Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 338

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 338
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 of 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 gross proceeds of $111.0 million (the “Series B Financing”). The Company incurred $0.2 million of issuance costs in connection with the
Series B Financing.

Upon the issuance of Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock (together with the
Series C Preferred Stock, the “Preferred Stock”), the Company assessed the embedded conversion and liquidation features of the shares and determined that the Company was not required to separately account for these features.

F-21

Preferred Stock consists of the following (in thousands, except share amounts):

|                             |     | December 31, 2023 |            |     |                 |            |     |                |         |     |             |         |     |                |            |
|:----------------------------|:----|:------------------|-----------:|:----|:----------------|-----------:|:----|:---------------|--------:|:----|:------------|--------:|:----|:---------------|-----------:|
|                             |     | Preferred         
 Stock             
 Authorized        |            |     | Preferred Stock 
 Issued and      
 Outstanding     |            |     | Carrying Value |         |     | Liquidation 
 Value       |         |     | Stock Issuable 
 Upon           
 Conversion     |            |
| Series Seed Preferred Stock |     |                   |  3,963,963 |     |                 |  3,963,963 |     | $              |  13,014 |     | $           |  13,200 |     |                |  2,712,998 |
| Series A Preferred Stock    |     |                   |  5,704,161 |     |                 |  5,704,161 |     |                |  25,235 |     |             |  25,263 |     |                |  3,904,017 |
| Series B Preferred Stock    |     |                   | 11,370,621 |     |                 | 11,370,621 |     |                | 110,791 |     |             | 111,000 |     |                |  7,782,230 |
| Total                       |     |                   | 21,038,745 |     |                 | 21,038,745 |