Diclazuril in the horse: its identification and detection and preliminary pharmacokinetics

Diclazuril (4-chlorophenyl [2,6-dichloro-4-(4,5-dihydro-3H-3,5-dioxo-1,2,4- triazin-2-yl)phenyl] acetonitrile), is a benzeneacetonitrile antiprotozoal agent (Janssen Research Compound R 64433) marketed as Clinacox(R). Diclazur il may have clinical application in the treatment of Equine Protozoal Myelo encephalitis (EPM), To evaluate its bioavailability and preliminary pharmac okinetics in the horse we developed a sensitive quantitative high-pressure liquid chromatography (HPLC) method for diclazuril in equine biological flu ids. MS/MS analysis of diclazuril in our HPLC solvent yielded mass spectral data consistent with the presence of diclazuril. After a single oral dose of diclazuril at 2.5 g/450 kg las 500 g Clinacox(R )), plasma samples from four horses showed good plasma concentrations of di clazuril which peaked at 1.077 +/- 0.174 mu g/mL, (meant SEM) with an appar ent plasma half-life of about 43 h. When this dose of Clinacox(R) was admin istered daily for 21 days to two horses, mean steady state plasma concentra tions of 7-9 mu g/mL were attained, Steady-state levels in the CSF ranged b etween 100 and 250 ng/mL. There was no detectable parent diclazuril in the urine samples of dosed horses by HPLC or by routine postrace thin layer chr omatography (TLC). These results show that diclazuril is absorbed after oral administration an d attains steady-state concentrations in plasma and CSF. The steady state c oncentrations attained in CSF are more than sufficient to interfere with Sa rcocystis neurona, whose proliferation is reportedly 95% inhibited by conce ntrations of diclazuril as low as 1 ng/mL, These results are therefore enti rely consistent with and support the reported clinical efficacy of diclazur il in the treatment of clinical cases of EPM.