Recommendations for spacing of test chemical concentrations in the mouse lymphoma tk mutation assay (MLA)

Recent test guidelines for the mouse lymphoma tk mutation assay (MLA) have highlighted the need to achieve 80-90% reduction in cell survival for a valid, robust assay with toxic chemicals. For many pharmaceuticals, under new ICH recommendations, this may be the only in vitro mammalian cell test that is performed. It was important to discover, therefore, how critical it is to achieve 80-90% toxicity, and how best to select the number and spacing of test concentrations to fulfil this requirement. We analysed data from 121 positive chemicals, provided by nine industrial and commercial laboratories, and found that for 17 chemicals (14%), the response profiles were so steep that using a conventional 2-fold dilution series of test concentrations would have failed to identify the active range (> 90% toxicity at one concentration, and no significant mutation at 50% of this dose), and positive responses would have been missed. Analysis of genotoxicity results in other test systems with these 17 chemicals revealed no differences in overall response profiles from the 104 chemicals that exhibited less steep MLA responses. The MLA results were therefore deemed to be equally biologically relevant. From this analysis, it is recommended that concentration spacing in the MLA needs to be closer than that obtained with a 2-fold dilution series, and a dilution factor where each concentration is 0.75 or 0.8 of the one above is recommended to identify the active range of positive mutagens.