Reductive Metabolism of the Nitroimidazole-Based Hypoxia-Selective Cytotoxin NLCQ-1 (NSC 709257)

The enzymatic cell-free metabolism of the novel hypoxia-selective cytotoxin 4-[3-(2-nitro-1-imidazolyl)-propylamino]-7-chloroquinoline hydrochloride (NLCQ-1) was investigated under hypoxic or aerobic conditions in the presence of purified reductive enzymes or isolated rat liver microsomes by monitoring the parent compound with HPLC-UV analysis. Enzymatic reduction of NLCQ-1 with isolated rat liver microsomes and NADPH or NADH showed that, only under hypoxic conditions, ca. 45% and 60% of the parent compound was reduced, respectively, within 1 h of incubation (37°C). Under identical conditions but in the presence of 2′-AMP (a P450 reductase inhibitor), 6-propyl-2-thiouracil or p-hydroxymercuribenzoate (cytochrome b₅ reductase inhibitors), NLCQ-1 reduction was inhibited. Enzymatic cell-free metabolism of NLCQ-1 with recombinant human DT-diaphorase (DTD) and NADPH or NADH under hypoxic or aerobic conditions showed that ≤5% of the compound was reduced within 2 h. Reduction kinetics with human P450 reductase-expressing microsomes showed ca. 75% or 50% reduction of NLCQ-1 under hypoxic or aerobic conditions, respectively, after 2-h incubation. These results suggest that DTD is not involved in the initial steps of the bioreductive metabolism of NLCQ-1, although it could be involved with metabolites of NLCQ-1, and that cytochrome P450 and cytochrome b₅ reductases play a significant role in the bioreductive metabolism of NLCQ-1.