Prediction of environmental carcinogens: A strategy for the mid-1980s

Mechanistic Insights Gained from an Analysis of Carcinogenic Polycyclic Aromatic Hydrocarbons with the Computer Automated Structure Evaluation System

The Computer Automated Structure Evaluation System (CASE) was used to identify the structural components responsible for either the mutagenicity or the carcinogenicity of polycyclic aromatic hydrocarbons (PAHs). These were found not to overlap and some of the structural components were endowed with much more biological activity than others.

A study was then undertaken to determine whether these structural features could explain the mutagenicity and carcinogenicity of 7,12-dimethylbenz(a)anthracene (DMBA). It was found that the structures identified by CASE could explain the mutagenicity and carcinogenicity qualitatively as well as quantitatively. Moreover, the identified structures were entirely consistent with recent findings on the metabolism and DNA adduct formation of DMBA.

These results are taken to indicate that CASE can be used to identify the structures in a molecule (e.g., PAH) that are sites of metabolism. Laboratory studies can then focus on that portion of the molecule.

Methods for predicting carcinogenic hazards: new opportunities coming from recent developments in molecular oncology and SAR studies

Without epidemiological evidence, and prior to either short-term tests of genotoxicity or long-term tests of carcinogenicity in rodents, an initial level of information about the carcinogenic hazard of a chemical that perhaps has been designed on paper, but never synthesized, can be provided by structure-activity relationship (SAR) studies. Herein, we have reviewed the interesting strategies developed by human experts and/or computerized approaches for the identification of structural alerts that can denote the possible presence of a carcinogenic hazard in a novel molecule. At a higher level of information, immediately below epidemiological evidence, we have discussed carcinogenicity experiments performed in new types of genetically engineered small rodents. If a dominant oncogene is already mutated, or if an allele of a recessive oncogene is inactivated, we have a model animal with (n-1) stages in the process of carcinogenesis. Both genotoxic and receptor-mediated carcinogens can induce cancers in 20-40% of the time required for classical murine strains. We have described the first interesting results obtained using these new artificial animal models for carcinogenicity studies. We have also briefly discussed other types of engineered mice (lac operon transgenic mice) that are especially suitable for detecting mutagenic effects in a broad spectrum of organs and tissues and that can help to establish mechanistic correlations between mutations and cancer frequencies in specific target organs. Finally, we have reviewed two complementary methods that, while obviously also feasible in rodents, are especially suitable for biomonitoring studies. We have illustrated some of the advantages and drawbacks related to the detection of DNA adducts in target and surrogate tissues using the 32P-DNA postlabeling technique, and we have discussed the possibility of biomonitoring mutations in different human target organs using a molecular technique that combines the activity of restriction enzymes with polymerase chain reaction (RFLP/PCR). Prediction of carcinogenic hazard and biomonitoring are very wide-ranging areas of investigation. We have therefore selected five different subfields for which we felt that interesting innovations have been introduced in the last few years. We have made no attempt to systematically cover the entire area: such an endeavor would have produced a book instead of a review article.

The influence of the extent of target organs on sensitivities of methods for screening rodent carcinogens

Two hundred and thirty-three rodent carcinogens from the Carcinogenic Potency Database (CPDB) were analyzed with CASE (Computer Automated Structure Evaluation), and a comparison of the extents of target organs with the sensitivities for long-term carcinogenic bioassays in rats and mice, Salmonella assay (Sty), electrophilic substructure alert analysis (ESAA) and CASE was made. The carcinogenicity of 233 chemicals was evaluated in both rat and mouse bioassays. The present study showed that the sensitivities of the five methods for screening carcinogens were related to the extents of target organs of carcinogens. Among the carcinogens that did not induce tumors (extent = 0) in rats, the sensitivities of Sty and ESAA were 46 and 53, respectively. Among the carcinogens which induced tumors at a single organ (extent = 1) in rats, the sensitivities were 57 and 64 respectively; and 71 and 80 at multiple organs (extent > 1) respectively. The sensitivities of CASE were 76, 82, and 89 respectively at these three different extents. Similar results were obtained with these carcinogens in mice. The results indicate that mutagenic or electrophilic carcinogens are more likely to induce tumors at multiple target organs; in contrast, most carcinogens which induced tumors at only a single target organ in one species are rarely mutagenic or electrophilic. The sensitivities of Sty and ESAA were lower than that of the CASE method in these carcinogens. CASE analyzed chemical structures of many carcinogens and non-carcinogens and then established a database of key fragments, and its parameters are not only based on mutagenicity or electrophilicity of chemicals, and this resulted in a more exact detection of the carcinogenicity of chemicals with the CASE method.

