Multinuclear NMR studies and the kinetics of formation of platinum(II)-adenine nucleotide complexes

Effect of the aniline fragment in Pt(II) and Pt(IV) complexes as anti-proliferative agents. Standard reduction potential as a more reliable parameter for Pt(IV) compounds than peak reduction potential

The problems of resistance and side effects associated with cisplatin and other chemotherapeutic drugs have boosted research aimed at finding new compounds with improved properties. The use of platinum(IV) prodrugs is one alternative, although there is some controversy regarding the predictive ability of the peak reduction potentials. In the work described here a series of fourteen chloride Pt(II) and Pt(IV) compounds was synthesised and fully characterised. The compounds contain different bidentate arylazole heterocyclic ligands. Their cytotoxic properties against human lung carcinoma (A549), human breast carcinoma (MCF7) and human colon carcinoma (HCT116 and HT29) cell lines were studied. A clear relationship between the type of ligand and the anti-proliferative properties was found, with the best results obtained for the Pt(II) compound that contains an aniline fragment, (13), thus evidencing a positive effect of the NH₂ group. Stability and aquation studies in DMSO, DMF and DMSO/water mixtures were carried out on the active complexes and an in-depth analysis of the two aquation processes, including DFT analysis, of 13 was undertaken. It was verified that DNA was the target and that cell death occurred by apoptosis in the case of 13. Furthermore, the cytotoxic derivatives did not exhibit haemolytic activity. The reduction of the Pt(IV) compounds whose Pt(II) congeners were active was studied by several techniques. It was concluded that the peak reduction potential was not useful to predict the ability for reduction. However, a correlation between the cytotoxic activity and the standard reduction potential was found.

Platinum(II) oxalato complexes involving adenosine-based N-donor ligands: synthesis, characterization and cytotoxicity evaluation

A one-step synthetic procedure using the reaction of potassium bis(oxalato)platinate(II) with the corresponding N6-benzyladenosine derivative (nL) provided the [Pt(ox)(nL)₂]∙1.5H₂O oxalato (ox) complexes 1-5, involving the nL molecules as monodentate coordinated N-donor ligands. The complexes were thoroughly characterized by elemental analysis, multinuclear (¹H, ¹³C, ¹⁵N, 1¹⁹⁵Pt) and two dimensional NMR, infrared and Raman spectroscopy, and mass spectrometry, proving their composition and purity as well as coordination of nL through the N7 atom of the purine moiety. Geometry of [Pt(ox)(4FL)₂] (5) was optimized at the B3LYP/LANLTZ/6-311G** level of theory. The complexes were screened for their in vitro cytotoxicity against two human cancer cell lines (HOS osteosarcoma and MCF7 breast adenocarcinoma), but they did not show any effect up to the concentration of 50.0 µM (compounds 1, 2) or 20.0 µM (compounds 3-5).

Equilibrium studies of the reactions of palladium(II) bis(imidazolin-2-imine) complexes with biologically relevant nucleophiles. The crystal structures of [(TLtBu)PdCl]ClO4 and [(BLiPr)PdCl2]

The kinetics and the mechanism of the substitution reactions of the complex [(TL(tBu))PdCl](+), where TL(tBu) is 2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine, with nucleophiles (guanosine-5'-monophosphate (5'-GMP), l-Methionine (l-Met) and l-Histidine (l-His)) were studied using variable-temperature stopped-flow techniques in aqueous 0.1 M NaClO(4) with 10 mM NaCl at 298 K. The order of reactivity is: l-Met > 5'-GMP > l-His. The formation equilibria of [(BL(iPr))Pd(H(2)O)(2)](2+), where BL(iPr) is 1,2-bis(1,3diisopropyl-4,5-dimethylimidazolin-2-imino)ethane, and [(TL(tBu))Pd(H(2)O)](2+) with some biologically relevant ligands (l-Met, 5'-GMP and l-His) were also studied. The stoichiometry and stability constants of the newly formed complexes are reported, and the concentration distribution of the various complex species has been evaluated as a function of pH. Comparing the values of logβ(1,1,0) for 5'-GMP, l-His and l-Met complexes, the most stable complex is with 5'-GMP followed by l-His and l-Met for both complexes, [(BL(iPr))Pd(H(2)O)(2)](2+) and [(TL(tBu))Pd(H(2)O)](2+). The crystal structures of [(TL(tBu))PdCl]ClO(4) and [(BL(iPr))PdCl(2)] were determined by X-ray diffraction. The coordination geometries around the palladium atoms are distorted square-planar, with the Pd-N1 distance to the central nitrogen atom of the TL(tBu) ligand, 1.944(2) Å, being shorter than those to the other two nitrogen atoms of TL(tBu), viz. 2.034(3) and 2.038(2) Å. The BL(iPr) complex displays similar Pd-N distances of 2.031(2) and 2.047(2) Å.

