Effects of geometric isomerism in dinuclear platinum antitumor complexes on aquation reactions in the presence of perchlorate, acetate and phosphate

Probing Disaccharide Binding to Triplatin as Models for Tumor Cell Heparan Sulfate (GAG) Interactions

In this study, we have used [1H, 15N] NMR spectroscopy to investigate the interactions of the trinuclear platinum anticancer drug triplatin (1) (1,0,1/t,t,t or BBR3464) with site-specific sulfated and carboxylated disaccharides. Specifically, the disaccharides GlcNS(6S)-GlcA (I) and GlcNS(6S)-IdoA(2S) (II) are useful models of longer-chain glycosaminoglycans (GAGs) such as heparan sulfate (HS). For both the reactions of 15N-1 with I and II, equilibrium conditions were achieved more slowly (65 h) compared to the reaction with the monosaccharide GlcNS(6S) (9 h). The data suggest both carboxylate and sulfate binding of disaccharide I to the Pt with the sulfato species accounting for <1% of the total species at equilibrium. The rate constant for sulfate displacement of the aqua ligand (kL2) is 4 times higher than the analogous rate constant for carboxylate displacement (kL1). There are marked differences in the equilibrium concentrations of the chlorido, aqua, and carboxy-bound species for reactions with the two disaccharides, notably a significantly higher concentration of carboxylate-bound species for II, where sulfate-bound species were barely detectable. The trend mirrors that reported for the corresponding dinuclear platinum complex 1,1/t,t, where the rate constant for sulfate displacement of the aqua ligand was 3 times higher than that for acetate. Also similar to what we observed for the reactions of 1,1/t,t with the simple anions, aquation of the sulfato group is rapid, and the rate constant k-L2 is 3 orders of magnitude higher than that for displacement of the carboxylate (k-L1). Molecular dynamics calculations suggest that extra hydrogen-bonding interactions with the more sulfated disaccharide II may prevent or diminish sulfate binding of the triplatin moiety. The overall results suggest that Pt-O donor interactions should be considered in any full description of platinum complex cellular chemistry.

1H and 195Pt NMR prediction for inclusion compounds formed by cisplatin and oxidized carbon nanostructures

Prediction of NMR chemical shifts can assist experimentalists in the characterization of drug delivery systems based on carbon nanocomposites. Chemical shifts are strongly correlated to the nucleus position and its chemical neighborhood. Therefore, to predict structures and NMR properties of complex chemical models, choosing a more consistent theoretical level capable of providing more realistic results and moderate computational demand is a major challenge. In this work, we predicted the NMR spectra of inclusion compounds formed by cisplatin (cDDP) and an oxidized carbon nanotube (CNTox) and nanocone (CNCox) considered by specialists as potential drug delivery systems. The ¹⁹⁵Pt NMR chemical shifts calculated at the DFT level with the new relativistic NMR-DKH basis set were −2314 ppm and −2192 ppm for cDDP@CNTox and cDDP@CNCox complexes, respectively, which are both high-field shifted relative to the free cDDP (−2110 ppm). ¹H NMR chemical shifts are also sensitive to the inclusion process. The H (NH₃) signals are found on average at +4.3 (cDDP), −5.1 (cDDP@CNTox) and +6.6 ppm (cDDP@CNCox). Interestingly, despite the similar inclusion modes in CNTox and CNCox cavities, the ¹H NMR shifts were in opposite directions. A possible reason might be the higher stability of cDDP@CNTox (ΔE_F = −19.9 kcal mol⁻¹) than that of cDDP@CNCox (ΔE_F = −5.7 kcal mol⁻¹), which suggests a short guest–host contact in the former and consequently, a more efficient shielding of hydrogen atoms due to the electron-rich carbon structure. These results may be helpful as comparison data in the NMR spectra assignment in solution and the inclusion compounds' structural elucidation.

¹⁹⁵Pt NMR chemical shifts of the cisplatin molecule in the inclusion complex formed by oxidized carbon nanotubes.

