Abstract

The acridine antitumor drug 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA), which is currently in Phase II clinical trial, is known to be an inhibitor of nucleic acid synthesis. The intercalative binding of this drug to native calf thymus DNA has been studied using equilibrium dialysis, spectrophotometry, and competition with ethidium. All three techniques indicate an intrinsic association constant of approximately 1.5 x 10⁵ M^-1 at an ionic strength of 0.01. The binding isotherm is adequately described by a neighboring-site exclusion model, and indicates a site size of approximately two base pairs. At elevated ionic strength the association constant is depressed by a factor which corresponds closely to that predicted for a monocationic ligand. Studies with a variety of synthetic polynucleotides indicate that association constants for binding sites of different sequence vary over a 10-fold range. m-AMSA binds to heat-denatured calf thymus DNA or ribosomal RNA with association constants, respectively, 5- and 25-fold lower than that for native DNA. Comparable measurements have been made with related acridinylaminomethanesulfonanilide (AMSA) drugs and simple aminoacridines; they reveal that the presence of an unsubstituted methanesulfonanilide ring does not noticeably interfere with binding to native DNA. However, addition of a methoxy substituent at the 3' position of this ring, as in m-AMSA, may sterically hinder intercalation of the acridine nucleus of the drug. The antitumor activity of the series of ligands studied could not be correlated in any straightforward fashion with nucleic acid binding parameters, although the conspicuous ability of AMSA drugs to discriminate sensitively between native DNA and RNA may result in efficient binding to DNA in vivo.