The unique role of rodents in the detection of possible human carcinogens and mutagens

Data-Driven Quantitative Structure-Activity Relationship Modeling for Human Carcinogenicity by Chronic Oral Exposure

Traditional methodologies for assessing chemical toxicity are expensive and time-consuming. Computational modeling approaches have emerged as low-cost alternatives, especially those used to develop quantitative structure-activity relationship (QSAR) models. However, conventional QSAR models have limited training data, leading to low predictivity for new compounds. We developed a data-driven modeling approach for constructing carcinogenicity-related models and used these models to identify potential new human carcinogens. To this goal, we used a probe carcinogen dataset from the US Environmental Protection Agency's Integrated Risk Information System (IRIS) to identify relevant PubChem bioassays. Responses of 25 PubChem assays were significantly relevant to carcinogenicity. Eight assays inferred carcinogenicity predictivity and were selected for QSAR model training. Using 5 machine learning algorithms and 3 types of chemical fingerprints, 15 QSAR models were developed for each PubChem assay dataset. These models showed acceptable predictivity during 5-fold cross-validation (average CCR = 0.71). Using our QSAR models, we can correctly predict and rank 342 IRIS compounds' carcinogenic potentials (PPV = 0.72). The models predicted potential new carcinogens, which were validated by a literature search. This study portends an automated technique that can be applied to prioritize potential toxicants using validated QSAR models based on extensive training sets from public data resources.

PET/MRI and PET/CT Radiomics in Primary Cervical Cancer: A Pilot Study on the Correlation of Pelvic PET, MRI, and CT Derived Image Features

PURPOSE

To evaluate the correlation of radiomic features in pelvic [2-deoxy-2-18F]fluoro-D-glucose positron emission tomography/magnetic resonance imaging and computed tomography ([¹⁸F]FDG PET/MRI and [¹⁸F]FDG PET/CT) in patients with primary cervical cancer (CCa).

PROCEDURES

Nineteen patients with histologically confirmed primary squamous cell carcinoma of the cervix underwent same-day [¹⁸F]FDG PET/MRI and PET/CT. Two nuclear medicine physicians performed a consensus reading in random order. Free-hand regions of interest covering the primary cervical tumors were drawn on PET, contrast-enhanced pelvic CT, and pelvic MR (T2 weighted and ADC) images. Several basic imaging features, standard uptake values (SUV_mean, SUV_max, and SUV_peak), total lesion glycolysis (TLG), metabolic tumor volume (MTV), and more advanced texture analysis features were calculated. Pearson's correlation test was used to assess the correlation between each pair of features. Features were compared between local and metastatic tumors, and their role in predicting metastasis was evaluated by receiver operating characteristic curves.

RESULTS

For a total of 101 extracted features, 1104/5050 pairs of features showed a significant correlation (ρ ≥ 0.70, p < 0.05). There was a strong correlation between 190/484 PET pairs of features from PET/MRI and PET/CT, 91/418 pairs of CT and PET from PET/CT, 79/418 pairs of T2 and PET from PET/MRI, and 50/418 pairs of ADC and PET from PET/MRI. Significant difference was seen between eight features in local and metastatic tumors including MTV, TLG, and entropy on PET from PET/CT; MTV and TLG on PET from PET/MRI; compactness and entropy on T2; and entropy on ADC images.

CONCLUSIONS

We demonstrated strong correlation of many extracted radiomic features between PET/MRI and PET/CT. Eight radiomic features calculated on PET/CT and PET/MRI were significantly different between local and metastatic CCa. This study paves the way for future studies to evaluate the diagnostic and predictive potential of radiomics that could guide clinicians toward personalized patients care.

Assessment of genotoxic potential of the insecticide Dichlorvos using cytogenetic assay

The possible genotoxic activity of Dichlorvos (2,2-Dichlorovinyl-O,O-dimethyl phosphate/DDVP, CAS No. 62-73-7), an organophosphorus insecticide was investigated employing three cytogenetic end points, i.e. micronucleus (MN) assay, mitotic indices (MI) and chromosome abberation (CA) analysis in vivo. The assays were carried out in hematopoietic bone marrow cells of Mus musculus at concentrations of 10, 20 and 30% of LD50 for intraperitoneal (ip) administration, corresponding to 0.06, 0.08 and 0.13 mg/kg Bwt, respectively. The normal control group received single ip dose of distilled water (2 ml/100 g Bwt), while animals of the positive group were injected with cyclophosphamide, a model mutagen (40 mg/kg Bwt) under identical conditions. The animals were sacrificed 24, 48 and 72 hrs post treatment. Under the present experimental conditions, there was no evidence of significant increase of MN frequencies at any dose or sampling time in polychromatic (PCE) and normochromatic (NCE) erythrocytes. The PCE/NCE ratio was not notably affected; however, a slight depression in prolonged exposure (48, 72 hr) intervals and a slight increase at the 24 hr interval were observed. Cells with various structural chromosome aberrations were noted but no significant (p<0.05; Man-Whitney U-test) differences in the frequencies of CA or mitotic indices (p<0.05; χ(2) test) were observed between Dichlorvos treated groups and the normal control group at doses or time intervals used. The results of the present investigation reflects a negative in vivo genotoxic potential of Dichlorvos at sublethal doses in bone marrow cells. Further studies are underway to confirm the presence or absence of genotoxic activity since compounds negative in genotoxic evaluation are susceptible of being carcinogens triggering cancer by genotoxic or non-genotoxic mechanisms.

