Pharmacokinetics of a FDA-labeled dose of diclazuril administered orally once weekly to adult horses.
Authors: Pusterla Nicola, Vaala Wendy, Bain Fairfield T, Chappell Duane E, Craig Bryant, Schneider Chrissie, Barnett D Craig, Gaughan Earl, Papich Mark G
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Weekly Diclazuril Dosing for EPM Prophylaxis in Adult Horses Equine protozoal myeloencephalitis remains a significant neurological threat to performance horses across the Americas, and prophylactic diclazuril has emerged as a strategy to reduce seroconversion and antibody titres against *Sarcocystis neurona* in at-risk populations. Researchers administered the FDA-labelled dose of 1 mg/kg diclazuril (Protazil™) once weekly to eight healthy adult horses over five consecutive weeks, measuring plasma concentrations via high-performance liquid chromatography immediately before dosing (trough) and 10 hours post-administration (peak), with extended sampling over seven days following the final dose. Peak plasma concentrations averaged 264 ng/mL with a terminal half-life of 3.6 days, demonstrating that steady-state levels sufficient to inhibit *S. neurona* in cell culture were consistently achieved on the weekly dosing schedule. These findings support the use of once-weekly diclazuril prophylaxis in adult horses with continuous parasite exposure, validating the FDA-approved regimen's pharmacokinetic basis and providing confidence in clinical efficacy for prevention programmes in high-risk facilities and competition environments.
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Practical Takeaways
- •Weekly oral diclazuril at FDA-labeled doses achieves therapeutic blood levels in adult horses for EPM prophylaxis, supporting the labeled dosing regimen
- •The 3.6-day half-life and weekly dosing schedule maintains inhibitory drug concentrations against Sarcocystis neurona between doses
- •This pharmacokinetic data supports using weekly diclazuril prophylaxis in healthy horses exposed to EPM risk as a seroprevalence and titer reduction strategy
Key Findings
- •FDA-labeled dose of 1 mg/kg diclazuril administered once weekly achieved mean peak plasma concentration of 264 ng/mL in healthy adult horses
- •Mean terminal half-life of diclazuril was 3.6 days, allowing steady-state plasma concentrations to be maintained with weekly dosing
- •Steady-state plasma concentrations achieved were known to inhibit S. neurona in vitro cell culture studies