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Toxicological information

Carcinogenicity

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Description of key information

Human studies support a conclusion of no substantive cancer risk, while animal carcinogenicity studies with rats, mice and hamsters only demonstrate carcinogenic potential of dichloromethane in the mouse (lungs and liver) via the inhalation but not the oral route. Mechanistic evidence shows that dichloromethane is a threshold carcinogen starting the carcinogenic process via GST pathway activity. In humans, the dichloromethane metabolising GST levels are even lower than in rats and hamsters. In those species, no treatment related malignant lung and liver tumours were observed up to a 2-year exposure to 12,400 mg/m3. At lower chronic exposures, non-neoplastic effects were observed. The benign mammary tumours found in rats are plausibly related to hyperprolactinaemia, which may be relevant to humans but represents a non-genotoxic, threshold-mediated mode of action.Therefore, the carcinogenic effects of dichloromethane in humans are not critical relative to the non-neoplastic effects.

MAK (2016) concluded: After long-term inhalation exposure to dichloromethane concentrations of 1000 ppm, benign mammary tumours occurred in rats, and liver and lung tumours in mice. The metabolism of dichloromethane via the reductive metabolic pathway, catalyzed by GSTT1 -1), produces a reactive metabolite, probably S-(chloromethyl)glutathione, which has genotoxic effects and is thought to be responsible for the carcinogenic effects of the substance in animals. Carcinogenic effects in humans could, to date, not be demonstrated.

Key value for chemical safety assessment

Carcinogenicity: via oral route

Link to relevant study records
Reference
Endpoint:
carcinogenicity: oral
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
according to
Guideline:
EU Method B.32 (Carcinogenicity Test)
GLP compliance:
not specified
Species:
rat
Strain:
Fischer 344
Sex:
male/female
Route of administration:
oral: drinking water
Vehicle:
water
Analytical verification of doses or concentrations:
yes
Duration of treatment / exposure:
104 weeks
Frequency of treatment:
daily
Post exposure period:
6 months for the recovery group
Remarks:
Doses / Concentrations:
0, 5, 50, 125, 250, 250 (recovery, 18 months exposure)
Basis:
nominal in water
Remarks:
Doses / Concentrations:
0, 6, 52, 125, 235, 232 (recovery, 18 months exposure) mg/kg bw/day (males)
Basis:
actual ingested
Remarks:
Doses / Concentrations:
0, 6, 58, 136, 263, 269 (recovery, 18 months exposure) mg/kg bw/day (females)
Basis:
actual ingested
No. of animals per sex per dose:
85 (main experiment), 25 (high dose, recovery), 50 (recovery control group)
Control animals:
yes, concurrent vehicle
Based on:
test mat.
Sex:
male/female
Basis for effect level:
other: No treatment-related tumours were found up to the highest level of 250 mg/kg bw

Pathology and tumour data

In the histopathological examinations, no treatment-related effects were noted in any of the tissues examined, with the exception of the liver. Histomorphological alterations of the liver were observed and are presented inTable 1. A dose-related positive trend in the incidences of foci/areas of cellular alteration was noted in both sexes treated with DCM. Group comparisons were statistically significant at all levels except 5 mg/kg/day. This finding was first noted after treatment for 78 wk and progressed until wk 104. In addition, an increased incidence of fatty change in the liver was observed in tissues stained with haematoxylin and eosin, and was confirmed by staining with Oil Red O. This effect was generally observed in both sexes treated with DCM levels of 50, 125 or 250 mg/kg/day at both wk 78 and wk 104. An increase in the incidence of foci/areas of cellular alteration, similar to that noted in the 50-250 mg/kg/day group, was observed in the recovery group receiving 250 mg/kg/day. However, the fatty change was less severe in the recovery group than in the group receiving 250 mg DCM/kg/day for 104 wk.

Table 1. Incidence of liver foci/areas of alteration in rats given dichloromethane (DCM) in the drinking-water for up to 2 yr.

