Aspartame is an artificial sweetener used throughout the world in food and beverages. Conventional 2-year rodent cancer studies of aspartame are considered negative, although a small number of neoplasms of the brain were observed in a rat study (Fed. Regist., 1981a,b). The NTP has explored the use of genetically altered mouse models as adjuncts to the 2-year rodent cancer assay. These models may prove to be more rapid, use fewer animals, and provide some mechanistic insights into neoplastic responses. As part of the evaluation of new mouse cancer screening models, aspartame was tested for potential toxicity and carcinogenicity in two relatively well-studied models, the Tg.AC hemizygous strain and the p53 haploinsufficient strain, and an uncharacterized model, the Cdkn2a deficient strain. Male and female Tg.AC hemizygous, p53 haploinsufficient, and Cdkn2a deficient mice were given feed containing aspartame (greater than 98% pure) for 9 months. Genetic toxicology studies were conducted in Salmonella typhimurium, rat bone marrow cells, and mouse peripheral blood erythrocytes.
9-Month Study in Tg.AC Hemizygous Mice
Groups of 15 male and 15 female Tg.AC hemizygous mice were fed diets containing 0, 3,125, 6,250, 12,500, 25,000, or 50,000 ppm aspartame (equivalent to average daily doses of approximately 490, 980, 1,960, 3,960, or 7,660 mg aspartame/kg body weight to males and 550, 1,100, 2,260, 4,420, or 8,180 mg/kg to females) for 40 weeks. Exposure to aspartame had no effect on survival. The mean body weights of 50,000 ppm females were greater than those of the controls from week 15 until the end of the study. Feed consumption by the exposed groups was similar to that by the control groups throughout the study. There were no neoplasms or nonneoplastic lesions that were attributed to exposure to aspartame.
9-Month Study in p53 Haploinsufficient Mice
Groups of 15 male and 15 female p53 haploinsufficient mice were fed diets containing 0, 3,125, 6,250, 12,500, 25,000, or 50,000 ppm aspartame (equivalent to average daily doses of approximately 490, 970, 1,860, 3,800, or 7,280 mg/kg to males and 630, 1,210, 2,490, 5,020, or 9,620 mg/kg to females) for 40 weeks. Exposure to aspartame had no effect on survival or mean body weights. Feed consumption by the exposed groups was similar to that by the control groups throughout the study. No neoplasms or nonneoplastic lesions were attributed to exposure to aspartame.
9-Month Study in Cdkn2a Deficient Mice
Groups of 15 male and 15 female Cdkn2a deficient mice were fed diets containing 0, 3,125, 6,250, 12,500, 25,000, or 50,000 ppm aspartame for 40 weeks (equivalent to average daily doses of approximately of approximately 490, 960, 1,900, 3,700, and 7,400 mg/kg to males and 610, 1,200, 2,390, 4,850, and 9,560 mg/kg to females). Survival of all exposed groups was similar to that of the control groups. Mean body weights of 3,125 and 6,250 ppm males were less than those of the controls after weeks 29 and 16, respectively. Mean body weights of female mice were similar to those of the controls throughout the study. The incidences of minimal to mild cytoplasmic vacuolization of periportal hepatocytes were significantly greater than controls in males exposed to 6,250, 25,000, or 50,000 ppm aspartame.
Aspartame was tested for induction of gene mutations in Salmonella typhimurium. No mutagenicity was detected in strains TA98, TA100, or TA1535 with or without exogenous metabolic activation (S9). In addition, a single test in TA1537 with 30% rat liver S9 gave negative results. In TA97 with 30% rat liver S9, however, a reproducible small increase in mutant colonies was observed, and this response was judged to be equivocal. No mutagenicity was detected in TA97 without S9 or with hamster liver S9.
An acute bone marrow micronucleus test was conducted with aspartame administered by gavage to male F344/N rats. No increase in micronucleated polychromatic erythrocytes was observed at any dose level.
Peripheral blood micronucleus tests were conducted after 9 months exposure of Tg.AC hemizygous, p53 haploinsufficient, and Cdkn2a deficient mice to aspartame in dosed feed. Negative results were obtained in male and female Tg.AC hemizygous and Cdkn2a deficient mice. Negative results were also obtained with male p53 haploinsufficient mice. In female p53 haploinsufficient mice, the results of the micronucleus test were judged to be positive, based on a significant trend test and a small but statistically significant increased frequency of micronucleated erythrocytes in the 50,000 ppm group.
Under the conditions of this 9-month feed study, there was no evidence of carcinogenic activity of aspartame in male or female p53 haploinsufficient mice exposed to 3,125, 6,250, 12,500, 25,000, or 50,000 ppm. Because this is a new model, there is uncertainty whether the study possessed sufficient sensitivity to detect a carcinogenic effect.