Experimental study of pharmacokinetics of methotrexate and its metabolite (7-hydroxy-methotrexate) in tissue

The metabolism of methotrexate was investigated in rabbits with six hours infusion of (3'5'7-3H) methotrexate (50 mg/kg). Methotrexate and its metabolites were analyzed by reverse-phase high-pressure liquid chromatography and by counting of radioactivity. 7-Hydroxymethotrexate was found to be the major metabolite in the plasma, urine, bile and tissues of rabbits. Cumulative production of 7-Hydroxymethotrexate during the first six hours was 31.8% of the total dose. The 7-Hydroxymethotrexate concentration in plasma was reached to peak level in six hours after the initiation of the infusion, and was declined slower than that of methotrexate. The highest total radioactivity was found in the kidney after six hours, which was 7.24 times higher than that in plasma. Ratio of 7-Hydroxymethotrexate/methotrexate increased in the liver, small intestine, lung, kidney and testis. This data suggested that conversion of methotrexate to 7-hydroxymethotrexate took place in lung and kidney as well as in liver. (author)