Oxymetholone: I. Evaluation in a comprehensive battery of genetic toxicology and in vitro transformation assays

Toxicol Pathol. 1999 Sep-Oct;27(5):501-6.

Comment on:
    Toxicol Pathol. 1999 Sep-Oct;27(5):507-12.

Oxymetholone: I. Evaluation in a comprehensive battery of genetic toxicology and
in vitro transformation assays.

Holden HE, Studwell D, Majeska JB.

Department of Toxicology and Safety Assessment, Boehringer Ingelheim
Pharmaceuticals, Ridgefield, Connecticut 06877, USA. heholden@midcoast.com

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 microg/plate in the
bacterial plate incorporation assay using 4 Salmonella strains and the WP2 uvrA
(pKM101) strain of Escherichia coil. 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 microg/ml reduced relative survival to
approximately 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 microg/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.