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.

Introduction: Not only the hepatic cytrochrome-P-450 enzymes, but also UGT (UDP-glucuronosyltransferase) and PGP (P-glycoprotein) are involved in the metabolism of many frequently prescribed drugs. Most of the drugs are metabolized by multiple enzymes, the metabolism might be affected by individual factors like hepatic metabolism or gender. Therapeutic drug monitoring is an essential tool to monitor drug therapy in clinical practise, but interpretation of individual drug-drug-interactions is only possible if as many metabolic pathways as possible are known. The existing sources of hepatic drug interactions contradict themselves partially, references are often missing. Methods: A comprehensvie review of the existing literature of drug metabolism of cytochrome-p-450-enzymes, UGT and PGP was performed, the results were summarised in an drug-interaction table. This table and the corresponding references were integrated to a special computer program (Konbest®) to interpret therapeutic drug monitoring (TDM) automatically. Results: More than 2000 references were found. Therapeutic drug monitoring in clinical practise was extended by a drug-interaction table, which demonstrates possible drug-interactions of each individual medication. Conclusion: Interpreting TDM by using automatically generated drug-interaction tables may help to increase drug safety in clinical practise.

In former years it was generally accepted to use therapeutic drug monitoring (TDM) to prevent toxic side effects of drugs with a narrow therapeutic window. Very often it was just used to confirm that a side effect that had already occurred was due to an elevated drug plasma concentration. For this purpose the quantified drug concentration was related to the therapeutic reference range of the drug. We relate the drug concentration to both the therapeutic reference range and the dose-related reference range. Dose-related reference ranges are calculated according to the mathematical equation De = Clt * c, where De is the maintenance dose and Clt the total clearance taken as x ± SD from phase II trials of the drug. A drug concentration outside this range is taken as a signal that the patient does not belong to the study population because of a comedication or a pharmacologically active food or drug component (drug-drug-interaction), an age below 18 years (children), an age above 65 years (old people), a genetically determined alteration in drug metabolism (fast/slow metabolizers), diseases in the elimination organs, or non-compliance. KONBEST, an internet platform that is programmed to allow clinical pharmacological commenting of drug concentrations in TDM, contains the relevant pharmacological data to calculate among others the dose-related reference ranges. By this approach we are able to prevent adverse drug reactions before the drug concentration is high enough to cause them.

Introduction: Citalopram is one of the most frequently prescribed selective serotonin reuptake-inhibitors (SSRI) in the AGATE hospitals (Arbeitsgemeinschaft Arzneimitteltherapie bei psychiatrischen Erkrankungen). It is metabolized by the cytochrome-P450 isoenzymes (CYP) 2C19, 2D6 and 3A4. Drug-drug interactions are rarely observed in clinical routine under citalopram and the substance is known not to influence the activity of CYP-isoenzymes. However, when quantifying drug concentrations in psychiatric patients we do see increased concentrations of various drugs that are metabolized via the same CYP isoenzymes. We therefore developed an in vitro method to study the influence of citalopram in comedication on the concentration of those drugs. Methods: Pooled human liver microsomes were incubated in various concentrations of 0.5 to’2.0 mg protein/ml with dipotassiumhydrogenphosphatebuffer (0.1 M, pH7.4), NADPH regenerating system (containing NADP+, glucose-6-phosphate, MgCl2 and glucose-6-phosphatedehydrogenase), drug under study (quetiapine in various concentrations from 250 to 500 ng/ml) and the comedication under investigation (citalopram in various concentrations from 50 to 500 ng/ml) at a temperature of 37°C. Starting at 0 min (baseline) the reaction was stopped by adding acetonitrile to precipitate the protein at multiple incubation times (0, 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, 165, 180, 195, 210, 225, 240, 255 and 270 min). After removing the human liver microsomes by centrifugation the concentrations of the drugs and their metabolites were measured by an HPLC online method with UV-VIS detection. Results: In these experiments, 1thinsp;mg protein/mL of human liver seems likely to to be a good concentration to monitor CYP metabolism. At this concentration, the decrease of the substrate and the increase of the metabolites could be kept in a concentration range above the limit of detection of our HPLC methods. After a time period of 270 min the monitoring of the reaction is stopped, because the enzymes in the human liver microsomes loose their activity. Maximum metabolic turnover was maintained for about 90 min and then decreases to 0 ng/ml/min/mg protein. In presence of an inhibiting comedication the metabolic turnover of the drug under study was reduced. The dimension of inhibition depended on the divergence in affinity between the drugs. Conclusion: With this assay, we could monitor the metabolism of drugs over a time period of about 90 min. In most of the analysis we were quantifying in our TDM measurements there was more than one medication prescribed. Now, we are able to study drug-drug-interactions between substances probably responsible to cause an unexpected plasma concentration.

Interpreting an individual patient's drug concentration in relation to a therapeutic and a dose-related reference range is suitable for controlling compliance, lack of clinical response, adverse effects at recommended doses, drug interactions and genetic variations of metabolism. Therefore it is an important contribution to complement pharmacovigilance programs. [1–2]

Aim: A novel high performance liquid chromatography(hplc)assay for the purification of rivastigmine from serum, its detection and estimation has been developed and validated. Methods: A solid phase extraction using Oasis HLB-columns(Waters Ltd,Ger)was optimized to purify rivastigmine from serum followed by HPLC separation with UV detection. Chromatographic analyses were performed on a Dionex system with a Phenomenex Luna Phenyl-Hexyl analytical column. The mobile phase constituted of 0,02mol/l K2HPO4/acetonitrile(80/20)and the flow rate was kept at 0,4ml/min. The detection wavelength was kept at 210nm. Results: The retention time was 10,5min for rivastigmine. Our method recovered >92% of rivastigmine from the serum samples. The calibration curve was linear (r=0,9999, n=9) over rivastigmine concentrations ranging from 5 to 640ng/ml. No endogenous compounds were found to interfere with the analyte. The method had an accuracy of >90%. Intra- and interday precision were <5% and <3%, resp., at three different concentrations of 5, 80 and 640ng/ml. The limit of quantification(LOQ) was found to be 2,5ng/ml. The accuracy and precision at the LOQ level are in agreement with the guidelines of GTFCh(Society of Tox. and Forensic Chemistry) in consideration of ISO 5725(International Organization for Standardization). The method reported here is simple, reliable, precise, and accurate and has the capacity to be used for detection of rivastigmine in serum samples in routine analysis.