Diaplacental carcinogenic effects of 5-azacytidine in NMRI-mice.

5-Azacytidine was applied to NMRI-mice (1-2 mg/kg) either on gestation day 12, 14, or 16. In the first case it mainly induced leukemias, while in the latter experiments leukemias, lung adenomas and soft tissue sarcomas represent the main effects. The experiments performed on gestation day 14 led to tumor rates below the spontaneously occurring tumor frequencies in NMRI-mice. There is a clear-cut inverse dose-response relationship in leukemia induction, as the higher dose principally shows a lower degree of effectiveness. This, as well as a reduction of tumor frequency below control levels after application of this drug on day 14, can be explained by an "overkill" effect. The cytotoxic and embryolethal efficacy of the agent thus surpasses the transformation effects at the cellular genome. While a negative correlation between the general embryotoxicity of azacytidine and the simultaneous tumor inducibility is to be observed, there is no correlation at the target organ level between the embryotoxic and the carcinogenic effects.