Flunixin in cattle

Following single daily intravenous injections of radiolabeled flunixin in cattle at the rate of 2.2 mg/kg bw for 3 consecutive days, the parent compound accounted for 50% of the extractable tissue radioactivity, composing the major residue in liver and kidney (94). Three metabolites were also present in both liver and kidney samples 4-hydroxyflunixin was present at higher levels, whereas 5-hydroxyflunixin and 2-methylhydroxyflunixin occurred at lower levels. 

Design and Conduct of Studies To Meet Residue Chemistry Requirements Residue Depletion and Metabolism of Flunixin in Cattle... 

The safe concentration of drug-related residue must be known in order to determine the withdrawal period for a veterinary product. Often the toxicity data is incomplete and an estimate must be made to progress with requisite residue studies. One approach is to conduct a total residue study with sufficiently widely-spaced sacrifice intervals to assess the rate of depletion of total residue over the projected range of probable safe concentrations. A zero-withdrawal sacrifice interval should be included. The target tissue and marker residue are identified and surveillance/confirmatory assays developed. If a major portion of residue is non-extractable (bound) and the marker is undetectable at times when total residue is still significant, a residue bioavailability study may be necessary. To complete the data package, final residue and comparative metabolism studies are conducted. Studies on the metabolism of flunixin in cattle will illustrate this approach. 

Second Total Residue Depletion Study vith Flunixin in Cattle. 

Despite use of alternate methods of tissue extraction, the percent of total radiolabeled residue extractable from the liver or kidney decreased with increasing length of time after dosing. In liver, the percent recovery of total radioactivity in the organic extract ranged from 70-942 at 12 hours, 61-722 at 24 hours, and had decreased to between 21 and 482 at 72 and 120 hours post final dose. These results suggest that bound residues may be formed during the metabolism of flunixin in cattle. 

Final Residue Depletion Study with Flunixin in Cattle. In the... 

Flunixin is registered or is under development in many countries for use in horses, cattle, and swine for treatment of equine colic, musculoskeletal disorders, acute endotoxin-induced mastitis in cattle, and respiratory disease (89-91). It is administered orally or parenterally for a maximum of 5 successive days (92). Flunixin is a genotoxic but not carcinogenic compound. Its mechanism of action is believed to be via the inhibition of cycloxygenase to reduce the presence of arachidonic acid metabolites produced during inflammation (93). 

Following development of the assays for the marker residue (flunixin), liver tissue from feeder cattle dosed intravenously daily for three days with C-flunixin NHG (second total residue depletion study) was assayed by the surveillance method. Based on this assay, mean values of flunixin in the liver at 12 and 24 hours post final dose were 531 and 36 ng/g tissue, respectively. Liver samples collected at 72 and 120 hours contained flunixin concentrations below the limit of quantitation of the assay. The flunixin concentrations at.12 and 24 hours represented less than one-third of the total u residues, and although no fluni n was detected at 72 and 120 hours, there were still detectable residues at these sacrifice intervals. 

The mean concentration of flunixin in the liver was 389 ng/g after 12 hours, 53 ng/g after 24 hours, and 13 ng/g after 48 hours of withdrawal from treatment. At 48 hours, only two of the five animals treated had residues above the limit of quantitation (8 ng/g). No flunixin was detected in the 72-hour withdrawal liver samples. Confirmation analyses by GC-MS indicated that detected residues were flunixin. As shown in Figure 2, the flunixin concentrations detected in the livers of treated cattle at 12 and 24 hours post final dose in both the total and final residue depletion studies are similar. However, in both studies, the concentrations of detected flunixin are low with respect to total radiolabeled residue. These results suggest that a significant portion of the total residue may be bound and/or the existing surveillance and confirmatory assays do not adequately extract flunixin residues from the tissue of treated animals. The need for further work on the bound residues and the existing assays will depend on the final safe concentration assigned to the drug. 

Results of the rat metabolism studies indicate that, as in cattle, the primary route of metabolism of flunixin is via oxidation of the aromatic ring system(s) and the methyl group on the phenyl ring. Flunixin and the two hydroxylated metabolites,... 

Three compounds, including flunixin and its 5-hydroxy- and 4 -hydroxy- metabolites were identified by co-elution with authentic standards in the liver and kidney extracts from male and female cattle. The structures of flunixin and its metabolites are presented in Figure 1. 

Profiling and identification of flunixin and its metabolites in the liver and kidneys of male and female feeder cattle was also carried out during the second total residue depletion study. Initially, liver tissue from cattle treated intravenously with... 

Figure 2. Depletion of total and flunixin residues in livers of cattle following intravenous administration of 2.2 mg flunixin/kg/day for three consecutive days.

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