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

Abstract

Diclazuril (4-chlorophenyl [2,6-dichloro-4-(4,5-dihydro-3H-3,5-dioxo-1,2,4-triazin-2-yl)pheny l] acetonitrile), is a benzeneacetonitrile antiprotozoal agent (Janssen Research Compound R 64433) marketed as Clinacox . Diclazuril may have clinical application in the treatment of Equine Protozoal Myeloencephalitis (EPM). To evaluate its bioavailability and preliminary pharmacokinetics in the horse we developed a sensitive quantitative high-pressure liquid chromatography (HPLC) method for diclazuril in equine biological fluids. 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 (as 500 g Clinacox), plasma samples from four horses showed good plasma concentrations of diclazuril which peaked at 1.077 +/- 0.174 microg/mL (mean +/- SEM) with an apparent plasma half-life of about 43 h. When this dose of Clinacox was administered daily for 21 days to two horses, mean steady state plasma concentrations of 7-9 microg/mL were attained. Steady-state levels in the CSF ranged between 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 chromatography (TLC). These results show that diclazuril is absorbed after oral administration and attains steady-state concentrations in plasma and CSF. The steady state concentrations attained in CSF are more than sufficient to interfere with Sarcocystis neurona, whose proliferation is reportedly 95% inhibited by concentrations of diclazuril as low as 1 ng/mL. These results are therefore entirely consistent with and support the reported clinical efficacy of diclazuril in the treatment of clinical cases of EPM.

Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment, and Prevention

“…The activity of benzeneacetonitrile compounds against S. neurona and N. caninum was initially shown in vitro 111, 112, 113. In horses, pharmacokinetic studies have established that therapeutic steady‐state concentrations of both diclazuril and ponazuril are achieved by day 7 using labeled doses 114, 115, 116. Moreover, use of a loading dose of ponazuril at 15 mg/kg resulted in steady‐state concentrations in blood and CSF by day 2 117.…”

Equine Protozoal Myeloencephalitis

“…88,89 In horses, pharmacokinetic studies have established that therapeutic steady-state concentrations of both diclazuril and ponazuril are achieved by day 7 using labeled doses. 90,91 Moreover, use of a loading dose of ponazuril at 15 mg/kg resulted in steady-state concentrations in blood and CSF by day 2. 92 The formulations of these drugs adapted for clinical use in horses are Protazil (1.56% diclazuril; Merck Animal Health, Kansas City, KS) and Marquis (15% ponazuril; Bayer Inc, Kansas City, KS).…”

Simultaneous Determination of Florfenicol and Diclazuril in Compound Powder by RP-HPLC-UV Method

“…HPLC methods have been widely used to determine florfenicol and diclazuril in samples at present [12][13][14][15][16][17]. To our knowledge, no HPLC methods have been developed in the literature for determination of florfenicol and diclazuril in compound powder simultaneously.…”

“…Florfenicol and diclazuril solid dispersions were prepared according to the method described by Dirikolu et al and Norambuena et al with PEG6000 as carriers [13][14][15]. After drug content test, equivalent 5 g florfenicol and 0.5 g diclazuril were transferred to a mechanical blender, followed by mixing at least 10 minutes.…”