OrbiMed fund linked to Sionna Therapeutics (SION) director sells 510K shares

Filing Impact

(Very High)

Filing Sentiment

(Very Negative)

Form Type

Rhea-AI Filing Summary

Sionna Therapeutics, Inc. director Peter A. Thompson reported indirect open-market sales of a total of 510,583 shares of common stock in three transactions. The shares were sold at prices between $44.51 and $45.34 per share pursuant to a Rule 10b5-1 trading plan.

The securities are held of record by OrbiMed Private Investments VIII, LP, an investment fund advised by OrbiMed entities with which Thompson is affiliated. Following these transactions, 3,020,119 shares of common stock remain indirectly held, reflecting a continuing sizable position.

Insights

Pre-planned fund sales reduce position but leave large indirect stake.

The filing shows OrbiMed Private Investments VIII, LP, an OrbiMed-managed fund associated with director Peter A. Thompson, sold 510,583 Sionna Therapeutics common shares in three open-market trades at $44.51–$45.34 per share under a Rule 10b5-1 plan.

Because these trades were executed pursuant to a pre-arranged plan, their timing carries less informational value than discretionary sales. After the transactions, 3,020,119 common shares remain indirectly held through the fund, indicating that a substantial exposure to SION is maintained despite the net reduction.

Insider Trade Summary 10b5-1

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

Net Sell 3 txns

Insider Thompson Peter A.

Role 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)

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

Total shares sold: 510,583 shares Shares sold on 2026-04-16: 226,906 shares Shares sold on 2026-04-15: 235,863 shares +4 more

7 metrics

Total shares sold 510,583 shares Aggregate open-market sales across three transactions

Shares sold on 2026-04-16 226,906 shares Common stock sold indirectly at $45.34 per share

Shares sold on 2026-04-15 235,863 shares Common stock sold indirectly at $44.64 per share

Shares sold on 2026-04-14 47,814 shares Common stock sold indirectly at $44.51 per share

Remaining indirect holdings 3,020,119 shares Common stock indirectly held after the latest transaction

Highest sale price $45.34/share Price for the 226,906-share sale on 2026-04-16

Lowest sale price $44.51/share Price for the 47,814-share sale on 2026-04-14

Key Terms

Rule 10b5-1 plan, open-market sale, beneficial ownership, 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.

open-market sale financial

"transaction_action: open-market sale of Common Stock"

An open-market sale is when a shareholder sells existing shares directly on a public exchange to any willing buyer, rather than through a private deal. Think of it like putting goods on a busy market stall where price is set by supply and demand; for investors it matters because such sales increase available supply, can put short-term downward pressure on the stock price, and signal changes in liquidity or investor confidence.

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.

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.

pecuniary interest financial

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

Form 4 Insider Transactions: What Each Field Means →

04/16/2026 - 07:15 PM

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).

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)
04/14/2026

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)
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	Code	V	Amount	(A) or (D)	Price		6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
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)
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)	Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	8. Price of Derivative Security (Instr. 5)		10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)

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	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)

Source: View Original Filing on SEC EDGAR

FAQ

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

Sionna Therapeutics disclosed that an OrbiMed-affiliated fund associated with director Peter A. Thompson sold 510,583 common shares in three open-market transactions. The sales occurred at prices around the mid-$40s per share and reduced, but did not eliminate, the fund’s indirect position.

At what prices were the Sionna Therapeutics (SION) shares sold in the latest insider trades?

The reported insider trades involved sales of Sionna Therapeutics common stock at prices between $44.51 and $45.34 per share. These were open-market transactions executed over three days and collectively totaled 510,583 shares sold by an OrbiMed-managed investment fund.

How many Sionna Therapeutics (SION) shares remain indirectly held after the reported sales?

After the reported transactions, 3,020,119 Sionna Therapeutics common shares remain indirectly held through OrbiMed Private Investments VIII, LP. This figure reflects the fund’s continuing position following the 510,583-share net reduction disclosed in the Form 4 insider trading report.

Were the recent Sionna Therapeutics (SION) insider sales made under a Rule 10b5-1 plan?

Yes. The footnotes state that the Sionna Therapeutics common shares were sold pursuant to a Rule 10b5-1 trading plan. Such pre-arranged plans are established in advance, which generally makes the timing of these sales less indicative of short-term views on the stock.

Who is the selling holder in the Sionna Therapeutics (SION) Form 4 transactions?

The securities are held of record by OrbiMed Private Investments VIII, LP, an investment fund managed through OrbiMed entities. Director Peter A. Thompson is a member of OrbiMed Advisors, and the filing attributes the reported Sionna Therapeutics share sales to this affiliated fund.