Micronuclei induced by modulators of methylation: analogs of 5-azacytidine.

Jones and coworkers demonstrated a qualitative correlation between 5-azacytidine and some of its analogs in inducing changes in cell morphology and their ability in preventing DNA methylation. Previously, we evaluated the same compounds to determine their ability to induce trifluorothymidine (TFT) resistance in L5178Y mouse cells and found that their mutagenic potency also correlated with their reported ability to induce morphological changes in C3H10T1/2 cells. Here, we examine four of the same analogs, 5-fluoro-2'-deoxycytidine, 5-azacytidine, 5,6-dihydro-5-azacytidine and 6-azacytidine, to find out if micronuclei induced by these compounds correlated with these effects. The most cytotoxic analog was 5-fluoro-2'-deoxycytidine, followed by 5-azacytidine. 5,6-Dihydro-5-azacytidine and 6-azacytidine were substantially less cytotoxic. All four compounds induced micronuclei. The lowest dose ranges at which responses were observed for micronucleus induction were -0.04 microM for 5-fluoro-2'-deoxycytidine, 0.2 microM for 5-azacytidine and 10-20 microM for 5,6-dihydro-5-azacytidine and 6-azacytidine. Lack of kinetochore staining in most of the micronuclei indicated that all four compounds were clastogenic. We note a general trend in the biological activity of these analogs: compounds that are specifically blocked at the 5 position such as 5-azacytidine and 5-fluoro-2'-deoxycytidine effect changes in cell morphology, cytotoxicity, TFT resistance and the induction of micronuclei at very low doses. 5-Azacytidine analogs that possess more chemically accessible 5 positions such as 5,6-dihydro-5-azacytidine and 6-azacytidine either require doses that are orders of magnitude greater to induce these effects or are unable to induce changes in cell morphology and TFT resistance at doses below which the compound is lethal to the cells.