Cytosine arabinoside, vinblastine, 5-fluorouracil and 2-aminoanthracene testing in the in vitro micronucleus assay with L5178Y mouse lymphoma cells at Sanofi Aventis, with different cytotoxicity measurements, in support of the draft OECD Test Guideline on

Cytosine arabinoside (a nucleoside analogue that inhibits the gap-filling step of excision repair), vinblastine (an aneugen that inhibits tubulin polymerisation), 5-fluorouracil (a nucleoside analogue with a steep response profile), and 2-aminoanthracene (a metabolism-dependent reference genotoxin) were tested in the in vitro micronucleus assay with L5178Y mouse lymphoma cells, without cytokinesis block. The four chemicals were independently evaluated in two Sanofi Aventis laboratories, one of which used an image analyser to score micronuclei, while the other scored micronucleated cells manually. Very similar results were obtained in the two laboratories, highlighting the robustness of the assay. The four test chemicals induced significant increases in the incidence of micronucleated cells at concentrations that produced no more than a 55 ± 5% reduction in survival growth, as measured with the three parameters recommended in the draft OECD Test Guideline on In Vitro Mammalian Cell Micronucleus Test (MNvit) for chemical testing, namely the relative increase in cell counts, relative population doubling, and the relative cell count. These results support the premise that the relative increase in cell counts and relative population doubling, that take into account both cell death and cytostasis, are appropriate measures of survival growth reduction in the in vitro micronucleus test conducted in the absence of cytokinesis block, as recommended in MNvit.