Inhibition of poly(ADP-ribose) polymerase enhances cell death and improves tumor growth delay in irradiated lung cancer models.

PURPOSE

Poly(ADP-ribose) polymerase-1 (PARP-1) is the founding member of a family of enzymes that catalyze the addition of ADP-ribose units to proteins that mediate DNA repair pathways. Ionizing radiation induces DNA strand breaks, suggesting that PARP-1 inhibition may sensitize tumor cells to radiation.

EXPERIMENTAL DESIGN

We investigated the combination of PARP-1 inhibition with radiation in lung cancer models. ABT-888, a novel potent PARP-1 inhibitor, was used to explore the effects of PARP-1 inhibition on irradiated tumors and tumor vasculature.

RESULTS

ABT-888 reduced clonogenic survival in H460 lung cancer cells, and inhibited DNA repair as shown by enhanced expression of DNA strand break marker histone gamma-H2AX. Both apoptosis and autophagy contributed to the mechanism of increased cell death. Additionally, ABT-888 increased tumor growth delay at well-tolerated doses in murine models. For a 5-fold increase in tumor volume, tumor growth delay was 1 day for ABT-888 alone, 7 days for radiation alone, and 13.5 days for combination treatment. Immunohistochemical staining of tumor sections revealed an increase in terminal deoxyribonucleotide transferase-mediated nick-end labeling apoptotic staining, and a decrease in Ki-67 proliferative staining after combination treatment. Matrigel assay showed a decrease in in vitro endothelial tubule formation with ABT-888/radiation combination treatment, and von Willebrand factor staining of tumor sections revealed decreased vessel formation in vivo, suggesting that this strategy may also target tumor angiogenesis.

CONCLUSIONS

We conclude that PARP-1 inhibition shows promise as an effective means of enhancing tumor sensitivity to radiation, and future clinical studies are needed to determine the potential of ABT-888 as a radiation enhancer.