Mechanistic toxicity studies

To support the mechanistic interpretation of safety studies (as an alternative or a complement to toxicity studies in normal animals). 

Many toxicity studies on contaminating metals show a ceUular oxidative stress situation not mechanistically defined in terms of the subcellular localization of the initial damage. Growing evidence shows that metals such as Zn, Pb-+, and Cd and ambient particulate matter may exert their toxic action involving mitochondrial oxidative stress and dysfunction [58]. 

Conventional rodent toxicity studies characterize adverse effects of a chemical primarily on apical endpoints such as clinical signs or pathological states. Evidence of organ toxicity in the form of an apical endpoint does not always provide mechanistic understanding of the toxicity involved (see Chapter 13). The exposure of rodents in a cancer bioassay model can result in species-specific responses that are not relevant to humans (e.g., alpha2u-globulin-induced rat renal tumors) (see Chapter 18) (EPA 1991). Rodents may also have increased sensitivity to a particular toxicity pathway relative to humans (e.g., disruption of thyroid homeostasis and thyroid follicular tumors in rodents) (EPA 1998 I ARC 2001). There are rodent responses to chemical treatment in tissues where there is a high spontaneous incidence to develop... 

Mechanistic studies - any study necessary to clarify effects reported in toxicity studies Studies on other routes of administration (parenteral routes)... 

Note 2 There may be extensive public information available regarding potential reproductive and/or developmental effects of a particular class of compounds (e.g., interferons) where the only relevant species is the non-human primate. In such cases, mechanistic studies indicating that similar effects are likely to be caused by a new but related molecule, may obviate the need for formal reproductive/developmental toxicity studies. In each case, the scientific basis for assessing the potential for possible effects on reproduction/development should be provided. 

Mechanistic studies to identify how endocrine disrupting chemicals interact with hormone systems are required. Although population effects coupled with biomarkers of exposure are strongly suggestive of endocrine disruption, the effect could be secondary to metabolic toxicity. Establishing mechanisms may avoid the need to make decisions on a weight of evidence approach alone. 

What is needed is an alternative approach which permits development of valid cause and effect relationships. This strategy, one involving intensive surveys, is referred to here as mechanistic. The Willamette River, Oregon, USA, is used as a case study to illustrate quantitative, semi-quantitative and qualitative approaches to mechanistic assessment of river water quality using, respectively, dissolved oxygen depletion, erosion/deposition and potentially toxic trace elements as examples. 

The goals of this chapter do not include a "state of the art" literature review which would be appropriate for a more in-depth discussion of one particular problem area. Rather the intent is to illustrate mechanistic approaches to river quality assessment using the three globally relevant water quality problem areas discussed in the previous chapter dissolved oxygen depletion, erosion/deposition, and potentially toxic trace elements. The information provided does not include all rationale, methology or approaches used in the study as this is beyond the scope of the chapter. Additional general information on application of the intensive river quality assessment approach in the Willamette River basin may be found elsewhere (4-9, 11-14, 17). 

Mechanistic studies have shown that TBT and certain other forms of trialkyltin have two distinct modes of toxic action in vertebrates. On the one hand they act as inhibitors of oxidative phosphorylation in mitochondria (Aldridge and Street 1964). Inhibition is associated with repression of ATP synthesis, disturbance of ion transport across the mitochondrial membrane, and swelling of the membrane. Oxidative phosphorylation is a vital process in animals and plants, and so trialkyltin compounds act as wide-ranging biocides. Another mode of action involves the inhibition of forms of cytochrome P450, which was referred to earlier in connection with metabolism. This has been demonstrated in mammals, aquatic invertebrates and fish (Morcillo et al. 2004, Oberdorster 2002). TBTO has been shown to inhibit P450 activity in cells from various tissues of mammals, including liver, kidney, and small intestine mucosa, both in vivo and in vitro (Rosenberg and Drummond 1983, Environmental Health Criteria 116). 

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