Absorption, tissue distribution, and excretion of tritium-labeled ivermectin in cattle, sheep, and rat

Tritium-labeled ivermectin was studied in cattle, sheep, and rat for absorption, tissue residue distribution, and excretion at doses of 0.3 mg/kg of body weight. The drug was absorbed by various dosing routes. By intraruminal and subcutaneous dosing routes, highest tissue residues were present in fat and liver of cattle, with half-lives of 6-8 and 4-5 days, respectively. Shorter half-lives (1-2 days) were observed in sheep and rat. The tissue residue distribution pattern was essentially the same for all species studied and similar in male and female rats. With doses of tritium-labeled avermectin B{sub 1a} ranging from 0.06 to 7.5 mg/kg of body weight, plasma and tissue residue concentrations increased proportionally with the dose. When ivermectin was administered by various routes (ip, sc, iv, oral, and intraruminal), blood residue levels converged to 20-50 ppb 4 h after dosing and then depleted at similar rate regardless of the dosing route. Ivermectin was excreted primarily in the feces, with only less than 2% of the doses being eliminated in the urine in all three species studied.