Total plasma concentrations, red blood cell concentrations, and radioreceptor assay values compared with unbound plasma concentrations of thioridazine and thioridazine metabolites in psychiatric patients.

Neuroleptic drug concentrations at the receptor sites are likely to be reflected more closely by the unbound than by the total plasma concentrations. The aim of this study was to establish whether or not the unbound plasma concentrations of thioridazine and its main nonconjugated metabolites show a stronger correlation to the red blood cell (RBC) concentration than to the total plasma concentration of the drug. The total and unbound plasma concentrations and the RBC concentrations of thioridazine and its metabolites were therefore determined in thioridazine-treated patients. "Calculated unbound concentration values" were derived from the determined total concentrations of the drug, the concentrations of drug-binding proteins, and previously determined kappa values. Since the RBC concentrations showed the best correlation to the unbound plasma values, they may be a more accurate tool than the total plasma concentrations for monitoring thioridazine treatment. The determined unbound plasma concentrations were better correlated to the calculated unbound concentrations than to the total plasma concentrations. The total plasma concentrations, but neither the unbound plasma nor the RBC concentrations, were significantly correlated to the concentrations of the drug-binding protein alpha 1-acid glycoprotein. Radioreceptor assay values were strongly correlated to the weighted sum of the total and unbound plasma concentrations of thioridazine and its metabolites.