Ibrance (Palbociclib)

Ibrance is a CDK4/6 inhibitor used in the management of hormone‑receptor‑positive, HER2‑negative breast cancer.

• Hormone‑receptor‑positive, HER2‑negative advanced breast cancer: used with endocrine therapy to delay disease progression.
• Hormone‑receptor‑positive, HER2‑negative metastatic breast cancer: improves progression‑free survival when added to hormonal agents.
• Combination therapy with aromatase inhibitors or fulvestrant: enhances overall response rates in patients whose disease has progressed on prior endocrine treatment.

• Palbociclib selectively inhibits cyclin‑dependent kinases 4 and 6, key regulators of the G1 to S phase transition in the cell cycle. By blocking CDK4/6 activity, the drug prevents phosphorylation of the retinoblastoma protein, leading to G1‑phase arrest and reduced proliferation of tumor cells that rely on this pathway for growth.

Common Side Effects

• Neutropenia: decreased neutrophil count, increasing infection risk.
• Leukopenia: reduced white blood cells.
• Anemia: lowered red blood cell levels causing fatigue.
• Fatigue: general tiredness and reduced stamina.
• Nausea: upset stomach, sometimes with vomiting.
• Alopecia: hair thinning or loss.

Serious or Rare Side Effects

• Febrile neutropenia: fever with low neutrophils, requiring urgent care.
• Severe infections: pneumonia, sepsis, or other opportunistic infections.
• Pulmonary embolism: blood clot in the lungs.
• Elevated liver enzymes: potential liver injury.
• Severe neutropenia: profound drop in neutrophils.
• Thromboembolic events: deep‑vein thrombosis or arterial clotting.

Before initiating therapy, patients should be evaluated for baseline blood counts and liver function.

• Monitor blood counts: perform complete blood count (CBC) regularly to detect neutropenia.
• Assess liver function: check hepatic enzymes before and during treatment.
• Avoid concomitant myelosuppressive agents: may increase risk of severe cytopenias.
• Pregnancy and lactation: use effective contraception; safety in pregnancy is unknown.
• Drug interactions: review concomitant medications for CYP3A4 inhibitors or inducers.
• Store at ambient temperature: keep tablets below 25 °C, in the original container, protected from moisture and light.

Palbociclib is metabolized primarily by CYP3A4, making it susceptible to drug‑drug interactions.

Major Interactions (Avoid)

• Strong CYP3A4 inhibitors (e.g., ketoconazole, clarithromycin): may increase palbociclib exposure.
• Strong CYP3A4 inducers (e.g., rifampin, phenytoin): may reduce efficacy.
• Concurrent myelosuppressive chemotherapy: can exacerbate hematologic toxicity.

Moderate Interactions (Monitor Closely)

• Moderate CYP3A4 inhibitors (e.g., fluconazole, diltiazem): may modestly raise drug levels.
• Antacids containing aluminum or magnesium: may affect absorption if taken simultaneously.
• CYP3A4 substrates with narrow therapeutic index (e.g., tacrolimus): monitor plasma concentrations.