Abemaciclib — Drug Monograph

Brand names: Verzenio

Drug class: CDK4/6 Inhibitor

Mechanism of Action

Selective cyclin-dependent kinase 4 (CDK4) and CDK6 inhibitor. CDK4/6, in complex with D-type cyclins, phosphorylate and inactivate retinoblastoma protein (Rb), releasing E2F transcription factors and driving cell cycle entry (G1 → S phase). Abemaciclib prevents Rb phosphorylation, maintaining Rb in its tumor-suppressive active state, arresting cells in G1. Compared to palbociclib, abemaciclib has greater selectivity for CDK4 over CDK6 and can be dosed continuously (not 3 weeks on / 1 week off), which may contribute to its distinct toxicity profile (more diarrhea, less myelosuppression) and…

FDA Indications

• HR+/HER2-negative breast cancer — advanced/metastatic, first-line in combination with an aromatase inhibitor (MONARCH 3 trial)
• HR+/HER2-negative breast cancer — advanced/metastatic, with fulvestrant in women who progressed on endocrine therapy (MONARCH 2 trial)
• HR+/HER2-negative breast cancer — early stage, high risk (node-positive, high-risk features), adjuvant treatment for 2 years with endocrine therapy (monarchE trial — 4-year invasive DFS benefit of 6.4%)

Common Side Effects

• Diarrhea (most common; 80–85% any grade; grade 3 in ~20% — diarrhea is more prominent than with palbociclib/ribociclib)
• Neutropenia (less severe than palbociclib)
• Fatigue
• Nausea
• Abdominal pain
• Infections (primarily due to neutropenia)
• Anemia
• Leukopenia
• Thrombocytopenia
• Decreased appetite

Clinical Pearl

Abemaciclib is unique among CDK4/6 inhibitors in two ways: (1) it can be dosed continuously (unlike palbociclib's or ribociclib's 3-weeks-on/1-week-off schedule), and (2) it has demonstrated meaningful single-agent activity in heavily pre-treated HR+ breast cancer. Diarrhea management is critical — patients should start loperamide at the FIRST loose stool (not wait for grade 2). Abemaciclib (monarchE, 2 years) and ribociclib (NATALEE, 3 years) both now hold FDA adjuvant approvals for high-risk…

Related Therapies & Mechanisms

• Palbociclib (Ibrance) — CDK4/6 Inhibitor · Breast
• Ribociclib (Kisqali) — CDK4/6 Inhibitor · Breast
• Trilaciclib (Cosela) — CDK4/6 Inhibitor
• 5-Fluorouracil (5-FU) (Adrucil) — Breast