Plasma disposition of toltrazuril and its metabolites, toltrazuril sulfoxide and toltrazuril sulfone, in rabbits after oral administration.

The objective of this study was to evaluate the pharmacokinetic profiles of toltrazuril (TZR), and its major metabolites toltrazuril sulfoxide (TZR x SO) and toltrazuril sulfone (TZR x SO(2)) in rabbits after oral administrations. Rabbits were dosed once with 10 and 20mg/kg TZR via stomach tube with manual restraint. The plasma concentrations of TZR, TZR x SO and TZR x SO(2) were determined by liquid chromatography/mass spectrometry. Plasma concentration-time data after single oral administration were analyzed by a non-compartmental analysis. Plasma peak concentrations of TZR, TZR x SO and TZR x SO(2) were 30.2+/-1.5microg/mL at 20.0+/-6.9h, 8.9+/-1.3microg/mL at 20.0+/-6.9h and 14.7+/-3.9microg/mL at 96.0+/-0.0h after oral administration of TZR with 10mg/kg bw, respectively. The terminal elimination half-lives for TZR, TZR x SO and TZR x SO(2) after oral dose of 10mg/kg were 52.7+/-3.6, 56.1+/-10.7 and 76.7+/-7.5h, respectively. Plasma peak concentrations of TZR, TZR x SO and TZR x SO(2) were 39.4+/-1.2microg/mL at 28.0+/-6.9h, 12.5+/-3.9microg/mL at 20.0+/-6.9h and 24.9+/-8.74microg/mL at 112.0+/-6.9h after oral administration of TZR with 20mg/kg bw, respectively. The terminal elimination half-lives for TZR, TZR x SO and TZR x SO(2) after oral dose of 20mg/kg were 56.7+/-1.9, 68.8+/-12.5 and 82.3+/-12.6h, respectively. In conclusion, TZR was very well-absorbed through the gastrointestinal tract and rapidly metabolized to TZR x SO and TZR x SO(2) in rabbits after oral administration. TZR x SO(2) was actually more slowly eliminated than TZR and TZR x SO.