The structural basis of the genotoxicity of nitroarenofurans and related compounds

The CASE (Computer-Automated Structure Evaluation) methodology has been applied to an investigation of the basis of the genotoxicity (sfiA induction) of 79 nitroarenofurans and related molecules examined with the E. coli PQ37 genotoxicity assay (SOS chromotest). CASE identified 9 major activating structural fragments (biophores) responsible for the probability of genotoxicity (SAR). With respect to quantitative features, CASE identified 8 major molecular subunits related to the genotoxic potency (QSAR). Both the SAR as well as the QSAR analysis indicate that a nitro group on position 2 of the furan ring is important for activity provided one or more aromatic rings are attached to the furan ring, i.e. 2-nitrobenzofuran, 2-nitronaphthofuran, 2-nitroanthrafuran and 8-nitropyrenofuran. Additionally, a small substituent at position 3 of the furan ring, i.e. the methyl group of R7371, the ethyl group of R7427 and the butyl group of R7429, enhance the activity of 2-nitronaphtho[2,1-b]furan (R6597), whereas longer aliphatic chains decrease activity. Moreover, the activity of the nitro group at position 2 of the furan ring was increased by substitution of a methoxy group at position 7 of the R6597 structure. Additionally the n-octanol/water partition coefficient (log P) was found to be an important descriptor for the genotoxic potency in E. coli PQ37. Using the identified descriptors CASE correctly predicted the probability of genotoxicity of all of the genotoxicants and non-genotoxicants in the data base. The calculated genotoxic potency was equally good: 94% of all predicted results were within plus/minus one order of magnitude of the experimental result. Using CASE in the predictive mode, the program correctly predicted the probability of sfiA induction in E. coli of 95.8% of 24 "unknown" nitroarenofurans which were not part of the learning set (QSAR with r = 0.88-0.97).

Relationship between molecular connectivity and carcinogenic activity: a confirmation with a new software program based on graph theory

For a database of 826 chemicals tested for carcinogenicity, we fragmented the structural formula of the chemicals into all possible contiguous-atom fragments with size between two and eight (nonhydrogen) atoms. The fragmentation was obtained using a new software program based on graph theory. We used 80% of the chemicals as a training set and 20% as a test set. The two sets were obtained by random sorting. From the training sets, an average (8 computer runs with independently sorted chemicals) of 315 different fragments were significantly (p < 0.125) associated with carcinogenicity or lack thereof. Even using this relatively low level of statistical significance, 23% of the molecules of the test sets lacked significant fragments. For 77% of the molecules of the test sets, we used the presence of significant fragments to predict carcinogenicity. The average level of accuracy of the predictions in the test sets was 67.5%. Chemicals containing only positive fragments were predicted with an accuracy of 78.7%. The level of accuracy was around 60% for chemicals characterized by contradictory fragments or only negative fragments. In a parallel manner, we performed eight paired runs in which carcinogenicity was attributed randomly to the molecules of the training sets. The fragments generated by these pseudo-training sets were devoid of any predictivity in the corresponding test sets. Using an independent software program, we confirmed (for the complex biological endpoint of carcinogenicity) the validity of a structure-activity relationship approach of the type proposed by Klopman and Rosenkranz with their CASE program.

Assessment of the carcinogenicity of the nonnutritive sweetener cyclamate

The weight of the evidence from metabolic studies, short-term tests, animal bioassays, and epidemiological studies indicates that cyclamate (CHS) is not carcinogenic by itself; however, there is evidence from in vitro and in vivo studies in animals that implies it may have cancer-promoting or cocarcinogenic activity. Epidemiological studies indicate that the use of nonnutritive sweeteners (CHS and saccharin) has not resulted in a measurable overall increase in the risk of bladder cancer in individuals who have ever used these products. No epidemiological information exists on the possible associations of these sweeteners and cancers other than those of the urinary tract. It is recommended that (1) no further studies on the metabolism of CHS to evaluate its carcinogenicity are required since no potentially hazardous metabolites have been appreciably detected in humans; (2) no further animal bioassays to test for the carcinogenicity of CHS by itself are necessary; (3) the studies in rodents that suggest a promotional or cocarcinogenic effect of CHS should be repeated because they cannot be ruled out; (4) because the significance to human health of a positive outcome of such studies is uncertain, additional research aimed at understanding the predictive value for human health of such results and more generic studies to develop well-validated systems that can be relied on in the assessment of cancer-promoting agents are recommended; (5) in populations where CHS continues to be used, epidemiological monitoring should be continued to determine whether there is an increased risk of cancer in humans who are heavy or long-term users or for those observed long after first exposure. In such monitoring, other cancer sites--in addition to the bladder--should be considered.

Structural requirements for the induction of the SOS repair in bacteria by nitrated polycyclic aromatic hydrocarbons and related chemicals

The CASE (computer-automated structure evaluation) methodology was used to investigate the structural basis of the SOS-inducing activity of 56 nitrated polycyclic aromatic hydrocarbons (nitroarenes, nPAH) and the unsubstituted parent PAH molecules. Based upon the presence and/or absence of structural features, CASE identified 5 activating (biophores) and 4 inactivating (biophobes) fragments responsible for the SOS-inducing activity. Based upon these fragments, CASE correctly calculated the genotoxicity of 94.6% of the molecules in the training set (sensitivity = 0.85, specificity = 1.0). Disregarding the questionable experimental results of the unexpected very weak direct-acting activity of the unsubstituted benzo[a]pyrene, dibenzo[a,h]anthracene and 7,12-dimethylbenz[a]anthracene, the concordance of the prediction was 100%, i.e., sensitivity = 1.0, specificity = 1.0. Additionally, the quantitative analysis of the SOS-inducing potency showed a good correlation between the experimental and predicted results. The present analyses indicate an identity in the structural determinants responsible for SOS induction in E. coli PQ37 (SOS chromotest) and mutagenicity in Salmonella typhimurium.

