Inhibition of poly(ADP-ribose) polymerase (PARP) by a novel, potent inhibitor, INO-1001, was examined in two rodent and one human fibroblast cell lines, after single and fractionated radiation treatments. Since PARP plays a role in the early events following DNA damage and influences the effectiveness of DNA repair, its inhibition has been proposed to constitute a drug target for the development of novel radiosensitizers. We found that INO-1001 effectively inhibited PARP activity at non-cytotoxic concentrations. Combination treatment of 10 microM INO-1001 and a single dose of radiation resulted in significant radiosensitization of all three cells lines (enhancement ratios 1.4-1.6). This radioenhancement was even greater when the drug and radiation were given as fractionated treatments (enhancement ratio 8.0). Apoptosis (as evaluated by TUNEL staining) was not enhanced by the treatments, suggesting that inhibiting PARP enzyme activity by INO-1001 enhanced radiation-induced cell killing by interfering with DNA repair mechanisms, resulting in necrotic cell death. INO-1001 therefore, appears to have potential as a potent enhancer of radiation sensitivity, without any intrinsic cytotoxicity from the drug alone.