Problems of dose-response studies in chemical carcinogenesis with special reference to N-nitroso compounds

Defining thresholds in occupational and environmental toxicology

When a chemical causes a defined form of toxicity, the threshold is the maximum exposure when this toxicity does not occur. It is an operational parameter and is limited in its interpretation and applicability. The aim of this paper is to consider biological parameters which influence exposure-response relationships. Biomonitoring of dose and effects has much potential for defining thresholds in human exposure; extension of their use in experimental studies on new compounds should help predictions to thresholds for human exposure. Intoxication initiated by both reversible and covalent interactions with targets are discussed and, as exposure is reduced and the time of exposure extended, changes in the shape of the dose-response curves examined for acute and delayed neuropathy (axonopathy) and for carcinogenesis.

Expression of exposure in negative carcinogenicity studies: dose/body weight, dose/body surface area, or plasma concentrations?

Evaluation of positive findings in a rodent carcinogenicity study and the subsequent extrapolation to humans is based on chemical structure, mutagenicity, pharmacology, hormone changes, chronic toxicity, and the nature of the tumors induced. For negative studies, adequacy of exposure may become an issue. The use of plasma concentrations as a metric for exposure assumes that each species responds in a similar manner to a given concentration; data are now available that demonstrate that this is not generally true for carcinogenicity. The use of the body surface area metric (i.e., mg/m2) is a special case of interspecies allometric scaling (i.e., W0.67). For a chemical to be amenable to such scaling in toxicology, it must satisfy 3 criteria: (a) the concentration-time profile of the putative toxicant at the site of action must be governed by a scalable pharmacokinetic process (e.g., glomerular filtration); (b) the mechanism of action and the susceptibility of each species to a given systemic exposure must be the same and, for example, be independent of lifespan, cellular repair mechanism/rate, and so forth; and (c) the biological response must depend only on size (e.g., not on race, strain, gender, age, or parity). Carcinogens rarely, if ever, meet these criteria. An empirical analysis of carcinogenic potency data in rodents and in humans shows that, in general, exposure is best expressed in terms of mg/kg body weight.

Carcinogen daily dose-dependence of the biological features and development rate of hepatocellular carcinomas induced by 3'-methyl-4-dimethylaminoazobenzene in the rat

Differences in biological features and the rate development of hepatocellular carcinomas (HCCs) induced with various daily doses of 3'-methyl-4-dimethylaminoazobenzene were investigated. Male Donryu rats at 21 days old were fed the carcinogen at concentrations ranging from 50 to 600 ppm in the diet continuously, or 600 ppm for 3 weeks followed by a dietary promoting regimen of 500 ppm phenobarbital. Large (> or = 10 mm) HCCs were monitored and the histological features and alpha-fetoprotein (AFP) production analysed. High doses of the carcinogen predominantly induced HCCs of high grade malignancy with AFP production in a short latent period whereas lower doses and the initiation-promotion protocol were primarily associated with low grade HCCs lacking AFP production and developing after long latent periods. Thus the experimental results clearly document that biological features of neoplasms, viewed as a spectrum, may markedly differ according to the daily dose of the carcinogen applied.

Carcinogens in Israeli milk: a study in regulatory failure

The potential danger to humans of exposure to chemicals shown to be carcinogenic in animals has become increasingly clear in the last 20 years. A gap still exists, however, between the appreciation of the risk by scientists and the willingness of public health authorities to reduce it. Three pesticides, shown repeatedly to produce over a dozen different types of cancer in rats and mice, were discovered in inordinately high concentrations in Israeli milk and dairy products. The three pesticides--alpha-BHC, gamma-BHC (lindane), and DDT--had been shown to be present for ten years or more at mean concentrations up to 100 times those found in U.S. dairy products--with resultant concentrations in breast milk being possibly 800 times greater than those in the United States--yet neither the Ministry of Health nor the Israel Cancer Association made any apparent moves either to warn the public or to rectify the situation. A small consumer organization, Consumer Shield, brought the issue into the open. Through public pressure, court action, and the threat of further legal redress--and despite repeated attacks in the media by the milk producers, the Ministry, and the Cancer Association--Consumer Shield forced the authorities to outlaw the use of alpha-BHC and lindane (DDT no longer being in general use). The ban resulted in a precipitous drop in the concentrations of these substances in Israeli milk. Recent epidemiological and laboratory findings suggest that the dramatic drop in breast cancer mortality rates subsequent to the pesticide ban could be a direct result of that ban.

What is an appropriate measure of exposure when testing drugs for carcinogenicity in rodents?

This discussion paper argues that in the carcinogenicity testing of drugs, biological measures of drug exposure may often be more relevant than the classical pharmacokinetic approaches applicable to reversible pharmacodynamic phenomena. Chemicals that produce tumors in rodents may do so (either directly or after bioactivation) by mechanisms involving inter alia a mutagenic, cytotoxic, or hormone-like effect. Such mechanisms may involve the formation of reactive metabolites of fleeting existence, and these are subject to the principles of irreversible pharmacokinetics. Examples are given of genotoxic and nongenotoxic substances for which the species and target site for tumor formation correlates not with plasma concentration, but with the amount metabolized and/or the rate of metabolism. Other compounds produce tumors in rodents, often in only one sex or species, in association with an exaggerated pharmacodynamic effect, with an increase in liver weight (by one of a diversity of mechanisms) or with hormonal or hormonal-like effects. In such cases the determinant of tumor formation is the degree of disturbance of homeostasis, not the plasma concentration of the parent substance. For drugs, the disturbance in question may have no relevance to the clinical use of the drug. Plasma concentrations of the parent substance are useful in assessing the proportion of an oral dose that is absorbed and the linearity of kinetics over the full dose range and for exploring the differences between dietary and gavage administration. They do not usually, however, provide information of direct relevance to assessment of "exposure" for the purposes of carcinogenic risk assessment.

