Sionna insider RA Capital sells 3.4M shares

RA Capital–affiliated funds reported significant indirect sales of Sionna Therapeutics (SION) common stock on September 17, 2026.

Rhea-AI Filing Summary

Sionna Therapeutics, Inc. (SION) had large shareholder entities affiliated with RA Capital report open-market sales of its common stock on September 17, 2026. RA Capital Healthcare Fund LP sold 3,415,011 shares at $6.32 per share and a further 33,800 shares at a weighted average price of $7.46 per share, in multiple trades between $7.30 and $7.68. The transactions were reported as indirectly held and the RA Capital adviser, its general partner and principals Peter Kolchinsky and Rajeev Shah disclaim beneficial ownership except to the extent of their pecuniary interest. No Rule 10b5-1 trading plan is reported.

Insider Trade Summary

Net Seller: 3,448,811 shares

Net Sell 5 txns

Insider RA CAPITAL MANAGEMENT, L.P., RA Capital Healthcare Fund LP, RA Capital Nexus Fund, L.P., RA Capital Nexus Fund III, L.P., Kolchinsky Peter, Shah Rajeev M.

Role Director, 10% Owner | Director | Director | Director | Director, 10% Owner | Director, 10% Owner

Sold 3,448,811 shs ($21.84M)

Type	Security	Shares	Price	Value
Sale	Common Stock F1, F2	3,415,011	$6.32	$21.58M
Sale	Common Stock F3, F1, F2	33,800	$7.46	$252K
holding	Common Stock F1, F4	--	--	--
holding	Common Stock F1, F5	--	--	--
holding	Common Stock F1, F6	--	--	--

Holdings After Transaction: Common Stock — 6,456,234 shares (Indirect, See footnotes)

Footnotes (6)

1. F1. RA Capital Management, L.P. (the "Adviser") is the investment manager for RA Capital Healthcare Fund, L.P. (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.
2. F2. Held directly by the Fund.
3. F3. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $7.30 to $7.68 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.
4. F4. Held directly by the Nexus Fund.
5. F5. Held directly by the Nexus Fund III.
6. F6. Held directly by the Account.

Key Figures

Shares sold (main block): 3,415,011 shares Sale price (main block): $6.32 per share Shares sold (weighted-average block): 33,800 shares +3 more

6 metrics

Shares sold (main block) 3,415,011 shares Common Stock sold indirectly on September 17, 2026 at $6.32 per share

Sale price (main block) $6.32 per share Price for 3,415,011 Sionna Therapeutics common shares sold on September 17, 2026

Shares sold (weighted-average block) 33,800 shares Additional Sionna Therapeutics common shares sold on September 17, 2026

Weighted average sale price $7.46 per share 33,800 shares sold in multiple trades between $7.30 and $7.68

Price range for weighted-average sales $7.30–$7.68 per share Range of prices for the 33,800 shares sold on September 17, 2026

Total shares sold 3,448,811 shares Aggregate Sionna Therapeutics common shares reported sold across both transactions

Key Terms

weighted average price, pecuniary interest, separately managed account

3 terms

weighted average price financial

"The price reported in Column 4 is a weighted average price."

Weighted average price is the average price of a security where each trade or component is counted according to its size, so bigger trades pull the average more than smaller ones. Think of it like calculating the average cost of a grocery haul where items you bought more of have greater influence on the final per-item cost. Investors use it to understand the true average price paid or received, judge execution quality, and compare trading performance against market movement.

pecuniary interest financial

"disclaim beneficial ownership of any of the reported securities, except to the extent of their pecuniary interest therein."

separately managed account financial

"and a separately managed account (the "Account")."

A separately managed account (SMA) is a personalized investment portfolio owned by a single investor and run by a professional manager who buys and sells securities on that investor’s behalf. It matters to investors because an SMA offers tailored asset selection, tax handling, and transparency—like hiring a personal chef who prepares meals to your dietary needs rather than sharing a set menu—so you can align holdings with your goals and see exactly what you own.

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What insider transactions in SION stock did RA Capital report on September 17, 2026?

RA Capital–affiliated entities reported selling 3,415,011 shares of Sionna Therapeutics common stock at $6.32 per share and an additional 33,800 shares at a weighted average price of $7.46 per share on September 17, 2026.