Equilibrium, kinetic and HPLC study of the reactions between platinum(II) complexes and DNA constituents in the presence and absence of glutathione

The complex formation equilibria of [Pt(SMC)(H(2)O)(2)](+) and [Pt(terpy)H(2)O](2+), where SMC =S-methyl-L-cysteine and terpy = 2,2':6',2"-terpyridine, with some biologically relevant ligands such as inosine (INO), inosine-5'-monophosphate (5'-IMP), guanosine-5'-monophosphate (5'-GMP) and glutathione (GSH) were studied. The stoichiometry and stability constants of the complexes formed are reported, and the concentration distribution of the various complex species have been evaluated as a function of pH. Also the kinetics and mechanism of the complex formation reactions were studied as a function of nucleophile concentration and temperature. For the complex [Pt(SMC)(H(2)O)(2)](+), two consecutive reaction steps, which both depend on the nucleophile concentration, were observed under all conditions. The negative entropies of activation support an associative complex formation mechanism. Reaction of guanosine-5'-monophosphate (5'-GMP) with Pt(II) complexes was carried out in the presence and absence of glutathione (GSH) at neutral pH. The rate constants clearly showed a kinetic preference toward GSH at neutral pH. The reactions were also monitored by HPLC. However, only a small amount of coordinated 5'-GMP was detected in the HPLC trace. The products were isolated and characterized by MALDI-TOF mass spectrometry.

Platinum(II) catalysis and radical intervention in reductions of platinum(IV) antitumor drugs by ascorbic acid

Reductions of four platinum(IV) amine complexes, cis-diamminetetrachloroplatinum(IV), tetraammine-cis-dichloroplatinum(IV), cis,cis,trans-diamminedichlorodihydroxoplatinum(IV), and cis,trans,cis-dichloro-dihydroxo-bis(isopropylamine)platinum(IV) by ascorbic acid were catalyzed by platinum(II) at pH 7.3 and 22 degrees C. Except for the first mentioned compound, initial slow uncatalyzed reductions yielded platinum(II) products which served as catalyst as revealed by the presence of induction periods and their disappearance by the addition of the platinum(II) products. The platinum(II) catalysis generated ascorbate bound platinum(IV) intermediates. An internal electron transfer process within these intermediates led to the formation of platinum(II) complexes. Although the rate constants for the uncatalyzed reductions vary greatly depending on the nature of the ligands and their spatial arrangements, the magnitudes of the platinum(II) catalyzed rate constants fall in the narrow range, 100 to 300 M(-2) s(-1). The values of the uncatalyzed reductions lie in the range 5 x 10(-2) to 15 M(-1) s(-1), the tetrachloroplatinum(IV) complex suffered the faster reduction. The reduction of iproplatin with two hydroxide ligands in trans configuration was the slowest. The internal electron transfer rate constants span two orders of magnitude, from 0.15 to 4 x 10(-3) s(-1). These reactions were accompanied by the formation of the ascorbate radical which persists throughout the entire reaction. Although the tetrachloro species exhibited simple second order reduction, first order in each of the reactants, the rate of reduction was also accelerated by the addition of cis-diamminedichoroplatinum(II) indicating the presence of catalysis in this reaction as well.

