Pharmacokinetics of erythromycin estolate and erythromycin phosphate afterintragastric administration to healthy foals

Objective - To determine pharmacokinetics and plasma concentrations of eryt hromycin and related compounds after intragastric administration of erythro mycin 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 admi nistration of erythromycin estolate (25 mg/kg of body weight; n = 8) and er ythromycin phosphate (25 mg/kg; 7). Four foals received both drugs with 2 w eeks between treatments. Plasma erythromycin concentrations were determined at various times after drug administration by use of high-performance liqu id chromatography. Maximum plasma peak concentrations, time to maximum conc entrations, area under plasma concentration versus time curves, half-life o f 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 administrat ion of erythromycin estolate (2.9 +/- 1.1 mu g/ml vs 1.0 +/- 0.82 mu g/ml). Time to maximum concentration was shorter after administration of enythrom ycin 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 afte r administration of erythromycin phosphate. Conclusions and Clinical Relevance - Plasma concentrations of erythromycin A remained > 0.25 mu g/ml (reported minimum inhibitory concentration for Rh odococcus equi) for at least 4 hours after intragastric administration of e rythromycin phosphate or erythromycin estolate, suggesting that the recomme nded dosage for either formulation (25 mg/kg, q 6 h) should be adequate for treatment of R equi infections in foals.