Which RA Capital entity sold SION shares in this Form 4 filing?

The filing states that the sold Sionna Therapeutics (SION) shares were held directly by RA Capital Healthcare Fund LP, with RA Capital Management, L.P. as investment manager and related entities and individuals disclaiming beneficial ownership except for their pecuniary interest.

What price range applied to the 33,800 SION shares sold at a weighted average price?

The 33,800 Sionna Therapeutics shares were sold in multiple transactions at prices ranging from $7.30 to $7.68 per share, resulting in the reported weighted average sale price of $7.46 per share.

Are these SION insider sales under a Rule 10b5-1 trading plan?

No. The Form 4 for Sionna Therapeutics (SION) indicates that the Rule 10b5-1 checkbox is not marked, and there is no footnote stating that the reported transactions were effected pursuant to a Rule 10b5-1 trading plan.

Do Kolchinsky and Shah report direct ownership of the SION shares sold?

No. The footnotes state that RA Capital Management, its general partner, the related funds, and Dr. Peter Kolchinsky and Mr. Rajeev Shah disclaim beneficial ownership of the reported Sionna Therapeutics securities except to the extent of their pecuniary interest.

What other RA Capital vehicles are mentioned in relation to SION holdings?

The filing references RA Capital Nexus Fund, L.P., RA Capital Nexus Fund III, L.P., and a separately managed account, stating that certain Sionna Therapeutics shares are held directly by each of these entities, with indirect reporting and a disclaimer of beneficial ownership.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Available on EDGAR 09/21/2026 - 06:57 PM Accepted by SEC EDGAR 09/21/2026 - 06:56 PM Learn about SEC filing dates

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 MASSACHUSETTS 02116

(City) (State) (Zip)

UNITED STATES

(Country)

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)

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
09/17/2026

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

	Form filed by One Reporting Person
X	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 Stock	09/17/2026		S		3,415,011	D	$6.32	3,482,612	I	See footnotes(1)(2)
Common Stock	09/17/2026		S		33,800	D	$7.46(3)	3,448,812	I	See footnotes(1)(2)
Common Stock								650,307	I	See footnotes(1)(4)
Common Stock								2,198,082	I	See footnotes(1)(5)
Common Stock								159,033	I	See footnotes(1)(6)

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 MASSACHUSETTS 02116

(City) (State) (Zip)

UNITED STATES

(Country)

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 MASSACHUSETTS 02116

(City) (State) (Zip)

UNITED STATES

(Country)

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, L.P.

(Last) (First) (Middle)

C/O RA CAPITAL MANAGEMENT, L.P.

200 BERKELEY STREET, 18TH FLOOR

(Street)

BOSTON MASSACHUSETTS 02116

(City) (State) (Zip)

UNITED STATES

(Country)

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 MASSACHUSETTS 02116

(City) (State) (Zip)

UNITED STATES

(Country)

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 MASSACHUSETTS 02116

(City) (State) (Zip)

UNITED STATES

(Country)

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 MASSACHUSETTS 02116

(City) (State) (Zip)

UNITED STATES

(Country)

Relationship of Reporting Person(s) to Issuer

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

Explanation of Responses:

1. RA Capital Management, L.P. (the "Adviser") is the investment manager for RA Capital Healthcare Fund, L.P. (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.

2. Held directly by the Fund.

3. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $7.30 to $7.68 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.

4. Held directly by the Nexus Fund.

5. Held directly by the Nexus Fund III.

6. Held directly by the Account.

Remarks:

Dr. 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.	09/21/2026
	/s/ Peter Kolchinsky, Manager of RA Capital Healthcare Fund GP, LLC, the General Partner of RA Capital Healthcare Fund, L.P.	09/21/2026
	/s/ Peter Kolchinsky, Manager of RA Capital Nexus Fund GP, LLC, the General Partner of RA Capital Nexus Fund, L.P.	09/21/2026
	/s/ Peter Kolchinsky, Manager of RA Capital Nexus Fund III GP, LLC, the General Partner of RA Capital Nexus Fund III, L.P.	09/21/2026
	/s/ Peter Kolchinsky, individually	09/21/2026
	/s/ Rajeev Shah, individually	09/21/2026
	** 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)