clarifying carcinogenicity of ethylbenzene

3
Commentary Clarifying carcinogenicity of ethylbenzene James Huff , Po Chan, Ronald Melnick 1 National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709, USA article info Article history: Available online 17 August 2010 Keywords: Cancer bioassay Carcinogenesis Chemicals Long-term study Occupational cancer Primary prevention abstract Ethylbenzene has been evaluated for carcinogenic activity in Fischer rats and B6C3F1 mice exposed by inhalation (Chan et al., 1998; Chan, 1999) and in Sprague–Dawley rats after oral exposure (Maltoni et al., 1985,1997). Bioassay findings are summarized below to expand on those not stated clearly or com- pletely in Saghir et al. (2010). Overall in these three studies animals exposed to ethylbenzene had increased tumors in rats for kidneys, testes, head (including rare neuroesthesioepitheliomas), and total malignant tumors, whilst in mice tumor incidences were increased in the lung and liver (Huff, 2002). Thus ethylbenzene was carcinogenic by two exposure routes to both sexes of two species of rodents, two strains of rats, and one strain of mice, causing collectively tumors in five different target organs and a composite of ‘‘total malignant” tumors. Published by Elsevier Inc. Saghir et al. (2010) insist that these ethylbenzene induced car- cinogenic responses in animals are not relevant to humans at envi- ronmental or occupational exposure levels, and hence mechanistically should not be considered a cancer risk. Also cited to discount these effects were ‘high doses’ and ‘cellular toxicity’ leading to or preconditioning the animals to cancer. Neither of these arguments has been shown germane to their thesis (e.g., Bu- cher, 2002; Hoel et al., 1988; Huff, 1992,1993,1995). Over and over again cancer bioassay findings do not support any remotely consis- tent influence of toxicity – irritation, inflammation, cellular degen- eration, cellular turnover, cell proliferation – and cancer (e.g., Farber, 1995,1996; Hoel et al., 1988; Huff, 1992, 1995; Huff et al., 1991; Melnick, 1992; Melnick and Huff, 1993; Melnick et al., 1993,1996,1998; Tomatis, 1993; Ward et al., 1993; Weinstein, 1991,1992,1993). As a counter example. among many, inhalation of tetranitromethane caused irritation of nasal passages and no tu- mors yet induced pulmonary carcinogenesis without any observed lung toxicity (Bucher, 1990; Bucher et al., 1991). These ‘anti-corre- lation’ cancers are not uncommon in NTP bioassays. Regarding the authors’ ‘‘high-dose only effects” claim, with inci- dences of 14 vs 20, 30, 38% for alveolar/bronchiolar adenoma/car- cinoma of the lung in male mice, and 6 vs 10, 16, 42% for renal tubule adenomas/carcinomas in male rats, anyone claiming this is a ‘‘high-dose only” effect is either oblivious to the concept of dose–response or has a vested agenda. Significant dose–response trends (P < 0.01) were evident for both these sites, and it is difficult to get much better linear dose–responses for animal tumor data. This is especially true given the uneven spread of exposure doses as chosen from shorter term studies (Chan, 1992); for the NTP studies: 0 vs 75, 250, or 750 ppm ethylbenzene by inhalation, 6 h per day, 5 days per week, for 103 weeks (Chan et al., 1998; Chan, 1999). Another posed argument is genetic toxicity and cancer. Again there is little convincing and dependable evidence that one can correlate either artificial ‘‘grouping” of genotoxic versus non-geno- toxic chemicals for or against producing cancer in rodents or hu- mans. Melnick et al. (1996) evaluated this issue of non-genotoxic carcinogens and concluded with these interpretations ‘‘(a) many chemicals considered to be non-genotoxic carcinogens actually possess certain genotoxic activities, and limiting evaluations of carcinogenicity to their non-genotoxic effects can be misleading; (b) some non-genotoxic activities may cause oxidative DNA dam- age and thereby initiate carcinogenesis; (c) although cell replica- tion is involved in tumor development, cytotoxicity and mitogenesis do not reliably predict carcinogenesis; (d) a threshold tumor response is not an inevitable result of a receptor-mediated mechanism. There are insufficient data on the chemicals reviewed here to justify treating their carcinogenic effects in animals as irrel- evant for evaluating human risk.” If ethylbenzene metabolism ‘saturates’ between 200 and 500 ppm (Saghir et al., 2010), then, in organs where ethylbenzene is metabolized, one would not expect to see much increase in tu- mor incidence in the top exposure (750 ppm) compared to the mid level exposure (250 ppm), because both exposures are in the ‘‘saturation zone”. However, significant increases between these concentrations were obvious for liver, lung, and kidney. Also, the purported requirement for high levels of metabolism of ethylben- zene in human lung tissue to influence lung tumor risk may be false, because there is no reason why reactive metabolites formed 0273-2300/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.yrtph.2010.08.011 Corresponding author. E-mail address: [email protected] (J. Huff). 1 Formerly with National Institute of Environmental Health Sciences. Regulatory Toxicology and Pharmacology 58 (2010) 167–169 Contents lists available at ScienceDirect Regulatory Toxicology and Pharmacology journal homepage: www.elsevier.com/locate/yrtph

