Effects of the tricyclic nucleoside 6-amino-4-methyl-8-(beta-D-ribofuranosyl)- pyrrolo[4,3,2-de]pyrimido[4,5-c]pyridazine on the viability and cell cycle distribution of L1210 cells in vitro.

TCN (1 microM) totally inhibited the growth of L1210 cells in culture and caused progressive loss of cellular viability, as indicated by a decreased clonogenicity and nigrosin dye exclusion. After 24 h or more of TCN treatment, a significant fraction of the cells had shrunk in size but did not fragment into dye-impermeable vesicles as reported previously for N1S1-67 hepatoma cells (P. G. W. Plagemann, J. Natl. Cancer Inst., 57: 1283-1295, 1976). TCN-induced growth inhibition was accompanied by a block of cell cycle progression in G1 or at the G1-S boundary. At all TCN concentrations studied, progression of cells out from behind this block was evident as a depletion of the early S-phase population in comparison to controls, while increasing the concentration of TCN (0.1 to 1 microM) led to a progressive retention of cells in S phase, suggesting a slowing of progression through S phase. The fraction of S-phase cells incorporating [methyl-3H]thymidine and the amount of [methyl-3H]thymidine incorporated per labeled cell were both decreased by TCN treatment. Increasing the concentration of TCN (0.1 to 1 microM) progressively decreased DNA synthesis and increased cell lethality. Thus it appeared that inhibition of DNA synthesis might cause the retention of cells in S phase which is associated with TCN lethality.