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

Rhea-AI Filing Summary

Sionna Therapeutics (SION): RA Capital–affiliated reporting persons disclosed open‑market sales of common stock. Trades occurred on 10/24/2025 at $39.50 and on 10/28/2025 at a weighted average price of $39.58, with individual transactions ranging from $39.50 to $39.81.

Following the 10/28 transactions, indirect holdings reported included 6,905,625 shares (RA Capital Healthcare Fund, L.P.) and 715,988 shares (RA Capital Nexus Fund, L.P.), plus 2,240,708 shares (Nexus Fund III) and 209,391 shares (a separately managed account). The filing identifies the reporting persons as Director and 10% Owner where indicated, and notes that a partner of the adviser serves on the board.

Insights

Equity Ownership Analyst

Large shareholder-director reported open-market sales across affiliated funds; ownership remains sizable across entities.

The filing reports multiple open-market sales of **Sionna Therapeutics, Inc.** common stock by affiliated entities managed by **RA Capital Management, L.P.** on 10/24/2025 and 10/28/2025. Reported sales include 14,116; 122,271; 66,672; and 107,941 shares at $39.5, and 3,624; 31,392; 17,118; and 10,476 shares at a weighted average price of $39.58 (range $39.50–$39.81).

Post-transaction holdings reported per entity remain substantial and indirect: Fund at 6,909,249 then 6,905,625 shares; Nexus Fund at 747,380 then 715,988; Nexus Fund III at 2,257,826 then 2,240,708; and a managed account at 219,867 then 209,391. The reporting persons disclaim beneficial ownership beyond pecuniary interest. The relationship boxes indicate status as **Director** and **10% Owner** for certain reporting persons, with board representation noted.

What it means: a significant holder and board-affiliated group reduced positions across several vehicles while still disclosing large residual stakes. Why it matters: changes by a 10% owner can alter float dynamics and governance influence. Items to watch include any subsequent Forms 4 and whether future transactions affect 10% owner status or board representation; the weighted-average disclosure offers price granularity upon request.

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).

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^*

RA CAPITAL MANAGEMENT, L.P.

(Last) (First) (Middle)

200 BERKELEY STREET, 18TH FLOOR

(Street)

BOSTON MA 02116

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

3. Date of Earliest Transaction (Month/Day/Year)
10/24/2025

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

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

	Form filed by One Reporting Person
X	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	10/24/2025		S		14,116	D	$39.5	6,909,249	I	See footnotes(1)(2)
Common Stock	10/24/2025		S		122,271	D	$39.5	747,380	I	See footnotes(2)(3)
Common Stock	10/24/2025		S		66,672	D	$39.5	2,257,826	I	See footnotes(2)(4)
Common Stock	10/24/2025		S		107,941	D	$39.5	219,867	I	See footnotes(2)(5)
Common Stock	10/28/2025		S		3,624	D	$39.58(6)	6,905,625	I	See footnotes(1)(2)
Common Stock	10/28/2025		S		31,392	D	$39.58(6)	715,988	I	See footnotes(2)(3)
Common Stock	10/28/2025		S		17,118	D	$39.58(6)	2,240,708	I	See footnotes(2)(4)
Common Stock	10/28/2025		S		10,476	D	$39.58(6)	209,391	I	See footnotes(2)(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

1. Name and Address of Reporting Person^*

RA CAPITAL MANAGEMENT, L.P.

(Last) (First) (Middle)

200 BERKELEY STREET, 18TH FLOOR

(Street)

BOSTON MA 02116

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

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

1. Name and Address of Reporting Person^*

RA Capital Healthcare Fund LP

(Last) (First) (Middle)

C/O RA CAPITAL MANAGEMENT, L.P.

200 BERKELEY STREET, 18TH FLOOR

(Street)

BOSTON MA 02116

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

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

1. Name and Address of Reporting Person^*

RA Capital Nexus Fund, L.P.

(Last) (First) (Middle)

C/O RA CAPITAL MANAGEMENT, L.P.

