Peculiar features in the crystal structure of the adduct formed between cis-PtI2(NH3)2 and hen egg white lysozyme

Digging into protein metalation differences triggered by fluorine containing-dirhodium tetracarboxylate analogues

Catalytic and biological properties of dirhodium tetracarboxylates ([Rh2(μ-O2CR)4L2], L=axial ligand, R=CH3-, CH3CH2-, etc) largely depend on the nature of the bridging carboxylate equatorial μ-O2CR ligands, which can be easily exchanged...

Interaction of Platinum-based Drugs with Proteins: An Overview of Representative Crystallographic Studies

Pt-based drugs are widely used in clinics for the treatment of cancer. The mechanism of action of these molecules relies on their interaction with DNA. However, the recognition of these metal compounds by proteins plays an important role in defining pharmacokinetics, side effects and their overall pharmacological profiles. Single crystal X-ray diffraction studies provided important information on the molecular mechanisms at the basis of this process. Here, the molecular structures of representative adducts obtained upon reaction with proteins of selected Pt-based drugs, including cisplatin, carboplatin and oxaliplatin, are briefly described and comparatively examined. Data indicate that metal ligands play a significant role in driving the reaction of Pt compounds with proteins; non-covalent interactions that occur in the early steps of Pt compound/protein recognition process play a crucial role in defining the structure of the final Pt-protein adduct. In the metallated protein structures, Pt centers coordinate few protein side chains, such as His, Met, Cys, Asp, Glu and Lys residues upon releasing labile ligands.

Strategies for the Improvement of Metal-Based Chemotherapeutic Treatments

This article provides an overview of the various research approaches we have explored in recent years to improve metal-based agents for cancer or infection treatments. Although cisplatin, carboplatin, and oxaliplatin remain the cornerstones in tumor chemotherapy, the discovery and approval of novel inorganic anticancer drugs is a very slow process. Analogously, although a few promising inorganic drugs have found clinical application against parasitic or bacterial infections, their use remains relatively limited. Moreover, the discovery process is often affected by small therapeutic enhancements that are not attractive for the pharmaceutical industry. However, the availability of increasing mechanistic information for the modes of action of established inorganic drugs is fueling the exploration of various approaches for developing effective inorganic chemotherapy agents. Through a series of examples, some from our own research experience, we focus our attention on a number of promising strategies, including (1) drug repurposing, (2) the simple modification of the chemical structures of approved metal-based drugs, (3) testing novel drug combinations, and (4) newly synthesized complexes coupling different anticancer drugs. Accordingly, we aim to suggest and summarize a series of reliable approaches that are exploitable for the development of improved and innovative treatments.

Thermodynamic Evaluation of the Interactions between Anticancer Pt(II) Complexes and Model Proteins

In this work, we have analysed the binding of the Pt(II) complexes ([PtCl(4′-phenyl-2,2′:6′,2″-terpyridine)](CF₃SO₃) (1), [PtI(4′-phenyl-2,2′:6′,2″-terpyridine)](CF₃SO₃) (2) and [PtCl(1,3-di(2-pyridyl)benzene) (3)] with selected model proteins (hen egg-white lysozyme, HEWL, and ribonuclease A, RNase A). Platinum coordination compounds are intensively studied to develop improved anticancer agents. In this regard, a critical issue is the possible role of Pt-protein interactions in their mechanisms of action. Multiple techniques such as differential scanning calorimetry (DSC), electrospray ionization mass spectrometry (ESI-MS) and UV-Vis absorbance titrations were used to enlighten the details of the binding to the different biosubstrates. On the one hand, it may be concluded that the affinity of 3 for the proteins is low. On the other hand, 1 and 2 strongly bind them, but with major binding mode differences when switching from HEWL to RNase A. Both 1 and 2 bind to HEWL with a non-specific (DSC) and non-covalent (ESI-MS) binding mode, dominated by a 1:1 binding stoichiometry (UV-Vis). ESI-MS data indicate a protein-driven chloride loss that does not convert into a covalent bond, likely due to the unfavourable complexes’ geometries and steric hindrance. This result, together with the significant changes of the absorbance profiles of the complex upon interaction, suggest an electrostatic binding mode supported by some stacking interaction of the aromatic ligand. Very differently, in the case of RNase A, slow formation of covalent adducts occurs (DSC, ESI-MS). The reactivity is higher for the iodo-compound 2, in agreement with iodine lability higher than chlorine.

