Carcinogens and human health: Part 2

A Systemic Exposure-Based Alternative to the Maximum Tolerated Dose for Carcinogenicity Studies of Human Therapeutics

A systemic exposure-based alternative to the maximum tolerated dose (MTD) for high-dose selection in carcinogenicity studies for human therapeutics was accepted at the Second International Conference on Harmonization (ICH-2). The systemic exposure-based alternative to the MTD is suitable for nongenotoxic compounds with low rodent toxicity that are metabolized similarly in rodents and humans. This is the first product of an evaluation of current standards for rodent carcinogenicity studies of therapeutics. The relative systemic exposure is the ratio of the rat plasma area under the plasma concentration-time curve (AUC) at the MTD/human plasma AUC at the maximum recommended daily dose. An appropriate systemic exposure ratio for high-dose selection in carcinogenicity studies was empirically derived from the distribution of systemic exposure ratios attained by 35 compounds from 11 therapeutic categories in a Food and Drug Administration (FDA) database. Approximately one-third achieved a relative systemic exposure ratio <1 and two-thirds attained an exposure ratio of 10 or less, at the MTD. A systemic exposure ratio of at least 25 was accepted for high-dose selection in carcinogenicity studies at ICH-2. This ratio is high enough to detect all compounds with positive studies in the FDA database and would detect IARC 1 and 2A carcinogenic drugs. A ratio of 25 exceeds the systemic exposure ratio attained by 75% of drugs tested at the MTD in the FDA database and represents an adequate margin of safety which can be attained by a significant proportion of drugs.

Chemicals studied and evaluated in long-term carcinogenesis bioassays by both the Ramazzini Foundation and the National Toxicology Program: in tribute to Cesare Maltoni and David Rall

The Ramazzini Foundation (RF) in Bentivoglio, Italy and the National Toxicology Program (NTP) in Research Triangle Park, North Carolina have carried out several hundred chemical carcinogenesis bioassays: 200 by RF and 500 by NTP. Of these, 21 have been evaluated by both laboratories. The 14 chemicals for which both laboratories have designed, conducted, and reported bioassay results are: acrylonitrile, benzene, chlorine, diesel fuel, ethylbenzene, methylene chloride (dichloromethane), propylene, styrene, styrene oxide, toluene, trichloroethylene, trichlorofluoromethane, vinylidene chloride, and xylenes. The other seven chemicals (two are fibers) were evaluated by both laboratories, but results have not yet been published. Results of these 14 interlaboratory studies were compared both to explore consistency of carcinogenic responses and to identify possible factors that may reveal reasons for any differences observed. Individual carcinogenesis results from each laboratory were duplicated and complementary. Of the 14 chemicals compared, 11 (80%) were either carcinogenic (9 chemicals) or noncarcinogenic (2 chemicals) in both studies. Eight of the paired chemicals had at least one carcinogenic target site in common. The other three were carcinogenic in one laboratory but not in the other. Possible explanations for these differences include dose, method of administration, duration of follow-up, and whether or not total tumors are counted. The collaboration between these two pioneering bioassay laboratory programs contributes greatly to our understanding of chemical carcinogenesis and results in better protection of workers and the general population from chemical diseases, especially cancers.

Applied epidemiology and environmental health: emerging controversies

This review article assesses the state of the science in environmental epidemiology, not by summarizing current scientific findings but rather by examining conceptual controversies in the study of how environmental factors influence human health. This approach seems necessary because environmental epidemiology presently stands at a crossroads-in fact, at a number of overlapping crossroads. The field teems with epistemologic debates concerning appropriate paradigms for framing research questions, interpreting data, and applying research findings to policy. The present review focuses on emerging controversies related to three questions: What is considered "environmental"? What counts as credible research in environmental epidemiology? And what does "applied epidemiology" mean in the context of environmental health? The goal is to organize the presently fragmented critical literature on these issues and to promote productive dialogue by identifying central themes in current conceptual debates.

Comparative assessment of DNA adduct formation, Salmonella mutagenicity, and chromosome aberration assays as short-term tests for DNA damage

DNA adduct formation assay (DAFA) was carried out to compare dose responses with the Ames test and chromosomal aberration test using aflatoxin B1 (AFB1) and benzo[a]pyrene (BaP). In the bacterial mutation test, AFB1 and BaP (0-1 microgram/plate) were all positive in TA97a and TA100 with dose-related revertants. However, the slopes of the dose-response curves were gradual (slope 0.55-3.73, r = .84-.98). In the chromosome aberration test, a significant increase in the percentage of chromosomal aberrations was obtained from male ICR mouse spleen cells treated with AFB1 and BaP, but a dose-related increase was insensitive (slope 0.09-0.23, r = .75-.78). The incidence of chromosomally aberrant spleen cells treated with BaP was significantly increased compared with AFB1. DAFA was performed in vitro with [3H]-AFB1 and [3H]BaP. These two carcinogens were able to induce genotoxicity and showed good dose-related increases in terms of DNA adduct formation (slope 0.78-1.28, r = 1.00). Coefficients of variation (CV) for the slope of each dose-response curve were much lower in DAFA in vitro (CV 15.09- 18.34%) than those in any other test (CV 19.69-99.33%, Ames test; 18.89-44.58%, chromosome aberration test). Furthermore, DAFA in vivo was performed to investigate organotropic DNA adduct formation and persistence in Sprague-Dawley rats ip or orally treated with AFB1 and BaP. DNA adducts were monitored for 48-96 h by enzyme-linked immunosorbent assay (ELISA) using corresponding monoclonal antibodies, 6A10 and 8E11. DAFA in vivo demonstrated that the liver and kidney might be the probable target organs for AFB1 with the highest formation and persistence of DNA adducts and the lung and liver for BaP regardless of the route of administration. The results suggest that DAFA in vitro could be useful for detecting genotoxic compounds, and DAFA in vivo should also be considered as a good alternative method for the screening of organ-specific chemical carcinogens.

