INTERSTRAND CROSS-LINKING OF DNA BY 1,3-BIS(2-CHLOROETHYL)-1-NITROSOUREA AND OTHER 1-(2-HALOETHYL)-1-NITROSOUREAS

Abstract

Bifunctional [antitumor] alkylating agents are known to cross link [Escherichia coli] DNA by simultaneously alkylating 2 guanine residues located on opposite strands. Despite this apparent requirement for bifunctionality, 1-(2-chloroethyl)-1-nitrosoureas bearing a single alkylating function were cross linked DNA in vitro. Cross linking was demonstrated by showing inhibition of alkali induced strand separation. Extensive cross linking was observed in DNA treated with 1-(2-chloroethyl)-1-nitrosourea, 1,3-bis(2-chloroethyl)-1-nitrosourea and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea. The reaction occurs in 2 steps, an initial binding followed by a 2nd step which can proceed after removal of unbound drug. The 1st step is chloroethylation of a nucleophilic site on 1 strand and the 2nd step involves displacement of Cl- by a nucleophilic site on the opposite strand, resulting in an ethyl bridge between the strands. Consistent with this possibility, 1-(2-fluoroethyl)-3-cyclohexyl-1-nitrosourea produced less cross linking, as expected from the known low activity of F-, compared with Cl-, as leaving group. 1-Methyl-1-nitrosourea, which is known to depurinate DNA, produced no detectable cross linking.