The ability to metabolize CYP2D6 substrates sparteine, debrisoquine, and dextromethorphan was studied in healthy Caucasian (n=20), Ghanaian (n=21), and Chinese (n=22) CYP2D6 extensive metabolizers. Genotype analysis for the CYP2D6*1, *3, *4, *5, *9, *10, and *17 alleles was performed. Interethnic differences in the disposition of the probe drugs were found among the extensive metabolizers; extensive metabolizer status was confirmed by phenotype and genotype analysis. The mean metabolic rate was lower for Caucasians than for Ghanaians for sparteine (P< 0.02) and for both Ghanaians and Chinese for debrisoquine (P< 0.02). Correlation comparisons resulted in lower pair wise correlation coefficients in Ghanaians compared with Chinese and Caucasians for every combination of probe substrates. In addition, in Chinese and Caucasians, metabolic rates for each pair of probe drugs were significantly correlated (P< 0.002), but in Ghanaians the dextromethorphan metabolic rates were not correlated to either sparteine or debrisoquine (P< 0.05). Even when only those with a CYP2D6*1 /*1 genotype were included in the correlation calculations, the Ghanaians had very low correlation coefficients (rs - 0.02-0.2, n=9); much lower than those found in Caucasian (rs 0.78-0.92, n=14) or Chinese (rs 0.54-0.96, n=7) individuals. Quinidine had significantly less affect on sparteine metabolic rates in Ghanaians than both Caucasians and Chinese (P<0.02). In addition, live of the 21 Ghanaian individuals had dextromethorphan metabolic ratios which were unaffected by quinidine. These individuals also had differences in urinary recovery of dextromethorphan and its metabolites when compared to the other Ghanaian individuals. These results confirm the large ethnic differences in probe drug metabolism and quinidine sensitivity among these ethnic groups. They also suggest that the Ghanaians have an additional unidentified allele(s) with altered substrate specificity and quinidine sensitivity which is currently genotyped as CYP2D6*1.
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