Single- and repeat-dose pharmacokinetic studies of chloramphenicol in horses: values and limitations of pharmacokinetic studies in predicting dosage regimens.

The research study investigates the pharmacokinetics, or how the drug chloramphenicol behaves in the body of horses, using a single-dose and then a repeat-dose regime. The findings help predict dosage regimens and assess the limitations of such predictions.

Experimental Design

• The experiment began with a single-dose study where chloramphenicol was administered intravenously (directly into the bloodstream) to six horses. The dosage was 22mg/kg.
• Serum drug concentrations were then measured at various predetermined time points. The data was determined through an ‘electroncapture gas-chromatographic technique’, which is a method used to identify and quantify how much of the drug is present.
• The data from this single-dose study was then modeled using a two-compartment open model. Variables including the elimination rate constant, half-life, apparent volume of distribution, and total body clearance were estimated.

Single-Dose Study Results

• The median elimination rate constant was found to be -0.0185 minute-1, indicating the rate at which the drug was eliminated from the body.
• The median half-life of the drug was 37.36 minutes. Half-life is the time taken for the drug concentration in the body to reduce by half.
• The apparent volume of distribution (the hypothetical volume that would be necessary to contain the total amount of drug in the body) was 1.46 L/kg, and the total body clearance (the volume of plasma from which the drug is completely removed per unit time) was 25.56 ml/kg/min.

Repeat-Dose Study Design and Results

• Based on the single-dose results, a repeat-dose study was conducted on another group of six horses. 22 mg/kg of the drug was administered every four hours for three days.
• Blood samples were taken for pharmacokinetic studies after the first and the last doses.
• The half-life, volume of distribution and total body clearance remained relatively unchanged after three days of therapy.

Implications and Limitations

• The research has helped to predict the intravenous dose schedule for treating bacterial infections with chloramphenicol in horses.
• The limitations of predicting dose schedules for chloramphenicol based on pharmacokinetic variables in horses are also discussed. While not detailed in the abstract, these limitations could include factors such as individual variation between horses, environmental factors, and potential changes in drug absorption and metabolism over time.