Regulatory agency considerations and requirements for validation of toxicity test alternatives

When developing an alternative toxicity test, one must first determine whether the alternative assay is to be used as a screen or as a replacement for the traditional toxicity test. An assay used as a screen will require less stringent acceptance criteria, for it is designed to answer fewer and less complex questions (e.g., the assessment of only potential teratogenicity). An assay used as a replacement will be used to establish hazard or lack thereof (safety). In other words, a replacement assay must clearly establish whether or not a chemical is a teratogen. One should also have knowledge of and experience with the in vivo assay to be replaced. This knowledge should be of not only the procedural aspects of the test but also the regulatory information it provides (i.e., how the results are used for hazard determination). Thorough consideration of the regulatory information is critical for a test intended to be used as a replacement. Validation should include intralaboratory and interlaboratory reproducibility of results from a standard protocol, an assessment of the qualitative and quantitative aspects of the test responses, and the use of a sufficient number of chemicals representative of the defined category of interest.