MICROSOMAL CODEINE N-DEMETHYLATION - COSEGREGATION WITH CYTOCHROME P45O3A4 ACTIVITY

Codeine is metabolized by glucuronidation, by O-demethylation to morph ine, and by N-demethylation to norcodeine. The enzyme responsible for the O-demethylation to morphine has been identified as cytochrome P450 2D6 (CYP2D6). The purpose of the present study was to identify the spe cific P450 enzyme responsible for codeine N-demethylation. Microsomal preparations (250 pmol of P450) obtained from 12 human liver donors we re incubated with 20 mu M codeine and analyzed for norcodeine formatio n. Codeine N-demethylation activity was linearly correlated with nifed ipine oxidation activity (r = 0.90, p <0.001), a marker of CYP3A4, but not with codeine O-demethylation, a marker of CYP2D6. Preincubation w ith troleandomycin (50 mu M), or gestodene (50 mu M) inhibitors of CYP 3A4, decreased the rate of production of norcodeine by 60 and 45% comp ared to control values, respectively. Similarly, ketoconazole (10 mu M ) and erythromycin (10 mu M) inhibited codeine N-demethylation by 75 a nd 35%, respectively. In contrast, the presence of quinidine, sulfaphe nazole, or diethyldithiocarbamate in the incubation mixture had no eff ect on norcodeine formation. Preincubation with antibodies raised to C YP3A4 (5 mg IgG/nmol P450) caused 96% inhibition of norcodeine product ion, whereas preimmune IgG or antibodies raised to CYP2A6 and CYP2C ha d no effect. Additionally, significant norcodeine production was obser ved with purified CYP3A4 derived from human liver microsomes. In concl usion, codeine N-demethylation activity cosegregates with CYP3A4 activ ity. Coadministration of codeine with selective inhibitors of CYP3A4 m ay result in increased morphine production and enhanced pharmacodynami c effects due to shunting down the CYP2D6 pathway.