This is an international, multicenter, randomized, double-blind, active-controlled, phase 3 clinical trial. The purpose of this trial is to compare the efficacy and safety of palazestrant in combination with ribociclib +letrozole -matching placebo (Arm A: investigational arm) with letrozole in combination with ribociclib + palazestrant-matching placebo (Arm B: control arm).
This trial is seeking adult participants with ER+, HER2- advanced breast cancer who have not received prior systemic anti-cancer treatment for advanced disease. Approximately 1,000 participants will be randomized in a 1:1 ratio to one of the two study arms.
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.
