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

Background: Quetiapine is one of the most frequently prescribed psychotropic drugs in the AGATE hospitals. It is metabolized by the CYP450 isoenzymes 2D6 and 3A4. When quantifying drug concentrations in psychiatric patients we see increased concentrations of various drugs that are metabolized via the same CYP isoenzymes and also increased levels of quetiapine when combined with different comedication. We developed an in vitro method to study the influence of citalopram, venlafaxine and mirtazapine as comedication to the concentration of quetiapine. Methods: Pooled human liver microsomes are incubated in various concentrations of 0.5 to 2.0mg protein/ml with dipotassiumhydrogenphosphatebuffer, NADPH Regenerating System, drug under study and the comedication under investigation (citalopram, venlafaxine and mirtazapine in various concentrations) at a temperature of 37°C. The reaction was stopped by precipitation of the protein at several times from 0 to 90min. After removing the HLM the concentrations of the drugs and its metabolites are measured by HPLC/UV-VIS. Results: 1 mg protein/ml showed up as good concentration to monitor CYP metabolism with the possibility to pursuit the changes in concentration of quetiapine and its metabolites. After 90min, the enzymes in the HLM loose their activity. In presence of an inhibiting comedication the metabolic turnover of the drug under study is reduced. The reaction velocity of quetiapine metabolism is reduced by up to 18% in combination with citalopram and up to 14% in combination with venlafaxine. In combination with mirtazapine the metabolism is incomplete. Incubation with recombinant CYP450 enzymes revealed that the metabolism of quetiapine was not affected by specific substrates or inhibitors of CYP2D6 but by mirtazapine which is a substrate of CYP1A2, CYP2D6 and CYP3A4. Conclusion: With this assay, we can monitor the metabolism of drugs over a time period of about 90min with constant enzyme activity. The observation of changes in the metabolic rate and velocity gives us information, which comedication is more or less suitable with quetiapine. Simultaneously we have the possibility to identify and when indicated avoid potentially dangerous or useful drug-drug-interactions between quetiapine and comedication.