CDK4/6-inhibitor or Chemotherapy, in Combination With ENDOcrine Therapy, for Advanced Breast Cancer / KENDO

Prospective, open label, multicenter, group sequential response adaptive randomized phase 2 study, comparing two treatments for locally advanced or metastatic luminal breast cancer: * Arm A: concomitant cyclin-dependent Kinase 4/6 (CDK4/6) inhibitor (palbociclib, ribociclib or abemaciclib) plus endocrine therapy (aromatase inhibitor \[AI\] or fulvestrant) * Arm B: chemotherapy plus endocrine therapy (AI or fulvestrant, administered either concomitantly from the beginning of chemotherapy or sequentially after 4-6 months of chemotherapy) Treatments will continue until disease progression or toxicity or patient refusal.