COMPARISON OF ORAL ERYTHROMYCIN FORMULATIONS IN THE HORSE USING PHARMACOKINETIC PROFILES

The pharmacokinetic properties of four erythromycin formulations were compared in six adult horses after administration of single and multip le oral doses. Formulations of erythromycin administered were estolate and phosphate given 37.5 mg/kg every 12 h and 25 mg/kg every 8 h, and stearate and ethylsuccinate given 2 5 mg/kg every 8 h. Areas under th e curve (AUC) and maximum plasma erythromycin concentrations (C(max)) were equal or greater (P greater-than-or-equal-to 0.05) following admi nistration of erythromycin phosphate and stearate compared with those values following administration of erythromycin estolate or ethylsucci nate. In comparing an 8 h vs. a 12 h dosage interval for multiple dose s of erythromycin phosphate or estolate, there were no significant dif ferences observed in AUC(24-28 h), peak-trough plasma concentrations o r duration that plasma concentrations exceeded the minimal inhibitory concentration (MIC) for Rhodococcus equi. Comparisons of pharmacokinet ic parameters between single and multiple doses were made for each for mulation. Differences in C(max), t(max), or t1/2beta between single an d multiple doses were demonstrated for erythromycin ethylsuccinate and estolate. Based on equivalent plasma antibiotic concentrations, eryth romycin phosphate or stearate could be substituted for estolate in the treatment of Rhodococcus equi pneumonia. Furthermore, there was no ad vantage of an 8-h interval, compared with an interval of 12 h.