Genetic toxicity tests

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. 

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. 

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. 

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. 

Pharmacokinetic profiling (ICH-S3B 1995) may prove useful. Review of the battery of genetic toxicity tests ICH-S2B (1997) and the ICH reproduction toxicity guidances (S5A) (1994) and S5B (1996, 2000) are valuable. 

Tennant, R. W., et al. Prediction of chemical carcinogenicity in rodents from in vitro genetic toxicity tests. Science 236 933-941, 1987. 

Dieldrin has been undergoing in vitro mutagenesis/genetic toxicity testing (CH0 cells) to have been completed in 1985. Testing on endrin for mammalian mutagenesis/genetic toxicity (mouse lymphoma cells) was to have started in 1985 (ref. 89, pp. 37 and 49). 

S-9 A metabolic activation mixture that is used with many in vitro genetic-toxicity tests to provide for the conversion of promutagens into mutagens the enzymatic activities of an S-9 mixture are those of a post-mitochondrial supernatant (i.e., microsomal and cytosolic enzymes) derived from a mammalian liver homogenate the expression "S-9" originally referred to supernatant from centrifugation at 9,000 rpm. 

Acute or repeated dose studies of up to 90 days are conducted in rats and mice on selected agents under the direction of NIEHS. Results from genetic toxicity testing and pharmacokinetic studies are frequently presented. 

FDA s recommendations. The regulations for submitting food contact notifications stipulate that all relevant toxicity data available to the notifier be made available to FDA. The rationale and utility of the recommended genetic toxicity tests are discussed in detail by the FDA s Redbook (FDA, 2004). 

For a constituent of an additive with equivocal (mixed positive and negative) battery of genetic toxicity tests and a EDI of < 150 pg/p/d, SAR analysis for SAs and predictive software such as MDL QSAR (Contrera et al, 2005) and MultiCASE s MC4PC (Rosenkranz and Klopman, 1988 Matthews and Contrera, 1998) may be used as part of the weight of evidence approach in assessing the safety of the compound. If a constituent were of potential concern, a quantitative SAR analysis might be feasible to characterize the expected risk. 

This theoretical example of a QSAR assessment is presented to exhibit FDA s current approach to using S AR as a tool in the safety evaluation of substances proposed for use as food contact materials. If anthrafurin (1,6-dihydroxy anthraquinone CASRN 117-12-4 Fig. 7.3) were expected to be an impurity in a food contact material, an immediate initial concern would be raised due to reports in the literature of positive results in the bacterial reverse mutation assay and other in vitro genetic toxicity tests. A literature search did not identify relevant carcinogenicity data for anthrafurin. 

The oral LD50 in rodents ranges from 1 to 7 g kg and intravenous LD50 in mice and rats is lOgkg. No acute lethality information is available following either dermal or inhalation exposures. Acetamide is not a developmental toxicant and is generally inactive in genetic toxicity tests. 

Shelby MD, Allen JW, Caspary WJ, et al. (1987) Results of in vitro and in vivo genetic toxicity tests on methyl isocyanate. Environmental Health Perspectives 72 183-187. 

Premise 1. The Salmonella mutation test is a necessary component of genetic toxicity testing schemes. 

Zeiger, E. (1998). Identification of rodent carcinogens and noncarcinogens using genetic toxicity tests premises, promises and performance. Regul Toxicol Pharmacol 28, 85—95. 

Zeiger, E. (2004). The history and rationale of genetic toxicity testing—An impersonal, and sometimes personal, view. Environ Mol Mutagen 44, 363—371. 

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