Biological relevance of interaction of platinum drugs with O-donor ligands

Platinum complexes with S and N-donor small molecule ligands have received much attention with respect to understanding of Pt-protein and Pt-DNA(RNA) interactions in biology. Oxygen-donor ligands have received less attention, partly due to the fact that as a hard Lewis base, oxygen-donor interactions are expected to be less favourable for the soft Lewis acid properties of Pt(II), especially. Yet, it is now clear that for a full understanding of the cellular fate of platinum complexes, a plethora of oxygen-donor interactions are possible, considering extracellular and intracellular concentrations of simple anions in buffer. Further, the importance of the general class of glycans, the third major class of biomolecules after proteins and nucleic acids, contain many specific examples of important biomolecules such as sialic acids and sulphated glycosaminoglycans capable of metal complex interactions. In this contribution we summarise some important kinetic and thermodynamic aspects of platinum-oxygen-donor ligand interactions and their relevance to examples of biomolecular interactions contributing to the overall profile of platinum (and metal complexes in general) biology.

Diplatinum(II) Catecholate of Photoactive Boron-Dipyrromethene for Lysosome-Targeted Photodynamic Therapy in Red Light

The binuclear platinum(II) boron-dipyrromethene (BODIPY) complex [{Pt(dach)}₂(μ-Dcrb)] (DP), where dach is 1,2-diaminocyclohexane and H₄Dcrb is a morpholine-conjugated BODIPY-linked dicatechol bridging ligand, was prepared for lysosome organelle targeting and near-IR (NIR) light (600-720 nm) induced photocytotoxic activity. The platinum complex [Pt(dach)(cat)] (CP), where H₂cat is catechol, was synthesized and used as a control complex without bearing the BODIPY unit. The complex DP displayed a band at 660 nm (ε = 2.1 × 10⁴ M^-1 cm^-1) in the red region of the UV-visible spectrum recorded in 10% dimethyl sulfoxide/Dulbecco's Modified Eagle's Medium (DMSO/DMEM, pH 7.2). The complex DP and the BODIPY ligand displayed emission in 10% DMSO-DMEM (pH 7.2) giving an λ_em value of 668 nm (λ_ex = 650 nm) with a Φ_F value of 0.02 for DP and 0.16 for H₄Dcrb (Φ_F, fluorescence quantum yield). Titration experiments using 1,3-diphenylisobenzofuran (DPBF) indicated that the complex DP and H₄Dcrb on irradiation with near-IR light of 600-720 nm generated singlet oxygen (¹O₂) as the ROS (reactive oxygen species). The complex DP showed significant lysosomal localization and remarkable apoptotic photodynamic therapy (PDT) effects, giving half-maximal inhibitory concentration values (IC₅₀) within 0.6-3.4 μM in HeLa cervical cancer, A549 lung cancer, and MDA-MB231 multidrug resistant cancer cells, while being essentially nontoxic in the dark and in the HPL1D immortalized lung epithelial normal cells. The acridine orange assay using A549 cells showed lysosomal membrane permeabilization by the complex DP under near-IR light (600-720 nm). This complex on near-IR light (600-720 nm) activation in A549 cells induced apoptotic cell death, as observed from an Annexin-V FITC assay.

Influence of geometric isomerism on the binding of platinum anticancer agents with phospholipids

Reported herein is a detailed NMR and DFT study of the interaction of the 15N-labelled dinuclear platinum anticancer compound [{cis-PtCl(NH3)2}2{μ-H2N(CH2)6NH2}]2+ (15N-1, 1,1/c,c) with 1,2-dihexanoyl-sn-glycero-3-phosphate (DHPA), as a comparison with an earlier study of the interaction of the same water-soluble phospholipid fragment with the geometric trans isomer (1,1/t,t). The reaction of 15N-1 with the sodium salt of DHPA was studied at 298 K, pH ∼ 5.6, by [1H,15N] HSQC 2D NMR spectroscopy. The NMR data, supported by DFT models, provide evidence that the monofunctional DHPA adduct of 15N-1 exists in two conformational forms, with different orientation of the (CH2)6 linker; one has an interaction between the unbound {PtN3Cl} moiety and the coordinated DHPA molecule. Similarly, two bifunctional adduct conformers are identified, in which one has an interaction between the phosphate groups of the two bound DHPA molecules. When compared to the previously reported reactions of 1,1/t,t with DHPA, equilibrium conditions of the 1,1/c,c reaction are reached more slowly (120 h), similar to the reaction with phosphate. The rate constant for the first step of DHPA binding (kL) is slightly lower (1.6 fold) for the cis-compared to the trans-isomer, whereas the rate constant for the reverse reaction is 4-fold lower, resulting in a much greater proportion of DHPA bound species at equilibrium.

