Preoperative Window Opportunity Study With Giredestrant or Tamoxifen in Premenopausal Women With ER+/HER2[-] & Ki67≥10%

This is a multicenter, international, open-label, two-arms, one stage, phase II, preoperative window of opportunity clinical trial to evaluate the efficacy of giredestrant (GDC-9545) as single agent in ER\[+\]/HER2\[-\] early breast cancer patients with Ki67 ≥ 10%. Upon meeting all selection criteria, 92 patients enrolled in the study will receive either giredestrant (GDC-9545) 30 mg or tamoxifen . A total of 92 patients will be enrolled as follows: * 46 patients in the investigational Arm A will receive giredestrant \[GDC-9545\] 30 mg. It will be administered orally once a day during 15 days. * 46 patients in the control Arm B will receive tamoxifen 20 mg. It will be administered orally once a day during 15 days. Patients can take both treatments at home. The main objective is to analyze the efficacy of giredestrant (GDC-9545) according to changes in tumor cell proliferation. This analysis will compare absolute changes for Ki67 expression between baseline score and the evaluation after 15 days of treatment. Total study duration is 15 days of treatment and until 28 days after the last dose of the study treatment (or discontinuation) of follow up.