The carcinogenic potential of ethylene

Information Section--Fd Chem. Toxic. Vol. 32, No. 2 Having established the identity of the sensitizing agent, many dermatologists seem to consider their job done. And yet, with just a little extra effort in the preparation of the report the whole scientific picture could be much clearer. [James Hopkins--BIBRA] References Alomar A. et al. (1985) Contact Dermatitis 12, 129. Andersen K. E. & Hamann K. (1984) Food and Chemical Toxicology 22, 655. Botham P. A. et al. (1991) Contact Dermatitis 25, 172.

THE CARCINOGENIC

Brown R. (1979) Contact Dermatitis 5, 340. Cronin E. (1980) Contact Dermatitis. Churchill Livingstone, Edinburgh. pp. 851 & 853. Damstra R. J. et al. (1992) Contact Dermatitis 27, 105. Dias M. et al. (1992) Contact Dermatitis 27, 205. Freeman S. (1984) Contact Dermatitis !1, 146. Greig D. E. (1991) Contact Dermatitis 25, 201. Pedersen N. B. (1976) Contact Dermatitis 2, 340. Roberts D. L. et al. (1981) Contact Dermatitis 7, 145. Sanz-Gallen P. et al. (1992) Contact Dermatitis 27, 271. Slovak A. J. M. (1980) Contact Dermatitis 6, 187. Stejskal V. D. M. et al. (1990) Journal o f Investigative Dermatology 94, 798.

POTENTIAL

Introduction

Is ethylene a potential human carcinogen? In the occupational setting, the present guidance from the HSE---cthylene is listed and controlled as a simple asphyxiant without a specific occupational exposure limit (HSE, 1993)--would suggest it is not. A more sophisticated question is whether metabolism of inhaled ethylene poses a carcinogenic risk. The question is a valid one if humans metabolize ethylene to ethylene oxide, a known genotoxic animal carcinogen for which the HSE has a set maximum exposure (8-hr time-weighted average) level of 5 ppm or 10mg/m '~ (HSE, 1993). After more than a decade of research interest in the metabolism of ethylene, how much better placed are we to answer these questions? Metabolism in animals

As a first step, let us review the metabolic link between ethylene and its oxide in laboratory animals. A series of studies (carried out at concentrations of up to I l p p m ethylene) conducted at Stockholm University indicated that a range of laboratory species probably convert in the order of 5-10% of inhaled ethylene into ethylene oxide. The studies involved measuring the degree of alkylation, or more specifically the ethylation, of the blood haemoglobin in mice exposed to ethylene (Ehrenberg et al., 1977; Segerb/ich, 1983) or in hamsters and rats exposed to a series of vehicle exhausts of which ethylene was present as one component (T6rnqvist et al., 1988). Ethylation of haemoglobin is not in itself a reaction of any direct genotoxic significance, but it provides evidence of the presence and activity of a potentially genotoxic moiety. Indeed, in the mouse DNA ethylation was detected in the liver, spleen and testes (Segerbfich, 1983). The assumption that the metabolite ethylene oxide was responsible for the alkylation is supported by the observations that in mice (i) similar adducts (2-hydroxyethyl derivatives of proteins and DNA) were seen after exposure to ethylene and ethylene oxide, (ii) the relative proportions of the types of alkylation product that occurred in haemoglobin

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OF ETHYLENE

following ethylene exposure were the same as those that occurred following exposure to the more potent ethylene oxide, and (iii) the ratio between haemoglobin alkylation and DNA alkylation in the liver/spleen/testes was approximately the same after exposure to either compound. This assumption is given further credence by recent US studies where ethylene oxide was detected in the blood (Maples and Dahl, 1993). and earlier German work where it was detected in the expired air (Filser and Bolt, 1983) of rats almost immediately on exposure to ethylene. Metabolism in humans

Humans and rodents may metabolize ethylene in a qualitatively similar manner, for on the evidence of another Stockholm University study (T6rnqvist et al., 1989) we too convert it to the genotoxic and carcinogenic oxide. The study is only a fairly limited one and involved the examination of blood haemoglobin ethylation in a small group of workers exposed to ethylene used in a fruit store to control the ripening of bananas. A low level of these same adducts was detected in the "'non-exposed" controls--a similar low baseline of ethylation is found in unexposed laboratory animals (perhaps due to endogenous ethylene production). Nevertheless, the investigators, with the aid of a number of pharmacokinetic assumptions, felt able to conclude that the additional adduct burden found in the storemen was "compatible with a metabolic conversion of 3% (1-10%) of the inhaled ethene to ethylene oxide". Metabolic production o f ethylene oxide w a carcinogenic threat?

As well as a demonstrable ability to aklylate DNA, the oxide has also shown its in vivo genotoxic potential in a number of other ways. It induced, in rodents, for example, chromosomal damage, dominant lethal mutations, and heritable translocations (IARC, 1987). In addition, daily atmospheric exposures in the order of 30-100 ppm for 2 yr produced mononuclear cell leukaemias and brain turnouts in rats (Garman et al., 1986; Lynch et al., 1984), while a variety of

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malignant tumours was induced in mice inhaling ethylene oxide in a long-term bioassay (NTP, 1987). Although the human cancer and genotoxicity data are by no means conclusive (IARC, 1987), they do not contradict the view that ethylene oxide is a conventional alkylating carcinogen. In the regulatory world, even in the less excitable UK circles, a threshold approach to cancer risk evaluation is not usually considered prudent for a compound that induces cancer by a predominantly genotoxic mechanism (Doll, 1991); as the dose reduces it is assumed that there is a corresponding reduction in risk, but that a cancer risk of some degree would remain even in the lower-dose region. In theory then there is undoubtedly a problem, even in a circumstance associated with low systemic burdens of ethylene oxide. What. in practice, is the current carcinogenic status of ethylene (as opposed to ethylene oxide) in humans and laboratory animals? Ethylene and human cancer

The first thing to say about ethylene's carcinogenicity in humans is that it is essentially unexamined, a surprising statement for such a huge tonnage chemical. Certainly no high quality epidcmiological work seems to have been published. There are a number of limitcd reports. One of these, a study of workers at a Texan chemical plant, found a higher than expected number of dcaths from brain cancer in those who had been exposed to [unspecified levels of] ethylene. Nevertheless, similarly increased brain cancer risks were found for a number of the plant chemicals, confidence intervals were wide, and the NIOSH investigators emphasized that the report was a preliminary one and that the various chemical associations were not convincing evidence of any causal links (Leffingwell et al., 1983). Ethylene and animal carcinogenicity

Ethylene's carcinogenic potential has at least been studied according, in the main, to modern protocol requirements in one species of laboratory a n i m a l - a rat study was conducted at Industrial Bio-Test Laboratories in the late 1970s under the auspices of toxicologists at CIlT. Animals were exposed to 0, 300, 1000 or 3000 ppm ethylene in air, 6 hr/day, 5 days/wk for 2 yr. The pathology section of the published report (Ham et al., 1984) notes that lesions occurred "with approximately equal frequency in control and treated groups". Animals were not treated at the maximum tolerated dose because of concerns over the explosive potential of higher concentrations. Risk assessment

The lack of any demonstrable carcinogenic activity in the pathology report of Hamm et al. (1984) has led some reviewers to express the opinion that the metabolism of ethylene to its oxide is unlikely to represent

"a significant contribution to the risk/hazard of the parent compound" (Gibson et al., 1987). However, this view may incorrectly undermine the significance of the metabolic insights and, not surprisingly, not all subscribe to this marginalization. It should not be forgotten that in statistical terms a long-term animal study, for all its huge expense, is a rather insensitive test system for detecting weak genotoxic carcinogenic potential even when carried out to modern-day standards. A commentary from German scientists illustrated some of these issues using ethylene as a case study (Bolt and Filser, 1987). Based on the pharmacokinetics of ethylene and its oxide in the rat, they estimated that exposure at an atmospheric concentration of 1000 ppm ethylene would correspond to a theoretical atmospheric exposure of 5.6 ppm ethylene oxide. Because of saturation kinetics, exposure concentrations of ethylene above 1000ppm would not result in further increases in systemic ethylene oxide concentrations. Thus, an ethylene bioassay could not expose rats to more than the equivalent of approximately 5.6 ppm ethylene oxide. By extrapolating the tumour/exposure data in the ethylene oxide rat studies to 5.6 ppm, the investigators concluded that high ethylene exposures would not result in a tumour incidence of more than 2% above the background incidence. This led to the conclusion that, should ethylene pose a carcinogenic threat to the rat by virtue of its conversion to the oxide, the group sizes normally used in even a modern high quality cancer study would be insufficient to produce statistically significant increases in tumour yield at attainable ethylene concentrations. With this line of scientific thought in mind, it is of interest to look again at the fine detail of the CIIT findings on ethylene as given in the full research report (CIlT, 1980). Microscopic examination of a comprehensive range of tissues from the controls and the group exposed to the highest ethylene concentration revealed a slight increase in the incidence of mononuclear cell leukaemia in the treated animals (32 affected compared with 20 of the controls; groups of 90 animals/sex in treated and control groups). Unfortunately, tissues from the lower-dose groups were not routinely examined microscopically (although gross lesions were examined in detail). While the difference in leukaemia incidences between the top-dose and control group was not of sufficient magnitude to achieve statistical significance, many would say that it was certainly worth more attention than it has received, particularly since the problematic metabolite has demonstrated an ability to produce leukaemia in rats. On a similar suspicious (some might say paranoid) note, the oxide's ability to induce brain tumours in rats does increase the concern that a preliminary epidemiological study of workers" exposure to ethylene (and a number of other chemicals) may have given weak indications of a real effect.

