Pharmacokinetics of enrofloxacin administered intravenously and orally to foals.

The research article investigates how foals’ bodies absorb and eliminate the drug enrofloxacin, an antibiotic, when given intravenously or orally. The study found that this process is more efficient in foals compared to adult horses and certain dosage recommendations are suggested based on computer modeling.

Research Methods

• Five clinically normal foals were involved in this study. Enrofloxacin was administered in two ways in a cross-over experiment: first intravenously at 5 mg/kg of body weight, then orally at 10 mg/kg after a 7-day washout period – time allowed for the drug to completely leave the foal’s system.
• Blood samples were collected over 48 hours after both methods of administration. The plasma samples were then analyzed using high-performance liquid chromatography, a technique used to identify amounts of specific substances, to determine presence and quantities of enrofloxacin and its derivative compound, ciprofloxacin.

Findings

• Several pharmacokinetic parameters were calculated from the collected data. For IV administration, overall exposure to the drug (measured as mean AUC – area under the curve) was approximately 48.54 microg x h/ml, clearance rate was 103.72 ml/kg/h, half-life around 17.10 hours, and volume of distribution was about 2.49 L/kg.
• For oral administration, these figures altered; AUC was around 58.47 microg x h/ml, half-life approximately 18.39 hours, peak concentration (Cmax) was about 2.12 microg/ml, time taken to reach this peak (time to Cmax) was 2.20 hours, and the average absorption time was 2.09 hours. The bioavailability (amount of enrofloxacin that entered the systemic circulation) was estimated at 42%.
• The levels of ciprofloxacin, a derivative compound of enrofloxacin, were negligible indicating that the foals’ bodies did not break down substantial amounts of the administered enrofloxacin into this compound.

Implications and Conclusion

• Comparatively, it was found that foals have higher total drug exposure (higher AUC), longer drug half-life, and slower drug clearance than adult horses administered with the identical dosage of 5 mg/kg enrofloxacin intravenously.
• These findings suggest that the biological system of foals processes enrofloxacin differently as compared to adult horses.
• Based on these pharmacokinetic data and computer modelling, the study proposes that administering between 2.5 to 10 mg of enrofloxacin/kg every 24 hours would effectively treat bacterial infections in foals, depending on the particular pathogen’s minimum inhibitory concentration – the lowest concentration of the drug which can prevent bacterial growth.