Pharmacokinetics of toltrazuril and its metabolites, toltrazuril sulfoxide and toltrazuril sulfone, after a single oral administration to pigs.

Toltrazuril (TZR) is a triazine-based antiprotozoal agent. Following a single oral administration of TZR at 10 and 20 mg/kg to male pigs, the mean TZR concentration in plasma peaked at 4.24 and 8.18 microg/ml at 15.0 and 12.0 hr post-dose, respectively. TZR absorbed was rapidly converted to the short-lived intermediary metabolite toltrazuril sulfoxide (TZR-SO), and then metabolized to the reactive toltrazuril sulfone (TZR-SO2). TZR-SO2 was actually more slowly eliminated, with average half-lives of 231 and 245 hr, compared with TZR (48.7 and 68.9 hr) or TZR-SO (51.9 and 53.2 hr) in the 10 and 20 mg/kg groups, respectively. This study demonstrates that TZR metabolizes to TZR-SO2 having a long-terminal half-life, enabling the persistent clinical efficacy in the treatment of I. suis infection. In contrast, special consideration should be given to the residual of TZR-SO2.