5-Azacytidine, a synthetic analogue of cytidine, has been used as an investigational anticancer drug in the United States since 1970.
A bioassay of 5-azacytidine for possible carcinogenicity was conducted by administering the test chemical by intraperitoneal injection to Sprague-Dawley rats and B6C3F1 mice.
Groups of 35 rats of each sex were administered 5-azacytidine at one of two doses, either 2.6 or 5.2 mg/kg body weight, in buffered saline three times per week for 34 weeks, and were then observed for 46 or 47 weeks. Controls consisted of groups of 15 rats of each sex that received injections of buffered saline (vehicle controls) and 15 rats of each sex that were untreated (untreated controls). All surviving rats were killed at 80 or 81 weeks.
Groups of 35 mice of each sex were administered the chemical at one of two doses, either 2.2 or 4.4 mg/kg body weight, in buffered saline three times per week for 52 weeks, and were then observed for 29 or 30 weeks. Controls consisted of groups of 15 mice of each sex that received injections of buffered saline (vehicle controls) and 15 mice of each sex that were untreated (untreated controls). All surviving mice were killed at 81 or 82 weeks.
5-Azacytidine was toxic to the animals in this bioassay, since mean body weights of both treated rats and treated mice were lower than those of the corresponding vehicle controls, and since none of the high-dose male and female rats and high-dose female mice lived to the end of the bioassay. In treated male and female rats and male mice, survival was inadequate for meaningful statistical analyses of the incidences of tumors.
Only one male and three female high-dose rats had tumors,and none of the tumors in the low-dose group of either sex were present at a significantly increased incidence using any of the statistical tests. Bone-marrow atrophy was present in both treated groups of both sexes of rats.
Only five high-dose male mice and one high-dose female mouse had neoplasms. In low-dose female mice, however, lymphocytic and granulocytic neoplasms of the hematopoietic system occurred in 17 animals, even though only 54% survived until week 81. Granulocytic neoplasms were observed in 10/29 low-dose female mice, but in no other group, and were significant (P=0.010) compared with the vehicle controls. The incidence of combined lymphoma and granulocytic neoplasms was highly significant in the low-dose females (vehicle controls 0/14, low-dose 17/29, P<0.001). No tumors were observed at a significant incidence in male mice. Bone-marrow atrophy was present in high-dose female mice.
It is concluded that under the conditions of this bioassay, the short life span and short duration of treatment of Sprague-Dawley rats of either sex and of male B6C3F1 mice precluded evaluation of the carcinogenicity of 5-azacytidine in these groups; however, the induction of tumors of the hematopoietic system in female B6C3F1 mice was associated with the administration of 5-azacytidine.