OrbiMed-backed Sionna (NASDAQ: SION) sees fund share sale

Rhea-AI Filing Summary

OrbiMed-affiliated funds reported open-market sales of Sionna Therapeutics, Inc. common stock. OrbiMed-related entities sold 52,397 shares on April 17 at $44.68 per share and 2,948 shares on April 18 at $44.52 per share, under a pre-arranged Rule 10b5-1 trading plan. After these indirect transactions through OrbiMed Private Investments VIII, LP, the reporting entities continued to hold 2,964,774 shares of Sionna Therapeutics common stock.

Insider Trade Summary 10b5-1

Net Seller: 55,345 shares ($2,472,343)

Net Sell 2 txns

Insider ORBIMED ADVISORS LLC, OrbiMed Capital GP VIII LLC

Role null | null

Sold 55,345 shs ($2.47M)

Type	Security	Shares	Price	Value
Sale	Common Stock	2,948	$44.52	$131K
Sale	Common Stock	52,397	$44.68	$2.34M

Holdings After Transaction: Common Stock — 2,964,774 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 17 Apr: 52,397 shares Price 17 Apr sale: $44.68/share Shares sold 18 Apr: 2,948 shares +3 more

6 metrics

Shares sold 17 Apr 52,397 shares Open-market sale of common stock at $44.68 per share

Price 17 Apr sale $44.68/share Open-market sale of 52,397 common shares

Shares sold 18 Apr 2,948 shares Open-market sale of common stock at $44.52 per share

Price 18 Apr sale $44.52/share Open-market sale of 2,948 common shares

Total shares sold 55,345 shares Net open-market sales across both reported transactions

Shares held after sales 2,964,774 shares Indirect common stock holdings after April transactions

Key Terms

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

5 terms

Rule 10b5-1 plan financial

"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 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.

indirect ownership financial

"ownership_type: "indirect", ownership_code: "I""

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^*

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)
04/17/2026

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	04/17/2026		S		52,397(1)	D	$44.68	2,967,722	I	See footnotes(2)(3)
Common Stock	04/18/2026		S		2,948(1)	D	$44.52	2,964,774	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

(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	04/21/2026
	/s/ Carl L. Gordon, Member of OrbiMed Capital GP VIII LLC	04/21/2026
	** 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)

FAQ

What insider activity did Sionna Therapeutics (SION) report in this Form 4?

Sionna Therapeutics reported that OrbiMed-affiliated entities executed open-market sales of common stock. They sold 52,397 shares on April 17 and 2,948 shares on April 18, while retaining an indirect holding of 2,964,774 shares after the transactions.

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

The shares were sold by OrbiMed-related investment entities, including OrbiMed Private Investments VIII, LP, with OrbiMed Advisors LLC and OrbiMed Capital GP VIII LLC as affiliated reporting persons. They report indirect beneficial ownership through their roles in the OrbiMed investment structure.

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

Two open-market sales were reported, totaling 55,345 shares. On April 17, 52,397 shares were sold at $44.68 per share, and on April 18, 2,948 shares were sold at $44.52 per share, according to the Form 4 disclosure.

How many Sionna Therapeutics (SION) shares do the OrbiMed entities hold after the sales?

Following the reported transactions, the OrbiMed-affiliated entities reported indirect ownership of 2,964,774 Sionna Therapeutics common shares. This post-transaction figure reflects their remaining position held through OrbiMed Private Investments VIII, LP as stated in the filing.

Were the Sionna Therapeutics (SION) insider sales part of a Rule 10b5-1 trading plan?

Yes. The footnotes state that the securities were sold pursuant to a Rule 10b5-1 trading plan. Such pre-arranged plans allow insiders and affiliated funds to schedule trades in advance, helping separate routine portfolio management from discretionary timing decisions.

Do OrbiMed Advisors and related entities claim full beneficial ownership of Sionna Therapeutics (SION) shares?

The reporting entities state they may be deemed beneficial owners through their roles but expressly disclaim beneficial ownership except to the extent of their pecuniary interest. Individuals on OrbiMed’s management committee also disclaim beneficial ownership of the fund-held shares.