DNA sequence selectivity of guanine-N7 alkylation by three antitumor chloroethylating agents

The DNA sequence selectivities of guanine-N7 alkylation produced by three chloroethylating antitumor agents, 1-(2-chloroethyl)-3-(cis-2-hydroxy) cyclohexyl-1-nitrosourea (cis-2-OH CCNU), 2-chloroethyl (methylsulfonyl)methanesulfonate, and 8-carbamoyl-3-(2-chloroethyl)imidazo-[5,1-d]-1,2,3,5-tetrazin-4(3H )-one (mitozolomide), were examined using a modification of the Maxam and Gilbert sequencing technique. In a region of pBR322 DNA, 2-chloroethyl (methylsulfonyl)methanesulfonate produced approximately the same degree of alkylation at all guanines. cis-2-OH CCNU, however, preferentially alkylated the middle guanines in runs of three or more guanines; the intensity of the reaction increased with the number of adjacent guanines in the DNA sequence. Mitozolomide produced the same pattern of preferential alkylation but not as intensely as cis-2-OH CCNU. Three other nitrosoureas, 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, 1-(2-fluorethyl)-3-cyclohexyl-1-nitrosourea, and 1-(2-chloroethyl)-1-nitrosourea gave similar patterns of alkylation to that of cis-2-OH CCNU at pH 7.2. The ratio of 7-hydroxyethylguanine to 7-chloroethylguanine was approximately the same following treatment of the synthetic polymers dGn X dCn and (dG X dC)n with cis-2-OH CCNU, indicating that 7-chloroethylation and 7-hydroxyethylation were enhanced similarly by the presence of adjacent guanines. Ethylnitrosourea produced relatively little alkylation preference. The results suggest that the alkylating intermediates, 2-chloroethyldiazohydroxide and 2-hydroxyethyldiazohydroxide, tend to react preferentially with those guanine-N7 positions the electronegativity of which is enhanced by the presence of neighboring guanines. This is consistent with the presence of cationic character in the alkylating centers of these intermediates. 2-Chloroethyl (methylsulfonyl)methanesulfonate and ethyldiazohydroxide would not be expected to have strong cationic character, in agreement with their lack of sequence selectivity.