Quantification of the predictivity of some short-term assays for carcinogenicity in rodents

A statistical procedure is described for assessing the predictive performance of short-term tests for carcinogenicity in which the actual number of chemicals tested is taken into consideration. The method is then applied to several widely used short-term assays.

The mutagenic activity and polycyclic aromatic hydrocarbon content of mineral oils

Naphthenic distillates (raw or acid-treated) and motor and emulsifiable aluminium rolling oils were tested for mutagenicity in the Salmonella/microsome assay using the TA98 and TA100 strains. The polycyclic aromatic hydrocarbon (PAH) content of oil samples was also determined in parallel. In the presence of metabolic activation, both untreated and acid-treated naphthenic distillates were found to be mutagenic on a modified Ames test. One untreated sample showed the highest value of mutagenic potency (50 net revertants/mg oil for strain TA98). The PAH content of naphthenic distillates was about 10% (w/w) and was slightly reduced by sulfuric acid/earth treatment (1%). Non-mutagenic paraffin- and solvent-extracted crankcase oils became active, both with and without enzyme activation, after long use as gasoline engine lubricants, whereby their PAH content doubled (from 1.5% to 3%, w/w). A refined emulsifiable mineral oil also became directly mutagenic in both Salmonella strains after prolonged use in an aluminium hot-rolling mill. As the PAH levels found in used rolling oils was very low, we cannot explain their mutagenic activity. Mutagenicity was greatly reduced following careful cleaning of the oil bath and of the entire rolling machine. The present data reveal both the potential risk of occupational exposure to unrefined or mildly treated oils and the formation of mutagens in highly refined oils if the latter are used at high temperatures. The formation of mutagens in oils used in the metal-working industry can be prevented by careful industrial hygiene measures.

Use of a composite polyfunctional model electrophile as a probe to analyze the performance of an artificial intelligence structure-activity method

The CASE structure-activity relational method was applied to the model polyfunctional electrophile proposed by Ashby and associates. The predicted activities from data bases of 'structural alerts', mutagenicity in Salmonella and rodent carcinogenicity were compared. It was thus found that the predictive efficacy of CASE was increased when it employed a combination of human and artificial intelligence, as exemplified by the CASE analysis of 'structural alerts.

Novel structural concepts in elucidating the potential genotoxicity and carcinogenicity of tetrandrine, a traditional herbal drug

Tetrandrine, a widely used remedy, is predicted to be a 'genotoxic' carcinogen. This finding suggests that the usage of this substance in non-life-threatening situations should be evaluated.

Structural basis of carcinogenicity in rodents of genotoxicants and non-genotoxicants

A set of 189 chemicals tested in the National Toxicology Program Cancer Bioassay was subjected to analysis by CASE, the Computer-Automated Structure Evaluation system. In the data set, 63% of the chemicals were carcinogens, approx. 40% of the carcinogens were non-genotoxic, i.e., they possessed neither "structural alerts" for DNA reactivity as defined by Ashby and Tennant, 1988, nor were they mutagenic for Salmonella. The data base can be characterized as a "combined rodent" compilation as chemicals were characterized as "carcinogenic" if they were carcinogenic in either rats or mice or both. CASE identified 23 fragments which accounted for the carcinogenicity, or lack thereof, of most of the chemicals. The sensitivity and specificity were unexpectedly high: 1.00 and 0.86, respectively. Based upon the identified biophores and biophobes, CASE performed exceedingly well in predicting the activity of chemicals not included among the 189 in the original set. CASE predicted correctly the carcinogenicity of non-genotoxic carcinogens thereby suggesting a structural commonality in the action of this group of carcinogens. As a matter of fact biophores restricted to non-genotoxic carcinogens were identified as were "non-electrophilic" biophores shared by genotoxic and non-genotoxic carcinogens. The findings suggest that the CASE program may help in the elucidation of the basis of the action of non-genotoxic carcinogens.

New structural concepts for predicting carcinogenicity in rodents: an artificial intelligence approach

The Computer Automated Structure Evaluation (CASE) method for studying structure-activity relationships has been applied to a data base of rodent carcinogens. It has been demonstrated that CASE is able to identify determinants embedded in the molecular structure which, with a high probability, predict rodent carcinogenicity. CASE has also identified determinants associated with the activity of non-genotoxic carcinogens, thereby suggesting that there is a structural commonality in the activity of these molecules. The present study reveals that there are "universal" as well as species-specific structural determinants of carcinogenicity. CASE was able to predict the carcinogenicity in rodents of certain endogenous pesticides in edible plants.

