Neoadjuvant Endocrine Therapy Combined With Chemotherapy in ER-positive, HER2-negative Stage I-III Breast Cancer

The goal of this investigator-initiated, open-label, phase II clinical trial is to evaluate the efficacy and safety of neoadjuvant chemotherapy combined with endocrine therapy in patients with hormone receptor-positive, human epidermal growth factor receptor 2 (HER2)-negative, Ki67 \> 20% invasive breast cancer. The main questions it aims to answer are: What is the objective response rate (ORR) assessed by Response Evaluation Criteria in Solid Tumours (RECIST) version 1.1 criteria at the completion of neoadjuvant treatment? What are the changes in Ki-67 proliferation index, pathological tumor response, pathological complete response (pCR) rate, disease-free survival (DFS), and safety profile? Participants will receive standard anthracycline- and taxane-based chemotherapy with letrozole. The chemotherapy regimen was either six cycles of docetaxel, epirubicin, and cyclophosphamide (TEC) every 3 weeks or four cycles of epirubicin plus cyclophosphamide followed by four cycles of a taxane (EC→T). The investigator could replace docetaxel or paclitaxel with nanoparticle albumin-bound paclitaxel (nab-paclitaxel), depending on the patient's clinical features and treatment tolerance. Patients took letrozole 2.5 mg orally once daily throughout neoadjuvant systemic therapy. Premenopausal patients also received ovarian function suppression with leuprorelin acetate (3.75 mg) subcutaneously every 28 days, from the start of neoadjuvant treatment until definitive surgery. A safety assessment will be conducted for each treatment cycle in the patient. During the neoadjuvant therapy phase, tumor assessment will be performed at least after 4 cycles and preoperatively; postoperatively, assessments will be conducted every 3 months using imaging modalities to evaluate the tumor status until disease progression, death, or completion of 3 years postoperatively occurs. Following surgery, all patients will receive adjuvant therapy selected by the investigator based on their individual cli