sourea, and l-(2-chlornethyl)-1-nifrosoureagave similar patterns of al kylation to that of cis-2-OH CCNU at pH 7.2. The ratio of 7-hydroxy

The DNA sequenceselectivitiesof guanine-N7alkylation producedby three chloroethylating antitumor agents, I-(2-chloroethyl)-3-(cis-2-hy droxy)cyclohexyl-l-nitrosourea(cis-2-OH CCNU), 2-chloroethyl(meth ylsulfonyl)methanesulfonate,and 8-carhamoyl-3-(2-chloroethyl)imidazo 15,l-dl-l,2,3,5-tetrazin-4(3H)-one (mitozolomide), were examined using a modification of the Maxam and Gilbert sequencing technique. In a region ofpBR322 DNA, 2-chloroethyl(methylsulfonyl)methanesulfonate produced approximately the same degree of alkylation at all guanines. cis-2-OH CCNU, however,preferentiallyalkylated the middleguanines in runs of three or more guanines the intensity of the reaction increased with the number of adjacent guanines in the DNA sequence. Mitozolom ide produced the same pattern of preferential alkylation but not as intensely as cis-2-OH CCNU. Three other nitrosoureas, 1-(2-chloro ethyl)-3-cyclohexyl-1-nitrosourea, l-(2-fluorethyl)-3-cyclohexyl-1-nitro sourea, and l-(2-chlornethyl)-1-nifrosoureagave similar patterns of al kylation to that of cis-2-OH CCNU at pH 7.2. The ratio of 7-hydroxy ethylguanine to 7-chloroethylguanine was approximately the same follow ing treatment of the synthetic polymersdG@•dC. and (dG.dC). with cis 2-OH CCNU, indicatingthat 7-chloroethylationand 7-hydroxyethylation were enhanced similarly by the presence of adjacent guanines. Ethylni trosourea produced relatively little alkylation preference. The results suggest that the alkylating intermediates, 2-chloroethyldiazohydroxide and 2-hydroxyethyldiazohydroxide, tend to react preferentiallywiththose guanine-N7 positions the electronegativity of which is enhanced by the presenceof neighboringguanines.This is consistent with the presenceof cationic character in the alkylating centers of these intermediates. 2Chioroethyl (methylsulfonyl)methanesulfonate and ethyldiazohydroxide would not be expected to have strong cationic character, in agreement with their lack of sequence selectivity.