Elacestrant + Everolimus in Patients ER+/HER2-, ESR1mut, Advanced Breast Cancer Progressing to ET and CDK4/6i.

Inclusion Criteria: Patients will be included in the study only if they meet ALL of the following criteria: 1. Patient must be capable to understand the purpose of the study and have signed written informed consent form (ICF) prior to beginning specific protocol procedures. 2. Female or male patients ≥ 18 years of age at the time of signing ICF. 3. Pre- or perimenopausal women, who do not meet the criteria for post-menopausal status (defined in continuation) and men must be concurrently receiving a LHRH analogue for at least 28 days (if shorter, post-menopausal levels of serum estradiol/follicle-stimulating hormone \[FSH\] must be confirmed analytically) prior to study randomization and are planning to continue LHRH agonist treatment during the study. Post-menopausal women as defined by any of the following criteria: 1. Age ≥ 60 years; 2. Age \< 60 years and cessation of regular menses for at least 12 consecutive months with no alternative pathological or physiological cause; and serum estradiol and/or FSH levels within the laboratory's reference range for post-menopausal females; 3. Documented bilateral surgical oophorectomy. 4. Histologically- or cytologically proven diagnosis of adenocarcinoma of the breast with evidence of either unresectable locally recurrent or metastatic disease confirmed by computerized tomography (CT) scan or magnetic resonance imaging (MRI) that is not amenable to resection with curative intent. 5. Documentation of ER\[+\] (≥10% positive stained cells) and HER2\[-\] (0-1+ by immunohistochemistry \[IHC\] or 2+ and negative by in situ hybridization \[ISH\] test) tumor according to the most recent American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines as per local assessment. ER\[+\]/HER2\[-\] status should be confirmed in metastatic setting, with exception of patients with bone and lung only disease. 6. Patients with ESR1 mutational status will be determined before patient randomization using Guardant360 CDx (Guardant Health) test. Note: Patients with previously determined ESR1 mutation using appropriately validated tests (Guardant360 CDx \[Guardant Health\], FoundationOne CDx, FoundationOne Liquid \[Foundation Medicine Inc\]) will be eligible for inclusion. This local determination can be performed either in blood or tumor samples. 7. Radiological or objective evidence of disease progression on prior treatment with a CDK4/6 inhibitor in combination with endocrine therapy for advanced disease after at least 6 months of treatment. Patients receiving CDK4/6 inhibitor-based therapy in the adjuvant setting are also eligible provided that disease progression is confirmed after at least 12 months of treatment but no more than 12 months following CDK4/6 inhibitor treatment completion in this scenario. 8. Patients must have previously received at least one and no more than two lines of endocrine therapy for ABC. Progression during or within 12 months of adjuvant endocrine therapy is considered as a line of endocrine therapy for advanced disease. 9. No prior elacestrant or other investigational SERDs, proteolysis targeting chimera (PROTAC), complete estrogen receptor antagonist (CERAN), or novel SERM, and/or PI3K/AKT/mTOR inhibitors, including everolimus, for advanced disease are permitted. Note: Fulvestrant is permitted if treatment was completed administered at least 28 days before randomization. 10. No prior chemotherapy for advanced disease is allowed. 11. Evidence of measurable disease as per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v.1.1), or non-measurable, but evaluable, disease, including bone-only disease with at least one lytic or mixed lytic-blastic bone lesion. 12. Willingness and ability to provide the most recently available formalin-fixed paraffin-embedded (FFPE) tumor tissue or block. If a newly obtained baseline biopsy of an accessible tumor lesion is not possible to be obtained prior randomization, an archival tissue sample will be accepted. 13. Fasting serum cholesterol ≤ 300 mg/dL or 7.75 mmol/L and fasting triglycerides ≤ 2.5 times the upper limit of normal (x ULN). 14. Adequate bone marrow and organ function: 1. Hematological (without platelet, red blood cell transfusion, and/or granulocyte colony-stimulating factor support within seven days before randomization): absolute neutrophil count (ANC) ≥ 1.5 x 109/L; platelet count ≥ 100.0 x109/L; and hemoglobin ≥ 9.0 g/dL. 2. Hepatic: Serum albumin ≥ 2.5 g/dL; total serum bilirubin \< 1.5 x ULN except for patients with Gilbert's syndrome who may be included if the total serum bilirubin is ≤ 3 x ULN or direct bilirubin ≤ 1.5 x ULN; alkaline phosphatase (ALP) ≤ 2.5 x ULN (≤ 3 x ULN in patients with liver and/or bone metastases); aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 1.5 x ULN (≤ 3 x ULN in patients with liver metastases). 3. Renal: Serum creatinine ≤ 1.5 x ULN or estimated creatinine clearance ≥ 50 mL/min as calculated by Cockcroft- Gault equation. 4. Coagulation: International normalized ratio (INR) ≤ 1.5 x ULN, unless that the patient meets the exception described in the exclusion criteria 16. 15. Resolution of all acute toxic effects of prior anti-cancer therapy to grade ≤ 1 as determined by the National Cancer Institute (NCI)-Common Terminology Criteria for Adverse Events (CTCAE) v.5.0 (except for toxicities not considered a safety risk for the patient at Investigator's discretion). Note: Patients with grade 2 alopecia are allowed. 16. Women of childbearing potential who are sexually active with a non-sterilized male partner must have a negative serum pregnancy test within 7 days before randomization. In addition, they agree to use one highly effective method of birth control 28 days prior to start of treatment until 120 days after the last dose of study treatments. Female patients must refrain from egg cell donation and breastfeeding during this same time period. 17. Male