Platinum nucleotides

It has been shown that cis - DDP, at low platinum/nucleotide ratios selectively inhibits the activity of several restriction endo - and exo nucleases when their cutting sites are adjacent to (d6)n (d )n sequences with n > 2 This demonstrates the sequence specificity of the binding of cis - DDP to DNA (17, 18, 19). GAG and GCG sequences also appear to be selectively involved in base - pair substitution mutagenesis and this suggests the possibility of platinum chelation by two guanines separated by a third base (20). 

Platinum coordination to C - N3 of Cyd and 5 - CMP gives smaller downfield shifts of the H6 and H5 signals, respectively 0.11-0.28 and 0.12-0.26 ppm (27, 29). In most cases, for the platinum- nucleotide complexes, one cannot observe, at high magnetic fields, the expected Pt - coupling constants because of a dominant Pt chemical shift anisotropy relaxation... 

Kasparkova, J. Zehnulova, J. Farrell, N. Brabec, V. DNA interstrand crosslinks of novel antitumor trinuclear platinum complex BBR. Conformation, recognition by HMG-domain proteins and nucleotide excision repair. J Biol Chem 2002, 277, 48076-48086. 

Kinetics and mechanisms of substitution at Pt(IV) are occasionally mentioned in relation to those complexes which may have anti-tumor properties. An article on molecular modeling of interactions between platinum complexes and nucleotides or DNA includes a brief mention of Pt(IV) (178). 

Rate constants for reaction of cis-[Pt(NH3)2(H20)Cl]+ with phosphate and with S - and 5/ -nucleotide bases are 4.6xl0-3, 0.48, and 0.16 M-1s-1, respectively, with ring closure rate constants of 0.17 x 10 5 and 2.55x10-5s-1 for subsequent reaction in the latter two cases 220). Kinetic aspects of interactions between DNA and platinum(II) complexes such as [Pt(NH3)3(H20)]2+, ds-[Pt(NH3)2(H20)2]2+, and cis-[Pt(NH3)2(H20)Cl]+, of loss of chloride from Pt-DNA-Cl adducts, and of chelate ring formation of cis-[Pt(NH3)2(H20)(oligonucleotide)]"+ intermediates implicate cis-[Pt(NH3)2(H20)2]2+ rather than cis-[Pt(NH3)2 (H20)C1]+, as usually proposed, as the most important Pt-binder 222). The role of aquation in the overall scheme of platinum(II)/DNA interactions has been reviewed 223), and platinum(II)-nucleotide-DNA interactions have been the subject of molecular modeling investigations 178). 

During in vivo studies under biologically relevant conditions, the cis-Pt loading of the DNA is much lower than for the above-mentioned in vitro studies. It has been calculated that mortality of HeLa cells occurs at an value of 10 5 (i.e., one bound cis-Pt molecule per 105 nucleotides) (64a). This excludes atomic absorption spectroscopy for identification of the in vivo adducts. Immunochemical techniques, however, have shown to be very promising, and high sensitivity and selectivity levels have been reached. At the moment, only a few studies in which antibodies are raised against cis-Pt-treated DNA (64) or against synthetic cis-Pt adducts with mono- or dinucleotides are available (64a). With the latter method, quantitation of the different platinum-DNA adducts formed under in vivo conditions is possible. At the moment, femtomole (10-15 mol) amounts of the adducts can be detected with competitive enzyme-linked immunosorbent assay (ELISA) techniques. It has been demonstrated in this manner that the GG-Pt adduct is also the predominant adduct under in vivo conditions. 

Molecular mechanics and dynamics studies of metal-nucleotide and metal-DNA interactions to date have been limited almost exclusively to modeling the interactions involving platinum-based anticancer drugs. As with metal-amino-acid complexes, there have been surprisingly few molecular mechanics studies of simple metal-nucleotide complexes that provide a means of deriving reliable force field parameters. A study of bis(purine)diamine-platinum(II) complexes successfully reproduced the structures of such complexes and demonstrated how steric factors influenced the barriers to rotation about the Pt(II)-N(purine) coordinate bonds and interconversion of the head-to-head (HTH) to head-to-tail (HTT) isomers (Fig. 12.4)[2011. In the process, force field parameters for the Pt(II)/nucleotide interactions were developed. A promising new approach involving the use of ab-initio calculations to calculate force constants has been applied to the interaction between Pt(II) and adenine[202]. 

The chemotherapeutic agent d.v-diammincdichloroplatinum(II), cis-DDP, or cisplatin, can form covalent adducts with many cellular macromolecules, but there is convincing evidence that its cytotoxic properties are a consequence of bifunctional-DNA adduct formation [ 1 ] [2]. Platinum binds to the N(7) position of purine nucleotides, resulting predominantly in 1,2-d(GpG) and l,2-d(ApG) intrastrand cross-links, but also in l,3-d(GpNpG) intrastrand, interstrand and protein-DNA cross-links [3][4], The 1,2-intrastrand cross-links, which comprise 90% of the DNA adducts, are not formed by the clinically inactive trans-DDP because of geometric constraints, and attention has therefore focused on these adducts as the active lesions in the anticancer activity of the drug. 

Cisplatin reacts preferentially with purine residues in DNA and forms mainly bifunctional lesions [3][10-12]. In vivo and in vitro, the major adducts are 1,2-intrastrand cross-links at the d(GpG) and d(ApG) sites (cis- Pt(NH3)2[d(GpG)-A7( 1 ),N7(2)] and cw- Pt(NH3)2[d(ApG)-A7(l), N7(2)] ) and they represent about 65 and 25% of the bound platinum respectively. Among the minor adducts are the interstrand cross-links between two guanine residues on opposite strands at the d(GpC) d(GpC) sites. The 1,3-intrastrand cross-links at the d(GpNpG) sites (N being a nucleotide residue) have been found, but their rate of formation is very slow [13] [14], The... 

More work is needed to clearly establish the role of outer-sphere association in DNA-platination. We can infer its influence on the rate of platination according to the relation kp = kK0 [N]/(l + K0 [N]) (with N = nucleotide-binding sites of Pt, i.e., N G, [N] [Pt]) (Scheme 3). It could also influence the selectivity of platination via selective association between the cationic species and the sites of higher negative electrostatic potential. To test this hypothesis one will have to analyze the influence of various sequences, of different types of platinum ligands, and of the ionic status of the DNA medium. 

Fig. 2), a nucleotide closely related to 5 -GMP (inosine is an analog of gua-nosine without the 2-NH2 group) [14]. This structure clearly shows the monodentate attachment of the purine to Pt through the N(7) atom. The Pt-N(7) distances are 2.02 A and the N(7)-Pt-N(7) angle is 89°. In this paper the authors speculated on the possible involvement of an N(7)-0(6) chelate from guanine to platinum, even though such a chelate was not actually present in the structure of cis- [Pt(NH3)2(5 -IMP)2]2. The issue of the N(7)-0(6) chelate will be discussed later in this article. 

In marked contrast to the relatively sparse structural results on Pt-nu-cleoside and Pt-nucleotide complexes, platinum complexes of modified nucleobases (i. e., without the sugar and phosphate groups) have been quite plentiful. They are surveyed in an extensive review by Lippert, Randaccio... 

Since the discovery of the antitumor activity of m-PtCNFhuC (cisplatin, cd-DDP) by Rosenberg et al. [1], the interactions of cisplatin with nucleotides and nucleobases have attracted attention towards gaining an understanding of the mechanism of the antitumor activity of cisplatin at a molecular level. In the course of such studies, dark-blue platinum complexes called platinum blues were obtained when hydrolysis products of cis-... 

---