Failure of the standard battery of short-term tests in detecting some rodent and human genotoxic carcinogens

Theoretical reasons and experimental evidence indicate that a no-effect level generally cannot be expected for genotoxic carcinogens; as a consequence, in quantitative risk assessment the capability of distinguishing genotoxic from non-genotoxic carcinogens is of fundamental importance in order to identify relevant levels of human exposure. According to generally accepted guidelines, the standard three-test battery for the detection of genotoxic compounds consists of: (i) an in vitro test for gene mutation in bacteria; (ii) an in vitro test in mammalian cells with cytogenetic evaluation of chromosomal damage and/or a test that detects gene mutations; (iii) an in vivo test for chromosomal damage using rodent hematopoietic cells. This test battery is designed to avoid the risk of false negative results for compounds with genotoxic potential, but it cannot be taken for granted that the risk is completely eliminated. As a matter of fact there are some chemicals, classified by the International Agency for Research on Cancer (IARC) as probably or possibly carcinogenic to humans, which gave consistent negative results in this test battery, and in contrast provided positive results in other not routinely employed genotoxicity assays. The failure of the standard test battery in detecting some genotoxic carcinogens is attributable to several causes, but the principal of them are the following ones: in vitro, the artificial metabolic activity of the liver S9-mix, and the different biotransformation of chemicals in cells of different type and from different animal species; in vivo, the pharmacokinetic behaviour of the test compound, and its possible species-, sex- and tissue-specificity.

Genotoxicity of an extract of Calendula officinalis L

A fluid extract of Calendula officinalis L. displayed genotoxic properties when assayed for mitotic segregation in the heterozygous diploid D-30 of Aspergillus nidulans. The extract of Calendula exhibited dose-dependent toxicity and genotoxicity (both mitotic crossing-over and chromosome malsegregation being observed) to Aspergillus in the range of five plate concentrations from 0.1 to 1.0 mg of solids/ml assayed. Mutagenicity testing with the Salmonella/microsome assay in strains TA 1535, TA 1537, TA 98 and TA 100 was negative in a plate incorporation protocol, with concentrations ranging from 50 to 5000 microg/plate (+/- S9). The mouse bone marrow micronucleus test, where the extract was dosed orally up to 1 g/kg for 2 days, was also negative.

Mustard oil and garlic extract as inhibitors of sodium arsenite-induced chromosomal breaks in vivo

Arsenic, a well-known human carcinogen present as a contaminant in ground water poses a serious threat to public health in various countries. The anticlastogenic properties of two dietary supplements, garlic and mustard oil, were screened against the clastogenic activity of sodium arsenite, since diet may contain factors which affect the process of mutagenesis and carcinogenesis. Aqueous extract of garlic (100 mg/kg b.w.) and mustard oil (0.643 mg/kg b.w.) were fed to Mus musculus for 30 consecutive days either singly or simultaneously. Sodium arsenite (0.1 mg/kg b.w.) was injected subcutaneously on days 7, 14, 21 and 30 of the experiment, singly and together with the dietary supplements. The animals were sacrificed 24 h after the last exposure to sodium arsenite and clastogenic effects were observed in the bone marrow cells. The degree of modulation of sodium arsenite-induced chromosomal aberrations was more pronounced in mustard oil than in garlic extract and simultaneous administration of both the dietary supplements reduced the clastogenic effects of sodium arsenite closer to the level of the negative control. The greater efficacy could be due to the interaction of the two dietary supplements and its radical scavenging property.

An in vivo micronucleus assay for detecting the clastogenic effect in rat kidney cells

A micronucleus assay in vivo has been developed that is based on the use of freshly isolated kidney cells from mononephrectomized rats. In this validation study, a statistically significant increase in the frequency of micronucleated cells was detected in rats given i.p. a single dose of four kidney carcinogens, N-nitrosodimethylamine, N-nitrosodiethylamine, N-ethyl-N-hydroxyethylnitrosamine and N-nitroso-N-methylurea. The clastogenic effect was more marked when the same dose was injected for 3 successive days. As compared to controls, treated rats displayed a reduction in the frequency of binucleated cells, presumably due to a toxicity-induced inhibition of cellular proliferation. The proposed method should be suitable for the detection of the clastogenic effect of procarcinogens biotransformed into reactive species in the kidney.

Comparison of micronucleus formation in mouse bone marrow and spleen

The frequencies of micronucleated erythrocytes were compared in bone marrow and spleen of mice killed 24 and 48 h after a single i.p. dose of one directly acting carcinogen, N-nitroso-N-ethylurea (NEU, 100 mg/kg), and two indirectly acting ones, N-nitrosodimethylamine (NDMA, 50 mg/kg) and 7,12-dimethylbenz[a]anthracene (7,12-DMBA, 50 mg/kg). The treated/control ratio of the incidence of micronucleated polychromatic erythrocytes (MnPCEs) was similar in the two tissues for NDMA at 24 h (sampling at 48 h was precluded by toxicity) and for 7,12-DMBA at 48 h, while it was higher in the bone marrow than in the spleen for NEU at both 24 and 48 h and for 7,12-DMBA at 24 h. Concerning micronucleated normochromatic erythrocytes (MnNCEs), their frequency in both tissues was always lower than that of MnPCEs; however, while in bone marrow a marked increase in their incidence was induced by NEU and 7,12-DMBA, any response was absent in spleen, thus suggesting that this organ does not sequester micronucleated erythrocytes. These results already indicate that the spleen is not a useful alternative to the bone marrow in the micronucleus assay. Moreover, counting of MnPCEs in the spleen is made more difficult and prone to error by the low frequency of PCEs, and by their greater toxicity-induced reduction. This last effect was found to be enhanced by the use of old mice.

Simultaneous evaluation of clastogenicity, aneugenicity and toxicity in the mouse micronucleus assay using immunofluorescence

An improved antikinetochore antibody technique was established in the mouse micronucleus assay to simultaneously evaluate toxicity, clastogenicity and aneugenicity induced by various test agents. The procedure involved the use of cellulose column fractionated cytospun slides for analysis. The staining method consisted of sequential treatment of slides with crest serum, fluorosceinated goat-antihuman and swine-antigoat antibodies, and propidium iodide. In this method, polychromatic erythrocytes (PCEs, dark red), normochromatic erythrocytes (NCEs, green), chromosome(s)/fragments/micronuclei (orange), and kinetochores (yellow), are identified using the same filter setting under blue excitation (440-490 nm) with a barrier filter at 520 nm. Using this method, three agents, cyclophosphamide, X-rays and vincristine were tested for micronucleus/aneuploidy induction and bone marrow toxicity. The aneugen, vincristine, and clastogens, X-rays and cyclophosphamide, induced predominantly kinetochore positive (K+) and negative (K-) micronucleated PCEs, respectively. At the doses tested, cyclophosphamide caused a slight but statistically significant decrease in PCEs in females, and other agents did not produce any severe bone-marrow toxicity in either male or female mice. These results are comparable with the results reported in the literature on these compounds with various methods and thus demonstrate the usefulness of this assay in distinguishing clastogenicity from aneugenicity and in evaluating toxicity.

