A Window-of-Opportunity Trial of Giredestrant +/- Triptorelin vs. Anastrozole + Triptorelin in Premenopausal Patients With ER-positive/HER2-negative Early Breast Cancer

Inclusion Criteria: * Premenopausal women age ≥18 years, premenopausal status defined as: Estradiol (E2) in the premenopausal range (according to institution parameters) or Patient has been menstruating regularly during the 6 months prior to screening and has not used any form of hormonal contraception or any other hormonal treatments during this time. * Histologically confirmed, operable invasive breast carcinoma. * Eligible for upfront breast conservative surgery or upfront mastectomy: stage I, stage II or operable stage III (excludes T4) (AJCC Cancer Staging Manual 8th edition 2017).46 Tumor size must be ≥1.0 cm Multicentric and multifocal tumors and bilateral breast cancers are allowed but investigators must ensure the same tumor foci is biopsied pre-treatment and post-treatment (e.g., via clipping of the biopsied tumor foci). * Documented estrogen receptor (ER)-positive tumor in accordance to ASCO/CAP guidelines (Allison et al. 2020),47 assessed locally and defined as ≥1% of tumor cells stained positive. * Documented human epidermal growth factor receptor-2 (HER2)-negative tumor in accordance to 2018 ASCO/CAP guidelines (Wolff et al. 2018)48, as determined per local assessment. * Ki 67 ≥10% in diagnostic biopsy as determined per local assessment. * Eastern Cooperative Oncology Group Performance Status 0-1. * Resting heart rate ≥40 bpm. * Normal hematologic status * Normal renal function * Normal liver function * INR \<1.5× ULN and PTT \<1.5x ULN Except for patients re…