SION Form 144: Proposed 9,753-Share Sale via Merrill Lynch

Rhea-AI Filing Summary

Sionna Therapeutics, Inc. (SION) filed a Form 144 notice reporting a proposed sale of 9,753 common shares valued at $292,687.53 to be sold through Merrill Lynch on 10/06/2025 on NASDAQ. The shares were acquired in a private placement on 03/04/2024 and paid for in cash.

The filing also lists recent open-market sales by related funds between 09/29/2025 and 10/03/2025, totaling approximately $3,499,940.95 across several transactions. The filer affirms no undisclosed material adverse information and includes the standard certification language regarding Rule 10b5-1 trading plans.

Positive

• Transaction transparency: Proposed sale disclosed under Rule 144 with broker and timing specified
• Prior sales disclosed: Recent related-party sales totaling $3,499,940.95 are itemized

Negative

• No Rule 10b5-1 adoption date provided, leaving uncertainty about reliance on a trading plan
• Concentrated recent selling by affiliated funds (multiple sales totaling $3.50M) could exert near-term share pressure

Insights

Securities Transactions Analyst

TL;DR: Insider-related sale notice for 9,753 shares ahead of immediate market transactions.

The notice states a proposed sale of 9,753 common shares with an aggregate market value of $292,687.53, to be executed via Merrill Lynch on 10/06/2025. The shares originate from a private placement on 03/04/2024, paid in cash, which means these are not newly issued in this filing.

Related funds reported multiple sales from 09/29/2025 to 10/03/2025 totaling roughly $3.50M, indicating prior liquidation activity by affiliated holders. Watch near-term trading volume and price impact around 10/06/2025 for potential short-term supply effects.

Corporate Securities Counsel

TL;DR: Filing follows Rule 144 formality with standard representations and potential Rule 10b5-1 reference.

The signer represents no knowledge of undisclosed material adverse information and references Rule 10b5-1 plan adoption language. The presence of that language indicates attention to insider trading safe-harbors; however, no plan adoption date is provided in the visible fields.

For compliance, confirm whether a written trading plan exists and its adoption date if relying on Rule 10b5-1; absent that, the representation is effective only as of the notice signature and could matter for enforcement timing.

Form 144 Filer Information	UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Form 144 NOTICE OF PROPOSED SALE OF SECURITIES PURSUANT TO RULE 144 UNDER THE SECURITIES ACT OF 1933
FORM 144

144: Filer Information

Filer CIK
Filer CCC
Is this a LIVE or TEST Filing?	LIVE TEST
Submission Contact Information
Name
Phone
E-Mail Address

144: Issuer Information

Name of Issuer

SEC File Number

Address of Issuer

Phone

Name of Person for Whose Account the Securities are To Be Sold

See the definition of "person" in paragraph (a) of Rule 144. Information is to be given not only as to the person for whose account the securities are to be sold but also as to all other persons included in that definition. In addition, information shall be given as to sales by all persons whose sales are required by paragraph (e) of Rule 144 to be aggregated with sales for the account of the person filing this notice.

Relationship to Issuer

144: Securities Information

Title of the Class of Securities To Be Sold	Name and Address of the Broker	Number of Shares or Other Units To Be Sold	Aggregate Market Value	Number of Shares or Other Units Outstanding	Approximate Date of Sale	Name the Securities Exchange
Common	Merrill Lynch 555 California Street 18th Floor San Francisco CA 94104	9753	292687.53	44139823	10/06/2025	NASDAQ

Furnish the following information with respect to the acquisition of the securities to be sold and with respect to the payment of all or any part of the purchase price or other consideration therefor:

144: Securities To Be Sold

Title of the Class	Date you Acquired	Nature of Acquisition Transaction	Name of Person from Whom Acquired	Is this a Gift?	Date Donor Acquired	Amount of Securities Acquired	Date of Payment	Nature of Payment *
Common	03/04/2024	Private Placement	Issuer			9753	03/04/2024	Cash

* If the securities were purchased and full payment therefor was not made in cash at the time of purchase, explain in the table or in a note thereto the nature of the consideration given. If the consideration consisted of any note or other obligation, or if payment was made in installments describe the arrangement and state when the note or other obligation was discharged in full or the last installment paid.

Furnish the following information as to all securities of the issuer sold during the past 3 months by the person for whose account the securities are to be sold.

144: Securities Sold During The Past 3 Months

Name and Address of Seller	Title of Securities Sold	Date of Sale	Amount of Securities Sold	Gross Proceeds
Atlas Venture Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/29/2025	54637	1644065.58
Atlas Venture Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/30/2025	10239	307762.84
Atlas Venture Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/01/2025	165	4941.75
Atlas Venture Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/03/2025	31198	935996.16
Atlas Venture Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/29/2025	11468	345080.15
Atlas Venture Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	09/30/2025	2149	64594.43
Atlas Venture Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/01/2025	35	1048.25
Atlas Venture Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/03/2025	6548	196451.79

144: Remarks and Signature

Remarks

Date of Notice

Date of Plan Adoption or Giving of Instruction, If Relying on Rule 10b5-1

ATTENTION:

The person for whose account the securities to which this notice relates are to be sold hereby represents by signing this notice that he does not know any material adverse information in regard to the current and prospective operations of the Issuer of the securities to be sold which has not been publicly disclosed. If such person has adopted a written trading plan or given trading instructions to satisfy Rule 10b5-1 under the Exchange Act, by signing the form and indicating the date that the plan was adopted or the instruction given, that person makes such representation as of the plan adoption or instruction date.

Signature

ATTENTION: Intentional misstatements or omission of facts constitute Federal Criminal Violations (See 18 U.S.C. 1001)

FAQ

What does the Form 144 filed by Sionna Therapeutics (SION) report?

It reports a proposed sale of 9,753 common shares valued at $292,687.53, to be sold through Merrill Lynch on 10/06/2025.

When were the shares to be sold originally acquired?

The shares were acquired in a private placement on 03/04/2024 and paid for in cash on that date.

Have related parties recently sold Sionna shares?

Yes. Related funds reported sales between 09/29/2025 and 10/03/2025 totaling approximately $3,499,940.95 in gross proceeds.

Does the filing state reliance on a Rule 10b5-1 trading plan?

The form includes the Rule 10b5-1 language but does not list a plan adoption date in the visible fields, so reliance is not confirmed.

On which exchange will the proposed sale occur?

The proposed sale is listed for NASDAQ with Merrill Lynch as the broker.