Detection, quantification and pharmacokinetics of triazine-based antiprotozoal agents for the treatment of equine protozoal myeloencephalitis

Equine protozoal myeloencephalitis (EPM) is an infectious neurological disease of horses which affects the central nervous system (CNS) and is caused by the apicomplexan parasite Sarcocystis neurona. Current therapeutic approaches to EPM are based on administration of pyrimethamine/sulfonamide combinations. While treatment is successful in many cases, it can be prolonged and relapse after cessation of treatment is not uncommon. Current treatments also carry significant toxicity risks, therefore, safer and more effective prophylactic and therapeutic procedures with little toxicity are under investigation.; Diclazuril, toltrazuril and toltrazuril sulfone are triazine-based antiprotozoal agents with well established antiprotozoal efficacy. These agents have been shown to be effective in vitro against S. neurona at very low concentrations. In this study, we developed highly sensitive analytical methods for detection, identification and quantification of these agents in the biological fluids of the horse. Triazine-based agents included in this study are well absorbed following oral administration and have relatively long plasma-half-lives, facilitating maintenance of effective steady state plasma and cerebrospinal fluid (CSF) concentrations. In vitro studies have shown that these agents are highly effective against S. neurona, with minimum inhibitory concentrations (MICs) being readily maintained in CSF. A clinical efficacy study with diclazuril has shown that a 28-day course of therapy is highly effective in the treatment of this disease as 71% of horses improved clinically post-treatment. The incidence of adverse reactions to these agents is also low, consistent with their highly selective toxicity for apicomplexans. Additionally, other work suggests that these agents can be combined with pyrimethamine/sulfonamide combinations to further increase the efficacy of therapy. In summary, we determined important pharmacokinetic parameters of these clazurils that can help clinicians and researchers design and test dosing schedules to maintain effective steady state plasma and CSF concentrations of these compounds in the horse. This information together with the in vitro sensitivity assay was utilized to find a safer, more effective treatment and prophylaxis protocol for EPM, a disease that is devastating the breeding and racing industries.