Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 249

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 249
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offering). The stock option grant is further detailed in the “Outstanding Equity Awards at Fiscal 2024 Year-End” section below. Ms. Ridloff is also eligible to participate in our
employee benefit plans, subject to the terms of such plans, and is entitled to severance benefits pursuant to her Severance and CIC Agreement (as defined below) and/or the New Severance and CIC Plan (as defined below), as applicable.

In addition to Ms. Ridloff’s severance entitlements under her Severance and CIC Agreement and/or the New Severance and CIC Plan, as
applicable, each as described below, under the Ridloff Offer Letter, in the event Ms. Ridloff’s employment ceases due to her death or disability (as defined in her Severance and CIC Agreement), she will be entitled to any earned but unpaid
bonus for the year ending prior to the date of such cessation and the prorated portion of her annual target bonus for the year in which such cessation takes place.

Charlotte McKee, M.D.

In May 2021, we entered into an offer
letter with Dr. McKee (the “McKee Offer Letter”). Under the McKee Offer Letter, Dr. McKee is entitled to receive an annual base salary and an annual bonus opportunity (each of which has subsequently been increased as described
above), and an initial equity grant consisting of 151,548 shares of restricted stock, which vested as to 25% on the first anniversary

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following Dr. McKee’s start date and vests as to the remaining 75% in 36 equal monthly installments thereafter, subject to Dr. McKee’s continued service with us through each
applicable vesting date; provided that, the vesting of such shares will fully accelerate upon a “sale event” (as defined in the 2020 Plan and which does not include this offering). The restricted stock grant is subject to certain tag along
rights upon a sale event. The restricted stock grant is further detailed in the “Outstanding Equity Awards at Fiscal 2024 Year-End” section below. Dr. McKee is also eligible to
participate in our employee benefit plans, subject to the terms of such plans, and is entitled to severance benefits pursuant to her Severance and CIC Agreement (as defined below) and/or the New Severance and CIC Plan (as defined below), as
applicable.

In addition to Dr. McK