Micronucleated reticulocyte induction by ethylating agents in mice

Six model ethylating agents were tested for clastogenic potency by means of a new technique of the micronucleus assay with mouse peripheral blood cells using acridine orange (AO)-coated slides, to evaluate the test. The alkylating agents were: N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), N-ethyl-N-nitrosourea (ENU), diethylsulfate (DES), ethyl methanesulfonate (EMS), epichlorohydrin (ECH) and ethylene dibromide (EDB). The animals were given a single intraperitoneal injection of the following doses of the chemicals: ENNG and ENU, 25, 50 and 100 mg/kg; EMS and DES, 100, 200 and 400 mg/kg body weight. For EDB and ECH, the doses were 50, 100 and 200 mg/kg, given twice, 24 h apart. Before and after the injection, blood samples were taken from the tails at 24-h intervals up to 72 h and preparations were made on AO-coated slides. For each dose group, 4 animals were used and 1000 reticulocytes were examined per slide for the presence of micronuclei. At the optimum induction time of 48 h, ENU induced micronucleated reticulocytes (MNRETs) at all 3 doses. ENNG and EMS induced MNRETs significantly at 2 dose levels each and DES only at the highest dose. ECH and EDB failed to induce MNRETs. On the basis of the dose of chemical needed to double the spontaneous frequency, the order of clastogenic potency was ENU greater than ENNG greater than EMS greater than DES. The results obtained compared favorably with those from other in vivo methods. The present technique proves to be simple, flexible and relatively sensitive. Shifts in the optimum induction peak in individual animals and by some chemicals can be picked up easily which is important when testing weak mutagens and chemicals with an unknown mechanism of action.

Evaluation in a battery of in vivo assays of four in vitro genotoxins proved to be noncarcinogens in rodents

2-Chlorethanol, 8-hydroxyquinoline, 2,6-toluenediamine, and eugenol, previously found to behave as genotoxins in in vitro systems and as noncarcinogens in rodents, were evaluated for their ability to induce genotoxic effects in vivo. Rats were given by gavage a single or two successive doses equal to one-half the corresponding LD50, killed at different times after treatment, and examined for the following end points: the frequency of both micronucleated polychromatic erythrocytes in the bone marrow and micronucleated hepatocytes (after partial hepatectomy); the in vivo-in vitro induction of DNA fragmentation, as measured by the alkaline elution technique, and of unscheduled DNA synthesis, as measured by autoradiography, in hepatocyte primary cultures. The two latter end points were also evaluated after in vitro exposure of hepatocytes to log-spaced subtoxic concentrations. 2-Chloroethanol, 8-hydroxyquinoline, and eugenol never produced effects indicative of genotoxic activity. The same happened with 2,6-toluenediamine, with the exception of a significant increase over controls in the amounts of DNA damage and repair displayed by hepatocyte cultures obtained from rats given two 1/2 LD50 separated by a 24 h interval. Our results, which, apart the above mentioned exception, are in concordance with the rodent carcinogenicity results, contribute to underline the role of in vivo short-term tests for the detection of potential genotoxic carcinogens.

Modifying role of Phyllanthus emblica and ascorbic acid against nickel clastogenicity in mice

Nickel, a major environmental pollutant is known for its clastogenic and carcinogenic potential. Dietary inhibitors of mutagenesis and carcinogenesis are of particular importance since they may have a role in cancer prevention. In the present investigation, aqueous extract of edible dried fruits of Phyllanthus emblica, a well known medicinal plant, was fed to Mus musculus for seven consecutive days prior to treatment with different doses of nickel chloride (10, 20 and 40 mg/kg body wt.); the fruit extract significantly reduced the frequency of CA/cell, the percentage of aberrant cells and the frequency of micronuclei induced by all doses of nickel in the bone marrow cells of mice. Ascorbic acid, a major constituent of the fruit, fed for 7 consecutive days in equivalent concentration as that present in the fruit, however, could only alleviate the cytotoxic effects induced by low doses of nickel; at the higher doses it was ineffective. The greater efficacy of the fruit extract could be due to the interaction of its various natural components rather than to any single constituent. The study assumes importance in view of the widespread human exposure to nickel compounds.

The application of a wedge perfusion technique to the in vivo-in vitro rat hepatocyte DNA-repair assay

The in vivo-in vitro rat hepatocyte DNA-repair assay is regarded as labour-intensive and time-consuming to perform. This has tended to impose limitations on its use as a routine procedure for assessing the potential genotoxicity of chemicals. We have developed a simple wedge-perfusion technique which enables hepatocytes to be isolated from several different rats simultaneously. Hepatocyte yield and metabolic capacity are comparable to those isolated by conventional whole-liver perfusion. Hepatocyte viability was generally superior to that obtained when performing multiple in situ perfusions for the rat hepatocyte UDS assay. The median lobe is routinely used but no difference was observed in the UDS response to the positive control genotoxic agents, methyl methanesulphonate (MMS, CAS No. 66-27-3) and 2-acetylaminofluorene (AAF, CAS No. 53-96-3), in hepatocytes isolated from the median or either lateral lobe. The use of Williams medium E or Leibovitz L15 culture medium did not influence the response. This perfusion technique greatly reduces the time, equipment and personnel required and therefore the cost for hepatocyte isolation. It also facilitates the inclusion of concurrent control groups at each time point of assay.

