Review of the pharmacokinetics and metabolism of triclopyr herbicide in mammals: Impact on safety assessments

A review of pharmacokinetic and metabolism studies show that triclopyr is well absorbed from the oral route in numerous species (>= 80%), primarily as parent compound. Absorption is quite rapid in rats, dogs and human volunteers. Plasma or blood clearance is also rapid (t(1/2) 3-9 h), except for dog (12-96 h). Systemic exposure is not dose-proportional: in the rat above 20 mg/kg (dietary) or between 3 and 60 mg/kg (gavage), or in dogs above 5 mg/kg, with systemic exposure in human more comparable to rat than dog. Triclopyr is highly bound to protein in rat, dog and human plasma (>= 97% at or below 7 mu g/mL), indicating that species differences in systemic exposure are not due to differences in the free fraction of this test material in plasma. An in vitro flux study in renal proximal tubule cells showed that net renal transport of triclopyr is in the direction of secretion in rat and human donors, while reabsorption predominated in the dog, possibly via organic anion transporters such as OAT1/3. These results fit well into the framework of utilizing metabolism and toxicokinetics across species and exposure levels to allow for toxicity testing in the most relevant species as well as at proper dose levels.