A unifying concept for carcinogenic risk assessments

Toxicological potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin relative to 100 other compounds: a relative potency analysis of in vitro and in vivo test data

A common definition of relative potency is the dose of a reference compound required to cause a particular incidence of a specific toxic response divided by the dose of a test compound needed to cause an equal incidence of that same effect. In this simple manner, toxicological assessments for a chemical of concern can be made in terms of another compound about which much is known from a human health perspective. Relative potency factors were used to compare 2,3,7,8-tetrachlorodibenzo-p-dioxin CAS # 1746-01-6 (TCDD) with 100 other compounds both individually and collectively. All results were standardized to a common scale that spanned many orders of magnitude and was indexed to an arbitrary potency of unity for benzo(a)pyrene [B(a)P]. From comparisons between 2,771 pairs of bioassay results (i.e., matched experimental design conditions) for TCDD compared with the 100 other compounds, it was found that TCDD is about 600 times as toxic as B(a)P (interquartile range of 130 to 1,900). The distribution of relative potency values is fitted accurately with a log-normal distribution function having an untransformed mean of 550 and an untransformed slope (i.e., the inverse of the standard deviation of the distribution) of 140. These factors combined with (a) a reference lifetime carcinogenic risk level of 1/100,000 and (b) a universal, potency-dependent risk coefficient (estimated from the collection of epidemiologically-based carcinogens) yielded estimates that equally toxic concentrations for TCDD should be in the range of 13 pg/m3 and 7 pg/L in air and water, respectively.

Effects of caloric restriction in animals on cellular function, oncogene expression, and DNA methylation in vitro

While the life-extending and disease-modulating effects of caloric restriction (CR) are well documented in whole animal studies and in correlative experiments using cells taken from CR animals, very few studies have used cells in culture after their removal from the CR-fed animal. In using this in vivo-->in vitro approach we have attempted to examine the proposition that the effects of CR can be transferred to individual cells by analyzing the cellular functions of proliferation and transformation, the activation of oncogenes, and the methylation of DNA as a function only of diet. Pancreatic acinar cells excised from CR-fed Brown-Norway rats and placed in rich medium showed different responses compared to cells from ad libitum (AL)-fed controls. CR had the effect of slowing growth rate and protecting against spontaneous and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced transformation over 14 passages of cells in culture. At the molecular level, cells from the CR animals showed reduced c-Ha-ras oncogene expression and mutation as well as reduced mutation of the p53 suppressor gene. CR also increased genomic methylation of ras DNA. We conclude that the effects of CR treatment of the animal are transferred to individual cells and note that these responses (decreased proliferation and transformation; depressed oncogene expression and mutation and decreased suppressor gene mutation; and increased oncogene methylation) are cellular and molecular analogs of in vivo weight loss, life extension, and carcinogenesis modulation, which are hallmarks of CR in the whole animal. The fact that these responses are seen generations after the cells are removed from the CR-treated animal indicates that CR causes a permanent predisposition of pancreatic acinar cells to these modulated responses and shows the value of the in vivo-->in vitro protocol in studies that relate diet to cellular and molecular function.

Oxidants and mitogenesis as causes of mutation and cancer: the influence of diet

A very high level oxidative damage to DNA occurs during normal metabolism. In each rat cell, the steady-state level of this damage is estimated to be about 10(6) oxidative adducts, and about 10(5) new adducts are formed daily. This endogenous DNA damage appears to be a major contributor to cancer and aging. The oxidative damage rate in mammalian species with a high metabolic rate, short life span, and high age-specific cancer rate such as in rats is much higher than the rate in humans, long-lived mammals with a lower metabolic rate, and a lower age-specific cancer rate. It is argued that deficiency of micronutrients, that protect against oxidative DNA damage, is a major contributor to human cancer. Epidemiological studies, a large body of experimental evidence, and theoretical work on the mechanisms of carcinogenesis point to mitogenesis as a major contributor to cancer. Dividing cells, compared to nondividing cells, are at an increased risk for mutations due to: 1.) conversion of DNA adducts to mutations; 2.) chance of mitotic recombination, gene conversion, and nondisjunction; and, 3.) increased exposure of DNA to mutagens. Mitogenesis also increases the probability of gene amplification and loss of 5-methylcytosine. Dietary interventions that lower mitogenesis, such as calorie restriction, decrease cancer incidence.

Oxidants are a major contributor to aging

Very high level oxidative damage to DNA occurs during normal metabolism. In each rat cell the steady-state level of this damage is estimated to be about 10(6) oxidative adducts, and about 10(5) new adducts are formed daily. This endogenous DNA damage appears to be a major contributor to aging and to the degenerative diseases associated with aging such as cancer. The oxidative damage rate in mammalian species with a high metabolic rate, short life span, and high age-specific cancer rate such as rats is much higher than the rate in humans, long-lived mammals with a lower metabolic rate and a lower age-specific cancer rate. It is argued that deficiency of micronutrients that protect against oxidative DNA damage is a major contributor to human cancer. Epidemiological studies, a large body of experimental evidence, and theoretical work on the mechanisms of carcinogenesis point to mitogenesis as a major contributor to cancer. Dividing cells compared to nondividing cells are at an enormously increased risk for mutations in part due to the conversion of DNA adducts to mutations. Mitogenesis also increases the probability of gene amplification and loss of 5-methylcytosine. Dietary interventions that lower mitogenesis, such as calorie restriction, decrease the incidence of cancer.

