Company: SION
Filing Date: 2025-02-03
Form Type: S-1/A
Source: 0001193125-25-018825
Chunk: 125

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-03
Form: S-1/A
Chunk 125
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 assumed initial public offering price per share, the midpoint of the estimated offering price range listed on the cover page of this prospectus in this offering, would increase or decrease, as applicable, the pro forma as adjusted amount 
 of each of cash, cash equivalents and investments in marketable securities, additional paid-in capital, total stockholders’ equity and total capitalization by $15.8 million, after deducting estimated                                                 
 underwriting discounts and commissions and estimated offering expenses payable by us.                                                                                                                                                                   |

The number of shares of common stock that will be outstanding after this offering on a pro forma and pro forma as adjusted basis is based on 31,932,467 shares of common stock (which includes 119,639 shares of restricted common stock) outstanding as of September 30, 2024 after giving effect to the automatic conversion of all outstanding shares of our preferred stock into the aggregate of 27,149,206 shares of 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 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 will 
 cease to be available for issuance at the time that our 2025 Plan becomes effective;                                   |

| • |     | 390,127 shares of common stock reserved for future issuance under our ESPP, which will become 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 will become                                                                                                                        
 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,