chloromethane; methyl chloride

Carcinogenicity

Description of key information

NOAEC carcinogenicity (rat, female mouse): 2065 mg/m³ (CIIT, 1981)
NOAEC carcinogenicity (male mouse): 465 mg/m³ (CIIT, 1981)

Key value for chemical safety assessment

Carcinogenicity: via oral route

Endpoint conclusion

Endpoint conclusion:

no study available

Carcinogenicity: via inhalation route

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEC

465 mg/m³

Study duration:

chronic

Species:

mouse

Carcinogenicity: via dermal route

Endpoint conclusion

Endpoint conclusion:

no study available

Justification for classification or non-classification

CLP: Carc. 2; H351 - Suspected of causing cancer

DSD: Carc. Cat. 3; R40 - Limited evidence of a carcinogenic effect

Additional information

The available data on the carcinogenicity of chloromethane are life-time studies in mice and rates (CIIT, 1981).

In a combined chronic toxicity/carcinogenicity study Fischer 344 rats and B6C3F1 mice (12 animals per sex per dose) were exposed to 50, 225 and 1000 ppm (corresponding to 103, 465 or 2065 mg/m³) for 6 hours/days, 5 days/week for 2 years with the objective to determine the potential toxicological and oncogenic effects of atmospheres containing chloromethane (CIIT, 1981). Control groups of animals were subjected to the same procedures as the test groups, except exposure was to air alone. Planned interim necropsies of experimental animals were completed at 6, 12, 18 and 24 months following initiation of the exposure. As a result of compound-related high mortality in the mouse high-dose group, the scheduled 24-months sacrifice was carried out after 21 or 22 months of exposure.

The liver and the urogenital tract could be identified as target organs of chloromethane, as well as the testes and epididymis in rats. Further significant findings from these long-term studies were disturbances of the nervous system in mice. Neoplastic lesions were detected in male mice only. Renal tubuloepithelial hyperplasia and karyomegaly were seen in male mice exposed to 2065 mg/m³ for 12 months and progressed in severity and prevalence throughout the study. A significant increase in renal tumors was noted in 2065 mg/m³ male mice sacrificed or dying between 12 and 21 months, including renal cortical adenoma, renal cortical adenocarcinoma, papillary cystadenoma, papillarx cystadenocarcinoma, and tubular cystadenoma. At the 24 month terminal sacrifice the only renal neoplasm noted at concentrations less than 2065 mg/m³ occurred in two 465 mg/m³ male mice. However, this effect was not statistically significant. Renal cortical cysts were predominantly seen in mice in the 2065 mg/m³ group, whereas microcysts were noted most frequently in the 103 mg/m³ at 24 months compared with the control males. Both occurrences were different from controls but were not statistically significant and did not occur in a dose-dependent manner. In addition, according to Johnson (1988) subsequent review indicates the purported increases in non-tumorous renal cortical microcysts in the 103 and 2065 mg/m³ groups were a likely procedural artifact due to multiple pathologists examining the tissues and using different nomenclature.

Neoplasia were neither found at any other site in the male B6C3F1 mouse nor at any site in female mice or rats of either sex. Thus, the NOAEC for carcinogenicity in male mice was considered to be 465 mg/m³ and 2065 mg/m³ for rats and female mice. The results indicated a species and sex-specific tumour formation after exposure to chloromethane.

 

Human data

There are few studies that have examined chloromethane’s carcinogenicity potential in humans. An epidemiological study by Holmes et al. (1986) summarized the limited data on causes of death in 852 butyl rubber workers including carcinogenic death. There was no increase in deaths due to cancer in this study population, but the study has only limited statistical power. The trawler cohort study (Rafnsson and Gudmundson, 1997), in which the authors investigated mortality and cancer patterns among a group of individuals accidentally exposed to chloromethane 32 years earlier and the Louisiana chemical worker study by Olsen et al., 1989 have failed to demonstrate any association between chloromethane exposure and carcinogenicity.

