Effect of cell cycle position on the survival of 9L cells treated with nitrosoureas that alkylate, cross-link, and carbamoylate.

The relationship between cell cycle position and cytotoxicity was studied in 9L rat brain tumor cells treated with nitrosoureas that, depending on their structures, can alkylate or alkylate and cross-link DNA and/or carbamoylate biomolecules. Because pure populations of G1-, S-, and G2-M-phase cells could not be obtained with the centrifugal elutriation methods used, drug sensitivity of cells in each phase of the cell cycle was estimated using a mathematical model that accounts for variation in enrichment of elutriated fractions. 1,3-Bis(2-chloroethyl)-1-nitrosourea, 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, 1-(2-chloroethyl)-3-(trans-4-methylcyclohexyl)-1-nitrosourea, 1-(2-chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea, which alkylate and cross-link DNA and carbamoylate biomolecules, and 2-[3-(2-chloroethyl)-3-nitrosoureido]-D-glucopyranose (chlorozotocin), which alkylates and cross-links DNA but cannot carbamoylate biomolecules, killed more cells in G1 and G2-M phases than in S phase. N-Ethylnitrosourea, which alkylates and carbamoylates but does not form DNA interstrand cross-links, was more toxic to cells in S phase than in other phases. Cell kill caused by N,N'-bis(trans-4-hydroxycyclohexyl)-N-nitrosourea, a compound that carbamoylates only, increased progressively through the cell cycle from G1 to M. Nitrosoureas that cross-link DNA were more cytotoxic than nitrosoureas that do not cross-link DNA, although the latter had phase specificity. The results suggest that the increased sensitivity of G1- and G2-M-phase cells to chloroethylnitrosoureas is related to the formation of DNA interstrand cross-links.