Upload: james-huff

Post on 13-Sep-2016

219 views

Category:

Documents


6 download

TRANSCRIPT

Page 1: Clarifying carcinogenicity of ethylbenzene

Regulatory Toxicology and Pharmacology 58 (2010) 167–169

Contents lists available at ScienceDirect

Regulatory Toxicology and Pharmacology

journal homepage: www.elsevier .com/locate /yr tph

Commentary

Clarifying carcinogenicity of ethylbenzene

James Huff ⇑, Po Chan, Ronald Melnick 1

National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709, USA

a r t i c l e i n f o

Article history:Available online 17 August 2010

Keywords:Cancer bioassayCarcinogenesisChemicalsLong-term studyOccupational cancerPrimary prevention

0273-2300/$ - see front matter Published by Elsevierdoi:10.1016/j.yrtph.2010.08.011

⇑ Corresponding author.E-mail address: [email protected] (J. Huff).

1 Formerly with National Institute of Environmental

a b s t r a c t

Ethylbenzene has been evaluated for carcinogenic activity in Fischer rats and B6C3F1 mice exposed byinhalation (Chan et al., 1998; Chan, 1999) and in Sprague–Dawley rats after oral exposure (Maltoniet al., 1985,1997). Bioassay findings are summarized below to expand on those not stated clearly or com-pletely in Saghir et al. (2010). Overall in these three studies animals exposed to ethylbenzene hadincreased tumors in rats for kidneys, testes, head (including rare neuroesthesioepitheliomas), and totalmalignant tumors, whilst in mice tumor incidences were increased in the lung and liver (Huff, 2002).Thus ethylbenzene was carcinogenic by two exposure routes to both sexes of two species of rodents,two strains of rats, and one strain of mice, causing collectively tumors in five different target organsand a composite of ‘‘total malignant” tumors.

Published by Elsevier Inc.

Saghir et al. (2010) insist that these ethylbenzene induced car-cinogenic responses in animals are not relevant to humans at envi-ronmental or occupational exposure levels, and hencemechanistically should not be considered a cancer risk. Also citedto discount these effects were ‘high doses’ and ‘cellular toxicity’leading to or preconditioning the animals to cancer. Neither ofthese arguments has been shown germane to their thesis (e.g., Bu-cher, 2002; Hoel et al., 1988; Huff, 1992,1993,1995). Over and overagain cancer bioassay findings do not support any remotely consis-tent influence of toxicity – irritation, inflammation, cellular degen-eration, cellular turnover, cell proliferation – and cancer (e.g.,Farber, 1995,1996; Hoel et al., 1988; Huff, 1992, 1995; Huff et al.,1991; Melnick, 1992; Melnick and Huff, 1993; Melnick et al.,1993,1996,1998; Tomatis, 1993; Ward et al., 1993; Weinstein,1991,1992,1993). As a counter example. among many, inhalationof tetranitromethane caused irritation of nasal passages and no tu-mors yet induced pulmonary carcinogenesis without any observedlung toxicity (Bucher, 1990; Bucher et al., 1991). These ‘anti-corre-lation’ cancers are not uncommon in NTP bioassays.

Regarding the authors’ ‘‘high-dose only effects” claim, with inci-dences of 14 vs 20, 30, 38% for alveolar/bronchiolar adenoma/car-cinoma of the lung in male mice, and 6 vs 10, 16, 42% for renaltubule adenomas/carcinomas in male rats, anyone claiming thisis a ‘‘high-dose only” effect is either oblivious to the concept ofdose–response or has a vested agenda. Significant dose–responsetrends (P < 0.01) were evident for both these sites, and it is difficultto get much better linear dose–responses for animal tumor data.

Inc.

Health Sciences.

This is especially true given the uneven spread of exposure dosesas chosen from shorter term studies (Chan, 1992); for the NTPstudies: 0 vs 75, 250, or 750 ppm ethylbenzene by inhalation, 6 hper day, 5 days per week, for 103 weeks (Chan et al., 1998; Chan,1999).

Another posed argument is genetic toxicity and cancer. Againthere is little convincing and dependable evidence that one cancorrelate either artificial ‘‘grouping” of genotoxic versus non-geno-toxic chemicals for or against producing cancer in rodents or hu-mans. Melnick et al. (1996) evaluated this issue of non-genotoxiccarcinogens and concluded with these interpretations ‘‘(a) manychemicals considered to be non-genotoxic carcinogens actuallypossess certain genotoxic activities, and limiting evaluations ofcarcinogenicity to their non-genotoxic effects can be misleading;(b) some non-genotoxic activities may cause oxidative DNA dam-age and thereby initiate carcinogenesis; (c) although cell replica-tion is involved in tumor development, cytotoxicity andmitogenesis do not reliably predict carcinogenesis; (d) a thresholdtumor response is not an inevitable result of a receptor-mediatedmechanism. There are insufficient data on the chemicals reviewedhere to justify treating their carcinogenic effects in animals as irrel-evant for evaluating human risk.”

If ethylbenzene metabolism ‘saturates’ between 200 and500 ppm (Saghir et al., 2010), then, in organs where ethylbenzeneis metabolized, one would not expect to see much increase in tu-mor incidence in the top exposure (750 ppm) compared to themid level exposure (250 ppm), because both exposures are in the‘‘saturation zone”. However, significant increases between theseconcentrations were obvious for liver, lung, and kidney. Also, thepurported requirement for high levels of metabolism of ethylben-zene in human lung tissue to influence lung tumor risk may befalse, because there is no reason why reactive metabolites formed

Page 2: Clarifying carcinogenicity of ethylbenzene

168 J. Huff et al. / Regulatory Toxicology and Pharmacology 58 (2010) 167–169

in the liver of humans don’t or couldn’t distribute through blood tothe lungs.

Additionally, several nonneoplastic lesions induced by ethyl-benzene were dose-related: male and female rats with renal tubulehyperplasia and severities of nephropathy, male mice showingalveolar epithelial metaplasia, syncytial alteration of hepatocytes,hepatocyte necrosis, and thyroid gland follicular cell hyperplasia,and female mice exhibiting pituitary gland pars distalis hyperpla-sia, and thyroid gland follicular cell hyperplasia. No tumor in-creases of the pituitary or thyroid glands were observed despitesignificant increases in hyperplasias.

The statement ‘‘very low binding activity of human lungmicrosomes could be attributed to low enzymatic activity of themicrosomal preparation due to autolysis of lung tissues from thetime of death of the donors and harvesting and processing oftissues” (Saghir et al., 2010, ,pg 13) clearly indicates that theirhuman data is virtually useless. Because of polymorphisms ingenes that code for metabolizing enzymes, use of a pooled humansamples gives no information on variability or the range of activitiesamong individuals. For these reasons alone this paper should havebeen better scrutinized, and perhaps could have been rejectedunless explained satisfactorily.