Important recent advances in the practice of health risk assessment: implications for the 1990s

Health risk assessments have been so widely adopted in the United States that their conclusions are a major factor in many environmental decisions. The procedure by which these assessments are conducted is one which has evolved over the past 10-15 years and a number of short-comings have been widely recognized. Unfortunately, improvements in the process have often occurred more slowly than advancements in technology or scientific knowledge. Recent significant advances for more accurately estimating the risks posed by environmental chemicals are likely to have a dramatic effect on the regulation of many substances. Each of the four portions of risk assessment (hazard identification, dose-response assessment, exposure assessment, and risk characterization) has undergone significant refinement since 1985. This paper reviews some of the specific changes and explains the likely benefits as well as the implications. Emphasis is placed on the improved techniques for (a) identifying those chemicals which may pose a human cancer or developmental hazard, (b) using statistical approaches which account for the distribution of interindividual biological differences, (c) using lognormal statistics when interpreting environmental data, (d) using physiologically based pharmacokinetic models for estimating delivered dose and for scaling up rodent data, (e) using biologically based cancer models to account for the seven or more apparently different mechanisms of chemical carcinogenesis, (f) describing the severity of the public health risks by considering those portions of the population exposed to various concentrations of a contaminant, and (g) reviewing how criteria for acceptable risk have been influenced by the number of exposed persons. The net benefit of these improvements should be a reduction in the uncertainty inherent in current estimates of the health risks posed by low level exposure to carcinogens and developmental toxicants.

Types and amounts of carcinogens as potential human cancer hazards

Current knowledge on the mechanisms of chemical carcinogenesis forms the basis for application of select short-term in vitro and in vivo tests to detect potential human carcinogens, for ultimate application to hazard assessment. Chemical carcinogenesis involves a series of distinct steps, proceeding from the initiation of a neoplastic cell, through its promotion, development, and progression to cancer. Some chemicals act in each of these stages as initiators, cocarcinogens, promoters, or inhibitors of carcinogenesis. Chemicals can be classified as operating by genotoxic or epigenetic mechanisms, and appropriate tests can be used to detect such properties. These abbreviated tests provide enhanced qualitative decision-making potential since they are based on mechanisms of action. Advances in molecular biology may provide additional tests to detect cancer risk. The quantitative data available from in vitro dose-response studies indicate that carcinogenic effects are dose dependent and, therefore, a threshold or no-effect level probably exists, which is low for potent carcinogens (especially genotoxins) and high for weaker ones (particularly epigenetic agents).

Structural basis of the mutagenicity of phenylazoaniline dyes

The CASE (Computer-Automated Structure Evaluation) methodology was used to gain an understanding of the basis of mutagenicity of phenylazoanilines. It was found that the activity of these molecules is dependent upon an intact moiety that spans the azo linkage, i.e., the azo bond must remain intact for mutagenicity. The study also addressed the effect of sulfonation on the activity of these azo dyes. It was revealed that sulfonation at only certain sites resulted in loss of mutagenicity. Sulfonation of the structural moiety responsible for the activity of phenylazoaniline dyes did not necessarily result in complete elimination of activity as this substitution could generate new structural moieties which contribute to the activity of the molecules.

Strategies for the use of computational SAR methods in assessing genotoxicity

The relationship between computational SAR studies and relevant data gathering and generation activities is complex. First, the chemical class to be studied is selected on the basis of information requirements for hazard identification and assessment. Membership in the class is determined by consideration of chemical structure and reactivity. Compilation of the existing bioassay data for this chemical class follows immediately from the specification of the class. Bioassay data, qualitative knowledge of general chemical reactivities in this class, and knowledge concerning potential interactions with biomolecular targets all contribute to the derivation of possible mechanisms for biological activity. Computational studies based on modeling the proposed mechanism of action and/or the existing data base can provide a quantitative basis for the differentiation between chemicals. There is the opportunity for continuing feedback between the quantitative computational studies and the development of a relevant bioassay data base for this chemical class. The qualitative and quantitative information on the potential biological responses obtained will provide a rational basis for extrapolation from the extant data base to the chemicals of interest, and to biological responses significant to the assessment for which complete data are unavailable. Knowledge concerning possible mechanisms of action and preexisting data determine the type of computational study that will be most useful.

CASE, the computer-automated structure evaluation system, as an alternative to extensive animal testing

CASE, an artificial intelligence system with demonstrated ability to predict biological activity based on structural considerations, correctly predicts animal carcinogenicity. It can, therefore, play a pivotal role in classifying chemicals as carcinogens and prioritizing them for further testing. Additionally, CASE shows promise in the design of pharmacologically active agents by reducing the number of drugs that need to be synthesized and tested. For both of these applications, CASE provides a mechanism to conserve animal and other testing resources.

The genotoxicity of a variety of aniline derivatives in a DNA repair test with primary cultured rat hepatocytes

The genotoxicity of a variety of aniline derivatives was examined by a DNA repair test with rat hepatocytes. Out of 37 aniline derivatives, 6 chemicals, i.e., 2,4,6-trimethylaniline (mesidine), 2,4-xylidine, 3,5-diaminobenzoic acid, 3,4-diaminochlorobenzene, 2-chloro-4-methylaniline and 4-chloro-N-methylaniline, elicited positive DNA repair responses. The results are in agreement with the bacterial mutagenicities with or without norharman of these compounds. Positive compounds of unknown carcinogenicity in the present assay, i.e., 3,5-diaminobenzoic acid, 2-chloro-4-methylaniline and 4-chloro-N-methylaniline are suspected of being potentially carcinogenic.

Chloramphenicol: magic bullet or double-edge sword?

The genetic and genotoxic potentials of chloramphenicol are reviewed and analyzed. Although this widely used antimicrobial agent appears to cause chromosomal effects in somatic cells, in view of the consistent absence of other genetic effects, these cytogenetic abnormalities are ascribed to non-genotoxic causes. It is pointed out that despite its widespread use in human medicine, chloramphenicol has not been systematically tested for genotoxicity.

