OrbiMed (NASDAQ: SION) fund logs small 101-share sale under 10b5-1 plan

Rhea-AI Filing Summary

OrbiMed-affiliated entities reported a small open-market sale of Sionna Therapeutics stock under a pre-set trading plan. An entity associated with OrbiMed sold 101 shares of Sionna Therapeutics, Inc. common stock at $44.49 per share on May 27, 2026, in an open-market transaction pursuant to a Rule 10b5-1 plan.

After this sale, OrbiMed-related holdings reported in this filing totaled 2,671,721 shares of common stock, held indirectly through OrbiMed Private Investments VIII, LP. The OrbiMed reporting entities and certain principals disclaim beneficial ownership beyond any pecuniary interest.

Insights

Equity ownership & insider-transactions analyst

OrbiMed-linked fund made a very small, pre-planned share sale.

The reporting entities tied to OrbiMed Advisors LLC disclosed an open-market sale of 101 shares of Sionna Therapeutics common stock at $44.49 per share. The transaction was executed pursuant to a Rule 10b5-1 trading plan, indicating it was pre-scheduled rather than opportunistic.

The shares are held indirectly by OrbiMed Private Investments VIII, LP, with OrbiMed Capital GP VIII LLC and OrbiMed Advisors LLC in control positions. Post-transaction holdings reported in this filing are 2,671,721 shares, so the sale represents a very small fraction of the position.

The reporting persons expressly disclaim beneficial ownership of the securities beyond any pecuniary interest, and the filing clarifies that OrbiMed Advisors exercises investment and voting power through a management committee. This structure and the small size of the sale point to a routine portfolio management move with limited informational value.

Insider Trade Summary 10b5-1

Net Seller: 101 shares ($4,493)

Net Sell 1 txn

Insider ORBIMED ADVISORS LLC, OrbiMed Capital GP VIII LLC

Role null | 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. 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: 101 shares Sale price: $44.49 per share Post-transaction holdings: 2,671,721 shares +2 more

5 metrics

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

Sale price $44.49 per share Price for Sionna Therapeutics common stock

Post-transaction holdings 2,671,721 shares Indirect holdings reported after the sale

Transaction type Open-market sale (Code S) Non-derivative Form 4 transaction

Trading plan Rule 10b5-1 plan Footnote states sale pursuant to 10b5-1 plan

Key Terms

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

5 terms

Rule 10b5-1 regulatory

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

Rule 10b5-1 is a regulation that allows company insiders to buy or sell their shares at predetermined times, even if they have access to non-public information. It acts like setting a schedule in advance for transactions, helping prevent accusations of unfair trading. This rule provides a way for insiders to plan trades transparently, giving investors confidence that these transactions are not based on hidden 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.

Form 4 regulatory

"This report on Form 4 is jointly filed by OrbiMed Advisors and GP VIII."

Form 4 is a official document that company insiders, such as executives or major shareholders, file with regulators whenever they buy or sell company shares. It provides transparency about how those with inside knowledge are trading, helping investors see if insiders are confident in the company's prospects or may be selling for personal reasons. This information can influence investor decisions by revealing insiders' perspectives on the company's value.

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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^*

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)
05/27/2026

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	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

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	05/29/2026
	/s/ Carl L. Gordon, Member of OrbiMed Capital GP VIII LLC	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 OrbiMed report for Sionna Therapeutics (SION)?

OrbiMed-linked entities reported selling 101 shares of Sionna Therapeutics common stock at $44.49 per share on May 27, 2026. The sale was an open-market transaction and was executed under a pre-arranged Rule 10b5-1 trading plan.

How many Sionna Therapeutics (SION) shares do OrbiMed entities hold after this Form 4?

After the reported transaction, OrbiMed-related entities reported holding 2,671,721 shares of Sionna Therapeutics common stock indirectly. These shares are held of record by OrbiMed Private Investments VIII, LP, with OrbiMed-affiliated entities exercising investment and voting power.

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

Yes. The Form 4 footnotes state that the 101-share sale of Sionna Therapeutics stock was conducted pursuant to a Rule 10b5-1 trading plan, indicating the trade was pre-planned rather than timed at the insider’s discretion.

Who is actually selling Sionna Therapeutics (SION) shares in this OrbiMed Form 4?

The securities are held of record by OrbiMed Private Investments VIII, LP. OrbiMed Capital GP VIII LLC and OrbiMed Advisors LLC may be deemed to have voting and investment power, but they and certain principals disclaim beneficial ownership beyond any pecuniary interest.

Do OrbiMed insiders claim beneficial ownership of all Sionna Therapeutics (SION) shares reported?

No. The reporting persons disclaim beneficial ownership of the securities reported, except to the extent of any pecuniary interest. The filing states it should not be deemed an admission of beneficial ownership for Section 16 or other purposes.