Glycans as Ligands in Bioinorganic Chemistry. Probing the Interaction of a Trinuclear Platinum Anticancer Complex with Defined Monosaccharide Fragments of Heparan Sulfate

We report herein a detailed NMR study of the aquation and subsequent covalent binding of the trinuclear clinical agent [{ trans-PtCl(¹⁵NH₃)₂}₂{μ- trans-Pt(¹⁵NH₃)₂(¹⁵NH₂(CH₂)₆¹⁵NH₂)₂}]⁴⁺ (1, 1,0,1/ t, t, t or Triplatin) with three d-glucosamine residues containing varied O-sulfate and N-sulfate or N-acetyl substitutions, which represent monosaccharide fragments present within the repeating disaccharide sequences of cell surface heparan sulfate (HS). The monosaccharides GlcNS(6S), GlcNS, and GlcNAc(6S) were synthesized in good yield from a common 4,6-diol α-methyl glucopyranoside intermediate. The reactions of ¹⁵N-1 with sodium sulfate, GlcNS(6S), GlcNS, and GlcNAc(6S) were followed by 2D [¹H,¹⁵N] heteronuclear single quantum coherence (HSQC) NMR spectroscopy using conditions (298 K, pH ≈5.4) similar to those previously used for other anionic systems, allowing for a direct comparison. The equilibrium constants (p K₁) for the aquation of 1 in the presence of GlcNS(6S) and GlcNS were slightly higher compared to that of the aquation in a sulfate solution, while a comparable p K₁ value was observed in the presence of GlcNAc(6S). A comparison of the rate constants for sulfate displacement of the aqua ligand showed preferential binding to 2- N-sulfate compared to 6- O-sulfate but a more rapid liberation. For disulfated GlcNS(6S), equilibrium conditions were achieved rapidly (9 h) and strongly favored the dichloro form, with <2% sulfato species observed. The value of k_L1 was up to 15-fold lower than that for binding to sulfate, whereas the rate constant for the reverse ligation ( k_-L1) was comparable. Equilibrium conditions were achieved much more slowly (∼ 100 h) for the reactions of 1 with GlcNS and GlcNAc(6S), attributed to covalent binding also to the N-donor of the sulfamate (GlcNS) group and the O-donor of the N-acetyl [GlcNAc(6S)] group. The rate constants ( k_L2) were 20-40-fold lower than that for binding to the 2- N- or 6- O-sulfate, but the binding was less reversible, so that their equilibrium concentrations (5-8%) were comparable to the 2- N- or 6- O-sulfate-bound species. The results emphasize the relevance of glycans in bioinorganic chemistry and underpin a fundamental molecular description of the HS-Pt interactions that alter the profile of platinum agents from cytotoxic to metastatic in a systematic manner.

Antiangiogenic platinum through glycan targeting

Heparan sulfate is identified as a ligand receptor for polynuclear platinum anti-cancer agents through sulfate cluster binding. We present a new biological role for platinum and coordination compounds and a new target for metal-based drugs while presenting a new chemotype for heparanase and growth factor inhibition through modulation (metalloshielding) of their interactions. Masking of extracellular (ECM)-resident heparan sulfate (HS) through metalloshielding results in very effective inhibition of physiologically critical HS functions including enzyme (heparanase, HPSE) and protein growth factor recognition. The interaction of the highly cationic polynuclear platinum complexes (PPCs) with the highly sulfated pentasaccharide Fondaparinux (FPX, in this case as a model HS-like substrate) results in inhibition of its cleavage by the HS-related enzyme heparanase. Binding of the fibroblast growth factor FGF-2 to HS is also inhibited with consequences for downstream signalling events as measured by a reduction in accumulation of phospho-S6 ribosomal protein in human colon tumor HCT-116 cells. The end-point of inhibition of HPSE activity and growth factor growth factor signaling is the prevention of cell invasion and angiogenesis. Finally these events culminate in inhibition of HCT-116 cell invasion at sub-cytotoxic concentrations and the process of angiogenesis. A competition assay shows that Fondaparinux can sequester the 8+ TriplatinNC from bound DNA, emphasising the strength of PPC-HS interactions. Altering the profile of platinum agents from cytotoxic to anti-metastatic has profound implications for future directions in the development of platinum-based chemotherapeutics.

