Thioguanine-induced S and G2 blocks and their significance to the mechanism of cytotoxicity.

The delayed cytotoxic effect of 6-thioguanine (TG) was studied using L1210 mouse leukemic cells in culture. The cell cycle distribution of a population treated continuously with 10(-5) M TG was compared to that of control cells using flow cytometric analysis. The TG-treated cells had an increase in the fraction of the population in G2-M, a decrease in G1, and a constant level in S phase. However, the [methyl-3H]thymidine-labeling index decreased dramatically during TG treatment. Thus, it appeared that some cells were arrested in S phase and that G1 cells did not enter S phase, due to failure to synthesize DNA. To examine the importance of the G2 and S cell progression blocks, cells were exposed to a lethal treatment of 10(-5) M TG for 12 hr and returned to normal medium. Under these conditions, the fraction of the population in both S and G1 decreased, and nearly one-half of the cells accumulated in G2 by 60 hr after TG addition, compared to a G2 fraction of less than one-tenth for the control cells. These results showed that the delayed cytotoxic effect of TG was associated with a cell progression block in the second G2 phase after TG addition, whereas the retention of cells in S phase appeared to be due to readily reversible secondary effects of TG.