 

 

Incidence of lesion* in rats given a DCM target dose (mg/kg/day) of:

Sex

0

0

5

50

125

250

250**

Wk78

Males

7/20

--

3/20

15/20

13/20

20/20

--

Females

3/20

--

11/20

14/20

16/20

17/20

--

Wk 104

Males

27/36

25/40

22/34

35/38

34/35

40/41

15/15

Females

17/31

17/36

12/29

30/41

34/38

31/34

17/20

 *No. of rats affected/no. examined. **Recovery group.Bold: statistically significant differences with control group.

With the exception of neoplastic nodules in the recovery group, males treated with DCM generally exhibited significantly lower incidences of neoplastic nodules and hepatocellular carcinomas than control groups. Conversely, both neoplastic nodules and hepatocellular carcinomas were noted in females receiving DCM, while a zero incidence for both of these lesions was observed in the control groups (see Table 2). However, there were no significant treatment-related increases in the incidence of hepatocellular carcinomas in any of the treated female groups, although the elevated incidence of neoplastic nodules, and of neoplastic nodules and hepatocellular carcinomas combined in females receiving DCM, compared to the zero incidence in controls, showed a significant positive trend. This trend, however, was inconsistent, since it lost monotonicity at the second highest dose (125 mg/kg/day), at which a single neoplastic nodule was found, the same incidence as in the lowest dose group (5 mg/kg/day).

Table 2.Liver tumour incidence for female rats given DCM in the drinking water for 2 yr

 

Target dose level (mg/kg bw/day)

 

0

0

5

50

125

250

250*

Number examined

85

50

85

83

85

85

25

Neoplastic nodules (NN)

0 (0.0)

0 (0.0)

1 (1.2)

2 (2.4)

1 (1.2)

4 (4.7)

2 (8.0)

Hepatocellular carcinomas (HC)

0 (0.0)

0 (0.0)

0 (0.0)

2 (2.4)

0 (0.0)

2 (2.4)

0 (0.0)

NN + HC

0 (0.0)

0 (0.0)

1 (1.2)

4 (4.8)

1 (1.2)

6 (7.1)

2 (8.0)

Between parentheses: percentage

The relevance and toxicological significance of the increase in neoplastic nodules and hepatocellular carcinomas in females should be examined further in terms of historical control data and dose-response relationships. Based upon the histopathological examination of 324 untreated female Fischer 344 rats for 104 wk at Hazleton Laboratories, the average historical incidences of neoplastic nodules and hepatocellular carcinomas were 6.3 and 1.7%, respectively. Individual control incidences ranged from 0-13 and 0-3% for neoplastic nodules and hepatocellular carcinomas, respectively. On the basis of published results (Goodman, Ward, Squire et al.1979), the historical incidences of neoplastic nodules and hepatocellular carcinomas were 2.7 and 0.39%, respectively. In both instances, a low incidence of both tumour types was reported. Compared with these historical data, the females in the two control groups and in two treated groups (5 and 125 mg/kg/day) of this study exhibited unusually low incidences of these tumour types. Furthermore, although the positive trend is statistically significant, the dose-response curve loses monotonicity at 125 mg/kg/day. On the basis of the unexpectedly low incidence of neoplastic nodules and hepatocellular carcinomas in the female control groups and the absence of a comparable increase in these tumour types in females dosed at 125 mg/kg/day, the increase in liver tumours observed in females given 50 or 250 mg/kg/day is not considered to be biologically meaningful in terms of DCM administration.

 Reference cited

Goodman, D. G., Ward, J. M., Squire, R. A., Chu, K. C., and Linhart, M. S. (1979). Neoplastic and nonneoplastic lesions in aging F344 rats. Toxicol.Appl.Pharmacol.48, 237-248.

 

Conclusions:
No treatment-related tumours were found up to the highest level of 250 mg/kg bw.
Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
NOAEL
6 mg/kg bw/day
Study duration:
chronic
Species:
rat
Quality of whole database:
No treatment-related tumours were observed in both rats and mice. Quality of whole database is OK.