Endogenous mutagens and the causes of aging and cancer

A very large oxidative damage rate to DNA occurs as part of normal metabolism. In each rat cell the steady-state level is estimated to be about 10(6) oxidative adducts and about 10(5) new adducts are formed daily. It is argued that this endogenous DNA damage is a major contributor to aging and the degenerative diseases of aging, such as cancer. The oxidative damage rate in mammalian species with a high metabolic rate, short life span, and high age-specific cancer rate is much higher than the rate in humans, a long-lived creature with a lower metabolic rate and a lower age-specific cancer rate. It is argued that deficiency of micronutrients, such as dietary antioxidants or folate, is a major contributor to human cancer and degenerative diseases. Understanding the role of mitogenesis in mutagenesis is critical for clarifying the mechanisms of carcinogenesis and interpreting high-dose animal cancer tests. High-dose animal cancer tests have been done mainly on synthetic industrial chemicals, yet almost all of the chemicals humans are exposed to are natural. About half of natural chemicals tested in high-dose animal cancer tests are rodent carcinogens, a finding that is consistent with the view that high-dose tests frequently increase mitogenesis rates. Animals have numerous defenses against toxins that make them very well buffered against low doses of almost all toxins, whether synthetic or natural.

Formaldehyde in drinking water: comparative hazard evaluation and an approach to regulation

Formaldehyde, a widely used industrial chemical to which humans are ubiquitously exposed, presented cause for concern when it was demonstrated to be carcinogenic in laboratory animals. Risk assessment protocols subsequently applied to formaldehyde are of questionable validity in light of the results of recent mechanistic investigations of biological responses to formaldehyde. Further, the hazard of ingested formaldehyde is not addressed in current assessment protocols. This paper addresses the potential human health risks accompanying low-level exposure to formaldehyde as a contaminant in drinking water. In this exposure scenario, noncarcinogenic risk from inhalation of formaldehyde from drinking water is evaluated through knowledge of the metabolism and biological effects of formaldehyde exposure. Noncarcinogenic risk from ingestion of formaldehyde in drinking water is evaluated from the perspective gained by comparison with dietary sources of formaldehyde. Carcinogenic risk to humans is evaluated in light of recent investigations into the mechanisms underlying biological responses to formaldehyde exposure. Finally, based on a comparison of ingestion of formaldehyde in drinking water with ingestion of naturally occurring formaldehyde in foods, a comparative hazard approach to formaldehyde regulation is offered as a supplement to the rigid evaluation protocols currently used.

Hazard evaluation for complex mixtures: relative comparisons to improve regulatory consistency

The traditional "absolute decision-making" process used by federal regulatory agencies to derive permissible exposure concentrations for hazardous substances is initiated by an evaluation of the "weight-of-evidence" that a substance is a potential human carcinogen. Subsequent conservative procedures applied variably to noncarcinogens and carcinogens yield exposure limits for individual substances based on "data-sparse, model-intensive" techniques which may lack consistency and have difficulty directly addressing the hazards from complex mixtures. This paper describes how a "relative decision-making" technique applicable to complex mixtures can supplement the "absolute" approach currently used. Estimates obtained through this "data-intensive, model-sparse" technique may be evaluated by comparisons to estimates representing a range of hazards "generally regarded as safe" derived through analyses of chlorinated drinking water, cigarette smoke condensate, and other common human exposures. Comparisons are also used to evaluate the relative degree of consistency in risk estimates between 58 suspect human carcinogens analyzed by the U.S. Environmental Protection Agency Carcinogen Assessment Group and by the authors.

Sulfur mustard as a carcinogen: application of relative potency analysis to the chemical warfare agents H, HD, and HT

A relative potency method for assessing potential human health effects from exposures to relatively untested chemicals is presented and documented. The need for such a method in evaluating the carcinogenic potential of the chemical warfare agent sulfur mustard (agent HD) from a limited data base is specifically addressed. The best-estimate potency factor for sulfur mustard relative to benzo[a]pyrene is 1.3, with an interquartile range of 0.6 to 2.9. The method is applied to (1) the estimated fence-boundary air concentrations of mustard during operation of a proposed agent incinerator at Aberdeen Proving Ground (APG), Maryland, and (2) the current approved general population exposure level of 1 X 10(-4) mg HD/m3 and the occupational exposure level of 3 X 10(-3) mg HD/m3. Maximum estimates of excess lifetime cancer risk for individuals at sites along the APG boundary range between 3 X 10(-8) and 1 X 10(-7). Lifetime cancer risk estimates less than or equal to 10(-6) are not now regulated by the U.S. Environmental Protection Agency or the Food and Drug Administration. Maximum estimates of excess lifetime cancer risk assuming daily exposure to the approved standards during the proposed 5 years of incinerator operation are on the order of 10(-5) for the general public and 10(-4) for the worker population. These values are considered upper limit estimates.

