Quantification of dextromethorphan and metabolites: a dual phenotypic marker for cytochrome P450 3A4/5 and 2D6 activity.

A sensitive and selective liquid chromatographic procedure using fluorimetric detection was developed to quantify dextromethorphan (DTM), 3-methoxymorphinan (3MM), dextrorphan (DT), 3-hydroxymorphinan (3OH) and two internal standards, codeine (COD) and ethylmorphine (ETM), in urine. Precision and accuracy of the assay were determined over a concentration range of 5-3200 ng/ml urine for DTM, 5-400 ng/ml urine for 3MM, 400-40 000 ng/ml urine for DT and 200-16 000 ng/ml urine for 3OH, by assaying freshly prepared calibration standards and replicates of six quality control (QC) samples on separate days. All of the inter-day and intra-day coefficients of variation (C.V.s) were less than 20% except for a low QC for 3MM. The inter-day and intra-day accuracies were less than 20% for the low QCs, less than 15% for the medium QCs and less than 12% for the high QCs, for all compounds. The limit of quantification (LOQ) was 2 ng/ml urine for DTM and 3MM, 250 ng/ml urine for DT, and 100 ng/ml urine for 3OH. Absolute recovery was 76% for DTM, 74% for 3MM, 77% for DT, 46% for 3OH, 73% for ETM, and 57% for COD. The frequency distribution of the CYP2D6 metabolic ratio (DTM/DT) illustrated a bimodal distribution whereas, the CYP3A metabolic ratio (DTM/3MM) exhibited a unimodal distribution in overnight urine samples of volunteers who ingested 30 mg dextromethorphan hydrobromide. The CYP2D6 metabolic ratio significantly correlated with 3MM/3OH (r=0.82) and DTM/3OH (r=0.95) but did not correlate with the CYP3A metabolic ratio (r=0.27).