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 ass umption 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 Nation al Toxicology Program (NTP) rat carcinogenicity bioassay. To provide a comp rehensive assessment of its genotoxicity in a standard battery of mutagenic ity assays, oxymetholone was tested in microbial and mammalian cell gene mu tation assays, in an in vitro cytogenetics assay (human lymphocytes), and i n an in vivo micronucleus assay. Oxymetholone was also tested in an ill vit ro 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 mu g/plate in the bacterial plate incorporation assay using 4 Salmonella st rains and the WP2 uvrA (pKM101) strain of Escherichia coli. There was no in duction of revertants up to the highest dose levels, which were insoluble a s well as toxic. In the L5178Y tk(+/-) mouse lymphoma assay, doses up to 30 mu g/ml reduced relative survival to similar to 30% with no increase in mu tants. Male or female human lymphocytes were exposed in vitro to oxymetholo ne 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 ora lly by gavage for 26 wk with 125, 625, or 1,250 mg/kg/day of oxymetholone w as evaluated for micronuclei in polychromatic and normochromatic erythrocyt es. There was no difference in micronuclei frequency between control and tr eated animals. These results confirm that oxymetholone is not genotoxic in a comprehensive battery of mutagenicity assays. In the SHE assay, oxymethol one produced a significant increase in morphologically transformed colonies at dose levels of 13-18 mu g/ml. The lack of genotoxicity of oxymetholone, the positive response in the in vitro transformation assay, and the result s of transgenic mouse carcinogenicity assays will provide an interesting pe rspective on the results of an on-going NTP rat carcinogenicity bioassay.