Acquired resistance to combination treatment with TMZ and ABT-888 is mediated by both BER and HR DNA repair pathways

Many established cancer therapies involve DNA‐damaging chemotherapy or radiotherapy. Gain of DNA repair capacity of the tumor represents a common mechanism employed by cancer cells to survive DNA‐damaging therapy. Poly (ADP‐ribose) polymerase‐1 (PARP‐1) is a nuclear enzyme that is activated by DNA‐damage and plays a critical role in base excision repair (BER). Inhibition of PARP represents an attractive approach for the treatment of cancer. Previously, we have described the discovery and characterization of a potent PARP inhibitor, ABT‐888. ABT‐888 potentiates the activity of DNA‐damaging agents such as temozolomide (TMZ) in a variety of preclinical models. Previously, we demonstrated that ABT‐888 potentiated the cytotoxic effect of TMZ by converting TMZ‐induced single strand DNA breaks (SSB) to double strand breaks (DSB). We report here the generation of HCT116 cells resistant to treatment with TMZ and ABT‐888 (HCT116R cells). HCT116R cells exhibit decreased H2AX phosphorylation in response to the treatment with TMZ and ABT‐888 relative to parental HCT116 cells. Microarray and western blot studies indicate that HCT116R cells have decreased PARP‐1 and elevated Rad51 expression levels. HCT116R cells are dependent on Rad51 for proliferation and survival, as demonstrated by inhibition of proliferation and induction of apoptosis upon treatment with Rad51 siRNA. In addition, HCT116R cells are more resistant to radiation than the parental HCT116 cells. Our study suggests that cancer cells up‐regulate the homologous recombination DNA repair pathway to compensate for the loss of BER, which may account for the observed resistance to the treatment with TMZ and ABT‐888. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A161.