PF-00477736 mediates checkpoint kinase 1 signaling pathway and potentiates docetaxel-induced efficacy in xenografts.

PURPOSE Checkpoint kinase 1 (Chk1) plays a critical role in the activation of mitotic spindle checkpoint and DNA damage checkpoint. We examined the preclinical use of the Chk1 inhibitor PF-00477736 as a docetaxel-sensitizing agent. Specifically, we investigated the correlation between PF-00477736-mediated modulation of biomarkers and the sensitization of docetaxel efficacy. EXPERIMENTAL DESIGN In vitro and in vivo studies using COLO205 and other cell lines were done to assess PF-00477736-induced enhancement of docetaxel efficacy and effects on associated biomarkers. RESULTS PF-00477736 significantly enhanced the docetaxel-induced efficacy in tumor cells and xenografts. Docetaxel induced dose- and time-dependent increase in the levels of phosphorylated Chk1 (Ser(345)), phosphorylated histone H3 (Ser(10)), and gammaH2AX foci and promoted the cytoplasmic localization of phosphorylated Cdc25C (Ser(216)). PF-00477736 cotreatment suppressed docetaxel-induced changes in phosphorylated histone H3 and cytoplasmic phosphorylated Cdc25C (Ser(216)) levels and concurrently sensitized the docetaxel-induced apoptosis. Docetaxel alone or in combination with PF-00477736 induced significant antiproliferative activity in xenografts, shown via [18F]FLT-PET imaging. However, changes in [18F]FLT uptake did not reflect the potentiation of docetaxel efficacy. In contrast, bioluminescence imaging showed that PF-00477736 sensitized docetaxel-induced suppression of tumor survival. CONCLUSIONS Docetaxel triggers mitotic spindle checkpoint activation at low concentrations and activates both the DNA damage checkpoint and the spindle checkpoint at high concentrations. In combination with docetaxel, PF-00477736 abrogates the mitotic checkpoint, as well as the DNA damage checkpoint, and results in sensitization to docetaxel. Chk1 inhibitor PF-00477736 offers a therapeutic potential for the enhancement of taxane therapy.