OrbiMed entity trims Sionna Therapeutics (SION) stake under 10b5-1 plan

Rhea-AI Filing Summary

OrbiMed Private Investments VIII, LP, an entity associated with Sionna Therapeutics director Peter A. Thompson, sold 101 shares of Sionna Therapeutics common stock at $44.49 per share in an open-market transaction on May 27, 2026.

The sale was executed pursuant to a Rule 10b5-1 trading plan. After this transaction, entities associated with OrbiMed continued to hold 2,671,721 shares indirectly, indicating that only a very small portion of the overall indirect position was sold.

Insider Trade Summary 10b5-1

Net Seller: 101 shares ($4,493)

Net Sell 1 txn

Insider Thompson Peter A.

Role null

Sold 101 shs ($4K)

Type	Security	Shares	Price	Value
Sale	Common Stock	101	$44.49	$4K

Holdings After Transaction: Common Stock — 2,671,721 shares (Indirect, See footnotes)

Footnotes (1)

1. These securities were sold pursuant to a 10b5-1 plan. These securities are held of record by OrbiMed Private Investments VIII, LP ("OPI VIII"). OrbiMed Capital GP VIII LLC ("GP VIII") is the general partner of OPI VIII and OrbiMed Advisors LLC ("OrbiMed Advisors") is the managing member of GP VIII. By virtue of such relationships, GP VIII and OrbiMed Advisors may be deemed to have voting and investment power with respect to the shares held by OPI VIII and as a result may be deemed to have beneficial ownership of such shares. OrbiMed Advisors exercises investment and voting power through a management committee comprised of Carl L. Gordon, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the shares held by OPI VIII. The Reporting Person is a member of OrbiMed Advisors. Each of the Reporting Person, OrbiMed Advisors, and GP VIII disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), except to the extent of its pecuniary interest therein, if any. This report shall not be deemed an admission that any such entity or person is a beneficial owner of such securities for purposes of Section 16 of the Exchange Act, or for any other purpose.

Key Figures

Shares sold: 101 shares Sale price: $44.49 per share Indirect holdings after sale: 2,671,721 shares

3 metrics

Shares sold 101 shares Open-market sale on May 27, 2026

Sale price $44.49 per share Common stock transaction

Indirect holdings after sale 2,671,721 shares Shares held indirectly following transaction

Key Terms

10b5-1 plan, beneficial ownership, pecuniary interest, Section 16, +1 more

5 terms

10b5-1 plan financial

"These securities were sold pursuant to a 10b5-1 plan."

A 10b5-1 plan is a pre-arranged strategy that allows company insiders to buy or sell their shares at predetermined times and prices, even while they are aware of confidential information. It acts like a scheduled appointment for trading, helping ensure transactions happen transparently and legally, which can reassure investors that trades are not based on insider knowledge.

beneficial ownership financial

"may be deemed to have beneficial ownership of such shares."

Beneficial ownership means the person or entity that actually enjoys the benefits of owning shares or other assets — such as receiving dividends, voting rights, or price gains — even if the legal title is held in another name. For investors it matters because knowing who truly controls and profits from a company reveals who can influence decisions, exposes potential conflicts of interest or hidden concentration of power, and affects transparency and risk in the stock.

pecuniary interest financial

"except to the extent of its pecuniary interest therein, if any."

Section 16 regulatory

"beneficial owner of such securities for purposes of Section 16 of the Exchange Act"

Section 16 is a U.S. securities law rule that governs the trading and disclosure obligations of company insiders — typically officers, directors and large shareholders — to promote transparency and deter unfair profit-taking. It requires insiders to publicly report their stock trades and allows companies or the issuer to reclaim quick, short-term profits from certain insider trades, like a scoreboard and a refund policy that help investors see and limit possible insider advantage.

Rule 16a-1(a) regulatory

"for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934"

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Form 4 Insider Transactions: What Each Field Means →

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^*

Thompson Peter A.

(Last) (First) (Middle)

C/O SIONNA THERAPEUTICS, INC.

21 HICKORY DRIVE, SUITE 500

(Street)

WALTHAM MASSACHUSETTS 02451

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

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
05/27/2026

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

X	Form filed by One Reporting Person
	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	05/27/2026		S		101(1)	D	$44.49	2,671,721	I	See footnotes(2)(3)

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. These securities were sold pursuant to a 10b5-1 plan.

2. These securities are held of record by OrbiMed Private Investments VIII, LP ("OPI VIII"). OrbiMed Capital GP VIII LLC ("GP VIII") is the general partner of OPI VIII and OrbiMed Advisors LLC ("OrbiMed Advisors") is the managing member of GP VIII. By virtue of such relationships, GP VIII and OrbiMed Advisors may be deemed to have voting and investment power with respect to the shares held by OPI VIII and as a result may be deemed to have beneficial ownership of such shares. OrbiMed Advisors exercises investment and voting power through a management committee comprised of Carl L. Gordon, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the shares held by OPI VIII. The Reporting Person is a member of OrbiMed Advisors.

3. Each of the Reporting Person, OrbiMed Advisors, and GP VIII disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), except to the extent of its pecuniary interest therein, if any. This report shall not be deemed an admission that any such entity or person is a beneficial owner of such securities for purposes of Section 16 of the Exchange Act, or for any other purpose.

	/s/ Peter A. Thompson	05/29/2026
	** 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)

FAQ

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

Sionna Therapeutics reported that an OrbiMed-affiliated entity sold 101 shares of common stock at $44.49 per share. The trade was an open-market sale linked to director Peter A. Thompson through his role at OrbiMed Advisors.

Who actually sold Sionna Therapeutics (SION) shares in this Form 4 filing?

The shares were sold by OrbiMed Private Investments VIII, LP, not directly by Peter A. Thompson. OrbiMed entities may be deemed to have voting and investment power, while individuals on the OrbiMed management committee disclaim beneficial ownership beyond any pecuniary interest.

How many Sionna Therapeutics (SION) shares were sold and at what price?

The filing shows a sale of 101 shares of Sionna Therapeutics common stock at $44.49 per share. This was classified as an open-market transaction and represents only a very small portion of the OrbiMed-affiliated indirect holdings.

How many Sionna Therapeutics (SION) shares remain held after this insider sale?

Following the 101-share sale, OrbiMed-affiliated entities continued to hold 2,671,721 shares of Sionna Therapeutics indirectly. The Form 4 notes these holdings are attributed through OrbiMed structures, with beneficial ownership disclaimed beyond any pecuniary interest.

Was the Sionna Therapeutics (SION) insider sale made under a Rule 10b5-1 plan?

Yes. A footnote explains that the 101 Sionna Therapeutics shares were sold pursuant to a Rule 10b5-1 plan. Such plans pre-schedule trades, indicating the timing of this sale was arranged in advance rather than decided opportunistically.

Does Peter A. Thompson claim beneficial ownership of these Sionna Therapeutics (SION) shares?

The Form 4 states that Peter A. Thompson, OrbiMed Advisors, and OrbiMed Capital GP VIII LLC each disclaim beneficial ownership, except to the extent of any pecuniary interest. The report should not be considered an admission of beneficial ownership for Section 16 purposes.