 

Discussion mode of action

The possible mechanisms of tumor production by chloromethane only in male mice under the highest concentration tested have been assessed by various authors (Jäger et al 1988; Bolt and Gansewendt 1993; Dekant et al., 1995).

 

Dekant and Colnot (2013) assessed recently for the members of the Methyl chloride REACh consortium the data on carcinogenicity (CIIT, 1981).

Dekant and Colnot (2013) concluded, the fact that renal tumors are observed in male mice only is due to the species‐and sex‐specific expression of a P450 enzyme closely related to CYP2E1 protein. Of the two rodent species evaluated in life‐time studies, only the male mouse has high levels of CYP2E1 in kidney and has the capacity to produce significant amounts of formaldehyde from chloromethane. CYP2E1 protein levels (and the capacity for oxidation of chloromethane to formaldehyde) are significantly higher in male mouse kidney as compared to kidney from female mouse. Kidney microsomes from Sprague‐Dawley rats did not catalyze the formation of formaldehyde from chloromethane. Additionally, depletion of GSH by chloromethane in rodents can lead to an accumulation of formaldehyde through inhibition of formaldehyde dehydrogenase (a GSH‐requiring enzyme that oxidizes formaldehyde to formate) and to an increase in lipid peroxidation in kidney (and brain fractions) in mice, but not in rats.

Thus, at the high concentrations reached in the kidney of the male mouse, less chloromethane is metabolized by the usually dominant GSH‐conjugation pathway, and more chloromethane is oxidized to formaldehyde due to the high levels of CYP2E1. The cytotoxic effects of the achieved high formaldehyde concentrations, coupled with the high capacity of the kidney for regenerative cell proliferation, support the conclusion that tumor induction is a progression from regenerative proliferation following chronic high‐concentration exposure, and not due to initiation by a direct genotoxic event (please also refer to the Section Genotoxicity and to the expert review by Dekant (2015) attached in IUCLID Section 13 for further details and discussion).

Mouse kidney tumors and also mouse lung tumors are induced by a number of other non‐genotoxic chemicals, which are substrates of mouse specific P450 enzymes. These P450 enzymes are selectively expressed in mouse lung/kidney and catalyze formation of cytotoxic metabolites specifically in the target organs for tumorigenicity of chloromethane in mice. As a consequence of tissue‐specific cytotoxicity thus occurring only in mice, regenerative cell proliferation finally results in tumors. Male mouse specific renal tumors depending on P450 activation are observed with chloroform and 1,1‐dichloroethene. Examples for mouse specific lung tumors due to bioactivation by a mouse lung specific P450 enzyme are styrene, trichloroethylene, naphthalene, and coumarin. These tumors were not considered critical endpoints for assessing human health risk in evaluations by several scientific advisory bodies.

In conclusion, the available data indicate that chloromethane-induced carcinogenicity to the kidney selectively in the male mouse is caused by a species‐specific and threshold‐based mode of action. This mode of action is considered as not relevant for humans at the significantly lower levels of exposure typical in the occupational environment. In proximal tubular cells of human kidney, CYP2E1 protein has not been detected. In addition, occupational exposure at or below the current MAK OEL likely will not result in GSH depletion in exposed workers, thus GSH-conjugation of chloromethane will remain efficient and formaldehyde will not accumulate.

 

In the recommendation REC-191 from the SCOEL for chloromethane (2017) the mode of action of chloromethane for the observed incidence of kidney tumors in male mice of the highest exposure group is also assessed., largely based on the assessment by DFG 1996 (=Greim 1996). The respective key passage of the SCOEL document is cited in the following:

>> – Kidney tumours developed only in male mice exposed to the highest chloromethane concentration of 1 000 ppm. No tumours were seen in the lower concentration groups, in female mice or in rats of either sex.

