On May 14, 2025 Atossa Therapeutics, Inc. (Nasdaq: ATOS) ("Atossa" or the "Company"), a clinical-stage biopharmaceutical company developing innovative medicines for breast cancer, reported full results from the Phase 2 Endocrine Optimization Pilot (EOP) sub‑study within the I‑SPY 2 TRIAL evaluating low‑dose oral (Z)‑endoxifen (10 mg daily) as a neoadjuvant treatment in 20 women with stage II/III estrogen‑receptor–positive (ER+), HER2‑negative breast cancer (Press release, Atossa Therapeutics, MAY 14, 2025, View Source [SID1234653098]).

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Key Findings:

Primary feasibility goal surpassed – 95 percent of participants (19/20) completed at least 75 percent of planned dosing, far exceeding the predefined 75 percent threshold.
Early anti‑proliferative activity – Median Ki‑67 fell from 10.5 percent at baseline to 5 percent by Week 3 (prior to ovarian suppression); 65 percent of patients achieved Ki‑67 ≤ 10 percent at that time point, and suppression was maintained at surgery.
Robust imaging response – Median functional tumor volume (FTV) measurement (performed at baseline, week 3, week 12, and at surgery) decreased 77.7 percent (range 9.0 cc → 1.2 cc) from baseline to surgery, and the longest tumor diameter from baseline to preoperative MRI was reduced by 36.8 percent.
Well‑tolerated safety profile – Adverse events were predominantly Grade 1; vasomotor symptoms (hot flushes) and fatigue were most common. Only three Grade 3 events (two skin infections, one post‑procedural infection) occurred in a single patient and were deemed unrelated to study drug; no Grade 4 or Grade 5 events were reported.
Pathologic Findings:
No participants achieved a pathologic complete response (pCR), and residual cancer burden (RCB) classes skewed toward RCB‑II/III, indicating moderate to extensive residual disease. This result was anticipated based on earlier window‑of‑opportunity (neoadjuvant) studies that showed significant pathologic clearance typically requires higher systemic exposures to (Z)-endoxifen (> 500 ng/mL) to enable inhibition of PKCβ1 in addition to ERα. The 10 mg dose in this pilot was intentionally selected to establish tolerability and early biologic activity in the endocrine‑naïve setting; therefore, limited pCR rates at this dose are consistent with prior dose–response observations.

"These results show that even at a low capsule strength (Z)‑endoxifen is bioactive, producing rapid Ki‑67 suppression and meaningful tumor shrinkage, while remaining highly tolerable. The study strengthens our scientific hypothesis that dual targeting of ERα and PKCβ1 is key to inducing a pathological response," said Steven Quay, M.D., Ph.D., President and Chief Executive Officer of Atossa. "Importantly, it paves the way for our ongoing I‑SPY 2 cohorts evaluating higher, potentially PKCβ1‑engaging doses of (Z)‑endoxifen alone and in combination with the CDK4/6 inhibitor abemaciclib, where we aim to translate early biomarker gains into deeper pathologic responses."

Atossa is actively enrolling participants into an additional I‑SPY 2 cohort testing (Z)‑endoxifen at a 40 mg daily dose and in combination with abemaciclib, with and without ovarian function suppression. Top‑line data from these arms are expected beginning in 2026.

About (Z)-Endoxifen
(Z)-endoxifen is a highly potent Selective Estrogen Receptor Modulator (SERM) with demonstrated ability to inhibit—and potentially degrade—estrogen receptors. It 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. Importantly, (Z)-endoxifen seems to deliver 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 is developing 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 ensures optimal bioavailability and therapeutic integrity. Clinical studies have shown Atossa’s (Z)-endoxifen to be well tolerated in both healthy women and those with breast cancer.

Atossa is prioritizing the development of (Z)-endoxifen for the treatment of metastatic breast cancer, where novel therapeutic options are urgently needed. The compound is currently being evaluated in three Phase 2 trials: one in women with ductal carcinoma in situ (DCIS) and two in women with ER+/HER2- breast cancer, including the EVANGELINE trial and the combination I-SPY study. Atossa’s (Z)-endoxifen program is supported by a growing global intellectual property portfolio, including three recently issued U.S. patents and numerous pending applications worldwide.