Tissue drug residues and their toxicological significance

Veterinary drugs no longer need testing for carcinogenicity in rodent bioassays

The putative carcinogenic risk to humans from ingestion of edible tissues containing traces of nongenotoxic veterinary drugs is so slight that the routine application of rodent cancer bioassays cannot be justified. This argument is based, first, on the pharmacological similarity of veterinary and human drugs: many of the latter that are carcinogenic to rodents have been deemed on mechanistic and/or potency grounds not to pose a cancer risk to humans. Second, the distribution of a veterinary drug through the target animal body before ingestion of a portion of edible tissue by humans days or weeks later means that the human dose from a residue is several orders of magnitude lower than the normal dose of human drugs. The dose of residue is also much lower than the exposure of humans to the most potent carcinogens.

Evaluation of the toxicity of drug residues in food: the case for 'relay' toxicity studies

The tissue residues of veterinary drugs given to food-producing animals are an ill-defined mosaic of parent drug and metabolites. Consumers are only ever exposed to this mixture in the presence of a vast excess of the excipient, food. Given the modifying effects of food on the absorption and disposition of co-administered xenobiotics, it follows that the toxicity of these residues can only be properly evaluated in the presence of a large excess of food. Adoption of a relay toxicity strategy addresses these two points. The proposed relay toxicity testing approach is as follows. The target species receives a recommended dosage regimen of the drug, but a dose level three- to five-fold higher than normal and the animals are then killed several days earlier than the projected withdrawal period. The tissues from these animals, containing the residue mixture at artificially high concentrations, are then administered to laboratory animals for conventional toxicological evaluation. In this approach the residues do not require individual identification and their potential toxicity is evaluated in the presence of the inescapable excipient, food. Determination of an 'exposure' level without observerable toxicity provides, in principle, the means of relating human safety to a No-Observed Effect Level in laboratory animals in the traditional manner.

Sulfadimidine metabolism in vitro: II. Comparative studies in cultured rat, goat, sheep and cattle hepatocytes

Hydroxylation and acetylation of sulphadimidine (SDD) and the deacetylation of N4-acetyl SDD was investigated in cultured hepatocytes from male and female rats, from male and female goats and from female sheep and cattle. Significant sex differences were observed for hydroxylation of SDD in hepatocytes from rat and goat. In goat, sheep and cow hepatocytes, the hydroxylation pathway is relatively important, whereas in rat hepatocytes, acetylation is predominant. Hepatocytes of all four species deacetylated N4-acetyl SDD. In ruminant hepatocytes, deacetylating activity was of considerable importance, whereas in rat hepatocytes, it appeared a minor pathway of metabolism. Similar to the in vivo situation, formation of N4-acetyl SDD in cultured hepatocytes results from an equilibrium of acetylation and deacetylation. A good correlation was found between results in isolated hepatocytes and previous findings in vivo, both in levels of species-related activities and in acetylation-hydroxylation ratios. In conclusion, cultured hepatocytes appear a useful in vitro model to study comparative sulfonamide metabolism.

Interspecies comparisons in toxicology: the utility and futility of plasma concentrations of the test substance

A classical dilemma in toxicology is how the dose administered relates to the dose delivered to the target site. Plasma concentrations of the test substance may be misleading since the concentration of any given substance in the plasma may not be representative of its concentration in tissues. Furthermore, a given tissue concentration of a xenobiotic can evoke responses which are highly species-dependent. While evaluating toxicity data within one species, plasma concentrations reflect the effects of route of administration, bioavailability, dose level, multiple dosing, age, gender, etc. However, when toxicity data is compared across species, the relevance of plasma concentrations depends on the nature of the toxicity. Reversible, pharmacodynamic effects often correlate with plasma concentrations, although there may be marked interspecies differences in dose-response relationships. Irreversible effects, if pharmacodynamic in origin, often correlate better with the intensity/duration of the pharmacodynamic response, rather than with plasma concentration. On the other hand, irreversible effects, if chemically mediated, may not correlate at all with plasma concentration, the lesions being caused by reactive metabolites of fleeting existence, which rarely survive long enough to leave their site of synthesis. They cannot be measured in the plasma nor predicted from plasma concentrations of the parent xenobiotic. The limitations of plasma concentrations in interpreting the toxicology of substances which are tissue-sequestered, which are subject to pharmacogenetic factors, or which show plasma concentrations that are not proportional to dose are also discussed. Mention is made of possible alternatives to plasma concentrations in assessing exposure in toxicology studies.