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Genetic toxicology: Strategies and implications for man
419 In those species of bacteria for which information is available, various cellular processes, including DNA repair, replication and recombination, determine whether or not potentially mutagenic lesions lead to mutations which can be expressed (i.e. passed from parent to offspring). This paper will be largely concerned with the role of the enzyme systems involved in DNA repair in mutation fixation, b u t since the replication and recombination systems also have a very important part to play in mutation fixation they t o o will be discussed. DNA repair can eliminate potentially mutagenic lesions before they can be converted to final mutations. In addition, potentially lethal lesions can be removed at the expense of creating potentially mutagenic lesions or final mutations. Thus repair can increase, decrease, or have no effect on, the frequency of induced mutants among a surviving population. Repair also interacts with other processes involved in the mutational pathway. For example, mutation fixation often but not always happens at replication. Elimination of potentially mutagenic lesions prior to replication will therefore prevent their fixation. Certain mutagens, including UV and ionizing radiations and a number of chemicals, induce the production of an error-prone cellular repair mechanism which seems to be able to promote recovery from potentially lethal damage at the expense of making errors which lead to fixed mutations. This mechanism was at first thought to be involved only in post-replication repair of potentially mutagenic lesions, but n o w appears also to have a role in pre-replication repair. Inducible error-prone repair (SOS repair) is thought to be responsible for most if not all of the mutations induced by UV light, ionizing radiations, and a large number of chemicals. There is good evidence, however, that some mutagens -- especially monofunctional alkylating agents and base analogues -- induce mutations by other processes. Even in these cases, repair processes (including an inducible error-free process which appears to operate on damage produced by monofunctional alkylating agents but not by UV) appear to play in important role.
Genetic toxicology: Strategies and implications for man IAIN F.H. PURCHASE The vast b o d y of knowledge of genetics, based both on experimental observations and an understanding of the key role of DNA in hereditary processes, has provided genetic toxicology with a theoretical basis u n k n o w n in other fields of toxicology. For these reasons there are a multitude of test systems using a variety of organisms in which genetic events can be studied. All of those experimental systems suffer from the f a c t that their relevance to man remains uncertain because chemically-induced heritable changes observed in them have n o t been confirmed in man. The fact that many chemicals are both mutagenic and carcinogenic has led to the somatic mutation theory of the induction of cancer. This hypothesis sug-
420 gests that carcinogens induce somatic mutations resulting in changes in cell behaviour leading to the development of cancer. Therefore, many of the testing systems used for detecting genetic events may also be useful for identifying potential carcinogens. In this case, several human carcinogens and many animal carcinogens are well recognised, allowing the accuracy of short-term tests to be studied. It has been found that most short-term tests are chemical-class-specific, but nevertheless there is an important place for such tests in schemes for identifying carcinogens. All biological systems have various sources of variability. Recent studies suggest that although there is a high level of reproducibility in these tests, there is some variability which should be acknowledged. There is also good evidence for both false positive and false negative results. Both of these factors need to be considered in determining how to interpret results from tiered or battery approaches to testing. Assessment of risk to man, the ultimate objective of toxicity testing, is a multi-stage process. Laboratory testing of the effects of chemicals is only the first stage which is followed by quantification of risk and finally evaluation of risk. Recent attempts to regulate using the dogma of no thresholds and speculative quantitative methods appear to be means of avoiding the task of assessing the value and limitations of biological data in extrapolating to man. The uncertainties of the techniques and their interpretation lead to the conclusion that there are no short cuts to good scientific experimentation and balanced judgement of all the biological data when considering the potential genetic effects of chemicals.
8 Mutagenic p o t e n c y and drug structure in some experimental anti-cancer drugs LYNNETTE F E R G U S O N and BRUCE BAGULEY m-AMSA is an anilino-acridine derivative which is currently showing clinical promise as a broad spectrum anti-cancer drug. Preliminary experiments have indicated that it is mutagenic, suggesting that the development of drug resistance and carcinogenicity might limit its usefulness in the future. Therefore, a study of the structural requirements for mutagenicity has been undertaken, in order to establish whether mutagenicity can be reduced or eliminated in related molecules which retain their anti-tumour activity. The parent molecule has been substituted with a range of groups at each of 4 positions of the acridine ring, giving a series of drugs which span a wide range of lipophilic--hydrophilic balance, base strength, DNA binding and anti-tumour activity. Quantitative measurements of mutagenicity in an Ames test have been compared with data for the other parameters mentioned above. Firm guidelines have been established, by which active drugs can be formulated to have no detectable activity in an Ames test.
Genetic toxicology: Strategies and implications for man IAIN F.H. PURCHASE The vast b o d y of knowledge of genetics, based both on experimental observations and an understanding of the key role of DNA in hereditary processes, has provided genetic toxicology with a theoretical basis u n k n o w n in other fields of toxicology. For these reasons there are a multitude of test systems using a variety of organisms in which genetic events can be studied. All of those experimental systems suffer from the f a c t that their relevance to man remains uncertain because chemically-induced heritable changes observed in them have n o t been confirmed in man. The fact that many chemicals are both mutagenic and carcinogenic has led to the somatic mutation theory of the induction of cancer. This hypothesis sug-
420 gests that carcinogens induce somatic mutations resulting in changes in cell behaviour leading to the development of cancer. Therefore, many of the testing systems used for detecting genetic events may also be useful for identifying potential carcinogens. In this case, several human carcinogens and many animal carcinogens are well recognised, allowing the accuracy of short-term tests to be studied. It has been found that most short-term tests are chemical-class-specific, but nevertheless there is an important place for such tests in schemes for identifying carcinogens. All biological systems have various sources of variability. Recent studies suggest that although there is a high level of reproducibility in these tests, there is some variability which should be acknowledged. There is also good evidence for both false positive and false negative results. Both of these factors need to be considered in determining how to interpret results from tiered or battery approaches to testing. Assessment of risk to man, the ultimate objective of toxicity testing, is a multi-stage process. Laboratory testing of the effects of chemicals is only the first stage which is followed by quantification of risk and finally evaluation of risk. Recent attempts to regulate using the dogma of no thresholds and speculative quantitative methods appear to be means of avoiding the task of assessing the value and limitations of biological data in extrapolating to man. The uncertainties of the techniques and their interpretation lead to the conclusion that there are no short cuts to good scientific experimentation and balanced judgement of all the biological data when considering the potential genetic effects of chemicals.
8 Mutagenic p o t e n c y and drug structure in some experimental anti-cancer drugs LYNNETTE F E R G U S O N and BRUCE BAGULEY m-AMSA is an anilino-acridine derivative which is currently showing clinical promise as a broad spectrum anti-cancer drug. Preliminary experiments have indicated that it is mutagenic, suggesting that the development of drug resistance and carcinogenicity might limit its usefulness in the future. Therefore, a study of the structural requirements for mutagenicity has been undertaken, in order to establish whether mutagenicity can be reduced or eliminated in related molecules which retain their anti-tumour activity. The parent molecule has been substituted with a range of groups at each of 4 positions of the acridine ring, giving a series of drugs which span a wide range of lipophilic--hydrophilic balance, base strength, DNA binding and anti-tumour activity. Quantitative measurements of mutagenicity in an Ames test have been compared with data for the other parameters mentioned above. Firm guidelines have been established, by which active drugs can be formulated to have no detectable activity in an Ames test.