– The exposure concentration of 1 000 ppm is close to the level that produced a marked increase in replication rate in the kidney tissue of mice exposed repeatedly (1 500 ppm).

– This exposure concentration (1 000 ppm) caused glutathione depletion in the kidney and liver of the mouse, reducing the concentration to less than 5 % of the initial value and so impairing the glutathione-dependent metabolism of chloromethane. The enzyme activity required for the alternative oxidative pathway, which converts chloromethane to formaldehyde, is present in the kidneys of the male B6C3F1 mouse at higher levels than in those of females.

– Glutathione depletion reduces the activity of formaldehyde dehydrogenase, which converts formaldehyde to formic acid using glutathione as cofactor.

– DNA-protein cross-links, lesions typically produced by formaldehyde, were found in the kidneys of male mice (but not in females) exposed once for 8 hours to a chloromethane concentration of 1 000 ppm. DNA single strand breaks were also observed. The latter kind of lesion could also be produced by reactive oxygen species, a proposal which is supported by the observation that lipid peroxidation occurred.

– In addition to these effects, secondary effects were also observed in the long-term study: retrograde urinary tract infections (inflammatory processes associated with the production of reactive oxygen species and increased cell replication).

– Unlike the structurally analogous compounds, methyl bromide and methyl iodide, chloromethane (methyl chloride) is not able to methylate DNA directly in vivo. This conclusion is supported by two negative DNA binding studies.

It was concluded that, in the long-term study, the formation of renal tumours in the male mouse occurred only under such conditions, which do not permit any extrapolation of the results to human workplace situations (Greim 1996).

Backed by this argumentation, SCOEL concludes that the tumours reported in the study of Battelle Columbus (1981) in male mice have no relevance for humans at the workplace.<<

 

 

References:

Amet Y. et al. (1997). Cytochrome P450 4A and 2E1 expression in human kidney microsomes. Biochem Pharmacol, 53:765-771 [as cited in: US EPA, Toxicological review of methyl chloride, 2001].

Bolt, H.M. and Gansewendt, B. (1993). Mechanisms of carcinogenicity of methyl halides. Crit. Rev. Toxicol. 23(3):237 -253 [as cited in: OECD SIDS, Chloromethane, 2004]

Dekant W. and Colnot, T. (2013) Expert Review: Can the German OEL-value (MAK) for Chloromethane (MeCl) be considered to be equivalent to a DNEL within the REACH-framework?

Dekant, W. (2015) Expert Review: Additional comments on mutagenicity and carcinogenicity of chloromethane: Human relevance of male mouse-specific renal tumors

Dekant, W., Frischmann, C. and Speerschneider, P. (1995) Sex, organ and species specific bioactivation of chloromethane by cytochrome P4502E1. Xenobiotica 25 (11): 1259 -1265 [as cited in: OECD SIDS, Chloromethane, 2004]

DFG, Deutsche Forschungsgemeinschaft (1996). Methyl chloride. In: The MAKCollection, Vol. 7, 173-191. Wiley-VCH. http://onlinelibrary.wiley.com/book/10.1002/3527600418/topics (accessed 24April 2013) [as cited in: SCOEL, chloromethane, 2014, Draft]

Holmes, T.M. et al. (1986). A mortality study of employees at a synthetic rubber manufacturing plant. Americal Journal of Industrial Medicine, 9(4):355-362 [as cited in: CICAD 28, Methyl Chloride, 2000]. Battelle (1981). A chronic inhalation toxicology study in rats and mice exposed to methyl chloride. CIIT, Research Triangle Park, North Carolina, USA.

SCOEL, Scientific Committee on Occupational Exposure Limits (2017), REC-191 - Chloromethane, Publications Office of the European Union, Luxembourg, ISBN: 978-92-79-66616-2

Justification for selection of carcinogenicity via inhalation route endpoint:
The selected study is an adequate and reliable study with the lowest dose descriptor.

Carcinogenicity: via inhalation route (target organ): urogenital: kidneys