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Atossa Therapeutics Announces Full Results from I‑SPY 2 Endocrine‑Optimization Sub‑Study Evaluating Low‑Dose (Z)‑Endoxifen
Feasibility endpoint achieved; rapid Ki‑67 suppression and substantial MRI‑confirmed tumor shrinkage observed with favorable safety profile SEATTLE, May 14,
About this update from Atossa Therapeutics, Inc.
[{"type":"text","content":"Feasibility endpoint achieved; rapid Ki‑67 suppression and substantial MRI‑confirmed tumor shrinkage observed with favorable safety profile\nSEATTLE, May 14, 2025 /PRNewswire/ -- Atossa Therapeutics, Inc. (Nasdaq: ATOS) (\"Atossa\" or the \"Company\"), a clinical-stage biopharmaceutical company developing innovative medicines for breast cancer, today 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.\n \n Key Findings: \nPrimary 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 w...