PARP inhibitors in combination with external beam and radioimmunotherapy in aggressive lymphoma

Objectives The enzyme poly(ADP-ribose) polymerase-1 (PARP-1) represents an important novel target in cancer therapy. Due to its role as a repair enzyme for single strand breaks it might be an interesting new adjunct to radiation or radioimmunotherapy.

Methods EBV-infected human Raji lymphoma cells with lentivirally transfected GFP in log-phase growth were incubated with various doses of AZD-2281 and ABT-888 24h prior to exposure to external-beam radiation. A 500nM concentration of AZD-2281 and ABT-888 was used to assess the growth curve of the Raji lymphoma cells over 5 days. Visual control of the amount of double stranded breaks was investigated using a H2AX antibody and confocal microscopy. The intracellular PARP activity was measured 24h after incubation with 500nM AZD-2281 and ABT-888 using a colorimetric PARP assay kit. The radiosensitizing effect of 500nM AZD-2281 with various doses of 131I-tositumomab was assessed after 24 and 72h.

Results 500nM AZD-2281 and 500nM ABT-888 in combination with 0, 4, 8 and 12 gray of external beam radiation showed a 5.2%, 7.1%, 10.1% and 33.1% resp. 7.3%, 9.8%, 5.7% and 9.4% reduction in cell viability. 500nM AZD-2281 and ABT-888 significantly reduced the percentage of viable cells at d3 to d5. The maximal relative reduction, 78.5% of viable cells, occurred with 500nM AZD-2281 at day 5. AZD-2281 revealed more double strand breaks with confocal microscopy than ABT-888. 24hours after incubation of Raji cells with 500nM AZD-2281 or ABT-888 the colorimetric PARP activity assay shows a reduction of 30.36% with ABT-888 and 47.8% with AZD-2281. Combining 500nM AZD-2281 with 0, 5, 10 and 20µCi of 131I-tositumomab reveals a significant reduction in cell viability after 24 hours with 5µCi (p<0.0001) and with 5µCi, 10µCi and 20µCi after 72 hours (p<0.0001).

Conclusions The PARP inhibitors AZD-2281 and ABT-888 are highly radiosensitizing agents in combination with external beam radiation and 131I-tositumomab