Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 126

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 126
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| Pro forma as adjusted net tangible book value per share after this offering                              |     |     |        |   |     |   |  8.35 |
| Dilution per share to new investors in this offering                                                     |     |     |        |   |     | $ |  9.65 |

If the underwriters exercise their option to purchase up to 1,588,234 additional shares of our common stock in full, the pro forma as adjusted net tangible book value after the offering would be $8.65 per share, the increase in pro forma as adjusted net tangible book value per share to existing stockholders would be $0.30 per share and the dilution per share to new investors would be $0.30 per share, in each case based on the initial public offering price of $18.00 per share. The following table summarizes on the pro forma as adjusted basis described above, as of September 30, 2024, the differences between the number of shares of common stock purchased from 91

us by our existing stockholders and common stock by new investors purchasing shares in this offering, the total consideration paid to us in cash and the average price per share paid by existing stockholders for shares of common stock issued prior to this offering and the price to be paid by new investors for shares of common stock in this offering. The calculation below is based on the initial public offering price of $18.00 per share, before deducting underwriting discounts, placement agent fees and commissions and estimated offering expenses payable by us.

|                                                  |     | Shares Purchased                                         
 Number                                                   
 (in thousands, except share, per share and percent data) |            |     | Percentage |       |   |     | Total Consideration 
 Amount              |         |     | Percentage |       |   |     | Weighted- 
 Average   
 Price Per 
 Share     |       |
|:-------------------------------------------------|:----|:---------------------------------------------------------|-----------:|:----|:-----------|------:|:--|:----|:--------------------|--------:|:----|:-----------|------:|:--|:----|:----------|------:|
| Existing stockholders before this offering       |     |                                                          | 31,932,467 |     |            |  75.1 | % |     | $                   | 330,537 |     |            |  63.4 | % |     | $         | 10.35 |
| New