Computational consideration of cisplatin hydrolysis and acid dissociation in aqueous media: effect of total drug concentrations

Cisplatin (cis-DDP) is subject to nucleophilic displacement of chloride in water, forming aquated species, subsequently liberating hydrogen ion(s) with increasing pH. This study intends to theoretically analyze the hydrolysis and polyprotic dissociation behavior of cis-DDP in various aqueous media. A mathematical model was expressed by nonlinear simultaneous equations in terms of the total drug concentration, pH and pCl based on the hydrolysis and acid dissociation constants already published. Some of the interesting simulation results include that (1) in water, cis-DDP behaves in a very complicated manner, highly depending on the total drug concentration, pH and pCl, (2) in normal saline, about 3% of the total concentration is a positively charged chloro-aqua that may be very reactive, (3) in assumed blood (pH 7.4, [Cl(-)]=0.11 mol/l, mu=0.15), the drug is stabilized at the level of 85% and the remnants are the chloro-hydroxo (11%) and the chloro-aqua (4%), (4) in assumed intracellular conditions (pH 7.1, [Cl(-)]=0.01 mol/l, mu=0.15), the drug is converted to a large extent to various species including the parent species (44%), the chloro-hydroxo (30%), hydroxo-aqua (2%), chloro-aqua (24%) diaqua (less than 1%) and dihydroxo (null). The results of this analysis may provide a useful preliminary knowledge of existing species in a system concerned and a rationale for re-evaluating the reactions between cis-DDP and various nucleophilic substances already reported while there are somewhat conflicting interpretations of some cis-DDP reactions.

Study on the amount of binding of anti-tumor metal complexes to different target sites of dGMP using trans-[en(2)Os(eta2-H(2))(CF(3)SO(3))](CF(3)SO(3)) in a competitive mode

In order to investigate the binding sites and the amount of binding of a number of anti-tumor metal complexes (cisplatin, Cp(2)TiCl(2) and (CH(3))(2)SnCl(2)) to target molecule DNA mononucleotides in aqueous solution, a 1H NMR recognition probe, trans-[en(2)Os(eta2-H(2))(CF(3)SO(3))](CF(3)SO(3)), was used in a competitive mode. The minimum percentages of binding of anti-tumor metal complexes to different sites of dGMP were also determined.

Inhibition of Escherichia coli DNA polymerase-I by the anti-cancer drug cis-diaminedichloroplatinum(II): what roles do polymerases play in cis-platin-induced cytotoxicity?

Activities of Escherichia coli DNA polymerase-I were examined in the presence of the anti-tumor drug cis-diaminedichloroplatinum(II) and its inactive geometric isomer trans-diaminedichloroplatinum(II). The trans-isomer did not inhibit the enzyme activity. The anti-tumor drug, on the other hand, retarded the enzyme in its ability to extend the primer strand of DNA. Two alternative mechanisms of inhibition, covalent binding of cis-diaminedichloroplatinum(II) to the polymerase and to the template DNA, were explored. Selective preincubations of the platinum drug with the polymerase and DNA reveal that the inhibition is primarily due to covalent binding to the enzyme. The rates of inhibition were found to be first order in enzyme and zeroth order in platinum in the concentration range 0.05-3.0 mM. A mechanism that deals with the formation of an initial platinum-polymerase-I complex with a binding constant > 10(5) M(-1) followed by a further reaction to form an inhibitory complex is consistent with the kinetic data. The rate limiting first order rate constant for the formation of the inhibitory complex is comparable to that observed for the thiol coordination of peptides containing cysteine residues. Analyses of known structures and functions of catalytic domains of various polymerases point to the direction that the inhibition is perhaps due to the distortion of the DNA binding domain of the enzyme due to platinum coordination.

Electron paramagnetic resonance, kinetics of formation and decomposition studies of (bis(hydroxyethyl)amino-tris(hydroxymethyl)-methane)oxochromate(V): a model chromium(V) complex for DNA damage studies

