A rare example of three abundant conformers in one retro model of the cisplatin-DNA d(GpG) intrastrand cross link. Unambiguous evidence that guanine O6 to carrier amine ligand hydrogen bonding is not important. possible effect of the Lippard base pair step adjacent to the lesion on carrier ligand hydrogen bonding in DNA adducts

"Head-to-head" double-hamburger-like structure of di-ruthenated d(GpG) adducts of mono-functional Ru-arene anticancer complexes

Guanine bases in DNA are targets for some Ru-arene anticancer complexes. We have investigated the structure of the novel di-ruthenated d(GpG) adduct Ru₂-GpG (where Ru = {(η⁶-biphenyl)-Ru(en)}²⁺ (1')) in aqueous solution. 2D NMR results indicate that there are two conformers, supported by modeling studies. The major conformer I is a novel double-hamburger-like structure with a "head-to-head" (HH) base arrangement involving hydrophobic interactions between neighboring arene rings, the first example of a HH d(GpG) adduct constructed by weak interactions. Hence there are significant differences compared to Pt-d(GpG) adducts formed by cisplatin. There is no obviously rigid bending for the major conformer I. The minor conformer II of Ru₂-GpG has a back-to-back structure, with two ruthenated guanine bases flipped away from each other. 19-23 base-pair oligodeoxyribonucleotides containing central TGGT sequences di-ruthenated by 1 show no directional bending, only slightly distorted di-ruthenated duplexes, consistent with the NMR data for conformer I. The structural differences and similarities of d(GpG) residues which are di-ruthenated or cross-linked by platination are discussed in the context of the biological activity of these metal complexes.

A Very Rare Example of a Structurally Characterized 3'-GMP Metal Complex. NMR and Synthetic Assessment of Adducts Formed by Guanine Derivatives with [Pt(Ltri)Cl]Cl Complexes with an N,N',N″ Tridentate Ligand (Ltri) Terminated by Imidazole Rings

[Pt(N(R)-1,1'-Me₂dma)Cl]Cl complexes with tridentate ligands (bis(1-methyl-2-methylimidazolyl)amine, R = H; N-(methyl)bis(1-methyl-2-methylimidazolyl)amine, R = Me) were prepared in order to investigate Pt(N(R)-1,1'-Me₂dma)G adducts (G = monodentate N9-substituted guanine or hypoxanthine derivative). Solution NMR spectroscopy is the primary tool for studying metal complexes of nucleosides and nucleotides because such adducts rarely crystallize. However, [Pt(N(H)-1,1'-Me₂dma)(3'-GMPH)]NO₃·5H₂O (5) was crystallized, allowing, to our knowledge, the first crystallographic molecular structure determination for a 3'-GMP platinum complex. The structure is one of only a very few structures of a 3'-GMP complex with any metal. Complex 5 has the syn rotamer conformation, with 3'-GMP bound by N7. All Pt(N(R)-1,1'-Me₂dma)G adducts exhibit two new downfield-shifted G H8 signals, consistent with G bound to platinum by N7 and a syn/anti rotamer mixture. Anticancer-active monofunctional platinum(II) complexes have bulky carrier ligands that cause DNA adducts to be distorted. Hence, understanding carrier-ligand steric effects is key in designing new platinum drugs. Ligand bulk can be correlated with the degree of impeded rotation of the G nucleobase about the Pt-N7 bond, as assessed by the observation of rotamers. The signals of syn and anti rotamers are connected by EXSY cross-peaks in 2D ROESY spectra of Pt(N(H)-1,1'-Me₂dma)G adducts but not in spectra of Pt(N(H)dpa)G adducts (N(H)dpa = bis(2-picolyl)amine), indicating that rotamer interchange is more facile and carrier-ligand bulk is lower in Pt(N(H)-1,1'-Me₂dma)G than in Pt(N(H)dpa)G adducts. The lower steric hindrance is a direct consequence of the greater distance of the G nucleobase from the H4/4' protons in the N(R)-1,1'-Me₂dma carrier ligand in comparison to that from the H6/6' protons in the N(H)dpa carrier ligand. Although in 5 the nucleotide is 3'-GMP (not the usual 5'-GMP) and the N(H)-1,1'-Me₂dma carrier ligand is very different from those typically present in structurally characterized Pt(II) G complexes, the rocking and canting angles in 5 adhere to long-recognized trends.

DNA fragment conformations in adducts with Kiteplatin

The anticancer-active platinum complex with cis-1,4-diaminocyclohexane has proved to be very valuable in detecting multiple conformers in adducts with oligonucleotides.

