Pharmacokinetics and metabolism study of firocoxib in camels after intravenous administration by using high-resolution bench-top orbitrap mass spectrometry.

In this study, we developed a high-resolution liquid chromatography mass spectrometry method for the pharmacokinetic study of firocoxib followed by full method validation. Following a solid-phase extraction, the firocoxib and internal standard (celecoxib) were separated on an Agilent Zorbax ZDB C18 column (50 mm × 2.1 mm i.d., 3.5 μm) with a gradient elution using methanol and 0.1% aqueous formic acid. Data acquisition was performed at 25,000 resolution with the automatic gain set to 1,000,000 and the maximum injection time of 100 ms. Data were acquired in full-scan mode over a mass range of 100-550 Da in positive electrospray mode. Linear calibration curves were obtained over the concentration ranges of 0.5-200 ng/mL and no interfering peaks were detected at the retention time of firocoxib and internal standard in blank camel plasma samples. The mean extraction recoveries of firocoxib at three concentrations of 5, 25 and 75 ng/mL ranged from 92 to 104%. Coefficient of variation of intra-day and inter-day precision were both <10%. The accuracy of the method ranged from 95 to 107%. The validated method was then successfully applied in evaluating the pharmacokinetics and metabolism of firocoxib in camels (Camelus dromedarus) (n=5) following intravenous (i.v.) administration of a dose of 0.1 mgkg/body weight. The results obtained (mean ± SD) were as follows: the terminal elimination half-life (t₁/₂β) was 5.75 ± 2.26 h, and total body clearance (ClT) was 354.1 ± 82.6 mL/kg/h. The volume of distribution at steady state (VSS) was 2344.4 ± 238.7 mL/kg. One metabolite of firocoxib was tentatively identified as desalkyl firocoxib (m/z 283). Firocoxib could be detected in plasma 3-5 days following i.v. administration in camels using a sensitive liquid chromatography high-resolution orbitrap mass spectrometry method.