Atossa Therapeutics Shares Surge 13.14% on FDA Validation of (Z)-Endoxifen Strategy

Shares of Atossa Therapeutics (NASDAQ: ATOS) surged 13.14% on Monday, reaching a peak unseen since July 2025, with an intraday rally of 19.51%. The sharp rise followed favorable regulatory developments regarding its (Z)-endoxifen program, a selective estrogen receptor modulator targeting estrogen receptor-positive, HER2-negative metastatic breast cancer.

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The U.S. Food and Drug Administration validated key components of Atossa’s clinical development strategy for (Z)-endoxifen, including its proposed dose optimization trial. This endorsement eliminated the need for a pre-Investigational New Drug (IND) meeting and paved the way for an IND submission by Q4 2025. The alignment with the FDA’s Project Optimus initiative—focused on data-driven dose exploration—accelerated timelines and reduced regulatory uncertainty, directly boosting investor confidence.

Regulators confirmed the adequacy of Atossa’s nonclinical safety data and supported the scientific rationale for combining (Z)-endoxifen with standard-of-care therapies, including CDK4/6 and mTOR inhibitors. The drug’s dual mechanism, which blocks estrogen receptors and inhibits oncogenic signaling protein PKCβ1, positions it to address tumors resistant to existing treatments. This differentiation, alongside a $65.1 million cash runway, strengthens Atossa’s financial resilience and competitive positioning in a market where 70% of metastatic breast cancer cases remain unmet.

Upcoming milestones include finalizing the target patient population, combination therapy backbone, and trial design for the dose-ranging study. The FDA’s endorsement streamlines development pathways and enhances the likelihood of regulatory success. While clinical outcomes and approvals will ultimately determine long-term potential, the current progress highlights Atossa’s ability to advance (Z)-endoxifen efficiently, minimizing dilution risks and maximizing shareholder value.