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.