Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 124

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 124
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 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 Plan became effective;                                                  |

| • |     | 390,127 shares of common stock reserved for future issuance under our 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 will be 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, 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.                                                         |

90

DILUTION If you invest in our common stock in this offering, your ownership interest will be diluted to the extent of the difference between the initial public offering price per share of our common stock and the pro forma as adjusted net tangible book value per share of our common stock immediately after this offering. Our historical net tangible book deficit as of September 30, 2024 was $(150.5) million, or $(31.46) per share of our common stock. Our historical net tangible book deficit represents the amount of our total tangible assets less our total liabilities and preferred stock. Historical net tangible book deficit per share represents historical net tangible book deficit divided by the number of shares of our common stock outstanding as of September 30,