Animal carcinogenicity studies: 3. Alternatives to the bioassay

Abstract

Conventional animal carcinogenicity tests take around three years to design, conduct and interpret. Consequently, only a tiny fraction of the thousands of industrial chemicals currently in use have been tested for carcinogenicity. Despite the costs of hundreds of millions of dollars and millions of skilled personnel hours, as well as millions of animal lives, several investigations have revealed that animal carcinogenicity data lack human specificity (i.e. the ability to identify human non-carcinogens), which severely limits the human predictivity of the bioassay. This is due to the scientific inadequacies of many carcinogenicity bioassays, and numerous serious biological obstacles, which render profoundly difficult any attempts to accurately extrapolate animal data in order to predict carcinogenic hazards to humans. Proposed modifications to the conventional bioassays have included the elimination of mice as a second species, and the use of genetically-altered or neonatal mice, decreased study durations, initiation-promotion models, the greater incorporation of toxicokinetic and toxicodynamic assessments, structure-activity relationship (computerised) systems, in vitro assays, cDNA microarrays for detecting changes in gene expression, limited human clinical trials, and epidemiological research. The potential advantages of non-animal assays when compared to bioassays include the superior human specificity of the results, substantially reduced time-frames, and greatly reduced demands on financial, personnel and animal resources. Inexplicably, however, the regulatory agencies have been frustratingly slow to adopt alternative protocols. In order to decrease the enormous cost of cancer to society, a substantial redirection of resources away from excessively slow and resource-intensive rodent bioassays, into the further development and implementation of non-animal assays, is both strongly justified and urgently required.