Interactions of Anticancer Active Platinum(II) Complexes with DNA

Sammanfattning: The kinetics and mechanism for interactions of the anticancer active platinum(II) complexes cis-[PtCl2(NH3)2)], cis-[PtCl2(NH3)(c-C6H11NH2)], and trans-[PtCl2-(NH3)(quinoline)] with DNA have been studied using a combination of HPLC, circular dichroism (CD), UV/vis spectroscopy, and differential scanning calorimetry (DSC). Reactions of the monoaqua forms (charge = +1) of these complexes with single-stranded DNA oligonucleotides, typically between 13-17 bases long, were found to depend on i) the composition of the reaction medium, ii) the exact nature and location of the platination target, and iii) on the DNA structure. The platination reactions were studied in the presence of Na+, Mg2+ and spermidine (spd3+) cations. Increasing concentrations and increasing charge of these electrolytes reduce the rate of platination significantly. The results suggest that the rate dependence on cation concentration occurs as a result of the polyanionic nature of the oligonucleotides, rather than variations of the oligomer structure. Analyses of the kinetic data as a function of the salt dependence suggest that reactions between charged platinum(II) complexes and mononucleotides, for example dGMP or the sulfur-containing nucleotides 4-thiouridine, s4-UMP, and 6-thioinosine, s6IMP, can be described using Brønsted-Debye-Hückel theory. In contrast, paltination reactions of oligonucleotides are better described using polyelectrolyte theory. Results from analyses according to the polyelectrolyte theory suggest that the platinum complex associates with the oligonucleotide during displacement of counter ions from its surface. The associated platinum complex diffuses in reduced dimensions along the oligonucleotide, thus increasing the probability of finding the preferred coordination site. The location dependent kinetics for the oligonucleotides d(Tnp(S)T16-n), where n = 1, 4, 8, 12, and 15, was investigated. The rate of platination is most rapid in the middle of the oligonucleotide, and the reactivity gradually decreases towards both ends, as a consequence of the variation of the charge density along the oligonucleotide. This location dependent kinetics is maximised at low salt concentration, and an increase of the concentration of salt and/or the charge of the electrolytes diminishes the variation in reactivity. Platination in the presence of spd3+ changes the mechanism to one that involves a direct attack of the target on the oligonucleotide. The hairpins are preferentially platinated in the most solvent accessible loop region. As a result the platinated structure destabilizes and the DNA structure changes. The last observation is particularly interesting considering the function of transient DNA hairpins as regulatory factors during gene expression.