OrbiMed fund tied to Sionna (NASDAQ: SION) director trims 55K shares

Rhea-AI Filing Summary

Sionna Therapeutics director-affiliated entity sells SION shares under 10b5-1 plan

Entities associated with director Peter A. Thompson reported open-market sales of a total of 55,345 shares of Sionna Therapeutics common stock at prices around $44.5 per share over two days. The transactions were executed pursuant to a Rule 10b5-1 trading plan, indicating they were pre-arranged.

The shares are held of record by OrbiMed Private Investments VIII, LP, with OrbiMed Capital GP VIII LLC and OrbiMed Advisors LLC potentially deemed to share voting and investment power. After these sales, OrbiMed’s reported indirect holdings stand at 2,964,774 shares of Sionna Therapeutics common stock, and all reporting parties expressly disclaim beneficial ownership beyond any pecuniary interest.

Insights

Equity market structure analyst

Pre-planned fund-level sales trim a small portion of an existing position.

The filing shows entities affiliated with OrbiMed, a healthcare investment firm, sold 55,345 Sionna Therapeutics shares via open-market trades around $44.5 per share. These are indirect holdings associated with director Peter A. Thompson, not his personal direct stake.

The sales were made under a Rule 10b5-1 trading plan, meaning the trades were scheduled in advance rather than timed opportunistically. After the transactions, the entities still hold 2,964,774 shares, so the sales represent a relatively small slice of the disclosed position.

Because the activity is both pre-planned and modest relative to remaining holdings, the informational signal is limited. Subsequent company filings may provide further context on fund-level portfolio management but, based solely on these figures, this appears to be routine position trimming.

Insider Trade Summary 10b5-1

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

Net Sell 2 txns

Insider Thompson Peter A.

Role 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. 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

Shares sold 2026-04-18: 2,948 shares at $44.52 Shares sold 2026-04-17: 52,397 shares at $44.68 Total shares sold: 55,345 shares +2 more

5 metrics

Shares sold 2026-04-18 2,948 shares at $44.52 Open-market sale of common stock

Shares sold 2026-04-17 52,397 shares at $44.68 Open-market sale of common stock

Total shares sold 55,345 shares Net shares sold across reported transactions

Indirect holdings after sales 2,964,774 shares Shares of Sionna Therapeutics common stock held indirectly

Transaction direction Net-sell Two open-market sale transactions, no purchases

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.

voting and investment power financial

"may be deemed to have voting and investment power with respect to the shares"

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^*

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)
04/17/2026

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)
			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

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	04/21/2026
	** 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 transactions in Sionna Therapeutics (SION) does this Form 4 report?

The Form 4 reports two open-market sales totaling 55,345 shares of Sionna Therapeutics common stock at about $44.5 per share. These transactions were executed over two days and are associated with entities linked to director Peter A. Thompson.

Who actually sold the Sionna Therapeutics (SION) shares reported in this Form 4?

The shares are held of record by OrbiMed Private Investments VIII, LP, with OrbiMed Capital GP VIII LLC and OrbiMed Advisors LLC potentially sharing voting and investment power. Director Peter A. Thompson is a member of OrbiMed Advisors, but all parties disclaim full beneficial ownership.

How many Sionna Therapeutics (SION) shares remain held after these insider sales?

After the reported sales, entities associated with OrbiMed continue to hold 2,964,774 shares of Sionna Therapeutics common stock indirectly. This indicates that the 55,345 shares sold represent only a small portion of their disclosed overall position in the company.

Were the Sionna Therapeutics (SION) insider sales discretionary or pre-planned?

The filing states the securities were sold under a Rule 10b5-1 trading plan. Such plans pre-schedule trades in advance, so the timing of these 55,345 shares’ sales is considered routine portfolio management rather than a discretionary market-timing decision.

Does Peter A. Thompson claim full beneficial ownership of the SION shares in this Form 4?

No. The filing explains that Peter A. Thompson, OrbiMed Advisors, and OrbiMed Capital GP VIII LLC each disclaim beneficial ownership of the reported securities, except to the extent of any pecuniary interest, and clarify this is not an admission of beneficial ownership under Section 16.