.98. The shares were sold to satisfy withholding tax obligations connected with settlement of vested restricted stock units under sell-to-cover elections pursuant to a Rule 10b5-1 plan adopted on March 14, 2024.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Available on EDGAR 09/29/2026 - 09:11 PM Accepted by SEC EDGAR 09/29/2026 - 09:10 PM Learn about SEC filing dates

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).

X

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^*

Sullivan Mark

(Last) (First) (Middle)

C/O DEFINIUM THERAPEUTICS, INC.

ONE WORLD TRADE CENTER, SUITE 8500

(Street)

NEW YORK NEW YORK 10007

(City) (State) (Zip)

UNITED STATES

(Country)

2. Issuer Name and Ticker or Trading Symbol
Definium Therapeutics, Inc. [ DFTX ]

5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

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

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
09/25/2026

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

X	Form filed by One Reporting Person
	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 Shares	09/25/2026		S(1)		14,051	D	$36.57(2)	494,228	D

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

Explanation of Responses:

1. Represents the number of Common Shares sold to satisfy withholding tax obligations in connection with the settlement of vested restricted stock units, pursuant to sell-to-cover elections under a Rule 10b5-1 plan adopted on March 14, 2024.

2. The reported price is a weighted average sales price. These shares were sold in multiple transactions on September 25, 2026 at prices ranging from $36.24 to $36.98. The Reporting Person undertakes to provide the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.

	/s/ Mark Sullivan	09/29/2026
	** 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)