An insulin-like growth factor-II intronic variant affects local DNA conformation and ovarian cancer survival

Insulin-like growth factor-II (IGF-II) may be a prognostic marker in ovarian cancer, and its intronic single nucleotide polymorphism (SNP) rs4320932 has been associated with risk of the disease. We determined whether rs4320932 is associated with IGF-II expression and patient survival in ovarian cancer, and explored whether the SNP variation affects DNA conformation both in the absence of and presence of carboplatin. IGF-II genotype (rs4320932) and phenotype were analyzed in 212 primary invasive epithelial ovarian cancer tissue samples with Taqman® SNP genotyping assays, quantitative reverse transcription-polymerase chain reaction and commercial enzyme-linked immunosorbent assay. DNA conformation was evaluated by circular dichroism (CD) spectra. Kaplan-Meier survival curves and Cox proportional hazard regression models were used to analyze the SNP associations with patient survival. The C allele of rs4320932, previously associated with decreased risk of ovarian cancer development, was here associated with significantly elevated risks of relapse (Ptrend = 0.0002) and death (Ptrend = 0.0006), remaining significant in multivariate analyses. The adjusted hazard ratios were 3.05 (95% confidence interval [CI]: 1.47-6.37) for relapse and 3.28 (95% CI: 1.64-6.57) for death, respectively. The variant was also significantly associated with chemotherapy response, but not with other clinicopathologic variables or with IGF-II expression. DNA with genotypes TT and CC had distinct CD spectra in both the absence of and presence of carboplatin. These findings suggest that the intronic SNP rs4320932 affects patient survival and chemotherapy response via alteration of DNA conformation, but not through regulation of IGF-II expression. This novel finding may have implications in individualized medicine for the design of specific molecules targeting DNA of specific conformations.

Cisplatin-related drugs for nongenomic targets: Forcing the reactivity with nucleobases

The products obtained by forcing the reaction with nucleosides (guanosine, Guo, and adenosine, Ado) of potential anticancer drugs for nongenomic targets [PtCl(O,O'-acac)(L)] (L = dimethyl sulfoxide, DMSO; dimethyl sulfide, DMS), closely related to their very powerful organometallic analogues [Pt(O,O'-acac)(γ-acac)(L)], have been studied. [PtCl(O,O'-acac)(L)] and [Pt(O,O'-acac)(γ-acac)(L)] complexes were reported unreactive toward nucleobases. Aquo species [Pt(O,O'-acac)H2O(L)]+, obtained from [PtCl(O,O'-acac)(L)] by Ag+ driven coordinated Cl– removal, gave access to [Pt(O,O'-acac)(L)(nucleoside)]+ ([Pt(O,O'-acac)(DMSO)(Guo)]+, [Pt(O,O'-acac)(DMS)(Guo)]+, [Pt(O,O'-acac)(DMSO)(Ado)]+). The effect of the chelate oxygen donor acac (with respect to a chelate diammine), the role of the sulfur ligand (DMSO, DMS), and the influence of the purinic nucleoside itself on the coordinated Guo or Ado dynamic motions in [Pt(O,O'-acac)(L)(nucleoside)]+ complexes have been investigated by NMR spectroscopy. Interestingly, a slow rotation of nucleobase around the Pt–N(7) bond with formation of two rotamers was observed already at room temperature only in the case of [Pt(O,O'-acac)(DMSO)(Guo)]+. On the other hand, no hindered rotation at room temperature was detected in the analogous [Pt(O,O'-acac)(DMS)(Guo)]+ and [Pt(O,O'-acac)(DMSO)(Ado)]+ complexes. Data suggest that rotation of the nucleoside in [Pt(O,O'-acac)(L)(nucleoside)]+ is very different with respect to the analogous [Pt(diammine)(L)(nucleoside)]2+ systems, due to specific interactions between the acac chelate ligand, the DMSO, and the nucleobase.

Guanine nucleobase adducts formed by [Pt(di-(2-picolyl)amine)Cl]Cl: evidence that a tridentate ligand with only in-plane bulk can slow guanine base rotation

Pt(II) complexes bind preferentially at N7 of G residues of DNA, causing DNA structural distortions associated with anticancer activity. Some distortions induced by difunctional cisplatin are also found for monofunctional Pt(II) complexes with carrier ligands having bulk projecting toward the guanine base. This ligand bulk can be correlated with impeded rotation about the Pt-N7(guanine) bond. Pt(N(H)dpa)(G) adducts (N(H)dpa = di-(2-picolyl)amine, G = 5'-GMP, 5'-GDP, 5'-GTP, guanosine, 9-EtG, and 5'-IMP) were used to assess whether a tridentate carrier ligand having bulk concentrated in the coordination plane can impede guanine nucleobase rotation. Because the Pt(N(H)dpa) moiety contains a mirror plane but is unsymmetrical with respect to the coordination plane, Pt(N(H)dpa)(G) adducts can form anti or syn rotamers with the guanine O6 and the central N-H of N(H)dpa on the opposite or the same side of the coordination plane, respectively. The observation of two sharp, comparably intense guanine H8 NMR signals provided evidence that these Pt(N(H)dpa)(G) adducts exist as mixtures of syn and anti rotamers, that rotational interchange is impeded by N(H)dpa, and that the key interactions involves steric repulsions between the pyridyl and guanine rings. The relative proximity of the guanine H8 to the anisotropic pyridyl rings allowed us to conclude that the syn rotamer was usually more abundant. However, the anti rotamer was more abundant for the Pt(N(H)dpa)(5'-GTP) adduct, in which a hydrogen bond between the 5'-GTP γ-phosphate group and the N(H)dpa central N-H is geometrically possible. In all previous examples of the influence of hydrogen bond formation on rotamer abundance in Pt(II) guanine adducts, these hydrogen bonding interactions occurred between ligand groups in cis positions. Thus, the role of a trans ligand group in influencing rotamer abundance, as found here, is unusual.

