Abstract
The O-demethylation of dextromethorphan (DM) to dextrorphan (DR) is catalysed by the polymorphic CYP2D6 (cytochrome P4502D6) isozyme in man. DM is commonly used as a probe for phenotyping subjects as either poor or extensive metabolizers for the debrisoquine/sparteine oxidative polymorphism via CYP2D6. The enzyme kinetics of DM O- and N-demethylation, and the N- and O-demethylations of the primary metabolites DR and 3-methoxymorphinan (3MM), respectively, were studied in liver microsomes from female Dark Agouti (DA) rats, the poor metabolizer counterpart, and female Sprague-Dawley (SD) rats, the extensive metabolizer counterpart. The formation of metabolites was quantified by HPLC with fluorescence detection and kinetic parameters were calculated. The intrinsic clearance ( V max/ K m ) of the O-demethylation of 3MM to 3-hydroxymorphinan (3OHM) was 180-fold lower in DA rats (0.11 vs 20.77 mL/hr/mg) due to a 60-fold higher K m (108.7 vs 1.76 μM) and 3-fold lower V max (11.5 vs 35.95 nmol/mg/hr). The kinetics for DR N-demethylation to 30HM did not differ between rat strains. The Michaelis-Menten constant ( K m ) for DM N-demethylation to 3MM was similar between SD and DA rats (85.04 vs 68.99 μM); however, SD rats displayed a 2-fold higher V max (83.37 vs 35.49 nmol/mg/hr) and intrinsic clearance (0.96 vs 0.51 mL/hr/mg). The O-demethylation of DM to DR in SD rats showed a high and low affinity enzyme component, with the high affinity intrinsic clearance contributing 98% of the total intrinsic clearance in these rats. DM O-demethylation in DA rats was characterized by a single enzyme system. The high affinity O-demethylating enzyme in SD rats showed a 20-fold lower K m (2.5 vs 55.6 μM) and a three-fold higher V max (51.04 vs 16.84 nmol/ mg/ hr) resulting in a 66-fold higher intrinsic clearance (20.04 vs 0.31 mL/hr/mg) compared to DA rats. Quinine; dextropropoxyphene, (±)methadone and (±)propafenone were shown to be potent inhibitors of 3MM and DM O-emethylation but did not inhibit DR or DM N-demethylation at similar concentrations. SD and DA rats showed a clear strain difference in 3MM O-demethylation and DM O-demethylation. In contrast, DR N-demethylation and DM N-demethylation do not appear to be under genetic control in the female SD-DA rat model. Kinetic parameters and inhibition studies suggest that 3MM and DM O-demethylation pathways in the rat may be mediated by the same cytochrome P450 isozyme.
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