CDK4/6 Inhibitors in Breast Cancer Treatment

Introduction to CDK4/6 Inhibitors

The American Society of Clinical Oncology recommends CDK4/6 inhibitors as standard first-line treatment for hormone receptor-positive, HER2-negative metastatic breast cancer when combined with endocrine therapy, demonstrating improvements in both progression-free survival and overall survival 1, 4

The European Society for Medical Oncology states that CDK4/6 inhibitors work by blocking the phosphorylation of retinoblastoma protein, preventing progression from G1 into S phase of the cell cycle, ultimately leading to cell cycle arrest, senescence, and apoptosis in estrogen receptor-positive breast cancer cells 1
The National Comprehensive Cancer Network guidelines list all three CDK4/6 inhibitors (palbociclib, ribociclib, and abemaciclib) as Category 1 options without preferential ranking, with selection based primarily on toxicity profiles and dosing schedules rather than efficacy differences 3, 2

Palbociclib was the first approved agent, establishing the foundation for this drug class with pivotal PALOMA trials demonstrating doubled progression-free survival compared to endocrine therapy alone 3
Ribociclib followed with similar efficacy demonstrated in the MONALEESA trial series 3
Abemaciclib is the third approved agent, uniquely possessing single-agent activity unlike palbociclib and ribociclib, which must be combined with endocrine therapy 1, 2

The American College of Physicians recommends CDK4/6 inhibitors combined with endocrine therapy as the standard first-line treatment for hormone receptor-positive, HER2-negative metastatic breast cancer, demonstrating improvements in both progression-free survival and overall survival 4, 1
For patients who did not relapse on an aromatase inhibitor or within 12 months of stopping adjuvant AI, a CDK4/6 inhibitor combined with an AI is recommended 1

Older age alone should not be used to select against CDK4/6 inhibitor therapy, though there may be higher incidence of hematologic adverse events in older patients 1

The cost-effectiveness of CDK4/6 inhibitors in the adjuvant setting remains particularly controversial given the lack of demonstrated overall survival benefit and the largely curable nature of early-stage disease 5, 4