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The pharmacokinetics, distribution, and excretion of radioactivity and the main metabolites, M-1, M-2 and M-3, were studied in male rat after a single intravenous administration of 14C-labeled roxatidine acetate hydrochloride (TZU) at a dose of 10 mg/kg. The concentrations of these three metabolites in plasma, tissues and urine were simultaneously quantified with LC/MS-SIM using corresponding deuterium-labeled compounds as internal standards.1) The total radioactivity in the blood decreased with the half-lives of 0.38 and 3.82 hr in α-phase and β-phase, respectively. The plasma concentration of M-1, the active metabolite, decreased with the half-lives of 0.17 hr in α-phase and 0.77 hr in β-phase.2) At 15 min after administration, a high radioactivity was found in the liver, kidney, stomach, small intestine, parotid and submaxillary glands. At 72 hr after administration, the radioactivity was less than 0.1% of the dose or below the detection limit in any tissues.3) The metabolites in tissues were qualitatively analyzed by radio-HPLC. The radioactivity of the M-1 fraction was the highest in the brain, stomach, small intestine and kidney, then followed by that of the M-2 and M-3, the total radioactivity mainly consisted of those three fractions.4) The concentrations of three metabolites were quantified with LC/MS-SIM in the brain, lung, stomach, small intestine, liver, kidney and testis. All these metabolites disappeared rapidly, the half-lives were below 3.4 hr, in every tissue except for the testis. Although the radioactivity in the testis slowly decreased with the half-life of 10.5 hr, it accounted for M-2 and M-3, because they disappeared similarly to the radioactivity.5) In the target organs, stomach and small intestine, the concentration of M-1 was the highest, confirming that TZU has an advantage as an H2-receptor antagonist.6) Only in the liver, the level of the phenolic metabolite, M-3, was the highest of the three metabolites. This finding indicated that the metabolite might be mainly produced through oxidative dealkylation in liver.7) Approximately 87 and 8 % of the dosed radioactivity were excreted within 48 hr after administration in the urine and feces, respectively. The main metabolite in the urine was M-3, consisting of 29% of the dose, and about half of that was in the conjugated form. In feces, the excretion ratios of these three metabolites were almost equal, 0.3% of the dose.8) About 20% of the dosed radioactivity was excreted into bile in billiary fistular rats. The radioactivities in the M-1 and M-2 fractions were each 0.3% of the dose. The M-3 fraction comprised about 8%, almost all of which was in the conjugated form.

yakubutsu dōtai

Metabolic Fate of Roxatidine Acetate Hydrochloride. (9). Blood and Plasma Concentration, Tissue Distribution and Excretion in Male Rat after a Single Intravenous Administration.