Effects of Dihydro-5-azacytidine on Cell Survival and Cell Cycle Progression of Cultured Mammalian Cells

Abstract

The effects of dihydro-5-azacytidine [5-AC(H); NSC 264880] on cell viability, growth, and colony formation was investigated in suspension (Friend leukemia and L1210) and adherent [Chinese hamster ovary (CHO)] cell systems as well as in mitogen-stimulated normal human peripheral blood lymphocyte cultures. Cell cycle progression and the terminal point of action of the drug were monitored by flow cytometry. Formation of CHO cell colonies was inhibited by 50% following either a 13-hr exposure of exponentially growing cells to 25 µg 5-AC(H) per ml or 24-hr exposure to 11 µg 5-AC(H) per ml. Stationary cultures required a drug concentration more than 10 times higher to reduce colony formation by an equivalent degree. CHO cells during S phase were twice as sensitive as mitotic and G₁ cells and three times as sensitive as G₂ cells to the cytotoxic action of the drug. Drug concentrations of 50 and 35 µg/ml inhibited cell growth by 50% in suspension cultures of Friend leukemia and L1210 cells, respectively. Whereas constant exposure of Friend leukemia cells to 5-AC(H) led to a decrease in S-phase cells and an increase in G₂-phase cells, the percentage of S-phase cells in L1210 cultures increased both with increasing time of exposure and increasing drug concentration; the higher the dose, the earlier in S phase was the block. Lymphocytes exposed to 5-AC(H) either prior to or subsequent to mitogen exhibited diminished stimulation at high (500 µg/ml) drug concentrations. Cellular RNA content was decreased by 20 to 35% in suspension cultures and in mitogenstimulated lymphocytes exposed to 200 to 500 µg 5-AC(H) per ml for 48 hr.

Detailed analysis of cell cycle progression in L1210 cells in the presence of the drug determined that, while G₂-phase cells were refractory to the drug, cell progression through S phase was either slowed or halted. The drug also decreased the probability of cell exit from the indeterminate (G_1A) state but had no effect on the deterministic (G_1B) portion of G₁ phase. These findings are compared with those observed by others on the in vitro effects of the analog 5-azacytidine (NSC 102816).