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

Rhea-AI Summary

Atossa Therapeutics (Nasdaq: ATOS) announced on December 11, 2025 that the U.S. FDA granted Rare Pediatric Disease (RPD) designation to (Z)-Endoxifen for the treatment of Duchenne Muscular Dystrophy (DMD). RPD designation targets therapies for serious diseases affecting patients birth to 18 and can make a drug eligible for a Priority Review Voucher (PRV) upon approval. The release cites disclosed PRV sales in the last 18–24 months ranging from $100–$160 million. Company statements note enhanced FDA interaction to define a DMD clinical path, emerging preclinical data supporting (Z)-Endoxifen as a potent SERM/D, and plans to advance the program to the clinic as a potentially broader, exon-agnostic DMD approach.

Positive

• FDA granted RPD designation to (Z)-Endoxifen (Dec 11, 2025)
• PRV market signal: disclosed sales ranged $100–$160M
• RPD may enable non-dilutive value via a future PRV
• Enhanced FDA interaction to define the DMD clinical path

Negative

• PRV eligibility requires a future marketing approval
• Program described as supported by emerging preclinical data

Key Figures

Price move 5.11% Pre-news 24h change for ATOS

Relative volume 1.59x Today’s volume vs 20-day average

PRV sale range $100–$160 million Disclosed Priority Review Voucher sales in last 18–24 months

Age range Birth to 18 years Population targeted by Rare Pediatric Disease designation

Time window 18–24 months Period referenced for recent PRV sale disclosures

52-week high distance 34.52% below ATOS price vs 52-week high before this news

52-week low distance 53.82% above ATOS price vs 52-week low before this news

Market cap $104,460,931 ATOS equity value before this announcement

Market Reality Check

$0.8500 Last Close

Volume Volume 1,008,489 is 1.59x the 20-day average of 635,819, indicating elevated interest ahead of this regulatory update. high

Technical Price at $0.85 is trading above the 200-day MA of $0.82, but remains 34.52% below the 52-week high.

Peers on Argus 1 Up

ATOS was up 5.11% while key biotech peers were mixed: ANRO up 8.18%, but ADAG down 7.65% and others (MGNX, PYXS, SPRO) modestly negative. One momentum peer (CRBP) moved up 7.80% without news, suggesting ATOS’s move was more stock-specific than broad sector rotation.

Historical Context

Date	Event	Sentiment	Move	Catalyst
Dec 09	Patent grant	Positive	+3.5%	USPTO issued new U.S. patent covering enteric oral (Z)-endoxifen formulations.
Dec 04	Regulatory strategy	Positive	+2.7%	Completed FDA Type C meeting clarifying expedited paths for (Z)-endoxifen in breast cancer.
Dec 02	AI study data	Positive	-5.7%	AI-driven study identified (Z)-endoxifen as glioblastoma candidate; stock fell despite data.
Nov 17	DMD opportunity	Positive	-1.9%	Highlighted peer-reviewed support for (Z)-endoxifen in DMD and female carriers.
Nov 12	Earnings update	Negative	-1.7%	Reported higher operating expenses and detailed development plans and leadership changes.

Pattern Detected

Recent (Z)-endoxifen news has generally produced modest price moves, with some positive catalysts selling off, indicating uneven reaction to R&D updates.

Recent Company History

Over the last month, Atossa has issued multiple updates centered on (Z)-endoxifen. A new U.S. patent on Dec 9, 2025 and FDA strategy details on Dec 4, 2025 both saw modest positive reactions. Earlier, glioblastoma and DMD scientific updates in November produced slight declines despite constructive scientific narratives. The current Rare Pediatric Disease designation for DMD extends this pattern of regulatory and scientific milestones aimed at broadening (Z)-endoxifen’s potential across oncology and neuromuscular disease.

Market Pulse Summary

The stock is surging +14.3% following this news. A strong positive reaction aligns with the regulatory significance of an FDA Rare Pediatric Disease designation for DMD, especially given ATOS’s focus on (Z)-endoxifen. The stock had already shown modest gains on prior patent and FDA-strategy news but also sold off on some scientific updates, underscoring inconsistent responses to R&D catalysts. With shares still 34.52% below the 52-week high and past volatility around data-driven headlines, investors faced ongoing risk that enthusiasm for this milestone could fade as development timelines and execution come into focus.

Key Terms

preclinical medical

"We are very encouraged by emerging preclinical data and by (Z)-Endoxifen's potential"

Preclinical describes the stage of drug or medical-product development when researchers test a candidate in the lab and in animals to see if it is safe, reaches intended targets, and shows basic effectiveness before any human trials begin. For investors, preclinical status signals an early, higher‑risk opportunity where positive lab results can increase value but significant work, time, and regulatory hurdles remain — like proofing a prototype in a workshop before road testing.

AI-generated analysis. Not financial advice.

Designation expands (Z)-Endoxifen program into rare pediatric neuromuscular disease and may qualify Atossa for a future Priority Review Voucher upon approval

SEATTLE, Dec. 11, 2025 /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 Duchenne Muscular Dystrophy ("DMD").

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 $100–$160 million.

"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

FAQ

What did Atossa (ATOS) announce on December 11, 2025 about (Z)-Endoxifen?

Atossa said the FDA granted Rare Pediatric Disease designation to (Z)-Endoxifen for DMD on Dec 11, 2025.

How could the RPD designation affect Atossa (ATOS) shareholders?

RPD designation could make (Z)-Endoxifen eligible for a Priority Review Voucher upon approval, which has sold for $100–$160M recently.

Does the RPD designation mean (Z)-Endoxifen is approved for Duchenne muscular dystrophy?

No. RPD designation is a regulatory status; PRV eligibility only occurs upon marketing approval.

What development stage is Atossa's DMD program for (Z)-Endoxifen?

Atossa describes emerging preclinical data and plans to advance the program to the clinic.

What is the strategic benefit of (Z)-Endoxifen not targeting exon defects for DMD?

Company states (Z)-Endoxifen is exon-agnostic, potentially offering a broader treatment approach than exon-specific therapies.