Application of a cellular test battery in the decision point approach to carcinogen identification

The assessment of the potential carcinogenicity of a chemical requires a systematic approach taking into account various types of data. Important information on the DNA reactivity and other genetic effects of chemicals can be obtained from a battery of cellular tests. A battery is described which includes DNA repair in hepatocytes, mutagenesis in Salmonella typhimurium, mutagenesis, chromosome alterations, and transformation in mammalian cells. The interpretation of findings in this battery for the identification of potential carcinogenicity of chemicals is discussed.

Utilization of the quantitative component of positive and negative results of short-term tests

In this paper we discuss the possibility of utilizing not only the qualitative component of the information obtained from long-term and short-term tests (as is customary), but also the quantitative component of the results. We suggest that there is probably a precise mathematical relationship between the qualitative and quantitative approaches. We show that utilizing the quantitative approach, it is possible to give confidence limits to a given prediction, a possibility potentially very useful for risk evaluation. We show that starting from a reasonable working hypothesis, it is possible to include even negative data in a unified quantitative approach. Incorporating the quantitative component of the information could offer appreciable gains in predictivity, especially when utilizing batteries of tests.

Quantifying genotoxicity and non-genotoxicity

Since the ability to induce genotoxicity is often equated with the potential for initiating the carcinogenic process, a method for quantitating genotoxicity would provide a useful measure for this potential. It is demonstrated herein that CPBS, the Carcinogenicity Prediction and Battery Selection method, provides a useful quantitative measure of genotoxicity as well as allowing for the detailed evaluation of the performance of batteries of short-term tests in order to select those predictive of carcinogenic potential.

Scientific and cost-effectiveness criteria in selecting batteries of short-term tests

The scientific and cost-effectiveness criteria introduced in this paper can be applied to published datasets and current and proposed batteries of short-term tests. The reports in the current volume will provide a wealth of additional material for such evaluations, but more systematically obtained information will be necessary to assess both the internal and external validity of these tests. Individual tests and batteries of tests should be standardized, employ positive controls, generate results capable of quantitative analyses that may make dichotomous classification as "positive" and "negative" obsolete, be interpreted in light of mechanisms of action, and be cost-effective on a grand scale. For regulatory purposes our long-term goal should be to replace the whole animal lifetime bioassay with an appropriate and cost-effective set of short-term tests.

Evaluating the potential for genotoxic carcinogenicity of methyl isocyanate

The carcinogenicity prediction and battery selection method was used to predict the probability of carcinogenicity of methyl isocyanate (MIC) based upon the results of short-term tests. The analysis predicts that MIC has a significant potential for inducing cancer in rodents. However, the pattern of response suggests that the carcinogenic potency would be low. Obviously, the realization of the identified risk would be dependent upon level, duration, and mode of exposure.

Genotoxicity of amebicide and anthelmintic drugs in Escherichia coli pol A+/pol A-

The amebicides dehydroemetine and chloroquine diphosphate and the anthelmintic pyrvinium pamoate, previously reported to be mutagenic in Salmonella typhimurium (Cortinas de Nava et al., 1983), were clearly shown to be genotoxic in the Escherichia coli pol A+/pol A- assay. Two other antiparasitic drugs, diiodohydroxyquin and 4-hexylresorcinol, were also found to be genotoxic in E. coli, while iodochlorhydroxyquin preferentially inhibited the pol A+ strain. From the 3 alternative testing methods employed, the liquid suspension succeeded in detecting 5 antiparasitic drugs as genotoxic; the microsuspension identified 2, and the disc diffusion method only 1. However, the metabolic activation system could only be coupled successfully and in a reproducible way to the microsuspension assay.

An approach to identifying specialized batteries of bioassays for specific classes of chemicals: class analysis using mutagenicity and carcinogenicity relationships and phylogenetic concordance and discordance patterns. 1. Composition and analysis of the overall data base. A report of phase II of the U.S. Environmental Protection Agency Gene-Tox Program

This report of the Gene-Tox Assessment Panel is a compilation of data that documents the chemical testing efforts in genetic toxicology through mid-1979. It thus provides an historical perspective of the major efforts in this field and the utility of test models. The total number of chemicals tested in assays reflects chemical availability, commercial interest in specific structural types, the ease or difficulty in assay performance, as well as methodological development resulting from testing experience. Other factors that have been important in assay selection and utility are the perceptions of relevance to hazard evaluation of chemicals and the role that genetic factors may have in other disease states as well as in heritable defects. The phylogenetic diversity of test systems attests to the tremendous effort that has been applied to the testing and evaluation of the effect chemicals can have on genetic structure. The data also illustrate the fact that certain chemicals have an intrinsic capability to alter the genetic structure of cells of diverse biological origin in an heritable manner, whereas others do not. Any attempt to summarize and analyze a data base of this magnitude is a formidable task that would be almost impossible without a computer capability. A computerized system of analysis has been developed at the Environmental Mutagen Information Center (EMIC) that makes it possible to examine the performance of any particular assay in any of 30 chemical classes and to make comparisons with all the other assays individually or in designated groupings. Components of this system include: A distribution of the 2622 chemicals into 30 chemical classes with results of testing in each class. A tabulation of assay results showing the total numbers of chemicals tested, with their definitive and nondefinitive results. A subdivision of assays and results of testing into four major groups: gene mutation, chromosomal aberrations, other genotoxic effects, and in vitro cell transformation assays. These major groups are further subdivided into phylogenetic categories and type of assay. A system of analysis of results utilizing mutagenicity and carcinogenicity comparisons and phylogenetic concordance and discordance. The major utility and/or benefit of this compilation will be derived from a chemical class by chemical class comparative analysis of individual assay performance. Obviously, the data base will serve as a resource for safety evaluation of chemicals through structural correlations and biological end point analyses.(ABSTRACT TRUNCATED AT 400 WORDS)