Single-stranded oligonucleotide adducts formed by Pt complexes favoring left-handed base canting: steric effect of flanking residues and relevance to DNA adducts formed by Pt anticancer drugs

Platinum anticancer drug binding to DNA creates large distortions in the cross-link (G*G*) and the adjacent XG* base pair (bp) steps (G* = N7-platinated G). These distortions, which are responsible for anticancer activity, depend on features of the duplex (e.g., base pairing) and of the cross-link moiety (e.g., the position and canting of the G* bases). The duplex structure stabilizes the head-to-head (HH) over the head-to-tail (HT) orientation and right-handed (R) over left-handed (L) canting of the G* bases. To provide fundamental chemical information relevant to the assessment of such duplex effects, we examine (S,R,R,S)-BipPt(oligo) adducts (Bip = 2,2'-bipiperidine with S,R,R,S chiral centers at the N, C, C, and N chelate ring atoms, respectively; oligo = d(G*pG*) with 3'- and/or 5'-substituents). The moderately bulky (S,R,R,S)-Bip ligand favors L canting and slows rotation about the Pt-G* bonds, and the (S,R,R,S)-BipPt(oligo) models provide more useful data than do dynamic models derived from active Pt drugs. All 5'-substituents in (S,R,R,S)-BipPt(oligo) adducts favor the normal HH conformer (∼97%) by destabilizing the HT conformer through clashes with the 3'-G* residue rather than through favorable H-bonding interactions with the carrier ligand in the HH conformer. For all (S,R,R,S)-BipPt(oligo) adducts, the S pucker of the 5'-X residue is retained. For these adducts, a 5'-substituent had only modest effects on the degree of L canting for the (S,R,R,S)-BipPt(oligo) HH conformer. This small flanking 5'-substituent effect on an L-canted HH conformer contrasts with the significant decrease in the degree of R canting previously observed for flanking 5'-substituents in the R-canted (R,S,S,R)-BipPt(oligo) analogues. The present data support our earlier hypothesis that the distortion distinctive to the XG* bp step (S to N pucker change and movement of the X residue) is required for normal stacking and X·X' WC H bonding and to prevent XG* residue clashes.

NMR studies of models having the Pt(d(GpG)) 17-membered macrocyclic ring formed in DNA by platinum anticancer drugs: Pt complexes with bulky chiral diamine ligands

The highly distorted Pt(d(G*pG*)) (G* = N7-platinated G) 17-membered macrocyclic ring formed by cisplatin anticancer drug binding to DNA alters the structure of the G*G* base pair steps, canting one base, and increases dynamic motion, complicating solution structural studies. However, the ring appears to favor the HH1 conformation (HH1 denotes head-to-head guanine bases, 1 denotes the normal direction of backbone propagation). Compared to cisplatin, analogues with NH groups in the carrier ligand replaced by bulky N-alkyl groups are more toxic and less active and form less dynamic adducts. To examine the molecular origins for the biological effects of steric bulk, we evaluate Me(4)DABPt(d(G*pG*)) models; the bulk and chirality of Me(4)DAB (N,N,N',N'-tetramethyl-2,3-diaminobutane with S,S or R,R configurations at the chelate ring carbons) impede dynamic motion and enhance the utility of NMR methods for identifying and characterizing conformers. Unlike past studies of adducts with such bulky carrier ligands, in which no HH conformer was found, the Me(4)DABPt(d(G*pG*)) adducts did form the HH1 conformer, providing compelling evidence that the sugar-phosphate backbone can impose constraints sufficient to overcome the alkyl-group steric effects. The HH1 conformer exhibits no significant canting. The (S,S)-Me(4)DABPt(d(G*pG*)) adduct has the least amount of the "normal" HH1 conformer and the greatest amount of the ΔHT1 conformer (ΔHT1 = head-to-tail G* bases with Δ chirality) ever observed (88% under some conditions). Thus, our results lead us to hypothesize that the low activity and high toxicity of analogues of cisplatin having carrier ligands with N-alkyl groups arise from the low abundance and minimal canting of the HH1 conformer and possibly from the adverse effects of an abundant ΔHT1 conformer. The new findings advance our understanding of the chemistry of the Pt(d(G*pG*)) macrocyclic ring and of the effects of carrier-ligand steric bulk on the properties of the ring.

Theoretical analysis of trans-[PtCl2(NH3)(thiazole)] and trans-[PtCl2(thiazole)2] binding to biological targets — Factors influence binding kinetics and adduct stability

Full reaction energy profiles for trans-[PtCl2(NH3)(thiazole)] and trans-[PtCl2(thiazole)2] binding to sulfur- and nitrogen-containing biorelevant ligands were constructed by the density functional theory (DFT) method. Calculated results demonstrate that trans-platinum complexes can interact with biological targets, affording cis and trans products via very similar transition states. For different substituents, sulfur-containing ligands constitute kinetically preferred targets for platination, whereas the platination of nitrogen-containing ligands is more favorable thermodynamically. This is consistent with previous experimental studies. Calculated results also suggest that the trans effect, the influence of the ligand, the size of the ligand, and hydrogen bonding play important roles in binding kinetics and stabilizing adducts.

