PHARMACOKINETICS OF TRICLOPYR (3,5,6-TRICHLORO-2-PYRIDINYLOXYACETIC ACID) IN THE BEAGLE DOG AND RHESUS-MONKEY - PERSPECTIVE ON THE REDUCED CAPACITY OF DOGS TO EXCRETE THIS ORGANIC-ACID RELATIVE TO THE RAT, MONKEY, AND HUMAN

The pharmacokinetics of triclopyr (3,5,6-trichloro-2-pyridinyl-oxyacet ic acid) were measured in the beagle dog and rhesus monkey and compare d with the kinetics observed in rats and humans, In addition, studies were conducted in anesthetized dogs to better understand the mechanism by which [C-14]triclopyr is eliminated in this species, Triclopyr was dissolved in distilled water, and administered as a single oral dose of 0.5, 5, or 20 mg/kg to three male dogs, A single male rhesus monkey was given an intravenous dose of 30 mg [C-14]triclopyr/kg body wt on two occasions separated by 10 days, Anesthetized male dogs, were impla nted with venous, arterial, and urethral catheters and given increasin g amounts of triclopyr to produce plasma triclopyr levels ranging from 0.3 to 27 mu g eq/mL. In the monkey, triclopyr was rapidly eliminated from the plasma (t1/2 = 6.3 hr) with >95% of the urinary C-14 activit y excreted within 24 hr postdosing, In the dog, orally administered tr iclopyr was rapidly and effectively absorbed at every dose level with virtually all of it excreted in the urine by 72 hr postdosing, However , the kinetics were slightly nonlinear, and the fraction of the dose e xcreted in the urine decreased with increasing dose. Several nonlinear processes may collectively contribute to the modest nonlinear pharmac okinetics in the dog. Plasma protein binding of triclopyr in the dog r anged from 94 to 99%, was nonlinear, and was an important determinant in the renal clearance of triclopyr, The nonlinear plasma protein bind ing indicates that glomerular filtration became disproportionately mor e important as plasma triclopyr concentration increased. There was goo d evidence for a high-affinity low-capacity active-secretory process t hat was saturated by low plasma triclopyr concentrations. As plasma tr iclopyr concentrations increased, tubular reabsorption begins to excee d secretion, resulting in decreased renal clearance, The volume of dis tribution, normalized for body weight, was constant across all species . While clearance and half-life could be allometrically scaled to body weight for the rat, monkey, and human, the dog had a much slower clea rance and longer half-life for triclopyr elimination than predicted al lometrically. These data demonstrate that the pharmacokinetics of tric lopyr in the dog are markedly different than in rat, monkey. and human . (C) 1997 Academic Press.