Liquid chromatographic assay of moxidectin in human plasma for application to pharmacokinetic studies

A sensitive and selective high-performance liquid chromatography (HPLC) method is presented for the analysis of moxidectin in human plasma. Solid phase extraction using Oasis HLB cartridges is used for sample preparation. The fluorescent derivative is obtained by a dehydrative reaction with trifluoroacetic anhydride and N-methylimidazole. Separation is achieved on a Bondapak C(18) reversed-phase column with a mobile phase composed of tetrahydrafuran-acetonitrile-water (40:40:20, v/v/v). Detection is by fluorescence, with excitation at 365 nm and emission at 475 nm. The retention times of moxidectin and internal standard, ivermectin are approximately 10.7 and 18.6 min, respectively. The assay is linear over the concentration range 0.2-1000 ng/ml for moxidectin in human plasma (r=0.9999, weighted by 1/concentration). Recoveries at concentrations 0.2, 400, 1000 ng/ml are 94, 75, and 71%, respectively. The analysis of quality control (QC) samples for moxidectin (0.2, 400, and 1000 ng/ml) demonstrates excellent precision with relative standard deviations of 11.9, 5.7, and 2.7%, respectively (n=6). The method is accurate with all intra- (n=6) and inter-day (n=18) mean concentrations within (5.0%) from nominal at all QC sample concentrations. Moxidectin was found to be stable after three free-thaw cycles, and with storage at -20 and -80 degrees C for 12 weeks. The method is suitable for pharmacokinetic studies of moxidectin after oral administration to humans.