On the many roles of NH3 ligands in mono- and multinuclear complexes of platinum

The role of the NH(3) ligands in the highly successful antitumour agents cisplatin and carboplatin is not fully understood. Suggestions that the ammonia ligands are involved in target recognition through hydrogen bond formation, e.g. with guanine-O6, have been questioned. Here, we review the roles and functions of NH(3) ligands of cis-PtCl(2)(NH(3))(2) and likewise of its trans-isomer in complexes with model nucleobases as well as other N-heterocyclic ligands. Specifically, their roles in hydrogen bonding interactions with nucleobases as well as anions, the influence on acid-base properties of co-ligands, their involvement in condensation reactions, as well as a variety of displacement reactions will be examined. As a result, it can be stated that the ammonia ligands in cis- and trans-Pt(II)(NH(3))(2) entities display additional features to those generally discussed in the last four decades since the discovery of the antitumour activity of cisplatin.

Unusual DNA binding modes for metal anticancer complexes

DNA is believed to be the primary target for many metal-based drugs. For example, platinum-based anticancer drugs can form specific lesions on DNA that induce apoptosis. New platinum drugs can be designed that have novel modes of interaction with DNA, such as the trinuclear platinum complex BBR3464. Also it is possible to design inert platinum(IV) pro-drugs which are non-toxic in the dark, but lethal when irradiated with certain wavelengths of light. This gives rise to novel DNA lesions which are not as readily repaired as those induced by cisplatin, and provides the basis for a new type of photoactivated chemotherapy. Finally, newly emerging ruthenium(II) organometallic complexes not only bind to DNA coordinatively, but also by H-bonding and hydrophobic interactions triggered by the introduction of extended arene rings into their versatile structures. Intriguingly osmium (the heavier congener of ruthenium) reacts differently with DNA but can also give rise to highly cytotoxic organometallic complexes.

Head-to-head right-handed cross-links of the antitumor-active bis(mu-N,N'-di-p-tolylformamidinato)dirhodium(II,II) unit with the dinucleotides d(GpA) and d(ApG)

Reactions of cis-[Rh(2)(DTolF)(2)(NCCH(3))(6)](BF(4))(2) with the dinucleotides d(GpA) and d(ApG) proceed to form [Rh(2)(DTolF)(2){d(GpA)}] and [Rh(2)(DTolF)(2){d(ApG)}], respectively, with bridging purine bases spanning the Rh-Rh unit in the equatorial positions. Both dirhodium adducts exhibit head-to-head (HH) arrangement of the bases, as indicated by the presence of H8/H8 NOE cross-peaks in the 2D ROESY NMR spectra. The guanine bases bind to the dirhodium core at positions N7 and O6, a conclusion that is supported by the absence of N7 protonation at low pH values and the notable increase in the acidity of the guanine N1H sites (pK(a) approximately 7.4 in 4:1 CD(3)CN/D(2)O), inferred from the pH-dependence titrations of the guanine H8 proton resonances. In both dirhodium adducts, the adenine bases coordinate to the metal atoms through N6 and N7, which induces stabilization of the rare imino tautomer of the bases with a concomitant substantial decrease in the basicity of the N1H adenine sites (pK(a) approximately 7.0-7.1 in 4:1 CD(3)CN/D(2)O), as compared to the imino form of free adenosine. The presence of the adenine bases in the rare imino form is further corroborated by the observation of DQF-COSY H2/N1H and ROE N1H/N6H cross-peaks in the 2D NMR spectra of [Rh(2)(DTolF)(2){d(GpA)}] and [Rh(2)(DTolF)(2){d(ApG)}] in CD(3)CN at -38 degrees C. The 2D NMR spectroscopic data and the molecular modeling results suggest the presence of right-handed variants, HH1R, in solution for both adducts (HH1R refers to the relative base canting and the direction of propagation of the phosphodiester backbone with respect to the 5' base). Complete characterization of [Rh(2)(DTolF)(2){d(GpA)}] and [Rh(2)(DTolF)(2){d(ApG)}] by 2D NMR spectroscopy and molecular modeling supports anti-orientation of the sugar residues for both adducts about the glycosyl bonds as well as N- and S-type conformations for the 5'- and 3'-deoxyribose residues, respectively.

Inhibition of transcription by platinum antitumor compounds

Cisplatin, carboplatin, and oxaliplatin are three FDA-approved members of the platinum anticancer drug family. These compounds induce apoptosis in tumor cells by binding to nuclear DNA, forming a variety of structural adducts and triggering cellular responses, one of which is the inhibition of transcription. In this report we present (i) a detailed review of the structural investigations of various Pt-DNA adducts and the effects of these lesions on global DNA geometry; (ii) research detailing inhibition of cellular transcription by Pt-DNA adducts; and (iii) a mechanistic analysis of how DNA structural distortions induced by platinum damage may inhibit RNA synthesis in vivo. A thorough understanding of the molecular mechanism of action of platinum antitumor agents will aid in the development of new compounds in the family.

