SION Form 144: Brokered sale of 16,727 shares on NASDAQ

Rhea-AI Filing Summary

Sionna Therapeutics filed a Form 144 notifying a proposed sale of 16,727 shares of common stock through Merrill Lynch on 10/10/2025 with an aggregate market value of $474,210.45. The shares were acquired in a private placement on 03/04/2024 and were paid for in cash. The filing also lists multiple open-market sales by affiliated funds in the prior three months, including several trades between 09/29/2025 and 10/06/2025 totaling tens of thousands of shares and gross proceeds in the low millions.

The filer certifies they know of no material nonpublic information and notes the statutory warning about false statements. The notice identifies Merrill Lynch as the broker and NASDAQ as the exchange for the planned sale.

Positive

• Securities were acquired via a documented private placement on 03/04/2024
• Sale will be executed through a major broker (Merrill Lynch) on NASDAQ, indicating standard market process
• Filer attests to no known material nonpublic information, per the Form 144 certification

Negative

• Related affiliated funds conducted multiple open-market sales between 09/29/2025 and 10/06/2025, totaling tens of thousands of shares and gross proceeds in the low millions
• Outstanding share count listed at 44,139,823 provides context that continued sales could add supply pressure if sustained

Insights

Securities Compliance Analyst

TL;DR: Planned sale is routine Rule 144 notice tied to a prior private placement and recent open-market dispositions.

The filing shows 16,727 shares to be sold under Rule 144 by a holder who bought those shares in a 03/04/2024 private placement and paid in cash. The use of a major broker (Merrill Lynch) and listing on NASDAQ are consistent with a standard registered resale process.

Recent sales by related funds from 09/29/2025 to 10/06/2025 generated gross proceeds in the low millions, which may indicate ongoing secondary liquidity but do not by themselves signal company operational changes. Monitor settlement dates and any additional Form 4/Form 144 filings within days for further disposition details.

Equity Capital Markets Specialist

TL;DR: The size and market value suggest a modest block sale unlikely to move market price materially.

The indicated block (16,727 shares; $474,210.45) is small relative to the reported outstanding shares figure (44,139,823) noted in the filing, implying limited dilution or immediate market impact from this single transaction. The brokered execution on NASDAQ suggests standard marketable-lot placement.

Investors may watch for clustering of similar notices or significant insider sales; absent that, this appears to be routine liquidity from pre-existing private placement holdings.

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	16727	474210.45	44139823	10/10/2025	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			16727	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 Fund XI, LP 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/06/2025	9753	292687.53
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
Atlas Venture Opportunity Fund II, L.P. 400 Technology Sq. 10th Floor Cambridge MA 02139	Common	10/06/2025	2047	61430.47

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 SION's Form 144 filed for 10/10/2025 disclose?

The Form 144 notifies a proposed sale of 16,727 common shares on 10/10/2025 through Merrill Lynch on NASDAQ, with aggregate market value $474,210.45.

When and how were the shares in SION being sold acquired?

The shares were acquired in a private placement on 03/04/2024 and payment was made in cash on the same date.

Have related parties recently sold SION shares?

Yes. Affiliated funds sold shares between 09/29/2025 and 10/06/2025, totaling multiple transactions with gross proceeds in the low millions.

Which broker and exchange are being used for the planned SION sale?

The filing lists Merrill Lynch as the broker and NASDAQ as the exchange for the proposed sale.

Does the filer assert possession of material nonpublic information in the Form 144?

No. By signing the notice the person represents they do not know any material adverse information that has not been publicly disclosed.