Palbociclib is an inhibitor of Cyclin-dependent kinases (CDK) 4 and 6. cyclin D1 and CDK4/6 are downstream of signaling pathways that lead to cellular proliferation. Palbociclib reduced cellular proliferation of Estrogen receptor (ER)-positive breast cancer cell lines by blocking progression of the cell from G1 into s phase of the cell cycle. Treatment of breast cancer cell lines with the combination of Palbociclib and antiestrogens leads to decreased retinoblastoma (RB) protein phosphorylation resulting in reduced E2F expression and signaling, and increased growth arrest compared to treatment with each drug alone. Treatment of ER-positive breast cancer cell lines with the combination of Palbociclib and antiestrogens led to increased cell senescence compared to each drug alone, which was sustained for up to 6 days following Palbociclib removal and was greater if antiestrogen treatment was continued. In vivo studies using a patient-derived ER-positive breast cancer xenograft model demonstrated that the combination of Palbociclib and letrozole increased the inhibition of RB phosphorylation, downstream signaling, and tumor growth compared to each drug alone.

Palbociclib is a kinase inhibitor indicated for the treatment of hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced or metastatic breast cancer in combination with:

• An aromatase inhibitor as initial endocrine-based therapy in postmenopausal women; or
• Fulvestrant in women with disease progression following endocrine therapy.

The recommended dose of Palbociclib is a 125 mg capsule taken orally once daily for 21 consecutive days followed by 7 days off treatment to comprise a complete cycle of 28 days. Palbociclib should be taken with food. Patients should be encouraged to take their dose of Palbociclib at approximately the same time each day. For men treated with combination Palbociclib plus aromatase inhibitor therapy, consider treatment with an LHRH agonist according to current clinical practice standards.

Recommended Dose Modification for Adverse Reactions:

• Recommended Starting Dose: 125 mg/day
• First Dose Reduction: 100 mg/day
• Second Dose Reduction: 75 mg/day

If further dose reduction below 75 mg/day is required, discontinue. Or, as directed by the registered physician.

Use in Children: Palbociclib is not indicated for use in children.

Common side effects of Palbociclib include: WBC decreased, Neutrophils decreased, Neutropenia, Platelets decreased, infections, AST increased, ALT increased, Leukopenia, Fatigue, Nausea, Hair loss, Inflammation of the mouth and lips, Diarrhea, Anemia, Rash, Weakness/lethargy, Vomiting, Thrombocytopenia, Dry skin, Fever.

Overdose Effects

There is no known antidote for Palbociclib. The treatment of overdose of Palbociclib should consist of general supportive measures.

Neutropenia: Neutropenia was the most frequently reported adverse reaction. Monitor complete blood counts prior to starting Palbociclib therapy and at the beginning of each cycle, as well as on day 15 of the first 2 cycles, and as clinically indicated. Dose interruption, dose reduction, or delay in starting treatment cycles is recommended for patients who develop grade 3 or 4 neutropenia. Febrile neutropenia has been reported in 1.8% of patients exposed to Palbociclib. One death due to neutropenic sepsis was observed. Physicians should inform patients to promptly report any episodes of fever.

Embryo-Fetal Toxicity: Palbociclib can cause fetal harm when administered to a pregnant woman. Advise pregnant women of the potential risk to a fetus. Advise females of reproductive potential to use effective contraception during treatment with Palbociclib and for at least 3 weeks after the last dose.

Pregnancy & Lactation

Palbociclib can cause fetal harm when administered to a pregnant woman. Advise pregnant women of the potential risk to a fetus. Risk of major birth defects and miscarriage in clinically recognized pregnancies is 2%-4% and 15%-20%, respectively.