Unprecedented head-to-head right-handed cross-links between the antitumor bis(mu-N,N'-di-p-tolylformamidinate) dirhodium(II,II) core and the dinucleotide d(ApA) with the adenine bases in the rare imino form

Reactions of the anticancer active compound cis-[Rh2(DTolF)2(CH3CN)6](BF4)2 with 9-ethyladenine (9-EtAdeH) or the dinucleotide d(ApA) proceed with bridging adenine bases in the rare imino form (A*), spanning the Rh-Rh bond at equatorial positions via N7/N6. The inflection points for the pH-dependent H2 and H8 NMR resonance curves of cis-[Rh2(DTolF)2(9-EtAdeH)2](BF4)2 correspond to N1H deprotonation of the metal-stabilized rare imino tautomer, which takes place at pKa approximately 7.5 in CD3CN-d3, a considerably reduced value as compared to that of the imino form of 9-EtAdeH. Similarly, coordination of the metal atoms to the N7/N6 adenine sites in Rh2(DTolF)2{d(ApA)} induces formation of the rare imino tautomer of the bases with a concomitant substantial decrease in the basicity of the N1H sites (pKa approximately 7.0 in CD3CN-d3), as compared to the imino form of the free dinucleotide. The presence of the adenine bases in the rare imino form, due to bidentate metalation of the N6/N7 sites, is further corroborated by DQF-COSY H2/N1H and ROE N1H/N6H cross-peaks in the 2D NMR spectra of Rh2(DTolF)2{d(ApA)} in CD3CN-d3 at -38 degrees C. Due to the N7/N6 bridging mode of the adenine bases in Rh2(DTolF)2{d(ApA)}, only the anti orientation of the imino tautomer is possible. The imino form A* of adenine in DNA may result in AT-->CG transversions or AT-->GC transitions, which can eventually lead to lethal mutations. The HH arrangement of the bases in Rh2(DTolF)2{d(ApA)} is indicated by the H8/H8 NOE cross-peaks in the 2D ROESY NMR spectrum, whereas the formamidinate bridging groups dictate the presence of one right-handed conformer HH1R in solution. Complete characterization of Rh2(DTolF)2{d(ApA)} by 2D NMR spectroscopy and molecular modeling supports the presence of the HH1R conformer, anti orientation of both sugar residues about the glycosyl bonds, and N-type conformation for the 5'-A base.

Head-to-head cross-linked adduct between the antitumor unit bis(mu-N,N'-di-p-tolylformamidinato)dirhodium(II,II) and the DNA fragment d(GpG)

Reactions of the compound cis-[Rh2(DTolF)2(CH3CN)6](BF4)2, a formamidinate derivative of the class of antitumor compounds [Rh2(O2CR)4] (R=Me, Et, Pr), with 9-ethylguanine (9-EtGuaH) or the dinucleotide d(GpG) proceed by substitution of the acetonitrile groups, with the guanine bases spanning the Rh--Rh bond, in a bridging fashion, through sites N7/O6. In the case of 9-EtGuaH, both head-to-head (HH) and head-to-tail (HT) isomers are formed, whereas with the tethered bases in d(GpG), only one right-handed conformer HH1R [Rh2(DTolF)2{d(GpG)}] is present in solution. For both cis-[Rh2(DTolF)2(9-EtGuaH)2](BF4)2 and [Rh2(DTolF)2{d(GpG)}], the absence of N7 protonation at low pH and the substantial decrease of the pKa values for N1-H deprotonation, support N7/O6 binding of the bases to the dirhodium core. The N7/O6 binding of the bases is further corroborated by the downfield shift by Deltadelta approximately 4.0 ppm of the 13C NMR resonances for the C6 nuclei as compared to the corresponding resonances of the free ligands. The HH arrangement of the guanine bases in [Rh2(DTolF)2{d(GpG)}] is indicated by the intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum. Complete characterization of the [Rh2(DTolF)2{d(GpG)}] conformer by 2D NMR spectroscopy supports anti-orientation and N (C3'-endo) conformation for both deoxyribose residues. The N-pucker for the 5'-G base is universal in such cross-links, but it is very unusual for platinum and unprecedented for dirhodium HH cross-linked adducts to have both deoxyribose residues in the N-type conformation. The bulk, the nonlabile character, and the electron-donating ability of the formamidinate bridging groups spanning the dirhodium core affect the nature of the preferred dirhodium DNA adducts. Molecular modeling studies performed on [Rh2(DTolF)2{d(GpG)}] corroborate the structural features obtained by NMR spectroscopy.

Loss of amine from platinum(II) complexes: implications for cisplatin inactivation, storage, and resistance

Potential consequences of the binding of the anticancer drug cisplatin to various biomolecules in the cell have been investigated by using a combined density functional theory and continuum dielectric model approach. Since the amine ligands remain coordinated at the metal upon formation of the most frequent DNA adducts, whereas they were found to be displaced from the metal upon formation of drug metabolites, we have analyzed the factors governing amine loss from platinum(II) complexes as a possible pathway of cisplatin inactivation. The calculations systematically show the effect of 1) the trans ligand, 2) the charge of complex, 3) the nucleophile, and 4) the environment on the thermodynamic instability and kinetic lability of the platinum-amine bonds. After initial binding of cisplatin hydrolysis products to thioethers or thiols, loss of the amine trans to this sulfur ligand rather than replacement of the sulfur ligand itself by other nucleophiles like guanine-N7 is predicted to be the predominant reaction. The results of this study contribute to an understanding of the modes of cisplatin inactivation prior to DNA binding, for example, by elevated glutathione levels in cisplatin-resistant cancer cells.

