Company: SION
Filing Date: 2025-02-04
Form Type: 8-A12B
Source: 0001193125-25-020023
Chunk: 1

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-04
Form: 8-A12B
Chunk 1
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 and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), which information is hereby incorporated by reference. The description of the Common Stock included in any form of prospectus subsequently filed by the Registrant with the Commission pursuant to Rule 424(b) under the Securities Act shall be deemed to be incorporated by reference herein.

| Item 2. | Exhibits. |

In accordance with the “Instructions as to Exhibits” with respect to Form 8-A,no exhibits are required to be filed as part of this registration statement because no other securities of the Registrant are registered on The Nasdaq Stock Market LLC and the securities registered hereby are not being registered pursuant to Section 12(g) of the Securities Exchange Act of 1934, as amended.

SIGNATURE Pursuant to the requirements of Section 12 of the Securities Exchange Act of 1934, the registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereto duly authorized.

| Date: February 4, 2025 |     | SIONNA THERAPEUTICS, INC. 
 By:                       |     | /s/ Michael Cloonan                          |
|:-----------------------|:----|:--------------------------|:----|:---------------------------------------------|
|                        |     |                           |     | Name: Michael Cloonan, M.B.A.                |
|                        |     |                           |     | Title: President and Chief Executive Officer |