The kinetics of imidazopyrimidines selected for development were investigated in the rat, dog, and cynomolgus monkey using high pressure liquid chromatography (HPLC) measurements of levels. These data were compared with levels estimated by benzodiazepine radioreceptor binding assays and with the in vivo binding of the compounds to these receptors in the brain. RU 32698, RU 33203, and RU 33543 were found in relatively high peak levels in the plasma of rats (1-6-mu-g/ml at 10 min-1 hr after 10-100 mg/kg orally [p.o.]), dogs (4-12-mu-g/ml at 2 hr after 20 mg/kg p.o.), and monkeys (0.4-10-mu-g/ml after 150-500 mg/kg p.o.). HPLC analysis showed one further prominent peak for either RU 33203 or RU 33543 which had similar characteristics in all three species and may represent a common route of metabolism. In the rat these putative metabolites were at low levels in the brain when compared with parent levels. Nuclear magnetic resonance (NMR) and mass spectra of these isolated metabolites identified them as the hydroxymethyl derivatives on the oxadiazole ring. The synthesised metabolites showed two to six times less affinity for H-3-flunitrazepam-labelled benzodiazepine receptors, and the RU 33203 metabolite, RU 34149, did not displace in vivo benzodiazepine receptor binding after administration to mice, despite high plasma levels. This, coupled with little activity in several in vivo pharmacological models, suggests that this metabolite only poorly crosses the blood brain barrier. A pharmacological effect in the stress-induced ultrasounds model is consistent with this view as the rat pups used have a poor blood brain barrier. Daily dosing of low, pharmacologically effective doses of all three compounds (25 or 30 mg/kg p.o.) in rats showed little change in the plasma levels of parents after the 14th dose. There were slight increases in the levels of the metabolites of RU 33203 and RU 33543. Higher doses showed some decrease of parent levels on day 14 but a clear increase in the metabolite levels, particularly with RU 33543, suggesting enhanced metabolism on repeated dosing. No marked differences in overall plasma level time courses of parent or metabolites were observed when comparing the first or last (28-30 days) daily dose in monkeys (150-500 mg/kg p.o.).
