Pharmacologypalbociclib is a reversible small molecule cyclin-dependent kinase (cdk) inhibitor which is selective for cdk 4 and 6. cdks have a role in regulating progression through the cell cycle at the g1/s phase by blocking retinoblastoma (rb) hyperphosphorylation (finn 2015). palbociclib reduces proliferation of breast cancer cell lines by preventing progression from the g1 to the s cell cycle phase. the combination of palbociclib with an antiestrogen provides for increased inhibition of rb phosphorylation, downstream signaling, and tumor growth compared with each agent alone.indications-breast cancer, advanced (initial endocrine-based therapy): 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) in postmenopausal women as initial endocrine-based therapy.breast cancer, advanced (with disease progression following endocrine therapy).