In vitro metabolism of the new cardiotonic agent denopamine (TA-064) by rat and rabbit liver preparations. Oxidation, methylation, and glucuronidation.

The metabolic pathways of the cardiotonic agent denopamine, (-)-(R)-1-(p-hydroxyphenyl)-2-[(3,4-dimethoxyphenethyl)amino]ethanol, were studied in vitro with rat and rabbit liver preparations. 4'-O-Demethylated (M-1), 3'-O-demethylated (iso-M-1), and 3-hydroxylated (M-4) metabolites of denopamine were formed by incubation of denopamine with the rat liver microsomal fraction containing the NADPH-generating system. The ratio of M-1 to iso-M-1 formed in this system was 33:1. 3-Methoxydenopamine (M-2) and 3-hydroxy-4-O-methyldenopamine (iso-M-2) were formed via the catechol intermediate M-4, when denopamine was incubated with the rat liver 9000g supernatant fraction in the presence of the NADPH-generating system and S-adenosyl-L-methionine. The ratio of M-2 to iso-M-2 in this system was 7:1. Conversion of iso-M-2 to M-2, i.e. 4-O-demethylation followed by 3-O-methylation, but not vice versa, took place in this system. M-2 was demethylated at 4' to form M-3 by the above microsomal system. M-1 was not ring-hydroxylated by this system, excluding the metabolic route to M-3 via M-1. Denopamine, M-1, M-2, and M-3 were glucuronidated in vitro by the rabbit liver microsomal fraction. The glucuronides of denopamine and M-2 were conjugated at the 4-phenolic hydroxy group, and the glucuronides of M-1 and M-3, which possess two phenolic hydroxy groups, were preferentially conjugated at the 4'-hydroxy group. The order of the rates of in vitro glucuronidation was M-3 greater than M-1 greater than M-2 greater than denopamine.