A new radioimmunoassay of thioridazine and its comparison with a high performance liquid chromatographic method and another radioimmunoassay.

A new radioimmunoassay (RIA) procedure for the determination of thioridazine in plasma is described. Antiserum was produced in rabbits immunized with N-(2-carboxyethyl)desmethylthioridazine-protein conjugate. The developed RIA procedure can measure as low as 80 pg of thioridazine in a 200-microliter human plasma sample with a coefficient of variation of less than 5%. This RIA procedure was compared with previously reported RIA and high performance liquid chromatographic (HPLC) methods by determining plasma concentrations of thioridazine in samples from human volunteers over 72 h after administration of single 50-mg oral doses of thioridazine hydrochloride. There was a good correlation between the assay values (n = 55) determined by the new RIA and HPLC methods (r2 = 0.916), and the slope of the regression line was not significantly different from 1.0 (p greater than 0.60, 95% confidence limits 0.981 +/- 0.081 when RIA values were plotted on the y axis, and HPLC values on the x axis; p greater than 0.10, 95% confidence limits 0.934 +/- 0.080 when HPLC values were plotted on the y axis and RIA values on the x axis). Also, the plot of the differences between these two assay values against the average of the assay values showed that the differences were independent of the concentration range studied. Similar favorable statistical comparisons were obtained when the assay values (n = 44) determined for thioridazine by the two RIA procedures were compared with one another.