Cytogenetic assays in genotoxic studies: somatic cell effects of benzene and germinal cell effects of dibromochloropropane

We have reviewed the results of our cytogenetic studies to evaluate the usefulness of these assays in genotoxic studies. In one study, we observed unusual dose-response in lymphocyte chromosome aberration frequencies after exposure of mice to low doses of a chemical mixture (benzene, chloroprene, epichlorohydrin, and xylene). The frequency in the high dose group is lower than those of the medium and low dose groups. This reduction of genotoxicity is correlated with a significant induction of a detoxifying enzyme, glutathione-S-transferase. The data also suggest that extrapolation of effects from high to low doses for risk assessment may be erroneous. Using benzene as a model clastogen, we found that the clastogenic effect persists for a long time after termination of exposure in mice. This phenomenon is probably due to a gradual release of benzene from absorption of the chemical in body fat. In an inhalation study, we observed that chromosome aberrations are induced in mice after exposure to benzene at below the occupational exposure limit of 1 ppm. Since benzene is a ubiquitous environmental contaminant, it may interact with other environmental agents to modify its genotoxic effects. We found that a nongenotoxic drug, praziquantel, and a free radical scavenger, DMSO, can enhance or reduce respectively the clastogenic activities of benzene in mice. Both modifying agents acted by altering the metabolic pathways of benzene. In a study with rats, we showed that carcinogenic doses of benzene can induce chromosome aberrations in lymphocytes of rats long before the expression of cancer. With dibromochloropropane, we observed that this chemical can induce dominant lethality in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigations on the use of EDTA-permeabilized E. coli cells in liquid suspension and animal-mediated genotoxicity assays

The potential use of EDTA-permeabilized E. coli cells for the investigation of genotoxic effects of compounds with a large molecular configuration in vitro and in animal-mediated differential DNA-repair assays was studied. The indicator for the induction of (repairable) DNA damage was a pair of E. coli K-12 strains (343/765 and 343/753) differing vastly in DNA-repair capacity (uvr+/rec+ vs. uvrB/recA). Investigations on the influence of EDTA treatment on the viability of these strains show that during short-term exposure (3 min), the EDTA level should not exceed 0.5 mmole/l in the pretreatment mix, since at higher concentrations a marginal titer reduction of the repair-deficient strain occurs, thus indicating a weak genotoxic activity of this chelating agent. Comparisons of the results gained in vitro with permeabilized and untreated cells demonstrate that EDTA exposure leads to a substantial enhancement of the sensitivity of the indicator bacteria towards DNA damage induced by B(a)P and N-Ac-2AAF which is essential for the detection of genotoxic activities of these polycyclic aromatic compounds. Experiments to elucidate the possibility of employing EDTA-treated cells in vivo show that following intravenous and oral administration the recovery rates of permeabilized indicator strains from various mouse organs are substantially lower than those found under identical conditions (exposure time 150 min) with untreated strains. Nevertheless enough viable cells can be recovered from liver, spleen, kidneys, lungs and stomach to allow the investigation of organ-specific genotoxicity. It is furthermore noteworthy that exposure of permeabilized indicator cells in control animals (for 150 min) resulted in a marginal reduction of the relative survival of the repair-deficient strain in all organs investigated, whereas with non-treated strains such effects are only detectable after extended exposure periods. The observation of a slightly elevated genotoxic background under in vivo conditions does not prevent the assessment of the organ distribution of genotoxic effects induced by mutagens and/or carcinogens: in the case of B(a)P, intraperitoneal administration to mice in the dose range of 10-50 mg/kg body weight resulted in a pronounced dose-dependent inactivation of the uvrB/recA cells in the liver. Also in the lungs differential killing effects occurred at the highest dose tested, whereas no genotoxic activities were detectable in stomach, kidneys and spleen of the host animals.

Induced hepatocytes as a metabolic activation system for the mouse-lymphoma assay

We have developed methods for the coculture of hepatocytes and mouse lymphoma cells and have shown that this system can be used for evaluating promutagens from several chemical classes (Brock et al., 1987). In the present study we investigated the use of hepatocytes isolated from rats pretreated with a cytochrome P-450 inducer (PB) or a P-448 inducer (BNF). CP-induced mutagenicity was higher in the presence of PB-induced hepatocytes than in control hepatocytes. Control and BNF-induced hepatocytes were evaluated with B(a)P, B(l)A, and BA. A dose-related positive response was observed with B(a)P and B(l)A both in the presence of control or induced hepatocytes; however, somewhat higher mutant frequencies were obtained in the presence of BNF-induced hepatocytes. BA induced a very weak positive response (approx. 2 X b.g.) in the presence of control hepatocytes and was weakly positive in the presence of BNF-induced hepatocytes. Benzene was tested using control and both PB- and BNF-induced hepatocytes. Neither of these approaches were successful in activating benzene to a mutagenic metabolite. These studies indicate that for some chemicals the mutagenic response of mouse lymphoma cells can be increased by inducing hepatocytes prior to isolation and cocultivation, and expands the use of hepatocytes for research evaluating chemicals requiring metabolic activation.

Benzene and the genotoxicity of its metabolites. II. The effect of the route of administration on the micronuclei and bone marrow depression in mouse bone marrow cells

Benzene (880 mg/kg) and 4 of its metabolites, i.e., phenol (265 mg/kg), hydroquinone (80 mg/kg), catechol (40 mg/kg), and p-benzoquinone (5-20 mg/kg) have been tested for their capability to induce micronuclei in bone marrow cells of male mice after oral administration or intraperitoneal injection. Oral administration of benzene shows more activity than intraperitoneal injection, whereas the metabolites show more activity if administered by the latter method. The respective genotoxic strengths of the benzene metabolites are the following: hydroquinone much greater than phenol greater than catechol = p-benzoquinone. This last is active when administered orally.

Carcinogenicity of chemicals: the weight of evidence

1. The evidence discussed here is derived from epidemiology, long-term bioassays in laboratory animals, and predictive short-term tests. 2. Epidemiological data are obtained directly from human studies and are most compelling when they demonstrate a large relative risk and a clear dose-response in association with a distinctive tumour type. Exposure to a suspected carcinogen and the doses involved are, however, often difficult to determine, and the most sophisticated epidemiological methods are relatively insensitive. There are no epidemiological data for most occupational/environmental chemicals. 3. Long-term bioassays can present major problems in design, interpretation and extrapolation. Particular difficulties are associated with the planning of appropriate dose levels and the occurrence of certain tumours at high incidence in both control and test groups. Results from animal bioassays set priorities for concern and action but they cannot be reliably used for quantitative assessment of human risk. 4. Evidence of potential carcinogenicity derived from short-term predictive tests, involving a wide variety of systems with diverse end-points, is increasingly important. Emphasis is placed on the need for more in vivo procedures with a broadening of the scope of somatic cell targets.