It is amazing that even Dow Chemical Company would suggestin vitro binding to microsomal protein is a possible mechanism ofmouse lung tumorigenesis. Based on the suggested mechanism of‘‘increased toxicity to the mouse lung”, why then were there no ad-verse effects on survival or body weight in the NTP 2-year studiesand no evidence of histopathologic injury to the lungs of mice ex-posed for 13 weeks to ethylbenzene at concentrations up to1000 ppm? In fact, in these 13-week studies, ‘‘Chemically relatedhistopathologic changes were not observed in any tissues of ratsor mice” (Chan, 1992). The Dow paper (Saghir et al., 2010) also failsto address binding to DNA by the ‘‘electrophilic reactive metabo-lites” that bind to microsomal proteins.

Certain chemicals, mixtures of chemicals, exposure circum-stances, life-styles and personal or cultural habits, occupations,viruses, living conditions, and physical agents have been causallyassociated with cancers in humans (IARC, 2010; RoC, NTP, 2010).Most of these varied exposures to chemicals however are not con-sidered potentially carcinogenic, and the proportion of ‘agents’eventually identified to cause cancer is projected to be relativelylow (Fung et al., 1995; Huff et al., 1985). However when a chemicalconvincingly causes cancers in animals, as does ethylbenzene, weshould quickly give serious attention to these multiple bioassay re-sults rather than conjuring ways to discount these warnings of po-tential cancers to humans, especially workers exposedoccupationally (Huff, 2010a, 2010b; Tomatis et al., 1997,2001).IARC, 2000 reviewed the available information on ethylbenzeneand decided ‘‘There is sufficient evidence in experimental animalsfor the carcinogenicity of ethylbenzene”, with an ‘‘Overall evalua-tion: Ethylbenzene is possibly carcinogenic to humans (Group2B).” Further, IARC states ‘‘it is biologically plausible that agents(like ethylbenzene) for which there is sufficient evidence of carcin-ogenicity (usually two species) in experimental animals also pres-ent a carcinogenic hazard to humans” (IARC, 2010).

Long-term carcinogenesis bioassays using experimental ani-mals are the most predictive method for identifying likely humancarcinogens. Since the 1960s, bioassays have proven to be a main-stay for identifying chemical carcinogens, establishing occupa-tional exposure standards, and primary cancer prevention. Thereasons, rationale, and validity are many (Huff, 2010b). Long-termbioassays are both predictive (prospective) and confirmatory (ret-rospective) for human carcinogens, and there has long been anagreeable association between carcinogenic outcomes from bioas-says and human cancer hazards (Huff, 1999; Tomatis, 2000). Thesecorrelations stem from accumulated evidence over the last

50 years during the modern era of experimental carcinogenesis(Tomatis and Huff, 2001,2002).

Conflict of interest

The authors declare that there are no conflicts of interest.

References

Bucher, J.R., 1990. NTP toxicology and carcinogenesis studies of tetranitromethane(CAS No. 509–14-8) in F344/N rats and B6C3F1 mice (inhalation studies). Natl.Toxicol. Program Tech. Rep. Ser. 386, 1–207.

Bucher, J.R., Huff, J.E., Jokinen, M.P., Haseman, J.K., Stedham, M., Cholakis, J.M., 1991.Inhalation of tetranitromethane causes nasal passage irritation and pulmonarycarcinogenesis in rodents. Cancer Lett. 57 (2), 95–101.

Bucher, J.R., 2002. The National Toxicology Program rodent bioassay: designs,interpretations, and scientific contributions. Ann. N. Y. Acad. Sci. 982, 198–207.

