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

The study provides a comparative assessment of the pharmacokinetics of erythromycin estolate and erythromycin phosphate in healthy foals, with findings suggesting that both formulations, when given in recommended dosages, should be sufficient for the treatment of Rhodococcus equi infections.

Methods

• The study was carried out on 11 healthy foals aged between 2 to 6 months.
• Prior to the experiment, the foals were not fed overnight.
• Erythromycin estolate and erythromycin phosphate were administered to the foals in a dosage of 25 mg/kg body weight.
• Eight foals received erythromycin estolate while seven received erythromycin phosphate.
• Four foals received both drugs with a two-week gap between the two treatments.
• The researchers tracked plasma erythromycin concentrations at different times post-treatment via high-performance liquid chromatography.
• Data like maximum plasma peak concentrations, time to peak concentration, area under plasma concentration-time curves, the half-life of elimination, and mean residence times were determined from the concentration-time curves.

Results

• After the administration of erythromycin phosphate, the maximum peak concentration of erythromycin was found to be significantly greater than after the administration of erythromycin estolate (2.9 +/- 1.1 microg/ml vs 1.0 +/- 0.82 microg/ml).
• The time to reach maximum concentration was also shorter for erythromycin phosphate than for erythromycin estolate (0.71 +/- 0.29 hours vs 1.7 +/- 1.2 hours).
• Anhydroerythromycin A concentrations recorded 1 and 3 hours after erythromycin estolate administration were significantly lesser than after erythromycin phosphate administration.

Conclusions

• Plasma concentrations of erythromycin A remained over 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.
• This observation suggests that the recommended dosage for either formulation (25 mg/kg, every 6 hours) should be adequate for the treatment of R equi infections in foals.