Selective radiosensitization of p53 mutant pancreatic cancer cells by combined inhibition of Chk1 and PARP1.

We have recently shown that inhibition of HRR (homologous recombination repair) by Chk1 (checkpoint kinase 1) inhibition radiosensitizes pancreatic cancer cells and others have demonstrated that Chk1 inhibition selectively sensitizes p53 mutant tumor cells. Furthermore, PARP1 [poly (ADP-ribose) polymerase-1] inhibitors dramatically radiosensitize cells with DNA double strand break repair defects. Thus, we hypothesized that inhibition of HRR (mediated by Chk1 via AZD7762) and PARP1 [via olaparib (AZD2281)] would selectively sensitize p53 mutant pancreatic cancer cells to radiation. We also used 2 isogenic p53 cell models to assess the role of p53 status in cancer cells and intestinal epithelial cells to assess overall cancer specificity. DNA damage response and repair were assessed by flow cytometry, γH2AX, and an HRR reporter assay. We found that the combination of AZD7762 and olaparib produced significant radiosensitization in p53 mutant pancreatic cancer cells and in all of the isogenic cancer cell lines. The magnitude of radiosensitization by AZD7762 and olaparib was greater in p53 mutant cells compared with p53 wild type cells. Importantly, normal intestinal epithelial cells were not radiosensitized. The combination of AZD7762 and olaparib caused G 2 checkpoint abrogation, inhibition of HRR, and persistent DNA damage responses. These findings demonstrate that the combination of Chk1 and PARP1 inhibition selectively radiosensitizes p53 mutant pancreatic cancer cells. Furthermore, these studies suggest that inhibition of HRR by Chk1 inhibitors may be a useful strategy for selectively inducing a BRCA1/2 'deficient-like' phenotype in p53 mutant tumor cells, while sparing normal tissue.