The conformation effect of the diamine bridge on the stability of dinuclear platinum(II) complexes and their hydrolysis

In this paper, the hydrolysis process of a bisplatinum complex containing the flexible chain 1,6-hexanediamine between the two metal centers was investigated through the use of density functional theory (DFT) with the analysis of the role of the spacing group arrangement on the values of free energy activation barrier. All structures were fully optimized in aqueous solution using implicit model for solvent at DFT level. The energy profiles for the hydrolysis reaction were determined by using the supermolecule approach. Five transition states were proposed differing by the conformation of the bridge group, and the activation free energy calculated as a weighted average within the selected forms. The Gibbs population for reactant was used as a statistical weight leading to the predicted value of 23.1kcalmol(-1), in good accordance with experiment, 23.8kcalmol(-1). Our results suggests that for 1,6-hexanediamine bridge ligand, the extend forms with average torsional angle over the carbon chain larger than 130° have the greatest contribution to the hydrolysis kinetics. The results presented here point out that the hydrolysis mechanism might follow different paths for each conformation and each of these contributes to the observed energy barrier.

Effects of noncovalent platinum drug-protein interactions on drug efficacy: use of fluorescent conjugates as probes for drug metabolism

The overall efficacy of platinum based drugs is limited by metabolic deactivation through covalent drug-protein binding. In this study the factors affecting cytotoxicity in the presence of glutathione, human serum albumin (HSA) and whole serum binding with cisplatin, BBR3464, and TriplatinNC, a "noncovalent" derivative of BBR3464, were investigated. Upon treatment with buthionine sulfoximine (BSO), to reduce cellular glutathione levels, cisplatin and BBR3464-induced apoptosis was augmented whereas TriplatinNC-induced cytotoxicity was unaltered. Treatment of A2780 ovarian carcinoma cells with HSA-bound cisplatin (cisplatin/HSA) and cisplatin preincubated with whole serum showed dramatic decreases in cytotoxicity, cellular accumulation, and DNA adduct formation compared to treatment with cisplatin alone. Similar effects are seen with BBR3464. In contrast, TriplatinNC, the HSA-bound derivative (TriplatinNC/HSA), and TriplatinNC pretreated with whole serum retained identical cytotoxic profiles and equal levels of cellular accumulation at all time points. Confocal microscopy of both TriplatinNC-NBD, a fluorescent derivative of TriplatinNC, and TriplatinNC-NBD/HSA showed nuclear/nucleolar localization patterns, distinctly different from the lysosomal localization pattern seen with HSA. Cisplatin-NBD, a fluorescent derivative of cisplatin, was shown to accumulate in the nucleus and throughout the cytoplasm while the localization of cisplatin-NBD/HSA was limited to lysosomal regions of the cytoplasm. The results suggest that TriplatinNC can avoid high levels of metabolic deactivation currently seen with clinical platinum chemotherapeutics, and therefore retain a unique cytotoxic profile after cellular administration.

Solution studies of dinuclear polyamine-linked platinum-based antitumour complexes

The aquation profiles of two novel dinuclear polyamine-linked, platinum-based antitumour complexes [{trans-PtCl((15)NH(3))(2)}(2){μ-((15)NH(2)(CH(2))(6)(15)NH(2)(CH(2))(6)(15)NH(2))}](3+) (BBR3007, 1,1/t,t-6,6, 1) and [{trans-PtCl((15)NH(3))(2)}(2){μ-((15)NH(2)(CH(2))(6)(15)NH(2)(CH(2))(2)(15)NH(2)(CH(2))(6)(15)NH(2))}](4+) (BBR3610, 1,1/t,t-6,2,6, 1') have been probed using 2D [(1)H, (15)N] HSQC NMR spectroscopy. Reported herein are the rate constants for the hydrolysis of 1 and 1', as well as the acid dissociation constants of the coordinated aqua ligands in their aquated derivatives. The aquation and anation rate constants for the single step aquation model in 15 mM NaClO(4) (pH 5.4) at 298 K are, for 1, k(1) = 7.2 ± 0.1 ×10(-5) s(-1), k(-1) = 0.096 ± 0.002 M(-1) s(-1) and, for 1', k(1) = 4.0 ± 0.2 × 10(-5) s(-1), k(-1) = 1.4 ± 0.1 M(-1) s(-1). The effect of the linker backbone (Pt(tetra(m)mine vs. polyamine) was evaluated by comparison with previous data for the trinuclear complex [{trans-PtCl(NH(3))(2)}(2)(μ-trans-Pt(NH(3))(2){NH(2)(CH(2))(6)NH(2)}(2))](4+) (1,0,1/t,t,t or BBR3464). The pK(1) for 1,0,1/t,t,t (3.44) is closest to that of 1 (3.12), while the pronounced difference for 1' (4.54), means that 1' is the least aquated of the three complexes at equilibrium. pK(a) values of 5.92 were calculated for the aquated forms of both 1 and 1', which are 0.3 pK units higher than for either 1,0,1/t,t,t, or the dinuclear 1,1/t,t. The higher pK(a) values for both polyamine-linked compounds may be attributed to the formation of macrochelates between the central NH(2) groups and the {PtN(3)O} coordination sphere of the aquated species.

