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.
This study is a window of opportunity clinical trial to evaluate the efficacy of giredestrant (GDC-9545) or tamoxifen in estrogen receptor-positive (ER[+])/human epidermal growth factor receptor 2-negative (HER2[-]) primary invasive adenocarcinoma of the breast with Ki67 level ≥ 10%.~A total of 92 patients will be enrolled in this trial and randomized 1:1 in the arm A with giredestrant (GDC-9545) and the arm B with tamoxifen, with a total duration of treatment of 15 days.~This study will analyze the efficacy of giredestrant (GDC-9545) as determined by Ki67 expression between baseline tumor biopsy samples and post-treatment biopsy samples.