[Toxicological considerations in the evaluation of veterinary drugs]. / Toxicologische overwegingen bij de beoordeling van diergeneesmiddelen.

Interactions between veterinary pharmacotherapy, toxicology of residues, prevention of residues of veterinary drugs and the evaluation of veterinary drug files are discussed on the basis of a number of examples. Sulphadimidine is used to treat atrophic rhinitis in medicated feeds which do not benefit the animal but are the cause of persistent sulphonamide residues in feed mills and husbandry. Carbadox is a potentially effective prophylactic feed additive for the prevention of swine dysentery, but is mostly used in high dosages which are almost toxic for the animals, and used during unnecessary prolonged periods. It is also prescribed as a therapeutic agent in which case a symptom of poisoning, dry faeces, is mistaken for a sign of recovery. Carbadox and/or its metabolites are carcinogenic and its use should be restricted to a bare minimum. Furazolidone is an example of an effective veterinary drug, the use of which should be limited by the fact that detoxification mechanisms of the animals, may result in the appearance of reactive metabolites which are available in the gastro-intestinal tract of the consumer. The central issue in a 'minimal residue' policy regarding the use of veterinary drugs should be the selection of effective drugs. Such a selection could result in a significant reduction of the incidence of veterinary drug residues. Second to this issue is the question of the extent to which residue toxicology should modulate the use of veterinary drugs.

Reversible interaction of a reactive intermediate derived from furazolidone with glutathione and protein.

Swine liver microsomes convert the nitrofuran furazolidone into N-(4-cyano-2-oxo-3-butenylidene)-3-amino-2-oxazolidone, a reactive open-chain acrylonitrile derivative. This derivative may be trapped with such thiol-group-containing agents as glutathione and mercaptoethanol. However, this reaction is reversible; e.g., adding an excess of mercaptoethanol to an aqueous solution (pH 7.4) of the glutathione conjugate results in conversion of 43% of this compound into the mercaptoethanol conjugate. In addition, when microsomal protein is added to the glutathione conjugate or the mercaptoethanol conjugate, 36 and 44%, respectively, become covalently bound to the protein. The amount of this covalently bound radioactivity decreases again on prolonged incubation at 37 degrees C (42% disappearance within 24 hr), suggesting that the acrylonitrile derivative also reacts reversibly with thiol groups of microsomal protein. Indeed an excess of mercaptoethanol could remove covalently bound radioactivity from microsomal protein resulting in the formation of the mercaptoethanol conjugate. The reversibility of the reaction is dependent on pH, as is demonstrated for the mercaptoethanol conjugate. Below pH 2 this conjugate is stable; optimal exchange to microsomal protein is found between pH 7 and 10. At very high pH (greater than 11) no binding to protein is found, although the conjugate disappears rapidly. The mercaptoethanol conjugate exhibits mutagenic activity in the Salmonella/microsome test indicating that the acrylonitrile derivative of furazolidone also interacts with DNA.

The elimination of furazolidone and its open-chain cyano-derivative from adult swine.

1. A sensitive method for the determination of 3-(4-cyano-2-oxobutylidene amino)-2-oxazolidone, the open-chain cyano-derivative of the veterinary drug furazolidone, in swine plasma and tissues is described. 2. After dosing adult swine orally with furazolidone (690 mg/animal per day) for 10 days no furazolidone was detected in liver, kidney and muscle (less than 2 ng/g). The half life of furazolidone as measured from the terminal phase of the plasma curves was 45 minutes. In urine, small amounts (less than 0.3% of total dose) of furazolidone were detected. 3. In contrast to other animals, 3-(4-cyano-2-oxobutylidene amino)-2-oxazolidone is a minor metabolite in swine with a plasma half life of 4 h. No cyano-derivative was detected in liver and kidney (less than 5 ng/g) 2 h after the last administration of furazolidone; 24 h after the last administration, the concentration in plasma was less than 2 ng/ml and in muscle less than 5/g. 4. The cyano-derivative was not mutagenic in the Salmonella/microsome test, with or without metabolic activation.

Identification of a reactive intermediate of furazolidone formed by swine liver microsomes.

Furazolidone (N-(5-nitro-2-furfurylidene)-3-amino-2-oxazolidone) is metabolized by swine liver microsomes under aerobic and anaerobic conditions (rate: 2.55 and 3.25 nmol/mg protein/min, respectively). Covalent binding to microsomal protein amounted aerobically to 0.29 nmol/mg protein/min. Of all amino acids tested, only addition of cysteine to the incubation mixture decreased microsomal protein binding of furazolidone, indicating that covalent binding may occur at protein thiol groups. Two known metabolites of furazolidone, 3-(4-cyano-2-oxobutylidene-amino)-2-oxazolidone and 2,3 dihydro-3-cyano-methyl-2-hydroxyl-5-nitro-1 alpha,2-di(2-oxo-oxazolidin-3-yl) iminomethyl-furo[2,3-b] furan, were minor metabolites. At least 50% of total metabolites is formed by swine liver microsomes via a reductive process of furazolidone as indicated by the formation of a furazolidone-mercaptoethanol conjugate after the addition of mercaptoethanol to the incubation mixture. The conjugate was identified as 3-(4-cyano-3-beta-hydroxyethylmercapto-2-oxobutylidene amino)-2-oxazolidone, indicating that the open-chain acrylonitrile-derivative is the reactive intermediate of furazolidone which also may be responsible for interaction with protein.

Quantitative studies of the metabolism of furazolidone by rat liver microsomes.

The nitrofuran, furazolidone, is metabolized by rat liver microsomes under both aerobic and anaerobic conditions, the rates for microsomes from 3-methylcholanthrene-induced male rats being 3.52 and 4.26 nmol/mg protein/min, respectively. The two major metabolites formed are the well-known 3-(4-cyano-2-oxobutylideneamino)-2-oxazolidone, and 2,3-dihydro-3-cyanomethyl-2-hydroxy-5-nitro-1alpha,2-di-(2-oxo-oxazolidin-3-yl)iminomethylfuro[2,3-b]furan (accounting for 16.6 and 16.4% of total extractable radioactivity, respectively). Cytochrome P-450 is not involved in the conversion of furazolidone, which was converted by rat liver microsomes to products identical to those obtained upon incubation with purified NADPH-cytochrome P-450 reductase, which is a microsomal reductase. During metabolism, a considerable amount of material (2-3% of total metabolites) became covalently bound to microsomal protein. This covalent binding could be inhibited by addition of glutathione, which also resulted in an almost complete shift from non-polar to water-soluble metabolites. No interaction of furazolidone with added calf thymus DNA was detected.

[Chronic furazolidone poisoning in swine? Practical problems and legal consequences]. / Chronische furazolidon vergiftiging van biggen? Praktijkproblemen en juridische gevolgen.

Long-term treatment (eighteen months) with a medicated feed containing 400 ppm of furazolidone gave rise to side-effects on a pig farm on which the animals were affected with persistent oedema disease. These side-effects disappeared on discontinuation of the medication. In view of this suspected case of chronic furazolidone intoxication the court formulated several questions for a panel of experts. The questions and the replies given are stated.

[Pharmacology and toxicology of furazolidone and carbadox; a literature study]. / Farmacologie en toxicologie van furazolidon en carbadox; een literatuuronderzoek.

A literature search on furazolidone in pigs was made in order to be able to answer a number of questions raised by a court regarding possible furazolidone intoxication. The following review of the literature is the result of this study. Attention is also paid to the residual toxicology of this drug. As one of the court's questions concerned carbadox alone and combined with furazolidone, a brief review of the pharmacology of this drug is included.