Carcinogenicity: via inhalation route

Link to relevant study records
Reference
Endpoint:
carcinogenicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
equivalent or similar to
Guideline:
OECD Guideline 451 (Carcinogenicity Studies)
GLP compliance:
not specified
Species:
mouse
Strain:
B6C3F1
Sex:
male/female
Details on test animals and environmental conditions:
TEST ANIMALS
- Source: Charles River Breeding Laboratories
- Age at study initiation: 8-9 weeks
- Housing: individually
- Diet: ad libitum (except during exposure periods)
- Water: ad libitum
- Acclimation period: 21 days

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 23-25
- Humidity (%): 45-65
- Air changes (per hr): 20
- Photoperiod (hrs dark / hrs light): 12/12

The B6C3F1 (C57BL/6N, female, X C3H/HeN MTV-, male) mice used in this study were produced under strict barrier conditions at Charles River Breeding Laboratories under a contract to the Carcinogenesis Program. Breeding stock for the foundation colonies at the production facility originated at the National Institutes of Health Repository. Animals shipped for testing were progeny of defined microflora-associated parents that were transferred from isolators to barrier-maintained rooms.
Animals were shipped to the testing laboratory at 4-6 weeks of age. The animals were quarantined at the testing facility for 3 weeks. Thereafter, a complete necropsy was performed on five animals of each sex and species to assess their health status.

Route of administration:
inhalation: vapour
Type of inhalation exposure (if applicable):
whole body
Vehicle:
unchanged (no vehicle)
Details on exposure:
Dichloromethane was vaporized at 38°-42° C, diluted with air, and introduced into the chambers.
Analytical verification of doses or concentrations:
yes
Details on analytical verification of doses or concentrations:
Concentrations in each exposure chamber were monitored 8-12 times per exposure period with a Hewlett-Packard 5840A Gas Chromatograph. The weekly mean vapor concentrations were within 10% of the target concentrations at all positions sampled within the chamber.
Duration of treatment / exposure:
102 weeks
Frequency of treatment:
6 h/d, 5 d/w
Post exposure period:
2 weeks
Remarks:
Doses / Concentrations:
0, 2000, and 4000 ppm
Basis:
other: target concentrations
Remarks:
Doses / Concentrations:
0, 2009, and 3982 ppm
Basis:
analytical conc.
No. of animals per sex per dose:
50
Control animals:
yes
Details on study design:
- Concentration selection rationale: based on deaths observed at 8400 ppm and the severity of histologic changes noted in mice exposed at 4000 ppm for13 weeks.
Groups of 50 mice of each sex were exposed to dichloromethane at concentrations of 0, 2,000, or 4,000 ppm 6 hours per day, 5 days per week for 102 weeks. During week 3 of the studies, the 2000 ppm mice (both sexes) were exposed at 1000 ppm.
Observations and examinations performed and frequency:
All animals were observed two times per day, and clinical signs were recorded once per week. Body weights by cage were recorded once per week for the first 12 weeks of the study and once per month thereafter. Mean body weights were calculated for each group.

Sacrifice and pathology:
Sacrifice:
Moribund animals were killed, as were animals that survived to the end of the study.
A necropsy was performed on all animals, including those found dead unless they were excessively autolyzed or cannibalized.

Pathology:
Examinations for grossly visible lesions were performed on major tissues or organs. Tissues were preserved in 10% neutral buffered formalin, embedded in paraffin, sectioned, and stained with hematoxylin and eosin.

Tissues examined microscopically:
gross lesions and tissue masses, regional lymph nodes, tracheobronchial lymph nodes, mandibular lymph nodes, salivary glands, sternebrae including marrow, thyroid gland, parathyroids, larynx, small intestine, colon, duodenum, liver, heart, trachea, prostate/testes or ovaries/uterus,
lungs and mainstem bronchi, skin, trachea gallbladder, kidneys, spleen, stomach, brain, thymus, colon, adrenal glands, urinary bladder, pituitary gland, nasal cavity/turbinates, mammary glands.

When the pathology examination was completed, the slides, individual animal data records, and summary tables were sent to an in-dependent quality assurance laboratory. Individual animal records and tables were compared for accuracy, slides and tissue counts were verified, and histotechnique was evaluated. All tumor diagnoses, all target tissues, and all tissues from a randomly selected 10% of the animals were evaluated by a quality assurance pathologist. Slides of all target tissues and those about which the original and quality assurance pathologists disagreed were submitted to the Chairperson of the Pathology Working Group (PWG) for evaluation. Representative coded slides selected by the Chairperson were re-viewed by PWG pathologists, who reached a consensus and compared their findings with the original and quality assurance diagnoses. When diagnostic differences were found, the PWG sent the appropriate slides and comments to the original pathologist for review. This procedure has been described, in part, by Maronpot and Boor-man (1982) and Boorman et al. (1985). The final diagnoses represent a consensus of contractor pathologists and the NTP Pathology Working Group. For subsequent evaluations, the diagnosed lesions for each tissue type are combined according to the guidelines of McConnell et al. (1986).