Iodido equatorial ligands influence on the mechanism of action of Pt(IV) and Pt(II) anti-cancer complexes: A DFT computational study

A detailed computational exploration of the most relevant steps of iodido Pt(IV) complexes reduction and Pt(II) drugs mechanism of action and eventual deactivation is presented here inspired by the recent findings on iodido Pt(II) complexes and surprising re-evaluation of their cytotoxic activity. Pt(II) and Pt(IV) model systems are investigated and compared with cisplatin and its Pt(IV) derivative. Both monodeprotonated ascorbic acid and l-cysteine are used as reducing agents in the inner-sphere reduction mechanism of Pt(IV) complexes. Aquation mechanism of iodido Pt(II) complexes, interaction with guanine and sulfur containing compounds and reaction with the model protein hen egg white lysozyme are explored, due to a detected different behavior with respect to classical platinum drugs. The outcomes of such exploration allow to shed light on the role that the increased soft character together with bridging and leaving abilities of iodide over chloride could play in determining the cytotoxic profile of iodido Pt drugs.

An Efficient Conjugation Approach for Coupling Drugs to Native Antibodies via the PtII Linker Lx for Improved Manufacturability of Antibody-Drug Conjugates

The PtII linker [ethylenediamineplatinum(II)]2+ , coined Lx, has emerged as a novel non-conventional approach to antibody-drug conjugates (ADCs) and has shown its potential in preclinical in vitro and in vivo benchmark studies. A crucial improvement of the Lx conjugation reaction from initially <15 % to ca. 75-90 % conjugation efficiency is described, resulting from a systematic screening of all relevant reaction parameters. NaI, a strikingly simple inorganic salt additive, greatly improves the conjugation efficiency as well as the conjugation selectivity simply by exchanging the leaving chloride ligand on Cl-Lx-drug complexes (which are direct precursors for Lx-ADCs) for iodide, thus generating I-Lx-drug complexes as more reactive species. Using this iodide effect, we developed a general and highly practical conjugation procedure that is scalable: our lead Lx-ADC was produced on a 5 g scale with an outstanding conjugation efficiency of 89 %.

Unusual Structural Features in the Adduct of Dirhodium Tetraacetate with Lysozyme

The structures of the adducts formed upon reaction of the cytotoxic paddlewheel dirhodium complex [Rh₂(μ-O₂CCH₃)₄] with the model protein hen egg white lysozyme (HEWL) under different experimental conditions are reported. Results indicate that [Rh₂(μ-O₂CCH₃)₄] extensively reacts with HEWL:it in part breaks down, at variance with what happens in reactions with other proteins. A Rh center coordinates the side chains of Arg14 and His15. Dimeric Rh–Rh units with Rh–Rh distances between 2.3 and 2.5 Å are bound to the side chains of Asp18, Asp101, Asn93, and Lys96, while a dirhodium unit with a Rh–Rh distance of 3.2–3.4 Å binds the C-terminal carboxylate and the side chain of Lys13 at the interface between two symmetry-related molecules. An additional monometallic fragment binds the side chain of Lys33. These data, which are supported by replicated structural determinations, shed light on the reactivity of dirhodium tetracarboxylates with proteins, providing useful information for the design of new Rh-containing biomaterials with an array of potential applications in the field of catalysis or of medicinal chemistry and valuable insight into the mechanism of action of these potential anticancer agents.

Recent advances in protein metalation: structural studies

Recent advances in structural studies unveiling the basis of the metal compounds/protein recognition process are discussed.

