Elacestrant vs Elacestrant Plus a CDK4/6 Inhibitor in Patients With ERpositive/HER2-negative Advanced or Metastatic Breast Cancer

Primary Objective I. Evaluate progression-free survival (PFS) of patients with advanced or metastatic ERpositive/HER2-negative breast cancer who have been treated with either elacestrant monotherapy or combination therapy of elacestrant with a CDK4/6 inhibitor (palbociclib, abemaciclib, or ribociclib). PFS of the elacestrant monotherapy arm will be compared with PFS of the combination therapy arm. Secondary Objectives I. Assess toxicity profile of elacestrant combination therapy with a CDK4/6 inhibitor (palbociclib, abemaciclib, or ribociclib) in advanced or metastatic ER-positive/HER2- negative breast cancer according to NCI-CTCAE v5.0. II. Assess duration of response (DOR) in patients with advanced or metastatic ERpositive/HER2-negative breast cancer who have been treated with either elacestrant monotherapy or combination therapy of elacestrant with a CDK4/6 inhibitor (palbociclib, abemaciclib, or ribociclib). DOR of the elacestrant monotherapy arm will be compared with DOR of the combination therapy arm. III. Determine overall survival (OS) in patients with advanced or metastatic ER-positive, HER2-negative breast cancer who have been treated with either elacestrant monotherapy or combination therapy of elacestrant with a CDK4/6 inhibitor (palbociclib, abemaciclib, or ribociclib). OS of the elacestrant monotherapy arm will be compared with OS of the combination therapy arm. OUTLINE: Patients will receive either elacestrant monotherapy or combination therapy with a CDK4/6 inhibitor (palbociclib, abemaciclib, or ribociclib) orally for each 28 day cycle until progressive disease, unacceptable toxicity, treating physician decision, or patient withdrawal of consent. Patients will be followed (either by routine clinic visit or by phone call) every 36 weeks for 2 years and then every 72 weeks up to 5 years total from time of registration to document survival and disease progression.