Company: SION
Filing Date: 2025-01-17
Form Type: S-1
Source: 0001193125-25-008474
Chunk: 364

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-01-17
Form: S-1
Chunk 364
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 court of appropriate jurisdiction the question whether such indemnification by it is against
public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

The registrant hereby undertakes
that:

(a) The Registrant will provide to the underwriter at the closing as specified in the underwriting agreement, certificates in such
denominations and registered in such names as 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 17 thof January, 2025.

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

POWER OF ATTORNEY AND SIGNATURES Each individual whose signature appears below hereby constitutes and appoints Michael Cloonan, M.B.A., and Elena Ridloff, C.F.A., as such person’s true and lawful attorney-in-factand agent with full power of substitution and resubstitution, for such person in such person’s name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this