Assessment of the genotoxicity of trichloroethylene and its metabolite, S-(1,2-dichlorovinyl)-L-cysteine (DCVC), in the comet assay in rat kidney.

Trichloroethylene (TCE) has been reported to give a small, but significant, increase in renal tumours in the rat. These tumours were always associated with nephrotoxicity which is most likely caused by the metabolism of TCE to S-(1,2-dichlorovinyl)-L-cysteine (DCVC) which accumulates in the proximal tubules. The genotoxicity of TCE and DCVC have been evaluated in vivo using the comet assay to assess DNA breakage in the proximal tubules of rat kidneys. Rats were exposed to TCE by inhalation or to DCVC by oral gavage at dose levels in excess of those which produced effects in long-term bioassays. Cell suspensions were produced from proximal tubules isolated from the kidneys of treated rats and the level of DNA damage assessed in these cells using the pH >13 comet assay. In vitro and in vivo positive controls were included and demonstrated the sensitivity of the assay. TCE gave a clearly negative response in the assay at all dose levels as did DCVC at the 16-h sampling time and at the 2-h sampling time with the lower dose level. At the 2-h sampling time following administration of DCVC at the higher dose level (10 mg/kg), there was limited evidence of DNA damage in a small number of animals, but this was considered insufficient to indicate a positive response in this assay. These data support an overall conclusion, based on these and other published data, that the renal tumours seen in bioassays are non-genotoxic in origin.

Mouse lymphoma thymidine kinase gene mutation assay: meeting of the International Workshop on Genotoxicity Testing, San Francisco, 2005, recommendations for 24-h treatment.

The Mouse Lymphoma Assay (MLA) Workgroup of the International Workshop on Genotoxicity Testing (IWGT), comprised of experts from Japan, Europe and the United States, met on September 9, 2005, in San Francisco, CA, USA. This meeting of the MLA Workgroup was devoted to reaching a consensus on issues involved with 24-h treatment. Recommendations were made concerning the acceptable values for the negative/solvent control (mutant frequency, cloning efficiency and suspension growth) and the criteria to define an acceptable positive control response. Consensus was also reached concerning the use of the global evaluation factor (GEF) and appropriate statistical trend analysis to define positive and negative responses for the 24-h treatment. The Workgroup agreed to continue their support of the International Committee on Harmonization (ICH) recommendation that the MLA assay should include a 24-h treatment (without S-9) in those situations where the short treatment (3-4 h) gives negative results.

Styrene monomer does not induce unscheduled DNA synthesis in the mouse liver following inhalation exposure.

Styrene monomer is a commercially important chemical used extensively in the production of plastics. It has been shown to induce lung tumours in the mouse via the inhalation route. Styrene monomer has shown a low reactivity with DNA and also a lack of genotoxic response in the mouse lung in vivo. Together with the fact that the mouse lung tumours were late occurring and mostly benign, which suggest a promotional effect rather than initiation, these factors have led to a suggestion that the tumours may not be of genotoxic origin. The studies examining the genotoxicity of styrene monomer in vivo have to date been predominantly cytogenetic assessments, although low levels of DNA adducts have been reported in the mouse liver and lung. In order to extend this evaluation, a mouse liver unscheduled DNA synthesis study has been performed to assess the ability of styrene monomer to induce DNA damage/repair. The negative response observed in this assay is consistent with the theory that tumours observed in mouse oncogenicity studies are non-genotoxic in origin.