Comparison of microbiologic and high-performance liquid chromatography assays to determine plasma concentrations, pharmacokinetics, and bioavailability of erythromycin base in plasma of foals after intravenous or intragastric administration.

The research measured the concentration, pharmacokinetics, and bioavailability of a dosage of Erythromycin base in foals, comparing the effects of intravenous and intragastric administration. It tracked plasma samples over time and compared microbiologic assays and HPLC.

Research Methodology

• The study was conducted on six healthy foals aged between 2 to 4 months old. It was a crossover study where the drug was administered to each foal twice—once intravenously and once intragastrically.
• The dosage was 10 mg/kg for intravenous (IV) erythromycin lactobionateand and 25 mg/kg for intragastric erythromycin base.
• Blood samples were taken at specific intervals after drug administration and plasma was harvested. Erythromycin concentrations were determined by both a microbiologic assay and a high-performance liquid chromatography (HPLC) method.
• The pharmacokinetic analysis of the plasma concentration-time data was performed and the results derived from each method were compared. The significant parameters being monitored were the area under the plasma concentration-time curve (AUC), the area under the first moment of the curve (AUMC), and the mean residence time of the drug in the body.

Research Findings

• The study revealed that the plasma concentration-time profiles for IV administration were best fitted by a two-compartment open model.
• There were substantial differences in pharmacokinetic results obtained by the two methods. The AUC and AUMC values were substantially higher when measured by the bioassay, indicating overestimation of plasma concentration-time data by this method.
• The derived rate transfer constants (K21 and K(e)1) and the average time the drug stayed in the body were markedly different when determined by the bioassay method.
• The systemic bioavailability of erythromycin base was low in all the foals, indicating that a smaller fraction of the ingested drug reached the systemic circulation.
• The researchers concluded that the bioassay technique overestimated erythromycin plasma concentrations compared to HPLC. Nevertheless, with intragastric administration, the erythromycin plasma concentrations for at least 4 hours exceeded the minimum inhibitory concentration of most Rhodococcus equi isolates, thereby offering potential therapeutic benefits.