EVALUATION OF THE GENETIC TOXICITY OF THE PEROXISOME PROLIFERATOR ANDCARCINOGEN METHYL CLOFENAPATE, INCLUDING ASSAYS USING MUTA(TM)MOUSE AND BIG BLUE(TM) TRANSGENIC MICE

The rodent liver carcinogen and hepatic peroxisome proliferator methyl clofenapate (MCP) has been evaluated for genetic toxicity in a range o f 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 preincubation assays), clastogenicity to c ultured human lymphocytes and CHO cells (+/- S9 mix), a mouse bone mar row micronucleus assay (24h and 48h sampling), a rat liver assay for U DS in vivo (12h sampling), assays for lac I (Big Blue(TM)) and lac Z ( Muta(TM) Mouse) mutations in the liver of transgenic mice, and an assa y of the ability of MCP to modify the mutagenicity to the liver of dim ethylnitrosamine 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 l evel. These nine daily administrations were shown to double the weight of the liver of non-transgenic, Big Blue(TM) and Muta(TM) Mice, as we ll as leading to a dramatic proliferation of peroxisomes (electron mic roscopy) in the livers of each strain, These changed parameters had re turned to control levels when the mutation analyses were conducted (10 days after the final dose of MCP). Despite the liver enlargement obse rved following MCP administration, no evidence of mitotic activity was observed in treated livers, although an increased number of cells wer e undergoing replicative DNA synthesis during the final 3 days of the 9 days of administration (BUdR assessment of S-phase). Liver biochemis try parameters (ALT, AST, AP, CK, GGT and albumin) were unaffected by the chronic (9 day) administration of MCP indicating an absence of hep atic toxicity. These combined observations favour a non-genotoxic mech anism of action for the hepatic carcinogenicity of MCP. The clastogeni city in vitro of the peroxisome proliferator Wyeth 14,643 has been con firmed in CHO cells, but it is noted that this chemical is more solubl e than is MCP. In particular, at the highest dose level at which MCP c ould be tested, Wy 14,643 was also nonclastogenic.