Palbociclib — Drug Monograph

Brand names: Ibrance

Drug class: CDK4/6 Inhibitor

Mechanism of Action

Selective inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/CDK6). CDK4/CDK6 in complex with cyclin D phosphorylate retinoblastoma protein (Rb), releasing E2F transcription factors and driving G1→S cell-cycle progression. Inhibition of CDK4/CDK6 maintains Rb in its hypophosphorylated state, causing G1 arrest and inhibiting tumor cell proliferation. Works upstream of, and synergistically with, endocrine therapy in HR+/HER2- breast cancer.

FDA Indications

• HR-positive, HER2-negative advanced or metastatic breast cancer — first-line with an aromatase inhibitor (postmenopausal) or fulvestrant ± GNRH agonist (premenopausal), or with fulvestrant after prior endocrine therapy

Common Side Effects

• Neutropenia (dose-limiting; Grade 3–4 in ~66% but febrile neutropenia <1%)
• Infections
• Fatigue
• Nausea
• Alopecia (diffuse thinning, not complete baldness)
• Anemia, thrombocytopenia
• Decreased appetite

Clinical Pearl

Palbociclib neutropenia is characteristic: it is reliably reversible by the end of the 7-day off-week, febrile neutropenia is rare (<1%), and it does not require G-CSF support routinely. The neutropenia mechanism is thought to be reversible cytostasis of bone marrow progenitors rather than cytotoxic destruction, explaining its rapid recovery.

Related Therapies & Mechanisms

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