| Abstract
| - We have used stopped-flow spectrophotometry and the sodium dodecyl sulfate sequestrationtechnique to study the kinetics of dissociation of DNA complexes of the mixed topoisomeraseI/II poison N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (termed DACA) and a range ofrelated linear tricyclic carboxamides with neutral chromophores. Complexes of DACA andrelated acridine and phenazinecarboxamides bearing an N,N-dimethylaminoethyl side chaindissociate from calf thymus DNA by a kinetic pathway involving four discernible steps in amanner similar to complexes of N-[(2-dimethylamino)ethyl]-9-aminoacridine-4-carboxamide(termed 9-amino-DACA). We infer from these findings that the side chains of DACA, itsphenazine homologue, and 9-amino-DACA make comparable interactions with the DNA basepairs. In the case of 9-amino-DACA, a selective topoisomerase II poison, these are known, bycrystallographic analysis, to involve hydrogen-bonding interactions between the protonateddimethylammonium group of the side chain and the O6/N7 atoms of guanine and to include abridging water molecule hydrogen bonded to the carboxamide group and a phosphate oxygen.By contrast, we find that other linear tricyclic carboxamides with neutral chromophores whichlack a peri nitrogen atom and are biologically inactive dissociate from DNA by a differentmechanism in which it appears their side chains fail to interact with guanine. We concludethat the ability of the carboxamide group to lie preferentially in the plane of the chromophore,so facilitating the dimethylammonium−guanine hydrogen bond and ensuring maintenance ofthe water-bridged carboxamide−phosphate interaction, is a critical requirement for antitumoractivity among ligands of the linear tricyclic carboxamide class. However, unlike the situationfor 9-amino-DACA, for ligands with uncharged chromophores containing peri nitrogen atomssuch as DACA, this outcome is possible with the 4-carboxamide group rotated cis or transwith respect to the ring nitrogen. This difference may have relevance to the ability of DACAto be a dual poison of both topoisomerases I and II.
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