Nonneoplastic lesions are not examined routinely by the quality assurance pathologist or PWG. Certain nonneoplastic findings are reviewed by the quality assurance pathologist and PWG if they are considered part of the toxic response to a chemical or if they are deemed of special interest.
Statistics:
Survival Analyses:
the product-limit procedure of Kaplan and Meier (1958). Statistical analyses for a possible concentration-related effect on survival used the method of Cox (1972) for testing two groups for equality and Tarone's (1975) life table test for a concentration-related trend.

Analysis of Tumor Incidence:
Three statistical methods are used to analyze tumor incidence data.
- Life Table Analyses - the life table method of Cox (1972) and of Tarone (1975).
- Incidental Tumor Analyses - Haseman, (1984)
- Unadjusted Analyses-- the Fisher exact test for pairwise comparisons and the Cochran-Armitage linear trend test (Armitage, 1971; Gart et al., 1979)
For studies in which compound administration has little effect on survival, the results of the three alternative analyses will generally be similar. When differing results are obtained by the three methods, the final interpretation of the data will depend on the extent to which the tumor under consideration is regarded as being the cause of death. All reported P values for tumor analyses are one-sided.
Clinical signs:
effects observed, treatment-related
Mortality:
mortality observed, treatment-related
Body weight and weight changes:
effects observed, treatment-related
Food consumption and compound intake (if feeding study):
not examined
Food efficiency:
not specified
Water consumption and compound intake (if drinking water study):
not specified
Ophthalmological findings:
not specified
Haematological findings:
not specified
Clinical biochemistry findings:
not specified
Urinalysis findings:
not specified
Behaviour (functional findings):
not specified
Organ weight findings including organ / body weight ratios:
not specified
Gross pathological findings:
effects observed, treatment-related
Histopathological findings: non-neoplastic:
effects observed, treatment-related
Histopathological findings: neoplastic:
effects observed, treatment-related
Details on results:
CLINICAL SIGNS
During exposure periods, high dose female mice (and to a lesser extent, high dose male mice and low dose female mice) were hyperactive. During the second year of the study, high dose female mice were lethargic.

SURVIVAL
The survival of both dosed groups of male mice (low dose after week 101, high dose after week 89) was significantly lower than that of the controls, and the survival of the high dose group was significantly lower than that of the low dose group (P=0.016). The survival of the high dose group of female mice was significantly lower than that of both the controls (after week 98) and the low dose group (P<0.01).

BODY WEIGHT AND WEIGHT GAIN
The initial weight of the high dose male mice was 15% lower than that of the controls. The mean body weight of the high dose group was generally comparable to that of the controls until week 90, after which they were 8%-11% lower than those of the controls. The initial mean body weight of the high dose female mice was 7% greater than that of the controls and remained greater until week 51.
From week 51 to week 95, mean body weights of the high dose female mice were 0%-9% lower than those of the controls; at week 99, the mean body weight of the high dose female group was 17% lower than that of the controls.

PATHOLOGY, NEOPLASTIC OR NON-NEOPLASTIC LESIONS
Lung: Alveolar/bronchiolar adenomas, alveolar/bronchiolar carcinomas, and alveolar/bronchiolar adenomas or carcinomas (combined) in male and female mice occurred with significant positive trends, and the incidences in the dosed groups were significantly greater than those in the controls.

