Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 274

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 274
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 grant.

The Initial Grants and the Annual Grants (including any pro rata portions thereof) are subject to full accelerated vesting upon a sale event (as defined
in the 2025 Plan).

The aggregate amount of compensation, including both equity compensation and cash compensation, paid to any non-employee director for service as a non-employee director in a calendar year period will not exceed $1,000,000 in the first calendar year such individual becomes a non-employee director and $750,000 in any other calendar year (or such other limits as may be set forth in the 2025 Plan or any successor plan).

We will reimburse all reasonable out-of-pocket expenses incurred by
non-employee directors for their attendance at meetings of our board of directors or any committee thereof.

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CERTAIN RELATIONSHIPS AND RELATED PERSON TRANSACTIONS In addition to the compensation arrangements, including employment, termination of employment and change in control arrangements, and indemnification arrangements discussed, when required, in the sections titled “ Management” and “ Executive Compensation” and the registration rights described in the section titled “ Description of Capital Stock—Registration Rights,” the following is a description of all transactions since January 1, 2021 and each currently proposed transaction in which:

| • |     | we 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 shares of common stock at a ratio of 1-for-1.4611 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: