Conclusions.

3. Conclusions

University of Leicester, UK

As indicated in the Introduction, the aim of this Report is to examine the ability of currently used biomarkers to reflect the chemical nature, level and duration of an individual’s exposure to environmental carcinogens and the degree of disease risk associated with this exposure. The more analytical aspects of biomarker validation, such as assay reliability, inter-laboratory varability, sampling strategy etc, are dealt with in an earlier ECNIS review [1]. Both aspects of validation assessment are equally important, and clearly it is not appropriate to embark on a human biomonitoring exercise if analytical validation has not been successfully completed. This unfortunately is not universally the case for biomarkers that have already been employed in human studies. With regard to the consideration of the validity of biomarkers for human risk assessment, which is dealt with in this Report, the ideal situation is when human data are available for comparison of biomarker level with exposure and/or adverse health effects, such as is the case, for example, for blood lead [2]. Such information on environmental genotoxins and cancer incidence is rarely available, one exception being aflatoxin B1 where the relationship of urinary DNA adducts with dietary exposure and with hepatocellular carcinoma has been investigated [3]. Of interest from these stu-dies is that no relationship was found between dietary aflatoxin intake and hepatocellular cancer risk, but the adduct was predictive for cancer, illustrating not only the impor-tance of using such a biomarker for human risk assessment, but also the necessity of ensuring that such biomarker approaches are truly validated for human studies.

Alternatively, if no or incomplete human data are available, experimental animal studies may provide supportive data for validation of the biomarker, e.g. by providing information on dose–response, exposure–time relationships, tissue comparisons (of parti-cular value when a surrogate tissue is planned for use in human studies) and the effect of modulating factors on the biological effects that may be associated with the exposure to the genotoxin. Physiologically based pharmacokinetic modelling studies may add greatly to the interpretation of animal (or human) biomarker studies [2]. However, although extremely valuable, such animal studies cannot provide all of the necessary information for validation of the biomarkers in humans, for example they cannot inform on inter-individual variation in humans caused by genetic polymorphisms, or on back-ground (or endogenous) levels of the biomarker in humans, which will affect the use of the biomarker in risk assessment processes.

This survey of the states of validation of the biomarkers related to genotoxic compounds in food and other environmental sources portrays a less than satisfactory situation, in that few of the essential parameters have been examined for most

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Peter Farmer

of the biomarkers (for summary, see Annex 1). Thus, for studies of bulky DNA adducts there are only a few examples where human health effects have been studied so that risk estimates could be made, and there are large inter-individual variations in adduct levels in subjects with similar exposure. There is also evidence for a non-linear dose–response relationship in humans for PAH adducts, which further impacts upon risk assessment and needs more investigation. For protein adducts there is still uncertainty about the source and relevance of several background adducts that are found in control subjects, and further knowledge of the comparison of protein adducts with target cell DNA adducts in humans is necessary. Chromosomal aberrations and micronuclei deter-mination are methodologically well validated and accumulating the data from these has shown their relationship to cancer risk [4,5]. However, as it is not normally possible to relate such chromosomal damage to actual exposure levels of particular genotoxins in humans it seems advisable to extend our knowledge of the relationship between biomarkers of exposure (e.g. DNA or protein adducts) and chromosomal aberrations or micronuclei. Many examples of measurements of DNA base oxidation products as oxidative stress biomarkers have been published, but there remains considerable uncertainty, not only related to analytical matters concerning artifactual production of the DNA lesion, but also concerning their value as biomarkers of adverse health effects. Although case–control studies support the role of DNA base oxidation in causing biological effects, it is hard to carry out prospective studies in view of the requirement to preserve DNA samples without the occurrence of further oxidation or decomposition. It is also important to be sure whether the biomarker is a predictor of disease or simply an effect thereof. Urinary oxidative DNA damage products or DNA lesions caused by interaction with lipid peroxidation products may be good alternatives with less potential for artifactual formation of the biomarker. However, both of these biomarkers need extensive analytical validation before extensive human use.

Although this has been referred to in an earlier ECNIS review [1], the importance of the availability of specific and sensitive high throughput methods for the determi-nation of DNA or protein adducts in large numbers of human samples for the develop-ment of fully validated biomarkers cannot be overemphasised. For determinations of chromosomal aberrations or micronuclei, development of automated procedures are a priority to avoid time-consuming manual counting. Another prerequisite for the use of biomarkers in human populations is that background levels in control human subjects must first be determined, together with the sources of variation in these levels, as without this knowledge the size of the study required to gain a valid risk estimate cannot be calculated.

Although this survey of the state of validation of biomarkers of exposure and early effects reveals that much remains to be done, it will provide a valuable tool for planning collaborative projects within and beyond ECNIS. Hence, this Report will contribute to the overall aim of ECNIS to overcome the fragmentation of activities within Europe and to enhance the relevance of research on environment, nutrition, and cancer risk.

83 State of validation of biomarkers of carcinogen exposure and effect: Conclusions

References

1. Farmer PB, Emeny JM, editors. Biomarkers of carcinogen exposure and early effects. ¸ódê, Poland: ECNIS Publications, Nofer Institute of Occupational Medicine; 2006.

2. National Research Council (US). Human biomonitoring for environmental chemicals. Washington (DC): The National Academies Press; 2006.

3. Ross RK, Yuan J-M, Yu MC, Wogan GN, Qian G-S, Tu J-T, et al. Urinary aflatoxin biomarkers and risk of hepatocellular carcinoma. Lancet 1992;339:943–6.

4. Norppa H, Bonassi S, Hansteen I-L, Hagmar L, Strömberg U, Rössner P, et al. Chromosomal aberrations and SCEs as biomarkers of cancer risk. Mutat Res 2006;600:37–45.

5. Bonassi S, Znaor A, Ceppi C, Lando L, Chang WP. An increased micronucleus frequency in peripheral blood lymphocytes predicts the risk of cancer in humans. Carcinogenesis 2007;28:625–31.