CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib) are approved for hormone receptor-positive (HR+), HER2-negative metastatic breast cancer. Their mechanism of action involves:
- A Blocking estrogen receptor-DNA binding, preventing transcription of proliferative genes
- B Inhibiting cyclin-dependent kinases 4 and 6, preventing Rb protein phosphorylation and maintaining G1-to-S phase cell cycle arrest in tumors driven by Rb pathway ✓
- C Inducing BRCA-like BRCAness by inhibiting homologous recombination repair
- D Activating intrinsic apoptosis by releasing cytochrome c from mitochondria via Bax activation
Explanation
CDK4/6, in complex with their partner cyclins D (D1, D2, D3), phosphorylate the retinoblastoma protein (Rb) during G1 phase of the cell cycle. Phospho-Rb releases E2F transcription factors that drive S-phase gene expression and DNA replication. By inhibiting CDK4/6, palbociclib/ribociclib/abemaciclib prevent Rb phosphorylation, maintaining it in its active (hypophosphorylated, growth-suppressive) form and arresting cells in G1. This is particularly effective in HR+ breast cancer where cyclin D1 overexpression drives CDK4/6 hyperactivity. Combination with endocrine therapy (letrozole, fulvestrant) produces synergistic benefit.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.