Carcinogens/mutagens Ames test

Bacterial mutagenicity Carcinogenicity Compound (Ames test) (rats)... 

The initial limit based arbitrarily upon a known carcinogen could be relaxed upon submission of favorable short-term testing data for carcinogenicity and mutagenicity (Ames test, cell transformation, DNA repair, chromosome damage. Drosophila etc.,) The test compound could be compared to a known carcinogen (and mutagen). The new limit for the test substance would be decreased in proportion to its potency as related to the standard (e.g., bis-chloromethyl ether). 

The application of pattern recognition (PR) methods to this field has been studied by Jurs and co-workers, using series of compounds which were examined as synthetic sweeteners, nitrosamines as carcinogens, and mutagens (Ames test data) 30y 44). Chu and colleagues at NCI studied the application of several PR methods to some antitumor agents with encouraging results (45). These studies are discussed in detail later in this article. 

Corrosive to skin, eyes, upper respiratory tract and mucous membranes. - Not mutagenic (Ames test). Not included in NTP 9th Report on carcinogens. 

These principles of reverse mutation are utilized in one important method, the Ames test (section 4.4.2), which is used to detect compounds that act as mutagens or carcinogens (most carcinogens are mutagens). 

The Ames test involves the reversion from a his— to his+ phenotype in any one of multiple bacterial strains (usually five strains are tested simultaneously). If the addition of test compound to a his— strain of bacteria allows them to grow on histidine deficient media, the obvious conclusion is compound-induced mutagenesis and a high potential hazard for the compound being carcinogenic. This test can also be conducted in the presence or absence of metabolic activation, in order to provide more information on potential risks (i.e., the parent compound may not be mutagenic, but the primary metabolite may present a safety risk). In practice, a positive Ames test almost always leads to discontinuing work on a compound of interest, and so these data are always collected prior to nomination of a compound for development. 

In vitro models could be used to study the molecular mechanisms involved in the process of neoplastic transformation and as screening tools for the classification of the carcinogenic potential of a substance. Among the in vitro tests available to the scientific community, the CTA may represent an important tool for the identification of carcinogens, particularly those that are not identifiable by classic mutagenicity tests such as the Ames test. 

A variety of methods are available to test a chemical for mutagenicity, i.e., its effect on the genetic material. The Ames Test has gained most recognition as a shortterm test [23],This is a bacterial test which allows fast performance and requires limited expense. Its correlation with the mutagenicity of mammals or even with a carcinogenic effect on mammals or humans has repeatedly been tested [24], but remains controversial. 

Am ino-1-methy 1-6-phenyl irnidazo 4,5-b]pyridine (29 PhIP), usually the most abundant product of food-derived mutagens, is formed by heating creatine and phenylalanine at 200 °C200. This compound is modestly mutagenic in the Ames test, but is a potent carcinogen in rats and mice, causing breast and colon cancers. 

Ames test analy chem A bioassay that uses a set of histidine auxotrophic mutants of Salmomlla typhimurium for detecting mutagenic and possibly carcinogenic compounds. amz, test ... 

Now there are two points here first, the Ames test is useful second, mutagenicity correlates with carcinogenicity. The second point provides a strong clue as to the molecular origins of cancer. However, the correlation between mutagenicity and... 

The Ames test is very useful in identifying potential carcinogens. It uses mutagenicity as a surrogate for carcinogenicity. 

Several short-term bioassay procedures (3-14) have been developed recently which are applicable to detecting mutagenic and potential carcinogenic activity of organic substances. The SaimoneJla/mammalian mlcrosome assay or Ames Test (13-13] has been the most frequently applied and its efficacy has been well documented. This assay has also been applied to complex mixtures (19-22) to reduce greatly the time... 

In Chapter 1 Bruce N. Ames, the plenary speaker at the symposium, describes the development of the Salmonella/microsome assay, which is better known as the Ames Test. This test was the first proven rehable and rapid in-vitro method for the mutagenic and carcinogenic screening of chemicals. Included is an excellent overview of the potential problem of human exposure to toxic substances as well as a description of how this important method is being used to test chemical substances. 

Before administration of a NME to man, a mutagenicity test in bacterial cells (Ames test), with and without metabolic activation, and tests for chromosomal aberrations in mammalian cells should be negative. Any positive or equivocal results will require additional tests to be performed before proceeding to man. Studies of embryo-foetal toxicity should be performed before administration of a NME to women of reproductive potential. Studies of fertility, early embryonic development and pre- and post-natal development are not required at this stage of development neither are carcinogenicity studies. 

DAB was genotoxic in the comet assay inducing DNA damage in the stomach, colon liver, bladder, lung, and bone marrow. It is also mutagenic to Salmonella in the Ames test. Because of its demonstrated carcinogenicity in animals, human exposure to DAB by any route should be avoided. In recent years, this compound has been used only in laboratories as a model of tumorigenic activity in animals. It is not produced commercially in the United States and is of little occupational health importance. 

Figure 6.55 Log-log plot of relative carcinogenic potencies for a group of chemicals against mutagenic potency as determined by the Ames test. The potency is the amount of chemical giving tumors in 50% of the animals or 100 mutant colonies (revertants) of Salmonella bacteria. Aflatoxin B, is the most potent and benzidine the least potent. Abbreviations Moca, 4,4,-methylene-b/s-(2-chloroaniline) mms, methylmethane sulfonate. Source From Ref. 16.

The exact mechanism underlying the carcinogenesis is less clear, but presumably involves inhibition of RNA synthesis or the production of abnormal ethylated nucleic acids and hence disruption of transcription, translation, or possibly replication. It is of interest that ethionine is not mutagenic in the Ames test, with or without rat liver homogenate. However, ethionine may be carcinogenic after metabolism to vinyl homocysteine (in which vinyl replaces ethyl), which is highly mutagenic. 

FIGURE 25-19 Ames test for carcinogens, based on their mutagenicity. A strain of Salmonella typhimurium having a mutation that... 

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