An unexpected in-solution instability of diiodido analogue of picoplatin complicates its biological characterization

Complex cis-[PtI2(NH3)(pic)] (1; pic = 2-methylpyridine), a diiodido analogue of clinically studied picoplatin (2), is unstable in solution, which is intriguingly connected with the release of its pic ligand. This observation complicates the biological testing of e.g. cytotoxicity in human cancer cells for 1.

A Combined Spectroscopic and Protein Crystallography Study Reveals Protein Interactions of RhI(NHC) Complexes at the Molecular Level

While most Rh-N-heterocyclic carbene (NHC) complexes currently investigated in anticancer research contain a Rh(III) metal center, an increasing amount of research is focusing on the cytotoxic activity and mode of action of square-planar [RhCl(COD)(NHC)] (where COD = 1,5-cyclooctadiene) which contains a Rh(I) center. The enzyme thioredoxin reductase (TrxR) and the protein albumin have been proposed as potential targets, but the molecular processes taking place upon protein interaction remain elusive. Herein, we report the preparation of peptide-conjugated and its nonconjugated parent [RhCl(COD)(NHC)] complexes, an in-depth investigation of both their stability in solution, and a crystallographic study of protein interaction. The organorhodium compounds showed a rapid loss of the COD ligand and slow loss of the NHC ligand in aqueous solution. These ligand exchange reactions were reflected in studies on the interaction with hen egg white lysozyme (HEWL) as a model protein in single-crystal X-ray crystallographic investigations. Upon treatment of HEWL with an amino acid functionalized [RhCl(COD)(NHC)] complex, two distinct rhodium adducts were found initially after 7 d of incubation at His15 and after 4 weeks also at Lys33. In both cases, the COD and chlorido ligands had been substituted with aqua and/or hydroxido ligands. While the histidine (His) adduct also indicated a loss of the NHC ligand, the lysine (Lys) adduct retained the NHC core derived from the amino acid l-histidine. In either case, an octahedral coordination environment of the metal center indicates oxidation to Rh(III). This investigation gives the first insight on the interaction of Rh(I)(NHC) complexes and proteins at the molecular level.

Reactions of cisplatin and cis-[PtI2(NH3)2] with molecular models of relevant protein sidechains: A comparative analysis

Quite surprisingly, cisplatin and cis-[PtI₂(NH₃)₂] were found to manifest significant differences in their reactions with the model protein lysozyme. We decided to explore whether these differences recur when reacting these two Pt compounds with other proteins. Notably, ESI-MS measurements carried out on cytochrome c nicely confirmed the reaction pattern observed for lysozyme. This prompted us to exploit a computational DFT approach to disclose the molecular basis of such behavior. We analyzed comparatively the reactions of cis-[PtCl₂(NH₃)₂] and cis-[PtI₂(NH₃)₂] with appropriate molecular models (Ls) of the sidechains of relevant aminoacids. We found that when Pt(II) complexes are reacted with sulfur ligands both quickly lose their halide ligands and then the resulting cis-[Pt(L)₂(NH₃)₂] species loses ammonia upon reaction with a ligand excess. In the case of imidazole, again cis-[PtCl₂(NH₃)₂] and cis-[PtI₂(NH₃)₂] quickly lose their halide ligands but the resulting cis-[Pt(L)₂(NH₃)₂] species does not lose ammonia by reaction with excess imidazole. These results imply that the two platinum complexes manifest a significantly different behavior in their reaction with representative small molecules in agreement with what observed in the case of model proteins. It follows that the protein itself must play a crucial role in triggering the peculiar reactivity of cis-[PtI₂(NH₃)₂] and in governing the nature of the formed protein adducts. The probable reasons for the observed behavior are critically commented and discussed.

