Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 123

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 123
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, par value $0.001 per share; 55,200,000 shares authorized, 4,783,261 shares issued and                                                                                                                                               
 4,663,622 shares outstanding, actual; 55,200,000 shares authorized, 31,932,467 shares issued and 31,812,828 shares outstanding, pro forma; 55,200,000 shares authorized, 42,520,700 shares issued and 42,401,061 shares outstanding, pro forma as 
 adjusted                                                                                                                                                                                                                                          |     |                          |        5 |   |     |           |       32 |   |     |             |       43 |   |
| Additional paid-in capital                                                                                                                                                                                                                        |     |                          |   15,911 |   |     |           |  346,252 |   |     |             |  519,288 |   |
| Accumulated other comprehensive loss                                                                                                                                                                                                              |     |                          |      773 |   |     |           |      773 |   |     |             |      773 |   |
| Accumulated deficit                                                                                                                                                                                                                               |     |                          | (165,238 | ) |     |           | (165,238 | ) |     |             | (165,238 | ) |
| Total stockholders’ (deficit) equity                                                                                                                                                                                                              |     |                          | (148,549 | ) |     |           |  181,819 |   |     |             |  354,866 |   |
| Total capitalization                                                                                                                                                                                                                              |     | $                        |  181,819 |   |     | $         |  181,819 |   |     | $           |  354,866 |   |

89

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