Development of a radioimmunoassay procedure for sulforidazine and its comparison with a high-performance liquid chromatographic method.

A radioimmunoassay (RIA) procedure for the determination of sulforidazine in human plasma was developed using an antiserum raised in rabbits immunized with N-(2-carboxyethyl)desmethylsulforidazine-porcine thyroglobulin conjugate. The RIA procedure can measure as low as 40 pg of sulforidazine in a 200-microliter human plasma sample with a coefficient of variation of less than 6%. The antiserum did not show any marked cross reaction with any available potential cross reactant. Also, there were no significant differences between the concentrations of sulforidazine determined in the presence or absence of thioridazine and/or mesoridazine. This RIA procedure was compared with a high performance liquid chromatographic (HPLC) method by determining concentrations of sulforidazine in plasma samples from human volunteers over 72 h after administration of single 50-mg oral doses of thioridazine hydrochloride. The two methods showed a good correlation for the assay values (n = 55; r2 = 0.926) and the slope of the regression line was not significantly different from 1.0 (p greater than 0.30 when RIA values were plotted on the gamma-axis and HPLC values on the chi-axis; p greater than 0.30 when HPLC values were plotted on the gamma-axis and RIA values on the chi-axis). 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. Also, statistically favorable comparisons were obtained for area under the plasma concentration-time curve to 48 h (AUC48(0)), Cmax, and Tmax calculated from the data obtained by the two methods.