AUTOMATED ONLINE DIALYSIS, TRACE ENRICHMENT AND HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY INHIBITION OF INTERACTION WITH THE DIALYSIS MEMBRANEAND DISRUPTION OF PROTEIN-BINDING IN THE DETERMINATION OF CLOZAPINE IN HUMAN PLASMA

Problems related to interaction of drugs with the dialysis membrane an d to protein binding must be overcome in order to develop automated me thods for drug analysis based on on-line dialysis, trace enrichment an d HPLC. In order to study these problems, clozapine and its active met abolite N-desmethylclozapine were chosen as model compounds because th ey were found to interact with the dialysis membrane, and clozapine is highly protein bound. Addition of a cationic surfactant, dodecylethyl dimethyl ammonium bromide, to the donor solution and to the plasma sam ples was found to inhibit interaction of the drugs with surfaces. The protein binding in plasma was disrupted prior to dialysis by lowering the pH with hydrochloric acid and the plasma proteins were solubilised with glycerol. The results obtained were used to develop a fully auto mated method for the determination of clozapine and N-desmethylclozapi ne in human plasma. More than 100 samples could be analysed within 24 h. The limit of detection in human plasma was 0.050 mu mol/l for cloza pine and 0.055 mu mol/l for N-desmethylclozapine. Linearity was found for drug concentrations between 0.25-3 mu mol/l. The relative standard deviations were between 1.2-6.7% and the method was applicable for th erapeutic drug monitoring.