In vitro evaluation of the PARP inhibitor ABT-888 in combination with temozolomide for the treatment of pediatric leukemia

Abstract

9528 Background: The alkylating agent temozolomide exerts its primary cytotoxic activity through the addition to O6 methyl adducts to guanine residues in DNA, and temozolomide resistance results either from increased expression of methyl-guanine DNA methyltransferase (MGMT), which removes O6 adducts, or from mutations in the mismatch repair system (MRS), which lead to microsatellite instability (MSI). Temozolomide also creates N3 and N7 methyl adducts that are efficiently removed by the base excision repair (BER) system. PARP inhibitors block BER and may potentiate the cytotoxic effects of temozolomide. Methods: The cytotoxicity of temozolomide in combination with the PARP inhibitor ABT-888 was evaluated in vitro in leukemia cell lines and primary leukemia cells using the MTT assay. PARP activity was measured using a commercially available PARP assay. Results: ABT-888 effectively enhanced temozolomide cytotoxicity in cell lines with elevated MGMT (Jurkat and HSB2 T-cell ALL cell lines) or MRS deficiencies (Jurkat and Molt-4 T cell ALL). The temozolomide IC50 decreased from 450 μM to 35 μM at an ABT-888 concentration of 5 μM in the Jurkat T-cell ALL cell line and from 340 μM to 7.5 μM in the HSB2 T-cell line. ABT-888 also enhanced temozolomide cytotoxicity in non-T-cell leukemia subtypes, decreasing the IC50 in the JM1 pre-B ALL cell line from 51 μM to 7.7 μM and decreasing the IC50 from 316 μM to 22 μM in tumor cells obtained from a patient with pre-B ALL. PARP activity was also examined. In contrast, the sensitivity of U937 AML cells to temozolomide showed no effect of the addition of a PARP inhibitor. These results are consistent with the findings that these cells have low or undetectable MGMT and no MSI to suggest MRS mutations. Conclusion: These results suggest that ABT-888 may enhance the cytotoxic activity of temozolomide in leukemia patients whose tumors are resistant to temozolomide because of elevated MGMT expression and mismatch repair defects. No significant financial relationships to disclose.