Assessment of the potential germ cell mutagenicity of industrial and plant protection chemicals as part of an integrated study of genotoxicity in vitro and in vivo

An approach is described that enables the germ cell mutagenicity of chemicals to be assessed as part of an integrated assessment of genotoxic potential. It is recommended, first, that the genotoxicity of a chemical be defined by appropriate studies in vitro. This should involve use of the Salmonella mutation assay and an assay for the induction of chromosomal aberrations, but supplementary assays may be indicated in specific instances. If negative results are obtained from these 2 tests there is no need for the conduct of additional tests. Agents considered to be genotoxic in vitro should then be assessed for genotoxicity to rodents. This will usually involve the conduct of a bone marrow cytogenetic assay, and in the case of negative results, a genotoxicity test in an independent tissue. Agents found to be non-genotoxic in vivo are regarded as having no potential for germ cell mutagenicity. Agents found to be genotoxic in vivo may either be assumed to have potential as germ cell mutagens, or their status in this respect may be defined by appropriate germ cell mutagenicity studies. The basis of the approach, which is supported by the available experimental data, is that germ cell mutagens will be evident as somatic cell genotoxins in vivo, and that these will be detected as genotoxins in vitro given appropriate experimentation. The conduct of appropriate and adequate studies is suggested to be of more value than the conduct of a rigid set of prescribed tests.

Reflections on the declining ability of the Salmonella assay to detect rodent carcinogens as positive

It is suggested that urgent attempts should be made to define and gain general agreement for the existence of two classes of animal carcinogen, those which are genotoxic and those which are not. In the absence of such a step, attempts to validate in vivo genotoxicity assays, and to derive a meaningful structure-activity database for chemical carcinogenesis, will be frustrated. These suggestions are supported by the preliminary findings of a detailed analysis of the carcinogen database accrued by the United States National Toxicology Program. The possibility that many non-genotoxic carcinogens should be regarded as tumour-promoting agents is considered.

Chemical structure, Salmonella mutagenicity and extent of carcinogenicity as indicators of genotoxic carcinogenesis among 222 chemicals tested in rodents by the U.S. NCI/NTP

A survey has been conducted of 222 chemicals evaluated for carcinogenicity in mice and rats by the United States NCI/NTP. The structure of each chemical has been assessed for potential electrophilic (DNA-reactive) sites, its mutagenicity to Salmonella recorded, and the level of its carcinogenicity to rodents tabulated. Correlations among these 3 parameters were then sought. A strong association exists among chemical structure (S/A), mutagenicity to Salmonella (Salm.) and the extent and sites of rodent tumorigenicity among the 222 compounds. Thus, a approximately 90% correlation exists between S/A and Salm. across the 115 carcinogens, the 24 equivocal carcinogens and the 83 non-carcinogens. This indicates the Salmonella assay to be a sensitive method of detecting intrinsic genotoxicity in a chemical. Concordance between S/A and Salm. have therefore been employed as an index of genotoxicity, and use of this index reveals two groups of carcinogens within the database, genotoxic and putatively non-genotoxic. These two broad groups are characterized by different overall carcinogenicity profiles. Thus, 16 tissues were subject to carcinogenesis only by genotoxins, chief among which were the stomach, Zymbal's glands, lung, subcutaneous tissue and circulatory system. Conclusions of carcinogenicity in these 16 tissues comprised 31% of the individual chemical/tissue reports of carcinogenicity. In contrast, both genotoxins and non-genotoxins were active in the remaining 13 tissues, chief among which was the mouse liver which accounted for 24% of all chemical/tissue reports of carcinogenicity. Further, the group of 70 carcinogens reported to be active in both species and/or in 2 or more tissues contained a higher proportion of Salmonella mutagens (70%) than observed for the group of 45 single-species/single-tissue carcinogens (39%). 30% of the 83 non-carcinogens were mutagenic to Salmonella. This confirms earlier observations that a significant proportion of in vitro genotoxins are non-carcinogenic, probably due to their non-absorption or preferential detoxification in vivo. Also, only 30% of the mouse liver-specific carcinogens were mutagenic to Salmonella. This is consistent with tumors being induced in this tissue (and to a lesser extent in other tissues of the mouse and rat) by mechanisms not dependent upon direct interaction of the test chemical with DNA. Detection of 103 of the 115 carcinogens could be achieved by use of only male rats and female mice.(ABSTRACT TRUNCATED AT 400 WORDS)

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC publication No. 13. The need for biological risk assessment in reaching decisions about carcinogens

The prudent assumption that carcinogen bioassays in rodents predict for human carcinogenicity is examined. It is suggested that in certain cases, as for example the induction of tumors against a high incidence in controls, or in situations in which high dose toxicity may be a critical factor in the induction of cancer, the probability that animal bioassays predict for humans may be low. The term 'biological risk assessment' is introduced to describe that part of risk assessment concerned with the relevance of specific animal results to the induction of human cancer. Biological risk assessment, which is almost entirely dependent on an understanding of carcinogenesis mechanisms, is an important addition to present mathematical modeling used to predict the effects of animal carcinogens that have been demonstrated after high dose exposure, to the effects of the much smaller doses to which humans are perceived to be exposed. Evidence for the conclusions reached by biological risk assessment may sometimes be supported by a careful review of human epidemiological data.

