Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 346

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 346
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                         |    262,149 |     |     |    484,531 |
| Options to purchase common stock |     |                         |  1,914,362 |     |     |  1,945,186 |
| Total                            |     |                         | 16,575,756 |     |     | 16,828,962 |

15. Employee Benefit Plan The Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers all employees and allows participants to defer a portion of their annual compensation on a pre-taxbasis. Company contributions to the plan may be made at the discretion of the Board. For the years ended December 31, 2023 and 2022, no contributions have been made to the plan by the Company. 16. Subsequent Events The Company has evaluated subsequent events after December 31, 2023, through September 12, 2024, the date these financial statements were available to be issued, and February 3, 2025, for the reverse stock split described below. F-28

Series C Financing

In March 2024, the Company authorized, issued and sold 18,628,970 shares of Series C Preferred Stock to certain investors at a purchase price of $9.762
per share, for gross proceeds of $181.9 million. The Series C preferred stockholders have senior liquidation preference to the holders of the Preferred Stock upon a deemed liquidation event. The Company is required to obtain certain approvals,
including the largest Series C preferred stockholders’ vote if a deemed liquidation event would result in a per share payment with respect to each share of Series C Preferred Stock that is less than the Series C Preferred Stock original
issuance price. All other material terms are substantively similar to the Preferred Stock (see Note 10). In connection with the financing, the Company increased its authorized shares to 55,200,000 of common stock.

Following the financing, the Company subsequently issued 1,631,350 stock options granted to employees and
non-employees.

AbbVie License Agreement

In July 2024, the Company entered into a license agreement (the “AbbVie License Agreement”) with AbbVie Global Enterprises Ltd.
(“AbbVie”), pursuant to which the Company has been granted an exclusive worldwide, royalty-bearing, sublicensable license to research and develop certain CFTR compounds. The licensed rights are directed, among other things, to