Company: SION
Filing Date: 2025-02-03
Form Type: S-1/A
Source: 0001193125-25-018825
Chunk: 300

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-03
Form: S-1/A
Chunk 300
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 then outstanding, which will equal approximately 407,560 shares immediately          
 after this offering, assuming no exercise of the underwriters’ option to purchase additional shares of common stock from us; or |

| • |     | the average weekly trading volume of our common stock on Nasdaq during the four calendar weeks preceding the filing of a 
 notice on Form 144 with respect to such sale.                                                                            |

Sales under Rule 144 by our affiliates or persons selling shares on behalf of our affiliates are also subject to certain manner of sale provisions and notice requirements and to the availability of current public information about us. 213

Rule 701

Rule 701 under the Securities Act (“Rule 701”) generally allows a stockholder who was issued shares under a written compensatory plan or
contract and who is not deemed to have been an affiliate of our company during the immediately preceding 90 days, to sell these shares in reliance on Rule 144, but without being required to comply with the public information, holding period,
volume limitation or notice provisions of Rule 144. Rule 701 also permits affiliates of our company to sell their Rule 701 shares under Rule 144 without complying with the holding period requirements of Rule 144. All holders of Rule 701 shares,
however, are required by that rule to wait until 90 days after the date of this prospectus before selling those shares under Rule 701, subject to the expiration of the lock-up agreements described below.

Form S-8Registration Statements

We intend to file one or more registration statements on Form S-8 under the Securities Act with the SEC to
register the offer and sale of shares of our common stock that are issuable under the 2020 Plan, the 2025 Plan and the ESPP. These registration statements will become effective immediately upon filing. Shares covered by these registration statements
will then be eligible for sale in the public markets, subject to vesting restrictions, any applicable lock-up agreements described below, and Rule 144 limitations applicable to affiliates.

Lock-UpArrangements

We, all of our directors and executive officers, and the holders of substantially all of our capital stock and securities convertible into or
exchangeable for our capital stock have entered into lock-up agreements with the underwriters and/or are subject to market standoff agreements or other agreements with us, which prevents them from transferring
any of our common stock or any securities convertible into or exercisable or exchangeable for common stock for a period of not less than 180 days from the date of this prospectus without