Induction of unscheduled DNA synthesis in liver and micronucleus in bone marrow of rats exposed in vivo to the benzidine-derived azo dye, Direct Black 38

The genotoxic activity of the benzidine-derived azo dye, Direct Black 38 (DB38), was studied in vivo, using two different genetic end-points: unscheduled DNA synthesis in liver (UDS) and bone marrow micronucleus (MN). Exposure times were 12, 24 or 36 h. Both assays were performed in the same rat, except for the 24-h exposure when only MN was investigated. For the liver UDS assay, the rat hepatocarcinogen, 6-dimethylaminophenylazobenzthiazole (6BT), was used as positive control and for the MN assay, cyclophosphamide (CP). In agreement with earlier results, 6BT gave rise to a dose-related increase in liver UDS after 12-h exposure to 25 or 50 mg/kg bw. After 36-h exposure, there was still an indication of a weak dose-response effect between 0 and 5 net nuclear grains (NG). DB38 induced liver UDS at the higher dose levels used (500 and 1000 mg/kg), and after both 12- and 36-h exposure. With the longer exposure time, a weak induction of UDS was also observed at 100 mg/kg. The strongest UDS induction (12.2 NG), was obtained in one rat after 36-h exposure to 500 mg/kg. DB38 also had a weak effect on the MN induction, which was statistically significant at the higher concentrations used. A dose-related response was observed at all exposure times used.

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.

The long-term study in rodents for identifying carcinogens: some controversies and suggestions for improvements

Despite increasing standardisation and international harmonization of the long-term carcinogenicity study in rodents there are still areas of controversy such as high-dose selection, duration of the test period, combining or not combining the chronic toxicity and the carcinogenicity study, the use of historical control data, conditions for redundancy of the study, and significance of studies of restricted design. It is discussed that the 'Maximum Tolerated Dose', or doses above and below the 'metabolic break-point' should be included. In general a test period of 24 months is adequate, but the protocol should be flexible enough to allow extension beyond 24 months. Major advantages of the combined chronic toxicity/carcinogenicity study are that the toxicity and carcinogenicity data are obtained with the same sample of the test compound using the same batch of animals kept on the same diet under the same environmental conditions. It is discussed that the use of historical control data will not lead to a final conclusion on carcinogenic or non-carcinogenic potential of a compound. A long-term carcinogenicity study is considered redundant when adequate (semi-chronic) toxicity studies including reproduction and mutagenicity tests and pre-screens for carcinogenicity, do not indicate mutagenic or carcinogenic activity, and when there is a wide margin (e.g. 1000) between the 'no-observed-adverse effect level' and the (presumed) exposure level in humans. Studies of restricted design and conduct may clearly demonstrate carcinogenicity but also may easily lead to inconclusive results.

Comparative in vivo and in vitro genotoxicity studies of airborne particle extract in mice

The genotoxicity of an acetone extract of locally collected airborne particles was evaluated both in vitro and in vivo using the sister-chromatid exchange (SCE) assay in mice. At the highest concentration (5.36 mg/5 ml culture), the extract caused approximately a 3-fold increase in SCEs over controls in mouse bone marrow and spleen primary cells in vitro. However, the same airborne particle extract did not induce a significant increase in the SCE level over controls in vivo in mouse bone marrow and spleen cells when administered intraperitoneally or through oral gavage. This indicates that bone marrow and spleen primary cell cultures can be used in in vitro genotoxicity studies of complex mixtures, and that the genotoxicity of airborne particles detected in the in vitro system cannot always be detected in vivo with the same cell types. In addition, the same acetone extract of airborne particles caused dose-related his+ revertants in the strain TA98 of Salmonella typhimurium, both with and without S9 activation. The significant finding of this study is that the in vitro genotoxicity results of airborne particle extract may not be very meaningful in an in vivo situation.

Slide preparation and sampling as a major source of variability in the mouse micronucleus assay

This paper describes the results of a study in which mouse bone marrow micronucleus assay slides were assessed for homogeneity of micronucleated polychromatic erythrocytes (MPE) among polychromatic erythrocytes (PE). The slides were prepared by 3 distinct methods and several methods of slide reading were assessed. Observations made using our slides were confirmed by re-analysis of slides from 3 independent laboratories. It is concluded that the method of slide preparation and assessment can significantly influence the variability of data obtained from a study. The extent of this variability casts doubt upon the validity of certain assumptions concerning this assay--such as sex differences in MPE incidence, responder variability, etc. Results are discussed within the context of the very recent literature for this assay. Some laboratories appear to have adequate methods of slide preparation and data accumulation, while others do not. Methods to improve the sensitivity of this assay are suggested within the context of the recommendations made by the Gene-Tox review group. In particular, it is suggested that individual investigators present evidence of the adequacy of their data accumulation techniques in order to enhance the value of future studies.

The effect of mixed-function oxidase and amine oxidase inhibitors on the activation of dialkylnitrosamines and 1,2-dimethylhydrazine to bacterial mutagens in mice

The effect of the mixed-function oxidase inhibitor phenylimidazole (PI) and the amine oxidase inhibitors iproniazid (IPRO) and aminoacetonitrile (AAN) on the mutagenic activity of various carcinogens was determined in intrasanguineous host-mediated assays, using mice as hosts and E. coli 343/113 as an indicator of mutagenic activity. The carcinogenic compounds dimethyl-, diethyl-, methylethyl-, and diethanolnitrosamine (DMNA, DENA, MENA, and DELNA respectively) and 1,2-dimethylhydrazine (SDMH) were administered i.p. to mice pretreated or not with one of the inhibitors. After 4 h exposure to each of the carcinogens, E. coli cells recovered from the liver of non-pretreated mice showed considerable induction of VALr mutations; after pretreatment of the hosts with the three inhibitors, significant reduction of the amounts of induced mutants in vivo was observed. Particularly, PI proved a very efficient inhibitor of DENA, MENA, DELNA, and SDMH mutagenicity (93%-97% reduction), suggesting that these carcinogens are mainly activated by cytochrome P-450-dependent enzymes. However, since PI might also inhibit the NAD-mediated activation of DELNA by alcohol dehydrogenase (ADH), the present experiments do not rule out an additional role of ADH in the in vivo mutagenic activation of DELNA. AAN and IPRO were less and much less effective, respectively, in reducing the mutagenic activity of all compounds. Surprisingly, PI showed less inhibition of the mutagenic activity of DMNA (60% reduction), as compared to the other carcinogens; this indicates that metabolic routes other than the cytochrome P-450-dependent enzyme system may be important for the activation of DMNA.

