Overview of the interaction between chemotherapeutic agents and DNA.

Some chemotherapeutic agents, such as the antibiotics mitomycin and bleomycin, modify the structure of DNA by chemical reactions involving the formation or breakage of covalent bonds. Others interact with the macromolecule reversibly to form a transient complex which may be intercalative or nonintercalative in character. Techniques are available to probe the extent of perturbation of DNA structure produced by these drugs, and they reveal subtle differences between the effects of various ligands. In recent years bifunctional intercalating agents such as the quinoxaline antibiotics have been discovered; their binding to DNA is often tighter than seen with simple (monofunctional) intercalators and there is evidence for nucleotide sequence-selectivity. Footprinting experiments have been employed to identify preferred ligand-binding sites in natural DNA fragments (CpG sequences in the case of echinomycin) and have revealed that local perturbations of the helical structure can be propagated into DNA regions flanking the antibiotic-binding sites. Crystallographic evidence suggests that echinomycin and its congeners recognise GC base-pairs by hydrogen bonding between the carbonyl groups of alanine residues in the antibiotic and the 2-amino groups of guanine nucleotides in the minor groove of the DNA helix. Kinetic studies support the hypothesis that sequence-selective antibiotic molecules "shuffle" between different binding sites in the process of locating their optimal (preferred) sites.