OrbiMed entities sell Sionna Therapeutics (SION) stock under 10b5-1 plan

Rhea-AI Filing Summary

OrbiMed-affiliated funds reported open-market sales of Sionna Therapeutics, Inc. common stock. OrbiMed entities sold a total of 510,583 shares over three days at prices between $44.51 and $45.34 per share under a pre-arranged Rule 10b5-1 trading plan, and continued to hold 3,020,119 shares indirectly after the latest transaction.

The shares are held of record by OrbiMed Private Investments VIII, LP, with OrbiMed Capital GP VIII LLC and OrbiMed Advisors LLC potentially deemed beneficial owners through their control relationships, subject to customary pecuniary-interest and beneficial-ownership disclaimers.

Insights

Equity capital markets analyst

OrbiMed funds executed planned share sales while retaining a large Sionna stake.

The filing shows OrbiMed-affiliated entities completed three open-market sales totaling 510,583 Sionna Therapeutics shares at prices around $44–$45 per share. All trades were effected pursuant to a Rule 10b5-1 plan, indicating they were pre-scheduled rather than opportunistic.

After these transactions, the entities still held 3,020,119 shares of common stock indirectly, so this is a partial reduction, not an exit. The filing also emphasizes that various OrbiMed entities and individuals disclaim beneficial ownership beyond their pecuniary interests, which is standard for institutional fund structures.

Insider Trade Summary 10b5-1

Net Seller: 510,583 shares ($22,945,044)

Net Sell 3 txns

Insider ORBIMED ADVISORS LLC, OrbiMed Capital GP VIII LLC

Role Director | Director

Sold 510,583 shs ($22.95M)

Type	Security	Shares	Price	Value
Sale	Common Stock	226,906	$45.34	$10.29M
Sale	Common Stock	235,863	$44.64	$10.53M
Sale	Common Stock	47,814	$44.51	$2.13M

Holdings After Transaction: Common Stock — 3,020,119 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

Total shares sold: 510,583 shares Shares sold on 2026-04-16: 226,906 shares at $45.34/share Shares sold on 2026-04-15: 235,863 shares at $44.64/share +2 more

5 metrics

Total shares sold 510,583 shares Aggregate open-market sales reported in this Form 4

Shares sold on 2026-04-16 226,906 shares at $45.34/share Open-market sale of common stock

Shares sold on 2026-04-15 235,863 shares at $44.64/share Open-market sale of common stock

Shares sold on 2026-04-14 47,814 shares at $44.51/share Open-market sale of common stock

Shares held after latest sale 3,020,119 shares Indirect holdings following 2026-04-16 transaction

Key Terms

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

5 terms

10b5-1 plan regulatory

"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 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.

Rule 16a-1(a) regulatory

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

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)
04/14/2026

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	04/14/2026		S		47,814(1)	D	$44.51	3,482,888	I	See footnotes(2)(3)
Common Stock	04/15/2026		S		235,863(1)	D	$44.64	3,247,025	I	See footnotes(2)(3)
Common Stock	04/16/2026		S		226,906(1)	D	$45.34	3,020,119	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	04/16/2026
	/s/ Carl L. Gordon, Member of OrbiMed Capital GP VIII LLC	04/16/2026
	** 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 did OrbiMed entities do in this Form 4 for SION?

OrbiMed-affiliated entities reported selling 510,583 shares of Sionna Therapeutics (SION) common stock in open-market transactions. The sales were executed over three days at prices around the mid-$40s per share under a pre-arranged Rule 10b5-1 trading plan.

How many Sionna Therapeutics shares do the OrbiMed entities still hold?

After the reported transactions, OrbiMed-related entities held 3,020,119 shares of Sionna Therapeutics common stock indirectly. This indicates the reported trades represent a partial position reduction, with a substantial stake remaining under the control of the OrbiMed fund structure.

At what prices were the SION shares sold by OrbiMed entities?

The OrbiMed entities sold SION common stock at prices of $44.51, $44.64, and $45.34 per share. These prices correspond to three trading days of open-market sales, as disclosed in the Form 4 transaction details for the non-derivative common stock.

Were the OrbiMed sales of Sionna Therapeutics shares pre-planned?

Yes. A footnote states the securities were sold pursuant to a Rule 10b5-1 trading plan. Such plans are set up in advance and allow insiders or affiliated funds to sell shares according to pre-established instructions, reducing the significance of trade timing signals.

Who is the actual holder of the SION shares linked to OrbiMed?

The shares are held of record by OrbiMed Private Investments VIII, LP. OrbiMed Capital GP VIII LLC is its general partner, and OrbiMed Advisors LLC is the managing member. Through these roles, they may be deemed beneficial owners, subject to standard pecuniary-interest disclaimers.

Does the Form 4 admit OrbiMed as beneficial owner of all reported SION shares?

No. The filing explains that the reporting entities disclaim beneficial ownership of the securities beyond their pecuniary interests. It states the report should not be deemed an admission of beneficial ownership for Section 16 or other legal purposes, which is common fund disclosure language.