Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 127

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 127
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 investors purchasing shares in this offering |     |                                                          | 10,588,233 |     |            |  24.9 | % |     | $                   | 190,588 |     |            |  36.6 | % |     | $         | 18.00 |
| Total                                            |     |                                                          | 42,520,700 |     |            | 100.0 | % |     | $                   | 521,125 |     |            | 100.0 | % |     |           |       |

The table above assumes no exercise of the underwriters’ option to purchase additional shares in this offering. If the underwriters’ option to purchase additional shares of our common stock is exercised in full, the number of shares of our common stock held by existing stockholders would be reduced to 72.4% of the total number of shares of our common stock outstanding after this offering, and the number of shares of common stock held by new investors purchasing common stock in this offering would be increased to 27.6% of the total number of shares of our common stock outstanding after this offering. The foregoing tables and calculations (other than the historical net tangible book value calculation) are 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 outstanding shares of our preferred stock into 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 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,