The pharmacokinetic aspects of tulathromycin (2.5 mg/kg b.w.) were studied following intravenous administration alone and in combination with flunixin meglumine (2.2 mg/kg b.w) in apparently healthy goats. Tulathromycin concentrations in serum were determined by microbiological assay technique using Bacillus subtiles (ATCC 66343) as test organism. The half-lives of distribution and elimination (t0.5(a)and to.5(p)) were 0.071, 0.046 and 6.43, and 5.05 h. following intravenous injection of tulathromycin alone and in combination with flunixin, respectively. Volume of distribution at steady state (Vdss) was 0.249 and 0.96l/kg., mean residence time (MRT) was 6.27 and 5.99 h and total body clearance (ClB) was 0.046 and 0.17 l/kg/hr., respectively. It was concluded that flunixin significantly altered the pharmacokinetics of tulathromycin by increase its distribution and accelerate its elimination from body. Therefore care should be taken during use of tulathromycin in goats concurrently with flunixin.

OBJECTIVE To describe plasma pharmacokinetic parameters and tissue elimination of flunixin in veal calves. ANIMALS 20 unweaned Holstein calves between 3 and 6 weeks old. PROCEDURES Each calf received flunixin (2.2 mg/kg, IV, q 24 h) for 3 days. Blood samples were collected from all calves before the first dose and at predetermined times after the first and last doses. Beginning 24 hours after injection of the last dose, 4 calves were euthanized each day for 5 days. Plasma and tissue samples were analyzed by ultraperformance liquid chromatography. Pharmacokinetic parameters were calculated by compartmental and noncompartmental methods. RESULTS Mean ± SD plasma flunixin elimination half-life, residence time, and clearance were 1.32 ± 0.94 hours, 12.54 ± 10.96 hours, and 64.6 ± 40.7 mL/h/kg, respectively. Mean hepatic and muscle flunixin concentrations decreased to below FDA-established tolerance limits (0.125 and 0.025 μg/mL, respectively) for adult cattle by 3 and 2 days, respectively, after injection of the last dose of flunixin. Detectable flunixin concentrations were present in both the liver and muscle for at least 5 days after injection of the last dose. CONCLUSIONS AND CLINICAL RELEVANCE The labeled slaughter withdrawal interval for flunixin in adult cattle is 4 days. Because administration of flunixin to veal calves represents extralabel drug use, any detectable flunixin concentrations in edible tissues are considered a violation. Results indicated that a slaughter withdrawal interval of several weeks may be necessary to ensure that violative tissue residues of flunixin are not detected in veal calves treated with that drug.