Ligand effects on the binding of cis- and trans-[PtCl(2)Am(1)Am(2)] to proteins

Comparative cisplatin reactivity towards human Zn7-metallothionein-2 and MTF-1 zinc fingers: potential implications in anticancer drug resistance

Cis-diamminedichloroplatinum(II) (cisplatin) is a widely used metal-based chemotherapeutic drug for the treatment of cancers. However, intrinsic and acquired drug resistance limit the efficacy of cisplatin-based treatments. Increased production of intracellular thiol-rich molecules, in particular metallothioneins (MTs), which form stable coordination complexes with the electrophilic cisplatin, results in cisplatin sequestration leading to pre-target resistance. MT-1/-2 are overexpressed in cancer cells, and their expression is controlled by the metal response element (MRE)-binding transcription factor-1 (MTF-1), featuring six Cys2His2-type zinc fingers which, upon zinc metalation, recognize specific MRE sequences in the promoter region of MT genes triggering their expression. Cisplatin can efficiently react with protein metal binding sites featuring nucleophilic cysteine and/or histidine residues, including MTs and zinc fingers proteins, but the preferential reactivity towards specific targets with competing binding sites cannot be easily predicted. In this work, by in vitro competition reactions, we investigated the thermodynamic and kinetic preferential reactivity of cisplatin towards human Zn7MT-2, each of the six MTF-1 zinc fingers, and the entire human MTF-1 zinc finger domain. By spectroscopic, spectrometric, and electrophoretic mobility shift assays (EMSA), we demonstrated that cisplatin preferentially reacts with Zn7MT-2 to form Cys4-Pt(II) complexes, resulting in zinc release from MT-2. Zinc transfer from MT-2 to the MTF-1 triggers MTF-1 metalation, activation, and binding to target MRE sequences, as demonstrated by EMSA with DNA oligonucleotides. The cisplatin-dependent MT-mediated MTF-1 activation leading to apo-MT overexpression potentially establishes one of the molecular mechanisms underlying the development and potentiation of MT-mediated pre-target resistance.

Interaction of Vanadium(IV) Species with Ubiquitin: A Combined Instrumental and Computational Approach

The interaction of V^IVO²⁺ ion and five V^IVOL₂ compounds with potential pharmacological application, where L indicates maltolate (ma), kojate (koj), acetylacetonate (acac), 1,2-dimethyl-3-hydroxy-4(1 H)-pyridinonate (dhp), and l-mimosinate (mim), with ubiquitin (Ub) was studied by EPR, ESI-MS, and computational (docking and DFT) methods. The free metal ion V^IVO²⁺ interacts with Glu, Asp, His, Thr, and Leu residues, but the most stable sites (named 1 and 2) involve the coordination of (Glu16, Glu18) and (Glu24, Asp52). In the system with V^IVOL₂ compounds, the type of binding depends on the vanadium concentration. When the concentration is in the mM range, the binding occurs with cis-VOL₂(H₂O), L = ma, koj, dhp, and mim, or with VO(acac)₂: in the first case, the equatorial coordination of His68, Glu16, Glu18, or Asp21 residues yields species with formula n[VOL₂]-Ub where n = 2-3, while with VO(acac)₂ only noncovalent surface interactions are revealed. When the concentration of V is on the order of micromolar, the mono-chelated species VOL(H₂O)₂⁺ with L = ma, koj, acac, dhp, and mim, favored by the hydrolysis, interact with Ub, and adducts with composition n[VOL]-Ub ( n = 1-2) are observed with the contemporaneous coordination of (Glu18, Asp21) or (Glu16, Glu18), and (Glu24, Asp52) or (Glu51, Asp52) donors. The results of this work suggest that the combined application of spectroscopic, spectrometric, and computational techniques allow the complete characterization of the ternary systems formed by a V compound and a model protein such as ubiquitin. The same approach can be applied, eventually changing the spectroscopic/spectrometric techniques, to study the interaction of other metal species with other proteins.

