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Methods and regulations on the genotoxicity screening

All living creatures are inseparably associated with chemicals. They are all around us, from food and cosmetics, to the drugs and different industrial products used in everyday life. Safety assessment of the novel compounds is very important as some chemicals are very “aggressive” and could produce devastating effect on the health. Compounds that interact with DNA, changing its structure and integrity are known as genotoxic. Most of those chemicals induce mutagenesis (alter genetic information) or cancerogenesis (tumor induction). Both somatic and germ cells could suffer damage. When germ cells are altered by genotoxic compound, mutagenic effect is seen in the offspring even though it has never been exposed to the genotoxin.

Genotoxic effect could be result of a point mutation, changes in the DNA structure or damage on the chromosome. Tests used for genotoxic screening are discovering the damage on different levels and couple of tests is usually applied to ensure an accurate final result.
In vivo and in vitro screening methods could measure and predict possible toxic effect both in the somatic and germ cells. Some in vivo tests are designed to measure DNA damage that will be passed to the offspring. Examples for that kind of tests are mouse heritable translocation test, mouse specific locus test and rodent dominant lethal test. Mammalian germ cells could be used for assessment of chemical potential to induce heritable damage. The most popular tests of this kind are unscheduled DNA synthesis using testicular cells, spermatogonial chromosome aberration test or oocyte chromosome aberration & aneuploidy test…Commonly used assays for prediction of genotoxicity in somatic cells are mammalian erythrocyte micronucleus test, bone marrow chromosome aberration test, unscheduled DNA synthesis in the liver or mouse spot test. Beside this, in vivo Comet assay and in vivo transgenic mutation assays could be used as well. Also, a lot of tests using in vitro non-animal related alternative methods (such as bacterial and mammalian cell assays) are often performed.

Regulatory agencies and international authorities determine the type and pattern of the testing that should be applied during genotoxic compounds evaluation. Globally respected and recognized organization that establishes standards for various chemical testing is The Organization for Economic Co-operation and Development or OECD. It is established in 1961. when Europe, USA and Canada joined their forces to promote global development. Today, 34 member countries are working on the improvement of the social and economic prosperity of the people around the world by adopting different policies and strategies. OECD assists in defining methods for molecular screening and toxicogenomics that could be applied in a regulatory context. Tests that are adopted and approved by the OECD are used by government, industry and laboratories for the safety assessment of chemicals and/or chemical preparations.

This is the OECD test guidelines for genotoxicity and mutagenicity testing:

1. Bacterial Reverse Mutation Test (Ames Test)
2. Genetic Toxicology: Escherichia coli, reverse assay
3. In Vitro Mammalian Chromosome Aberration Test
4. Mammalian Erythrocyte Micronucleus Test
5. Mammalian Bone Marrow Chromosome Aberration Test
6. In Vitro Mammalian Cell Gene Mutation Test
7. Genetic Toxicology: Sex-linked Recessive Lethal Test in Drosophila melanogaster
8. Genetic Toxicology: Rodent Dominant Lethal Test
9. Genetic Toxicology: In Vitro Sister Chromatid Exchange Assay in Mammalian Cells
10. Genetic Toxicology: Saccharomyces cerevisiae, Gene Mutation Assay
11. Genetic Toxicology: Saccharomyces cerevisiae, Mitotic Recombination Assay
12. Genetic Toxicology: DNA Damage and Repair, Unscheduled DNA Synthesis in Mammalian Cells In Vitro
13. Mammalian Spermatogonial Chromosome Aberration Test
14. Genetic Toxicology: Mouse Spot Test
15. Genetic Toxicology: Mouse Heritable Translocation Assay
16. Unscheduled DNA Synthesis (UDS) Test with Mouse Liver Cells In Vitro
17. In Vitro Mammalian Cell Micronucleus Test

A lot of money is spent on the research and development of methods that could detect genotoxic compounds quickly and efficiently. Pharmaceutical industry is especially interested for these techniques, because a lot of drugs (around 12%) in the late stage development could show genotoxic potential. They need to be modified and improved before they enter clinical trials. Estimated cost of delaying clinical trials and bringing the drug to the market later than expected could vary from 5 – 30 million dollars.

In vitro toxicity testing is important for one more reason. Animal testing is considered cruel and unethical and it will cease completely as soon as alternative methods become efficient enough. Joined collaboration of scientific methods (in vitro tests) and predictive methods (software based) should become standard in the future safety assessment.