National Industrial Chemicals Notification and Assessment Scheme

12.3.5Reproductive effects

Reproductive effects following oral administration have been well investigated in animals. Oral administration resulted in reduced sperm motility and slight reductions in neonatal bodyweight and survival in mice at doses at which general toxic effects were produced. In rats there was a reduction in the number of litters born and in the litter size. General toxic effects were also observed at these levels.

Several developmental studies have been conducted in animals. In inhalation studies on rats, mice and rabbits no clear evidence of developmental toxicity was reported at doses up to 1800 ppm. A series of oral studies from one laboratory suggest that trichloroethylene may induce developmental neurotoxicity following maternal exposure.

Classification

Trichloroethylene does not meet the Approved Criteria for teratogenicity nor the EC Criteria (European Commission Directive 93/21/EEC, 1993 27 April,) for effects on fertility or developmental toxicity as animal studies do not provide any evidence of fertility or developmental effects.

12.3.6Genotoxicity

Trichloroethylene in the vapour state tested positive in several bacterial assays using Salmonella typhimurium in the presence of metabolic activation. In several fungal studies, trichloroethylene tested positive to Saccharomyces cerevisiae, also in the presence of metabolic activation. Trichloroethylene also tested positive in a mouse lymphoma gene mutation assay and induction of unscheduled DNA synthesis (UDS) was reported in several studies. On this evidence, trichloroethylene can be regarded as weakly mutagenic in vitro.

In a host-mediated gene mutation assay in mice, trichloroethylene tested positive to S. cerevisiae in the kidneys and liver but not the lungs

In somatic cell studies in vivo, both positive and negative results were obtained in rat and mouse micronucleus tests, with some doubts about two of the studies with positive results. Negative results were obtained in rat and mouse studies for chromosomal aberrations, sister chromatid exchange and UDS, however, trichloroethylene induced DNA single strand breaks in the liver of rats and mice in one study, and in mouse liver and kidneys in a second study. A mouse spot test was equivocal, however, a preliminary test for mouse pink-eyed unstable mutation was clearly positive. In germ cell assays, dominant lethal tests were either negative or inconclusive.

Studies conducted in occupationally exposed groups have been considered to be inconclusive. These studies had limitations such as small group size and potential confounders not being considered. A study investigating somatic mutations in the von Hippel-Lindau (VHL) gene obtained from tumour tissue of patients with renal cell cancer reported that trichloroethylene specifically acts on the VHL gene which has been identified as a renal tumour suppressor gene. It is possible that renal cell cancers following trichloroethylene exposure develop with somatic mutation of the tumour suppressor gene being one of the events. The findings of this report are preliminary as all the VHL genes had not been confirmed by sequencing. Limitations of this study include exposure not being determined precisely for each individual, cases not selected from a well-defined study base and controls were not selected for the same base. Further work is underway in Europe to confirm the effects of trichloroethylene on the VHL gene.

Classification

Trichloroethylene meets the Approved Criteria for classification as a category 3 mutagen (R40M3) on the basis of

positive results in a variety of tests in somatic cells in vivo, described above;

and, supported by:

the study of mutations in VHL tumour suppressor gene;

positive results from a number of in vitro mutagenicity assays; and,

mutagenicity of known metabolites.

According to the Approved Criteria category 3 are those substances which cause concern for humans owing to possible mutagenic effects, but in respect of which available information does not satisfactorily demonstrate heritable genetic damage (Appendix 4).

12.3.7Carcinogenicity

A high incidence of renal cancer in workers from a cardboard factory was reported by Henschler et al (1995) following exposure to high levels of trichloroethylene for long periods. This study appears to be a cluster investigation and has some weaknesses. The findings of this study are supported by a case control study by Vamvakas et al (cited in Deutsche Forchungsgemeinschaft, 1996) who have demonstrated an association between renal cell carcinomas and occupational exposure to trichloroethylene. The findings of these two studies are limited by weaknesses in the design of the studies, however they cannot be dismissed.

In animals trichloroethylene induces tumours at several sites and in different species. Tumours have been seen in mouse liver and lung and rat kidney and testis.