Effect of secondary interactions, steric hindrance and electric charge on the interaction of VIVO species with proteins

The effect of secondary interactions (hydrogen bonds and van der Waals contacts), steric hindrance and electric charge, on the binding of V^IV complexes formed by pipemidic and 8-hydroxyquinoline-5-sulphonic acids with ubiquitin and lysozyme is studied.

Protein metalation by metal-based drugs: X-ray crystallography and mass spectrometry studies

The combined use of X-ray crystallography and mass spectrometry represents a valuable strategy to investigate and characterize protein metalation induced by anticancer metal-based drugs. Here, we summarize a series of significant results recently obtained in our laboratories upon the examination of the structures of several adducts of proteins with representative metallodrugs (mostly containing ruthenium, gold and platinum). The general mechanisms of protein metalation that emerge from a careful comparative analysis of these structures are illustrated and their mechanistic implications are discussed. Possible directions for future work in the field are delineated.

Application of mass spectrometric techniques to delineate the modes-of-action of anticancer metallodrugs

Mass spectrometry (MS) has emerged as an important tool for studying anticancer metallodrugs in complex biological samples and for characterising their interactions with biomolecules and potential targets on a molecular level. The exact modes-of-action of these coordination compounds and especially of next generation drug candidates have not been fully elucidated. Due to the fact that DNA is considered a crucial target for platinum chemotherapeutics, metallodrug-DNA binding studies dominated the field for a long time. However, more recently, alternative targets were considered, including enzymes and proteins that may play a role in the overall pharmacological and toxicological profile of metallodrugs. This review focuses on MS-based techniques for studying anticancer metallodrugs in vivo, in vitro and in situ to delineate their modes-of-action.

Palladium(II) and platinum(II) bind strongly to an engineered blue copper protein

Studies of palladium(II) and platinum(II) binding to well-characterized proteins contribute to understanding the influence of these metals in the environment and body. The well-characterized apoprotein of azurin has a soft-metal binding site that may be exposed to solvent by mutation of a coordinating His-117 residue to glycine (H117G). Palladium(II) and platinum(II) form strong 1:1 adducts with the apo form of H117G azurin. A combination of UV-vis, circular dichroism, and inductively coupled plasma mass spectrometry techniques suggests that the metal binds specifically at His-46 and Cys-112 of the protein.

A new class of antitumor trans-amine-amidine-Pt(II) cationic complexes: influence of chemical structure and solvent on in vitro and in vivo tumor cell proliferation

The reactions of cyclopropylamine, cyclopentylamine, and cyclohexylamine with trans-[PtCl2(NCMe)2] afforded the bis-cationic complexes trans-[Pt(amine)2(Z-amidine)2]2+[Cl-]2, 1-3. The solution behavior and biological activity have been studied in different solvents (DMSO, water, polyethylene glycol (PEG 400), and polyethylene glycol dimethyl ether (PEG-DME 500)). The biological activity was strongly influenced by the cycloaliphatic amine ring size, with trans-[Pt(NH2CH(CH2)4CH2)2{N(H) horizontal lineC(CH3)N(H)CH(CH2)4CH2}2]2+[Cl-]2 (3) being the most active compound. Complex 3 overcame both cisplatin and MDR resistance, inducing cancer cell death through p53-mediated apoptosis. Alkaline single-cell gel electrophoresis experiments indicated direct DNA damage, reasonably attributable to DNA adducts of trans-[PtCl(amine)(Z-amidine)2][Cl] species, which can evolve to produce disruptive and nonrepairable lesions on DNA, thus leading to the drug-induced programmed cancer cell death. Preliminary in vivo antitumor studies on C57BL mice bearing Lewis lung carcinoma highlighted that complex 3 promoted a significant and dose-dependent tumor growth inhibition without adverse side effects.

Multifunctional nanocarriers for cell imaging, drug delivery, and near-IR photothermal therapy

Multifunctional nanocarriers based on chitosan/gold nanorod (CS-AuNR) hybrid nanospheres have been successfully fabricated by a simple nonsolvent-aided counterion complexation method. Anticancer drug cisplatin was subsequently loaded into the obtained hybrid nanospheres, utilizing the loading space provided by the chitosan spherical matrix. In vitro cell experiments demonstrated that the CS-AuNR hybrid nanospheres can not only be utilized as contrast agents for real-time cell imaging but also serve as a near-infrared (NIR) thermotherapy nanodevice to achieve irradiation-induced cancer cell death owing to the unique optical properties endowed by the encapsulated gold nanorods. In addition, an effective attack on the cancer cells by the loaded anticancer drug cisplatin has also been observed, rendering the obtained nanocarriers an all-in-one system possessing drug delivery, cell imaging, and photothermal therapy functionalities.

