Atossa Therapeutics, a clinical-stage biopharmaceutical company, has announced a strategic shift to prioritize the development of its lead compound (Z)-endoxifen for metastatic breast cancer, positioning the therapy to address a critical treatment gap for patients with advanced disease.
The Seattle-based company revealed this strategic direction as part of its first quarter 2025 financial results, emphasizing the significant unmet medical need in metastatic breast cancer where current treatment options often provide limited durability of response and come with substantial side effects.
Strategic Pivot to Metastatic Breast Cancer
The decision to focus on metastatic breast cancer represents a calculated move to potentially accelerate (Z)-endoxifen's path to market through a more streamlined regulatory pathway while targeting patients with the most urgent treatment needs.
"Our focus remains firmly on advancing (Z)-endoxifen as a next-generation therapy for breast cancer patients across the full spectrum of care—including a strategic emphasis on metastatic breast cancer, where therapeutic innovation is urgently needed," said Steven Quay, M.D., Ph.D., President and Chief Executive Officer of Atossa.
According to Dr. Quay, (Z)-endoxifen has demonstrated strong tolerability and therapeutic versatility across multiple clinical trials involving hundreds of patients, suggesting potential applications from early-stage disease to more advanced stages of breast cancer.
Strengthened Intellectual Property Position
In a significant development for the company's commercial prospects, Atossa has secured three new U.S. patents for (Z)-endoxifen, expanding its intellectual property portfolio to more than 200 patent claims. The new patents cover:
- Sustained release compositions of (Z)-endoxifen (U.S. Patent No. 12,201,591)
- Enteric oral formulations of (Z)-endoxifen and their use in treating hormone-dependent breast and reproductive tract disorders (U.S. Patent No. 12,275,684)
- Various (Z)-endoxifen formulations, including specifications for purity, stability, and methods of use (U.S. Patent No. 12,281,056)
This robust patent protection strengthens Atossa's market position and provides extended exclusivity for its novel therapeutic approach.
Mechanism of Action and Clinical Development
(Z)-endoxifen is characterized as a highly potent selective estrogen receptor modulator (SERM) with demonstrated ability to inhibit and potentially degrade estrogen receptors. The compound has shown activity even in tumors that have developed resistance to other endocrine therapies.
Beyond its anti-estrogenic properties, (Z)-endoxifen also targets protein kinase C beta 1 (PKCβ1), an oncogenic signaling protein, at clinically achievable blood levels. The company reports that the compound delivers comparable or superior bone-protective effects relative to tamoxifen while exhibiting minimal or no endometrial proliferative activity—addressing key limitations of current standard-of-care therapies.
Atossa has developed a proprietary oral formulation of (Z)-endoxifen that is enteric-coated to bypass stomach acid, which would otherwise convert the active (Z)-isomer to its inactive (E)-form. This innovation is designed to maintain optimal bioavailability and therapeutic integrity.
The compound is currently being evaluated in three Phase 2 clinical trials:
- One trial in women with ductal carcinoma in situ (DCIS)
- Two trials in women with estrogen receptor positive (ER+) / human epidermal growth factor receptor 2 negative (HER2-) breast cancer, including the EVANGELINE study and an I-SPY study
Financial Position
Atossa reported a solid financial foundation to support its clinical development programs, ending the first quarter of 2025 with $65.1 million in cash and cash equivalents and no debt. The company reported operating expenses of $7.4 million for the quarter, representing a modest increase of $0.4 million compared to the same period in 2024.
Research and development expenses totaled $4.2 million, an 11% increase from the previous year, primarily due to increased headcount and regulatory consulting services. General and administrative expenses remained relatively stable at $3.3 million.
Breast Cancer Treatment Landscape
Breast cancer remains one of the most common cancers worldwide, with metastatic disease presenting particular challenges for effective treatment. When breast cancer metastasizes, it spreads beyond the breast to distant organs, significantly complicating treatment approaches and reducing survival rates.
Current treatment options for metastatic breast cancer, particularly for hormone receptor-positive disease, include endocrine therapies, targeted agents, and chemotherapy. However, these treatments often provide only temporary disease control before resistance develops, and many come with significant side effects that impact quality of life.
The development of (Z)-endoxifen represents a potential advance in addressing these limitations, particularly for patients with ER+ breast cancer who have developed resistance to existing endocrine therapies.
Looking Ahead
As Atossa advances its clinical programs for (Z)-endoxifen, the company appears positioned to potentially address significant unmet needs across the breast cancer treatment spectrum, with a strategic focus on the metastatic setting.
"As we look ahead to the remainder of 2025 and beyond, we are energized by the many opportunities to position (Z)-endoxifen as a potentially safer, more effective endocrine therapy for breast cancer patients worldwide," Dr. Quay stated.
The company's progress with (Z)-endoxifen will be closely watched by clinicians and investors alike, as positive clinical outcomes could potentially reshape treatment paradigms for patients with metastatic breast cancer and potentially earlier stages of the disease.
