Modifications of DNA by platinum complexes. Relation to resistance of tumors to platinum antitumor drugs

Pt-bridges in various single-strand and double-helix DNA sequences. DFT and MP2 study of the cisplatin coordination with guanine, adenine, and cytosine

In this study, various platinum cross-links in DNA bases were explored. Some of these structures occur in many cis/trans-platinated double-helixes or single-stranded adducts. However, in the models studied, no steric hindrance from sugar-phosphate backbone or other surroundings is considered. Such restrictions can change the bonding picture partially but hopefully the basic energy characteristics will not be changed substantially. The optimization of the structures explored was performed at the DFT level with the B3LYP functional and the 6-31G(d) basis set. Perturbation theory at the MP2/6-31++G(2df,2pd) level was used for the single-point energy and 6-31+G(d) basis set for the electron-property analyses. It was found that the most stable structures are the diguanine complexes followed by guanine-cytosine Pt-cross-links, ca 5 kcal mol(-1) less stable. The adenine-containing complexes are about 15 kcal mol(-1) below the stability of diguanine structures. This stability order was also confirmed by the BE of Pt-N bonds. For a detailed view on dative and electrostatic contributions to Pt-N bonds, Natural Population Analysis, determination of electrostatic potentials, and canonical Molecular Orbitals description of the examined systems were used.

More pronounced salt dependence and higher reactivity for platination of the hairpin r(CGCGUUGUUCGCG) compared with d(CGCGTTGTTCGCG)

The DNA interference pathways exhibited by cisplatin and related anticancer active metal complexes have been extensively studied. Much less is known to what extent RNA interaction pathways may operate in parallel, and perhaps contribute to both antineoplastic activity and toxicity. The present study was designed with the aim of comparing the reactivity of two model systems comprising RNA and DNA hairpins, r(CGCGUUGUUCGCG) and d(CGCGTTGTTCGCG), towards a series of platinum(II) complexes. Three platinum complexes were used as metallation reagents; cis-[PtCl(NH3)2(OH2)]+ (1), cis-[PtCl(NH3)(c-C6H11NH2)(OH2)]+ (2), and trans-[PtCl(NH3)(quinoline)(OH2)]+ (3). The reaction kinetics were studied at pH 6.0, 25 degrees C, and 1.0 mM < or = I < or = 500 mM. For both types of nucleic acid targets, compound 3 was found to react about 1 order of magnitude more rapidly than compounds 1 and 2. Further, all platinum compounds exhibited a more pronounced salt dependence for the interaction with r(CGCGUUGUUCGCG). Chemical and enzymatic cleavage studies revealed similar interaction patterns with r(CGCGUUGUUCGCG) after long exposure times to 1 and 2. A substantial decrease of cleavage intensity was found at residues G4 and G7, indicative of bifunctional adduct formation. Circular dichroism studies showed that platinum adduct formation leads to a structural change of the ribonucleic acid. Thermal denaturation studies revealed platination to cause a decrease of the RNA melting temperatures by 5-10 degrees C. Our observations therefore suggest that RNA is a kinetically competitive target to DNA. Furthermore, platination causes destabilization of RNA structural elements, which may lead to deleterious intracellular effects on biologically relevant RNA targets.

Interactions of platinum complexes containing cationic, bicyclic, nonplanar piperidinopiperidine ligands with biological nucleophiles

The determination of the structures and DNA interactions and the reactions with GSH and ubiquitin of complexes of the general formula trans-[PtCl2(Am)(pip-pip)] x HCl, where pip-pip is 4-piperidinopiperidine and Am is NH3, methylamine (MA), dimethylamine (DMA), n-propylamine (NPA), isopropylamine (IPA), n-butylamine (NBA), or cyclohexylamine (CHA), were performed. X-ray structures and NMR studies of the NH3 and MA complexes showed that both pip rings were in the chair conformation and that the second pip ring is fluxional. The DNA binding studies showed that these complexes bind to calf thymus DNA nearly an order of magnitude more quickly than cisplatin and form covalent adducts that stabilize the double helix. The binding of the pip-pip complexes to DNA results in high unwinding angles (approximately 30 degrees) and in the formation of approximately 25% interstrand cross-links. The pip-pip complexes reacted with GSH more quickly than cisplatin and transplatin, and the rate of reaction decreased with increasing steric bulk of the ligand trans to the pip-pip. The reactions with ubiquitin resulted in monofunctional binding to Met1. Only the NH3, MA, and DMA complexes reacted with ubiquitin in a slower and less efficient fashion than cisplatin.

Modification and uptake of a cisplatin carbonato complex by Jurkat cells

The interactions of Jurkat cells with cisplatin, cis-[Pt(15NH3)2Cl2]1, are studied using 1H-15N heteronuclear single quantum coherence (HSQC) NMR and inductively coupled plasma mass spectrometry. We show that Jurkat cells in culture rapidly modify the monocarbonato complex cis-[Pt(15NH3)2(CO3)Cl]- (4), a cisplatin species that forms in culture media and probably also in blood. Analysis of the HSQC NMR peak intensity for 4 in the presence of different numbers of Jurkat cells reveals that each cell is capable of modifying 0.0028 pmol of 4 within approximately 0.6 h. The amounts of platinum taken up by the cell, weakly bound to the cell surface, remaining in the culture medium, and bound to genomic DNA were measured as functions of time of exposure to different concentrations of drug. The results show that most of the 4 that has been modified by the cells remains in the culture medium as a substance of molecular mass <3 kDa, which is HSQC NMR silent, and is not taken up by the cell. These results are consistent with a hitherto undocumented extracellular detoxification mechanism in which the cells rapidly modify 4, which is present in the culture medium, so it cannot bind to the cell. Because there is only a slow decrease in the amount of unmodified 4 remaining in the culture medium after 1 h, -1.1 +/- 0.4 microM h(-1), the cells subsequently lose their ability to modify 4. These observations have important implications for the mechanism of action of cisplatin.

Emerging cancer therapeutic opportunities target DNA-repair systems

DNA-damaging agents have a central role in non-surgical cancer treatment. The balance between DNA damage and repair determines the final therapeutic consequences. An elevated DNA-repair capacity in tumor cells leads to drug or radiation resistance and severely limits the efficacy of these agents. Interference with DNA repair has emerged as an important approach in combination therapy against cancer. Anticancer targets in DNA-repair systems have emerged, against which several small-molecule compounds are currently undergoing clinical trials.

Drug resistance in ovarian cancer: The emerging importance of gene transcription and spatio-temporal regulation of resistance

Resistance to carboplatin plus paclitaxel, one of the most active drug combinations in ovarian cancer, is the major barrier to the successful long-term treatment of this disease. Understanding the mechanisms involved is a first step towards rational strategies to overcome drug resistance and is an area of intense research effort. Recent work has identified several gene families which appear to contribute to the evolution of drug resistance and which are involved in regulating DNA damage, apoptosis and survival signalling. These genes may be co-ordinately regulated as part of a gene expression program that confers drug resistance through multiple pathways. The subcellular localisation of the gene products and their kinetic regulation following exposure to chemotherapeutic agents may also play a part in the development of drug resistance. This provides a more complex paradigm for drug resistance in which the steady-state expression of a single gene may not be predictive of response to therapy. Nevertheless, the identification of critical genes, most relevant to the development of clinical drug resistance, is now feasible through microarray analysis of tumour samples, and strategies aimed at the circumvention of resistance can be developed using these data.