Company: SION
Filing Date: 2025-01-24
Form Type: CORRESP
Source: 0001193125-25-012256
Chunk: 9

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-01-24
Form: CORRESP
Chunk 9
---
 August 9, 2024 to October 29, 2024 was the increase in the probability weighting of the near-term IPO scenario from 5.0% to 25.0%, reflecting progress made by the Company since August 9, 2024, including the following:

| • |     | The Company, in consultation with its Board and the underwriters of the potential IPO, further solidified its                                                                                            
 plans for an IPO to be consummated in early 2025, pending market conditions and review by the Commission, including incurring additional professional advisor fees, and otherwise preparing for post-IPO 
 operations; and                                                                                                                                                                                          |

| • |     | Given the progression of the Company towards a potential IPO event, the DLOM applied in IPO scenarios was reduced 
 to 8.0% in the near-term scenario and 10.0% in the base case scenario.                                            |

Between October 29, 2024 and November 14, 2024, the Company continued to operate its business in the ordinary course and there were no significant developments in its business. Given minimal changes in the guideline public peers stock prices, biotechnology indices and overall market conditions, the Board determined that the key assumptions used in the October 29, 2024 valuation were still accurate as of November 14, 2024. The Company does not intend to grant any additional equity awards under the existing 2020 Stock Option and Grant Plan, as amended, leading up to the completion of the proposed IPO, assuming the Company’s current timeline. FOIA CONFIDENTIAL TREATMENT REQUESTED BY SIONNA THERAPEUTICS, INC.

Division of Corporation Finance Office of Life Sciences U.S. Securities and Exchange Commission January 24, 2025 Page 7 In connection with the proposed IPO, the Company intends to issue certain options to purchase shares of Common Stock to certain of the Company’s employees, including its executive officers, and options to purchase shares of Common Stock to certain of the Company’s non-employeedirectors. The effectiveness of this grant of stock options is contingent on, and effective immediately following, the effectiveness of the Registration Statement. The stock options will have a per share exercise price equal to the IPO price set forth on the cover page of the final prospectus included in the Registration Statement, which will be the fair market value of a share of the Common Stock on the grant date of the stock options. The stock options will be subject to the terms and conditions of the Company’s