A Phase 2 Bridging Study of Elacestrant Monotherapy in Advanced or Metastatic Breast Cancer

This is a multicenter, open-label, Phase 2 bridging study to evaluate the efficacy and safety of elacestrant in Japanese post-menopausal women and men with estrogen receptor positive (ER+)/human epidermal growth factor receptor 2 negative (HER2-) advanced or metastatic breast cancer with estrogen receptor 1 gene mutation (ESR1-mut) whose disease has relapsed or progressed on at least 1 and no more than 2 prior lines of endocrine therapy for metastatic breast cancer, which must have included cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) therapy in combination with fulvestrant or an aromatase inhibitor.

Key Inclusion Criteria: * Must have a histologically or cytologically proven diagnosis of adenocarcinoma of the breast with evidence of either locally advanced disease not amenable to resection or radiation therapy with curative intent or metastatic disease not amenable to curative therapy * Female participants must be post-menopausal (as defined by the protocol) * Must have ER+ and HER2- tumor status confirmed per local laboratory testing * Participants must be ESR1-mutant positive determined via testing with the Guardant360 Companion Diagnostic panel test prior to enrollment * Must have disease progression during or within 28 days of completion of prior treatment with a CDK4/6 inhibitor in combination with either fulvestrant or an aromatase inhibitor (this counts as a line of prior endocrine therapy) for metastatic breast cancer Key Exclusion Criteria: * Prior treatment with: elacestrant or investigational or approved selective estrogen receptor degrader or ER antagonist; antibody-drug conjugate therapy for advanced/metastatic breast cancer; anti-cancer or investigational drug treatment * Radiation therapy within 14 days (28 days for brain lesions) before the first dose of study drug * Intact uterus with a history of endometrial intraepithelial neoplasia (atypical endometrial hyperplasia or higher-grade lesion) * Diagnosis of any other malignancy within 5 years before enrollment, except for adequately treated basal cell or squamous cell skin cancer, carcinoma in situ of the cervix, or second primary breast cancer Note: Other protocol-defined inclusion/exclusion criteria may apply.