Contribution of toxicology towards risk assessment of carcinogens

In the last decade many tests have been designed to detect possible carcinogenicity of compounds. Presently, many more or less simple and convenient systems are available to detect mutations, effects on chromosomes, DNA binding and damage and malignant transformation. These systems, which have been extensively refined during the last years, often show reasonably good relevance to carcinogenicity. Although inconsistencies in the patterns of response do indicate that their role as predictive indicators of carcinogenicity remains still uncertain, the use of such short-term tests in carcinogen risk assessment does seem feasible. Factors other than these tests should also be taken into consideration, since other characteristics like chemical structure, biotransformation, toxicokinetics, qualitative and quantitative physiological and/or morphological effects, species, strains, organ specificity, dose-response relation and information on studies in man, if available, are of importance too. In conjunction with the results of adequately performed carcinogenicity tests in mammals, one may attempt to classify carcinogens. Current knowledge does not permit a rigid classification, but may warrant a subclassification into carcinogens acting via a genetic or a non-genetic mechanism. It is emphasized that on theoretical and practical grounds a different extrapolation system should be used for the different types of carcinogens in risk assessment procedures. Evaluations on individual compounds should be made to decide whether such genotoxic or non-genotoxic compounds should be permitted in the human environment.

Computer-assisted structure-activity relationships of nitrogenous cyclic compounds tested in salmonella assays for mutagenicity

Study of the relationship between mutagenicity and molecular structure for a data set of nitrogenous cyclic compounds is reported. A computerized SAR system (ADAPT) was utilized to classify a data set of 114 nitrogenous cyclic compounds with 19 molecular descriptors. All of the descriptors represented at least 10% of the compounds in the data sets. The average correct predictability of the data base was calculated to be 89% after evaluating 100 training/prediction subsets. The actual predictive ability of the discriminants generated by the ADAPT system was demonstrated by predicting the mutagenicity of structurally similar compounds not in the data set. Weight vectors generated in the pattern recognition programs were used to predict the bacterial mutagenicity of 10 compounds which were not included in the data set. All of the compounds were predicted correctly which was actually better than the 89% calculated by the system. This displayed the ability of the system of classify compounds as mutagenic or nonmutagenic by using molecular descriptors and to predict the biological activity of untested chemicals with a high degree of confidence. This paper presents the uses of this type of SAR approach in a research laboratory.

Computer automated structure evaluation of the carcinogenicity of N-nitrosothiazolidine and N-nitrosothiazolidine 4-carboxylic acid

N-Nitrosothiazolidine 4-carboxylic acid (NTCA), a sulphur-containing nitrosamine present in human urine, was predicted to be non-carcinogenic by CASE, the newly developed Computer Automated Structure Evaluating system. On the other hand, N-nitrosothiazolidine (NTHZ), a nitrosamine present in food products, was predicted to be carcinogenic. The putative non-carcinogenic NTCA may be metabolized to NTHZ, a predicted carcinogen.

Evaluation of the genotoxicity of theobromine and caffeine

Published data on the mutagenicity and genotoxicity of theobromine and caffeine were analysed by the Carcinogen Prediction and Battery Section (CPBS) method. In spite of some positive responses, these analyses did not predict for theobromine a potential for causing cancer by virtue of a genotoxic mechanism. Caffeine, on the other hand, clearly has potential for genotoxic carcinogenicity. The predictive performance of cost-effective batteries consisting of selected combinations of four assays was also evaluated. The predictions were similar to those derived when all the available test results were considered.

Invited contribution: an objective approach to the development of short-term tests predictive of carcinogenicity

The Carcinogenicity Prediction and Battery Selection procedure was developed to address two problems: (1) the identification of highly predictive, yet cost-effective, batteries of short-term tests and (2) the objective prediction of the potential carcinogenicity of chemicals based upon the results of short-term tests even when a mixture of positive and negative results is obtained. In the present report the usefulness of the Carcinogenicity Prediction and Battery Selection procedure is demonstrated using benzo[a]pyrene, benzoin and diethylstilbestrol as examples. In addition, its applicability in the analysis of all the possible outcomes of a battery is illustrated together with an analysis of the worth of additional testing.

