Bioassay of pyrazinamide for possible carcinogenicity.
A bioassay of the tuberculostatic drug pyrazinamide 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 pyrazinamide at one of two doses, either 5,000 or 10,000 ppm, for 78 weeks, and then observed for an additional 26 or 27 weeks. Matched controls consisted of groups of 15 untreated rats and 15 untreated mice of each sex. High-dose male mice died or were killed by week 92; all other surviving animals were killed at weeks 104 or 105. Mean body weights of the dosed male rats were slightly lower than those of the matched controls, while mean body weights of the dosed females were more nearly comparable to those of the controls. A sufficient number of rats in each group was at risk to termination of the study at weeks 104-105 for the development of late-appearing tumors. In mice, administration of pyrazinamide had no consistent effect on mean body weights. Survival to termination of the study was low, particularly among the control groups. In rats, no lesions could clearly be related to administration of the chemical. In mice, interstitial and suppurative myocarditis in the dosed animals and suppurative bronchopneumonias in both dosed and matched control mice of each sex were associated with increased deaths. In the females, there was a significant positive dose-related trend (P=0.037) in the incidence of lymphoma (matched controls 0/13, low-dose 2/25, high-dose 6/29); however, the incidences in each of the dosed groups were not significant when compared with that in the matched controls. In addition, the poor survival and the small size of the control group precluded making a clear association of the incidence of these tumors with administration of the chemical. It is concluded that under the conditions of this bioassay, the early deaths and small size of the control group precluded a conclusion regarding the carcinogenicity of pyrazinamide in female B6C3F1 mice. Pyrazinamide was not carcinogenic for Fischer 344 rats or for male mice.