[A Multicenter Retrospective Study of Predictors of Duration of Treatment with Palbociclib]

Palbociclib(PAL), which is a small molecule with inhibitory activity against cyclin-dependent kinase 4/6, is used in endocrine combined therapy for the treatment of estrogen receptor(ER)-positive and HER2-negative inoperable and recurrent breast cancer. We retrospectively investigated the factors associated with prolonged treatment in inoperable and recurrent breast cancer in a multicenter study. The median time-to-treatment failure(TTF)after PAL was 5.6 months(0.2-22.5). A total of 28 patients in the fulvestrant(FUL)group and 21 patients in the aromatase inhibitor(AI)group received concomitant endocrine therapy. The median TTF was 2.6 vs 6.7 months(p=0.015)for white blood cell(WBC), 3.7 vs 6.6 months (p=0.021)for neutrophils(Neu), and 2.8 vs 7.5 months(p=0.007)for lymphocytes(Lym). The treatment period tended to be prolonged in the group with higher WBC, Neu, and Lym levels than that of the standard values. The median treatment duration of the FUL group was 7.5 months vs 4.2 months(p=0.162); however, the difference was not statistically significant. The WBC, Neu, and Lym levels upon PAL introduction may be factors affecting the prolonged treatment. Further analysis of the data and further investigation of the prolongation-related factors of PAL treatment period are necessary.