Relationship between mutagenic potency in Salmonella typhimurium strains and the chemical structure of nitro biphenyls

Most of the positional isomers of mono-, di-, tri- and tetranitrobiphenyls were synthesized and assayed for their mutagenicity in Salmonella typhimurium strains TA98, TA98NR and TA98/1,8DNP6 in the absence of S9 mix. In mono- and dinitrobiphenyls, the structure requirements favoring mutagenic activity are the presence of a nitro group at the 4-position and its absence at the 2-position. TA98 and TA98/1,8DNP6 were reverted by 2-position-free 4-nitro analogues, but TA98NR was not reverted. The results suggest that direct-acting mutagenicity involves the reduction of the nitro group by bacterial nitroreductase but does not involve specific esterification enzymes. Some of the tri- and tetranitrobiphenyls e.g. 3,4,3'-, 3,4,4'-, 3,4,3',4'- and 3,4,2',4'-derivatives reverted not only TA98 and TA98/1,8DNP6 but also TA98NR. Those derivatives commonly have 2 nitro groups at an adjoining position (3,4-dinitro group), whereas 2,4,2',4'-tetranitrobiphenyl, which has strong potency not only in TA98 and TA98/1,8DNP6 but also in TA98NR, possesses 2 nitro groups at the 2-position of each benzene ring.

Deployment of short-term assays for the detection of carcinogens; genetic and molecular considerations

The deployment of short-term assays for the detection of carcinogens inevitably has to be based on the genetic alterations actually involved in carcinogenesis. This paper gives an overview of oncogene activation and other mutagenic events connected with cancer induction. It is emphasized that there are indications of DNA alterations in carcinogenicity, which are not in accordance with "conventional" mutations and mutation frequencies, as measured by short-term assays of point mutations, chromosome aberrations and numerical chromosome changes. This discrepancy between DNA alterations in carcinogenicity and the endpoints of short-term assays in current use include transpositions, insertion mutations, polygene mutations, gene amplifications and DNA methylations. Furthermore, tumourigenicity may imply an induction of a genetic instability, followed by a cascade of genetic alterations. The evaluation of short-term assays for carcinogenesis mostly involves two correlations that is, between mutation and animal cancer data on the one hand and between animal cancer data and human carcinogenicity on the other. It should be stressed that animal bioassays for cancer in general imply tests specifically for the property of chemicals to function as complete carcinogens, which may be a rather poor reflection of the actual situation in human populations. The primary aim of short-term mutagenicity assays is to provide evidence as to whether a compound can be expected to cause mutations in humans, and such evidence has to be considered seriously even against a background of negative cancer data. For the evaluation of data from short-term assays the massive amount of empirical data from different assays should be used and new computer systems in that direction can be expected to provide improved predictions of carcinogenicity.

Quantitative structure-activity relationships of beta-adrenergic agents. Application of the computer automated structure evaluation (CASE) technique of molecular fragment recognition

A quantitative structure-activity analysis of adenylate cyclase-coupled beta-adrenergic receptor agonists and antagonists in the frog erythrocyte membrane was made. On the basis of molecular structural fragment descriptors, automatically generated by the CASE methodology, catecholamine derivatives were correctly classified as agonists or antagonists. The potency of these agents in each category, as well as their binding-affinity for the beta receptor was correlated through a multivariate regression analysis.

Short-term test results for NTP noncarcinogens: an alternate, more predictive battery

A battery of short-term tests used to predict whether or not a chemical is a carcinogen must be both sensitive (correctly identifying carcinogens) and specific (correctly identifying noncarcinogens). A recent publication [Shelby and Stasiewicz, 1984, Environ Mutagen 6:871-876] of results in four short-term tests for 70 noncarcinogens tested under the aegis of the National Toxicology Program (NTP) indicates that the battery of short-term tests lacked specificity. We have analyzed these results using the Carcinogen Prediction and Battery Selection (CPBS) procedure and calculated that the specificity of the NTP battery is indeed very low, i.e., 0.50. Using published data from NTP, the Gene Tox program of EPA, and the collaborative study of the WHO International Programme on Chemical Safety, we have constructed an alternate battery that has fewer false positives; this battery has a specificity of 0.80. Thus, the lack of specificity of the original NTP battery does not imply that no set of short-term tests is able to predict carcinogenicity accurately.

Short-term tests are unable to distinguish between human carcinogens and noncarcinogens

Previously published results of short-term tests (STT) applied to chemicals, identified as carcinogens and noncarcinogens, are compared and discussed. Inspection of the data shows that carcinogens and noncarcinogens are about equally STT-positive. Although it may be possible to observe suggestive indications in the data, it is difficult to be certain that these indications are necessarily relevant to or predictive of human carcinogenic risk. The assumption that STT are predictive, if incorrect and accepted, would seem to have the potential for causing harm to public health by leading to falsely recognized carcinogens as noncarcinogens and noncarcinogens as carcinogens. Technological and legal preparations have been made for rapid regulatory decisions on the basis of STT. The data, however, appear to caution against utilizing these means too quickly.

NPPD (spy dust) is predicted to be a mutagen

With the aid of CASE, the Computer Automated Structure Evaluation system, a new artificial intelligence procedure to study structure-activity relationships and a data base consisting of 233 monocyclic nitroarenes, it is predicted that 5-(4-nitrophenyl)-2,4-pentadienal will be mutagenic for Salmonella typhimurium but that the activity will be very low.