Mechanisms of multistep carcinogenesis: keys to developing in vitro approaches for assessing the carcinogenicity of chemicals

The process of neoplastic development is a complex combination of genetic and non-genetic changes within a tissue. It is, therefore, not surprising that a number of mechanisms exist by which chemicals influence this process. In designing short-term tests to detect potential carcinogens, the major emphasis to date has been on genetic assays. This is justified on both theoretical and practical grounds. Substantial evidence exists to support the hypothesis that genetic damage increases the probability of tumour development. Mutational assays for gene mutations, clastogenicity and aneuploidy induction are therefore extremely important tests for carcinogens. Unfortunately, few assays exist for detecting chemicals that induce gene amplification, and thus, if these are carcinogens that specifically induce this type of genetic change, they may go undetected. Because of a variety of possible differences between a chemical's activity in vivo and in vitro, it is to be expected that chemicals that are mutagenic in vitro may not have a significant carcinogenic effect in vivo. However, the potential harm of these chemicals to man does exist and may be expressed under different conditions (e.g. species- or organ-specific carcinogenic effects) or as non-carcinogenic hazards.

The value and limitations of short-term genotoxicity assays and the inadequacy of current criteria for selecting chemicals for cancer bioassays

The Salmonella gene mutation assay is established as the primary in vitro test for genotoxicity. It does not, however, detect all known genotoxic carcinogens, and several mammalian cell genotoxicity assays are now used to complement the Salmonella test--the most usual being cytogenetic analysis in vitro. Use of these two tests provides an adequate screen for genotoxins, but leaves undetected a variety of weak, DNA-unreactive animal carcinogens such as DDT, saccharin and diethylhexyl phthalate. A reappraisal of the present confused situation examines the underlying problems and options for action. It is suggested that the most effective way of focussing resources on the detection of possible new human carcinogens involves in vitro evaluation of genotoxicity, followed where appropriate by further evaluation in vivo, with urgent attention being paid to the methods currently used to select chemicals for cancer bioassays and to the resolution of the current debate on the validity of some recent classifications of carcinogenicity.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC Working Paper No. 3. The concept of negativity in experimental carcinogenesis

The problems that arise in the interpretation of experimental data on chemical carcinogenesis are addressed. In particular, the difficulties in demonstrating negative results are shown to present problems in delineating carcinogens from noncarcinogens. The use of the virtually safe dose estimated under the assumption of low dose linearity is shown to lead to potentially anomalous results if used indiscriminately in bioassays in which no statistically significant increase in tumor occurrence is induced. It is suggested that there is a need to establish an operational definition of negativity in carcinogenesis, with the realization that this definition may be revised in light of new information. The establishment of negativity in aligning data from positive and negative experiments and in considering possible thresholds is also discussed.

Comparison of S9 mix and hepatocytes as external metabolizing systems in mammalian cell cultures: cytogenetic effects of 7,12-dimethylbenzanthracene and aflatoxin B1

Two external metabolizing systems, S9 mix from Aroclor-induced rat livers and freshly isolated hepatocytes, were used for activation in cultures of human lymphocytes and V79 cells. 7, 12-dimethylbenzanthracene (DMBA) and aflatoxin B1 (AFB1) were employed as indirectly acting reference mutagens. Mutagenic effects were measured by induction of sister chromatid exchange (SCE). With DMBA, SCE-inducing effects were found to be quite similar after activation by S9 mix and activation by hepatocytes. In human lymphocytes nearly the same dose-effect relationships were found with both metabolizing systems; in V79 cells the hepatocyte-mediated induction of SCE was detectable at slightly lower concentrations than the S9-mediated SCE induction. In contrast with AFB1, S9 activation led to a stronger SCE induction than hepatocyte activation in both target cells. The induction of chromosomal aberrations by AFB1 after activation by the two metabolizing systems was also analysed in V79 cells. This experiment again revealed that AFB1 was more efficiently activated by S9 mix than by hepatocytes, and it appeared that AFB1 is a more potent inducer of chromosomal aberrations than of SCE. The different activation capacities of the two metabolizing systems for AFB1 may be due to the maintenance of inactivation mechanisms in hepatocytes or to the Aroclor induction of the S9 fraction. Our experiments have shown that the suitability of hepatocytes as an activation system is not restricted to microbial or eukaryotic point mutation assays, but that hepatocyte metabolism can also be successfully included in cytogenetic tests with short- and long-term cultures of mammalian target cells.

The mouse spot test. Evaluation of its performance in identifying chemical mutagens and carcinogens

The published results on 60 chemicals and X-rays investigated in the mouse spot test were compared with data on the same chemicals tested in the bacterial mutation assay (Ames test) and lifetime rodent bioassays. The performance of the spot test as an in vivo complementary assay to the in vitro bacterial mutagenesis test reveals that of 60 agents, 38 were positive in both systems, 6 were positive only in the spot test, 10 were positive only in the bacterial test and 6 were negative in both assays. The spot test was also considered as a predictor of carcinogenesis; 45 chemicals were carcinogenic of which 35 were detected as positive by the spot test and 3 out of 6 non-carcinogens were correctly identified as negative. If the results are regarded in sequence, i.e. that a positive result in a bacterial mutagenicity test reveals potential that may or may not be realized in vivo, then 48 chemicals were mutagenic in the bacterial mutation assay of which 38 were active in the spot test and 31 were confirmed as carcinogens in bioassays. 12 chemicals were non-mutagenic to bacteria of which 6 gave positive responses in the spot test and 5 were confirmed as carcinogens. These results provide strong evidence that the mouse coat spot test is an effective complementary test to the bacterial mutagenesis assay for the detection of genotoxic chemicals and as a confirmatory test for the identification of carcinogens. The main deficiency at present is the paucity of data from the testing of non-carcinogens. With further development and improvement of the test it is probable that the predictive performance of the assay in identifying carcinogens should improve, since many of the false negative responses may be due to inadequate testing.

