Development of an LC-MS/MS-Based Analytical Method, In Vitro Characterization, and Pharmacokinetic Study of Rafoxanide Nanosuspension in Sheep

This study aimed to establish a more sensitive and reliable quantitative method for determining rafoxanide (RFX) in ovine plasma. The method was applied in an 84-day long-term comparative pharmacokinetic trial to evaluate the performance of a rafoxanide nanosuspension (RFX-NS) versus its conventional suspension (RFX-S) in sheep. Characterization of the two formulations revealed the following results: RFX-NS had an average particle size of 484.93 ± 43.11 nm, a uniform size distribution (PDI 0.06 ± 0.07), and a Zeta potential of −43.59 ± 0.67 mV, which were significantly superior to those of RFX-S (particle size 2379.67 ± 121.71 nm, PDI 0.93 ± 0.10, Zeta potential −38.10 ± 0.55 mV), demonstrating excellent physical stability. In vitro dissolution tests indicated a higher dissolution rate for RFX-NS (97.6% at 60 min). By integrating formulation characterization, in vitro dissolution assessment, and pharmacokinetic studies, this research provided a comprehensive analysis of the differences between the two formulations. Pharmacokinetic results showed that, compared to RFX-S, RFX-NS had a significantly reduced apparent volume of distribution (Vz/F: 856.02 ± 274.00 vs. 1404.17 ± 285.1 mL/kg, p < 0.01), a shorter elimination half-life (t1/2: 4.05 ± 1.18 vs. 5.32 ± 0.94 days, p < 0.05), and a relative bioavailability of 128.98%. These findings suggest that RFX-NS is an improved novel formulation superior to the conventional suspension, as it enhances drug efficacy and efficiency by improving drug dissolution and absorption.