Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 144

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 144
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 by purchases of marketable securities for $23.5 million and purchases of
property and equipment of $1.1 million.

Net cash used in investing activities was $29.3 million during the year ended December 31,
2022, which was primarily driven by purchases of marketable securities for $40.4 million and property and equipment of $1.9 million, partially offset by cash provided from maturities of marketable securities of $13.0 million.

Financing Activities

Net cash provided by financing
activities was $180.4 million during the nine months ended September 30, 2024, as compared to $31,000 during the nine months ended September 30, 2023. The increase in net cash provided by financing activities was primarily due to the
Series C Financing which occurred in March 2024, compared to no financings completed during 2023, partially offset by $1.1 million of payments of offering costs.

Net cash provided by financing activities was $31,000 during the year ended December 31, 2023, as compared to $110.9 million during the year
ended December 31, 2022. The decrease in net cash provided by financing activities was primarily due to the fact that the Series B financing occurred in 2022 and no financings occurred in 2023.

Future Funding Requirements

As of September 30,
2024, we had cash, cash equivalents and marketable securities of $180.9 million. Based upon our current operating plans, we believe that the estimated net proceeds

105

from this offering, together with our existing cash, cash equivalents and investments in marketable securities, will be sufficient to fund our operations into 2028. However, our forecast for the period of time through which our financial resources will be adequate to support our operations is a forward-looking statement that involves risks and uncertainties, and actual results could vary materially. Additionally, conducting preclinical studies and clinical trials is costly, and the timing of progress and expenses in these studies and trials is uncertain. We will need to raise substantial additional capital in the future. Our future funding requirements will depend largely on:

| • |     | the type, number, scope, progress, expansions, results, costs and timing of, discovery, preclinical studies and clinical 
 trials of our current and future product candidates;                                                                     |

| • |     | the costs and timing of manufacturing for our current and future product candidates and commercial manufacturing; |

| • |     | the