The effects of exposure to "synthetic" chemicals on human health: a review

This article examines how scientists use human, animal, and bacterial evidence to develop policy recommendations about the health consequences of human exposure to modern chemicals. Human evidence is limited because many epidemiological studies are contaminated with selection effects or unobserved heterogeneity. Changes in the aggregate incidence of morbidity (such as cancer) in the population over time are not a substitute for the lack of good individual-level data because incidence data are contaminated by the medicalization of cancer. Animal tests are also problematic because the expense of conducting experiments leads researchers to use only enough animals to allow detection of large differences in cancer incidence between controls and experimental animals that can only arise if the exposure doses are large. Predictions about the cancer incidence that would result in humans at much lower exposure levels, thus, require statistical inferences that implicitly make choices between false positive and false negative inference errors. Policy recommendations about carcinogens, therefore, are as much the product of value choices as "scientific" knowledge.

Historical perspective on risk assessment in the federal government

This article traces the evolution of risk assessment as an essential analytical tool in the federal government. In many programs and agencies, decisions cannot be made without the benefit of information from risk assessment. Although this analytical tool influences important public health and economic decisions, there is widespread dissatisfaction with the day-to-day practice of risk assessment. The article describes the sources of dissatisfaction that have been voiced by scientists, regulators, interest groups and ordinary citizens. Problems include the use of arbitrary exposure scenarios, the misuse of the 'carcinogen' label, the excessive reliance on animal cancer tests, the lack of formal uncertainty analysis the low priority assigned to noncancer endpoints, the poor communication of risk estimates and the neglect of inequities in the distribution of risk. Despite these limitations, the article argues that more danger rests in efforts to make decisions without any risk assessment. Recent Congressional and Administration interest in risk assessment is encouraging because it offers promise to learn from past mistakes and set in motion steps to enhance the risk assessment process.

Research to strengthen the scientific basis for health risk assessment: a survey of the context and rationale for mechanistically based methods and models

Assessment of health risks is an integral part of regulatory decision-making that occurs at the interface between science (e.g. facts) and policy (e.g. values). Because existing scientific knowledge and understanding are often inadequate to answer the most critical risk-related questions, regulatory agencies have developed sets of formalized 'science policies' to extrapolate from existing data to real-life events and situations. These science policies, as, for example, the use of default assumptions or exposure scenarios, can introduce significant uncertainties into the final risk estimate. We survey the rationale for research to reduce extrapolation-related uncertainties, focusing specifically on the need to develop mechanistically based methods and models, including test methods to identify and characterize health effects, integrated human exposure models, physiologically based pharmacokinetic (PBPK) models and biologically based dose-response (BBDR) models.

Induction of the human growth hormone gene placed under human hsp70 promoter control in mouse cells: a quantitative indicator of metal toxicity

An in vitro test method for general metal toxicity screening was designed, based on the cellular response to stress. The expression of a transfected human growth hormone gene sequence driven by the human heat-shock protein 70 promoter in NIH/3T3 cells was used as marker of noxious contact with metal compounds. Out of a series of 31 metals, 17 were competent for inducing this stress response system. According to the effective concentration and to the intensity of the response, three different clusters of positive compounds emerged and were ranked as strong, intermediate strength and weak inducers. These results correlated well with data from other in vivo and in vitro metal toxicity studies, including LD50 in mice. Apparently the positive/negative compounds also fitted well with data from genotoxicity and carcinogenesis studies on metal salts.

A new approach to evaluating carcinogenic risk

Carcinogenic risk assessments are based on extrapolating from high-dose chronic rodent-feeding studies to human-exposure levels. A serious problem is that about half of all substances tested at their respective maximum tolerated dose (MTD) are found to induce cancer. The MTD as currently defined has been criticized because it may stimulate cell proliferation in susceptible tissues. Such chemically induced mitogenesis is postulated to increase the probability that neoplasia will develop at the affected site. It is proposed that, in the development of an MTD for a given substance, chemically induced mitogenesis be considered an undesirable toxic manifestation. Hence, mitogenesis should not be induced by a substance fed at its true MTD. Since MTDs determined in this fashion are likely to be lower than those developed using current criteria, an added level of protection is introduced by employing a safety factor similar to that used now in determining acceptable daily intakes for noncarcinogenic food additives. In calculating acceptable daily intakes, the usual safety factor is 100; i.e., the acceptable daily intake is set at 1% of the no-observed-effect level. Hence it is proposed that the acceptable daily level of exposure to a substance that does not induce cancer at its MTD as defined herein be set at 1% of that MTD. On the other hand, a chemical that induces cancer at its MTD as defined herein would continue to be regulated as is customary now.

An analysis of the 1987 list of IARC-identified human carcinogens and the correlated animal studies

The thesis that the use of long-term, appropriately conducted and interpreted laboratory studies with rats and/or mice to test materials for potential carcinogenicity in humans is supported by the evidence on human carcinogens developed by Working Groups of the International Agency for Research on Cancer (IARC) is examined by a review of the Working Group reports on IARC-designated human carcinogens. The conclusion from that review is that the list of carcinogens does not constitute a sufficiently diverse group of agents to warrant a generalization; nor do the correlated animal studies justify the implied claim that most human carcinogens are demonstrated animal carcinogens, when the test for animal carcinogenicity is a properly conducted and interpreted study with rats and/or mice.