Radiosensitization of hypoxic tumour cells by S-nitroso-N-acetylpenicillamine implicates a bioreductive mechanism of nitric oxide generation

The radiosensitizing activity of S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide (NO) donor, was assessed in a model of non-metabolic hypoxia a chieved in an atmosphere of 95% nitrogen-5% carbon dioxide. A 10 min preinc ubation of hypoxic EMT-6 cells (10 x 10(6) ml(-1)) with 0.1 and 1 mM SNAP b efore radiation resulted in an enhancement ratio of 1.6 and 1.7 respectivel y. The level of spontaneous NO release, measured by a NO specific microsens or, correlated directly with the concentration of SNAP and was enhanced 50 times in the presence of cells. Dilution of the cell suspension from 10 to 0.1 x 10(6) ml(-1) resulted in a 16-fold decline in NO release, but only a twofold decrease in radiosensitization was observed. Preincubation of hypox ic cells with SNAP for 3 min up to 30 min caused an increasing radiosensiti zing effect. Extended preincubation of 100 min led to the loss of radiosens itization although the half-life of SNAP is known to be 4-5 h. Taken togeth er, these observations suggest that SNAP generates NO predominantly by a bi oreductive mechanism and that its biological half-life is unlikely to excee d 30 min. The lack of correlation between free NO radical and radiosensitiz ing activity may reflect a role of intracellular NO adducts which could con tribute to radiosensitization as well.