chloromethane; methyl chloride

Endpoint summary

Additional information

Under normal conditions chloromethane exists as a gas. Therefore, the only relevant route of exposure is via inhalation. Tests referred to below, were mostly conducted in exposure chambers.

Following in vitro studies have been conducted with chloromethane:

In bacterial reverse gene mutation assays with Salmonella typhimurium strains according to Ames, chloromethane was shown to induce gene mutations in the test strains detecting base pair substitution mutations, namely TA 100 (Gericke, 1977, Simmon et al., 1977, NTP, 1991 and Haskell Laboratories, 1978) and TA 1535 (Andrews et al., 1976, Gericke, 1977 and Haskell Laboratories, 1978) with and without metabolic activation system. In contrast, no mutation induction was observed using TA 98 (Gericke, 1977 and NTP, 1991) and TA 1537 (Gericke, 1977) both in the presence and absence of metabolic activation. Further, a concentration-related increase in the 8-azaguanine-resistent fraction in S. typhimurium TM 677 was observed (Fostel et al., 1985).

Using a mammalian gene mutation assay in established TK6 human lymphoblasts, Fostel et al.(1985) detected a dose-related increase in the mutant frequency after exposure to cytotoxic concentrations of chloromethane. In addition, Fostel et al. (1985) demonstrated that chloromethane exposure caused a statistically significant concentration-related induction of sister chromatid exchange (SCE) frequency in the same cell line; again chloromethane caused a significant decline in the mitotic index and inhibited cell-cycle kinetics. Unscheduled DNA synthesis (UDS) could be detected in spermatocyte and hepatocyte primary cell cultures after exposure to 1-10% chloromethane, but not in tracheal epithelial cells (Working et al., 1986). In general, the increases in UDS occurred in combination with elevated cytotoxicity.

Although chloromethane did induce UDS in vitro, no induction of UDS was evident in spermatocytes, hepatocytes, or tracheal epithelial cells from rats exposed to concentrations of 6195-7228 mg/m³ for 6 h/day for 5 days (Working et al., 1986). However, acute exposure to a concentration, which was close to the maximum tolerated dose (30975 mg/m³, 3 h), did cause a marginal increase in UDS in hepatocytes, but not in spermatocytes and tracheal epithelial cells (Working et al., 1986).

Jäger et al. (1988) and Ristau et al. (1990) performed alkaline elution assays in mice after inhalation exposure to chloromethane at 1000 ppm for 1-4 days and could detect transient DNA cross-links and/or DNA single strand breaks in renal tissue of males but not female mice or in male and female hepatic tissues. At 48 h post-exposure, all observed lesions had disappeared. This effect could be explained by the increased susceptibility for nephrotoxicity observed in male B6F3C1 mice upon chloromethane exposure and is therefore considered as secondary effect due to cytotoxicity.

In dominant lethal assays, dose independent increases of pre- and post-implantation losses could be detected after chloromethane exposure (Working et al., 1985, Chellman et al., 1986). It was demonstrated in parallel experiments with concomitant treatment with an anti-inflammatory agent (BW755C) that cytotoxic rather than genotoxic effects were the underlying causes (Chellman et al., 1986).

In macromolecular binding studies, radio labelled chloromethane (14C) could be detected in lipid, RNA, DNA, and protein from isolated liver, kidneys, lungs, and testes of male rats, but methylation was not evident (Kornbrust et al., 1982, Peter et al. 1985). However, the detected incorporation of 14C into these macromolecules was most likely due to its metabolism via the one carbon pool. Nevertheless, this does not exclude the possibility that chloromethane might to a lesser extent bind directly to macromolecules.

 

Available studies demonstrate that at high concentrations chloromethane appears to be directly genotoxic in in vitro systems, both in bacteria and mammalian cells. However, in in vivo studies chloromethane exhibited cytotoxic rather than genotoxic effects.

 

 

Critical assessment of data on genotoxicity

Dekant and Colnot (2013) assessed recently for the members of the Methyl chloride REACh consortium the data on mutagenicity. Considering the above summarized genotoxicity data, Dekant and Colnot (2013) concluded, that only at excessively high and toxic concentrations, chloromethane induces genotoxicity in bacteria and mammalian cells in-vitro. Available information indicates that chloromethane exposure does not result in DNA alkylation (i.e. there is no evidence of methylated products). Therefore, it is unlikely that a direct genotoxic activity of chloromethane (for further details please refer to the expert review by Dekant (2015) attached in IUCLID Section 13) contributes to tumor formation, which is due to the high concentration of cytotoxic intermediates formed in the target organ by male mouse specific mechanism.

 

SCOEL (2017) also summarized in their recommendation the genotoxicity data as follows: “Chloromethane is weakly mutagenic in in vitro tests; in vivo, however, genotoxicity effects are noticed only at very high and already toxic doses. But there is no evidence of DNA alkylation by chloromethane in vivo. The likely reason for this discrepancy is the rapid metabolism of chloromethane in vivo.

 

References:

Andrews, A.W. et al. (1976) A comparison of the mutagenic properties of vinyl chloride and methyl chloride.Mutation Research, 40(3):273-276 [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

Haskell Laboratories, E.I. Dupont de Nemours and Company; Health and Safety Study Report (1978), EPA Document No. 877800204, Fiche No. OTS0200336 [as cited in: OECD SIDS Chloromethane, 2004].

Peter, H. et al. (1985). DNA-binding assay of methyl chloride. Archives of Toxicology; 57(2):84-87 [as cited in: OECD SIDS Chloromethane, 2004].

Ristau, C. et al. (1990). Formation and repair of DNA lesions in kidneys of male mice after acute exposure to methyl chloride. Archives of Toxicology; 64(3):254-256 [as cited in: OECD SIDS Chloromethane, 2004].

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

Vaughan, P. et al. (1993). Induction of the adaptive response of Escherichia coli to alkylation damage by the environmental mutagen, methyl chloride. Mutation Research; 293(3):249-257 [as cited in: OECD SIDS Chloromethane, 2004].

Justification for selection of genetic toxicity endpoint
No study was selected. Hazard assessment is based on the weight of evidence of all available studies.

Short description of key information:
IN VITRO
Bacterial reverse mutation assay (Gericke, 1977): positive
Mammalian cell gene mutation assay (Fostel et al., 1985): positive
Sister chromatid exchange assay in mammalian cells (Fostel et al., 1985): positive
IN VIVO
Unscheduled DNA synthesis (Working et al. 1986): negative
DNA binding study (Peter et al. 1985): negative
DNA-protein cross link (alkaline elution) (Ristau et al., 1989): ambigous (indications in kidneys of male mice (B6F3C1) directly after exposure, effect transient due to repair mechanisms)
Dominant lethal assay (Chellman et al. 1986; Working et al. 1985): negative (positive effects due to cytotoxicity)

Endpoint Conclusion: No adverse effect observed (negative)

Justification for classification or non-classification

The available data is conclusive but not sufficient for classification according to DSD (67/548/EEC) and CLP (1272/2008/EC).