Multiple human cytochromes contribute to biotransformation of dextromethorphan in-vitro: Role of CYP2C9, CYP2C19, CYP2D6, and CYP3A

Cytochromes mediating the biotransformation of dextromethorphan to dextrorp han and 3-methoxymorphinan, its principal metabolites in man, have been stu died by use of liver microsomes and microsomes containing individual cytoch romes expressed by cDNA-transfected human lymphoblastoid cells. In-vitro formation of dextrorphan from dextromethorphan by liver microsomes was mediated principally by a high-affinity enzyme (K-m (substrate concent ration producing maximum reaction velocity) 3-13 mu M). Formation of dextro rphan from 25 mu M dextromethorphan was strongly inhibited by quinidine (IC 50 (concentration resulting in 50% inhibition) = 0.37 mu M); inhibition by sulphaphenazole was approximately 18% and omeprazole and ketoconazole had m inimal effect. Dextrorphan was formed from dextromethorphan by microsomes f rom cDNA-transfected lymphoblastoid cells expressing CYP2C9, -2C19, and -2D 6 but not by those expressing CYP1A2, -2E1 or -3A4, Despite the low in-vivo abundance of CYP2D6, this cytochrome was identified as the dominant enzyme mediating dextrorphan formation at substrate concentrations below 10 mu M. Formation of 3-methoxy-morphinan from dextromethorphan in liver microsomes proceeded with a mean K-m of 259 mu M. For formation of 3-methoxymorphinan from 25 mu M dextromethorphan the IC50 for ketoconazole was 1.15 mu M; sul phaphenazole, omeprazole and quinidine had little effect. 3-Methoxymorphina n was formed by microsomes from cDNA-transfected lymphoblastoid cells expre ssing CYP2C9, -2C19, -2D6, and -3A4, but not by those expressing CYP1A2 or -2E1, CYP2C19 had the highest affinity (K-m = 49 mu M) whereas CYP3A4 had t he lowest (K-m = 1155 mu M). Relative abundances of the four cytochromes we re determined in liver microsomes by use of the relative activity factor ap proach. After adjustment for relative abundance, CYP3A4 was identified as t he dominant enzyme mediating dominant enzyme mediating 3-methoxymorphinan f ormation from dextromethorphan, although CYP2C9 and -2C19 were estimated to contribute to 3-methoxymorphinan formation, particularly at low substrate concentrations. Although formation of dextrorphan from dextromethorphan appears to be suffi ciently specific to be used as an in-vitro or in-vivo index reaction for pr ofiling of CYP2D6 activity, the findings raise questions about the specific ity of 3-methoxymorphinan formation as an index of CYP3A activity.