A capillary GC-MS method for analysis of phenytoin and [¹³C₃]-phenytoin from plasma obtained from pulse dose pharmacokinetic studies¹

Stable isotope analogues of phenytoin are useful for pulse dose pharmacokinetic studies in epilepsy patients. A simultaneous assay was developed to quantitate phenytoin (5,5-diphenylhydantoin) and its stable isotope analogue [¹³C₃]-phenytoin (5,5-diphenyl-2,4,5-¹³C₃-hydantoin) from plasma. Quantitation was achieved by GC-MS analysis of liquid/liquid extracted plasma samples, with [²H₁₀]-phenytoin (5,5-di(pentadeuterophenyl)-hydantoin) as an internal standard. The total coefficients of variance (C.V.(t)) were <7% for phenytoin (2.5-40 μg ml^-1) and <10.3% for [¹³C₃]-phenytoin (0.1-6.0 μg ml^-1). The accuracy of the assay varied from 87.8-100.1% (phenytoin, 2.5-40 μg ml^-1) and 89.6-116.3% ([¹³C₃]-phenytoin, 0.02-6.0 μg ml^-1). The assay was tested under in vivo conditions by administration of a pulse dose of the stable isotope analogue to a single rat dosed to steady-state with fosphenytoin, a phenytoin prodrug. The results of the in vivo experiment demonstrate the usefulness of this assay for future pharmacokinetic studies in special population epilepsy patients. Copyright (C) 1998 Elsevier Science B.V.