Determination of the kinetic profile of a dinuclear platinum anticancer complex in the presence of sulfate: introducing a new tool for the expedited analysis of 2D [(1)H,( 15)N] HSQC NMR spectra

Two-dimensional (2D) [(1)H, (15)N] heteronuclear single-quantum coherence (HSQC) NMR experiments of the kinetics of aquation and sulfation of the dinuclear platinum anticancer complex [{trans-PtCl(NH(3))(2)}(2)(μ-NH(2)(CH(2))(6)NH(2))](2+) (1,1/t,t, 1) in 15 mM sulfate solution are reported using conditions (298 K, pH 5.4) identical to those previously used for other anionic systems (phosphate and acetate), allowing for a direct comparison. Sulfate is the fourth most abundant anion in human plasma. The rate constant for the aquation step (k(H)) is higher than that previously found in the presence of phosphate, but the anation rate constants are similar. The rate constant for sulfate displacement of the aqua ligand (k(L)) is approximately three times higher than that of phosphate, and a further major difference between these two anions is the very high k(-L) for loss of sulfate, suggesting that when formed in plasma the sulfato species will be substitution labile. We also introduce a novel (free) plug-in, '2D NMR analysis', developed for the expedited integration and analysis of 2D [(1)H, (15)N] HSQC NMR spectra. We have found that this plug-in significantly reduces the amount of time taken in the analysis of experiments with no loss to the quality of the data.

The trans influence in the modulation of platinum anticancer agent biology: the effect of nitrite leaving group on aquation, reactions with S-nucleophiles and DNA binding of dinuclear and trinuclear compounds

To examine the effect of leaving group and trans influence on the general reactivity of polynuclear platinum antitumor agents we investigated substitution of the chloride leaving groups with nitrite ion, which forms strong bonds to Pt. It was of interest to explore whether nitrite could be used to modulate biological properties of these agents, in particular the deactivating reactions that occur on reaction with S-nucleophiles, involving loss of the linking diamine under the trans influence of sulfur. Reported herein is a study of the synthesis, aquation, DNA binding and reactions with glutathione (GSH), methionine (Met) and acetylmethione (AcMet) of nitrito derivatives of di- and trinuclear platinum antitumor compounds: [{trans-PtNO(2)(NH(3))(2)}(2)(mu-NH(2)(CH(2))(6)NH(2))](NO(3))(2) (1-NO(2)) and [{trans-PtNO(2)(NH(3))(2)}(2)(mu-trans-Pt(NH(3))(2){NH(2)(CH(2))(6)NH(2)}(2))](NO(3))(4) (1'-NO(2)). {(1)H,(15)N}-HSQC NMR studies revealed that 1-NO(2) is inert to aquation reactions, even after prolonged incubation at physiological pH. Monitoring of the interaction of 1-NO(2) with the duplex 5'-d(ATATGTACATAT)(2) (I) showed only unreacted complex, consistent with activation by aquation being a requirement for covalent DNA binding. The reaction of 1-NO(2) with GSH was studied by (1)H, (195)Pt, (15)N and {(1)H,(15)N}-HSQC NMR spectroscopy. For the parent dichlorido compounds (1 and 1') substitution of chloride by GS(-) leads to drug degradation involving liberation of the diamine linker. While the same final products trans-[Pt(SG)(2)(NH(3))(2)] (5) and trans-[{Pt(SG)(NH(3))(2)}(2)-mu-SG] (6) are formed, different mechanisms are involved, consistent with the trans influence NO(2)(-) > Cl(-); the half-life is slightly longer for 1-NO(2) (1.8 h) compared with 1 (1.3 h). Identification of the intermediate trans-[Pt(NH(3))(2)(NO(2))(SG)] (4) shows that the nitrito group remains coordinated while the linker amine is substituted by coordination of GS(-), and then trans labilization of the nitrito group occurs leading to 5 and 6. Reaction of the trinuclear 1'-NO(2) with GSH follows essentially the same reaction pathway. Reaction of 1-NO(2) with Met and AcMet is much slower and only 20 % liberated amine was observed after reaction with Met for 24 h at 37 degrees C. The final product from reaction with AcMet is trans-[Pt(NH(3))(2)(NO(2))(AcMet)], as in this case coordination of the S-nucleophile does not lead to trans labilization of the nitrito group.

