Coordinately targeting cell cycle checkpoint functions in integrated models of pancreatic cancer

Purpose: Cancer cells often have deficiencies in cell cycle control mechanisms and could be dependent on specific cell cycle checkpoints to maintain viability. Due to the documented role of KRAS in driving replication stress, we targeted the checkpoint governing DNA replication using CHK1 kinase inhibitors in pancreatic ductal adenocarcinoma (PDAC) models and examined mechanisms of resistance. Experimental Design: Single agent efficacy of CHK1 inhibition was investigated in established and primary PDAC lines. Drug screening was performed to identify cooperative agents. In vitro and in vivo studies were employed to interrogate combination treatment efficacy and mechanisms of resistance. Results: Many PDAC models evade single agent inhibition through mechanisms that allow S-phase progression with CHK1 inhibited. Gene expression analysis revealed FOXM1 as a potential marker of CHK1 sensitivity and defined a form of pancreatic cancer with poor prognosis. Drug screen analysis identified WEE1 as a cooperative agent with CHK1 and was effective in cell culture. In vivo experiments validated the combination efficacy; however, resistance could evolve. Resistance was due to selection of a stable sub-clone from the original PDX tumor, which harbored high baseline replication stress. In vitro analysis revealed that gemcitabine could eliminate viability in the resistant models. The triplet regimen of gemcitabine, CHK1 and WEE1 inhibition provided strong disease control in all xenograft models interrogated. Conclusions: These results demonstrate the therapeutic resiliency of pancreatic cancer and indicate that coordinately targeting cell cycle checkpoints in concert with chemotherapy could be particularly efficacious.

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