200 BERKELEY STREET, 18TH FLOOR

(Street)

BOSTON MA 02116

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

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

1. Name and Address of Reporting Person^*

RA Capital Nexus Fund III, L.P.

(Last) (First) (Middle)

C/O RA CAPITAL MANAGEMENT, L.P.

200 BERKELEY STREET, 18TH FLOOR

(Street)

BOSTON MA 02116

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

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

1. Name and Address of Reporting Person^*

Kolchinsky Peter

(Last) (First) (Middle)

C/O RA CAPITAL MANAGEMENT, L.P.

200 BERKELEY STREET, 18TH FLOOR

(Street)

BOSTON MA 02116

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

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

1. Name and Address of Reporting Person^*

Shah Rajeev M.

(Last) (First) (Middle)

C/O RA CAPITAL MANAGEMENT, L.P.

200 BERKELEY STREET, 18TH FLOOR

(Street)

BOSTON MA 02116

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

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

Explanation of Responses:

1. Held directly by RA Capital Healthcare Fund, L.P. (the "Fund").

2. RA Capital Management, L.P. (the "Adviser") is the investment manager for the Fund, RA Capital Nexus Fund, L.P. (the "Nexus Fund"), RA Capital Nexus III Fund, L.P. (the "Nexus Fund III"), and a separately managed account (the "Account"). The general partner of the Adviser is RA Capital Management GP, LLC (the "Adviser GP"), of which Dr. Peter Kolchinsky and Mr. Rajeev Shah are the managing members. The Adviser, the Adviser GP, the Fund, the Nexus Fund, the Nexus Fund III, Dr. Kolchinsky and Mr. Shah disclaim beneficial ownership of any of the reported securities, except to the extent of their pecuniary interest therein.

3. Held directly by the Nexus Fund.

4. Held directly by Nexus Fund III.

5. Held directly by the Account.

6. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $39.50 to $39.81 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 range set forth in this footnote.

Remarks:

Joshua Resnick, a Partner of the Adviser, serves on the Issuer's board of directors.

	/s/ Peter Kolchinsky, Manager of RA Capital Management, L.P.	10/28/2025
	/s/ Peter Kolchinsky, Manager of RA Capital Healthcare Fund GP, LLC, the General Partner of RA Capital Healthcare Fund, L.P.	10/28/2025
	/s/ Peter Kolchinsky, Manager of RA Capital Nexus Fund GP, LLC, the General Partner of RA Capital Nexus Fund, L.P.	10/28/2025
	/s/ Peter Kolchinsky, Manager of RA Capital Nexus Fund III GP, LLC, the General Partner of RA Capital Nexus Fund III, L.P.	10/28/2025
	/s/ Peter Kolchinsky, individually	10/28/2025
	/s/ Rajeev Shah, individually	10/28/2025
	** 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 RA Capital report in the SION Form 4 filing?

Open‑market sales of Sionna Therapeutics common stock on 10/24/2025 and 10/28/2025, with disclosed post‑transaction indirect holdings by several RA Capital entities.

On what dates did SION insider sales occur and at what prices?

Trades occurred on 10/24/2025 at $39.50 and on 10/28/2025 at a weighted average price of $39.58, within a $39.50–$39.81 range.

How many SION shares did the RA Capital Healthcare Fund report holding after the trades?

6,905,625 shares were reported as indirectly held following the 10/28/2025 transactions.

What were the post‑trade holdings for the RA Capital Nexus funds and the account?

Following 10/28/2025, RA Capital Nexus Fund reported 715,988 shares, Nexus Fund III 2,240,708 shares, and a separately managed account 209,391 shares.

What roles do the reporting persons have relative to Sionna (SION)?

The filing indicates Director and 10% Owner where applicable; it also notes a partner of the adviser serves on the issuer’s board.

How were the 10/28/2025 prices reported in the Form 4?

As a weighted average price of $39.58, with multiple trades executed between $39.50 and $39.81.