Insights into the Use of CDK 4/6 Inhibitors in Patients with HR-positive Advanced or Metastatic Breast Cancer

Hormone receptor(HR)-positive breast cancer(BC)is the most common subtype of BC and some patients with such tumors experience recurrences.Endocrine-based therapy(ET)(e.g.,tamoxifen,aromatase inhibitors(AIs),and fulvestrant)that has improved outcomes in such patients represents the initial therapy for women with HR-positive/human epidermal growth factor receptor 2(HER2)-negative BC(considering no evidence of visceral crisis).However,the resistance to ET can occur in almost 50%of HR-positive BCs.In order to improve outcomes of patients with HR-positive metastatic BC,new treatment strategies are required.One such therapy is the new class of medications,cyclin-dependent kinase(CDK)4/6 inhibitors,that have improved the outcomes in such patients(both endocrine-sensitive and endocrine-resistant).This article presents evidence from the main clinical trials,which led to the approval of palbociclib,ribociclib,and abemaciclib.These three CDK 4/6 inhibitors have shown a significant improvement of the progression-free survival(PFS)in patients with HR-positive/HER2-negative metastatic BC when used in combination with selected ETs.In addition,some important patient management considerations,when choosing a particular CDK 4/6 inhibitor for an individual patient are presented.Furthermore,a need to find biomarkers for CDK 4/6 inhibitor sensitivity,efficacy,and resistance,to be able to precisely select the best patientcandidates for this treatment is highlighted.