BINDING OF PLATINUM AND PALLADIUM METALLOINTERCALATION REAGENTS AND ANTITUMOR DRUGS TO CLOSED AND OPEN DNAS

The interaction of Pt and Pd complexes with closed and nicked circular and linear DNA was investigated by a variety of methods. Cationic metal complexes containing flat, aromatic ligands, such as 2,2'',2"-terpyridine, [terpy], o-phenanthroline and 2,2''-bipyridine [bipy] interfere with the usual fluorescence enhancement of ethidium bromide by competing for intercalation sites on calf thymus DNA. Metal complexes having kinetically exchangable ligands, including the antitumor drugs cis-[(NH3)2PtCl2] and [(en) [ethylene diamine] PtCl2], inhibit noncompetitively the DNA associated ethidium fluroescence enhancement by binding covalently to the bases and blocking potential intercalation sites. Only the metallointercalators were capable of altering the DNA duplex winding, as judged by the effects of these reagents on the electrophoretic mobility and sedimentation behavior of PM-2 DNA. Long-term (t > 120 h) interactions of metal complexes with PM-2 DNAs I, I0 and II, corresponding to superhelical, closed relaxed and nicked circles, respectively, showed that covalent binding occurs the most readily to DNA I, possibly because of the presence of underwound duplex regions in this tightly wound superhelical DNA. The active antitumor drugs cis-[(NH3)2PtCl2] and [(en)PtCl2] bind covalently to DNA I under conditions where the inactive trans-[(NH3)2PtCl2] does not. Most of the complexes studied were capable of producing chain scissions in PM-2 DNA I. Exceptions are the kinetically inert complexes [(bipy)Pt(en)]2+ and [(terpy)Pt(SCH2CH2OH)]+, suggesting that covalent binding might be a prerequisite for nicking.