Dose-dependence and related studies on the pharmacokinetics of misonidazole and desmethylmisonidazole in mice

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Abstract
An understanding of lipophilicity and pharmacokinetics is important in developing alternative radiosensitizers to misonidazole (MIS). Analogues more hydrophilic that MIS, including its O-demethylated metabolite DEMIS (Ro 05-9963), appear promising candidates. In vivo testing is usually carried out in mice, and the present paper reports dose-dependence and related studies on the comparative kinetics of MIS and DEMIS in this species. The data are consistent with a model in which MIS is eliminated mainly by metabolism, including demethylation to DEMIS, and saturable (non-linear) kinetics are exhibited. Apparent t1/2 increased with dose. But DEMIS is eliminated mainly by renal clearance exhibiting first-order (linear) kinetics. Phenobarbitone reduced the t1/2 and toxicity of MIS but not of DEMIS. SKF 525A increased the t1/2 of MIS. The following were not responsible for the non-linear kinetics of MIS: short-term enzyme induction by MIS; potent enzyme inhibition by the product DEMIS; decrease in body temperature by MIS; injection volume; and protein binding. For both MIS and DEMIS peak blood concentrations were about 2-fold lower for IP than IV injection. The IP bioavailability of DEMIS (1.07 mmol/kg) was 100%, but that of MIS (1 mmol/kg) was 80%, suggesting some first-pass metabolism. Both MIS and DEMIS were absorbed more slowly and gave lower peak blood concentrations after IP injection in a large (40 ml/kg) as against a small (10ml/kg) volume. Peak concentrations were lower for equimolar IP DEMIS, but this was less marked at lower doses. With both MIS and DEMIS, tumour/blood and brain/blood ratios were slightly increased at higher doses. Some kinetic differences were also observed between male and female mice. The above findings, particularly the major differences in elimination kinetics between MIS and DEMIS, should be considered in in vivo experiments with nitroimidazoles.