SIMULTANEOUS RAPID HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC DETERMINATION OF PHENYTOIN AND ITS PRODRUG, FOSPHENYTOIN IN HUMAN PLASMA AND ULTRAFILTRATE

A reversed-phase high-performance liquid chromatographic assay for the simultaneous determination of phenytoin and fosphenytoin, a prodrug f or phenytoin, in human plasma and plasma ultrafiltrate is described. F or plasma, the method involves simple extraction of drugs with diethyl ether and evaporation of solvent, followed by injection of the recons tituted sample onto a reversed-phase C-18 column. Plasma ultrafiltrate is injected directly into the HPLC column. Compounds are eluted using an ion-pair mobile phase containing 20% acetonitrile. The eluent is m onitored by UV absorbance at 210 nm. The fosphenytoin standard curves are linear in the concentration range 0.4 to 400 mu g/ml for plasma an d 0.03 to 80 mu g/ml for ultrafiltrate. Phenytoin standard curves are linear from 0.08 to 40 mu g/ml for plasma and from 0.02 to 5.0 mu g/ml for ultrafiltrate. No interferences with the assay procedure were fou nd in drug-free blank plasma or plasma ultrafiltrate. Relative standar d deviation for replicate plasma or ultrafiltrate samples was less tha n 5% at concentrations above the limit of quantitation for both within - and between-run calculations.