Rifampin disposition in the horse: effects of age and method of oral administration

The effects of time and method of administration of rifampin with respect to feeding were evaluated in five mature horses. There was a significant (P less than or equal to 0.05) delay in time of maximum serum concentration and an apparent but not significant decrease in oral absorption when rifampin was given as a top dressing on grain as compared with administration in corn syrup 2 h before or 2 h after feeding. Although there were no differences between administration before or after feeding, administration 2 h prior to feeding was selected as the method of choice for future experiments. The effects of age on rifampin disposition were subsequently examined using this method of administration in six, 1-week old foals. Rifampin (10 mg/kg) was given at increasing age from 1 through 10 weeks and the pharmacokinetic disposition parameters compared. There were significant differences in the slope of the elimination phase (beta) and area under the curve (AUC) at 1 week through 6 weeks compared with 10 weeks or with values in the five mature horses.