Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 17

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 17
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 Series A preferred stock and Series B preferred stock, the “preferred stock”) into an aggregate of 27,149,206 shares of our common stock immediately prior to the completion of this offering, and excludes:

| • |     | 3,652,051 shares of common stock issuable upon exercise of outstanding stock options as of September 30, 2024 under        
 our 2020 Stock Option and Grant Plan, as amended (“2020 Plan”), with a weighted average exercise price of $5.77 per share; |

| • |     | 63,745 shares of common stock issuable upon exercise of outstanding stock options granted after September 30, 2024 
 pursuant to our 2020 Plan, with a weighted average exercise price of $10.33 per share;                             |

| • |     | 1,099,636 shares of common stock reserved for future issuance as of September 30, 2024 under the 2020 Plan, which             
 ceased to be available for issuance at the time that our 2025 Stock Option and Incentive Plan (“2025 Plan”) became effective; |

| • |     | 390,127 shares of common stock reserved for future issuance under our 2025 Employee Stock Purchase Plan (“ESPP”),                                                                                                                                      
 which became effective on the date immediately prior to the effectiveness of the registration statement of which this prospectus forms a part, as well as any automatic increases in the number of shares of common stock reserved for future issuance 
 under the ESPP; and                                                                                                                                                                                                                                    |

| • |     | 5,060,000 shares of our common stock that will become available for future issuance under our 2025 Plan, which became                                                                                                                                     
 effective on the date immediately prior to the effectiveness of the registration statement of which this prospectus forms a part, which includes an aggregate of 1,946,388 shares of our common stock, which were granted to certain of our executive     
 officers, directors and employees at the time of effectiveness of the 2025 Plan with an exercise price equal to $18.00 per share, the initial public offering price per share, as well as any automatic increases in the number of shares of common stock 
 reserved for future issuance under the 2025 Plan and any shares underlying outstanding stock awards granted under the 2020 Plan that expire or are repurchased, forfeited, cancelled or withheld.                                                         |

Unless otherwise indicated, the information in this prospectus reflects or assumes the following:

| • |     | the automatic