Stereoselectivity in trans-tramadol metabolism and trans-O-demethyltramadol formation in rat liver microsomes

AIM: To study the stereoselectivity in trans-tramadol [(+/-)-trans-T] metabolism and trans-O-demethyltramadol (M1) formation. METHODS: (+)-, (-)-, Or (+/-)-trans-T was separately incubated with rat liver microsomes in vitro. The concentrations of (+/-)-trans-T and M1 enantiomers were determined by high performance capillary electrophoresis (HPCE). RESULTS: When each enantiomer of (+/-)-trans-T was incubated with rat liver microsomers, the metabolic rate of (+)-trans-T was lower than that of (-)-trans-T. The kinetics of (+)-, (-)-M1 formation was found to fit the single-enzyme Michaelis-Menten model. The V-max and CLint of (-)-M1 formation were lower than those of (-)-M1 formation. When (+/-)-trans-T was used as the substrate, the metabolic rates of (+)-, (-)-trans-T, and the formation rates of (+)-M1, (-)-M1 decreased to different extents. Dextromethorphan (Dex), propafenone (Pro), and fluoxetine (Flu) could inhibit both the metabolism of (+/-)-trans-T enantiomers and the formation of M1 enantiomers. Pro and Flu were shown to enhance the stereoselectivity in both (+/-)-trans-T metabolism and M1 formation, and Dex could only enhance that in M1 formation. CONCLUSION: (+/-)-Trans-T metabolism and M1 formation were stereoselective, (-)-trans-T being preferentially metabolized and (-)-M1 being preferentially formed. There was interaction in metabolism between (+/-)-trans-T enantiomers. Dex, Pro, and Flu had different effects on the stereoselectivity.