High-performance liquid chromatographic analysis of diflunisal in plasma and urine: application to pharmacokinetic studies in two normal volunteers.

A high-performance liquid chromatographic (HPLC) assay with fluorescence detection has been developed for the determination of diflunisal in plasma and urine. The plasma or urine, containing naproxen as the internal standard, was extracted with ether-hexane (1:1). The samples were analyzed on a microparticulate column, and the compounds were eluted using a mobile phase of 0.05 M phosphate buffer (pH 3) and methanol. Plasma samples were analyzed from two healthy male subjects who received a 250- and 750-mg oral dose of diflunisal 3 weeks apart. The data were analyzed according to a two-compartment open model. There was a disproportionate increase in the area under the plasma concentration-time curves (AUC 750 mg/AUC 250 mg was 3.84 for subject A and 4.22 for subject B) and a reduction in plasma clearance after the 750-mg dose of diflunisal. These data suggest that the kinetics of diflunisal may be dose dependent.