OrbiMed entities trim Sionna Therapeutics (SION) stake via 115,844-share 10b5-1 sale

Rhea-AI Filing Summary

OrbiMed-affiliated entities reported an open-market sale of Sionna Therapeutics stock under a pre-arranged trading plan. An entity associated with OrbiMed sold 115,844 shares of Sionna Therapeutics, Inc. common stock at a price of $45.50 per share pursuant to a Rule 10b5-1 plan. After this transaction, the OrbiMed-related holder continues to own 2,555,877 shares indirectly through OrbiMed Private Investments VIII, LP, reflecting a large remaining position. OrbiMed Capital GP VIII LLC and OrbiMed Advisors LLC may be deemed to share voting and investment power over these shares, but they each disclaim beneficial ownership beyond any pecuniary interest.

Insider Trade Summary 10b5-1

Net Seller: 115,844 shares ($5,270,902)

Net Sell 1 txn

Insider ORBIMED ADVISORS LLC, OrbiMed Capital GP VIII LLC

Role null | null

Sold 115,844 shs ($5.27M)

Type	Security	Shares	Price	Value
Sale	Common Stock	115,844	$45.50	$5.27M

Holdings After Transaction: Common Stock — 2,555,877 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. This report on Form 4 is jointly filed by OrbiMed Advisors and GP VIII. Each of the Reporting Persons 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. The Reporting Persons have designated a representative, Peter A. Thompson, a member of OrbiMed Advisors, to serve on the Issuer's board of directors. 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: 115,844 shares Sale price: $45.50 per share Shares held after sale: 2,555,877 shares +1 more

4 metrics

Shares sold 115,844 shares Open-market sale of Sionna Therapeutics common stock

Sale price $45.50 per share Price received in the 115,844-share sale

Shares held after sale 2,555,877 shares Indirect holdings following the reported transaction

Form type Form 4 Insider transaction report under Section 16

Key Terms

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

5 terms

Rule 10b5-1 plan regulatory

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

A Rule 10b5-1 plan is a prearranged, written schedule that lets corporate insiders buy or sell company stock at set times or amounts, even if they later learn material nonpublic information. Think of it like setting an automatic thermostat for trades: it creates a clear record that trades were planned in advance, reducing the risk of insider-trading accusations and helping investors trust that insider transactions are routine rather than based on secret information.

beneficial ownership regulatory

"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

"for purposes of Section 16 of the Exchange Act, or for any other purpose."

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.

voting and investment power financial

"may be deemed to have voting and investment power with respect to the shares held by OPI VIII"

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Form 4 Insider Transactions: What Each Field Means →

Available on EDGAR 07/02/2026 - 04:42 PM Accepted by SEC EDGAR 07/02/2026 - 04:42 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).

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^*

ORBIMED ADVISORS LLC

(Last) (First) (Middle)

601 LEXINGTON AVENUE

54TH FLOOR

(Street)

NEW YORK NEW YORK 10022

(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)
06/30/2026

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	06/30/2026		S		115,844(1)	D	$45.5	2,555,877	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

1. Name and Address of Reporting Person^*

ORBIMED ADVISORS LLC

(Last) (First) (Middle)

601 LEXINGTON AVENUE

54TH FLOOR

(Street)

NEW YORK NEW YORK 10022

(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^*

OrbiMed Capital GP VIII LLC

(Last) (First) (Middle)

601 LEXINGTON AVENUE

54TH FLOOR

(Street)

NEW YORK NEW YORK 10022-4629

(City) (State) (Zip)

UNITED STATES

(Country)

Relationship of Reporting Person(s) to Issuer

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

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.

3. This report on Form 4 is jointly filed by OrbiMed Advisors and GP VIII. Each of the Reporting Persons 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. The Reporting Persons have designated a representative, Peter A. Thompson, a member of OrbiMed Advisors, to serve on the Issuer's board of directors. 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/ Carl L. Gordon, Member of OrbiMed Advisors LLC	07/02/2026
	/s/ Carl L. Gordon, Member of OrbiMed Capital GP VIII LLC	07/02/2026
	** 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 OrbiMed report for Sionna Therapeutics (SION)?

OrbiMed-related entities reported an open-market sale of 115,844 shares of Sionna Therapeutics common stock at $45.50 per share. The transaction was executed under a Rule 10b5-1 trading plan and reflects a scheduled disposition rather than a discretionary market-timing decision.

How many Sionna Therapeutics (SION) shares does the OrbiMed-affiliated holder still own?

Following the reported sale, the OrbiMed-affiliated holder continues to own 2,555,877 shares of Sionna Therapeutics common stock indirectly. These shares are held by OrbiMed Private Investments VIII, LP, with OrbiMed Capital GP VIII LLC and OrbiMed Advisors LLC potentially sharing voting and investment power.

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

Yes, the sold Sionna Therapeutics shares were disposed of pursuant to a Rule 10b5-1 trading plan. Such plans are pre-arranged schedules for trading, designed to allow insiders to sell shares over time without making ad hoc market-timing decisions.

Who is the actual holder of the Sionna Therapeutics (SION) shares linked to OrbiMed?

The Sionna Therapeutics shares are held of record by OrbiMed Private Investments VIII, LP. OrbiMed Capital GP VIII LLC is the general partner of this fund, and OrbiMed Advisors LLC is the managing member, giving them potential voting and investment authority over the position.

Do OrbiMed Advisors LLC and OrbiMed Capital GP VIII LLC claim full beneficial ownership of SION shares?

No, OrbiMed Advisors LLC and OrbiMed Capital GP VIII LLC each disclaim beneficial ownership of the Sionna Therapeutics securities beyond any pecuniary interest. The filing states this disclaimer is for purposes of the Exchange Act’s Section 16 beneficial ownership rules.

Who represents the OrbiMed entities on Sionna Therapeutics (SION) board of directors?

The OrbiMed entities designated Peter A. Thompson, a member of OrbiMed Advisors, to serve on Sionna Therapeutics’ board of directors. This board representation aligns with their investment role but does not by itself confer full beneficial ownership of all related shares.