PROMETEO II is a single-arm window of opportunity trial to evaluate biologic and anti-proliferative effects of palbociclib and letrozole in HR+/HER2-negative operable breast cancer (BC) patients with residual disease after neoadjuvant chemotherapy (NAC) and help to identify biomarkers for better patient selection.

This is a single-arm window of opportunity trial to evaluate biologic and anti-proliferative effects of palbociclib and letrozole in HR+/HER2-negative operable BC patients with residual disease after NAC and help to identify biomarkers for better patient selection. The primary endpoint will be the Complete Cell Cycle Arrest (CCCA) determined by Ki67\<2.7%, centrally assessed at surgery after 4 weeks of palbociclib and letrozole. Tumor measurement will be performed by ultrasound (US) for disease evaluation and confirmation of residual disease will be performed at screening at the end of NAC. The biopsy after chemotherapy will only be done after confirmation of residual disease by US. Ki67% ≥ 5% after NAC by local determination will be necessary to be included in the study. Patients will be administered palbociclib at a dose of 125 mg once daily, day 1 to day 21 followed by 7 days off treatment in a 28-day cycle and letrozole: oral, 2.5 mg per day continuously, one cycle of treatment. After the finalization of the neoadjuvant treatment, patients will undergo surgery. Surgery specimens will be collected for histological examination and biomarker analysis The end of the study is defined as the date of post-surgery visit and will take place 4 weeks (+/- 7days) after the surgery in order to monitor the patient's safety and collect the surgery information.