Loss of DNA mismatch repair (MMR) is associated with cancer susceptibility and acquired resistance to chemotherapy following treatment of chemo-responsive tumors. In cell line models, the association with resistance to alkylating agents and cisplatin appears to be due to damage tolerance and reduced engagement of apoptosis following drug-induced DNA damage. We have used matched ovarian tumor cell lines that either express or lack MLH1 in a screen of an ICR/CRUK cancer therapeutics compound library to identify compounds that show preferential toxicity to drug resistant hMLH1 deficient tumor cells. Of 60,000 compounds in the primary screen 140 hits were identified and tested for selective toxicity in 2 independently derived MLH1 deficient cisplatin resistant variants of cell line A2780 (A2780/cp70 and A2780/mcp1). MMR201 was selected for further study since it retained selectivity between derivatives of A2780/cp70 with human chromosome 3 introduced and which differentially express MLH1. Chromosome 3 transfectant clone A2780cp70/A1 re-expresses MLH1, consistent with the location of hMLH1 on chromosome 3. Clone A2780cp70/A1 is more sensitive to cisplatin, but more resistant to MMR201 (IC50: cisplatin 0.7μM; MMR201 119nM) than clone A2780cp70/A2 which does not express MLH1 (IC50: cisplatin 2μM; MMR201 18nM). MMR201 sensitivities determined for a panel of 9 colon cell lines (IC50: 38 - 1500nM) show that lines that lack the MLH1 component of MMR are the most sensitive to MMR201 compared to lines that lack other components such as MSH2 or MSH6 or are proficient in MMR. Furthermore, a 2 fold increase in sensitivity was observed in MLH1-deficient HCT116 cells that also have a genetically inactivated p53 compared to matched p53 proficient HCT116 cells. However, MMR201 does not induce a G2/M cell cycle arrest suggesting that MMR201 does not induce DNA damage that is recognized by the G2/M cell cycle checkpoint pathways. It does induce apoptosis, as measured by cleavage of either PARP or cytokeratin 18, in a dose dependent manner in the ovarian cell line models. MMR201 (2mg/kg i.v.) shows inhibition of tumor growth at non-toxic doses against xenografts of MMR deficient HCT116 colon and A2780/cp70 ovarian tumors, both of which are resistant to the MTD for cisplatin. It is also active but less so against xenografts of cisplatin sensitive MMR proficient A2780. Thus, we show the potential of chemical genetics, with a cell phenotype based screen, to identify compounds which target tumor cells deficient in hMLH1 and consequent loss of MMR, a phenotype that has a central role both in tumor development and acquired drug resistance of cancer.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]