Tax-cover sale by Definium (DFTX) legal chief totals 10,702 shares

Rhea-AI Filing Summary

Definium Therapeutics, Inc. Chief Legal Officer Mark Sullivan sold 10,702 Common Shares on March 25, 2026 at a weighted average price of $18.47. According to the filing, the shares were sold to satisfy withholding tax obligations tied to vested restricted stock units under a Rule 10b5-1 sell-to-cover plan adopted on March 14, 2024. Following this tax-related sale, he directly holds 271,079 Common Shares, which includes 793 shares acquired under the company’s 2024 Employee Share Purchase Plan.

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).

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)
03/25/2026

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	03/25/2026		S(1)		10,702	D	$18.47(2)	271,079(3)	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 March 25, 2026 at prices ranging from $18.36 to $18.80. 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.

3. Amount includes 793 shares acquired under the Definium Therapeutics, Inc. 2024 Employee Share Purchase Plan.

	/s/ Mark Sullivan	03/26/2026
	** 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 insider transaction did Definium Therapeutics (DFTX) report for Mark Sullivan?

Definium Therapeutics reported that Chief Legal Officer Mark Sullivan sold 10,702 Common Shares. The sale occurred on March 25, 2026 and was executed under a Rule 10b5-1 sell-to-cover arrangement tied to vested restricted stock units and related tax withholding.

How many DFTX shares did Mark Sullivan sell and at what price?

Mark Sullivan sold 10,702 Common Shares of Definium Therapeutics. The reported weighted average sales price was $18.47 per share, with individual trades executed between $18.36 and $18.80 on March 25, 2026, according to the detailed pricing footnote.

Why were Mark Sullivan’s Definium Therapeutics shares sold in this Form 4?

The filing states the 10,702 Common Shares were sold to satisfy withholding tax obligations from the settlement of vested restricted stock units. The transactions were made pursuant to sell-to-cover elections under a Rule 10b5-1 trading plan adopted on March 14, 2024.

How many Definium Therapeutics (DFTX) shares does Mark Sullivan own after the sale?

After the reported sale, Mark Sullivan directly owns 271,079 Common Shares of Definium Therapeutics. The filing notes this amount includes 793 shares that were acquired through the Definium Therapeutics, Inc. 2024 Employee Share Purchase Plan, reflecting his ongoing equity position.

Were Mark Sullivan’s DFTX share sales part of a pre-planned trading arrangement?

Yes. The filing explains that the sale of 10,702 Common Shares was made pursuant to sell-to-cover elections under a Rule 10b5-1 plan. That trading plan was adopted on March 14, 2024, and governed the tax-related sales linked to vested restricted stock units.

What pricing details were disclosed for Mark Sullivan’s DFTX share sale?

The filing reports a weighted average sales price of $18.47 per Common Share. It further explains that multiple transactions occurred on March 25, 2026, at prices ranging from $18.36 to $18.80, with full trade-level details available upon request from the company or regulators.