Relationship among particle size distribution, dissolution profile, plasma values, and anthelmintic efficacy of oxfendazole.

Three mean particle sizes of oxfendazole raw material (1.65 micron, lot A; 3.2 micron, 10t B; 12.0 micron, lot C) were prepared and identically formulated as corresponding (A, B, and C) suspensions at 2.26% (W/V) concentration. Studies involving microscopic examination, scanning electron microscope analysis, particle size distribution, and surface area measurement were carried out on raw materials. In vitro dissolution profiles were obtained for the suspensions. A comparative bioavailability study of these 3 suspensions was performed in 12 sheep with each sheep given each formulation in a Latin square crossover study design; oxfendazole was dosed at rate of 5 mg/kg of body weight. Plasma-value measurements were made followed by an analysis of various bioavailability studies. Plasma area values indicated that suspension C (dw = 12.0 micron) was significantly (P less than 0.05) less bioavailable than was suspension A (dw = 1.65 micron); there was no difference between suspension A and suspension B. Significant differences were not seen in biological half-life and maximum plasma concentrations. The term dw refers to that particle diameter (determined by Coulter counting) at which 50% of the oxfendazole mass was in the form of particles having a lesser diameter and 50% was in the form of particles having a greater diameter. In a separate study involving 20 Merino weaner sheep infected with benzimidazole-resistant Haemonchus contortus larvae, oxfendazole's anthelmintic efficacy was demonstrated in the 2.26% suspension dosage form (90% particles less than 10 micron) at a dosing rate of 5 mg/kg. A correlation was found between its anthelmintic activity and plasma area values when compared in individual sheep. Data demonstrated that substantial differences in particle size distribution of oxfendazole could influence its dissolution rate, plasma concentrations, and absorption characteristics, thus indicating that oxfendazole's absorption could be dissolution-rate limited.