Methyl trans-styryl ketone is used as a synthetic flavoring agent and a fragrance additive in food and personal care products. Methyl trans-styryl ketone was nominated for study by the National Cancer Institute due to widespread human exposure as a flavoring and fragrance additive, positive results in the Ames/Salmonella assay and the mouse lymphoma L5178Y/tk+/- assay, and as a representative of the α,β-unsaturated ketone chemical class. Male and female F344/N rats and B6C3F1 mice received methyl trans-styryl ketone (98.6% pure) in feed for 3 months and dermally for 3 months or 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium, Escherichia coli, and mouse peripheral blood erythrocytes. Two-year studies were conducted to provide data for assessment of possible toxicity due to exposure to methyl trans-styryl ketone. The dermal route was chosen since this is the route for highest human exposure and due to studies demonstrating systemic exposure following dermal application to methyl trans-styryl ketone. 3-MONTH FEED STUDY IN RATS Groups of 10 male and 10 female rats were fed diets containing 0%, 0.025%, 0.05%, 0.1%, 0.2%, or 0.4% methyl trans-styryl ketone (equivalent to average daily doses of approximately 18, 36, 72, 145, or 290 mg methyl trans-styryl ketone/kg body weight to males and 19, 38, 77, 150, or 300 mg/kg to females) for 14 weeks. Groups of 10 male and 10 female clinical pathology rats were fed the same concentrations for 24 days. All core study rats survived to the end of the study. Final mean body weights of males and females receiving 0.4% and mean body weight gains of males receiving 0.4% were significantly less than those of the controls. Feed consumption by exposed groups was similar to that by the controls. Clinical findings included diarrhea and hyperactivity in males and females. Results of sperm motility and vaginal cytology evaluations indicated methyl trans-styryl ketone is unlikely to be a reproductive toxicant in male rats; however, it exhibits potential for reproductive toxicity in female rats based upon an increased probability of extended diestrus at the highest exposure concentration. In all exposed groups of males, there were treatment-related increased incidences of goblet cell hyperplasia of the respiratory epithelium of the nose and nephropathy of the kidney. In females, there was an increased incidence of goblet cell hyperplasia of the respiratory epithelium of the nose in the group receiving 0.4%. 3-MONTH FEED STUDY IN MICE: Groups of 10 male and 10 female mice were fed diets containing 0%, 0.025%, 0.05%, 0.1%, 0.2%, or 0.4% methyl trans-styryl ketone (equivalent to average daily doses of approximately 55, 110, 220, 400, or 750 mg/kg to males and 50, 100, 200, 350, or 600 mg/kg to females) for 14 weeks. One male receiving 0.2% and one control female died before the end of the study. Mean body weights of males and females receiving 0.4% were significantly less than those of the controls. Feed consumption by exposed groups was similar to that by the controls. Hyperactivity in both sexes was the only clinical finding. Results of sperm motility and vaginal cytology evaluations indicated methyl trans-styryl ketone is unlikely to be a reproductive toxicant in male mice; however, it exhibits potential for reproductive toxicity in female mice based upon an increased probability of extended diestrus at the lowest and the highest exposure concentrations. There were significantly increased incidences of olfactory epithelial atrophy of the nose in males and females receiving 0.4%. 3-MONTH DERMAL STUDY IN RATS: Groups of 10 male and 10 female rats were dermally administered 0, 22, 44, 87.5, 175, or 350 mg methyl trans-styryl ketone/kg body weight in 95% ethanol, 5 days per week for 14 weeks. Groups of 10 male and 10 female clinical pathology rats were administered the same doses for 23 days. All rats survived to the end of the study. Mean body weights of 175 and 350 mg/kg males were significantly less than that of the vehicle controls. Clinical findings in groups administered 175 or 350 mg/kg included dermal irritation, thickened skin, and ulceration at the site of application. Results of sperm motility and vaginal cytology evaluations indicated methyl trans-styryl ketone is unlikely to be a reproductive toxicant in male or female rats at the doses used in this study. Histologically, there were significantly increased incidences of epidermal hyperplasia, hyperkeratosis, chronic active inflammation, epidermal necrosis, and sebaceous gland hypertrophy in the skin at the site of application in males and/or females. There were significantly increased incidences of goblet cell hyperplasia of the nose in 350 mg/kg males and 22, 175, and 350 mg/kg females. 