Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 278

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 278
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 capital stock.                                                                                                                                                                                                                                                                                                              |

Employment Arrangements We have entered into employment agreements with certain of our named executive officers, and granted stock options to our named executive officers and certain of our directors, as more fully described in “ Executive Compensation” and “ Director Compensation.” 200

Investors’ Rights Agreement

In March 2024, we entered into a third amended and restated investors’ rights agreement (the “investors’ rights agreement”) with
certain holders of more than 5% of our outstanding capital stock.

This investors’ rights agreement provides certain holders of our preferred
stock with a participation right to purchase their pro rata share of new securities that we may propose to sell and issue, subject to certain exceptions. Such participation right will terminate upon the completion of this offering. The
investors’ rights agreement further provides certain holders of our preferred stock with certain rights, including certain registration rights with respect to the registrable securities held by them. See the section titled “Description of Capital Stock—Registration Rights” for additional information.

Voting Agreement

In March 2024, we entered into a second amended and restated voting agreement (“voting agreement”) with certain holders of more than 5% of our
outstanding capital stock.

Our voting agreement provides for drag-along rights in respect of sales by certain holders of our capital stock. The
voting agreement also contains provisions with respect to the elections of our board of directors and its composition. The rights under the voting agreement will terminate upon the completion of this offering.

Right of First Refusal and Co-SaleAgreement

In March 2024, we entered into a second amended and restated right of first refusal and co-sale agreement
(“right of first refusal and co-sale agreement”) with certain holders of more than 5% of our outstanding capital stock.

Our right of first refusal and co-sale agreement provides for rights of first refusal and co-sale rights in respect of sales by certain holders of our capital stock. The rights under the right of first refusal and co-sale agreement will terminate upon the
completion of this offering.

Indemnification Agreements

Our fifth amended and restated certificate of incorporation will contain provisions limiting the liability of directors and officers, and our amended and
restated bylaws provide that we will indemnify each of our directors and officers to the fullest extent permitted under Delaware law. Our fifth amended and restated certificate of incorporation and amended and restated bylaws also provide our board
of directors with discretion to