Non-genotoxicity of 2,4,6-trinitrotoluene (TNT) to the mouse bone marrow and the rat liver: implications for its carcinogenicity

2,4,6-Trinitrotoluene (TNT) is structurally related to the rat liver carcinogen 2,4-dinitrotoluene (technical grade), and both compounds are known to be mutagenic to bacteria in vitro. TNT is therefore established as a potential rodent carcinogen; the present paper describes experiments designed to assess if this potential is likely to be expressed in appropriately exposed animals. TNT gave a negative response in the mouse bone marrow micronucleus assay and in an in vivo/in vitro rat liver assay for unscheduled DNA synthesis (UDS). In the latter assay animals are exposed to the test chemical in vivo and their hepatocytes subsequently evaluated for UDS in vitro. The negative response observed for TNT in the liver assay at dose-levels up to 1000 mg/kg was accompanied by a positive response for the hepatocarcinogen 2,4-dinitrotoluene at the lower dose-level of 200 mg/kg. In contrast, the dinitro compound gave a negative response in the micronucleus assay, as was also observed for TNT. It is concluded that the negative response observed for TNT in the liver assay indicates that it is unlikely to be a rat hepatocarcinogen. Nonetheless, high levels of methaemoglobin were observed in the TNT-treated rats and their urine was coloured red. These facts, together with the known toxicities of this agent suggest a possible carcinogenic hazard to the haemopoetic and urinary tissues of animals exposed chronically to it at toxic dose-levels.

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.

Quantitative comparative mutagenesis in bacteria, mammalian cells, and animal-mediated assays. A convenient way of estimating genotoxic activity in vivo?

The accumulation of environmental compounds which exhibit genotoxic properties in short-term assays and the increasing lag of time for obtaining confirmation or not in long-term animal mutagenicity and carcinogenicity tests, makes it necessary to develop alternative, rapid methodologies for estimating genotoxic activity in vivo. In the experimental approach used here, it was assumed that the genotoxic activity of foreign compounds in animals, and ultimately humans, is determined among others by exposure level, organ distribution of (DNA) dose, and genotoxic potency per unit of dose, and that knowledge about these 3 parameters may allow to rapidly determine the expected degree of genotoxicity in various organs of exposed animals. In view of the high degree of qualitative correlation between mutagenic activity of chemicals in bacteria and in cultured mammalian cells, and their mutagenic and carcinogenic properties in animals, and in order to be able to distinguish whether mutagenic potency differences were due to differences in (DNA) dose rather than other physiological factors, the results of mutagenicity tests obtained in the present experiments using bacteria and mammalian cells were compared on the basis of DNA dose rather than exposure concentrations, with the following questions in mind: Is there an absolute or a relative correlation between the mutagenic potencies of various ethylating agents in bacteria (E. coli K12) and in mammalian cells (V79 Chinese hamster) after treatment in standardized experiments, and can specific DNA adducts be made responsible for mutagenicity? Is the order of mutagenic potency of various ethylating agents observed in bacteria in vitro representative of the ranking of mutagenic potency found in vivo? Since the answer to this last question was negative, a further question addressed to was whether short-term in vivo assays could be developed for a rapid determination of the presence (and persistence) of genotoxic factors in various organs of mice treated with chemicals. In quantitative comparative mutagenesis experiments using E. coli K12 and Chinese hamster cells treated under standardized conditions in vitro with 5 ethylating agents, there was no indication of an absolute correlation between the number of induced mutants per unit of dose in the bacteria and the mammalian cells. The ranking of mutagenic potency was, however, identical in bacteria and mammalian cells, namely, ENNG greater than ENU greater than or equal to DES greater than DEN congruent to EMS, the mutagenic activity of DEN being dependent on the presence of mammalian liver preparations.(ABSTRACT TRUNCATED AT 400 WORDS)

Concomitant observations of UDS in the liver and micronuclei in the bone marrow of rats exposed to cyclophosphamide or 2-acetylaminofluorene

Oral dosing of between 5-30 mg/kg of cyclophosphamide (CP) to Alderley Park rats induced micronuclei in the bone marrow between 12 and 36 h after dosing, but failed to induce unscheduled DNA synthesis (UDS) in the liver at similar dose levels and treatment periods. Dose levels of greater than 30 mg/kg were toxic to the liver. In contrast, 2-acetylaminofluorene (2AAF) induced UDS in the rat liver between 4-36 h after dosing, but gave only a weak response in the bone marrow assay at dose levels between 0.5 and 2 g/kg. Selected observations were made for each chemical using both tissues of the same test animal. It is concluded that an assessment of the genotoxicity in vivo of chemicals defined as genotoxic in vitro will contribute to an assessment of their possible mammalian carcinogenicity, and that these should involve assays conducted using both the bone marrow and the liver of rodents. Due to its relative ease of commission, the bone marrow micronucleus assay will usually be conducted first; in the case of negative results it is recommended that a liver genotoxicity assay should be conducted. The case for employing in vivo short-term genotoxicity tests to predict the possible organotropic carcinogenicity or germ cell mutagenicity of a new in vitro genotoxin is discussed.

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.

Chloracetamide-N-metholol: an example of an in vitro and in vivo clastogen which is non-mutagenic to Salmonella

The industrial biocide chloracetamide-N-metholol (CAM) has been shown to be non-mutagenic to 6 strains of Salmonella using both the plate-incorporation and a pre-incubation test protocol. Its biocidal activity is unlikely to have influenced these results since Kathon 886, a more potent biocide, was concomitantly detected as mutagenic to strain TA100. In contrast, CAM was weakly clastogenic to human lymphocytes cultured in vitro and elicited a positive response in the mouse bone marrow micronucleus test when assayed using the intraperitoneal, but not the oral route of administration. A positive response was concomitantly observed for the rodent carcinogen and formaldehyde-releasing agent hexamethylphosphoramide (HMPA) in these 2 clastogenicity assays. Data are presented showing the slow hydrolysis of CAM to formaldehyde in vitro, and both [carbonyl-14C]CAM and [metholol-14C]CAM have been shown to interact covalently with calf-thymus DNA in vitro. It is concluded that CAM may be a direct-acting carcinogen to rodents, but that both the qualitative and quantitative outcome of its bioassay for carcinogenicity will be influenced critically by the bioassay protocol adopted; in particular, by the route of administration selected. These findings emphasize the need to complement the Salmonella gene-mutation assay with an in vitro assay for the induction of chromosomal aberrations if in vivo genotoxins are to be detected efficiently in vitro.