New Findings in the Signaling Pathways of cis and trans Platinum Iodido Complexes' Interaction with DNA of Cancer Cells

We have selected a series of aliphatic amine platinum compounds bearing chloride and/or iodide as the leaving groups. The complexes' cytotoxicity and interaction with DNA indicated differences in the reactivity. Now, we are reporting on the analysis of their molecular mechanism of action on gastric cancer cells. Our data reveals differences between them. Chlorido drugs showed similar behavior to cisplatin; they both required p53 to induce apoptosis but only cis-ipa showed DNA damage requirement for apoptosis induction. On the contrary, cis and trans iodido induced cell death independent of p53 activity, and they induced cell death through Bid activation, so their toxicity could be enhanced in a combined treatment with novel Bcl-2 protein family inhibitors. We also report the structural features of the DNA adduct for one of the complexes by X-ray diffraction. These findings represent a step forward in the search for new platinum-derived agents more specific and effective in the treatment of cancer.

Major conformational changes in the structure of lysozyme obtained from a crystal with a very low solvent content

A new crystal form of lysozyme with a very low solvent content (26.35%) has been obtained in the orthorhombic space group P212121 (with unit-cell parameters a = 30.04, b = 51.68, c = 61.53 Å). The lysozyme structure obtained from these crystals does not show the typical overall fold. Instead, major conformational changes take place in some elements of the secondary structure and in the hydrophobic core of the protein. At the end of the central α-helix (α2), Glu35 is usually buried in the catalytic site and shows an abnormally high pKa value, which is key to the activity of the enzyme. The high pKa value of this glutamate residue is favoured by the hydrophobic environment, particularly by its neighbour Trp108, which is important for structural stability and saccharide binding. In this new structure, Trp108 shows a 90° rotation of its side chain, which results in the rearrangement of the hydrophobic core. Conformational changes also result in the exposure of Glu35 to the solvent, which impairs the catalytic site by increasing the distance between Glu35 and Asp52 and lowering the pKa value of the glutamate. Altogether, this new lysozyme structure reveals major conformational changes in the hydrophobic core and catalytic site that might play a role in the folding and bactericidal function of the protein.

Reaction with Proteins of a Five-Coordinate Platinum(II) Compound

Stable five-coordinate Pt(II) complexes have been highlighted as a promising and original platform for the development of new cytotoxic drugs. Their interaction with proteins has been scarcely studied. Here, the reactivity of the five-coordinate Pt(II) compound [Pt(I)(Me) (dmphen)(olefin)] (Me = methyl, dmphen = 2,9-dimethyl-1,10-phenanthroline, olefin = dimethylfumarate) with the model proteins hen egg white lysozyme (HEWL) and bovine pancreatic ribonuclease (RNase A) has been investigated by X-ray crystallography and electrospray ionization mass spectrometry. The X-ray structures of the adducts of RNase A and HEWL with [Pt(I)(Me)(dmphen)(olefin)] are not of very high quality, but overall data indicate that, upon reaction with RNase A, the compound coordinates the side chain of His105 upon releasing the iodide ligand, but retains the pentacoordination. On the contrary, upon reaction with HEWL, the trigonal bi-pyramidal Pt geometry is lost, the iodide and the olefin ligands are released, and the metal center coordinates the side chain of His15 probably adopting a nearly square-planar geometry. This work underlines the importance of the combined use of crystallographic and mass spectrometry techniques to characterize, in detail, the protein–metallodrug recognition process. Our findings also suggest that five-coordinate Pt(II) complexes can act either retaining their uncommon structure or functioning as prodrugs, i.e., releasing square-planar platinum complexes as bioactive species.

