Pharmacological profile and clinical findings of palbociclib (IBRANCE® capsule 25 mg/125 mg)

Palbociclib is the world's first cyclin dependent kinase (CDK) 4 and 6 inhibitor. It's believed that palbociclib stops the cell cycle progression by inhibiting the activity of the complex consisting of CDK 4/6 and cyclin D, and suppresses tumor growth. In preclinical study using nonclinical model, it was confirmed that most of the cell lines sensitive to palbociclib are estrogen receptor (ER) positive. In addition, it was suggested that the expression of retinoblastoma protein (Rb) is needed for palbociclib to show its antitumor effect. By the preclinical studies using ER positive human breast cancer cell lines in combination administration of palbociclib and anti-estrogen drugs, it was confirmed that the antitumor effect was enhanced as compared with single agent administration of each drug. Based on these findings, the clinical studies in which hormone receptor positive/human epidermal growth factor receptor 2 negative (HR+/HER2-) advanced or metastatic breast cancer were conducted in combination with endocrine therapy. The PALOMA-2 study showed that progression-free survival (PFS) was longer with palbociclib plus letrozole than with placebo plus letrozole in the initial treatment of postmenopausal women with ER+/HER2- advanced breast cancer. In the PALOMA-3 study, the combination of palbociclib and fulvestrant was associated with significant improvements in PFS compared with fulvestrant plus placebo in patients with metastatic breast cancer. The rate of dose reduction or interruption of dosing by adverse events is higher in palbociclib group compared with placebo group in both studies while the rate of discontinuation of treatment was comparable.