Mechanistic insight into the inhibition of matrix metalloproteinases by platinum substrates

Platinum compounds are among the most used DNA-damaging anticancer drugs, however they can also be tailored to target biological substrates different from DNA, for instance enzymes involved in cancer progression. We recently reported that some platinum complexes with three labile ligands inhibit matrix metalloproteinase activity in a selective way. We have now extended the investigation to a series of platinum complexes having three chlorido or one chlorido and a dimethylmalonato leaving ligands. All compounds are strong inhibitors of MMP-3 by a noncompetitive mechanism, while platinum drugs in clinical use are not. Structural investigations reveal that the platinum substrate only loses two labile ligands, which are replaced by an imidazole nitrogen of His224 and a hydroxyl group, while it retains one chlorido ligand. A chlorido and a hydroxyl group are also present in the zinc complex inhibitor of carboxypeptidase A, whose active site has strong analogies with that of MMP-3.

A bulky platinum triamine complex that reacts faster with guanosine 5'-monophosphate than with N-acetylmethionine

A bulky platinum triamine complex, [Pt(Me(5)dien)(NO(3))]NO(3) (Me(5)dien=N,N,N',N',N''-pentamethyldiethylenetriamine) has been prepared and reacted in D(2)O with N-acetylmethionine (N-AcMet) and guanosine 5'-monophosphate (5'-GMP); the reactions have been studied using (1)H NMR spectroscopy. Reaction with 5'-GMP leads to two rotamers of [Pt(Me(5)dien)(5'-GMP-N7)](+). Reaction with N-AcMet leads to formation of [Pt(Me(5)dien)(N-AcMet-S)](+). When a sample with equimolar mixtures of [Pt(Me(5)dien)(D(2)O)](2+), 5'-GMP, and N-AcMet was prepared, [Pt(Me(5)dien)(5'-GMP-N7)](+) was the dominant product observed throughout the reaction. This selectivity is the opposite of that observed for a similar reaction of [Pt(dien)(D(2)O)](2+) with 5'-GMP and N-AcMet. To our knowledge, this is the first report of a platinum(II) triamine complex that reacts substantially faster with 5'-GMP than with N-AcMet; the effect is most likely due to steric clashes between the methyl groups of the Me(5)dien ligand and the N-AcMet.

Reactivity of platinum-based antitumor drugs towards a Met- and His-rich 20mer peptide corresponding to the N-terminal domain of human copper transporter 1

Cellular uptake of platinum-based antitumor drugs is a critical step in the mechanism of the drug action and associated resistance, and deeper understanding of this step may inspire development of novel methods for new drugs with reduced resistance. Human copper transporter 1 (hCtr1), a copper influx protein, was recently found to facilitate the cellular entry of several platinum drugs. In the work reported here, we constructed a Met- and His-rich 20mer peptide (hCtr1-N20) corresponding to the N-terminal domain of hCtr1, which is the essential domain of hCtr1 for transporting platinum drugs. The interactions of the peptide with cisplatin and its analogues, including transplatin, carboplatin, oxaliplatin, and [Pt(L: -Met)Cl(2)], were explored at the molecular level. Electrospray ionization (ESI) mass spectrometry (MS) data revealed that all of the platinum(II) complexes used in present study can bind to hCtr1-N20 in 1:1 and 2:1 stoichiometry. Four Met residues should be involved in binding to cis-platinum complexes on the basis of the tandem MS spectrometry and previously reported data. Time-dependent 2D [(1)H,(15)N] heteronuclear single quantum coherence NMR spectra indicate the reaction of cisplatin with hCtr1-N20 is a stepwise process. The intermediate, however, is transient, which is consistent with the ESI-MS results. Time-dependent ESI-MS data revealed that the geometry and the properties of both the leaving and the nonleaving groups of platinum(II) complexes play essential roles in controlling the reactivity and formation of the final products with hCtr1-N20.

