Palazestrant in Combination With Ribociclib for the First-line Treatment of ER+/HER2- Advanced Breast Cancer

Inclusion Criteria: * Adult female or male participants. * ER+, HER2- locally advanced or metastatic breast cancer that is not amenable to curative therapy. * Evaluable disease (measurable disease per RECIST 1.1 or bone-only disease). * De novo advanced breast cancer or with disease recurrence occurring after 12 months of completing adjuvant endocrine therapy (with or without CDK4/6 inhibitors) * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. * Adequate hematologic, hepatic, and renal functions. * Female participants can be pre-, peri- or postmenopausal. * Male and pre- or peri-menopausal female participants must be willing to take a GnRH (or LHRH) agonist. Exclusion Criteria: * Disease recurrence during adjuvant endocrine therapy * Currently receiving or previously received systemic anti-cancer therapy for ER+, HER2- advanced breast cancer. * Previously received treatment with fulvestrant, elacestrant or an investigational endocrine therapy in any setting. * History of allergic reactions to study treatment. * Any contraindications to letrozole and ribociclib. * Symptomatic central nervous system metastases, carcinomatous meningitis, leptomeningeal disease, or a spinal cord compression that require immediate treatment.