A Fluorescent Silver(I) Carbene Complex with Anticancer Properties: Synthesis, Characterization, and Biological Studies

The silver(I) N-heterocyclic carbene (NHC) complex bis(1-(anthracen-9-ylmethyl)-3-ethylimidazol-2-ylidene) silver chloride ([Ag(EIA)₂ ]Cl), bearing two anthracenyl fluorescent probes, has been synthesized and characterized. [Ag(EIA)₂ ]Cl is stable in organic solvents and under physiological conditions, and shows potent cytotoxic effects in vitro toward human SH-SY5Y neuroblastoma cells. The interactions of [Ag(EIA)₂ ]Cl with a few model biological targets have been studied as well as its ability to be internalized in cells. The in vitro anticancer activity is apparently related to the level of drug internalization. Notably, [Ag(EIA)₂ ]Cl does not react with a few model proteins, but is capable of binding the C-terminal dodecapeptide of thioredoxin reductase hTrxR(488-499) and to strongly inhibit the activity of this enzyme. Binding occurs through an unconventional process leading to covalent binding of one or two carbene ligands to the C-terminal dodecapeptide with concomitant release of the silver cation. To the best of our knowledge, this mode of interaction is reported here for the first time for Ag(NHC)₂ complexes.

Novel structures of platinum complexes bearing N‑bisphosphonates and study of their biological properties

Novel bisphosphonate platinum complexes: [Pt(isopropylamine)₂(BP)]NO₃ (BP = pamidronate and alendronate) have been synthesized and characterized. Their monomeric structure contains a bisphosphonate acting as chelate ligand through its oxygen atom donors, conferring the compound's cationic structure with a good solubility in water. The study of the compounds in solution showed high stability up to 24 h. The cytotoxicity in cancer cell lines has been assessed. We also present preliminary studies on the evaluation of the affinity towards biological targets such as DNA (both calf thymus DNA and supercoiled plasmid DNA) and hydroxyapatite where the complexes showed a low DNA interaction, but a clear affinity for hydroxyapatite comparing to their precursors.

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.

Reactions of a tetranuclear Pt-thiosemicarbazone complex with model proteins

The tetranuclear Pt complex (PtL)₄ (where L^2- is the anion derived from para-isopropyl thiosemicarbazone) was first described in A.G. Quiroga et al., J. Med. Chem. 41, 1998, 1399-1408. (PtL)₄ manifests antiproliferative properties toward various cancer cell lines being a promising anticancer drug candidate. Yet, details of its reactivity with biomolecules have not been elucidated. To this end, we investigated the reactions of (PtL)₄ with a few model proteins, i.e. bovine pancreatic ribonuclease (RNase A), cytochrome c (Cyt c) and hen egg white lysozyme (Lysozyme), through electrospray ionization mass spectrometry and other biophysical methods. A rich reactivity of (PtL)₄ with the above-mentioned model proteins is observed, leading to the formation of numerous metallodrug-protein adducts. The tetranuclear complex breaks down and various fragments bind proteins up to high metal/protein ratios; this typically results into very complicated mass spectral patterns. However, some of the main mass peaks could be assigned in the case of the Lysozyme adduct. In addition, crystallographic data were obtained for the (PtL)₄/Lysozyme and (PtL)₄/RNase A adducts pointing at His side chains as the primary binding sites for monometallic Pt fragments. Notably, a few selected features of the interactions observed in the (PtL)₄/protein adducts were reproduced by reacting (PtL)₄ with a small molecule, i.e. N-methylimidazole. In conclusion, the present study confirms the prodrug nature of the tetraplatinum complex, clarifies one possible pathway for its activation through cluster disassembly and allows initial identification of adducts formed with a representative protein.

Interaction of a gold(i) dicarbene anticancer drug with human telomeric DNA G-quadruplex: solution and computationally aided X-ray diffraction analysis

Solution and solid-state data give a quite clear picture for a bis carbene gold(i) complex having perspective anticancer properties.

Exploring the interactions between model proteins and Pd(ii) or Pt(ii) compounds bearing charged N,N-pyridylbenzimidazole bidentate ligands by X-ray crystallography

X-ray structure of the adducts formed between lysozyme and Pd(ii) and Pt(ii) compounds bearing N,N-pyridylbenzimidazole derivatives with an alkylated sulfonate or phosphonium side chain are reported.

A case of extensive protein platination: the reaction of lysozyme with a Pt(ii)–terpyridine complex

The interaction between a Pt(ii)–terpyridine cytotoxic compound and the model protein lysozyme has been investigated by X-ray crystallography and electrospray mass spectrometry under different experimental conditions. The compound shows a high reactivity with the model protein.