Towards Antitumor Active trans-Platinum Compounds

Substitution of NH(3) by a range of amines in trans-[PtCl(2)(NH(3))(2)] produces compounds with cytotoxicity significantly improved over the parent transplatin and in many cases equivalent to that of cisplatin. This microreview summarizes the chemistry and biology of trans-platinum compounds containing principally planar amines and succinctly reviews the current status of anticancer relevance of the trans-platinum geometry. The nature of bifunctional DNA adducts (intrastrand, interstrand) is remarkably dependent on the nature of the amine. Further, the stability of monofunctional adducts allows for competitive production of DNA-protein crosslinks and overall the results suggest that the trans-platinum chemotype may offer significant potential for design of selective DNA-protein crosslinking agents. A subset of proteins known to bind to DNA modified by trans-platinum is that comprised of zinc fingers - model studies show the potential for formation of heteronuclear thiolate-bridged species as precedent for zinc displacement from the biomolecule.

"Platinum on the road": Interactions of antitumoral cisplatin with proteins

When the antitumor activity of cisplatin was discovered, no one would have thought of the existence of specific proteins able to transport Pt across the cell membrane or to specifically recognize DNA modified by this drug. However, such proteins do exist and, furthermore, are specific for the Pt substrate considered. It follows that proteins are deeply involved in managing the biological activity of cisplatin. It is expected that, after the first 20 years in which most of the efforts were devoted to understanding its mode of interaction with DNA and consequent structural and functional alterations, the role of proteins will be more deeply scavenged. How cisplatin can survive the attack of the many platinophiles present in the extracellular and intracellular media is the issue addressed in this article. Significantly, differences are observed between cisplatin, carboplatin, and oxaliplatin.

Tuning the hydrophobicity of ruthenium(II)-arene (RAPTA) drugs to modify uptake, biomolecular interactions and efficacy

The antitumour activity of the organometallic ruthenium(ii)-arene mixed phosphine complexes, [Ru(eta(6)-p-cymene)Cl(PTA)(PPh(3))]BF(4) and [Ru(eta(6)-C(6)H(5)CH(2)CH(2)OH)Cl(PTA)(PPh(3))]BF(4) (PTA = 1,3,5-triaza-7-phosphaadamantane), have been evaluated in vitro and compared to their RAPTA analogues, [Ru(eta(6)-p-cymene)Cl(2)(PTA)] and [Ru(eta(6)-C(6)H(5)CH(2)CH(2)OH)Cl(2)(PTA)] . The results show that the addition of the PPh(3) ligand to increases the cytotoxicity towards the TS/A adenocarcinoma cancer cells, which correlates with increased uptake, but also increases cytotoxicity to non-tumourigenic HBL-100 cells, thus decreasing selectivity. The decrease in selectivity has been correlated to increased DNA interactions relative to proteins, demonstrated by reactivity of the compounds with a 14-mer oligonucleotide and the model proteins ubiquitin and cytochrome-c.

Reaction of Zn7metallothionein with cis- and trans-[Pt(N-donor)2Cl2] anticancer complexes: trans-Pt(II) complexes retain their N-donor ligands

Intrinsic and acquired resistance are major drawbacks of platinum-based cancer therapy. The protein superfamily of cysteine- and ZnII-rich proteins, metallothioneins (MT), efficiently inactivate these antitumor drugs because of the strong reactivity of platinum compounds with S-donor molecules. In this study the reactions of human Zn7MT-2 with twelve cis/trans-[Pt(N-donor)2Cl2] compounds and [Pt(dien)Cl]Cl, including new generation drugs, were investigated and the products characterized. A comparison of reaction kinetics revealed that trans-PtII compounds react faster with Zn7MT-2 than cis-PtII compounds. The characterization of the products showed that while all ligands in cis-PtII compounds were replaced by cysteine thiolates, trans-PtII compounds retained their N-donor ligands, thus remaining in a potentially active form. These results provide an increased understanding of the role of MT in the acquired resistance to platinum-based anticancer drugs.