Platinum speciation used for elucidating activation or inhibition of Pt-containing anti-cancer drugs

This article reviews approaches on platinum speciation with respect to Pt drugs in anti-cancer therapies. The paper starts with the introduction of available platinum-based drugs and describes their assumed principle of action. It is now generally accepted that these Pt complexes exhibit their therapeutic action by coordination to DNA which leads to bending of the DNA structure and to an inhibition of the DNA polymerase progression. But dose-limiting side effects, including nephrotoxicity as well as resistance to some of these Pt compounds, are still a major problem. Platinum speciation moved increasingly into the focus of interest when it became clear that (1) the active drugs were the hydrolyzation products rather than the originally administered ones and (2) that the parallel formation of inactive Pt-protein complexes, which additionally reduce the efficacy of Pt anti-tumor agents, compete with the formation of the cytotoxic Pt-DNA lesions. Speciation analysis methods were employed based on chromatography or capillary electrophoresis respectively, each coupled to inductively coupled plasma (ICP)-mass spectrometry (MS) or electrospray ionization (ESI)-MS. The paper describes these Pt-speciation investigations, which started with exploring hydrolyzation kinetics in aqueous solutions. These experiments were followed by the speciation investigations in model solutions containing proteins or other sulphur-containing ligands, which could also be responsible for deactivation of the Pt agent in vivo. The experiments improved the understanding of the metabolite form, by which the metal complex enters the tumor cells, and whether and how this metabolized complex is already inactivated at this time. As an example, reaction kinetics of cisplatin (cis-[diamminedichloroplatinum(II)]) with albumin, transferrin, myoglobin, ubiquitin, and metallothionein were investigated and reaction products were speciated. Finally, Pt-speciation in serum of medicated cancer patients was conducted by several research groups, which are outlined in the Section "Investigations in serum". The section "Investigations in urine of cancer treated patients" deals with speciation experiments on the Pt-metabolites excreted by the organism. By these means an assessment of the in vivo metabolism of Pt-drugs may be possible. Finally, the development of new anti-cancer metallodrugs needs the respective analytical techniques reported in the last section of the paper.

Solution behaviour and biomolecular interactions of two cytotoxic trans-platinum(II) complexes bearing aliphatic amine ligands

A novel trans-platinum(II) complex bearing one dimethylamine (dma) and one methylamine (ma) ligand, namely trans-[PtCl(2)(dma)(ma)], recently synthesised and characterised in our laboratory, displayed relevant antiproliferative properties in vitro, being more active than the parent complex, trans-[PtCl(2)(dma)(ipa)], which has isopropylamine (ipa) in place of methylamine. We have analysed comparatively the solution behaviour of these two complexes under various experimental conditions, and investigated their reactivity with horse heart cytochrome c by mass spectrometry, inductively coupled plasma-optical emission spectroscopy (ICP-OES), 2D [(1)H,(15)N],[(1)H,(13)C] HSQC and [(1)H,(1)H] NOESY NMR. Some important changes that occurred in the [(1)H,(13)C] HSQC NMR spectrum of cytochrome c treated with trans-[PtCl(2)(dma)(ma)] in water, after two days' incubation, most probably arose from direct platinum coordination to the protein side chain; this was proved conclusively by [(1)H,(1)H] NOESY NMR and [(1)H,(15)N] HSQC NMR measurements. Met65 was identified as the primary Pt binding site on cytochrome c. Electrospray mass spectrometry (ESIMS) results provided evidence for extensive platinum-protein adduct formation. A fragment of the [Pt(amine)(amine')] type was established to be primarily responsible for protein metalation. ICP-OES analysis revealed that these trans-platinum(II) complexes bind preferentially to the serum proteins albumin and transferrin rather than to calf thymus DNA. Pt binding to DNA was found to be far lower than in the case of cisplatin. The implications of the results for the mechanism of action of novel cytotoxic trans-platinum complexes are discussed.

Influence of amine ligands on the aquation and cytotoxicity of trans-diamine platinum(II) anticancer complexes

Three (15)N-labelled trans-Pt(ii) amine complexes with isopropylamine ((15)N-ipa), methylamine ((15)N-ma) and dimethylamine ((15)N-dma) have been prepared and characterized. 2D [(1)H,(15)N] HSQC NMR spectroscopy was used to obtain the rate and equilibrium constants for the aquation of trans-[PtCl(2)((15)N-ipa)((15)N-ma)] ((15)N-1), trans-[PtCl(2)((15)N-dma)((15)N-ma)] ((15)N-2) and trans-[PtCl(2)((15)N-dma)((15)N-ipa)] ((15)N-) in 100 mM NaClO(4) solutions at 298 K. New (15)N shift ranges for H(2)N-Pt(II)-N and HN-Pt(II)-N groups are reported. Formation of the diaqua complex was not observed for and accounted for <2% of the species at equilibrium for 1 and 2 . The first aquation step is significantly faster for 2 (k(1) = 14 x 10(-5) s(-1)) than for the two complexes with the bulkier ipa ligand (k(1) = 5.5 x 10(-5) s(-1) (), 6.1 x 10(-5) s(-1) (3)), but 2 is the least aquated of the three complexes at equilibrium. The pK(a) values for the monoaqua adducts of 1-3 are similar (5.98, 5.85 and 5.91, respectively) and 0.4 pH units lower than the related cis complex cis-[PtCl(2)(dma)(2)], indicating a smaller proportion of more reactive aqua species will exist at physiological pH. The pK(a) values for the diaqua adduct of 2 (4.59 and 7.98) are 0.3-0.6 pH units higher than those of 1(4.31 and 7.30) and 3 (4.28 and 7.29), which have very similar values. The speciation profiles of 1-3 , calculated on the basis of the calculated equilibrium and dissociation constants, indicate that <1% hydrolyzed species will exist under physiological conditions in cancer cells. The cytotoxicity of 1-3 (non-(15)N-labelled) was assessed in three cancer lines (SF268, MCF-7 and NCI-H460). The new trans-Pt(ii) diamine complex 2 is more active than 1 and 3 in all cases and is more potent than cisplatin in the MCF-7 adenocarcinoma cell line.

Effects of geometric isomerism and anions on the kinetics and mechanism of the stepwise formation of long-range DNA interstrand cross-links by dinuclear platinum antitumor complexes

Reported herein is a detailed study of the kinetics and mechanism of formation of a 1,4-GG interstrand cross-link by the dinuclear platinum anticancer compound [15N][{cis-PtCl(NH3)2}2{mu-NH2(CH2)6NH2}]2+ (1,1/c,c (1)). The reaction of [15N]1 with 5'-{d(ATATGTACATAT)2} (I) has been studied by [1H,15N] HSQC NMR spectroscopy in the presence of different concentrations of phosphate. In contrast with the geometric trans isomer (1,1/t,t), there was no evidence for an electrostatic preassociation of 1,1/c,c with the polyanionic DNA surface, and the pseudo-first-order rate constant for the aquation of [(15)N]1 was actually slightly higher (rather than lower) than that in the absence of DNA. When phosphate is absent, the overall rate of formation of the cross-link is quite similar for the two geometric isomers, occurring slightly faster for 1,1/t,t. A major difference in the DNA binding pathways is the observation of phosphate-bound intermediates only in the case of 1,1/c,c. 15 mM phosphate causes a dramatic slowing in the overall rate of formation of DNA interstrand cross-links due to both the slow formation and slow closure of the phosphate-bound monofunctional adduct. A comparison of the molecular models of the bifunctional adducts of the two isomers shows that helical distortion is minimal and globally the structures of the 1,4 interstrand cross-links are quite similar. The effect of carrier ligand was investigated by similar studies of the ethylenediamine derivative [15N]1-en. A pKa value of 5.43 was determined for the [15N]1,1/c,c-en diaquated species. The rate of reaction of [15N]1-en with duplex I is similar to that of 1,1/c,c and the overall conformation of the final adduct appears to be similar. The significance of these results to the development of "second-generation" polynuclear platinum clinical candidates based on the 1,1/c,c chelate (dach) series is discussed.

Dinuclear Platinum Complexes with Biological Relevance Based on the 1,2-Diaminocyclohexane Carrier Ligand

The synthesis of bifunctional dinuclear platinum complexes, [{PtCl(dach)}(2)-mu-Y](n+)Cl(n) (1-3; Y = H(2)N(CH(2))(3)NH(2)(CH(2))(4)NH(2), H(2)N(CH(2))(6)NH(2)(CH(2))(6)NH(2), and H(2)N(CH(2))(6)NH(2)(CH(2))(2)NH(2)(CH(2))(6)NH(2), respectively; Figure 1) is reported. There was no labilization of the polyamine linker groups of the cis-1,2-diaminocyclohexane complexes in the presence of sulfur-containing species at physiological pH, in contrast to previous studies preformed on trans complexes. Metabolism reactions are somewhat dependent on the nature of the polyamine: at physiological pH, the spermidine complex 1 forms an inert (tetraamine)platinum species in which one platinum is chelated by a central and terminal amino group. The stability of cis-geometry complexes may make them viable second-generation polynuclear platinum clinical candidates.

