Preclinical testing of the PARP inhibitor ABT-888 in microsatellite instable colorectal cancer

Abstract

11028 Background: Microsatellite instability (MSI) represents approximately 15% of colorectal cancer (CRC) cases. MSI due to hypermethylation or mutation in DNA mismatch repair genes leads to genetic instability and a mutator phenotype. Genetic instability is particularly high at repetitive sequences such as those located in MRE11, RAD50, CtIP and MBC. Each of these genes are implicated in the double strand break (DSB) repair pathway. PARP inhibitors induce single strand breaks that remain unrepaired and then will be converted to DSB during DNA replication. Our objective was to assess the preclinical activity of a novel PARP inhibitor ABT-888 in MSI cell lines deficient in the DSB repairing pathway and compare it to Microsatellite Stable (MSS) lines. Methods: We used the systems biology tool “Connectivity Map” to synthesize data from 5 different published studies of expression profiling of MSI CRC phenotype and to identify target compounds. We assessed the mutational status of MRE11, RAD50, CtIP and MBC in a panel of 10 CRC cell lines displaying either MSI or MSS, and measured the expression of MRE11 by quantitative RT-PCR. We tested the cytotoxic activity of single-agent ABT-888 for 6 days in MSS and MSI cell lines, stratified by mutational status. Flow cytometry was performed after 24 hours. Results: Systems biology studies identified PARP inhibitors as a candidate compound relevant for MSI CRC. Mutational status of MRE11 was perfectly correlated with MSI status. ABT-888 shows a preferential activity on those MSI cell lines harboring mutations in both MRE11 and RAD50 genes compared to MSS cell lines (wild-type for both genes). A significant correlation exists between MRE11 expression levels and cytotoxicity to ABT-888 at 10 μM (R2=0.915, P<0.001). Flow cytometry analyses show a G1 arrest following to the treatment with ABT-888 that is higher in MSI cell lines with mutations in MRE11 and RAD50 compared to MSS cell lines. Conclusions: This is the first report of the preclinical activity of a PARP inhibitor in CRC models. MSI colorectal tumors deficient in DSB repair show a higher sensitivity to PARP inhibition. Further clinical investigation of ABT-888 as a single agent or in combination with other chemotherapy drugs inducing DSB is warranted in MSI CRC with mutations in MRE11 and RAD50. No significant financial relationships to disclose.