DNA Alkylation, Cross-Linking, and Cancer Cell Killing by a Quinoxaline-<i>N</i>-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 (<b>Chb</b>). 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 <b>Chb</b> against a variety of cancer cell lines. Evidence is presented, showing that both the quinoxaline-mustard and the clinically used drug <b>Chb</b> 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.