Fixing the conformations of diamineplatinum(II)-GpG chelates: NMR and CD signatures of individual rotamers

The bulky, asymmetric analog of the antitumor drug cisplatin, [PtCl(2)(tmen)] (tmen = N,N,N'-trimethylethylenediamine), was used to produce crosslinks with the dinucleotide d(GpG), modeling the most frequent lesions that cisplatin and its analogs cause to DNA. The ligand tmen was chosen because it is expected to constrain the guanine cis to the NMe(2) group in the adduct [Pt(tmen){d(GpG)}](+) to an orientation perpendicular to the coordination plane and to stabilize the other guanine in an oblique orientation, thus maintaining a head-to-head geometry typical of cisplatin-d(GpG) crosslinks within single- and double-stranded DNA. Of the four possible combinations of tmen chirality (R or S symmetry of the coordinated NHMe group) and crosslink direction (5'-G bound cis to the secondary or the tertiary amino group of tmen), two isomers were preponderantly formed, [Pt(R-tmen){d(GpG)}](+) with 5'-G bound cis to NMe(2) and [Pt(S-tmen){d(GpG)}](+) with 5'-G bound cis to NHMe. The former was shown to have a right-handed R2 orientation of guanines similar to that found in duplex DNA, whereas the latter had a left-handed L1 orientation that modeled cisplatin-d(GpG) adducts within single-stranded DNA. The R2 rotamer was found to be in an equilibrium (as observed using EXSY spectroscopy) with a minor fraction (< or =4%) of a Delta-HT rotamer related to R2 by rotation of the 3'-G about the Pt-N7 bond. The major rotamers R2 and L1 were isolated using reverse-phase HPLC, and their NMR and CD signatures were compared to those of the corresponding rotamers of the less hindered adduct [Pt(dmen)(GpG)](+) (dmen = N,N-dimethylethylenediamine). From this and other comparisons with previously reported platinum dinucleotide complexes, and from molecular modeling, it could be concluded that both steric repulsion between guanine and substituents of the cis amino group and N-H...O6 hydrogen bonding are significant effects favoring the oblique orientation of one guanine base typical of the HH rotamers of [Pt(diamine){d(GpG)}](+) and [Pt(diamine)(GpG)](+) complexes.

New water-soluble platinum(ii) phenanthroline complexes tested as cisplatin analogues: first-time comparison of cytotoxic activity between analogous four- and five-coordinate species

Four- and five-coordinate platinum(II) complexes, cis-[PtCl2(A2)] (1) and [PtCl2(A2)(eta2-ethylene)] (2) {A2 = 4,7-diphenyl-1,10-phenanthroline disulfonic acid disodium salt, BPS (mixture of isomers) (a); 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline disulfonic acid disodium salt, BCS (mixture of isomers) (b)} have been synthesized and characterized by 1H, 13C, and 195Pt NMR spectroscopy. The stability and high water solubility of complexes 1a, 1b and 2b, due to the presence of the polar SO3- groups on the ligands skeleton, allowed to test their in vitro cytotoxicity on HeLa tumour cells in a wide range of drug concentration. At low and medium incubation doses (<200 microM) 1a, 1b and 2b all showed similar in vitro cytotoxicity, negligible or much lower with respect to cisplatin. At doses higher than 200 microM their activity increased and 1b, the most active among the new complexes, exhibited a cytotoxicity comparable, although still lower, with respect to cisplatin. GFAAS Platinum analytical data showed that the tested compounds 1a, 1b and 2b, although carrying sulfonate charged groups, may undergo cellular uptake, which, in the case of 1b and 2b, is even higher with respect to cisplatin. Furthermore, in the case of 1b and 2b it has been possible to compare, for the first time, the cytotoxic activity for square-planar four-coordinate and trigonal-bipyramidal five-coordinate platinum(II) complexes having the same carrier ligand. The tendency of the five-coordinate species 2b to give at longer incubation time similar cytotoxicity with respect to the square-planar compound 1b suggests a possible use of the trigonal-bipyramidal five-coordinate complexes as prodrugs.

Novel binding interactions of the DNA fragment d(pGpG) cross-linked by the antitumor active compound tetrakis(mu-carboxylato)dirhodium(II,II)

