Pharmacokinetics of a low dose and FDA-labeled dose of diclazuril administered orally as a pelleted topdressing in adult horses

The purpose of this study was to determine the pharmacokinetics of the FDA-approved labeled dose of diclazuril and compare it to a low dose in plasma and CSF in adult horses. During each research period, six healthy adult horses received 0.5mg/kg of 1.56% diclazuril pellets (Protazil(TM), Merck Animal Health) compared to the approved labeled dose of 1mg/kg orally once in two separate phases. A dose of 0.5mg/kg was calculated to each horse's weight. Blood was then collected immediately before diclazuril administration and then at regular intervals up to a 168h. After the last blood collection following the single dose at hour 168, a once daily oral dose was administered for the next 10days to ensure the drug's concentration reached steady-state. To determine the CSF concentration at steady-state, CSF samples were collected after the 9th oral dose. Blood was then collected after the 10th dose and then at regular intervals up to 168h. A washout period of 4weeks was allowed before repeating this protocol for the FDA-labeled dose at 1mg/kg. Plasma and CSF samples were analyzed by high-pressure liquid chromatography. A one-compartment pharmacokinetic model with first-order oral absorption was fitted to the single administration data. Steady-state pharmacokinetics was performed using noncompartmental analysis for steady-state analysis. The mean (standard deviation) concentration of diclazuril in CSF following the low dose was 26ng/mL (5ng/mL), while CSF in the FDA-labeled dose was 25ng/mL (4ng/mL), P=0.3750. Substantial accumulation in plasma occurred at steady-state after the 10th dose for both doses. The results of this study show that diclazuril pellets given at the approved label dose and a lower dose both produce similar plasma drug concentrations at steady-state and attain plasma and CSF concentrations known to inhibit Sarcocystis neurona in cell culture.