Platinum complexes can inhibit matrix metalloproteinase activity: platinum-diethyl[(methylsulfinyl)methyl]phosphonate complexes as inhibitors of matrix metalloproteinases 2, 3, 9, and 12

Platinum complexes able to inhibit matrix metalloproteinases (MMPs) through a noncompetitive mechanism are reported for the first time in this study. [PtCl2(SMP)] and [Pt(dimethylmalonato)(SMP)], characterized by the bisphosphonate-analogue ligand diethyl[(methylsulfinyl)methyl]phosphonate (SMP), are slight inhibitors of MMP-2 (IC50 = 258 +/- 38 and 123 +/- 14 microM, respectively) but markedly inhibit MMP-9 (IC50 = 35.5 +/- 6 and 17 +/- 4 microM), MMP-3 (IC50 = 5.3 +/- 2.9 and 4.4 +/- 2.2 microM), and MMP-12 (IC50 = 10.8 +/- 3 and 6.2 +/- 1.8 microM). In contrast, cisplatin, carboplatin, and the SMP ligand are inactive, and the bisphosphonate clodronate shows a broad-spectrum inhibitory activity in the high micromolar range (mean IC50 > 200 microM). These results, along with mechanistic investigations (DNA interaction and tumor cell growth inhibition), demonstrate that ligand modifications of platinum compounds can be exploited to target also biological substrates distinct from DNA.

Syntheses, characterization, and photo-hydrogen-evolving properties of tris(2,2′-bipyridine)ruthenium(ii) derivatives tethered to a cis-Pt(ii)Cl2unit: insights into the structure–activity relationship

The photo-hydrogen-evolving activity (activity to enhance the photochemical EDTA-reduction of water into molecular hydrogen) was evaluated for three different Ru(II)Pt(II) dimers with a general formula of [(bpy)2Ru(micro-bridge)PtCl2]2+(bpy = 2,2'-bipyridine; bridge = 4,4'-bis(N-(3-aminopropyl)carbamoyl)-2,2'-bipyridine (L1), 2,3-bis(2-pyridyl)pyrazine (L2), and 4,4'-bis(N-(4-pyridyl)methylcarbamoyl)-2,2'-bipyridine (L3); EDTA = ethylenediaminetetraacetic acid disodium salt). A new Ru(II)Pt(II) complex, [(bpy)2Ru(micro-L3)PtCl2]2+, was synthesized and characterized. It was confirmed that all three compounds are ineffective towards photochemical H2 production. In each case, an acetate-buffer solution (pH = 5) containing the Ru(II)Pt(II) dimer and EDTA was photolysed using a 350-W Xe lamp under an Ar atmosphere, during which the amount of H2 evolved was analysed by gas chromatography. Additional photolysis experiments were carried out by adding [Ru(bpy)3]2+ and methylviologen (N,N'-dimethyl-4,4'-bipyridinium) to the photolysis solutions described above to test the H2-evolving activity of the Pt(II) unit involved in these Ru(II)Pt(II) dimers. As a result, the Pt(II) units involved in the L1 and L2 compounds were found to be active as an H2-evolving catalyst, while that of the L3 compound was found to show no activity at all. The extent of intramolecular electron-transfer quenching from the 3MLCT excited state of the [Ru(bpy)3]2+ derivative to the tethering Pt(II) catalyst centre was investigated by comparison of the luminescence spectra of these compounds, together with the related compounds. The results showed that the quenching of the 3MLCT luminescence is not at all enhanced in either the L1 or the L3 compounds. On the other hand, the L2 compound is strongly quenched as previously reported. In addition to the above studies, the H2-evolving activity of some Pt(II) monomers, cis-PtCl2(NH3)2, PtCl2(en)(en = ethylenediamine), cis-PtCl2(4-methylpyridine)2, PtCl2(2,2'-bipyrimidine), PtCl2(4,4'-dicarboxy-2,2'-bipyridine), and [PtCl(terpy)]+(terpy = 2,2':6',2''-terpyridine), were similarly investigated in the presence of EDTA, [Ru(bpy)3]2+ and methylviologen, since they were regarded as structural analogues of the Pt(II) units involved in the L1-L3 compounds. The compounds having a cis-Pt(II)Cl2 unit were generally found to show high H2-evolving activity. This was interpreted in terms of the ligation of negatively charged chloride anions leading to the destabilization of the Pt(II) dz2 orbital responsible for the hydrogenic activation. Importantly, cis-PtCl2(4-methylpyridine)2 exhibited relatively high activity as an H2-evolving catalyst, suggesting the importance of the flexible rotation of the pyridyl ligands for efficient hydrogenic activation at the axial site of the Pt(II) ion. The DFT calculations also showed the validity of the structure-activity relationship discussed above for the L3 compound.

