Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 360

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 360
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 adjustment to the conversion price for each series of its convertible Preferred Stock, and to the exercise prices and number of shares of common stock underlying the outstanding stock options, which became effective on January 31, 2025. Accordingly, all share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been retroactively adjusted, where applicable, to reflect the reverse stock split. 2025 Equity Plans In January 2025, the Board adopted and approved the 2025 Stock Incentive Plan (the “2025 Plan”), which will become effective upon the date immediately preceding the date on which the registration statement for the proposed IPO is declared effective by the SEC. The 2025 Plan provides for the grant of awards with respect to an additional 5,060,000 shares of common stock. Subject to the effectiveness of the 2025 Plan, the Company will cease the grant of additional awards under the 2020 Plan. F-44

In January 2025, the Board adopted and approved the 2025 Employee Stock Purchase Plan (the “2025
ESPP”), which will become effective upon the date immediately preceding the date on which the registration statement for the proposed IPO is declared effective by the SEC. The 2025 ESPP initially provides participating employees with the
opportunity to purchase up to an aggregate of 390,127 shares of common stock.

Grant of stock options under the 2025 Plan

In January 2025, the Board approved the grant of options with service-based vesting criteria for the purchase of an aggregate of 1,946,388 shares of common stock, at an
exercise price per share equal to the per share “price to the public” or (equivalent), under the 2025 Plan.

F-45

10,588,233 Shares Common Stock Prospectus

| Goldman Sachs & Co. LLC |     | TD Cowen |     | Stifel |     | Guggenheim Securities |

Until March 3, 2025 (25 days after the date of this prospectus), all dealers that effect transactions in these securities, whether or not participating in this offering, may be required to deliver a prospectus. This delivery requirement is in addition to the dealers’ obligation to deliver a prospectus when acting as underwriters and with respect to their unsold allotments or subscriptions. February 6, 2025