3-MONTH DERMAL STUDY IN MICE Groups of 10 male and 10 female mice were dermally administered 0, 87.5, 175, 350, 700, or 1,400 mg methyl trans-styryl ketone/kg body weight in 95% ethanol, 5 days per week for 13 weeks. All mice in the 700 and 1,400 mg/kg groups were sacrificed moribund before the end of the study. The final mean body weights of surviving groups of dosed males and females were similar to those of the vehicle controls; however, the mean body weight gains of the 175 mg/kg groups were significantly less than those of the vehicle controls. Clinical findings at the site of application included dermal irritation in 350 mg/kg males and crust formation in all 700 and 1,400 mg/kg mice except one female. Results of sperm motility and vaginal cytology evaluations indicated methyl trans-styryl ketone is unlikely to be a reproductive toxicant in male or female mice at the doses used in this study. There were treatment-related increased incidences of epidermal hyperplasia, hyperkeratosis, chronic active inflammation, epidermal necrosis, sebaceous gland hypertrophy, and hair follicle hyperplasia in the skin at the site of application in males and females. There were increased incidences of olfactory epithelial atrophy of the nose in groups of males and females administered 350 mg/kg or greater. 2-YEAR DERMAL STUDY IN RATS: Groups of 50 male and 50 female rats were dermally administered 0, 10, 30, or 90 mg methyl trans-styryl ketone/kg body weight in 95% ethanol, 5 days per week for 105 weeks. Survival of all dosed groups was similar to that of the vehicle controls. Mean body weights of dosed groups were within 10% of those of the vehicle control groups throughout the study. In the skin at the site of application, there were increased incidences of epidermal hyperplasia and hyperkeratosis in males and females administered 30 or 90 mg/kg. 2-YEAR DERMAL STUDY IN MICE: Groups of 50 male and 50 female mice were dermally administered 0, 10, 30, or 90 mg methyl trans-styryl ketone/kg body weight in 95% ethanol, 5 days per week for 105 weeks. Survival of all dosed groups was similar to that of the vehicle controls. Mean body weights of dosed groups were within 10% of those of the vehicle control groups throughout the study. In the skin at the site of application in males and females, there were treatment-related increased incidences of epidermal hyperplasia, hyperkeratosis, chronic inflammation, and melanocyte hyperplasia. RESULTS Methyl trans-styryl ketone was mutagenic in Salmonella typhimurium strain TA100 when testing was conducted in the presence of rat liver microsomes (S9). No mutagenic activity was seen with methyl trans-styryl ketone in strain TA98 with or without S9 or in Escherichia coli strain WP2 uvrA/pKM101 in the absence of S9. With S9, inconsistent responses were seen in the E. coli tester strain. No increases in the frequencies of micronucleated normochromatic erythrocytes were seen in peripheral blood samples from male or female mice administered methyl trans-styryl ketone for 3 months via dosed feed or dermal application. CONCLUSIONS Under the conditions of these 2-year dermal studies, there was no evidence of carcinogenic activity of methyl trans-styryl ketone in male or female F344/N rats or in male or female B6C3F1 mice administered 10, 30, or 90 mg/kg. Administration of methyl trans-styryl ketone resulted in nonneoplastic lesions of the skin at the site of application in male and female rats and mice. Synonyms: Acetocinnamone; benzalacetone; benzylideneacetone; methyl 2-phenylvinyl ketone; methyl styryl ketone; methyl β-styryl ketone; MSK; 4-phenyl-3-butene-2-one; 4-phenylbutenone; 2-phenylvinyl methyl ketone; styryl methyl ketone Systematic name: (3E)-4-Phenylbut-3-en-2-one.