Prediction of the interaction of metallic moieties with proteins: An update for protein-ligand docking techniques

In this article, we present a new approach to expand the range of application of protein-ligand docking methods in the prediction of the interaction of coordination complexes (i.e., metallodrugs, natural and artificial cofactors, etc.) with proteins. To do so, we assume that, from a pure computational point of view, hydrogen bond functions could be an adequate model for the coordination bonds as both share directionality and polarity aspects. In this model, docking of metalloligands can be performed without using any geometrical constraints or energy restraints. The hard work consists in generating the convenient atom types and scoring functions. To test this approach, we applied our model to 39 high-quality X-ray structures with transition and main group metal complexes bound via a unique coordination bond to a protein. This concept was implemented in the protein-ligand docking program GOLD. The results are in very good agreement with the experimental structures: the percentage for which the RMSD of the simulated pose is smaller than the X-ray spectra resolution is 92.3% and the mean value of RMSD is < 1.0 Å. Such results also show the viability of the method to predict metal complexes-proteins interactions when the X-ray structure is not available. This work could be the first step for novel applicability of docking techniques in medicinal and bioinorganic chemistry and appears generalizable enough to be implemented in most protein-ligand docking programs nowadays available. © 2017 Wiley Periodicals, Inc.

ESI-MS studies of the reactions of novel platinum(II) complexes containing O,O'-chelated acetylacetonate and sulfur ligands with selected model proteins

A group of mixed-ligand Pt(II) complexes bearing acetylacetonate and sulphur ligands were recently developed in the University of Lecce as a new class of prospective anticancer agents that manifested promising pharma-cological properties in preliminary in vitro and in vivo tests. Though modelled on the basis of cisplatin, these Pt(II) complexes turned out to exhibit a profoundly distinct mode of action as they were found to act mainly on non-genomic targets rather than on DNA. Accordingly, we have explored here their reactions with two representative model proteins through an established ESI-MS procedure with the aim to describe their general interaction mechanism with protein targets. A pronounced reactivity with the tested proteins was indeed documented; the nature of the resulting metallodrug-protein interactions could be characterised in depth in the various cases. Preferential binding to protein targets compared to DNA is supported by independent ICP-OES measurements. The implications of these findings are discussed.

Mitochondria Targeting with Luminescent Rhenium(I) Complexes

Two new neutral fac-[Re(CO)₃(phen)L] compounds (1,2), with phen = 1,10-phenanthroline and L = O₂C(CH₂)₅CH₃ or O₂C(CH₂)₄C≡CH, were synthetized in one-pot procedures from fac-[Re(CO)₃(phen)Cl] and the corresponding carboxylic acids, and were fully characterized by IR and UV-Vis absorption spectroscopy, ¹H- and ¹³C-NMR, mass spectrometry and X-ray crystallography. The compounds, which display orange luminescence, were used as probes for living cancer HeLa cell staining. Confocal microscopy revealed accumulation of both dyes in mitochondria. To investigate the mechanism of mitochondrial staining, a new non-emissive compound, fac-[Re(CO)₃(phen)L], with L = O₂C(CH₂)₃((C₅H₅)Fe(C₅H₄), i.e., containing a ferrocenyl moiety, was synthetized and characterized (3). 3 shows the same mitochondrial accumulation pattern as 1 and 2. Emission of 3 can only be possible when ferrocene-containing ligand dissociates from the metal center to produce a species containing the luminescent fac-[Re(CO)₃(phen)]⁺ core. The release of ligands from the Re center was verified in vitro through the conjugation with model proteins. These findings suggest that the mitochondria accumulation of compounds 1-3 is due to the formation of luminescent fac-[Re(CO)₃(phen)]⁺ products, which react with cellular matrix molecules giving secondary products and are uptaken into the negatively charged mitochondrial membranes. Thus, reported compounds feature a rare dissociation-driven mechanism of action with great potential for biological applications.