Liver: Cytologic degeneration was observed at increased incidences in high dose male mice and dosed female mice (male: control, 0/50; low dose, 0/49; high dose, 22/49, 45%; female: control, 0/50; low dose, 23/48, 48%; high dose, 21/48, 44%). Hepatocellular adenomas, hepatocellular carcinomas, and hepatocellular adenomas or carcinomas (combined) in male and female mice occurred with significant positive trends. The incidences of hepatocellular adenomas in high dose male and high dose female mice, hepatocellular carcinomas in high dose male and dosed female mice, hepatocellular adenomas and hepatocellular adenomas or carcinomas (combined) in dosed male mice, and hepatocellular adenomas or carcinomas (combined) in dosed female mice were significantly greater than those in the controls.

Circulatory System: Hemangiosarcomas in male mice occurred with a significant positive trend by the life table test; the incidence in the 4,000 ppm group was significantly greater than that in the controls in life table pairwise comparisons. The following incidences of hemangioma or hemangiosarcoma (combined) were observed in female mice: control, 0/50; low dose, 2/49 (4%); high dose, 2/49 (4%).

Testis: Testicular atrophy was observed at increased incidences in dosed male mice (control, 0/50; low dose, 4/50, 8%; high dose, 31/50, 62%).

Ovary and Uterus: Ovarian atrophy was observed at increased incidences in dosed female mice (control, 6/50, 12%; low dose, 28/47, 60%; high dose, 32/43, 74%). Uterine atrophy was observed at increased incidence in high dose female mice (control, 0/50; low dose, 1/48, 2%;
high dose, 8/47, 17%).

Kidney: The incidence of kidney/tubule casts was increased in high dose male mice (male: control, 6/50, 12%; low dose, 11/49, 22%; high dose, 20/50, 40%; female: control, 8/49, 16%; low close, 23/48, 48%; high dose, 23/47, 49%).

Stomach: Dilatation of the stomach was observed at an increased incidence in high dose male and female mice (male: control, 3/49, 6%; low dose, 7/47, 15%; high close, 9/49, 18%; female: control, 1/49, 2%; low dose, 2/47, 4%; high dose, 10/48, 21%).

Spleen: Atrophy of the splenic follicles was observed at increased incidence in high dose male mice (male: control, 0/49; low dose, 3/49, 6%; high dose, 7/48, 15%; female: control, 0/49; low dose, 0/48; high dose, 1/47, 2%).
















Dose descriptor:
LOAEC
Effect level:
2 000 ppm
Sex:
male/female
Basis for effect level:
other: increased incidences of lung and liver tumours

The survival of exposed males and high dose females was reduced relative to that of the chamber controls. This reduced survival may have been because of the high incidences of liver and lung neoplasia in dosed animals. Total deaths increased in a concentration-related manner during the final 16 weeks (male: control, 4/43, 9%; low dose, 9/37, 24%; high dose, 12/33, 36%; female: control, 10/36, 28%; low dose, 15/41, 37%; high dose, 19/31, 61%). Final mean body weights of high concentration group male and female mice were 10%-17% lower than those of the chamber controls. These differences in body weight occurred during the last 16 weeks of the studies.

Pulmonary Effects

Exposure to dichloromethane increased the incidences of alveolar/bronchiolar adenomas in both male and female mice (male: control, 3/50; low concentration, 19/50; high concentration, 24/50; female: control, 2/50; low concentration, 23/48; high concentration, 28/48) and carcinomas (male: control, 2/50; low concentration, 10/50; high concentration, 28/50; female: control, 1/50; low concentration, 13/48; high concentration, 29/48). The observed incidences of alveolar/bronchiolar neoplasia in the chamber control groups were lower than those reported for other chamber control groups at this laboratory and for untreated controls throughout the Program.

In addition to concentration-related increases in the number of male and female mice with lung tumors, there were concentration-related increases in the incidences of exposed animals bearing multiple lung tumors. No chamber control animal had more than one lung tumor, whereas 38% of all exposed male mice and 42% of all exposed female mice had multiple lung tumors; in those mice with lung tumors, 38/67 (57%) exposed males and 40/71(56%) exposed females had multiple lung tumors. Lung tumor multiplicity included both alveolar/bronchiolar adenomas and carcinomas. In cases of lung tumormultiplicity, it was not possible to differentiate definitively between multiple primary carcinomas and primary carcinomas with multiple intrapulmonary metastatic lesions. However, the presence of numerous cases of multiple adenomas (15/100 exposed males and 16/96 exposed females) suggests that some of the carcinomas were multiple primary lesions rather than metastatic lesions.