Chan, P., 1992. NTP technical report on the toxicity studies of ethylbenzene (Cas No. 100–41-4) in F344/N rats and B6C3F1 mice (Inhalation Studies). Toxic. Rep. Ser. 10, 1–B7.

Chan, P.C., Haseman, J.K., Mahleri, J., Aranyi, C., 1998. Tumor induction in F344/Nrats and B6C3F1 mice following inhalation exposure to ethylbenzene. Toxicol.Lett. 99 (1), 23–32.

Chan, P., 1999. NTP Toxicology and Carcinogenesis Studies of ethylbenzene (CAS No.100–41-4) in F344/N rats and B6C3F1 mice (Inhalation Studies). Natl. Toxicol.Program Tech. Rep. Ser. 466, 1–231.

Farber, E., 1995. Cell proliferation as a major risk factor for cancer: a concept ofdoubtful validity. Cancer Res. 55 (17), 3759–3762.

Farber, E., 1996. Cell proliferation is not a major risk factor for cancer. Mod. Pathol. 9(6), 606.

Fung, V.A., Barrett, J.C., Huff, J., 1995. The carcinogenesis bioassay in perspective:application in identifying human cancer hazards. Environ. Health Perspect. 103(7–8), 680–683.

Hoel, D.G., Haseman, J.K., Hogan, M.D., Huff, J., McConnell, E.E., 1988. The impact oftoxicity on carcinogenicity studies: implications for risk assessment.Carcinogenesis 9 (11), 2045–2052.

Huff, J., McConnell, E.E., Haseman, J.K., 1985. On the proportion of positive results incarcinogenicity studies in animals. Environ. Mutagen. 7 (4), 427–428.

Huff, J., Haseman, J., Rall, D., 1991. Scientific concepts, value, and significance ofchemical carcinogenesis studies. Annu. Rev. Pharmacol. Toxicol. 31, 621–652.

Huff, J., 1992. Chemical toxicity and chemical carcinogenesis. Is there a causalconnection? A comparative morphological evaluation of 1500 experiments.IARC Sci. Publ. (116), 437–475.

Huff, J., 1993. Absence of morphologic correlation between chemical toxicity andchemical carcinogenesis. Environ. Health Perspect. 101 (Suppl 5), 45–53.

Huff, J., 1995. Mechanisms, chemical carcinogenesis, and risk assessment: cellproliferation and cancer. Am. J. Ind. Med. 27 (2), 293–300.

Huff, J., 1999. Long-term chemical carcinogenesis bioassays predict human cancerhazards. Issues, controversies, and uncertainties. Ann.N. Y. Acad. Sci. 895, 56–79.

Huff, J., 2002. Chemicals studied and evaluated in long-term carcinogenesis bioassaysby both the Ramazzini foundation and the national toxicology program: in tributeto Cesare Maltoni and David Rall. Ann. N. Y. Acad. Sci. 982, 208–230.

Huff, J., 2010a. Occupational cancer and social inequities. Eur. J. Public Health (Epubahead of print).

Huff, J., 2010b. Predicting chemicals causing cancer in animals as humancarcinogens. Occup. Environ. Med. 67 (10), 720.

IARC, 2000. Ethylbenzene. IARC Monogr. Eval. Carcinog. Risks Hum. 77, 227–266.IARC, 2010. International Agency for Research on Cancer. August 2010. Available

from: <http://monographs.iarc.fr/ENG/Classification/index.php> and <http://monographs.iarc.fr/>.

Maltoni, C., Conti, B., Cotti, G., Belpoggi, F., 1985. Experimental studies on benzenecarcinogenicity at the Bologna Institute of Oncology: Current results andongoing research. Am. J. Ind. Med. 7, 415–446.

Maltoni, C., Ciliberti, A., Pinto, C., Soffritti, M., Belpoggi, F., Menarini, L., 1997. Results oflong-term experimental carcinogenicity studies of the effects of gasoline,correlated fuels, and major gasoline aromatics on rats. Ann. N. Y. Acad. Sci. 837,15–52.

