Abstract
The treatment landscape of hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer has been modernized by the identification of cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors. Because the majority of HR+ breast cancers will develop resistance to endocrine therapies (tamoxifen and aromatase inhibitors), newer treatment options are necessary to restore endocrine sensitivity and prolong survival. Ribociclib and abemaciclib are two of three CDK4/6 inhibitors currently approved for first- and second-line treatment of HR+/HER2- metastatic breast cancer. Data from large, phase III clinical trials have demonstrated an improvement in both progression-free and overall survival with the addition of ribociclib or abemaciclib to endocrine-based therapy, establishing a new frontline standard of care. Treatment with ribociclib and abemaciclib provide a convenient oral treatment option that is both efficacious and well tolerated.