Quantitative analysis of naltrexone and 6beta-naltrexol in human, rat, and rabbit plasma by liquid chromatography-electrospray ionization tandem mass spectrometry with application to the pharmacokinetics of Depotrex in rabbits.

To improve the analysis of naltrexone and its primary metabolite 6beta-naltrexol, a sensitive and specific method for the analysis of subnanogram-per-milliliter concentrations of these analytes in human, rat, and rabbit plasma was developed utilizing liquid chromatography (LC) coupled to electrospray ionization (ESI) tandem mass spectrometry (MS-MS). Plasma samples were extracted utilizing a liquid-liquid extraction technique. Chromatographic separation was achieved using an isocratic solvent system consisting of dilute formic acid and methanol pumped through an ODS-AQ HPLC column. ESI-MS-MS was in the positive ion mode followed by collision-induced dissociation of the protonated molecular ions for naltrexone, 6beta-naltrexol, and their deuterated analogues. This method was validated using Good Laboratory Practice approved methods and was compared to an existing gas chromatography (GC)-MS method by analyzing plasma samples collected from a clinical study. Specificity determined from comparing blank plasma fortified with internal standard to samples fortified with internal standard and analyte at the lower limit of quantitation (LLOQ) from six different human, rat, and rabbit sources demonstrated sufficient signal-to-noise to set the LLOQ at 0.1 ng/mL. This assay has a quantitative range of 0.1-100 ng/mL. The inter- (human only) and intra-assay precision and accuracy in plasma varied by less than 13, 11, and 16% at the LLOQ for both analytes and by less than 10, 10, and 9% at higher concentrations for human, rat, and rabbit plasma, respectively. No loss of analyte was observed after 24 h of room temperature storage in human, rat, and rabbit plasma or three cycles of freezing and thawing of human plasma prior to extraction. Human samples that had been extracted were stable for at least five days when stored frozen at -20 degrees C or for at least two days when stored at room temperature on an autosampler. The GC-MS and LC-MS-MS methods correlated in the measured plasma concentrations of both naltrexone and 6beta-naltrexol. This method has been validated and subsequently used in the determination of the pharmacokinetics of Depotrex in rabbits. In rabbits, the parent compound shows dose-dependent pharmacokinetics as seen in humans, but rabbits have much lower unconjugated metabolite, 6beta-naltrexol, than that seen in humans.