Comparison of high pressure liquid chromatography and fluorescence polarization immunoassay to assess quinidine pharmacokinetics

High pressure liquid chromatography (HPLC) and fluorescence polarization immunoassay (FPIA) were compared in a quinidine pharmacokinetic study. Six healthy male subjects received single oral doses of regular release (RR) quinidine sulfate, sustained release (SR) quinidine bisulfate and the same dose of the SR product with food (SR-F). Serum was collected for 49 h after each dose and analysed for quinidine by HPLC and FPIA. Using HPLC, there were no statistically significant differences between dosing regimens with respect to area under the curve (AUC) or terminal rate constant (K). The RR dose resulted in a higher peak plasma concentration (Cp) and shorter time to peak (Tp) than either of the SR doses (p less than 0.01). Food had no apparent effect on the bioavailability of the SR product. When using FPIA, food administration was found to increase the AUC for the SR product (p less than 0.05) and all three dosage regimens resulted in a different Cp and Tp (p less than 0.001). When comparing all pharmacokinetic parameters determined by each assay, FPIA resulted in a significantly lower K (p less than 0.01). Orthogonal regression of all serum quinidine concentrations showed that FPIA = 0.926 (HPLC) + 0.06 (r = 0.971, p less than 0.001). Analysis of quinidine concentrations in different concentration ranges revealed that FPIA overestimated HPLC for concentrations less than 1 micrograms ml-1 (p less than 0.001) and underestimated HPLC for concentrations greater than 2 micrograms ml-1 (p less than 0.01). Although the use of FPIA is appropriate for quinidine therapeutic drug monitoring, HPLC is the preferred assay method for assessing pharmacokinetic parameters in single dose bioavailability studies.