524^ Background: WSG-ADAPT aims to optimize early breast cancer therapy within a genomically classified (by OncotypeDX) intermediate-risk group using individual endocrine sensitivity. Methods: WSG-ADAPT HR+/HER2- analyzes biomarker changes after 3 weeks of preoperative ET [aromatase inhibitors (AI) in postmenopausal, tamoxifen (Tam) in premenopausal women]. Overall, n=1760 patients (HR+/HER2-, pN0-1) with Recurrence Score (RS) 0-11 or RS 12-25 and post-Tx Ki-67<10% are treated by ET alone. Other RS 12-25 and all RS ≥26 patients are included in phase III CTx design (n=2200). Results: 1118 patients from 61 centers have been enrolled (01/2014); run-in phase analysis included 383 patients (median age 54 years, 175 Tam, 208 AI). RS distribution (≤11/12-25/≥26) was 23%/57%/20%; median relative Ki-67 decreases were 0.67/0.60/0.40 by RS groups (p=0.017). Median relative Ki-67 decrease was more pronounced in post- vs. premenopausal patients (75% vs. 38%; p<0.001). Mean PR (not ER) expression changes were also more...