National Institute of Environmental Health Sciences Research Triangle Park, North Carolina 27709 National Institute of Environmental Health Sciences Research Triangle Park, North Carolina 27709 Permanent Address: Cave 25, r-34011 Aurisina (Trieste), Italy
To the Editor:
Takayama and colleagues reported the results of their toxicity and carcinogenicity study of sodium cyclamate in nonhuman primates (Takayama et al., 2000). Like their study on DDT (Takayama et al., 1999
; Tomatis and Huff, 2000
), this was again admirable for its duration and detailed reporting of pathological findings. Once more, however, we disagree with the conclusions made by these authors who state that their results do not provide evidence of toxic or carcinogenic effects of sodium cyclamate in monkeys.
The beginning experimental groups consisted of 10 monkeys (4 cynomolgus, 4 rhesus, and 2 African green) receiving 100 mg/kg, and 11 monkeys (5 cynomolgus, 5 rhesus, and 1 African green) given 500 mg/kg of cyclamate daily for 5 days a week, starting shortly after birth and continuing for up to 24 years. The sodium cyclamate was dissolved in water and mixed with daily vitamins and placed on "sandwiches". An age-matched control group consisted of 16 monkeys (8 cynomolgus and 8 rhesus). All groups contained various mixtures of males and females. The authors indicated that the doses chosen represented respectively 9 and 45 times higher than the Acceptable Daily Intake (ADI) of 11 mg/kg/day established for humans by JECFA and SCF in 1982 and 1985.
At the end of the 22.524 year experiments, 8/10 monkeys in the 100 mg/kg group, 6/11 in the 500 mg/kg group, and 16/16 of the controls were alive. Of the seven monkeys dying early, the two early-death monkeys from the 100 mg/kg group died at 15 and 16 years (another survived 21.3 years), whereas the 5 early deaths in the 500 mg/kg group occurred at 2, 7, 14, 15, and 16 years, leaving a total of 14/21 monkeys between the two cyclamate groups for long term observation. None of the early deaths could be attributed to cyclamate. Two of the 15-year female monkeys (one in each dose group) were euthanized due to severe symptoms from extensive pelvic endometriosis.
Regarding carcinogenic effects, five cyclamate monkeys had six tumors: two of the 6 monkeys (33%) in the 500 mg/kg group each had a malignant metastatic tumor (one hepatocellular carcinoma and one adenocarcinoma of the colon), and the monkey with colon cancer also had a leiomyoma of the uterus. In the 100 mg/kg group, one malignant tumor (adenocarcinoma of the prostate) occurred among the eight long-term survivors, with two additional animals having benign tumors: a leiomyoma of the uterus and an adenoma of the thyroid gland. No benign or malignant tumors were observed in the 16 monkeys of the untreated contemporary control group, nor were any tumors observed in the 17 control monkeys of the previous DDT study from these same authors (Takayama et al., 1999).
The authors report that the incidence of "spontaneous" (causes unknown) tumors found among 373 untreated monkeys (historical controls) varied between 1.5% in cynomolgus, 2.8% in rhesus, and 8% in African green monkeys. Of the 6 tumors reported in this study, 2 occurred in cynomolgus and 4 in rhesus monkeys, the two strains with the lower "spontaneous" tumor rates. Thus, compared to the contemporary controls in the cyclamate and DDT studies (no tumors in 33 monkeys), as well as to the historical controls, the effective tumor incidence rates among treated monkeys appear remarkably conspicuous, and are unlikely to be a chance occurrence: 2/6 or 33% in the 500 mg/kg group, 3/8 or 38% in the 100 mg/kg group, and 5/14 or 36% in the two experimental groups combined.
Nonetheless, the study might be considered somewhat limited (especially for a `negative' study) because of the small number of animals involved, because the monkeys could have tolerated larger amounts of cyclamate, and because the observed tumors are of different types. However, especially considering the low background tumor rates, the occurrence of 3 malignant and 3 benign tumors in cyclamate-exposed monkeys appears to be of considerable biological significance and must be taken as a serious warning toward the possible carcinogenicity of cyclamate.
REFERENCES
Takayama, S., Renwick, A. G., Johansson, S. L., Thorgeirsson, U. P., Tsutsumi, M., Dalgard, D. W., and Sieber, S. M. (2000). Long-term toxicity and carcinogenicity study of cyclamate in nonhuman primates. Toxicol. Sci. 53, 3339.
Takayama, S., Sieber, S. M., Dalgard, D. W., Thorgeirsson, U. P., and Adamson, R. H. (1999). Effects of long-term oral administration of DDT on nonhuman primates. J. Cancer Res. Clin. Oncol. 125, 219225.[ISI][Medline]
Tomatis, I., and Huff, J. (2000). Evidence of carcinogenicity of DDT in nonhuman primates. J. Cancer Res. Clin. Oncol. 126, 246.[ISI][Medline]