Platinum(II) Iodido Complexes of 7-Azaindoles with Significant Antiproliferative Effects: An Old Story Revisited with Unexpected Outcomes

A series of platinum(II) diiodido complexes containing 7-azaindole derivatives, having the general formula cis-[PtI₂(naza)₂] (1–8), has been prepared and thoroughly characterized, including X-ray structure analysis of cis-[PtI₂(2Me4Claza)₂]∙DMF (8∙DMF; 2Me4Claza = 2-methyl-4-chloro-7-azaindole). Complexes showed high in vitro cytotoxicity against nine human cancer cell lines (IC₅₀ ranging from 0.4 to 12.8 μM), including the cisplatin-resistant ovarian cancer cell line (A2780R; IC₅₀ = 1.0–3.5 μM). The results of in vivo testing, using the L1210 lymphocytic leukaemia model, at the equimolar doses of Pt with cisplatin (2 mg/kg) confirmed the activity of complex 8 comparable to cisplatin. From the mechanistic point of view, evaluated ex vivo by Western blot analyses on the samples of isolated tumour tissues, the treatment of the animals with complex 8, contrary to cisplatin, decreased the levels of tumour suppressor p53 and increased significantly the amount of intracellular anti-apoptotic protein MCL-1_L (37 kDa). Additionally, the active form of caspase 3 was significantly elevated in the sample of tumour tissues treated with complex 8, indicating that the activation of p53-independent cell-death pathway was initiated. The light and electron microscopy observations of the cancerous tissues revealed necrosis as a dominant mechanism of cell death, followed by scarce signs of apoptosis. The additional results (e.g. in vitro interaction experiments with selected biomolecules, cell cycle perturbations, gel electrophoretic studies on pUC19 plasmid DNA) supported the hypothesis that the complexes might be involved in the mechanism of action quite different from cisplatin.

cis-Pt I2(NH3)2: a reappraisal

The investigation of cis-PtI2(NH3)2, the diiodido analogue of cisplatin (cisPtI2 hereafter), has been unjustly overlooked so far mainly because of old claims of pharmacological inactivity. Some recent - but still fragmentary - findings prompted us to reconsider more systematically the chemical and biological profile of cisPtI2 in comparison with cisplatin. Its solution behaviour, interactions with DNA and cytotoxic properties versus selected cancer cell lines were thus extensively analysed through a variety of biophysical and computational methods. Notably, we found that cisPtI2 is highly cytotoxic in vitro toward a few solid tumour cell lines and that its DNA platination pattern closely reproduces that of cisplatin; cisPtI2 is also shown to completely overcome resistance to cisplatin in a platinum resistant cancer cell line. The differences in the biological actions of these two Pt complexes are most likely related to slight but meaningful differences in their solution behaviour and reactivity. Overall, a very encouraging and unexpected pharmacological profile emerges for cisPtI2 with relevant implications both in terms of mechanistic knowledge and of prospective clinical application. An ab initio DFT study is also included to support the interpretation of the solution behaviour of cisPtI2 under physiological and slightly acidic pH conditions.

A first-in-class and a fished out anticancer platinum compound: cis-[PtCl2(NH3)2] and cis-[PtI2(NH3)2] compared for their reactivity towards DNA model systems

Contrary to what was believed for many years, cis-PtI2(NH3)2, the diiodido analogue of cisplatin, displays high in vitro antiproliferative activity toward a set of tumour cell lines, overcoming resistance to cisplatin in a platinum-resistant cancer cell line. In the context of a general reappraisal of iodinated Pt(ii) derivatives, aiming at a more systematic evaluation of their chemical and biological profiles, here we report on the reactivity of cis-PtI2(NH3)2 with selected DNA model systems, in single, double strand or G-quadruplex form, using cisplatin as a control. A combined approach has been exploited in this study, including circular dichroism (CD), UV-visible spectroscopy and electrospray mass spectrometry (ESI-MS) analyses. The data reveal that cis-PtI2(NH3)2 shows an overall reactivity towards the investigated oligonucleotides significantly higher than cisplatin.

