COMPARISON OF MUTAGENIC RESPONSES IN THE L5178Y/ tk+/- MOUSE LYMPHOMA ASSAY USING OLD AND NEW EVALUATION CRITERIA

Abstract

The National Cancer Institute (NCI) shares the responsibility for selecting the majority of the most significant chemicals for carcinogenicity testing by the National Toxicology Program (NTP), and uses two short-term in vitro mutagenicity tests (in Salmonella and mouse lymphoma L5178Y tk+/- cells) to assign priorities for chemicals whose carcinogenic potential should be evaluated in 2-year rodent studies. For almost 20 years, the mouse lymphoma (ML) assay data were evaluated based on a two-fold increase in mutant frequency as the criteria for a positive response. According to an international consensus agreement (Clive et al. 1995), the new evaluation criteria include: for a positive response, concentration-related increase in mutant frequency (MF) and one or more dose levels with 10% or greater total growth exhibit MF of >=100 mutants per 10(6) clonable cells over the background level; equivocal response, MF between 55 and 99 mutants per 106 clonable cells over the background level; negative response, MF fewer than 55 mutants per 106 clonable cells over the background level. Based on the re-evaluation of 408 chemicals using the new criteria, equivocal responses increased from 24 to 65 (171% increase) with exogenous metabolic activation (+S9), and from 17 to 38 (124% increase) without activation (-S9). The number of negative responses remained approximately the same (+S9, 43% of total; -S9, 50% of total). The number of positive or weakly positive responses decreased from 204 to 115 (+S9, 44% decrease), and from 196 to 108 (-S9, 45% decrease). The decrease in positive responses and concomitant increase in equivocal responses are indicative of increased stringency in the ML assay since the advent of the new criteria.