Genetic toxicants

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. 

SCGE technique to assess genetic toxicity of allelochemicals... 

By the time Phase III testing is completed, some additional preclinical safety tests must also generally be in hand. These include the three separate reproductive and developmental toxicity studies (Segments I and III in the rat, and Segment II in the rat and rabbit) and carcinogenicity studies in both rats and mice (unless the period of therapeutic usage is intended to be very short). Some assessment of genetic toxicity will also be expected. 

As a Screen. An agent that is positive in one or more genetic toxicity tests may be more likely than one that is negative to be carcinogenic and, therefore, may not warrant further development. 

To Provide Mechanistic Insight. For example, if an agent is negative in a wide range of genetic toxicity screens, but still produces tumors in animals, then one could hypothesize that an epigenetic mechanism was involved. 

While not officially required, the FDA does have the authority to request, on a case-by-case basis, specific tests it feels may be necessary to address a point of concern. A genetic toxicity test could be part of such a request. In general, therefore, companies deal with genetic toxicity (after screening ) on a case-by-case basis, dictated by good science. If more than a single administration is intended, common practice is to perform the tests prior to submitting an IND. 

In summary, genetic toxicity tests with both bacterial and mammalian cells are normally carried out with rat liver cell-free systems (S9 fraction) from animals pretreated with enzyme inducers. However, investigations should not slavishly follow this regimen there may be sound scientifically based reasons for using preparations from different species or different organs, or for using whole cells such as hepatocytes. 

There are a number of test systems that use cultured mammalian cells, from both established and primary lines, that now have a large database of tested chemicals in the literature, are relatively rapid, and are feasible to use for genetic toxicity screening. These are discussed in the next section. 

Tennant, R.W., Margolin, B.H., Shelby, M.D., Zeiger, E., Haseman, J.K., Spalding, J., Caspary, W., Resnick, M., Stasiewicz, S., Anderson, B. and Minor, R. (1987). Prediction of chemical carcinogenicity in rodents from in vitro genetic toxicity assays. Science 236 933-941. 

Legator MS, Ward JB Jr. 1984. Genetic toxicity Relevant studies with animals ahd humans. In Reproduction lie new frontier in occupational and environmental health research. Prog Clin Biol Res 160 491-525. 

There is no clear evidence for genetic toxicity of mescaline. Chromosomal damage in lymphocytes was not evident in Huichol Indians, who have a 1,600-year cultural tradition of religious peyote use (Dorrance et al. 1975). 

Tennant RW, Margolin BH, Shelby MD, et al. 1987. Prediction of chemical carcinogenicity in rodents from in vitro genetic toxicity assays. Science 236 933-941. 

Then, in the mid 1970s, Professor Bruce Ames of the University of California at Berkeley came along. We discussed in Chapter 5 Professor Ames role in the development of tests for genetic toxicity, tests that tell us something about mechanisms of carcinogenicity. 

McKee RH, Amoruso MA, Freeman JJ, Przygoda RT. 1994. Evaluation of the genetic toxicity of middle distillate fuels. Exxon Biomedical Services, Inc., East Millstone, NJ. 

Today, in the United States, the FDA is responsible for examining safety and efficacy data before an antibiotic or synthetic chemical may be commercialized for livestock use. This includes studies on formulations, product stability, conventional and genetic toxicity, environmental safety, metabolism, residue studies in target animals, studies on antibiotic resistance in gut microflora and on salmonella shedding in target animals. Similar requirements are part of registering these products in overseas markets. In general, after a product is discovered in the laboratory, many... 

ICCVAM has evaluated alternative test methods for acute oral toxicity, genetic toxicity, biologies, immunotoxicity, dermal corrosion and irritation, ocular toxicity, developmental toxicity, pyrogeni-city, and endocrine disrupter effects (ICCVAM 2007). As examples, alternative test systems for dermal corrosion and irritation are described in the following text. 

Metabolism and genetic toxicity have been reported to differ with the isomer of nitro-toluene. p-Nitrotoluene was not mutagenic in bacterial assays, but it did increase sister chromatid exchange frequencies and chromosomal aberrations in vitro-, in vivo it did not increase the frequency of micronuclei in bone marrow of treated rodents. Similar findings were reported for the ortho isomer, except that it did not induce chromosomal aberrations in vitro. Only the ortho isomer induces DNA excision repair in the in vivo-in vitro hepatocyte unscheduled DNA synthesis assay. Furthermore, ort/jo-nitrotoluene binds to hepatic DNA to a much greater extent than meta- or para-nitrotoluene, and investigators suggest that it may act similarly to the rodent hepatocarcino-gen 2,6-dinitrotoluene. ... 

Genetic Toxicity In Vitro Approaches for Hit and Lead Profiling... 

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