Sionna Therapeutics files automatic shelf with $250,000,000 ATM sales agreement

Sionna Therapeutics, Inc. filed an automatic shelf registration statement on Form S-3 on March 2, 2026. The filing contains a base prospectus covering an unspecified amount of common stock, preferred stock, debt securities, warrants and units, and a sales agreement prospectus covering up to $250,000,000 of common stock. The company entered into the sales agreement with Leerink Partners LLC, as sales agent, on the same day. Its common stock is listed on The Nasdaq Global Market under the symbol SION. The company reported a closing price of $38.03 per share on February 25, 2026.

The sales agent's terms

Under the sales agreement, Leerink Partners will sell shares at the market, meaning through its normal trading and sales practices on mutually agreed terms. Leerink is not required to sell any specific number or dollar amount of shares, and it will act using commercially reasonable efforts. The prospectus says Leerink will be entitled to a commission of up to 3.0% of the gross sales price per share sold. No funds will be held in escrow, trust or a similar arrangement. Sionna has also agreed to reimburse up to $75,000 of Leerink's actual outside legal expenses plus certain ongoing expenses, and either party can end the sales agreement at any time by written notice.

The prospectus says Leerink will be deemed an underwriter under the Securities Act of 1933, and that its compensation will be treated as underwriting commissions or discounts. The company has agreed to indemnify Leerink and provide contribution against certain liabilities, including liabilities under the Securities Act. The prospectus does not name counsel for Leerink. It says any counsel for underwriters, dealers or agents would be named in a prospectus supplement.

The prospectus says Goodwin Procter LLP of Boston will pass on the validity of the securities offered. The S-3 lists Goodwin Procter attorneys among those to receive copies of communications.

What the money is for

The prospectus says the company currently intends to use the net proceeds from any sale for general corporate purposes. Those purposes may include research and development, manufacturing, preclinical studies and clinical trials, and in-licensing or acquiring complementary businesses, technologies or assets. Management retains broad discretion over the use of proceeds, and the company has not decided how much of any net proceeds will go to a particular purpose.

The company's losses

XBRL data from the company's annual report filed March 2, 2026 show a 2025 net loss of $75.3 million, up from $61.7 million in 2024. Research and development expense was $60.3 million in 2025 and $57.3 million in 2024. Cash used in operations was $66.3 million in 2025, against $52.8 million in 2024. The prospectus does not say how much the company expects to sell under the $250 million program. It says 44,723,585 shares were outstanding as of December 31, 2025, so shares sold at the market would add to that count.

The business

Sionna describes itself as a clinical-stage biopharmaceutical company developing medicines for cystic fibrosis that aim to normalize the function of the CFTR protein. The annual report says at least two-thirds of patients on the current standard of care do not have normal CFTR function, defined as sweat chloride levels below 30 mmol/L. That is the company's own characterization of the unmet need.

The company's goal, as its annual report puts it, is to restore CFTR function to as close to normal as possible by directly stabilizing the nucleotide binding domain 1 (NBD1) of the protein. The lead NBD1 stabilizer, SION-719, is in a Phase 2a proof-of-concept trial in CF patients called PreciSION CF, given with Vertex's Trikafta, with topline data anticipated in mid-2026. SION-451 is in a Phase 1 trial in healthy volunteers in combination with SION-2222 and SION-109, also with topline data anticipated in mid-2026. The company announced positive topline data in June 2025 from Phase 1 trials in healthy volunteers, which measured pharmacokinetics, safety and tolerability, not benefit to patients.

What the filing does not say

The S-3 does not name the amount of any sale under the sales agreement, and it does not state how many shares Leerink has sold.

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