4-amino-1,8-naphthalimide: a novel inhibitor of poly(ADP-ribose) polymerase and radiation sensitizer

Purpose: Poly(ADP-ribose) polymerase (PARP; EC 2.4.2.30) is a chromatin-bou nd enzyme which is known to regulate chromatin structure by poly(ADP-ribosy l)ation of nuclear proteins, to facilitate DNA base excision repair, and to contribute to cellular recovery following DNA damage. Because inhibitors o f PARP are able to potentiate the cell-killing effects of some DNA-damaging agents and to inhibit the repair of induced DNA strand breaks, such compou nds may enhance the anti-tumour efficacy of radiotherapy or cytotoxic drug treatment. The PARP-inhibitory effects and radiosensitization of a new comp ound, 4-amino-1,8-naphthalimide (ANI), were examined. Materials and methods: The inhibition of radiation-induced poly(ADP-ribosyl )ation (50 Gy; Co-60 gamma-radiation) was evaluated by immunofluorescence a ssay using MoAb 10H directed against poly(ADP-ribose). Cell survival was as sessed by colony forming assay (CFA) to determine the cytotoxicity of radio sensitization potential in exponentially growing hamster lung fibroblasts ( V79), rat prostate carcinoma (R3327-AT1) and human prostate carcinoma (DU14 5) cells. Results: At concentrations above 30 nmol dm(-3) ANI, radiation-induced poly (ADP-ribose) was not detectable by immunofluorescence in V79, AT1 and DU145 cells. At the highest concentration tested for chronic exposure (20 mu mol dm(-3)), ANI was not cytotoxic and significantly potentiates the cytotoxic ity of gamma-irradiation. The level of radiation enhancement was directly p roportional to drug concentration. Survival curves for the three cell lines using 20 mu mol dm(-3) ANI revealed sensitizer enhancement ratios of 1.3 f or V79, 1.5 for AT1 and 1.3 for DU145. Conclusions: In living cells, ANI is about 1000-fold more potent at inhibit ing PARP activity compared with 3-aminobenzamide (3-ABA). CFA studies demon strated that ANI is a radiation sensitizer at non-toxic and lower concentra tions (20 mu mol dm(-3)) than 3-ABA (10 mmol dm(-3)).