Optimization of an exogenous metabolic activation system for FETAX. I. Post-isolation rat liver microsome mixtures

The developmental toxicity of cyclophosphamide, coumarin, 2-acetyl-aminofluorine (2-AAF), and trichloroethylene (TCE) was assessed with Frog Embryo Teratogenesis Assay: Xenopus (FETAX). Late Xenopus laevis blastulae were exposed to each test material for 96-h in two separate static-renewal tests with and without the presence of five differently induced exogenous metabolic activation systems (MAS). The MAS consisted of Aroclor 1254- (Aroclor 1254 MAS), isoniazid- (INH MAS), phenobarbital- (PB MAS), or beta-naphthoflavone- (beta-NF MAS), or a post-isolation mixture (mixed MAS) of INH-, PB-, and beta-NF-induced rat liver microsomes. Addition of the Aroclor 1254 MAS bioactivated cyclophosphamide, coumarin, 2-AAF, but not TCE. Addition of the PB MAS bioactivated cyclophosphamide, weakly bioactivated coumarin and 2-AAF, but had no effect on TCE developmental toxicity. The beta-NF MAS bioactivated coumarin and 2-AAF, weakly bioactivated cyclophosphamide, but did not alter the developmental toxicity of TCE. Addition of the INH-induced MAS only bioactivated TCE, whereas the post-isolation mixed MAS bioactivated each test material. Based on LC50 and EC50 (malformation) values, embryo growth, and types and severity of induced malformations, each test material was developmentally toxic. Use of post-microsome isolation mixtures from differentially induced rat livers increased the efficacy of the exogenous MAS routinely used by FETAX.