2-chlorobuta-1,3-diene

Carcinogenicity

Description of key information

The carcinogenicity of chloroprene has been investigated in a number of studies in mice, rats, and hamsters, following inhalation exposures and in a single study in rats following oral gavage. The results of the studies indicate that chloroprene is a multi-site carcinogen in B6C3F1 mice and Fischer F344 rats, following inhalation exposures. but not in Wistar rats or Syrian hamsters. Key carcinogenicity studies are 2-year inhalation studies by NTP (1998) in which F344/N rats and B6C3F1 mice were exposed to chloroprene vapour at 0, 12.8, 32 or 80 ppm. Critical local effects following chronic inhalation exposures to chloroprene were non-neoplastic lesions of the nose and the lungs, whereas critcial systemic effects were tumours occuring at various sites. Incidence of tumours of the lungs (alveolar and bronchiolar adenoma or carcinomas), the skin (sarcomas), the mammary gland (adenocarcinomas, carcinomas) and of the hardarian gland (adenomas or carcinomas) were statistically increased in mice, whereas incidence of tumours of the oral cavity (papillomas or carcinomas), the thyroid gland (follicular cell adenomas or carcinomas), the mammary gland (adenocarcinomas, carcinomas) and of the lung (alveolar and bronchiolar adenoma or carcinomas) were statistically increased in rats.

Justification for classification or non-classification

According to Annex VI to Regulation (EC) No 1272/2008, chloroprene is classified for carcinogenicity as: Carc 1B (H350) – May cause cancer.

Additional information

Carcinogenicity

The carcinogenicity of chloroprene has been investigated in a number of studies in mice, rats, and hamsters, following inhalation exposures and in a single study in rats following oral gavage. The results of the studies indicate that chloroprene is a multi-site carcinogen in B6C3F1 mice and Fischer F344 rats following inhalation exposures, but not in Wistar rats or Syrian hamsters.

The carcinogenic potential of chloroprene in rats and mice was investigated in studies conducted as part of the US Department of Health and Human Services National Toxicology Program (NTP). Choroprene was selected for study as part of the NTP on the basis that it is a high-volume production chemical with limited information on its carcinogenic potential and is a 2-chloro analogue of 1,3-butadiene; a potent, multi-species, multi-organ carcinogen.

Studies in rats

In a 2-year inhalation carcinogenicity study, three groups of 100 Wistar rats per sex were exposed to chloroprene vapour at 0, 10 or 50 ppm 6 hours/day, 5 days/week. (In week 72 the majority of low dose animals were accidentally killed as a result of a procedural error that resulted in suffocation of 87 males and 73 females).

All treated rats were slightly restless during the first few weeks of exposure. An increased incidence in alopecia for high dose rats was evident during the first year but reduced in frequency in the second year of treatment. Mortality was not affected by treatment with chloroprene. Reduced growth was apparent for male and female rats treated at 10 ppm, although the effects were less pronounced in the second year of dosing, such that terminal bodyweights for treated and control groups were similar. A decrease in relative lung weight was recorded in both treated groups but chronic respiratory disease was more prevalent in the controls than treated rats. An increase in focal hepatocellular alterations apparent in the high dose group was confirmed by additional histopathological investigations. Carcinogenic effects were not observed in rats following a 2 years of inhalation exposure to chloroprene at 50 ppm.

Ponomarkov and Tomatis (1980) investigated the carcinogenic potential of chloroprene administered orally (gavage route) as a single dose (100 mg/kg bw) to pregnant female rats on day 17 of gestation and as a weekly oral dose (50 mg/kg bw) in off-spring throughout their lifetime. Survival rates, bodyweight gain, litter sizes and pre-weaning mortality were similar in exposed and control groups. Severe congestion of the lungs and kidneys was observed in offspring that died within the first 23 -35 weeks of treatment and multiple liver necroses occurred in treated offspring after 80 -90 weeks of treatment. Liver, soft tissue, mammary gland, ovary and other unspecified tumours and hyperplastic liver nodules were observed following chloroprene treatment. However, the total incidence of tumours was similar for treated and control rats. Although several tumours were apparent among the treated males that were not observed in the controls and subcutaneous fibromas were more numerous in chloroprene treated males than among the controls, these differences were not considered significant nor indicative of a carcinogenic effect associated with long-term chloroprene exposure

Studies in hamsters

No indications of carcinogenesis were found in an 18 month inhalation carcinogenicity study in which three groups of 100 Syrian Golden hamsters per sex were exposed to chloroprene vapours at 0, 10 or 50 ppm for 6 hours/day, 5 days/week.

NTP studies

In a 2-year inhalation carcinogenicity study, groups of 50 F344/N rats per sex were exposed to chloroprene vapour (greater than 96% pure) at 0, 12.8, 32, or 80 ppm for 6 hours per day, 5 days per week. Two-year survival probabilities were significantly reduced in male rats exposed at 32 and at 80 ppm. In the study, evidence of the carcinogenicity of chloroprene in male rats was provided based on increased incidences of neoplasms (papillomas, adenomas or carcinomas) of the oral cavity, the thyroid gland , the lung, and the kidney attributed to chloroprene exposure. Evidence of carcinogenic activity in female rats was provided based on increased incidences of neoplasms (papillomas, adenomas, carcinomas or fibroadenomas) of the oral cavity the thyroid gland, the mammary gland, and the kidney attributed to exposure to chloroprene. It was proposed that low incidences of urinary bladder neoplasms in male and female rats and lung neoplasms in female rats may also have been related to exposure to chloroprene. An association was found between chloroprene exposure and increased incidences of alveolar epithelial hyperplasia in the lung; nephropathy; and several non neoplastic effects in the nose including olfactory epithelial atrophy, fibrosis, adenomatous hyperplasia, basal cell hyperplasia, chronic inflammation, respiratory metaplasia, and necrosis.

