Definition
CDK4/6 inhibitors are targeted oral antineoplastic agents that inhibit cyclin-dependent kinases 4 and 6, which are key regulators of the G1 to S phase cell cycle transition. By blocking CDK4/6, these drugs prevent phosphorylation of Rb protein, halting cell cycle progression and reducing proliferation of ER-positive/HER2-negative breast cancer cells.
Approved Agents
| Drug | Brand Name | Common Indication |
|---|---|---|
| Palbociclib | Ibrance | HR+/HER2- advanced/metastatic breast cancer |
| Ribociclib | Kisqali | HR+/HER2- advanced/metastatic breast cancer |
| Abemaciclib | Verzenio | HR+/HER2- advanced/metastatic breast cancer; adjuvant in high-risk early-stage disease |
Mechanism of Action
- Inhibits CDK4/6 → prevents Rb phosphorylation → G1 arrest → inhibits tumor cell proliferation
- Works synergistically with endocrine therapy (e.g., aromatase inhibitors, fulvestrant)
Dosing & Administration Highlights
- Palbociclib / Ribociclib: Usually 21 days on / 7 days off per 28-day cycle.
- Abemaciclib: Continuous daily dosing, no break.
- Taken with or without food (specific instructions vary by agent).
Common Toxicities / Pharmacist Considerations
| Toxicity | Notes / Monitoring |
|---|---|
| Neutropenia | Dose-dependent, reversible; CBC monitoring required; higher with palbociclib/ribociclib |
| Leukopenia / Thrombocytopenia / Anemia | CBC monitoring essential |
| Diarrhea | More common with abemaciclib; supportive antidiarrheals often needed |
| QT prolongation | Ribociclib may prolong QT; ECG and electrolytes monitoring recommended |
| Hepatotoxicity | Liver function tests recommended for all agents |
| Fatigue / nausea | Manage symptomatically |
| Drug interactions | CYP3A4 substrates; avoid strong inhibitors/inducers |
Pharmacist Counseling Points
- Emphasize adherence, especially with continuous dosing (abemaciclib).
- Monitor CBC and LFTs per protocol.
- Educate patients about diarrhea, neutropenia, infection signs.
- Avoid concomitant strong CYP3A4 modulators.
- Ribociclib: counsel regarding QT interval and ECG monitoring.