A dinuclear monofunctional platinum(II) complex with an aromatic linker shows low reactivity towards glutathione but high DNA binding ability and antitumor activity

Multinuclear Pt(II) complexes represent a novel class of antitumor agents. In this work, a dinuclear monofunctional Pt(II) complex {[cis-Pt(NH(3))(2)Cl](2)(4,4'-methylenedianiline)}(NO(3))(2) (1) was synthesized and characterized by (1)H NMR, electrospray mass spectrometry, and elemental analysis. The 2D [(1)H,(15)N] heteronuclear single quantum coherence NMR spectra of (15)N-labeled 1 revealed that the cationic core of this water-soluble complex hardly hydrolyzes in aqueous solution and reacts very slowly with glutathione. Hydrolysis appears not to be an essential step for the formation of Pt-guanosine-5'-monophosphate (5'-GMP) or Pt-DNA adducts because the complex can react readily with 5'-GMP and partially transform B-DNA into its Z form. Such properties are desired to achieve the goal of enhancing cytotoxicity and lowering side effects of Pt(II) complexes. In fact, complex 1 is highly cytotoxic against the murine leukemia (P-388) and the human non-small-cell lung cancer (A-549) cell lines, and it is more cytotoxic than cisplatin at most concentrations tested.

Mechanism of the Membrane Interaction of Polynuclear Platinum Anticancer Agents. Implications for Cellular Uptake

The interaction between phospholipids and polynuclear platinum drugs was studied as a mechanism model for cellular uptake of anticancer drugs. The interaction was studied by differential scanning calorimetry (DSC), 31P nuclear magnetic resonance spectroscopy (NMR), inductively coupled plasma optical emission spectroscopy (ICP-OES), and electrospray ionization mass spectrometry (ESI-MS). The transition temperature, enthalpy, and entropy of negatively charged phospholipids DPPS, DPPA, and DPPG were changed upon reaction with the trinuclear platinum complex [{trans-PtCl(NH3)2}2mu-Pt(NH3)2{H2N(CH2)6NH2}2](NO3)4 (I, BBR3464) and the dinuclear analogue [{trans-PtCl(NH3)2}mu-{(NH2)(CH2)3NH2(CH2)4(NH2)}Cl3 (II, BBR3571). This suggests that these platinum complexes interacted not only with the phosphate headgroup but also with the region of the fatty acid tail of liposomes and finally changed the fluidity of the membrane. Both noncovalent (presumably electrostatic and hydrogen bonding) and covalent interactions were involved in the reactions of the negatively charged phospholipids DPPA, DPPS, and DPPG with the highly positively charged platinum complexes. In contrast, few differences were seen for the zwitterionic phospholipids DPPC and DPPE. The binding ratio of BBR3464 to DPPA liposomes was higher than the ratio of BBR3464 to DPPS liposomes, and similar differences were seen for BBR3571. The binding ratios of the platinum complexes to negatively charged phospholipids DPPA, DPPS, and DPPG were slightly lower in a 100 mM chloride solution than in a chloride-free solution. The binding of BBR3464 and BBR3571 with the liposomes was significantly stronger than that with cis-[PtCl2(NH3)2], cisplatin. ESI-MS confirmed that the products of the incubation of BBR3464 with DPPA and DPPS correspond to chloride displacement and formation of [Pt3(NH3)6{NH2(CH2)6NH2}2(DPPA)2]2+ (1) and [Pt3(NH3)6{NH2(CH2)6NH2}2(DPPS)2]2+ (2), respectively. Similar observations were made for BBR3571. 31P NMR spectra confirmed that the site of binding for DPPA was the phosphate oxygen, whereas for DPPS, a binding site of the nitrogen of the serine side chain is indicated. Noncovalent interactions were also confirmed by use of the analogue [{Pt(NH3)3}2mu-Pt(NH3)2{H2N(CH2)6NH2}2](NO3)6 (III, 0,0,0/t,t,t). The implications of these results for the mechanism of cellular uptake of polynuclear platinum complexes are discussed.