Pharmacokinetics of erythromycin estolate and erythromycin phosphate after intragastric administration to healthy foals

OBJECTIVE: To determine pharmacokinetics and plasma concentrations of erythromycin and related compounds after intragastric administration of erythromycin phosphate and erythromycin estolate to healthy foals. ANIMALS: 11 healthy 2- to 6-month-old foals. PROCEDURE: Food was withheld from foals overnight before intragastric administration of erythromycin estolate (25 mg/kg of body weight; n = 8) and erythromycin phosphate (25 mg/kg; 7). Four foals received both drugs with 2 weeks between treatments. Plasma erythromycin concentrations were determined at various times after drug administration by use of high-performance liquid chromatography. Maximum plasma peak concentrations, time to maximum concentrations, area under plasma concentration versus time curves, half-life of elimination, and mean residence times were determined from concentration versus time curves. RESULTS: Maximum peak concentration of erythromycin A after administration of erythromycin phosphate was significantly greater than after administration of erythromycin estolate (2.9 +/- 1.1 microg/ml vs 1.0 +/- 0.82 microg/ml). Time to maximum concentration was shorter after administration of erythromycin phosphate than after erythromycin estolate (0.71 +/- 0.29 hours vs 1.7 +/- 1.2 hours). Concentrations of anhydroerythromycin A were significantly less 1 and 3 hours after administration of erythromycin estolate than after administration of erythromycin phosphate. CONCLUSIONS AND CLINICAL RELEVANCE: Plasma concentrations of erythromycin A remained > 0.25 microg/ml (reported minimum inhibitory concentration for Rhodococcus equi) for at least 4 hours after intragastric administration of erythromycin phosphate or erythromycin estolate, suggesting that the recommended dosage for either formulation (25 mg/kg, q 6 h) should be adequate for treatment of R equi infections in foals.