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

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

Cationic Nonsymmetric Transplatinum Complexes with Piperidinopiperidine Ligands. Preparation, Characterization, in Vitro Cytotoxicity, in Vivo Toxicity, and Anticancer Efficacy Studies

A series of complexes of the general formula trans-[PtCl2(Am)(pip-pip)] x HCl where pip-pip is 4-piperidinopiperidine and Am is NH3, methylamine (MA), dimethylamine (DMA), n-propylamine (NPA), isopropylamine (IPA), n-butylamine (NBA), or cyclohexylamine (CHA) were prepared and characterized, and their cytotoxic properties against ovarian and colon cancer cells were evaluated. The trans-[PtCl2(NH3)(pip-pip)] x HCl was significantly more potent than cisplatin in all the cisplatin-resistant ovarian cancer cell lines and was nearly as cytotoxic as cisplatin against colon cancer cells. In vivo studies in mice showed that the pip-pip complexes are significantly less toxic than cisplatin. Cisplatin was more efficacious than both trans-[PtCl2(NH3)(pip-pip)] x HCl and trans-[PtCl2(NBA)(pip-pip)] x HCl in the A2780 and A2780cisR tumor xenograft models, consistent with its lower IC50 values in A2780 cells but contrary to the higher IC50 values in A2780cisR cells. In the colon cancer cell studies, trans-[PtCl2(NH3)(pip-pip)] x HCl was slightly less potent than cisplatin in the in vitro studies but had efficacy comparable to that of cisplatin in the in vivo xenograft model.

Preparation, cytotoxicity and interactions with nucleophiles of three isomeric transplatinum complexes containing methylpiperidine ligands

Three isomeric complexes, trans-[PtCl2(NH3)(2-methylpiperidine)], trans-[PtCl2(NH3)(3-methylpiperidine)] and trans-[PtCl2(NH3)(4-methylpiperidine)], were prepared and their cytotoxicities against six ovarian cancer cell lines, three sensitive and three resistant to cisplatin, were measured. There were no significant differences in the cytotoxicities of the three isomers against these cell lines. The interactions of the three complexes with reduced glutathione (GSH) and with ubiquitin (Ub), as a model protein, were studied. The trans-[PtCl2(NH3)(2-methylpiperidine)] reacted approximately twice as slowly with GSH as did the other two isomers. In the 1:1 interactions of the three complexes with ubiquitin (Mr = 8565 amu), trans-[PtCl2(NH3)(3-methylpiperidine)] and trans-[PtCl2(NH3)(4-methylpiperidine)] attained 100% modification while trans-[PtCl2(NH3)(2-methylpiperidine)] reached only less than 50% modification. Trans-[PtCl2(NH3)(2-methylpiperidine)] reacts significantly less efficiently with GSH and proteins than the other two isomers yet this is not reflected in the cytotoxicity values. These results indicate that for these complexes, in these cell lines, cytosolic detoxification probably does not play a dominant role in determining the cytotoxicity of the complexes.