Hepatic Effects

Dichloromethane produced cytologic degeneration of the liver in both male and female mice; this change was not observed in chamber control animals. Exposure to dichloromethane increased the incidence of hepatocellular carcinomas and of adenomas or carcinomas (combined) in male mice exposed to dichloromethane at 4,000 ppm. In female mice, dichloromethane produced concentration-related increases in the incidences of both hepatocellular adenomas and hepatocellular carcinomas. The incidences of these tumors in the chamber control group were consistent with historical control incidences at this laboratory and in the overall Program.

As was the case for lung tumors in mice, multiplicity of hepatocellular tumors in dichloromethane-exposed male and female mice was common. The incidence of animals with multiple hepatocellular tumors was in-creased in both males and females in a concentration-related manner (male: control, 2/50; low concentration, 11/49; high concentration, 16/49; female: control, 0/50; low concentration, 3/48; high concentration, 28/48). Hepatocellular tumor multiplicity was found in 4% of the male chamber control mice and in none of the female chamber controls, whereas 28% of all exposed males and 32% of all exposed females had multiple liver tumors. In the chamber control groups, 2/22 (9%) hepatocellular tumor-bearing males and 0/3 (0%) hepatocellular tumor-bearing females had multiple liver tumors. In contrast, 27/57 (47%) liver tumor-bearing dichloromethane-exposed males and 31/56 (55%) liver tumor-bearing exposed females had hepatocellular tumor multiplicity.

In earlier studies, the National Coffee Association (1983) found no association between dichloromethane administration and liver tumors in mice. In the studies conducted by Burek et al. (1984) and by Nitschke et al. (1982), mice were not used as experimental animals.

Other Effects

Increased incidences of testicular atrophy in males and ovarian and uterine atrophy in females were detected in dichloromethane exposed mice. These changes may be secondary to the extensive lung and liver neoplasia produced by the inhalation exposures.

An increase in the incidence of hemangiomas or hemangiosarcomas (combined) was detected in high concentration male mice (control, 2/50; low concentration, 2/50; high concentration, 6/50). Five of the six tumors in the high dose group were hemangiosarcomas of the liver. The apparent increase in hemangiosarcomas was not considered to be clearly compound related.

Conclusion: Under the conditions of these inhalation studies, there was clear evidence of carcinogenicity of dichloromethane for male and female B6C3F1 mice, as shown by increased incidences of alveolar/bronchiolar neoplasms and of hepatocellular neoplasms.

Conclusions:
Conclusion: Under the conditions of these inhalation studies, there was clear evidence of carcinogenicity of dichloromethane for male and female B6C3F1 mice, as shown by increased incidences of alveolar/bronchiolar neoplasms and of hepatocellular neoplasms.
Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
LOAEC
7 000 mg/m³
Study duration:
chronic
Species:
mouse
Quality of whole database:
Increased incidence of lung and liver tumours in mice at 2000 ppm. Quality of whole database is OK.

Carcinogenicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available
Quality of whole database:
No studies available.

Additional information

Humans

Oral and dermal

No oral and dermal human carcinogenicity studies were reported, although some dermal exposure cannot be excluded in the occupational exposure studies summarised below.

 

Inhalation

From occupational studies it was concluded that no strong or consistent finding for any site of cancer was apparent despite several studies of large occupational cohorts of workers potentially exposed to high concentrations of dichloromethane. Sporadic and weak associations were reported for cancers of the pancreas, liver and biliary passages, breast and brain. The results of these human studies cannot completely rule out the possibility

of carcinogenic effects caused by dichloromethane and there is some evidence from animal studies that it may cause cancer, based either on a metabolism less relevant for humans or by a non-genotoxic, threshold-mediated mode of action.

Animals

Oral

In two chronic drinking water studies, one with rats and one with mice no increased tumour incidences were observed.

Dermal

No dermal animal carcinogenicity studies were reported.

 

Inhalation

Several inhalation carcinogenicity studies were performed in rats, mice and hamsters. In the mouse malignant lung and liver tumours were observed. In rats, dichloromethane induced increased incidences of mammary gland neoplasms.

