Company: SION
Filing Date: 2025-04-29
Form Type: DEF 14A
Source: 0001193125-25-101830
Chunk: 3

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-04-29
Form: DEF 14A
Chunk 3
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402(v) of Regulation S-K.We will remain an “emerging growth company” until the earliest of (i) the last day of the fiscal year following the fifth anniversary of our initial public offering in February 2025; (ii) the last day of 1

the fiscal year in which our total annual gross revenue is equal to or more than $1.235 billion; (iii) the date on which we have issued more than $1 billion in nonconvertible debt during the previous three years; or (iv) the date on which we are deemed to be a large accelerated filer under the rules of the Securities and Exchange Commission (the “SEC”). Even after we are no longer an “emerging growth company,” we may remain a “smaller reporting company.”

<div align='center'>**Important Notice Regarding the Availability of Proxy Materials for

the Annual Meeting of Stockholders to be Held on June 12, 2025:

This proxy statement and our 2024 Annual Report to Stockholders are

available for viewing, printing and downloading athttps://materials.proxyvote.com/829401.**</div>

A copy of our 2024 Annual Report, as filed with the SEC, except for exhibits, will be furnished without charge to any stockholder upon written request to Sionna Therapeutics, Inc., 21 Hickory Drive, Suite 500, Waltham, MA 02451, Attention: Chief Legal Officer. This proxy statement and our 2024 Annual Report are also available on the SEC’s website atwww.sec.gov.

<div align='center'>**2

SIONNA THERAPEUTICS, INC.

PROXY STATEMENT

FOR THE 2025 ANNUAL MEETING OF STOCKHOLDERS

GENERAL INFORMATION**</div>

**When are this proxy statement and the accompanying materials scheduled to be sent to stockholders?

We have elected to provide access to our proxy materials to our stockholders via the Internet. Accordingly, on or about April 29, 2025, we will begin mailing a Notice of Internet Availability of Proxy Materials (the “Notice”). Our proxy materials, including the Notice of the 2025 Annual Meeting of Stockholders, this proxy statement and the accompanying proxy card or, for shares held in street name (i.e., held for your account by a broker or other nominee), a voting instruction form, and the 2024 Annual Report, will be mailed or made available to stockholders on the Internet on or about the same date.