A new chromium complex, (bis(hydroxyethyl)amino-tris(hydroxymethyl)methane)oxochromate(V), has been characterized by epr spectroscopy. The chromium(V) complex was formed by the ligand displacement reaction of bis(2-ethyl-2-hydroxybutanato) oxochromate(V) with bis(hydroxyethyl)amino-tris(hydroxy-methyl)methane (BT). Both epr and kinetic data indicate that the reaction proceeds through a chromium(V) intermediate. Kinetics of formation of the intermediate exhibit a rate saturation at higher [BT] (> 30 mM) indicating a rate law constituting an equilibrium between the parent Cr(V) complex and the bis-tris ligand followed by a pure first order process. The g-value of the intermediate is consistent with a Cr(V) complex in which the BT is coordinated in a bidentate fashion replacing a coordinated hydroxy butanoic acid ligand, affording a mixed ligand complex. The equilibrium step (K = 36 M-1) consists of monodentate coordination by the BT ligand and the limiting first order rate constant (1.9 x 10(-2) s-1) manifests the rate of chelation by the polydentate ligand. The intermediate is converted to the product upon further chelation through the complete displacement of the remaining 2-ethyl-2-hydroxy butanoic acid by a first order process (k = 0.023 s-1). The epr data support a pair of products that are in rapid equilibrium. In these products, BT functions either as a tetra or a penta-dentate ligand coordinating through four or five alkoxy sites. The enthalpy and entropy of activations related to the two chelation steps were found to be 32 +/- 2 kJ/mol and -(1.7 +/- 0.2) x 10(2) J/mol K for the intermediate, and 36 +/- 1 kJ/mol and -(1.5 +/- 0.2) x 10(2) J/mol K for the product. Our data support an associative mechanism for the chelation steps. The Cr(V)-BT product is more stable than the parent complex. The second order disproportionation rate constant for the Cr(V)-BT complex was evaluated to be 0.1 M-1 s-1 compared to 8.0 M-1 s-1 for the parent complex. This is the first example of a chromium(V) complex with a non-macrocyclic ligand coordinating through oxygen donor atoms which is stable in aqueous solution at neutral pH over a long period of time.

Kinetic analysis of the cis-diamminedichloroplatinum(II)--cysteine reaction: implications to the extent of platinum--DNA binding

The reaction between cis-diamminedichloroplatinum(II) (cis-DDP) and L-cysteine was examined at neutral pH at 37 degrees C. The reaction proceeds through a Pt(NH3)2 (cys)Cl intermediate which undergoes parallel reactions with a second molecule of cysteine to form a bis(cysteine) complex, Pt(NH3)2(cys)2 and with the starting platinum complex to form a cysteine-bridged dinuclear complex. In the presence of excess cysteine, the product is predominantly the bis(cysteine) complex. The intermediate is formed by the direct reaction of the platinum complex with cysteine with a bimolecular rate constant 2.2 +/- 0.2 x 10(-2) M-1.s-1 at 37 degrees C as well as through a rapid reaction with the mono aqua-platinum complex. The rate constant for the formation of the dimer was evaluated to be 0.24 +/- 0.4 M-1.s-1, an order of magnitude higher than that for the mononuclear complex formation. The intermediate reacts with a second cysteine molecule with a bimolecular rate constant, 5.6 +/- 0.4 x 10(-2) M-1.s-1. The rate constant for the equation of Pt(NH3)2(cys)Cl was evaluated to be 1.8 +/- 0.2 10(-4) s-1. The Pt-195 chemical shifts for the mono(cysteine), bis(cysteine), and cysteine bridged dimer were found to be -3308, -3705, and -3104 ppm. The bis(cysteine) complex at neutral pH undergoes slow reaction (t1/2 approximately equal to four days) to form a secondary product, presumably Pt(NH3)(cys)2, in which one cysteine acts a bidentate chelating agent. In acidic solution, with equimolar concentrations of cysteine and diaqua-platinum complex, the reaction predominantly yielded a cysteine bridged dimeric complex. When cysteine concentration was increased fourfold over the platinum complex, the bis(cysteine) chelate with complete removal of coordinated ammonia appeared as the dominant product. The platinum-195 chemical shift for this chelate was found to be -3290 ppm. Considering the abundance of thiols in amino acids/peptides and replication enzymes in the cellular milieu, it remains to be seen how platinum complexes react with DNA. Direct platination to replication enzymes as a possible mechanism for antineoplactic activity is yet to be ruled out.

