Company: SION
Filing Date: 2025-01-17
Form Type: S-1
Source: 0001193125-25-008474
Chunk: 357

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-01-17
Form: S-1
Chunk 357
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,894 |     |      | 4,238,855 |
| Net loss per share of common shares, basic and diluted                                 |     | $                               |      8.76 |     | $    |      8.35 |

The Company’s potentially dilutive securities, which include convertible Preferred Stock, options to purchase common stock, and unvested restricted common stock, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted-average number of common stock outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following shares from the computation of diluted net loss per share attributable to common stockholders because including them would have had an anti-dilutive effect:

|                                  |     | Nine Months Ended September 30, |            |     |      |            |
|                                  |     |                            2024 |            |     | 2023 |            |
| Convertible Preferred Stock      |     |                                 | 39,667,715 |     |      | 21,038,745 |
| Options to purchase common stock |     |                                 |  5,336,077 |     |      |  2,924,938 |
| Unvested restricted common stock |     |                                 |    174,807 |     |      |    464,261 |
| Total                            |     |                                 | 45,178,599 |     |      | 24,427,944 |

13. Subsequent Events The Company has evaluated subsequent events after September 30, 2024, through November 26, 2024, the date these financial statements were available to be issued. F-44

Shares Common Stock Preliminary Prospectus

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

Until , 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. , 2025

PART II INFORMATION NOT REQUIRED IN PROSPECTUS Unless otherwise indicated, all references to “Sionna,” the “company,” “we,” “our,”