Pharmacokinetics of erythromycin ethylsuccinate after intragastric administration to healthy foals

Plasma concentrations and pharmacokinetics of erythromycin and related compounds were determined after administration of erythromycin ethylsuccinate to six healthy male foals 3 to 5 months of age. Hay was withheld from the foals overnight and erythromycin ethylsuccinate (25 mg/kg of body weight) was administered intragastrically. Plasma erythromycin concentrations were determined at specific times after drug administration by high-performance liquid chromatography assay. Maximum peak plasma concentrations, time to maximum concentrations, area under plasma concentration versus time curves, elimination half-life, and mean residence time were determined from concentration versus time curves. Maximum peak concentration of erythromycin A (0.45 ± 0.27 μg/ml) after administration of erythromycin ethylsuccinate was observed at 2.38 ± 1.54 hours after treatment. Concentrations of anhydroerythromycin A were maximal at 2.2 ± 2.0 hours and reached a maximum of 2.6 ± 1.9 μg/ml. Plasma concentrations of the ester parent drug (erythromycin ethylsuccinate) were below the limit of quantitation (0.1 μg/ml) at all times except 2.5, 2.75, and 5.5 hours. Levels at those times ranged from 0.1 to 0.144 μg/ml. Erythromycin ethylsuccinate appears to be poorly absorbed after oral administration to fasted foals. Plasma concentrations of erythromycin A remained below 0.25 μg/ml (reported minimum inhibitory concentration for Rhodococcus equi) for less than 4 hours after intragastric administration of erythromycin ethylsuccinate, suggesting that the recommended dosage (25 mg/kg every 6 hours) would be suboptimal for treatment of R. equi infections.