The use of nitrofurans as veterinary drugs has been banned in the EU since 1993 due to doubts on the safety of the protein-bound residues of these drugs in edible products. Following treatment of pigs with the veterinary drug furazolidone free 3-amino-2-oxazolidinone (AOZ), the side-chain of the drug, could be detected in the blood in concentrations up to 0.3 g/ml. The identity of the free AOZ was confirmed by LC/MS. This shows that the side-chain can be released from the parent drug, most likely under the acidic conditions in the stomach. Free AOZ was also detected in the blood of rats fed pig liver with protein-bound residues of furazolidone. Incubation of isolated pig hepatocytes with radiolabeled AOZ, resulted in the formation of protein-bound metabolites, to a similar extent as observed with furazolidone itself. Much lower levels were formed in the presence of dimethylsulfoxide or 4-chlorobenzenesulfonamide, most likely due to inhibition of the enzyme involved in the metabolic activation of AOZ. These compounds also prevented the inhibition by AOZ of monoamine-oxidase (MAO) activity in pig hepatocytes. These data strongly indicate that the protein-bound metabolites of furazolidone in tissues of treated animals are derived following metabolic activation of furazolidone itself, but also of the free AOZ side-chain, following its release from the parent drug. In addition to the MAO-inhibition and formation of protein-adducts, AOZ gave a dose-related positive respons in the Salmonella/microsome mutagenicity test especially in the presence of rat liver S9-mix, in tester strains TA 1535 and TA 100. Furthermore, a positive response was obtained in the chromosome aberration test with human lymphocytes and in the bone marrow micronucleus test with mice treated intraperitoneally with AOZ. It is concluded that ingestion of protein-bound residues of furazolidone results in the release and absorption of AOZ, a compound with potential mutagenic properties. This is the first report showing that protein-bound residues of veterinary drugs can be of toxicological significance.
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