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

Rhea-AI Filing Summary

Insider sales by a director were disclosed for Sionna Therapeutics (SION). The Form 4 shows Director Bruce Booth reported multiple sales under Rule 10b5-1 trading plans on 10/03/2025 and 10/06/2025

On 10/03/2025 the filing reports sales of 31,198 and 6,548 shares at a weighted price around $30. On 10/06/2025 it reports sales of 9,753 and 2,047 shares at a weighted price around $29.95. The reported beneficial ownership after the transactions is 2,780,301 shares held by Atlas Venture Fund XI, L.P. and 785,480 shares held by Atlas Venture Opportunity Fund II, L.P.

The filing notes the shares sold were held indirectly by the two Atlas Venture funds and that the reporting person disclaims beneficial ownership except for any pecuniary interest. The transactions were executed pursuant to trading plans adopted on 02/25/2025 and the Form is signed by an attorney-in-fact on 10/07/2025.

Positive

• Transactions were executed under Rule 10b5-1 plans, indicating preplanned sales
• Full disclosure given with weighted-average prices and explanatory footnotes
• Beneficial ownership remains concentrated in Atlas Venture Fund XI and Atlas Venture Opportunity Fund II after sales

Negative

• Reported sales reduced indirect holdings by the specific sold share counts: 31,198; 6,548; 9,753; and 2,047
• Sales occurred at prices near $29.95–$30.11, which may modestly reduce aggregate fund exposure over time

Insights

Corporate Governance Analyst

Director disclosed planned sales under 10b5-1 plans; holdings remain largely with Atlas funds.

The sales were executed under Rule 10b5-1 trading plans established on 02/25/2025, which provides an affirmative defense to accusations of trading on material nonpublic information when plans meet required conditions. Reporting shows the shares sold came from interests held by two Atlas Venture funds rather than direct personal holdings.

This structure reduces governance concerns tied to opportunistic insider sales, but investors may watch periodic Form 4 filings for any shift in the funds' aggregate stake over the next 12 months.

Insider Transactions Specialist

Sales totaled discrete blocks at approximately $29.95–$30.11, disclosed with weighted-average pricing.

The filing lists four sale entries: 31,198, 6,548, 9,753, and 2,047 shares on 10/03/2025 and 10/06/2025. Footnotes state the reported prices are weighted averages across multiple executions within the given ranges.

Because the sales were made under pre-established plans, monitor future Form 4s for additional planned-plan activity or any changes in the funds' reported beneficial ownership in the coming quarters.

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)
10/03/2025

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	10/03/2025		S(1)		31,198	D	$30(2)	2,790,054	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/03/2025		S(4)		6,548	D	$30(2)	787,527	I	By Atlas Venture Opportunity Fund II, L.P.(5)
Common Stock	10/06/2025		S(1)		9,753	D	$29.95(6)	2,780,301	I	By Atlas Venture Fund XI, L.P.(3)
Common Stock	10/06/2025		S(4)		2,047	D	$29.95(6)	785,480	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) and (6).

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 $30.00 inclusive.

	/s/ Ommer Chohan, Attorney-in-Fact	10/07/2025
	** 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 Bruce Booth (SION) sell and when?

Bruce Booth reported sales of 31,198 and 6,548 shares on 10/03/2025 and 9,753 and 2,047 shares on 10/06/2025.

Were the sales part of a 10b5-1 trading plan for SION?

Yes. The filing states the transactions were made pursuant to Rule 10b5-1 plans adopted on 02/25/2025.

What prices were the SION shares sold at?

The filing reports weighted-average prices in ranges from $29.95 to $30.11, with specific weighted averages of $30 and $29.95 for the reported trades.

Who holds the remaining reported shares after these trades?

The filing shows Atlas Venture Fund XI, L.P. held 2,780,301 shares and Atlas Venture Opportunity Fund II, L.P. held 785,480 shares after the reported transactions.

Did the reporting person claim direct beneficial ownership of the sold shares?

No. The reporting person disclaims beneficial ownership of the shares held by the Atlas funds except for any pecuniary interest, per the filing.