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  4. [Form 4] Sionna Therapeutics, Inc. Insider Trading Activity

OrbiMed fund tied to Sionna (SION) director sells 292,952 shares under plan

Filing Impact
(Very High)
Filing Sentiment
(Very Negative)
Form Type
4

Rhea-AI Filing Summary

Sionna Therapeutics, Inc. director reporting person Peter A. Thompson reported indirect open-market sales of company common stock by an affiliated OrbiMed investment entity. Over three transactions on May 7, 8, and 11, 2026, the entity sold a total of 292,952 shares at prices between $44.58 and $46.22 per share under a pre-arranged Rule 10b5-1 trading plan. The shares are held of record by OrbiMed Private Investments VIII, LP, for which OrbiMed Capital GP VIII LLC and OrbiMed Advisors LLC may be deemed to share voting and investment power. Following these sales, the OrbiMed entity associated with the reporting person continues to hold 2,671,822 shares of Sionna Therapeutics common stock indirectly, with beneficial ownership disclaimed except to the extent of any pecuniary interest.

Positive

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Negative

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Insider Thompson Peter A.
Role null
Sold 292,952 shs ($13.36M)
Type Security Shares Price Value
Sale Common Stock 184,639 $46.22 $8.53M
Sale Common Stock 59,767 $44.58 $2.66M
Sale Common Stock 48,546 $44.61 $2.17M
Holdings After Transaction: Common Stock — 2,671,822 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.
Total shares sold 292,952 shares Aggregate open-market sales reported in this Form 4
Shares sold May 11, 2026 184,639 shares Common stock at $46.22 per share, indirect ownership
Shares sold May 8, 2026 59,767 shares Common stock at $44.58 per share, indirect ownership
Shares sold May 7, 2026 48,546 shares Common stock at $44.61 per share, indirect ownership
Shares held after transactions 2,671,822 shares Indirect holdings by OrbiMed entity following May 11, 2026 trade
Highest reported sale price $46.22 per share Common stock sale on May 11, 2026
Lowest reported sale price $44.58 per share Common stock sale on May 8, 2026
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.
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.
voting and investment power financial
"may be deemed to have voting and investment power with respect to the shares"
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.
Form 4 Insider Transactions: What Each Field Means →
SEC Form 4
FORM 4UNITED 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)
XDirector10% Owner
Officer (give title below)Other (specify below)
2a. Foreign Trading Symbol
3. Date of Earliest Transaction (Month/Day/Year)
05/07/2026		6. Individual or Joint/Group Filing (Check Applicable Line)
XForm 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)
CodeVAmount(A) or (D)Price
Common Stock05/07/2026S48,546(1)D$44.612,916,228ISee footnotes((2)(3)
Common Stock05/08/2026S59,767(1)D$44.582,856,461ISee footnotes((2)(3)
Common Stock05/11/2026S184,639(1)D$46.222,671,822ISee 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)
CodeV(A)(D)Date ExercisableExpiration DateTitleAmount 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. Thompson05/11/2026
** Signature of Reporting PersonDate
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 activity did Sionna Therapeutics (SION) report in this Form 4?

Sionna Therapeutics reported indirect insider stock sales linked to director Peter A. Thompson. An affiliated OrbiMed investment fund sold common shares in three open-market transactions, all executed under a pre-arranged Rule 10b5-1 trading plan, rather than discretionary same-day trading decisions.

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

The affiliated OrbiMed entity sold 292,952 SION common shares across three trades. Reported average prices were $44.61 on May 7, $44.58 on May 8, and $46.22 on May 11, 2026, reflecting routine open-market sale transactions disclosed in the Form 4.

Who actually holds the Sionna Therapeutics (SION) shares involved in these transactions?

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 manages it. The reporting person is a member of OrbiMed Advisors, and all parties disclaim beneficial ownership except for any pecuniary interest.

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

Yes. The Form 4 footnotes state that these securities were sold pursuant to a Rule 10b5-1 plan. Such plans are pre-arranged trading programs, which can indicate that the timing of the trades was scheduled in advance rather than based on short-term market developments.

How many Sionna Therapeutics (SION) shares does the OrbiMed entity hold after these sales?

After the reported transactions, the affiliated OrbiMed fund holds 2,671,822 SION common shares indirectly associated with the reporting person. The Form 4 clarifies that OrbiMed entities and the reporting person disclaim beneficial ownership except to the extent of any pecuniary interest in those securities.

What is the nature of Peter A. Thompson’s interest in Sionna Therapeutics (SION) shares?

Peter A. Thompson is a director and member of OrbiMed Advisors. The filing explains that OrbiMed entities may be deemed to share voting and investment power, but each of them, including the reporting person, disclaims beneficial ownership of the shares except for any pecuniary interest under applicable Exchange Act rules.