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Conclusion

References

What is our view of the current status of ethylene's carcinogenicity? Certainly we would not disagree that the CI1T study provides no convincing evidence of laboratory carcinogenicity in animals. Moreover, there is only scant support for a carcinogenic action in humans from the NIOSH epidemiological study. While the ethylene molecule per se may not have significant carcinogenic potential, we do believe that the data on metabolic conversion to a genotoxic carcinogen provide a fairly good basis for suggesting that an expert, detailed reconsideration of the possible carcinogenic risks of inhaling ethylene would be timely. The rat study and the available cancer ¢pidemioiogy do not fully negate the concerns raised. On the basis of this argument, we believe a re-evaluation of the need for a specific industrial limit should be given a high priority, as should the funding of additional metabolic and epidemiological studies in humans. In recent years the efforts of the toxicological profession have led to the increased sophistication of the regulatory process. There is now a far greater likelihood that the evaluation of a chemical cancer hazard (and risk) will be based on the totality of the available biological information rather than on turnout data alone. Much scientific investigation has been authorized with the sensible objective of ensuring that there is no automatic assumption on the part of the regulators that a finding of cancers at high doses in a rodent study necessarily has a direct relevance to lower exposures and to the human perspective. That is why the ethylene story is an interesting test of the credibility of the new era. In this case there may be the opposite danger, that is a too literal interpretation of the rodent cancer study may lead to an underestimation of the low-dose rodent and human risks. [James Hopkins--BIBRA]

Bolt H. M. and Filser J. G.(1987) Archires of Toxicology 60, 73. CIIT (1980) Ethylene. Chronic 24-month Final Report. Chemical Industry Institute of Toxicology. EH40/93. Occupational Exposure Limits 1993. Health and Safety Executive. HMSO, London. Ehrenberg L. et al. (1977) Mutation Research 45, 175. Filscr J. G. and Bolt H. M. (1983) Mutation Research 120, 57. Garman R. H. et al. (1986) Food and Chemical Toxicology 24, 145. Gibson G. G. et al. (1987) Ethel Browning's Toxicity and Metablolism o f Industrial Solrents. 2nd Ed. Vol. l: Hydrocarbons. Chapter 4. I. Ethene. Edited by R. Snyder. Elsevier, Amsterdam. Guidelines for the Evaluation of Chemicals for Carcinogenicity (1991) Report on Health and Social Subjects No. 42. Committee on Carcinogenicity of Chemicals in Food, Consumer Products and the Environment. Department of Health. HMSO, London. Harem T. E., Jr et al. (1984) Fundamental and Applied Toxicology 4, 473. IARC (1987) IARC Monographs on the Evaluation of

THE

ROLE OF CANCER

MECHANISM

Carcinogenic Risks to Humans. Orerall Eraluations o f Carcinogenicity: An Updating of IARC Monographs. Volumes i to 42. Suppl. 7. International Agency for

Research on Cancer, Lyon. Lcflingwell S. S. et al. (1983) Neurocpidemiology 2, 179. Lynch D. W. et al. (1984) Toxicology and Applied Pharmacology 76, 69. Maples K. R. and Dahl A. R. (1993) Inhalation 7bxicalogy 5, 43. NTP (1987) Toxicology and carcinogenesis studies of ethylene oxide in B6C3FI mice (inhalation studies). NTP TR 326. NIH Publication 88-2582. Segerb/ick D. (1983) Chemico-Biological Interactions 45, 139. T6rnqvist M. et al. (1988) Journal of Applied Toxicology 8, 159. T6rnqvist M. A. et al. 0989) Scandinm,ian Journal o f Work Environment and ttealth 15, 436.

IN IARC CARCINOGEN

Whenever a group of toxicologists regularly gathers together, sooner or later their thoughts will turn to carcinogen classification. As well as the inherent intellectual satisfaction of tackling this complex subject, the major incentive to devise a robust classification scheme must be to widen the understanding amongst their colleagues within the company and the legislature that chemical carcinogenicity is not a rare property and that, since not all carcinogens are alike, they may not warrant the same degree of industrial and regulatory attention. Up until quite recently a possible further stimulus to the periodic pronouncements in this sphere (1) has been a certain amount of disgruntlement within industry over the efforts of the International Agency for Research on Cancer, the 'market leader" in carcinogen hazard evaluation.

CLASSIFICATION

It was the perceived reluctance of IARC to fully embrace mechanistic precepts in cancer evaluation, when mechanism appeared to be a critical aspect, that probably prompted some of industry's grumbles. Is there any basis to the accusation that IARC is a reluctam player of a mechanistic tune? The first thing to note about IARC's carcinogen assessment programme is its longevity: the first set of monographs originated at a Lyon meeting of invited experts in December 1971 (2). Whereas in those earliest days the conclusions on a compound's carcinogenicity were expressed in free text, the late 1970s saw the introduction of the standardized terms that are still used today (3). The aim of each IARC exercise then (as now) was the evaluation of human cancer hazard, the potential to cause cancer, and