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

Rhea-AI Filing Summary

Sionna Therapeutics (SION) disclosed insider activity on a Form 4. A director reported open‑market sales of common stock on 10/10, 10/13, 10/14, and 10/15/2025, executed under Rule 10b5‑1 trading plans adopted on February 25, 2025 by Atlas Venture Fund XI, L.P. and Atlas Venture Opportunity Fund II, L.P.

Examples: on 10/15, Atlas Venture Fund XI, L.P. sold 65,726 shares at a weighted average price of $30.37 (range $29.95–$30.94) and 30,032 shares at $31.12 (range $30.95–$31.30). Atlas Venture Opportunity Fund II, L.P. sold 13,795 shares at $30.37 and 6,303 shares at $31.12. Following the reported transactions, indirect holdings were 2,619,185 shares (Atlas Venture Fund XI, L.P.) and 751,664 shares (Atlas Venture Opportunity Fund II, L.P.).

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)
10/10/2025

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	10/10/2025		S(1)		16,727	D	$29.98(2)	2,763,574	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/10/2025		S(4)		3,511	D	$29.98(2)	781,969	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/13/2025		S(1)		44,048	D	$30.32(6)	2,719,526	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/13/2025		S(4)		9,245	D	$30.32(6)	772,724	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/13/2025		S(1)		4,082	D	$31.03	2,715,444	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/13/2025		S(4)		857	D	$31.03	771,867	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/14/2025		S(1)		501	D	$29.95	2,714,943	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/14/2025		S(4)		105	D	$29.95	771,762	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/15/2025		S(1)		65,726	D	$30.37(7)	2,649,217	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/15/2025		S(4)		13,795	D	$30.37(7)	757,967	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/15/2025		S(1)		30,032	D	$31.12(8)	2,619,185	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/15/2025		S(4)		6,303	D	$31.12(8)	751,664	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 $29.95 to $30.11 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 $29.95 to $30.93 inclusive.

7. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $29.95 to $30.94 inclusive.

8. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $30.95 to $31.30 inclusive.

	/s/ Ommer Chohan, Attorney-in-Fact	10/15/2025
	** 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 SION's Form 4 report?

It reported a director’s open‑market sales of common stock on 10/10, 10/13, 10/14, and 10/15/2025.

Were the sales under a Rule 10b5-1 plan for SION?

Yes. Sales were made pursuant to Rule 10b5‑1 trading plans adopted on February 25, 2025 by Atlas Venture Fund XI, L.P. and Atlas Venture Opportunity Fund II, L.P.

What were notable SION share sales on 10/15/2025?

Atlas Venture Fund XI, L.P. sold 65,726 shares at a weighted average of $30.37 (range $29.95–$30.94) and 30,032 shares at $31.12 (range $30.95–$31.30).

What are the post-transaction SION holdings?

Indirect holdings were 2,619,185 shares for Atlas Venture Fund XI, L.P. and 751,664 shares for Atlas Venture Opportunity Fund II, L.P.

How were prices reported for these SION transactions?

Prices are weighted averages, with detailed ranges provided; full breakdowns are available upon request as stated in the filing footnotes.

What is the reporter’s relationship to SION?

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