Evaluation of the Developmental Toxicity of Caffeine and Caffeine Metabolites using the Frog Embryo Teratogenesis Assay—Xenopus (FETAX)

The developmental toxicities of caffeine and 13 metabolites, including theophylline, and paraxanthine and a synthetic methylxanthine analogue 3-isobutyl-methylxanthine (IBMX) were evaluated using the Frog Embryo Teratogenesis Assay—Xenopus (FETAX). Young X. laevis embryos were exposed to these compounds in each of two separate concentration–response experiments with and without an exogenous metabolic activation system (MAS). Results obtained from these studies indicated that relative teratogenic potencies of caffeine and each of its di- and monomethylxanthine metabolites were similar. Representatives of both the substituted uric and uracil metabolites were less developmentally toxic on an equimolar basis than the methylxanthines, suggesting that they may have represented detoxification metabolites. IBMX, a phosphodiesterase inhibitor also known to be an adenosine receptor antagonist, was the most potent developmental toxicant of the materials evaluated. In conclusion, none of the caffeine metabolites tested was found to be significantly more potent than caffeine itself in the FETAX assay.