Neutropenia And Response To Single Agent Palbociclib

1043Background: CDK 4/6 inhibitors have been practice-changing in the treatment of metastatic breast cancer. Their activity against retinoblastoma protein positive (Rb+) breast cancer cells is mediated through cell-cycle arrest at the G1/S checkpoint. The most frequent dose-limiting toxicity is neutropenia (NTP), which, in preclinical studies, also occurs via reversible cell-cycle inhibition. The goal of this study is to determine whether NTP is associated with disease response to single agent palbociclib (PAL) as a predictive biomarker of efficacy. Methods: Blood count and response data were analyzed from two phase II clinical trials at different institutions using single agent PAL in 1) advanced Rb+ solid tumors (the “Penn trial”, n = 137) and 2) advanced liposarcoma (the “MSK trial”, n = 59). The majority of subjects in the Penn trial had breast cancer (45%). The primary endpoint was progression free survival (PFS). For each patient, NTP was examined as cycle 1 nadir absolute neutrophil count (C1ANC) a...