Enzyme assay to study the metabolism of quetiapine and the influence of psychotropic comedication on the concentration of quetiapine

Background: In our TDM-measurements we often see increased levels of quetiapine in combination whith comedication that is also metabolized via CYP2D6 and CYP3A4. We developed an in vitro method to study the influence of citaloprame, venlafaxine and mirtazapine to the concentration of quetiapine. Methods: Pooled human liver microsomes respectively recombinant CYP isoenzymes are incubated with K2HPO4-buffer (pH7.4), NADPH Regenerating System, quetiapine and citaloprame, venlafaxine or mirtazapine at 37°C. Reaction was stopped by adding acetonitrile at times from 0 to 270 min. After centrifugation the concentrations are measured by HPLC/UV-VIS. Results: 1[mg protein/ml] of HLM resulted as adequate concentration to monitor CYP metabolism. At this concentration the decrease and increase of substrate and metabolites were above the detection limit of our HPLC methods. After 270 min the enzymes loose activity. Maximum metabolic turnover is maintained for about 90 min. In presence of inhibiting comedication the metabolic turnover of quetiapine is reduced by up to 18% in combination with citaloprame and up to 14% with venlafaxine. Conclusion: With this assay we can monitor drug metabolism over about 90 min with constant enzyme activity. In most of the analysis in our TDM measurements there is more than one medication prescribed. Now we are able to study drug-drug-interactions between substances we consider to cause unexpected plasma concentration.