In a 2-year inhalation carcinogenicity study, groups of 50 B6C3F1 mice per sex were exposed to chloroprene vapour (greater than 96% pure) 0, 12.8, 32, or 80 ppm by inhalation, 6 hours per day, 5 days per week. In the study, evidence of carcinogenic activity of chloroprene in male mice was provided based on increased incidences of neoplasms (papillomas, adenomas or carcinomas) of the lung, harderian gland and kidney and the circulatory system (hemangiomas and hemangiosarcomas) attributed to exposure to chloroprene. Evidence of carcinogenic activity in female mice was provided based on increased incidences of neoplasms (papillomas, adenomas or carcinomas) of the lung, harderian gland, mammary gland, liver, skin, and mesentery; forestomach and Zymbal's gland and the circulatory system (hemangiomas and hemangiosarcomas), were also attributed to exposure to chloroprene. In the study an association was found between exposure to chloroprene and increased incidences of bronchiolar hyperplasia and histiocytic cell infiltration in the lung; epithelial hyperplasia in the forestomach; renal tubule hyperplasia (males only); several effects in the nose including olfactory epithelial atrophy, respiratory metaplasia, and adenomatous hyperplasia; and hematopoietic cell proliferation in the spleen.

Discussion

In a chronic inhalation toxicology study of chloroprene in rats and mice, significant increases in neoplasms of the lung and kidney are reported (Melnick et al., 1999). The observation of respiratory tract tumors in rodents is noteworthy in that it suggests that these neoplasms are relevant to humans since the exposure route of greatest concern in the workplace is the lungs via inhalation (Scott 1995). Since the rodent respiratory tract tumors may represent either direct acting effects on the lung from inhalation or indirect effects arising from circulatory exposure to chloroprene metabolites from the liver, lung tumors will be considered to represent systemic toxicity.

Using Benchmark Dose analysis, Melnick et al. (1999) concluded that the most sensitive sex, species and target organ was female mouse lung tumours. Similar observations have been observed in studies by the NTP (1998) which indicated that the critical systemic effects of chloroprene after chronic inhalation exposures are tumours occurring at different sites (including the lung and liver), with greater sensitivity towards tumour formation being seen in mice.Studies on the in vitro metabolism of chloroprene (Cottrell et al. 2001, Munter et al 2003 and Himmelstein 2004a,b) suggest that the the mouse is at considerably more risk for chloroprene toxicity than other species. Although the full mechanism leading to tumor development in the lung or other tissues is not completely understood, chloroprene metabolism, particularly its greater oxidation leading to epoxides or other reactive metabolites, slower epoxide hydrolysis, and the potential for glutathione depletion in the mouse relative to the other rodent species indicates that the mouse is not an appropriate model for human health risk assessment. Species differences in tumor outcome are also supported by the lack of lung and liver tumors in other rodent species where observed activation:detoxication ratios are lower than for mice; e.g. Wistar rat and Syrian Golden hamster (Trochimowicz et al., 1998). This lack of species similarity in response is also seen in the outcome of the latest human epidemiology studies were no significant increase in lung or liver cancer deaths were reported (Marsh et al. 2007). Collectively, these data show that the mouse is not a good predictor for human outcome. While quantitative response factors cannot be derived with certainty, intrinsic clearance values for lung and liver address species and tissue specific concerns for sensitivity. The activation:detoxication ratios suggest that a modification factor of at least 10 for liver effects and at least 100 for lung effects appears appropriate if the mouse model is used as the basis for predicting effects in humans.

For chloroprene, animal data points to a carcinogenic potential with greater sensitivity being shown by some species than others based, on differences in rates of chloroprene metabolism. There is limited evidence in mammalian systems in vivo to support a possible role for genoxtoxicity. The mode of action by which chloroprene induces carcinogenic effects in animal species and the relevance of this to humans is not currently known.

In contrast to the animal data, observational epidemiology studies of workers exposed at chloroprene manufacturing sites do not show chloroprene to be causally associated with cancer mortality.  The occupational epidemiological study by Marsh et al (2007a,b:) of workers from four chloroprene production cohorts was considered to be the most methodologically rigerous study by a recent qualitative assessment using published EPA criteria for the quality assessment of epidemiology studies (Bukowski 2009). The qualitative review by Bukowski concluded that the Marsh study provided the highest quality evidence and should serve as the principal study for chloroprene risk assessment. This review study of available observational epidemiological studies concluded the weight of evidence does not support a substantial association between chloroprene exposure and cancer (Bukowski 2009).

Taking these issues into consideration there is insufficent evidence to presume chloroprene is a human carcinogen, since observations of carcinogenic effects in animals are unsubstantiated by large, high quality occupational studies and studies indicate that rodents may be more susceptible to chloroprene toxicty due to metabolic differences. Given the current weight of evidence, a conservative approach would regard chloroprene as suspected of causing cancer.