Comparative study of the interactions of cisplatin and carboplatin with nucleotides using UV resonance Raman spectroscopy

We have obtained the UV excited resonance Raman spectra of five mononucleotides bound to cisplatin and to carboplatin using excitation in resonance with the first electronic absorption bands of the nucleotide bases. Substantial changes in the spectra are observed following interaction with both platinum drugs, indicating modifications to nucleotide structure. Pt (II) binds to base portions of the nucleotide molecules, altering their normal modes of vibration significantly. We present comparative data of cisplatin and carboplatin, and discuss the implications of these results. The kinetics of the drug/nucleotide reactions differ, but final products are found to be similar.

Enhanced levels of cyclic AMP, adenosine(5')tetraphospho(5')adenosine and nucleoside 5'-triphosphates in mouse leukemia P388/D1 after treatment with cis-diamminedichloroplatinum(II)

As part of the exploration of the mechanism of platinum(II) complex-induced growth inhibition and/or cytotoxicity, we studied the intracellular levels of several nucleotides during treatment of mouse leukemia P388/D1 at selected concentrations of 1 microM cis-diamminedichloroplatinum(II) (cis-DDP) and 20 microM of its trans-isomer (trans-DDP). The effects and their time-dependences are correlated with those on cell growth parameters previously published (Just G and Holler E, Cancer Res 49: 7072-7078, 1989). The effects of cis-DDP are strong and irreversible, whereas those of trans-DDP are marginal and reversible, in parallel with similar effects on cell growth parameters. Concentrations of nucleoside 5'-di- and 5'-triphosphates increase in parallel with cellular DNA and protein content by three- to four-fold after 60 hr of treatment. The nucleoside monophosphates dAMP, dGMP and dTMP reveal concentration maxima during exponential cell growth that are two- to six-fold higher than in the control cultures. Levels of cyclic AMP, adenosine(5')tetraphospho(5')adenosine (Ap4A) and CDP rise three- to five-fold above those in the control cultures within a few hours of the start of treatment. The level of coenzyme NAD+ falls below that of the control, concomitantly with an arrest of cells in the G2 phase of the cell cycle and with the appearance of giant cells. Due to the high reactivity of cis-DDP and the continuous concentration increase during the treatment, purine nucleoside 5'-triphosphates provide a possibility for the acquisition of resistance to cis-DDP. The correlation of responses of metabolically and regulatory active nucleotides with biological effects suggests their function in antitumorigenesis.

Ultrastructural localization of cisplatin in Ehrlich ascites tumor cells

The incorporation of cis-diammine Dichloro Platinum (II) (cisplatin) on the Ehrlich ascites carcinoma (EAT) cells has been studied in this paper. Ultrastructural study of cells treated 'in vivo' with cisplatin showed that a new treatment with this substance after fixation, blocks uranyl acetate staining with the consequent lack of heterochromatin contrast.

Synthesis and 1H NMR spectroscopic characterization of trans-[Pt(NH3)2[d(ApGpGpCpCpT)-N7-A(1),N7-G(3)]]

The reaction of trans-[Pt(NH3)2Cl2] with the sodium salt of [d(ApGpGpCpCpT)]2 in aqueous solution at 37 degrees C was monitored by reversed-phase high-performance liquid chromatography and UV spectroscopy. Two intermediates, most likely monofunctional adducts, were observed, which subsequently formed one predominant single-stranded product, as well as several polymeric species proposed to be interstrand cross-linked products. The single-stranded adduct was structurally characterized by 1H NMR spectroscopy. From the pH dependence of the chemical shifts, two-dimensional homonuclear chemical shift correlation (COSY) spectroscopy, and one- and two-dimensional nuclear Overhauser effect (NOESY) experiments, the platinum(II) moiety was found to be coordinated to the N7 positions of adenine(1) and guanine(3), with the intervening guanine(2) base destacked from its neighboring residues. This intrastrand 1,3 adduct induces changes in the backbone torsion angles and causes the deoxyribose ring of adenine(1) to switch from a C2'-endo to a predominantly C3'-endo conformation. The other deoxyribose rings retain B DNA type conformations. The structure of trans-[Pt(NH3)2[d(ApGpGpCpCpT)-N7-A(1),N7-G(3)]] differs from those previously reported for cis-DDP 1,2- and 1,3-intrastrand oligonucleotide adducts but is consistent with the structures of trans-DDP 1,3-intrastrand adducts of two previously reported trinucleotides.