[Form 4] Sionna Therapeutics, Inc. Insider Trading Activity

Rhea-AI Filing Summary

Sionna Therapeutics (SION): Director Form 4 discloses Rule 10b5‑1 sales. On 10/21/2025, indirect holdings associated with the reporting person sold multiple blocks of common stock via pre‑planned trades. Examples include 22,339 shares at a weighted average $32.69, 19,948 shares at $33.87, 5,763 shares at $34.91, and 25,327 shares at $35.69. Prices reflect weighted averages from disclosed ranges. Following these transactions, shares indirectly held by funds were 2,309,035 for Atlas Venture Fund XI, L.P. and 686,567 for Atlas Venture Opportunity Fund II, L.P., as reported.

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^*

Booth Bruce

(Last) (First) (Middle)

C/O SIONNA THERAPEUTICS, INC.

21 HICKORY DRIVE, SUITE 500

(Street)

WALTHAM MA 02451

(City) (State) (Zip)

2. Issuer Name and Ticker or Trading Symbol
Sionna Therapeutics, Inc. [ SION ]

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

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

3. Date of Earliest Transaction (Month/Day/Year)
10/21/2025

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

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

X	Form filed by One Reporting Person
	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	10/21/2025		S(1)		22,339	D	$32.69(2)	2,360,156	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/21/2025		S(4)		4,688	D	$32.69(2)	697,297	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/21/2025		S(1)		19,948	D	$33.87(6)	2,340,208	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/21/2025		S(4)		4,187	D	$33.87(6)	693,110	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/21/2025		S(1)		5,763	D	$34.91(7)	2,334,445	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/21/2025		S(4)		1,210	D	$34.91(7)	691,900	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/21/2025		S(1)		25,327	D	$35.69(8)	2,309,118	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/21/2025		S(4)		5,316	D	$35.69(8)	686,584	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/21/2025		S(1)		83	D	$36.42	2,309,035	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/21/2025		S(4)		17	D	$36.42	686,567	I	By Atlas Venture Opportunity Fund II, L.P.(5)

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. This transaction was made pursuant to a Rule 10b5-1 trading plan adopted by Atlas Venture Fund XI, L.P. on February 25, 2025.

2. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $32.19 to $33.057 inclusive. The Reporting Persons undertake to provide to 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 ranges set forth in footnotes (2), (6), (7) and (8).

3. These shares are held directly by Atlas Venture Fund XI, L.P. ("AVF XI "). The general partner of AVF XI is Atlas Venture Associates XI, L.P. ("AVA XI LP"). Atlas Venture Associates XI, LLC ("AVA XI LLC") is the general partner of AVA XI LP. The Reporting Person is a member of AVA XI LLC and disclaims beneficial ownership of such securities held by AVF XI, except to the extent of his pecuniary interest therein, if any.

4. This transaction was made pursuant to a Rule 10b5-1 trading plan adopted by Atlas Venture Opportunity Fund II, L.P. on February 25, 2025.

5. These shares are held directly by Atlas Venture Opportunity Fund II, L.P. ("AVOF II"). The general partner of AVOF II is Atlas Venture Associates Opportunity II, L.P. ("AVAO II LP"). Atlas Venture Associates Opportunity II, LLC ("AVAO II LLC") is the general partner of AVAO II LP. The Reporting Person is a member of AVAO II LLC and disclaims beneficial ownership of such securities held by AVOF II, except to the extent of his pecuniary interest therein, if any.

6. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $33.305 to $34.28 inclusive.

7. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $34.30 to $35.29 inclusive.

8. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $35.30 to $36.14 inclusive.

	/s/ Ommer Chohan, Attorney-in-Fact	10/21/2025
	** 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.

FAQ

What did Sionna Therapeutics (SION) disclose in this Form 4?

A director reported multiple open‑market sales of common stock on 10/21/2025, executed under a Rule 10b5‑1 trading plan.

What were the sale prices reported for SION shares?

Weighted average prices included $32.69, $33.87, $34.91, and $35.69, each based on multiple trades within disclosed ranges.

How many SION shares were sold in the individual transactions?

Examples include 22,339, 19,948, 5,763, and 25,327 shares, as listed in the transaction table.

Were the sales pre‑planned under Rule 10b5‑1?

Yes. Footnotes state plans adopted on February 25, 2025 for both Atlas Venture Fund XI, L.P. and Atlas Venture Opportunity Fund II, L.P.

What are the reported indirect holdings after these transactions?

Reported amounts were 2,309,035 shares for Atlas Venture Fund XI, L.P. and 686,567 shares for Atlas Venture Opportunity Fund II, L.P.

What relationship does the reporting person have to SION?

The reporting person is a Director of Sionna Therapeutics, Inc., per the filing.

Who held the shares that were sold?

Shares were held indirectly via Atlas Venture Fund XI, L.P. and Atlas Venture Opportunity Fund II, L.P., with beneficial ownership disclaimed except for any pecuniary interest.