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In this study, the basis for the diminished capacity of CYP2D6.17 to metabolise CYP2D6 substrate drugs and the possible implications this might have for CYP2D6 phenotyping studies and clinical use of substrate drugs were investigated in vitro. Enzyme kinetic analyses were performed with recombinant CYP2D6.1, CYP2D6.2, CYP2D6.17 and CYP2D6.T107I using bufuralol, debrisoquine, metoprolol and dextromethorphan as substrates. In addition, the intrinsic clearance of 10 CYP2D6 substrate drugs by CYP2D6.1 and CYP2D6.17 was determined by monitoring substrate disappearance. CYP2D6.17 exhibited generally higher K(m) values compared to CYP2D6.1. The V(max) values were generally not different except for metoprolol alpha-hydroxylation with the V(max) value for CYP2D6.17 being half that of CYP2D6.1. CYP2D6.1 and CYP2D6.2 displayed similar kinetics with all probe drugs except for dextromethorphan O-demethylation with the intrinsic clearance value of CYP2D6.2 being half that of CYP2D6.1. CYP2D6.17 exhibited substrate-dependent reduced clearances for the 10 substrates studied. In a clinical setting, the clearance of some drugs could be affected more than others in individuals with the CYP2D6(*)17 variant. The CYP2D6(*)17 allele might, therefore, contribute towards the poor correlation of phenotyping results when using different probe drugs in African populations. To investigate effects of CYP2D6(*)17 mutations on the structure of the enzyme, a homology model of CYP2D6 was built using the CYP2C5 crystal structure as a template. The results suggest an alteration in position of active-site residues in CYP2D6.17 as a possible explanation for the reduced activity of the enzyme.

The metabolism of praziquantel (PZQ) was studied in microsomes isolated from livers of differently pretreated rats and in the presence of various inhibitors of cytochrome P450 (P450) isoforms. Microsomes from phenobarbitone (PB)-pretreated rats metabolised PZQ to its major metabolite 4OH-praziquantel (4OH-PZQ) at a greater rate than those from 20-methylcholanthrene (MC) and saline (SA) pretreated rats. The Vmax for the PB microsomes was 600 nmol 4OH-PZQ formed/mg/min x 10(-3) compared to 91.4 nmol/mg/min x 10(-3) for MC and 238 nmol/mg/min x 10(-3) for SA microsomes. These results indicate that PZQ is metabolised by PB-inducible isoforms of P450. Inhibitor studies were conducted with microsomes from SA-pretreated animals. In these studies, caffeine, disulfuram, and tolbutamide were poor inhibitors of the metabolism of PZQ to 4OH-PZQ, with I50 values not determinable. Quinidine and quinine inhibited the hydroxylation of PZQ but with high Ki values. 17 alpha-Ethynylestradiol, cimetidine and diphenylhydramine were effective inhibitors of the formation of 4OH-PZQ, with 17 alpha-ethynylestradiol being the most potent with a Ki of 0.5 +/- 0.05 microM. From the known specificities of these P450 inhibitors, it is therefore concluded that cytochromes P450 1A2, 2E1, 2C9-10, and 2D6 probably do not contribute significantly to the metabolism of PZQ to its major metabolite in rats. It is likely that cytochromes P450 2B1 and 3A, both inducible by PB, are predominantly responsible for the formation of 4OH-PZQ.