Mechanism of action of non-cisplatin type DNA-targeted platinum anticancer agents: DNA interactions of novel acridinylthioureas and their platinum conjugates

The DNA binding of two novel acridinylthioureas, ACR-NH-(CH2)(2)-C(S)-NHCH3 (1) and ACR-N(CH3)-C(S)-NHCH3 (3), and their platinum conjugates 4 and 5-derived from [PtCl2(en)]-was studied in cell-free model systems using various physico-chemical and biophysical methods. These included: spectrophotometric drug-DNA titrations, ethidium-DNA fluorescence quenching, competitive drug displacement, high-resolution NMR spectroscopy, and unwinding of plasmid DNA monitored by agarose gel electrophoresis. The acridinium cation of 1 showed strong binding to native DNA with K-i = 1.5 x 10(6) M-1 and an excluded site size (n) of 2 by (McGhee-von Hippel fits of absorbance data). Compound 3 showed no measurable association with DNA. Binding of 1 was an order of magnitude stronger than that of simple 9-methylaminoacridine (2). In alternating copolymers, 1 exhibited slight AT preference. In poly(dA-dT)(2), enhanced association was accompanied by an increased binding site (approximately 3 bp), while parameters in poly(dG-dC)(2) were consistent with classical intercalation. Displacement of 1 by distamycin from calf thymus DNA was suggestive of non-intercalating thiourea in 1 being located in the minor groove of the duplex. H-1 NMR data of d(GGAGCTCC)(2) modified with 1 indicated intercalative binding of planar acridine, based on upfield shifts of aromatic proton signals relative to those in unbound 1 (Deltadelta approximate to -0.5 to -1 ppm). Finally, 4 and 5 were found to unwind negatively supercoiled pUC19 plasmid by 21degrees and 7degrees per adduct, respectively (electrophoretic gel mobility assays). The difference in DNA binding modes of 4 and 5 is discussed as the ultimate source of the distinctly different biological activities of the conjugates. (C) 2002 Elsevier Science Inc. All rights reserved.