In Vitro Approach to Addressing Violative Drug Residues in Cattle

Abstract

Violative residues of the veterinary drug flunixin (FNX) are sometimes found in cattle that were presumably slaughtered in accordance with the withdrawal period prescribed by federal guidelines. A possible cause for violative residues may be slow rates of FNX metabolism associated with sex, age or illness of animals. Experiments were designed to investigate the effects of sex and age on rates of flunixin metabolism using liver microsomal, S9, cytosol, and homogenate fractions from heifers (n=5) and steers (n=6) less than 30 months of age, and cows (n=8) older than 48 months of age. Validation of subcellular fractions was achieved through enzyme analyses: cytochrome P450 (P450), NADPH‐P450 reductase and glucose‐6‐phosphate dehydrogenase (G6PDH) activities under optimized conditions. Similarly, conditions for flunixin metabolism were optimized and fractional activity and metabolite formation was determined. Cows had lower (P < 0.05) concentrations of P450, NADPH‐P450 reductase activity, and FNX metabolism than their younger counterparts (heifers or steers). Using 15 min incubations, 5‐hydroxy FNX was the major metabolite detected by LC MS‐MS. Variables including exact animal age, cattle breed, diet, health and physiological status of animals at slaughter could have influenced results but were not assessed because of limitations associated with tissue collection.Support or Funding InformationAll funding of work was provided by USDA‐ARS.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.