Atossa Therapeutics Receives FDA Rare Pediatric Disease Designation for (Z)-Endoxifen for McCune-Albright Syndrome

Designation expands (Z)-endoxifen program into rare pediatric endocrine disorder and qualifies Atossa for a Priority Review Voucher award upon approval

SEATTLE, May 4, 2026 /PRNewswire/ -- Atossa Therapeutics, Inc. (Nasdaq: ATOS) ("Atossa" or the "Company"), a clinical-stage biopharmaceutical company developing novel therapies in oncology and other areas of high unmet clinical need, today announced that the U.S. Food and Drug Administration ("FDA") has granted Rare Pediatric Disease ("RPD") designation to (Z)-endoxifen for the treatment of McCune-Albright Syndrome ("MAS") in females.

RPD designation is granted to drug candidates intended to treat serious or life-threatening diseases that primarily affect individuals from birth to 18 years of age. Upon approval of a qualifying marketing application, drugs with RPD designation may be eligible for a Priority Review Voucher ("PRV"), which can be used to obtain priority review for a future application or may be sold or transferred to another sponsor. In the last 18–24 months, disclosed PRV sales have ranged from approximately $100–$205 million.

"This designation is an important regulatory milestone for Atossa and provides further validation of the potential of (Z)-endoxifen beyond oncology," said Steven Quay, M.D., Ph.D., Atossa Therapeutics President and Chief Executive Officer. "McCune-Albright Syndrome is a rare and serious pediatric disorder with significant unmet medical need, particularly in young girls affected by hormone dysregulation and early puberty. We believe (Z)-endoxifen has the potential to address key disease drivers and improve outcomes for these patients, while also creating potential non-dilutive value through the Rare Pediatric Disease program.

Dr. Quay continued, "We are pleased to have participated in the Fibrous Dysplasia, McCune-Albright Syndrome Alliance ("FD/MAS Alliance") (https://fdmasalliance.org/) Research Priorities Workshop at Children's Hospital of Philadelphia, with leading clinicians, researchers, and patients, their caregivers, and patient advocacy groups. This workshop was structured to inform and shape the FD/MAS research agenda by integrating the perspectives and challenges faced by patients and their care givers with insight from clinicians and researchers into the current state of science, identifying unmet medical needs and fostering collaboration to accelerate development of new therapeutic options for patients.

Our engagement in this forum underscores our commitment to collaborating with the clinical and patient community as we advance development of (Z)-endoxifen for MAS."

"RPD designation provides a valuable regulatory pathway and opportunities for enhanced engagement with the FDA as we evaluate development strategies for MAS," said Janet Rea, Senior Vice President of Research and Development at Atossa. "We are encouraged by the scientific rationale supporting (Z)-endoxifen in this setting, particularly given its mechanism as a potent SERM/D and its potential to modulate estrogen-driven disease manifestations. We look forward to advancing this program as we further define the clinical development path."

About Rare Pediatric Disease Designation

The FDA's RPD 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 under Section 526 of the Federal Food, Drug & Cosmetic Act ("FD&C Act"). MAS qualifies as a rare pediatric disease due to its serious manifestations, including gonadotropin-independent precocious puberty, accelerated bone maturation, and endocrine abnormalities affecting children and adolescents.

Drugs granted RPD designation may be eligible for a PRV upon FDA approval of a qualifying New Drug Application or Biologics License Application, provided all statutory criteria are met. A PRV may be used by the sponsor or sold or transferred to another company.

About McCune-Albright Syndrome

MAS is an extremely rare genetic disorder caused by activating mutations in the GNAS gene, leading to mosaic endocrine dysregulation. Because it is so rare and has a broad spectrum of symptoms, diagnosis can be challenging. The disease is characterized by a triad of symptoms: Polyostotic Fibrous Dysplasia: replacement of normal bone with weak, fibrous tissue, leading to fractures, deformities, and pain; Café-au-lait Spots: hyperpigmented skin patches with irregular, "jagged" borders (often described as the "Coast of Maine") that typically respect the body's midline; and Hyperfunctioning Endocrinopathies: commonly known as precocious puberty, where children (especially girls) may begin puberty as early as age two. In pediatric patients, MAS commonly presents with gonadotropin-independent precocious puberty, particularly in females, which can result in accelerated growth, premature epiphyseal closure, and reduced adult height. Additional complications may include thyroid dysfunction, growth hormone excess, and other endocrine abnormalities. There are currently limited effective treatment options, highlighting the need for new therapeutic approaches.

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.

(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 has previously received Orphan Drug Designation, as well as RPD for (Z)-endoxifen for the treatment of Duchenne Muscular Dystrophy from the FDA. Upon approval of a qualifying marketing application, drugs with RPD designation may be eligible for a PRV, which can be used to obtain priority review for a future application or may be sold or transferred to another sponsor. In the last 18-24 months, disclosed PRV sales have ranged from $100-$205 million.

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.

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, including 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 Company's potential eligibility for and the value of a Rare Pediatric Disease 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 MAS 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 orphan-drug exclusivity for (Z)-endoxifen for MAS; our ability to 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 PRV 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.

SOURCE Atossa Therapeutics Inc