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.

Permissible concentrations of chemicals in air and water derived from RTECS entries: a "rash" chemical scoring system

Many chemicals are of concern to human health, but only a few have epidemiologically derived risk estimates. About 45,000 chemicals are listed in RTECS, most of which have had some testing in subhuman models. RTECS entries range from cellular effects through organoleptic damage to lethality, with many pathological endpoints listed, including mutagenic changes, irritation, teratogenesis, cancer, mortality, etc. However, it is difficult to extend any biological test results to human risk assessments. If the results are extended, the degree of validity is highly uncertain. This paper describes a logical basis for using the entire complex spectrum of test results to evaluate the overall toxicological potency of a chemical to be assayed (i.e., an interviewing chemical) and describes how to derive tentative, permissible concentrations in air and water for any particular chemical for which no regulatory guidance exists. This approach has been tested for 16 reference chemicals discussed in NIOSH Criteria Documents, EPA-CAG reports, etc. The evaluations are uncomplicated, but occasionally it is difficult to match RTECS entries for two different chemicals. Difficult comparisons may require some familiarity with experimental design and the toxicological literature. One important product of this novel approach is that a distribution or array of potency values is obtained for any chemical evaluated. This distribution reflects many uncertainties stemming from low statistical power, experimental design, pharmacological processes, interspecies variability, dose rate, biological effect monitored, route of treatment, etc. The array of relative values for a particular chemical reflects many different biological and physical conditions. The distribution of the array helps to index a composite toxicological profile for many different biological effects resulting from numerous treatment protocols. To minimize the effect of extreme sensitivity of certain (perhaps novel) biological test models, possible errors in the RTECS data-base, and possible human pharmacological insensitivity to a particular chemical and/or a particular route of administration, we consider the interquartile range (i.e., the central 50%) of the array of relative potency values between two chemicals being compared as a practical measure of uncertainty. Thus, the range in response derived from variability in relative potency should be useful in addressing the range of response in man as estimated from extrapolations of test data.

Animal studies and prediction of human tumors can be aided by graphical sorting of animal data: neoplastic risk from B(a)P, benzene, benzidine, and chromium

This work is a graphical study of all known dose-response data for neoplasia induced by B(a)P, benzene, benzidine, and chromium administered to test animals. Doses are put in units of lifetime intake given in micromoles of chemical per kilogram body weight, and responses are in percent increased effect per unit dose. Space limitations do not permit experiment-by-experiment critiques; however, computer graphics have been used to compare the relationship of any individual dose-response point estimate to other such point estimates for the chemical of interest. Graphics are also used to study variability resulting from different experimental parameters such as species, route of intake, number of treatments, pathological classification of neoplasia, etc. Graphical sorting, according to various physical and biological classification parameters, permits one to judge, from visual inspection, such questions as whether mice as a species are more sensitive than rats as a species, whether intravenous injection is generally more effective than inhalation, whether a single well-defined dose-response function, which ignores these classification parameters, can be evaluated numerically from the composite data base deriving from all oncogenic studies with a given chemical, etc.

Role of tumor promotion in affecting the multi-hit nature of carcinogenesis

The assessment of risk from low-level exposure to radiation and chemicals is hindered by the basic lack of scientific understanding of the complex nature of the multiple levels of protective or synergistic interactions. Radiation and chemicals have the potential of inducing mutations, cell death and altered gene expression. The biological consequences of each of these effects is again complex, since many factors can enhance or mask the effect of a single mutation, cytotoxic or gene modulatory change. Carcinogenesis, representing but one chronic disease state, is a multi-step process, involving the clonal expansion of a single altered cell (that is, the initiation event). Radiation and chemicals appear to contribute to the initiation process by their ability to induce viable mutations. Insufficient theoretical and empirical knowledge precludes a determination of whether mutagenesis and, hence, initiation exhibit a threshold phenomenon. Because of a variety of redundancy mechanisms on the genetic and cellular levels, the physiological impact of a single dysfunctional cell is felt only after it is amplified to a large number (the promotion phase of carcinogenesis). Radiation and chemical-induced cytotoxicity, as well as noncytotoxic chemical induction of mitogenesis, can induce surviving single dysfunctional stem cells to multiply. If during this multiplication of dysfunctional cells, the initiated cells are further exposed chronically to low levels of mutagens, there is an enhanced probability of additional genetic changes. The accumulation of foci of dysfunctional cells can occur in any stem cell population of any tissue. As with mutagens, the existence of threshold levels for promoting conditions and chemicals is still not yet scientifically validated. However, specific examples of the actions of a few promoters does seem to be consistent with that idea. The concepts of initiation and promotion imply the existence of anti-initiation and antipromotion conditions. Together with genetic factors, the complex and unpredictable interactions of radiation and chemicals as initiators, anti-initiators, promoters and antipromoters, defy prospects of an easy means to predict the consequences of exposure to chronic low levels of radiation and chemicals.