REPORT OF THE BTS UKEMS WORKING GROUP ON DOSE SETTING IN INVIVO MUTAGENICITY ASSAYS

The BTS/UKEMS Working Group was established to consider dose selection for in-vivo mutagenicity assays, and the feasibility of establishing criteria for identifying maximum dose levels that did not involve rela ting these to a high fraction (50-80%) of the estimated LD50 value. In view of the importance attached by regulatory authorities to negative results from in-vivo tests, namely reassurance that mutagenic potenti al seen in vitro could not be expressed in the whole animal, the need for the use of some form of MTD was accepted. The crucial question fac ing the group was whether the use of 'evident toxicity' rather than so me index of lethality would result in any meaningful loss of sensitivi ty of the assays. The group endorsed the concept of a limit dose for r elatively non-toxic compounds and supported the use of a value of 2 g kg-1 for single oral dosing and 1 g kg-1 for repeated dosing, in line with the general values used by the OECD and EEC. In order to assess t he question of sensitivity of the assays the group considered the avai lable data from the published literature, and from 'in-house' studies, on dose-response for mutagenicity and for toxicity, using the same do sing regime. It rapidly became apparent that few data were available, and that these were limited essentially to the micronucleus test; ther e were inadequate data to consider any other methods, In addition, the re was the added complication that most of the available data related to protocols which were less rigorous than those currently recommended . The group thus concentrated on the micronucleus test because of its relatively wide use and since it had given rise to specific concerns d ue to a recent recommendation from the relevant EPA Gen-Tox group name ly that the top dose should be 50-80% of the estimated LD50 value. It was noted that the EPA appear to be considering this approach as one a lternative when dose setting, with the possibility of the use of a dos e producing overt toxicity as another. Available data indicate that ar ound 90% of tested mutagens would have been identified using an MTD ba sed on non-lethal criteria. Moreover the percentage would be expected to be higher if all tests had been carried out to current protocol sta ndards. However, the possibility of missing compounds could not be com pletely eliminated. Furthermore, it was important to put the bone marr ow study in context. In the UK, Regulatory Authorities would not accep t negative data from one tissue as providing adequate reassurance rega rding the absence of in-vivo activity. Data from at least one other as say using a different tissue would be needed. It would not therefore b e necessary to use 'heroic' and unrealistically high doses in the bone marrow assay in a misguided attempt to obtain absolute assurance from the one study. It is believed that Regulatory Authorities in most oth er countries would seek data from more than one in-vivo assay before d iscounting positive data from in-vitro studies. The group also conside red in quantitative terms, the actual difference in MTD in the mouse i f based on 'evident toxicity' or on lethality (an estimate of a dose e quivalent to 50-80% the LD50 value). There was relatively little diffe rence between the two levels, due to the steep dose response for toxic ity seen in the mouse with most compounds.