The calculation and use of carcinogenic potency: a review

Six methods of estimating carcinogenic potency now in use to some extent by regulatory agencies are examined. It is concluded that none of these methods is adequate for regulatory decision making when used alone, as is usually the case, but that all are useful as a contribution to the whole risk assessment process. The needs for further development and improvement of these methods, where appropriate to human risk, are identified and discussed in view of the growing use of potency estimates in regulatory decision making.

Structure-genotoxic activity relationships of pesticides: comparison of the results from several short-term assays

The Computer-Automated Structure Evaluation (CASE) program has been applied to the analysis of the genotoxic activity of 54 pesticides (31 insecticides, 15 herbicides and 8 fungicides) in 5 different short-term test systems measuring gene mutation and DNA damage. The database contains compounds presenting diverse structures including carbamates, thiocarbamates, organophosphates, halo-aromatics and other functionalities. Some significant relationships between common structural features and the genotoxic activity displayed by these chemicals have been found. Among the most relevant fragments, automatically selected by the program, a methoxyphosphinyl and a chlorovinyl group appear as the common structural subunits responsible for the activities detected in the battery composed of the Salmonella typhimurium histidine reversion assay, the mouse lymphoma gene mutation assay and recombination in the yeast Saccharomyces cerevisiae.

Artificial intelligence and Bayesian decision theory in the prediction of chemical carcinogens

Two procedures for predicting the carcinogenicity of chemicals are described. One of these (CASE) is a self-learning artificial intelligence system that automatically recognizes activating and/or deactivating structural subunits of candidate chemicals and uses this to determine the probability that the test chemical is or is not a carcinogen. If the chemical is predicted to be carcinogen, CASE also projects its probable potency. The second procedure (CPBS) uses Bayesian decision theory to predict the potential carcinogenicity of chemicals based upon the results of batteries of short-term assays. CPBS is useful even if the test results are mixed (i.e. both positive and negative responses are obtained in different genotoxic assays). CPBS can also be used to identify highly predictive as well as cost-effective batteries of assays. For illustrative purposes the ability of CASE and CPBS to predict the carcinogenicity of a carcinogenic and a non-carcinogenic polycyclic aromatic hydrocarbon is shown. The potential for using the two methods in tandem to increase reliability and decrease cost is presented.

The carcinogenicity prediction and battery selection (CPBS) method: a Bayesian approach

Recently, a large number of relatively inexpensive in vitro short-term tests have been developed to help predict the carcinogenicity of chemicals. The carcinogenicity prediction and battery selection (CPBS) method utilizes the results of such short-term tests to screen for chemicals that are most likely to cause cancer. The method is an integrated approach for analyzing large, often sparsely filled, data bases containing short-term test results, which often have only marginal representation of known non-carcinogens. The CPBS method is developed for the purpose of (i) determining the reliability and predictive capability of individual and batteries of short-term tests, and (ii) developing a strategy for formulating and selecting optimally preferred batteries of short-term tests for screening chemicals for further testing. The term 'optimally preferred' connotes the best acceptable combination of tests in terms of trade-offs among the multiple attributes of each test and resulting battery (e.g., cost, sensitivity, specificity, etc). The CPBS method consists of 5 major tasks: (1) data consolidation, (2) parameter estimation, (3) predictivity calculation, (4) battery selection and (5) risk assessment. Although there is a great need for more research and improvement, the CPBS method at its present stage should add an important method to the maze of the thousands of new chemicals that are introduced into drugs, foods, consumer goods and to the environment every year. This method should also provide an enhanced identification procedure for classifying chemicals more accurately as suspected carcinogens or non-carcinogens.

Assembly and preliminary analysis of a genotoxicity data base for predicting carcinogens

With a view to developing methodologies for predicting the carcinogenicity of chemicals on the basis of the results of short-term assays and selecting highly predictive batteries of short-term tests, a data base was assembled. The present is a compilation of data extracted from the reports of Gene-Tox working groups, Salmonella mutagenicity data obtained from the U.S. National Toxicology Program and the Environmental Mutagen Information Center and results from BHK21 transformation assays.

Application of the carcinogenicity prediction and battery selection (CPBS) method to the Gene-Tox data base

The carcinogenicity prediction and battery selection (CPBS) method (Chankong et al., 1985) utilizes the results of short-term tests to predict the carcinogenicity of chemicals and select batteries of tests that are capable of giving accurate predictions at reasonable costs. The CPBS method has been applied to the data compiled under the aegis of the Gene-Tox Program of the U.S. Environmental Protection Agency as a demonstration of the method on a typical data base. A number of batteries were selected by the methodology as having superior performance characteristics. The Bayesian predictions resulting from most of the selected 3-assay batteries were very good (greater than 90% of the carcinogens were correctly identified). It was also found that the 3-assay batteries of specified composition gave generally more accurate predictions than batteries of 4 or more assays of unspecified composition. A number of problems which may have affected our results have been identified: (1) the reliability of the sensitivities and specificities of the individual assays, (2) the prior probability that a chemical is a carcinogen was assumed to be 0.5, and (3) we have not (as yet) taken into account that some of the carcinogens are non-genotoxic and will produce false negative assays results. We are currently investigating approaches to take these factors into consideration. Our analysis also indicates that more testing of chemicals for carcinogenicity (especially probable non-carcinogens) is needed to further enhance the predictive capability of the CPBS method.