Company: SION
Filing Date: 2025-02-03
Form Type: S-1/A
Source: 0001193125-25-018825
Chunk: 370

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-03
Form: S-1/A
Chunk 370
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 required by the underwriter to permit prompt delivery to each purchaser.

(b) For purposes of
determining any liability under the Securities Act, as amended, the information omitted from a form of prospectus filed as part of this registration statement in reliance upon Rule 430A and contained in the form of prospectus filed by the Registrant
pursuant to Rule 424(b)(1) or (4) or 497(h) under the Securities Act, as amended, shall be deemed to be part of this registration statement as of the time it was declared effective.

(c) For the purpose of determining any liability under the Securities Act, as amended, each post-effective amendment that contains a form of prospectus
shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

II-5

SIGNATURES Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant has duly caused this Registration Statement on Form S-1to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Waltham, Massachusetts, on the 3 rdof February, 2025.

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

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement and Power of Attorney has been signed by the following person in the capacities and on the date indicated.

| Signature                        
 /s/ Michael Cloonan Michael      
 Cloonan, M.B.A.                  
 /s/ Elena Ridloff Elena Ridloff, 
 C.F.A.                           |     | Title                                                                                                                    
 President, Chief Executive Officer and Director (Principal Executive Officer)                                            
 Chief Financial Officer and Head of Corporate Development (Principal Financial Officer and Principal Accounting Officer) |     | Date             
 February 3, 2025 
 February 3, 2025 |
|:---------------------------------|:----|:-------------------------------------------------------------------------------------------------------------------------|:----|:-----------------|
| *                                |     | Chair of the Board of Directors                                                                                          |     | February