Polymorphism of dextromethorphan oxidation in South Indian subjects.

One hundred fifty-six unrelated healthy South Indian subjects were phenotyped according to their ability to metabolize dextromethorphan to its O-demethylated metabolite dextrorphan. Each volunteer was administered 25 mg oral dextromethorphan hydrobromide (19.3 mg dextromethorphan). Urine was collected during an 8-hour period after drug administration and was analyzed for dextromethorphan and dextrorphan by HPLC with fluorescence detection. This analysis was performed with and without previous deconjugation. The log10 (metabolic ratio), calculated as the ratio of dextromethorphan to dextrorphan, was bimodally distributed, and it was inferred that the frequency of occurrence of poor metabolizers of dextromethorphan in South Indian subjects is 3.2%. Phenotype assignment remained the same with both methods of analysis. Furthermore, a fairly good correlation (Spearman rank order correlation coefficient [r(s)] = 0.61; P < .0001) was observed between the log-transformed metabolic ratio derived from both methods.