Melnick, R.L., 1992. Does chemically induced hepatocyte proliferation predict livercarcinogenesis? FASEB J. 6 (9), 2698–2706.

Melnick, R.L., Huff, J., Barrett, J.C., Maronpot, R.R., Lucier, G., Portier, C.J., 1993. Cellproliferation and chemical carcinogenesis: a symposium overview. Mol.Carcinog. 7 (3), 135–138.

Melnick, R.L., Huff, J., 1993. Liver carcinogenesis is not a predicted outcome ofchemically induced hepatocyte proliferation. Toxicol. Ind. Health 9 (3), 415–438.

Melnick, R.L., Kohn, M.C., Portier, C.J., 1996. Implications for risk assessment ofsuggested nongenotoxic mechanisms of chemical carcinogenesis. Environ.Health Perspect. 104 (Suppl 1), 123–134.

Melnick, R.L., Kohn, M.C., Dunnick, J.K., Leininger, J.R., 1998. Regenerativehyperplasia is not required for liver tumor induction in female B6C3F1 miceexposed to trihalomethanes. Toxicol. Appl. Pharmacol. 148 (1), 137–147.

RoC, NTP, 2010. Report on Carcinogens. National Toxicology Program. August 2010.Available from: <http://ntp.niehs.nih.gov/ntpweb/index.cfm?objectid=035E5806-F735-FE81-FF769DFE5509AF0A>.

Page 3: Clarifying carcinogenicity of ethylbenzene

J. Huff et al. / Regulatory Toxicology and Pharmacology 58 (2010) 167–169 169

Saghir, S.A., Zhang, F., Rick, D.L., Kan, L., Bus, J.S., Bartels, M.J., 2010. In vitrometabolism and covalent binding of ethylbenzene to microsomal protein aspossible a mechanism of ethylbenzene-induced mouse lung tumorigenesis.Regul. Toxicol. Pharmacol. 57 (2–3), 129–135.

Tomatis, L., 1993. Cell proliferation and carcinogenesis: a brief history and currentview based on an IARC workshop report. international agency for research oncancer. Environ. Health Perspect. 101 (Suppl 5), 149–151.

Tomatis, L., Huff, J., Hertz-Picciotto, I., Sandler, D.P., Bucher, J., Boffetta, P., Axelson,O., Blair, A., Taylor, J., Stayner, L., Barrett, J.C., 1997. Avoided and avoidable risksof cancer. Carcinogenesis 18 (1), 97–105.

Tomatis, L., 2000. The identification of human carcinogens and primary preventionof cancer. Mutat. Res. 462 (2–3), 407–421.

Tomatis, L., Melnick, R.L., Haseman, J., Barrett, J.C., Huff, J., 2001. Allegedmisconceptions’ distort perceptions of environmental cancer risks. FASEB J. 15(1), 195–203.

Tomatis, L., Huff, J., 2001. Evolution of cancer etiology and primary prevention.Environ. Health Perspect. 109 (10), A458–A460.

Tomatis, L., Huff, J., 2002. Evolution of research on cancer etiology. In: Coleman,W.B., Tsongalis, G.J. (Eds.), The Molecular Basis of Human Cancer: GenomicInstability and Molecular Mutation in Neoplastic Transformation. HumanaPress Inc., Totowa, NJ, pp. 189–201 (Chapter 9).

Ward, J.M., Uno, H., Kurata, Y., Weghorst, C.M., Jang, J.J., 1993. Cell proliferation notassociated with carcinogenesis in rodents and humans. Environ. HealthPerspect. 101 (Suppl 5), 125–135.

Weinstein, I.B., 1991. Mitogenesis is only one factor in carcinogenesis. Science 251(4992), 387–388.

Weinstein, I.B., 1992. Toxicity, cell proliferation, and carcinogenesis. Mol. Carcinog.5 (1), 2–3.

Weinstein, I.B., 1993. Cell proliferation: concluding remarks. Environ. HealthPerspect. 101 (Suppl 5), 159–161.