NTP Toxicity Studies of Methyl Ethyl Ketoxime Administered in Drinking Water to F344/N Rats and B6C3F1 Mice (CAS No. 96-29-7).

Methyl ethyl ketoxime is used primarily as an antiskinning agent in alkyd coating resins. Methyl ethyl ketoxime was selected for study because of the potential for human exposure and because of interest in oximes as a chemical class. Toxicity studies of methyl ethyl ketoxime (greater than 99% pure) were carried out in male and female F344/N rats and B6C3F1 mice. The compound was administered in drinking water for 14 days or 13 weeks. In addition, the genetic toxicity of methyl ethyl ketoxime was evaluated by determining mutagenicity in Salmonella typhimurium and induction of sister chromatid exchanges and chromosomal aberrations in cultured Chinese hamster ovary cells in vitro, with and without S9 activation. The frequency of micronucleated normochromatic erythrocytes in the peripheral blood of mice from the 13-week study was also determined. In the 14-day studies, groups of five male and five female rats and mice were given drinking water containing 0, 106, 312, 625, 1,250, or 2,500 ppm methyl ethyl ketoxime. The mean body weight gain of male rats in the 2,500 ppm group was significantly less than that of the controls; the final mean body weight of male mice in the 2,500 ppm group was also less than that of the controls. Spleen weights were increased in male and female rats in the 1,250 and 2,500 ppm groups. No chemical-related gross lesions were observed. Microscopic tissue evaluations were not performed. In the 13-week studies, groups of 10 male and 10 female rats were given drinking water containing 0, 312, 625, 1,250, 2,500, or 5,000 ppm and groups of 10 male and 10 female mice were given drinking water containing 0, 625, 1,250, 2,500, 5,000, or 10,000 ppm. Mean body weights and body weight gains of 2,500 and 5,000 ppm male rats and 10,000 ppm male and female mice were less than those of the controls; mean body weight gains of male rats in the 1,250, 2,500 and 5,000 ppm groups and females in the 2,500 and 5,000 ppm groups were also less than those of the controls. Hematology results of this drinking water study indicate that methyl ethyl ketoxime induces a methemoglobinemia and a responsive Heinz body anemia. Liver and spleen weights were generally significantly greater than those of the controls in male and female rats exposed to 1,250 ppm or greater; spleen weights were also increased in male and female mice in the 10,000 ppm groups. Kidney weights were significantly greater in male rats in the 5,000 ppm group and in female rats exposed to 1,250 ppm or greater than those of the controls. Microscopically, there were exposure-related increases in the incidences and severities of hematopoietic cell proliferation in the spleen at exposure concentrations of 625 ppm or greater in male and female rats and at 5,000 and 10,000 ppm in male and female mice. A significant increase in the incidence of hematopoietic proliferation in the bone marrow was observed in rats exposed to 625 ppm or greater. Liver Kupffer cell erythrophagocytosis and hemosiderin pigmentation, as well as renal tubule hemosiderin pigmentation, occurred in exposed rats and mice. Other lesions observed include hyperplasia of the transitional epithelial lining of the urinary bladder in male and female mice exposed to 2,500 ppm or greater and degeneration of the nasal olfactory epithelium in male and female rats in the 2,500 and 5,000 ppm groups, male mice in the 5,000 and 10,000 ppm groups, and female mice exposed to 2,500 ppm or greater. Methyl ethyl ketoxime is extensively metabolized and does not accumulate in tissues. Single gavage doses of 2.7, 27, or 270 mg/kg administered to rats were primarily converted to carbon dioxide, mostly in the first 24 hours after dosing. After intravenous administration, less radioactivity on a percentage basis was excreted as carbon dioxide than in the gavage study, and more of the administered dose was excreted in urine and as volatiles. Following dermal administration, significantly greater amounts of volatiles were excreted than after gavage or intravenous administration. The 270 mg/kg gavage dose may result in saturation of a metabolic pathway(s). There is some evidence that the ketoxime is metabolized to the ketone and, presumably, hydroxylamine. Methyl ethyl ketoxime was mutagenic in Salmonella typhimurium strain TA1535 when tested in the presence of hamster liver S9 activation enzymes; results of mutagenicity testing were negative in strains TA97, TA98, and TA100, with and without exogenous metabolic activation. No induction of sister chromatid exhanges or chromosomal aberrations was observed in cultured Chinese hamster ovary cells treated with methyl ethyl ketoxime, with or without S9, and no increase in the frequency of micronucleated erythrocytes was noted in peripheral blood obtained from male and female mice administered methyl ethyl ketoxime in drinking water for 13 weeks. In summary, the major target of methyl ethyl ketoxime is the erythrocyte; the no-effect level for erythrotoxicity is 625 ppm in male rats and 312 ppm in female rats based on erythrocyte counts after 13 weeks of exposure. The no-effect level for hematopoietic toxicity is 312 ppm in rats based on erythroid cell hyperplasia in bone marrow and 2,500 ppm in mice based on hematopoietic cell proliferation in the spleen. Hematology results of this drinking water study indicate that methyl ethyl ketoxime induces a methemoglobinemia and a responsive Heinz body anemia. Methyl ethyl ketoxime was at most weakly genotoxic; it induced mutations in S. typhimurium under very specific conditions and increased the frequency of sister chromatid exchanges in cultured Chinese hamster ovary cells, but it did not induce sister chromatid exchanges or chromosomal aberrations in cultured Chinese hamster ovary cells in vitro or increase the frequency of micronucleated erythrocytes in mice treated in vivo.