The main hypothesis is that Elacestrant induces a significant proliferative arrest (Ki67 ≤ 2.7%) in ER+ and HER2-negative BC naïve to Endocrine Therapy (ET). To test this hypothesis, a total of 24 patients will be recruited.~Secondary objectives will include correlative molecular and histological analyses at the tumor tissue, changes in Ki67 and the identification of molecular biomarkers and gene signatures of response to Elacestrant. Molecular analysis of tumor circulating DNA (ctDNA) in blood will be also performed.~The study population consists of postmenopausal women with cT1c-3 (minimal 15 mm of largest diameter) cN0 cM0, ER+ and HER2-negative breast operable tumors.~Patients who fulfil all eligibility criteria will then start treatment in Elacestrant monotherapy cohort as follows: Elacestrant continuously at 400 mg given orally (PO), once a day, in a continuous schedule (QD). After 4 weeks (+/- 2 days) of Elacestrant treatment, surgery will be performed in accordance to local practice. If the tumor is not surgically removed after 4 weeks (+/-3 days), a tumor biopsy will be obtained. In this case, patients may continue further neoadjuvant treatment under physicians' criteria.~Two biopsies of the same lesion will be mandatory: first the screening sample and second the surgical sample. A core biopsy will be collected if surgery is not performed for any reason. Complete Cell Cycle Arrest (CCCA) (determined by Ki67 ≤ 2.7%) and relative changes in the PAM50 subtypes in tumor samples will be analyzed at baseline and after 4-weeks of Elacestrant therapy at central laboratory. Ki67 status at baseline will be performed at local sites for inclusion criteria (Ki67 at diagnostic ≥10%). Extensive RNA-based and blood analyses will be performed in tumor specimens at the same timepoints.~Safety assessments will include the incidence, nature, and severity of Adverse Events (AEs) and laboratory abnormalities graded per the NCI CTCAE v.5.0
ELIPSE is a window of opportunity, prospective, multicenter, phase 0 trial which evaluates the effect of Elacestrant on proliferation after 4 weeks of treatment in postmenopausal women with Estrogen Receptor-positive (ER+) and Human Epidermal Growth Factor Receptor 2-negative (HER2-negative) early breast cancer (BC) amenable to surgery.