A bioassay of pyrimethamine, a prophylactic antimalarial, for possible carcinogenicity was conducted by administering the test chemical in feed to Fischer 344 rats and B6C3F1 mice.
Groups of 35 rats and 35 mice of each sex were administered pyrimethamine 5 days per week at one of two doses, either 200 or 400 ppm for the rats and either 500 or 1,000 ppm for the mice. The animals were administered the chemical for 78 weeks, then observed for 26 or 27 additional weeks. Matched controls consisted of 15 untreated rats and 15 untreated mice of each sex; pooled controls consisted of the matched controls combined with 30 untreated rats and 30 untreated mice from similar bioassays of two other test compounds. All surviving rats and mice were killed at 102-105 weeks.
Mean body weights of the rats and mice fed diets containing pyrimethamine were slightly lower than those of the matched controls. Survival of the rats was not affected adversely by the chemical. In mice, survival rates of both dosed and matched-control males were low, with nearly two-thirds of the dosed and one-half of the control mice dying by week 52. Some of the deaths were associated with respiratory infections and may not have been related to administration of the chemical. Numbers of animals at risk in the dosed and control groups of female mice were adequate, however, for the development of late-appearing tumors.
In rats of each sex, no neoplastic lesions were found at a statistically significant incidence in the groups fed the pyrimethamine as compared with control groups. An increased frequency of bone-marrow atrophy occurred in both male and female dosed groups.
In male mice, the markedly decreased life spans may have prevented the observation of late-appearing tumors, since only two tumors were observed, one in a high-dose mouse and one in a low-dose mouse. In female mice, no neoplastic lesions were found at a statistically significant incidence in the groups fed the pyrimethamine as compared with control groups.
It is concluded that under the conditions of this bioassay, pyrimethamine was not carcinogenic for male or female Fischer 344 rats or for female B6C3F1 mice. The carcinogenic potential of pyrimethamine for male B6C3F1 mice cannot be assessed by this bioassay, because of the markedly reduced life span.