BACKGROUND: The 2015 American Society of Clinical Oncology guidelines recommend first-line treatment of hormone receptor (HR)-positive breast cancer with endocrine therapy plus or minus palbociclib, a selective cyclin-dependent kinase (CDK)4/6 inhibitor. In 2018, the U.S. Food and Drug Administration approved ribociclib, a new orally available selective CDK4/6 inhibitor. While gains in progression-free survival (PFS) and overall