Studies have shown that mouse liver tumours are likely to be due to peroxisome proliferation induced by the metabolite trichloroacetic acid. Trichloroacetic acid does not induce peroxisome proliferation in human hepatocytes.

Mouse lung tumours are also thought to be related to the metabolism of trichloroethylene. Green et al (1997) have demonstrated that chloral hydrate levels were twenty times higher in mouse lung microsomal incubates than in rat lung microsomes and could not be detected in human lung incubates. Rat lung cytosol was found to be most active in metabolising chloral hydrate to trichloroethanol in this study, followed by mouse lung and then human lung. This study provides evidence that accumulation of chloral hydrate is unlikely to occur in human lung.

Testicular tumours were observed only in one strain of rats with a high incidence in the control group. These tumours are rare in men and are often associated with peroxisomal proliferators.

Renal cytotoxicity was observed in rodent studies with trichloroethylene at concentrations or doses that did not cause renal tumours. Renal tumours were observed in rats, only in the presence of cytotoxicity at very high concentrations of trichloroethylene. It has been proposed that a likely mechanism of renal tumours seen in rats exposed to trichloroethylene is repeated cytotoxicity and regeneration (United Kingdom, 1996).

The mechanism by which trichloroethylene causes rat kidney cytotoxicity is still unclear. It has been postulated that cytotoxicity could be due to formation of the metabolite dichlorovinyl cysteine (Henschler 1995). Dichlorovinyl cysteine has been identified in the urine of workers exposed to 50 ppm of trichloroethylene. Renal tumours have been reported in one study in workers exposed occupationally to high levels of trichloroethylene. However, other well conducted epidemiological studies failed to show an association between occupational exposure to trichloroethylene and renal cancer under the conditions of exposure in these studies.

A recent study has assessed quantitatively the metabolic pathway leading to the formation of dichlorovinyl cysteine in rats in vivo and in rats, mice and humans in vitro (Green et al, 1997a). The in vitro studies have shown that the rate of conjugation of trichloroethylene with glutathione is higher in the mouse (2.5 pmol/min/mg protein) than in the rat (1.6 pmol/min/mg protein) and is very low in human liver (0.02-0.37 pmol/min/mg protein). The  lyase activity in rat kidney was found to be ten-fold greater than in the mouse and the metabolic clearance through this pathway was found to be greater in rat kidney than in human kidney. In vivo studies have shown that the mouse is more sensitive to the nephrotoxic effects of DCVC than rats.

Green (1997) have postulated an alternative mechanism for the renal toxicity of trichloroethylene. Rats administered trichloroethylene, trichloroethanol and trichloroacetic acid excreted high levels of formic acid. This was also observed in mice exposed to trichloroethylene, though the amount of formic acid was lower than in rats. Formic acid excretion may be responsible for renal toxicity. Formic acid is not a metabolite of trichloroethylene and the source of formic acid needs to be studied further.

The mechanism of renal toxicity is being investigated further by several workers. Renal toxicity in rats is considered to be of concern to human health until the mechanism is elucidated.

Classification

Trichloroethylene meets the Approved Criteria for classification as a Carcinogen Category 2 (National Occupational Health and Safety Commission (NOHSC), 1994), that is, a substance regarded as if it is carcinogenic to humans, on the basis of the occurrence of tumours in experimental animals and limited evidence in workers. Thus the available data provides suspicions of carcinogenic potential in humans (R45).

The carcinogen classification categories adopted by the various countries/agencies are similar. There are five major categories. Most countries/agencies have the first three categories listed below and several have all five categories. Although the classifications are similar, the reader is referred to the relevant country/agency classification system for further information on the criteria and basis for inclusion of a chemical into the various categories.

The five categories in the classification of carcinogens include:

known human carcinogen;

probable human carcinogen;

possible human carcinogen;

not a human carcinogen and

insufficient information to classify.

The carcinogenicity of trichloroethylene has been reviewed recently by a number of countries and agencies principally with a view to classification. A brief description of the outcome of the reviews and the classification adopted by the countries/agencies is provided below.