Mechanistic studies of enhanced in vitro radiosensitization and hypoxic cell cytotoxicity by targeting radiosensitizers to DNA via intercalation.

In an effort to increase the molar efficiency of 2-nitroimidazoles as hypoxic cell radiosensitizers and cytotoxins, they have been linked to a DNA intercalating group. The lead compound in this series, NLP-1, is a 2-nitroimidazole, linked at the one position to a phenanthridine ring system via a three carbon chain. Studies of the hypoxic cell specific radiosensitizing properties and hypoxic cell selective toxicity of NLP-1 toward CHO AA8-4 cells show the drug does have enhanced efficiency compared to the untargeted 2-nitroimidazole, misonidazole, based on external drug concentrations. To see if this enhanced efficiency was due to the proposed mechanism, targeting to DNA, or to a general increase in the intracellular concentration of the drug, its uptake and accumulation intracellularly were determined. Radioactive NLP-1 was synthesized labelled with 14C at the 2 position of the imidazole ring. Cells were incubated with the radioactive drug under aerobic and hypoxic exposure conditions, and intracellular levels of the drug were determined by a spin-through-oil technique. Results indicated that, at a drug concentration of 0.5 mM, there was no net accumulation of the drug over the external drug levels after aerobic exposure. Under hypoxic conditions, the drug did accumulate intracellularly, presumably because of hypoxia specific drug metabolism. Experiments with radioactive misonidazole labelled with 14C in the 2 position of the imidazole ring were run as controls. These results suggest that, under the conditions used, NLP-1 has an increased molar efficiency as a hypoxic cell radiosensitizer and cytotoxin, compared to misonidazole, based on intracellular drug concentrations.