Nitrates, nitrites and N-nitrosocompounds: a review of the occurrence in food and diet and the toxicological implications

Data on occurrence of nitrate, nitrite and N-nitrosocompounds in food and drinking water, and on total dietary intakes are reviewed. Metabolic, toxicological and epidemiological studies are surveyed and the implications with respect to safety evaluation are addressed. It is concluded that, on the basis of recent long-term animal studies and of clinical experience in man, the current Acceptable Daily Intake (ADI) allocated to nitrate by the Joint FAO/WHO Expert Committee on Food Additives of 0-5 mg/kg body weight/day (expressed as sodium nitrate) might be increased to 0-25 mg/kg body weight/day. Based on similar criteria, the ADI for nitrite would be 0-0.1 mg/kg body weight/day (expressed as sodium nitrite). In view of the known carcinogenicity of N-nitrosocompounds, exposure to these compounds in food should be minimized by appropriate technological means, such as lowering the nitrite concentration in preserved foods to the minimum required to ensure microbiological safety and use of inhibitors of nitrosation like alpha-tocopherol or ascorbic acid. Further work is needed to define the minimal levels of nitrite in foods needed to inhibit outgrowth of Clostridium botulinum and toxin production.

Assessment of the risk of formation of carcinogenic N-nitroso compounds from dietary precursors in the stomach

A literature review has shown that the daily intakes of various N-nitroso-precursor classes in a typical European diet span five orders of magnitude. Amides in the form of protein, and guanidines in the form of creatine and creatinine, are the nitrosatable groups found most abundantly in the diet, approaching levels of 100 g/day and 1 g/day, respectively. Approximately 100 mg of primary amines and amino acids are consumed daily, whereas aryl amines, secondary amines and ureas appear to lie in the 1-10 mg range. The ease of nitrosation of each precursor was estimated, the reactivities being found to span seven orders of magnitude, with ureas at the top and amines at the bottom of the scale. From this information and an assessment of the carcinogenicity of the resulting N-nitroso derivatives, the potential health risk due to gastric in vivo nitrosation was calculated. The combined effects of these risk variables were analysed using a simple mathematical model: Risk = [daily intake of precursor] X [gastric concentration of nitrite]n X [nitrosatability rate constant] X [carcinogenicity of derivative]. The risk estimates for the various dietary components spanned nine orders of magnitude. Dietary ureas and aromatic amines combined with a high nitrite burden could pose as great a risk as the intake of preformed dimethylnitrosamine in the diet. In contrast, the risk posed by the in vivo nitrosation of primary and secondary amines is probably negligibly small. The risk contribution by amides (including protein), guanidines and primary amino acids is intermediate between these two extremes. Thus three priorities for future work are a comprehensive study of the sources and levels of arylamines and ureas in the diet, determination of the carcinogenic potencies of key nitrosated products to replace the necessarily vague categories used so far, and the development of short-term in situ tests for studying the alkylating power of genotoxicity of N-nitroso compounds too unstable for inclusion in long-term studies.

Carcinogenic N-nitroso compounds and their environmental significance

A short review is given on the potent and organ-specific carcinogenic effects of N-nitroso compounds. Like many other chemical carcinogens, nitrosamino compounds require activation in vivo via enzymatic alpha-hydroxylation to form alkylating agents as ultimate carcinogens. Since no epidemiological data in man are available, extrapolation of animal data to man are important as well as dose-response studies in risk evaluations. Important aspects of these problems are presented. Finally the environmental impact of N-nitroso compounds is summarized.

Effect of the intermittent administration of N-nitrosopyrrolidine (NPYR) on the tumor incidence in Sprague-Dawley rats

N-Nitrosopyrrolidine (NPYR) was administered orally to 320 Sprague-Dawley rats. An additional group of 80 rats served as an untreated control. There were four experimental groups in which the effects of different periods of dosing and varied intervals without treatment were compared. The total dose for all the groups was constant and was 792 mg/kg body weight. The schedules of carcinogen application had a significant effect on the observed cancer risk. The different tumor indicences in the individual groups could not be attributed to differences in the survival pattern, but were related to the changes in the different treatment schedules.

Limitations of in vitro short-term tests as prescreening models for carcinogenicity in industry: a theoretical approach

The validity of in vitro short-term tests was investigated using published data from 10 investigations on a total of more than 800 compounds. The predictive value strongly depends on the number of carcinogens investigated in a test and, therefore, is not suited to describe the reliability of a short-term test. The usefulness of short-term tests as prescreening models for possible carcinogenicity to be used as a routine procedure in industry is very limited. If it is assumed that most of the compounds tested are non-carcinogenic, other criteria than results of short-term tests should be used additionally in deciding which compounds require investigation in long-term bioassays for possible tumorigenic hazards.