Nitrogen mustard alkylating agents are important
cancer drugs. Much interest has been focused on redirecting their covalent adducts from the N7 atoms of
guanine in the major groove of
DNA to the N3 atoms of
adenine in the minor groove by attaching mustard groups to AT-selective minor groove binding
ligands. Here we describe the use of electrospray ionization and matrix-assisted
laser desorption ionization/time-of-flight mass spectrometry to study the structure of the
DNA complexes of two minor groove binding polybenzamide mustards,
alkamin and alkamini; the former is a bis-half-mustard in which reactive groups are disposed at each end of the
ligand, and the latter is its monofunctional analog.
Alkamin is potently cytotoxic and active in experimental mouse
tumor models, whereas alkamini is not. We have studied their interaction with the
DNA dodecamer d(CGCGAATTCGCG)(2), designated A2T2, and we provide a detailed analysis of the observed
DNA-
ligand adduct
ions and their fragmentation products. We find that alkamini alkylates A2T2 at
guanine G4 and adenines A5 and A6 in a manner consistent with covalent attack on
purine N3 atoms from the minor groove of the AT tract.
Alkamin also forms monofunctional adducts at G4 and both adenines in which the second mustard arm is hydrolyzed but, in addition, forms a variety of interstrand cross-links between adenines A5/A6 and A5'/A6', an interstrand cross-link between G4 and A6', and an intrastrand cross-link between G4 and A6. We conclude that the marked cytotoxicity of
alkamin and its experimental antitumor activity could be the consequence of its ability to cross-link cellular
DNA at AT tract sequences.