Carbonyl Reduction of Bupropion in Human Liver Cytosol

Human carbonyl reductases are mainly cytosolic phase I drug metabolizing enzymes (e.g. carbonyl reductase, aldo‐keto reductase) that catalyze the reduction of numerous clinically relevant drugs. Patients with pharmacogenetic variations in carbonyl reductase (CR) have suffered serious adverse consequences when administered drugs that are metabolized by these enzymes (e.g. anthracyclines). To date, a phenotypic probe substrate for CR (analogous to dextromethorphan and CYP2D6) is unknown. A novel probe substrate for CR may be bupropion, which is reduced to two diastereomers: erythrohydrobupropion (EBUP) and threohydrobupropion (TBUP). Although unknown at this time, we speculate that CR catalyzes bupropion reduction in human liver. Hence, the major objective of this study was to examine the ketone reduction of bupropion in human liver cytosol (HLC). The formation of EBUP and TBUP from bupropion was measured using a mixture of bupropion (0.1 mM to 1.0 mM), NADPH, phosphate buffer and HLC. Reaction mixtures were incubated and analyzed by high performance liquid chromatography (HPLC). The mean rate of formation (±s.d.) of EBUP and TBUP from bupropion in fresh cytosol was 2.27 (0.82) pmole/mg/min and 17.4 (0.35) pmole/mg/min, respectively. No metabolite formation was observed in reactions containing boiled cytosol. Bupropion reduction to EBUP and TBUP was significantly decreased by the presence of the selective carbonyl reductase (CR) inhibitor menadione (100 μM), but not by the selective aldo‐keto reductase (AKR) inhibitor flufenamic acid (10 μM). To summarize, bupropion is metabolized to TBUP (and to a lesser amount EBUP) in HLC by CR, and shows promise as a novel probe substrate for identifying pharmacogenetic variations in CRs.