Environmental Factors in Glial Tumor Response to 1-(2-Chloroethyl)-3-cyclohexyl-1-nitrosourea: Effect of Cell Confluence2

Abstract

The action of the chemotherapeutic agent 1-(2-chloroethyl)-3-cyclohexyl-1-nltrosourea (CCNU) on C-S rat glioma growing in monolayer culture was studied. C-8 cells were slightly more sensitive to CCNU when exposed during the phase of the cell cycle in which DNA was synthesized. The sensitivity of the cells was Inversely related to the cell densities of the monolayers. Cells in confluent regions of the cultures became labeled with [³H]thymidine at about 70% of the frequency of relatively isolated cells in the same cultures as determined in radioautographs of the monolayers. CCNU (100 μM, 1 hr) caused a moderate decrease of thymidine incorporation into confluent cells while effectively halting all incorporation into nonconfluent cells. The effect of the confluence on sensitivity to the drug was much greater than could be accounted for by the small decrease in growth rate caused by confluence and which also decreases C-8 sensitivity to CCNU. Since confluence varies with cell density, this factor could make a significant contribution to the increasing resistance with increasing density. Studies of the effect of plateau-phase growth on CCNU response, in which growth slowing is caused by high cell density, could also be affected by the confluence factor independently of the effect of proliferation rate on drug response.