Insight into the N7/O6 equatorial binding interactions of the antitumor active complex Rh(2)(OAc)(4)(H(2)O)(2) (OAc(-) = CH(3)CO(2)(-)) with the nucleotide 5'-GMP and the DNA fragment d(pGpG) has been obtained by one- (1D) and two-dimensional (2D) NMR spectroscopy. The lack of N7 protonation at low pH values and the significant increase in the acidity of N1-H (pK(a) approximately 5.6 as compared to 8.5 for N7 only bound platinum adducts), indicated by the pH dependence study of the H8 (1)H NMR resonance for the HT (head-to-tail) isomer of Rh(2)(OAc)(2)(5'-GMP)(2), are consistent with bidentate N7/O6 binding of the guanine. The H8 (1)H NMR resonance of the HH (head-to-head) Rh(2)(OAc)(2)(5'-GMP)(2) isomer, as well as the 5'-G and 3'-G H8 resonances of the Rh(2)(OAc)(2) [d(pGpG)] adduct exhibit pH-independent titration curves, attributable to the added effect of the 5'-phosphate group deprotonation at a pH value similar to that of the N1 site. The enhancement in the acidity of N1-H, with respect to N7 only bound metal adducts, afforded by the O6 binding of the bases to the rhodium centers, has been corroborated by monitoring the pH dependence of the purine C6 and C2 (13)C NMR resonances for Rh(2)(OAc)(2)(5'-GMP)(2) and Rh(2)(OAc)(2) [d(pGpG)]. The latter studies resulted in pK(a) values in good agreement with those derived from the pH-dependent (1)H NMR titrations of the H8 resonances. Comparison of the (13)C NMR resonances of C6 and C2 for the dirhodium adducts Rh(2)(OAc)(2)(5'-GMP)(2) and Rh(2)(OAc)(2) [d(pGpG)] with the corresponding resonances of the unbound ligands at pH 8.0, showed substantial downfield shifts of Deltadelta approximately 11.0 and 6.0 ppm, respectively. The HH arrangement of the bases in the Rh(2)(OAc)(2) [d(pGpG)] adduct is evidenced by intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum. The presence of the terminal 5'-phosphate group in d(pGpG) results in stabilization of one left-handed Rh(2)(OAc)(2) [d(pGpG)] HH1 L conformer, due to the steric effect of the 5'-group, favoring left canting in cisplatin-DNA adducts. Complete characterization of the Rh(2)(OAc)(2[d(pGpG)] adduct revealed notable structural features that resemble those of cis-[Pt(NH(3))(2) [d(pGpG)]]; the latter involve repuckering of the 5'-G sugar ring to C3'-endo (N-type) conformation, retention of C2'-endo (S-type) 3'-G sugar ring conformation, and anti orientation with respect to the glycosyl bonds. The superposition of the low energy Rh(2)(OAc)(2) [d(pGpG)] conformers, generated by simulated annealing calculations, with the crystal structure of cis-[Pt(NH(3))(2) [d(pGpG)]], reveals remarkable similarities between the adducts; not only are the bases almost completely destacked upon coordination to the metal in both cases, but they are favorably poised to accommodate the bidentate N7/O6 binding to the dirhodium unit. Unexpectedly, the two metal-metal bonded rhodium centers are capable of engaging in cis binding to GG intrastrand sites by establishing N7/O6 bridges that span the Rh-Rh bond.

NMR and theoretical investigations on the structures and dynamics of octahedral bis(chelate)dichloro V(III) compounds isolated by an unusual reduction of non-oxo V(IV) species

Reaction of the non-oxo V(IV) species [V(IV)Cl(2)(L(OO))(2)] [L(OO) = acetylacetonate (acac(-)) or benzoylacetonate (bzac(-))] with a chelate nitrogen-donor ligand L(NN) in acetonitrile leads to the reduction of V(IV) to V(III) and the formation of the mononuclear V(III) compounds of the general formula [V(III)Cl(2)(L(OO))(L(NN))] (L(OO) and L(NN) are acac(-) and bipy for 1; acac- and 5,5'-me(2)bipy for 2; acac(-) and 4,4'-tb(2)bipy for 3; acac(-) and phen for 4; bzac(-) and bipy for 5; bzac(-) and phen for 6). The reduction of the V(IV) complexes was monitored by GC-MS and (1)H NMR spectroscopy. Both one- and two-dimensional (2D COSY and 2D EXSY) (1)H NMR techniques were used to assign the observed (1)H NMR resonances of 1-6 in CD(2)Cl(2) or CDCl(3) solution. It appeared that in solution these V(III) complexes form two isomers which are in equilibrium: cis-[V(III)Cl(2)(L(OO))(L(NN))] <==> trans-[V(III)Cl(2)(L(OO))(L(NN))]. 2D EXSY cross-peaks were clearly observed between bipy- and acac-hydrogen atoms of the two geometrical isomers of 1-3 as well as between bipy and acac(-) protons of the cis isomer, indicating a dynamic process that corresponds to cis-trans isomerization and a cis-cis racemization. The thermodynamic and kinetic parameters of the equilibrium between these two isomers were calculated for compounds 1 and 2 by using variable temperature (VT) NMR data. Both cis-trans isomerization and cis-cis racemization processes probably proceed with an intramolecular twist mechanism involving a trigonal prismatic transition state. Density functional calculations (DFT) also indicated such a rearrangement mechanism.

Synthesis, characterization, and DNA binding properties of a series of Ru, Pt mixed-metal complexes

A series of mixed-metal complexes coupling ruthenium light absorbers to platinum reactive metal sites through polyazine bridging ligands have been prepared of the form [(tpy)RuCl(BL)PtCl(2)](PF(6)) (BL = 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq), 2,3-bis(2-pyridyl)benzoquinoxaline (dpb); tpy = 2,2':6',2' '-terpyridine). These systems possess electron-rich Ru metal centers bound to five polyazine nitrogens and one chloride ligand. This leads to complexes with low-energy Ru --> BL charge-transfer bands that are tunable with BL variation occurring at 544, 632, and 682 nm for dpp, dpq, and dpb, respectively. This tuning of the charge-transfer energy results from a stabilization of the BL(pi) orbitals in this series as evidenced by the cathodic shift in the first reduction of these complexes occurring at -0.50, -0.32, and -0.20 V vs Ag/AgCl, for dpp, dpq, and dpb, respectively. The chlorides bound to the Pt(II) center are substitutionally labile giving these complexes the ability to covalently bind to DNA. All three title bimetallics, [(tpy)RuCl(BL)PtCl(2)](PF(6)), avidly bind double-stranded DNA with t(1/2) = 1-2 min, substantially reducing the migration of DNA through an agarose gel. Details of the synthetic methods, FAB MS data, spectroscopic and electrochemical properties, and DNA binding studies are presented.

