Statistical Review and Evaluation of in Vitro Mutagenicity Study Data *

Abstract

This paper deals with the procedures used in the statistical review and evaluation of in vitro mutagenicity assay data from a regulatory reviewer's perspective. Emphasis is placed on those used in analyzing revenant colony count data from Ames Salmonella/Microsome mutagenicity assays. Statistical procedures used in analyzing data from other widely employed in vitro mutagenicity assays are also described briefly due to space limitation. Due to uncontrollable factors in the conduct of Salmonella test assays, there is a large plate-to-plate variability in revenant colony count data. The Poisson distribution is judged as not suitable for modeling this variability since in most cases, distributions of such data show that the variance is greater than the mean. The negative binomial distribution is proposed to replace the Poisson distribution for modeling this plate-to-plate variability.

Two groups of procedures have been proposed to model the dose-response curve of the number of revenant colonies. The first group includes procedures which are based on biological mechanisms of reverse mutation of bacteria from auxotrophic cells to prototrophic cells. Those procedures consider the toxic effect in addition to the mutagenic effect of the test compound to reflect the possible nonmonotonic or downturn phenomenon in revenant count. They also consider the multigeneration phenomenon of the reverse mutation process. The second group consists of empirically-based procedures. Most procedures in this group also include terms for mutagenic and toxic effects of the test compound.

The unweighted and weighted least squares, the maximum likelihood method, and the quasi likelihood method are used to estimate the parameters in the dose-response curve. Standard procedures based on normal approximation and the likelihood ratio test are used to test the mutagenic and toxic effects.