Indices and graphical approaches for the detection of interindividual and interethnic variations in codeine metabolism

Aims To evaluate the use of different graphical methods and statistical tests in the detection of interindividual and interethnic variations in codeine metabolism. Various urinary metabolic ratios (MR) for codeine O -demethylation were also compared for their ability to determine phenotype.
Methods Frequency histograms, normal test variable (NTV) plots and admixture analysis were used to examine the distributions of the urinary MRs for codeine O -demethylation, N -demethylation and glucuronidation in 132 Caucasian and 222 Chinese subjects.
Results In the Caucasian population, apparent bimodality was shown in both a frequency histogram and NTV plot of the log MR of codeine O -demethylation (codeine/(morphine (M)+M-3 and M-6-glucuronide (M3G and M6G)+ normorphine (NM)). Admixture analysis confirmed the co-segregation of codeine O -demethylation and debrisoquine hydroxylation. The antimode for the codeine O -demethylation MR between extensive and poor metabolisers was located between 5.5 and 8.3. A simplified MR for codeine O -demethylation (codeine/M3G) demonstrated a similar correlation with the debrisoquine MR to the more complex MR, allowing a simplification of the analytical method for phenotyping. The Chinese population had significantly higher median MRs for codeine N -demethylation, O -demethylation and glucuronidation, which was shown clearly in the frequency histograms, but not in the NTV plots.
Conclusion A histogram seems preferable over a NTV plot for assigning phenotype using the codeine O -demethylation MR, because it is clear and simple. Interethnic difference in the metabolism of codeine are also better visualised from the histograms.