Evaluation of the Genetic Toxicity of the Peroxisome Proliferator and Carcinogen Methyl Clofenapate, Including Assays Usin Muta TM Mouse and Big Blue™ Transgenic Mice

Abstract

The rodent liver carcinogen and hepatic peroxisome proliferator methylclofenapate (MCP) has been evaluated for genetic toxicity in a range of in vitro and rodent genotoxicity assays. It gave a negative response in each of the following assays: mutagenicity to S. typhimurium and E. coli (+/- S9 mix, plate and pre-incubation assays), clastogenicity to cultured human lymphocytes and CHO cells (+/- S9 mix), a mouse bone marrow micronucleus assay (24h and 48h sampling), a rat liver assay for UDS in vivo (12h sampling), assays for lac I (Big Blue) and lac Z (Muta Mouse) mutations in the liver of transgenic mice, and an assay of the ability of MCP to modify the mutagenicity to the liver of dimethylnitrosamine in both transgenic mutation assays. The micronucleus and UDS assays were conducted using a single administration of MCP at its maximum tolerated dose, while the transgenic assays were conducted using nine daily administrations of MCP at its cancer bioassay dose level. These nine daily administrations were shown to double the weight of the liver of non-transgenic, Big Blue and Muta Mice, as well as leading to a dramatic proliferation of peroxisomes (electron microscopy) in the livers of each strain. These changed parameters had returned to control levels when the mutation analyses were conducted (10 days after the final dose of MCP). Despite the liver enlargement observed following MCP administration, no evidence of mitotic activity was observed in treated livers, although an increased number of cells were undergoing replicative DNA synthesis during the final 3 days of the 9 days of administration (BUdR assessment of S-phase). Liver biochemistry parameters (ALT, AST, AP, CK, GGT and albumin) were unaffected by the chronic (9 day) administration of MCP indicating an absence of hepatic toxicity. These combined observations favour a non-genotoxic mechanism of action for the hepatic carcinogenicity of MCP. The clastogenicity in vitro of the perixisome proliferator Wyeth 14,643 has been confirmed in CHO cells, but it is noted that this chemical is more soluble than is MCP. In particular, at the highest dose level at which MCP could be tested, Wy 14,643 was also non-clastogenic.