DNA Alkylation, Cross-Linking, and Cancer Cell Killing by a Quinoxaline-N-Mustard Conjugate

Nitrogen mustards are a family of clinically used anticancer drugs that contain a DNA-alkylating bis(2-chloroethyl)amino group. Appending the bis(2-chloroethyl)amino alkylating agent to noncovalent DNA-binding groups such as intercalators, polyamides, or polyamines has the potential to yield DNA-targeted anticancer agents with improved potency. In the work reported here, substituted quinoxaline groups were explored as minimal intercalators expected to confer noncovalent DNA-binding properties on a bis(2-chloroethyl)anilino mustard alkylating unit. A quinoxaline unit with a cationic dimethylamino-containing side chain was found to be a more potent DNA-alkylating and cross-linking agent than the clinically used mustard chlorambucil (Chb). The results of dye displacement and multiple DNA alkylation assays showed that the quinoxaline ring binds noncovalently to duplex DNA, likely via intercalation. The quinoxaline-mustard conjugate was more active than Chb against a variety of cancer cell lines. Evidence is presented, showing that both the quinoxaline-mustard and the clinically used drug Chb formed aggregates in aqueous buffer; however, the results clearly show that the propensity to form aggregates clearly does not abrogate the DNA-alkylating properties or bioactivity of these compounds.