The disposition of 14C-levamisole in the lactating cow.

1. 14C-Levamisole 1(-)-2,3,5,6-tetrahydro-6-phenyl[U-14C]imidazo[2,1-b]-thiazole was administered orally and subcutaneously to lactating cows (8 mg/kg body weight). Urine, faeces, milk and blood samples were collected from 0-48 h after dosing and tissues were collected 48 h after dosing. 2. 14C-Labelled residues (ppm 14C-levamisole equivalents) in blood were highest at 3 h (2.2 ppm, oral dose) or 6 h (2.1 ppm, subcutaneous dose) and then declined to less than 0.5 ppm 48 h after dosing. 3. 14C-Labelled residues in milk were highest in samples collected from 0-12 h after dosing (1.55 ppm and 1.86 ppm of levamisole equivalents from oral and subcutaneously dosed animals, respectively) and declined to 0.06 ppm in milk collected from 36-48 h after dosing. Milk collected from 0-48 h after dosing accounted for 0.2% (oral dose) and 0.6% (subcutaneous dose) of the total 14C-activity administered as 14C-levamisole. The parent compound, 14C-levamisole, accounted for 12% or less (declined with time after dosing) of the total 14C-activity in the milk. Three 14C-labelled metabolites (formed by oxidation of imidazoline ring and/or opening of thiazolidine ring) in the milk were isolated and identified. 4. Urinary excretion accounted for 83% and 84% and faecal excretion accounted for 11% and 9% of the total 14C-activity given orally and subcutaneously, respectively, as 14C-levamisole. No 14C-levamisole was detected in the urine; the major urinary metabolite (formed by opening of thiazolidine ring) was isolated and identified. 5. The 14C-activity remaining in the animals 48 h after dosing was widely distributed in body tissues; however, the concentration in the liver was substantially higher than in all other tissues examined. Less than 5% of the 14C-activity in the liver was present as 14C-levamisole.