Atossa Therapeutics Receives FDA Rare Pediatric Disease Designation for (Z)-Endoxifen for Duchenne Muscular Dystrophy

SEATTLE, Dec. 11, 2025

"This designation is an important regulatory milestone for Atossa, and we believe a strong validation of the science supporting the potential of (Z)-Endoxifen as a treatment for Duchenne Muscular Dystrophy," said Steven Quay, M.D., Ph.D., Atossa Therapeutics President and Chief Executive Officer. "DMD is one of the most devastating childhood diseases. Families urgently need better options beyond steroids and gene-targeted approaches. While oncology remains our core focus, this milestone highlights (Z)-Endoxifen's potential as a platform therapy in both cancer and rare diseases, opening the door to potential non-dilutive value creation through the Rare Pediatric Disease program."

"RPD designation provides a regulatory framework and an enhanced level of interaction with the FDA as we define the clinical development path in DMD," said Janet Rea, MSPH, Senior Vice President of Research and Development at Atossa. "We are very encouraged by emerging preclinical data and by (Z)-Endoxifen's potential to be a differentiated mechanism as a potent SERM/D, and look forward to our planned advancement of this program to the clinic for boys living with DMD. Unlike more recent therapeutic approaches, (Z)-Endoxifen does not target specific exon defects, thus potentially offering a broader and more accessible treatment approach for this patient population. Having previously secured IND clearance for what is now the DMD treatment, Exondys 51® (eteplirsen), I am excited to further Atossa's DMD (Z)-Endoxifen program."

About Rare Pediatric Disease Designation

The FDA's Rare Pediatric Disease designation is reserved for serious or life-threatening diseases that primarily affect individuals from birth to 18 years old and that meet the definition of a rare disease or condition within the meaning of Section 526 of the Federal Food, Drug & Cosmetic Act ("FD&C Act").

Currently, the FDA may not award any new rare pediatric disease PRVs unless the application is (i) for a drug that is designated as a drug for a rare pediatric disease not later than December 20, 2024, and (ii) is approved under the program not later than September 30, 2026. The House has passed the Mikaela Naylon "Give Kids a Chance Act" to extend voucher-award authority to 2029, with retroactive effect, and the bill is now pending Senate action.

Assuming the program is renewed, drugs granted RPD designation may be eligible for a PRV upon FDA approval of a qualifying New Drug Application or Biologics License Application, provided it meets all statutory criteria under Section 529(a)(4) of the FD&C Act. A PRV may be used by the sponsor or sold or transferred to another company.

About Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy is a rare, progressive, X-linked neuromuscular disorder caused by mutations in the dystrophin gene. Symptoms typically emerge in early childhood and include progressive muscle weakness, loss of ambulation, respiratory compromise, and cardiomyopathy. DMD is uniformly fatal, often in early adulthood, and despite recent therapeutic advances, there remains a substantial unmet medical need for safe, effective, and accessible treatments.

About (Z)-Endoxifen

(Z)-Endoxifen is a potent Selective Estrogen Receptor Modulator/Degrader (SERM/D) with demonstrated activity across multiple mechanisms of interest. Atossa is evaluating its potential applications in oncology and rare diseases. The Company's proprietary oral formulation has shown a favorable safety profile and pharmacology distinct from tamoxifen, including ER-targeted effects and PKC inhibition. Atossa's (Z)-Endoxifen is not approved for any indication.

Atossa's (Z)-Endoxifen program is supported by a growing global intellectual property portfolio, including multiple recently issued U.S. patents and numerous pending applications worldwide.

About Atossa Therapeutics

Atossa Therapeutics, Inc. (Nasdaq: ATOS) is a clinical-stage biopharmaceutical company developing innovative medicines in oncology and other areas of significant unmet need. The Company's lead product candidate, (Z)-Endoxifen, is currently in development across several clinical settings. More information is available at https://atossatherapeutics.com.

Forward-Looking Statements

This press release contains certain "forward-looking statements" within the meaning of applicable securities laws, including but not limited to, our expectations regarding the Company's development and regulatory strategy and related milestones, the potential indications that the Company may pursue for (Z)-Endoxifen, the potential for (Z)-Endoxifen to receive regulatory approval and the timing thereof, the potential extension of the RPD Priority Review Voucher program and the Company's potential eligibility for and value of a future Priority Review Voucher, and the potential market and growth opportunities for the Company. Words such as "expect," "potential," "continue," "may," "will," "should," "could," "would," "seek," "intend," "plan," "estimate," "anticipate," "believe," "design," "predict," "future," or other similar expressions or statements regarding intent, belief or current expectations, are forward-looking statements.

Forward-looking statements in this press release are subject to risks and uncertainties that may cause actual results, outcomes, or the timing of actual results or outcomes to differ materially from those projected or anticipated, including, without limitation, risks and uncertainties associated with: our ability to successfully execute our strategy to shorten our clinical development timelines and pursue a metastatic breast cancer indication, DMD indication or other indications for our lead program, (Z)-Endoxifen; expected timing, completion and results of our preclinical studies, clinical trials and research and development programs; the unpredictable relationship between preclinical study results and clinical study results; the timing or likelihood of regulatory filings and approvals; the outcome or timing of necessary regulatory approvals; our ability to receive a Priority Review Voucher for (Z)-Endoxifen, if approved, which is subject to, among other factors, the U.S. federal government's extension of the RPD PRV program and such extension being effective retroactively; our ability to regain and maintain compliance with Nasdaq listing requirements; our ability to establish and maintain intellectual property rights covering our products; the impact of general macroeconomic conditions on our business; our ability to raise capital; and other risks and uncertainties detailed from time to time in Atossa's filings with the SEC, including, without limitation, its Annual Reports on Form 10-K and Quarterly Reports on Form 10-Q.

The market value of a Priority Review Voucher is variable and subject to a number of factors beyond our control and reported past PRV sale amounts are not necessarily indicative of PRV sale amounts in the future.

Forward-looking statements are presented as of the date of this press release. Except as required by law, we do not intend to update any forward-looking statements.

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SOURCE Atossa Therapeutics Inc