Platinum metallodrug-protein binding studies by capillary electrophoresis-inductively coupled plasma-mass spectrometry: characterization of interactions between Pt(II) complexes and human serum albumin

Characterizing how platinum metallocomplexes bind to human serum albumin (HSA) is essential in evaluating anticancer drug candidates. Using cisplatin as a reference complex, the application of capillary electrophoresis (CE) to reliably assess drug/HSA interactions was validated. Since this complex is small compared to the size of the protein, the binding response could only be recognized when applying CE coupled to a (platinum) metal-specific mode of detection, namely inductively coupled plasma-mass spectrometry (ICP-MS). This coupling allowed for confirmation of a specific affinity of cisplatin and novel Pt complexes to HSA, measurement of the kinetics of binding reactions, and determination of the number of drug molecules attached to the protein. As the cisplatin/HSA molar ratio increased, the reaction rate became faster with a maximum on the kinetic curve appearing at about 50 h of incubation at 20 times excess of cisplatin. The reaction was characterized as a pseudo-first order reaction with the rate constant k = 0.003 min(-1) at 37 degrees C. When incubated with a 20-fold excess of cisplatin, HSA bound up to 10 mol of Pt per mol of the protein. This is indicative for a strong metal-protein coordination occurring at several HSA sites other than the only protein cysteine residue. Structural analogs of cisplatin, bearing aminoalcohol ligands, showed comparable protein binding reactivity and stoichiometry but a common equilibrium was not reached even after one week of incubation. Also apparent was a two-step mechanism of the binding reaction. Results demonstrated the suitability of CE-ICP-MS as a rapid assay for high-throughput studying of drug/HSA interactions.

Water soluble cationic trans-platinum complexes which induce programmed cell death in the protozoan parasite Leishmania infantum

We have evaluated the cytotoxic properties against the protozoan Leishmania infantum of four water soluble cationic trans-Pt(II)Cl(2) compounds containing as inert groups NH3 and piperazine (1), 4-picoline and piperazine (2), n-butylamine and piperazine (3), and NH3 and 4-piperidino-piperidine (4). The leishmanicidal activity of compounds 3 and 4 against promastigotes of the parasite Leishmania infantum was 2.5- and 1.6-times higher than that of the cytotoxic drug cis-diamminedichloroplatinum(II), respectively. Interestingly, compounds 3 and 4 produce in Leishmania infantum promastigotes a higher amount of programmed cell death than cisplatin, which is associated with cell cycle arrest in G2/M. In contrast to cis-diamminedichloroplatinum(II), binding of compounds 3 and 4 to calf thymus DNA induces conformational changes more similar to those of trans-diamminedichloroplatinum(II) that may be attributed to denaturation of the double helix. Similarly to cis-diamminedichloroplatinum(II) and trans-diamminedichloroplatinum(II), the interaction of compounds 3 and 4 with ubiquitin results in an increase of the alpha-helix content of the protein as observed by circular dichroism spectroscopy. However, fluorescence studies indicate that compounds 3 and 4 produce a decrease in the fluorescence of the tyrosine 59 residue of ubiquitin higher than both cis-diamminedichloroplatinum(II) and trans-diamminedichloroplatinum(II). Altogether, our results suggest that the biochemical mechanism of cytotoxic activity of compounds 3 and 4 against Leishmania infantum must be different from that of cis-diamminedichloroplatinum(II). To the best of our knowledge, compounds 3 and 4 are the first reported trans-platinum complexes that show antiparasitic activity.

Mass spectrometric studies of the formation and reactivity of trans-[PtCl2(Am)(piperidinopiperidine)] x HCl complexes with ubiquitin

trans-[PtCl2(Am)(pip-pip)] x HCl complexes, where Am = ammine, methylamine and dimethylamine, react with ubiquitin to form 1:1 covalent adducts. The platinum complexes bind exclusively to Met1 of ubiquitin forming trans-[PtCl(S-Met1-Ub)(Am)(pip-pip)] adducts. These adducts are reactive towards nucleophiles and react with deoxyguanosine (dGMP) to form the ternary trans-[Pt(dGMP)(S-Met1-Ub) (Am)(pip-pip)] complex which is stable in water and even in the presence of excess glutathione (GSH). Reaction of trans-[PtCl(S-Met1-Ub)(Am)(pip-pip)] with GSH resulted in the rapid formation of the ternary complex trans-[Pt(GS)(S-Met1-Ub)(Am)(pip-pip)] which was not stable and slowly lost the platinum moiety; after 7 days the platinum moiety was completely removed and the native ubiquitin was regenerated.