Determination of acyclovir in horse plasma and body fluids by high‐performance liquid chromatography combined with fluorescence detection and heated electrospray ionization tandem mass spectrometry

Two methods are presented for the determination of ‘respectively’ the plasma protein unbound and total concentration of acyclovir in horse plasma and body fluids: first, a liquid–liquid extraction was performed on plasma, combined with HPLC‐fluorescence detection for the total plasma concentration; second a more sensitive method using high‐performance liquid chromatography combined with heated electrospray ionization tandem mass spectrometry (LC‐HESI‐MS/MS) was described for plasma and for body fluids analysis. To obtain the unbound concentration of acyclovir in plasma, a simple deproteinization step using a Microcon® filter was performed. Ganciclovir was used as an internal standard. Analysis was carried out on an Inertsil 5 ODS‐3 column for the HPLC‐fluorescence method. For the LC‐HESI‐MS/MS method a PLRP‐S column was used. The limit of quantification (LOQ) for the total concentration was set at 50 and 2 ng mL −1 for the HPLC‐fluorescence method and the LC‐HESI‐MS/MS method, respectively. The limit of quantification for the unbound concentration was set at 5 ng mL −1 and at 2 ng mL −1 for body fluids. The methods were successfully used to perform pharmacokinetic and clinical studies in horses after intravenous and oral dosage of acyclovir and its prodrug valacyclovir. Copyright © 2008 John Wiley & Sons, Ltd.