Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 291

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 291
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 us by means of a merger, tender offer, proxy contest or otherwise. For example, if in the due exercise of its fiduciary obligations, our board of directors were to determine that a takeover proposal is not in the best interests
of our stockholders, our board of directors could cause shares of preferred stock to be issued without stockholder approval in one or more private offerings or other transactions that might dilute the voting or other rights of the proposed acquirer
or insurgent stockholder or stockholder group. In this regard, our fifth amended and restated certificate of incorporation will grant our board of directors broad power to establish the rights and preferences of authorized and unissued shares of
preferred stock. The issuance of shares of preferred stock could decrease the amount of earnings and assets available for distribution to holders of shares of common stock. The issuance may also adversely affect the rights and powers, including
voting rights, of these holders and may have the effect of delaying, deterring or preventing a change in control of us.

Section 203 of the Delaware General Corporation Law

Upon completion of this offering, we will be subject to the provisions of Section 203 of the Delaware General Corporation
Law (the “DGCL”). In general, Section 203 prohibits a publicly held

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Delaware corporation from engaging in a “business combination” with an “interested stockholder” for a three-year period following the time that this stockholder becomes an interested stockholder, unless the business combination is approved in a prescribed manner. Under Section 203, a business combination between a corporation and an interested stockholder is prohibited unless it satisfies one of the following conditions:

| • |     | before the stockholder became interested, our board of directors approved either the business combination or the 
 transaction which resulted in the stockholder becoming an interested stockholder;                                |

| • |     | upon consummation of the transaction which resulted in the stockholder becoming an interested stockholder, the interested                                                                                                                                 
 stockholder owned at least 85% of the voting stock of the corporation outstanding at the time the transaction commenced, excluding for purposes of determining the voting stock outstanding, shares owned by persons who are directors and also officers, 
 and employee stock plans, in some instances, but not the outstanding voting stock owned by the interested stockholder; or                                                                                                                                 |

| • |     | at or after the time the stockholder became interested, the business combination was approved by our board of directors and                                                                     
 authorized at an annual or special meeting of the stockholders by