Reference genotoxic compounds 2-aminoanthracene, diethylstilboestrol and vinblastine were tested in the in vitro micronucleus assay using Chinese hamster V79 derived cells in the laboratories of British American Tobacco in the UK. The work was conducted in support of the cytotoxicity measures recommended in the 2007 version of the OECD Test Guideline 487. The three compounds were positive in the assay in the presence and absence of the cytokinesis blocking agent cytochalasin B at concentrations that did not exceed the recommended cytotoxic limits determined by relative population doubling, relative increase in cell counts, relative cell counts and cytokinesis block proliferation index. Consequently, this work supports the hypothesis that relative population doubling, relative increase in cell counts and relative cell counts are appropriate measures of cytotoxicity for the non-cytokinesis blocked in vitro micronucleus assay.

The reference genotoxic agents 2-aminoanthracene (a metabolism dependent weak clastogen), 5-fluorouracil (a nucleoside analogue, characterised by a steep dose response profile), colchicine (an aneugen that inhibits tubulin polymerisation), benzo[a]pyrene (a polycyclic aromatic hydrocarbon requiring metabolic activation), cadmium chloride (an inorganic carcinogen), and cytosine arabinoside (a nucleoside analogue that inhibits the gap-filling step of excision repair) were tested in the in vitro micronucleus assay using the Chinese hamster ovary (CHO) cell line at Covance Laboratories, Harrogate, UK. All chemicals were treated in the absence and presence of cytokinesis block (via addition of cytochalasin B) with this work forming part of a collaborative evaluation of the toxicity measures recommended in the draft OECD Test Guideline 487 on the In vitro Mammalian Cell Micronucleus Test (MNvit). The toxicity measures used, detecting a possible combination of both cytostasis and cell death (though not cell death directly), were relative population doubling, relative increase in cell counts and relative cell counts for treatments in the absence of cytokinesis block, and replication index in the presence of cytokinesis block. All of the chemicals tested either gave marked positive increases in the percentage of micronucleated cells with and without cytokinesis block, or did not induce micronuclei at concentrations giving approximately 50-60% toxicity (cytostasis and cell death) or less by all of the toxicity measures used. The outcome from this series of tests supports the use of relative increase in cell counts and relative population doubling, as well as relative cell counts, as appropriate measures of cytotoxicity for the non-cytokinesis blocked in vitro micronucleus assay.

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.

The reference genotoxic agents Cytosine arabinoside, Vinblastine, Diethylstilboestrol and 2-Aminoanthracene were tested in the in vitro micronucleus assay, in human lymphoblastoid TK6 cells, without cytokinesis block, at the laboratories of Institut Pasteur de Lille, France. This was done in support of the toxicity measures recommended in the late 2007 version of the draft OECD Test Guideline 487 for the testing of chemicals. All four reference agents were positive in the assay at concentrations giving approximately 50% toxicity or less as assessed by draft Test Guideline 487 recommended measures, relative population doublings and relative increase in cell counts. Accordingly, this work supports the premise that relative population doublings and relative increase in cell counts are appropriate measures of toxicity for the non-cytokinesis blocked in vitro micronucleus assay.

At the laboratories of AstraZeneca, Alderley Park, UK the reference genotoxic agents etoposide (a topoisomerase II inhibitor), cadmium chloride (an inorganic carcinogen), colchicine (an aneugen that inhibits tubulin polymerisation), benzo[a]pyrene (a metabolism dependent reference genotoxin) and cyclophosphamide (a metabolism dependent reference genotoxin) were tested in the in vitro micronucleus assay (MNvit), using mouse lymphoma L5178Y cells, with and without cytokinesis block. Further, 2-aminoanthracene (a metabolism dependent weak clastogen) was tested in the MNvit, using TK6 cells, without cytokinesis block. This was done in support of the toxicity (cell death and cytostasis) measures recommended in the late 2007 version of the draft OECD Test Guideline 487 for the testing of chemicals. All six reference agents were positive in the MNvit without cytokinesis block at concentrations giving approximately 50% toxicity or less as defined by draft Test Guideline 487 recommended measures, relative population doublings and relative increase in cell counts. Furthermore, the five agents tested with cytokinesis block were positive in the MNvit at concentrations giving approximately 50% toxicity or less as assessed by replicative index. Accordingly, this work supports the premise that relative population doublings and relative increase in cell counts are appropriate measures of toxicity for the non-cytokinesis-blocked in vitro micronucleus assay.