Isotope Effects During Metabolism of (±)-Tramadol Isotopomers by Human Liver Microsomes

Tramadol (T), racemic 1(e)-(m-methoxyphenyl)-2(e)-(dimethylaminomethyl)-cyclohexane-1(a)-ol is an effective analgesic drug. Metabolites were formed by O- and N-demethylation. Six deuterated tramadol isotopomers have been synthesized; their kinetic isotope effects in oxidative demethylation reactions were investigated in vitro using human liver microsomes. In comparison to unlabelled (±)-tramadol, (±)-T-OCD3 and (±)-T-D9 displayed an unequivocal (< 3), (±)-T-ND6 and (±)-T-ND3 a noticeable, and (±)-T-cyclohexyl-D3 as well as (±)-T-15ND2 no measurable isotope effect. Metabolic switching (favoring the N-demethylation) was observed only after incubation of a tramadol with a trideuterated methoxy group. Additional N-CD3-groups prevented this metabolic switching. Metabolic switching favoring the O-demethylation was not observed. The isoenzyme responsible for the O-demethylation was always saturated under the experimental conditions required to detect the metabolites. The two tramadol isotopomers containing deuterium in metabolic inert positions (other than the methyl groups, i.e., (±)-T-15ND2 and (±)-T-cyclohexyl-D3) were expected to display no isotope effects. This expectation could be verified in this study.