Development and Validation of LC-MS/MS Methods for Quantification of Fluralaner and Moxidectin in Cat Plasma and Its Application in a Pharmacokinetic Study

This study was conducted to support the development of a novel generic veterinary formulation by performing a comparative pharmacokinetic analysis against an approved reference product (Bravecto® Plus). Sensitive and selective liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods were developed and validated for the quantification of fluralaner and moxidectin in cat plasma. A single topical dose (40 mg/kg fluralaner and 2 mg/kg moxidectin) of either the test or reference formulation was administered to 20 healthy cats in a parallel study. A validated LC-MS/MS method was used to determine plasma concentrations. Key pharmacokinetic parameters—including peak plasma concentration (Cmax), time to Cmax (tmax), and area under the curve (AUC0–∞)—were calculated and statistically compared. The Cmax of fluralaner was 1527.62 ± 905.94 ng/mL, tmax was 14.00 (interquartile range (IQR): 5.00–28.00) days and AUC0–∞ was 1,415,464.8 ± 822,873.6 ng·h/mL. The Cmax of moxidectin was 27.85 ± 38.78 ng/mL, tmax was 6.00 (IQR: 1.00–14.00) days, AUC0–∞ was 16,807.2 ± 12,885.6 ng·h/mL. Compared with the reference veterinary drug (Bravecto® Plus), the relative bioavailability (F) of both fluralaner (118.89% (90% CI: 96.5–145.3%)) and moxidectin (102.00% (90% CI: 85.2–118.8%)) exceeded 100%, and there was no statistical difference in the time to peak concentration (tmax). The test and reference formulations exhibit similar pharmacokinetic profiles in cats. These results provide critical pharmacokinetic data supporting the bioequivalence and therapeutic potential of the new generic formulation, thereby facilitating its further development and regulatory evaluation.