Enantioselectivity in the Methylation of the Catecholic Phase I Metabolites of Methylenedioxy Designer Drugs and Their Capability To Inhibit Catechol-O-methyltransferase-Catalyzed Dopamine 3-Methylation

The designer drugs MDMA, MDEA, and MBDB are chiral compounds whose metabolism is enantioselective, favoring the S-enantiomer. This study investigated whether the elimination of their catecholamine metabolites via O-methylation by catechol-O-methyltransferase (COMT) is also enantioselective. Using human liver cytosol and microsomes, the S-enantiomers of all three catecholamines were preferentially O-methylated by both soluble and membrane-bound COMT. The membrane-bound COMT had 10-fold higher affinity for substrates, while the soluble form had 10-fold higher turnover rate. All tested catechols uncompetitively inhibited dopamine methylation. Enantioselective elimination may contribute to different pharmacokinetic properties of the enantiomers.

Study at a glance

Abstract

The designer drugs R,S-3,4-methylenedioxy-methamphetamine (MDMA, Ecstasy), R,S-3,4-methylenedioxy-ethylamphetamine (MDEA, Eve), and R,S-N-methyl-benzodioxolyl-butanamine (MBDB, Eden) are chiral compounds, and their in vitro and in vivo metabolism is enantioselective with a preference for the S-enantiomer caused in part by P450-mediated demethylenation. As the elimination of the catecholamine metabolites could also be enantioselective, the aim of the present study was to investigate the O-methylation to the corresponding methoxy derivatives catalyzed by the soluble or membrane-bound form of the catechol-O-methyltransferase (COMT). As all three compounds showed substrate inhibition effects during the incubation, their inhibition potential was quantified using the methylation of dopamine as a marker reaction. For investigation of the catechol-O-methylation catalyzed by the soluble form of the COMT (sCOMT), incubations with human liver cytosol (HLC) were performed. Human liver microsomes (HLM) were used for investigation of the membrane-bound form. For inhibition studies, 3-hydroxytyramine (dopamine) was incubated in HLC. The respective catechols were added at various concentrations to check whether they influence the methylation of 3-hydroxytyramine. Our data showed that the S-enantiomers of all studied catecholamines were preferably O-methylated by both types of COMT. Comparing the resulting kinetics of the HLC and HLM assays, the affinity for all substrates was 10-fold higher for the membrane-bound COMT, whereas the turnover rate was 10-fold higher for the soluble COMT. Uncompetitive inhibition of dopamine methylation could be observed for all tested catechols. In conclusion, elimination of the catecholamine metabolites of MDMA, MDEA, and MBDB was shown to be enantioselective and might therefore contribute to the different pharmacokinetic properties observed for both enantiomers. Furthermore, the catecholic metabolites were identified to be uncompetitive inhibitors of the sCOMT localized in HLC.