Jenness B. Majeska

A direct comparison of mouse and rat bone marrow and blood as target tissues in the micronucleus assay

Rats and mice were treated concurrently with mitomycin C at a dose of 1 mg/kg/day i.p. for 3 days, a regimen known to induce micronuclei in polychromatic erythrocytes (MN-PCE) in the bone marrow of rats and mice and the peripheral blood of mice. The incidence of micronuclei was evaluated in the peripheral blood and the bone marrow of both species. Early reports suggested that the efficiency of the rat spleen in removing micronuclei from the circulation precluded the use of rat peripheral blood in the detection of chemically-induced micronuclei. The data in the present study demonstrate that the induction of micronuclei in polychromatic erythrocytes as the result of treatment with a clastogen can be demonstrated equally well in the bone marrow or the peripheral blood of both rats and mice.

Oxymetholone: I. Evaluation in a Comprehensive Battery of Genetic Toxicology and In Vitro Transformation Assays:

Oxymetholone is generally assumed to be a nongenotoxic carcinogen. This assumption is based primarily on the results of an Ames test, existing data in repeat-dose toxicology studies, and the predicted results of a 2-yr National Toxicology Program (NTP) rat carcinogenicity bioassay. To provide a comprehensive assessment of its genotoxicity in a standard battery of mutagenicity assays, oxymetholone was tested in microbial and mammalian cell gene mutation assays, in an in vitro cytogenetics assay (human lymphocytes), and in an in vivo micronucleus assay. Oxymetholone was also tested in an in vitro morphologic transformation model using Syrian hamster embryo (SHE) cells. These studies were initiated and completed prior to the disclosure of the results of the NTP bioassay. Oxymetholone was tested at doses up to 5,000 μg/plate in the bacterial plate incorporation assay using 4 Salmonella strains and the WP2 uvrA (pKM101) strain of Escherichia coli. There was no induction of revertants up to the highest dose levels, which were insoluble as well as toxic. In the L5178Y tk+/- mouse lymphoma assay, doses up to 30 μg/ml reduced relative survival to ∼30% with no increase in mutants. Male or female human lymphocytes were exposed in vitro to oxymetholone for 24 hr without S9 or 3 hr with S9 and evaluated for the induction of chromosomal aberrations. There was no increase in aberration frequency over control levels and no difference between male and female cells. Peripheral blood from Tg.AC transgenic mice treated dermally for 20 wk with 0, 1.2, 6.0, or 12.0 mg/day of oxymetholone and from p53 transgenic mice treated orally by gavage for 26 wk with 125, 625, or 1,250 mg/kg/day of oxymetholone was evaluated for micronuclei in polychromatic and normochromatic erythrocytes. There was no difference in micronuclei frequency between control and treated animals. These results confirm that oxymetholone is not genotoxic in a comprehensive battery of mutagenicity assays. In the SHE assay, oxymetholone produced a significant increase in morphologically transformed colonies at dose levels of 13-18 μg/ml. The lack of genotoxicity of oxymetholone, the positive response in the in vitro transformation assay, and the results of transgenic mouse carcinogenicity assays will provide an interesting perspective on the results of an on-going NTP rat carcinogenicity bioassay.

Selection of agar for use in Salmonella typhimurium and Escherichia coli mutation assays

The spontaneous and induced revertant frequency of four Salmonella typhimurium strains (TA1535, TA1537, TA98, and TA100) and Escherichia coli [WP2 uvrA (pKM101)] was evaluated using Vogel Bonner minimal plates prepared with ten different agars. In addition to the Difco Bacto agar originally recommended by Ames, Difco Noble, granulated and Bitek agars; BD grade A, BBL granulated and purified agars; Oxoid purified and No. 1 agars; and GIBCO select agar were tested. Several of these agars have been reported as acceptable alternatives for these Salmonella strains, but comparable studies with E. coli have not been done. The bacteria were treated with DMSO or an appropriate positive control in the presence or absence of an Aroclor 1254‐induced rat liver activation system. With the exception of Noble agar in the presence of S9, there was little difference among the responses of the Salmonella strains on any of the agars. However, with E. coli the responses include either a reduction or an increase in spontaneous revertants numbers as well as a reduction in absolute and relative induced revertant frequency. Difco Bacto agar appears to be the most consistent agar for use with these strains. As an alternative, only BBL purified agar resulted in consistent results for all of these strains under all testing conditions. These results emphasize the need to evaluate the components of the standard mutation assay when incorporating additional bacterial strains. Suboptimal responses related to the agar or other components could compromise the detection of weak mutagens. Environ. Mol. Mutagen. 32:192–196, 1998