Metabolite toxicity testing

The full extent of the toxicity of pesticides to aquatic life is not known. Although chronic toxicity testing is required for new substances, little is known about the long-term effects of older pesticides. Also, very little is known about the toxicity and occurrence of the products formed when pesticides break down (metabolites) or the many non-pesticidal additives (co-formulants and adjuvants) used in pesticide formulations. However, the future is looking brighter. New modelling techniques, EQS development, and the involvement of the NRA in the pesticide registration process, coupled with the development of newer, less persistent pesticides with lower dose rates, all should help to reduce the risk of pesticide pollution. 

The presence of chemically reactive structural features in potential drug candidates, especially when caused by metabolism, has been linked to idiosyncratic toxicity [56,57] although in most cases this is hard to prove unambiguously, and there is no evidence that idiosyncratic toxicity is correlated with specific physical properties per se. The best strategy for the medicinal chemist is avoidance of the liabilities associated with inherently chemically reactive or metabolically activated functional groups [58]. For reactive metabolites, protein covalent-binding screens [59] and genetic toxicity tests (Ames) of putative metabolites, for example, embedded anilines, can be employed in risky chemical series. 

If the systemic exposure for a major human circulating metabolite is equivalent to that observed in nonclinical toxicological species, then the metabolite levels may be sufficient to limit additional toxicity testing using the major human metabolite. 

The nature of metabolic reactions and their variations between species is detailed in Chapters 7, 8, and 9 with some aspects of toxicokinetics in Chapter 6. The methods used for the measurement of toxicants and their metabolites are detailed in Chapter 25. The present section is concerned with the general principles, use, and need for metabolic and toxicokinetics studies in toxicity testing. 

The majority of toxicity tests (which particularly are subject to ethical criticism) are firmly based on studies in whole animals, because only in them is it possible to approach the complexity of organisation of body systems in humans, to explore any consequences of variable absorption, metabolism and excretion, and to reveal not only direct toxic effects but also those of a secondary or indirect nature due to induced abnormalities in integrative mechanisms, or distant effects of a toxic metabolite produced in one organ that acts on another. ... 

The potential for unusual health effects of chemical mixtures due to the interaction of chemicals or their metabolites (e.g., metabolites of trichloroethylene and benzene) in or with the biosystem constitutes a real issue in the public health arena. However, toxicity testing to predict effects on humans has traditionally studied one chemical at a time for various reasons convenient to handle, physiochemical properties readily defined, dosage could easily be controlled, biologic fate could easily be measured, and relevant data were often available from human occupational exposures. Chemicals are known to cause disease for example, arsenic and skin cancer, asbestos and lung cancer, lead and decrements of IQ, and hepatitis B predisposes to aflatoxin-induced liver cancer but the link between the extent of human exposure to even well-defined chemical mixtures and disease formation remains relatively unexplored, but of paramount importance to public health. 

A basic tenet of current practice is to assume that the same cellular exposure concentration leads to the same effects, both qualitatively and quantitatively, in vitro and in vivo. That may be approximately correct for small and early effects or for effects for which intercellular communications do not take part. In that case, it is enough to (1) develop a simple PD model of the dose-response relationship observed in vitro and (2) transpose it without changes to predict in vivo effects, with an in vivo input concentration profile reconstructed using a PBPK model, as mentioned above. Louisse et al. [45] used that approach to predict in vivo dose-response curves for developmental toxicity. The embryotoxicity of four glycol ether metabolites was tested in vitro. The concentration-response curves obtained were then used in conjunction with a PBPK model to reconstruct and predict the in vivo dose-response curves for the developmental toxicity of the parent glycol ethers to rats and humans. A good agreement was found in rats, for this endpoint and those substances, between the dose-response curves predicted and the embryotoxic dose levels reported in classical in vivo studies. 

The Corley model (Corley et al. 1994) is an expansion of the model of human inhalation exposure by Johanson (1986). In the Corley model, disposition of 2-butoxyacetic acid was included, and the model was expanded to include data on rats, the most commonly used species in experimental toxicity tests of 2-butoxyethanol. This model was further modified on the basis of data on human metabolites and dermal absorption of 2-butoxyethanol vapor (Corley et al. 1997). Other routes of exposure, including oral, dermal, and intravenous infusion, were included (Figure 2-11). This is a new model, however, and has not yet been subjected to extensive testing. 

The requirement of this study is to demonstrate that the toxicity test animal has been exposed to all of the major metabolites seen in the food animal. If a major food animal metabolite is not detected in the excreta of the test animal, residues in organs such as the liver and kidney of those animals are then profiled. If major metabolites from the food animal are still not found, then the sponsor must examine the metabolite profiles of the other laboratory species until all of the major food animal metabolites have been accounted for. If a majw metabolite from the food producing species is not detected in the test species, then separate feeding studies of the untest major metabolite must be undertaken unless its toxicity can be evaluated by some other means. Chromatographic techniques such as high performance liquid chromatography are often used for proHling metabolites. This type of metabolic evaluation is commonly found in the literature (6),... 

---