Transition States of Cisplatin Binding to Guanine and Adenine: ab initio Reactivity Study

Fully optimised HF and DFT transition states of cisplatin binding to adenine and guanine are presented for the first time. They have similar structure as the recently published transition states for cisplatin hydrolysis with the angle of about 70° between entering and leaving ligands and corresponding bonds prolonged up to 0.5 Å. Calculated activation energies are in the range of 10.5-18 kcal/mol. The lowest activation energies were found for the binding of cis-Pt[(NH3)2(H2O)(OH)]+ to guanine. The role of hydrogen bonds in recognition of binding sites, stabilisation of reactants and final yields of individual cisplatin-DNA adducts is discussed.

2.4-A crystal structure of the asymmetric platinum complex [Pt(ammine)(cyclohexylamine)]2+ bound to a dodecamer DNA duplex

cis-trans-cis-Ammine(cyclohexylamine)diacetatodichloroplatinum(IV) is an oral analog of the platinum anti-cancer drug cisplatin that is currently in phase III clinical trials. Its active form, [Pt(ammine)(cyclohexylamine)]2+, binds to DNA similarly to cisplatin, forming intra- and interstrand cross-links between adjacent purine bases. Since [Pt(ammine)(cyclohexylamine)]2+ contains two different ligands, it can form two isomeric 1,2-d(GpG) intrastrand cross-links. Here we report the 2.4-A resolution x-ray crystal structure of the major adduct between [Pt(ammine)(cyclohexylamine)]2+ and a DNA dodecamer, using the same sequence as previously reported for crystal structures of cisplatin-DNA (Takahara, P. M., Rosenzweig, A. C., Frederick, C. A., and Lippard, S. J. (1995) Nature 377, 649-652) and oxaliplatin-DNA (Spingler, B., Whittington, D. A., and Lippard, S. J. (2001) Inorg. Chem. 40, 5596-5602). Both duplexes in the asymmetric unit contain 1,2-intrastrand cross-links in which the cyclohexylamine ligand is directed toward the 3'-end of the platinated strand. The chair conformation of the cyclohexyl group is clearly resolved. Platination distorts the duplex, resulting in a global bend angle of about 38(o) and a dihedral angle between platinated guanine bases of approximately 31(o). Both end-to-end and end-to-groove packing interactions occur in the crystal lattice, the latter positioned in the minor groove across from the site of the platinum cross-link. A high degree of homology observed between this structure and the previously reported platinum-DNA structures suggests that these platinum complexes distort the DNA duplex in a very similar manner. These results suggest that differences in activity between these drugs are unlikely to result from gross conformational distortions in DNA structure following platinum intrastrand cross-link formation.

On the competition of the purine bases, functionalities of peptide side chains, and protecting agents for the coordination sites of dicationic cisplatin derivatives

The Pt-L bond energies of simple triammineplatinum(II) complexes, [Pt(NH(3))(3)L](2+), with oxygen-, nitrogen-, and sulfur-containing donor ligands L have been predicted and rationalized using density functional theory. The ligands L have been chosen as models for functionalities of peptide side chains, for sulfur-containing protecting agents, and for adenine and guanine sites of the DNA as the ultimate target of platinum anticancer drugs. Calculation of the Pt-L bond energy in [Pt(NH(3))(3)L](2+) reveals that the soft metal center of triammineplatinum(II) prefers N ligands over S ligands. This remarkable result has been discussed in light of several interpretations of the hard and soft acids and bases principle. The concept of orbital-symmetry-based energy decomposition has been employed for the determination of the contributions from sigma and pi orbital interactions, electrostatics, and intramolecular hydrogen bonding to the Pt-L bond energy. The calculations show that considerable differences in the bond energies of the triammineplatinum(II) complexes with N-heterocycles such as 1-methylimidazole, 9-methyladenine, and 9-methylguanine arise from electrostatics rather than from orbital interactions. Surprisingly, the net stabilization by hydrogen bonding between the (Pt)N-H group and the oxygen of 9-methylguanine is as weak as the intramolecular hydrogen bond in the aqua complex [Pt(NH(3))(3)(H(2)O)](2+), challenging the common hypothesis that DNA-active anticancer drugs require carrier ligands with N-H functionalities because of their hydrogen-bonding ability. The influence of a polarizable environment on the stability of the complexes has been investigated systematically with the dependence of the dielectric constant epsilon. With increasing epsilon, the complexes with S-containing ligands are more strongly stabilized than the complexes of the N-containing heterocycles. At epsilon = 78.4, the dielectric constant of water, 9-methylguanine remains the only purine derivative investigated which is competitive to neutral sulfur ligands. These findings are particularly important for a rationalization of the results from recent experimental studies on the competition of biological donor ligands L for coordination with the metal center of [Pt(dien)L](2+) (dien = 1,5-diamino 3-azapentane).