Differences of cyclin-dependent kinase 4/6 inhibitor, palbociclib and abemaciclib, in breast cancer

Both palbociclib and abemaciclib are, oral, highly selective inhibitors of cyclin-dependent kinase 4 and 6, which are proteins involved in cell differentiation and growth. In pivotal phase III trials (PALOMA and MONARCH), they demonstrated a significant improvement in median progression-free survival in combination with a nonsteroidal aromatase inhibitor in the first-line, and with a fulvestrant in the second-line in hormone receptor-positive and HER2-negative metastatic breast cancer, respectively. Both palbociclib and abemaciclib were approved, however, ribociclib, the third cyclin-dependent kinase 4/6 inhibitor, has not been approved in Japan. The overall benefits from palbociclib and abemaciclib seem to be equivalent. Subsets analyses suggest that clinical benefits of palbociclib are associated with bone-only disease at baseline, no measurable disease, sensitive to previous endocrine therapy and longer disease-free interval. In contrast, additional benefits from abemaciclib in combination with nonsteroidal aromatase inhibitor or fulvestrant seem to have a relationship with visceral disease, liver metastasis, primary resistant to endocrine therapy, and short treatment-free interval. Abemaciclib induces senescence and apoptosis more than palbociclib does in a time-dependent manner and has potential to produce tumor shrinkage by single use. Neutropenia is more frequent in palbociclib, in contrast, diarrhea, nausea, and liver dysfunction are frequent in abemaciclib. In this review, we provide an overview of the two kinds of cyclin-dependent kinase 4/6 inhibitor, which were already approved in Japan. These differences might be useful information for the proper use in daily practice.