Mechanistic studies have shown that glutathione-S-transferase-mediated metabolism of dichloromethane -producing reactive intermediates that are held responsible for the liver and lung tumour formation- is expressed to a greater extent in mouse tissues than in rat, hamster or human tissues, explaining the development of liver and lung tumours in mice. Mechanistic studies in rats demonstrating dichloromethane- induced elevation of serum prolactin provide evidence that mammary tumours found in rats are plausibly related to hyperprolactinaemia. In addition, dichloromethane does not show alkylation of DNA nor DNA binding, both in vitro as well as in vivo. It should be noted, however, that binding to proteins and lipids in vitro, and DNA-protein cross links in mouse liver were observed. The DNA-protein cross links are considered having a causal relationship with the mouse GST pathway activity, which probably starts the carcinogenic process. As this mode of action does not involve inheritable changes in DNA sequences, it is epigenetic in nature. Therefore, taking into account the negative in vivo genotoxicity test results, the lack of DNA-binding by dichloromethane and the sequence of events leading to dichloromethane carcinogenesis, the mode of action of dichloromethane carcinogenicity probably occurs via a non genotoxic pathway. As such, a threshold approach may be assumed.

In vitro investigations have demonstrated only mouse hepatocytes form detectable amounts of DNA-protein cross links when incubated with dichloromethane, while those of rats, Syrian golden hamsters and humans do not. Furthermore, the markedly lower levels of the dichloromethane metabolising GST in rats and hamsters are consistent with the lack of liver and lung tumours in these species. The levels of this enzyme in humans are even lower than found in rats and hamsters. Therefore, humans are probably less sensitive to the carcinogenesis induced by dichloromethane. This is corroborated by the fact that no epidemiological study so far has shown a link between dichloromethane exposure and increased tumour incidence. Therefore, the mouse is not a quantitatively good model for carcinogenesis of dichloromethane in humans. Rat and hamster are better models, as their sensitivity in this respect seems to be comparable to that of humans, based on their GST activity.

In rats, in contrast, mammary gland tumours tumours were found following inhalation exposure; it also resulted in elevation of serum prolactin levels. Hyperprolactinaemia is associated with CNS depression and stress in rodents (discussed in Dow, 1986), evidence of which was noted in both male as well as female rats in this study. Given the increase in serum prolactin observed in females, it is not unreasonable to conclude that similar findings may have been noted in males exposed to dichloromethane under the same circumstances. Hyperprolactinaemia is associated with increased incidence of mammary tumours in rodents and this is a plausible mode of action for the induction of mammary tumours in rats by dichloromethane.

 

Justification for selection of carcinogenicity via oral route endpoint:

Well performed chronic drinking water studies in rats and mice (Serota et al., 1986).

Justification for selection of carcinogenicity via inhalation route endpoint:

Well performed chronic inhalation study in mice (and rats).

Justification for selection of carcinogenicity via dermal route endpoint:

No long term repeated dermal toxicity studies available.

Carcinogenicity: via oral route (target organ): digestive: liver

Carcinogenicity: via inhalation route (target organ): digestive: liver; respiratory: lung

Justification for classification or non-classification

Inhalation exposure to dichloromethane resulted in tumors in the lung and liver in mice. No such tumours were found in rats. Mechanistic studies have shown glutathione-S-transferase-mediated metabolism of dichloromethane (producing reactive intermediates held responsible for the liver and lung tumour formation), is utilized to a greater extent in mouse tissues than in rat, hamster or human tissues, explaining the development of liver and lung tumours in mice. In rats, benign tumours in the mammary gland were found. Mechanistic studies in rats demonstrating dichloromethane-induced elevation of serum prolactin provide evidence that the benign mammary tumours found in rats are plausibly related to hyper-prolactinaemia. This may be relevant to humans but represents a non-genotoxic, threshold-mediated mode of action. No consistent relationships between dichloromethane and cancer at any particular site have been reported in studies of workers exposed to dichloromethane (see section 7.10).

Overall, according to the EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) Regulation (EC) No. 1272/2008, based on the current data set in animals and humans, this would correspond to“Carcinogenicity Cat. 2” and H351 (Suspected of causing cancer).