Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 0

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 0
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| Filed pursuant to Rule 424(b)(4) 
 Registration No. 333-284352      
 10,588,233 Shares                |

Common Stock This is the initial public offering of shares of common stock of Sionna Therapeutics, Inc. We are offering 10,588,233 shares of our common stock. Prior to this offering, there has been no public market for our common stock. The initial public offering price per share is $18.00. Our common stock has been approved for listing on The Nasdaq Global Market (“Nasdaq”) under the symbol “SION.” We are an “emerging growth company” and “smaller reporting company” as defined under the United States (“U.S.”) federal securities laws and, as such, we have elected to comply with certain reduced reporting requirements. See the section titled “ Risk Factors” beginning on page 17 to read about factors that you should consider before deciding to invest in shares of our common stock.

|                                           |     | Per Share |       |     | Total |             |
|:------------------------------------------|:----|:----------|------:|:----|:------|------------:|
| Initial public offering price             |     | $         | 18.00 |     | $     | 190,588,194 |
| Underwriting discounts and commissions(1) |     | $         |  1.26 |     | $     |  13,341,174 |
| Proceeds, before expenses, to us          |     | $         | 16.74 |     | $     | 177,247,020 |

| (1) | See the section titled “Underwriting” for additional information regarding compensation payable to the 
 underwriters.                                                                                          |

We have granted the underwriters the option to purchase up to an additional 1,588,234 shares of common stock from us, at the initial public offering price, less the underwriting discounts and commissions. The underwriters expect to deliver the shares of common stock on or about February 10, 2025. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

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

February 6, 2025

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