Company: SION
Filing Date: 2025-01-17
Form Type: S-1
Source: 0001193125-25-008474
Chunk: 274

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-01-17
Form: S-1
Chunk 274
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 have been or are to be a participant; |

| • |     | the amounts involved exceeded or will exceed the lesser of $120,000 or 1% of the average of our total assets at the year-end for the last two completed fiscal years; and |

| • |     | any of our directors, executive officers or holders of more than 5% or more of our outstanding capital stock, or any                                                                    
 immediate family member of, or person sharing the household with, any of these individuals or entities or affiliated entities, had or will have a direct or indirect material interest. |

Financings Series B Preferred Stock Financing On February 2, 2022, we issued and sold an aggregate of 11,370,621 shares of our Series B convertible preferred stock, par value $0.001 per share (“Series B preferred stock”), at a purchase price of $9.762 per share, for an aggregate purchase price of approximately $111.0 million. Each outstanding share of Series B preferred stock will convert into one share of common stock immediately prior to the completion of this offering. The following table summarizes the shares of our Series B preferred stock issued to our related parties:

| Purchasers(1)                             |     | Shares of Series B 
 Preferred Stock    |           |     | Total Purchase Price |            |
|:------------------------------------------|:----|:-------------------|----------:|:----|:---------------------|-----------:|
| Entities affiliated with Atlas Venture(2) |     |                    | 1,298,332 |     | $                    | 12,674,317 |
| OrbiMed Private Investments VIII, LP(3)   |     |                    | 2,560,951 |     | $                    | 25,000,004 |
| Entities affiliated with RA Capital(4)    |     |                    | 3,079,242 |     | $                    | 30,059,560 |
| Entities affiliated with TPG Growth(5)    |     |                    | 2,275,528 |     | $                    | 22,213,704 |

| (1) | Additional details regarding these stockholders and their equity holdings are included in this prospectus under the 
 section “Principal Stockholders.”                                                                                   |

| (2) | Consists of 1,298,332 shares of Series B preferred stock held by Atlas Venture Fund XI, L.P. Atlas Venture Fund XI, L.P.                                                                                                                   
 is an affiliate of