Real-world treatment patterns of palbociclib and blood count monitoring in patients with advanced breast cancer in Japan.

To reveal the treatment patterns of palbociclib and complete blood count (CBC) monitoring in a Japanese real-world setting. Deidentified data of patients with advanced breast cancer who received palbociclib from 2017 to 2020 were examined from a Japanese claims database. We identified 1074 patients. Palbociclib was commonly prescribed as second- or later-line treatment in 2017/2018; thereafter its first-line treatment increased. Regardless of treatment lines, fulvestrant was most commonly prescribed in combination with palbociclib (57-66% in the first-third-line), and this finding differed from that in the USA. Most patients initiated palbociclib at 125 mg/day; however, over a half of patients reduced doses within the first 8 weeks. Although CBC was regularly monitored, some patients did not undergo blood tests. Early dose reduction and CBC monitoring should be performed cautiously to minimize safety concern and prevent early treatment discontinuation.