The metabolism of metiamide (1, Scheme l), the first orally active histamine H2-receptor antagonist, has already been described (Taylor, 1973). Cimetidine (2, Scheme 1) is a new H,-receptor antagonist described by Brimblecombe et al. (1975), in which the thioureido group of metiamide (1) is replaced by a cyanoguanidino group. The present communication reports some initial studies on the metabolism of cimetidine, labelled in the 2-position of the imidazole ring with either 3H or 14C in rat, dog and man. When cimetidine was given orally to male rats, 58 % of the dose (30mg/kg) of 14C was excreted in the urine in 2411, and approx. 50% of this was unchanged cimetidine; after intravenous dosing, 71 7; of 14C was excreted in the urine, 55 % of which was unchanged compound. Similarly, female rats excreted 64% of the 14C oral dose and 73% of the intravenous dose in the urine in 24h but metabolized the compound to a lesser extent, about 74% of the urinary radioactivity being present as unchanged drug. T.1.c. and radioautography showed the presence of unchanged compound together with one major and two minor metabolites. The difference in metabolism between male and female rats was due largely to the greater conversion into the major metabolite by the male rats.

The absorption, distribution, metabolism and excretion of [14C]oxmetidine in rat, dog and man has been studied following both i.v. and oral administration. Excretion is rapid and essentially complete in all three species. The biliary route is predominant. Distribution of radioactivity is widespread although none is seen in the brain. Metabolite patterns in urine from rat, dog and man have been compared by thin-layer chromatography. Metabolite patterns in urine and bile from rat and dog have been compared by high pressure liquid chromatography. Six major metabolites have been isolated and identified including two O-glucuronides and one N-glucuronide.

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