Abstract:The low-spin 5d 6

Modifications of natural DNA in a cell-free medium by antitumor monodentate Ru(II) arene compounds of the general formula [(eta(6)-arene)Ru(en)Cl](+) (arene = biphenyl, dihydroanthracene, tetrahydroanthracene, p-cymene, or benzene; en = ethylenediamine) were studied by atomic absorption, melting behavior, transcription mapping, circular and linear dichroism, plasmid unwinding, competitive ethidium displacement, and differential pulse polarography. The results indicate that these complexes bind preferentially to guanine residues in double-helical DNA. The data are consistent with DNA binding of the complexes containing biphenyl, dihydroanthracene, or tetrahydroanthracene ligands that involves combined coordination to G N7 and noncovalent, hydrophobic interactions between the arene ligand and DNA, which may include arene intercalation and minor groove binding. In contrast, the single hydrocarbon rings in the p-cymene and benzene ruthenium complexes cannot interact with double-helical DNA by intercalation. Interestingly, the adducts of the complex containing p-cymene ligand, which has methyl and isopropyl substituents, distort the conformation and thermally destabilize double-helical DNA distinctly more than the adducts of the three multiring ruthenium arene compounds. It has been suggested that the different character of conformational alterations induced in DNA, and the resulting thermal destabilization, may affect differently further "downstream" effects of damaged DNA and consequently may result in different biological effects of this new class of metal-based antitumor compounds. The results point to a unique profile of DNA binding for Ru(II) arene compounds, suggesting that a search for new anticancer compounds based on this class of complexes may also lead to an altered profile of biological activity in comparison with that of metal-based antitumor drugs already used in the clinic or currently on clinical trials.

The DNA-binding profile of a novel, trinuclear platinum Phase I clinical agent (BBR3464) is summarized. The structure of BBR3464 is best described as two trans-[PtCl(NH3)2] units linked by a tetra-amine [trans-Pt(NH3)2{H2N(CH2)6NH2}2]2+ unit. The +4 charge of BBR3464, the presence of at least two Pt coordination units capable of binding to DNA, and the consequences of such DNA binding are remarkable departures from the cisplatin structural paradigm. The chemical and biological features argue that the drug should be considered the first clinical representative of an entirely new structural class of DNA-modifying anticancer agents. The high charge on BBR3464 facilitates rapid binding to DNA with a t1/2 of approximately 40 min, significantly faster than the neutral cisplatin. The melting temperature of DNA adducted by BBR3464 increased at low ionic strength but decreased in high salt for the same rb. This unusual behavior is in contrast to that of cisplatin. BBR3464 produces an unwinding angle of 14 degrees in negatively supercoiled pSP73 plasmid DNA, indicative of bifunctional DNA binding. Quantitation of interstrand DNA-DNA cross-linking in plasmid pSP73 DNA linearized by EcoRI indicated approximately 20% of the DNA to be interstrand cross-linked. While this is significantly higher than the value for cisplatin, it is, interestingly, lower than that for dinuclear platinum compounds such as [{trans-PtCl(NH3)2}2H2N(CH2)6NH2]2+ (BBR3005) where interstrand cross-linking efficiency may be as high as 70-90%. Either the presence of charge in the linker backbone or the increased distance between platinating moieties may contribute to this relatively decreased ability of BBR3464 to induce DNA interstrand cross-linking. Fluorescence experiments with ethidium bromide were consistent with the formation of long-range delocalized lesions on DNA produced by BBR3464. The sequence preference for BBR3464 on plasmid DNA was determined to the exact base pair by assaying extension of the polynucleotide by VentR(exo+) DNA polymerase. Strong sequence preference for single dG or d(GG) sites was suggested. The presence of relatively few blocks on DNA in comparison to either cisplatin or BBR3005 was indicative of high sequence selectivity. The following appropriate sequence where stop sites occur was chosen: [sequence: see text] molecular modeling on 1,4 interstrand (G'30 to G33) and 1,5 intrastrand (G33 to G29) cross-links further confirmed the similarity in energy between the two forms of cross-link. Finally, immunochemical analysis confirmed the unique nature of the DNA adducts formed by BBR3464. This analysis showed that antibodies raised to cisplatin-adducted DNA did not recognize DNA modified by BBR3464. In contrast, DNA modified by BBR3464 inhibited the binding of antibodies raised to transplatin-adducted DNA. Thus, the bifunctional binding of BBR3464 contains few similarities to that of cisplatin but may have a subset of adducts recognized as being similar to the transplatinum species. In summary, the results point to a unique profi...