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Home Ames Test Mouse Lymphoma Assay Chromosome Aberration Test Micronucleus Test Rat Liver UDS Test Comet Assay ICH and FDA recommendations Links |
Introduction to Most human carcinogens are identified by epidemiological studies. These studies are necessarily long term, as no effect is expected to be observed until decades after the carcinogenic event or events. However convinving, these studies are costly and exposure levels and effects are difficult to quantify. A few multiple generation mutation assays have been carried out using rodents:
Short-Term Screening Tests Short-term screening tests rely on detection of DNA damage (or its immediate outcome) as an indicator for potential human mutagenicity and carcinogenicity. Because they detect carcinogenicity quite directly can help identify virtually all human mutagens, but only carcinogens which act directly on DNA (initiating agents). This is an important distinction. They cannot detect indirect carcinogens such as certain hormones and asbestos.The actual nature of the damage measured is referred to as an "endpoint." Such different endpoints would include chromosome breakage or point mutations. Originally, it was thought that a range of tests would be needed to detect different types of DNA damage. Years of observations have shown, empirically, that this is not the case, and that nearly every chemical that causes chromosome breakage in short-term screening tests will show evidence of point mutation induction in those same tests, and vice-versa. Thus, the standard battery of tests involved a mixture of sensitive in vitro tests using cultured cells to measure potential effects and realistic in vivo tests designed to measure risk. There exists a wide variety of testing methods, but few are routinely used. In vitro Tests In vitro tests include the Ames test (bacterial mutation test), the mouse lymphoma assay (mammalian cell mutation test) and the chromosome aberration test. These tests are highly sensitive to DNA-damaging effects and can be expected to detect a large majority of agents likely to be a genetic hazard in animals. Nearly all mutagens are detectable in in vitro genotoxicity tests. However, many substances that are positive in vitro for genotoxic effects are weakly active, or inactive in animals. This is expected to be because the compound does not interact with DNA in vivo. Conversely, some compounds that yield negative in vitro genotoxicity tests are acivated in vivo where they will produce positive results. In vivo Tests In vivo tests include the micronucleus test and the rat liver UDS test. These in vivo tests are important when high levels of human exposure are likely, where the compound has shown evidence of activity in an invitro test, or where the compound has similar structure to a known mutagen. A preliminary toxicity test is advisable to set dose levels. The highest level used int he actual test is the lowest of the maximum tolerated dose (MTD), 2000 mg/kg, or the maximum administrable amount. The most commonly examined target organ is the rodent bone marrow. The micronucleus test is sensitive to many aneuploidy-inducing agents. Consequently, the mouse bone marrow micronucleus test is the most widely used short-term assay for identification of genotoxic effects (chromosome damage and aneuploidy) associated with mutagens and carcinogens, and allows consideration of various factors including pharmacokinetics, metabolism and DNA repair that cannot be accurately modeled in an in vitro system. 
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