Cisplatin and its dibromido analogue: a comparison of chemical and biological profiles

The dibromido analogue of cisplatin, cis-PtBr2(NH3)2 (cisPtBr2 hereafter), has been prepared and characterised. Its solution behaviour in standard phosphate buffer, at pH 7.4, was investigated spectrophotometrically and found to reproduce quite closely that of cisplatin; indeed, progressive sequential release of the two halide ligands typically occurs as in the case of cisplatin, with a roughly similar kinetics. Afterward, patterns of reactivity toward model proteins and standard ctDNA were explored and the nature of the resulting interactions elucidated. The antiproliferative properties were then evaluated in four representative cancer cell lines, namely A549 (human lung cancer), HCT116 (human colon cancer), IGROV-1 (human ovarian cancer) and FLG 29.1 (human acute myeloid leukaemia). Cytotoxic properties in line with those of cisplatin were highlighted. From these studies an overall chemical and biological profile emerges for cisPtBr2 closely matching that of cisplatin; the few slight, but meaningful differences that were underscored might be advantageously exploited for clinical application.

Effect of temperature on the interaction of cisplatin with the model protein hen egg white lysozyme

The products of the reaction between cisplatin (CDDP) and the model protein hen egg white lysozyme (HEWL) at 20, 37 and 55 °C in pure water were studied by UV-Vis absorption spectroscopy, intrinsic fluorescence and circular dichroism, dynamic and electrophoretic light scattering and inductively coupled plasma mass spectrometry. X-ray structures were also solved for the adducts formed at 20 and 55 °C. Data demonstrate that high temperature facilitates the formation of CDDP-HEWL adducts, where Pt atoms bind ND1 atom of His15 or NE2 atom of His15 and NH1 atom of Arg14. Our study suggests that high human body temperature (fever) could increase the rate of drug binding to proteins thus enhancing possible toxic side effects related to CDDP administration.

Platinum(II) Complexes with O,S Bidentate Ligands: Biophysical Characterization, Antiproliferative Activity, and Crystallographic Evidence of Protein Binding

We recently characterized a series of novel platinum(II) compounds bearing a conserved O,S binding moiety as a bifunctional ligand and evaluated their solution behavior and antiproliferative properties in vitro against a representative cancer cell line. On the whole, those platinum compounds showed an appreciable stability in mixed dimethyl sulfoxide-aqueous buffers and promising in vitro cytotoxic effects; yet they manifested a rather limited solubility in aqueous media making them poorly suitable for further pharmaceutical development. To overcome this drawback, four new derivatives of this series were prepared and characterized based on a careful choice of substituents on the O,S bidentate ligand. The solubility and stability profile of these novel compounds in a reference buffer was determined, as well as the ligands' log P(o/w) value (P(o/w) = n-octanol-water partition coefficient) as an indirect measure for the complexes' lipophilicity. The antiproliferative properties were comparatively evaluated in a panel of three cancer cell lines. The protein binding properties of the four platinum compounds were assessed using the model protein hen egg white lysozyme (HEWL), and the molecular structures of two relevant HEWL-metallodrug adducts were solved. Overall, it is shown that a proper choice of the substituents leads to a higher solubility and enables a selective fine-tuning of the antiproliferative properties. The implications of these results are thoroughly discussed.

Interactions of carboplatin and oxaliplatin with proteins: Insights from X-ray structures and mass spectrometry studies of their ribonuclease A adducts

Oxaliplatin and carboplatin are two platinum(II) drugs in widespread clinical use for the treatment of various types of cancers; yet, structural information on their interactions with proteins is scarce. Here, the X-ray structures of the adducts formed upon reaction of carboplatin and oxaliplatin with bovine pancreatic ribonuclease (RNase A) are reported and compared with results obtained for the structure of the RNase A-cisplatin adduct derived from isomorphous crystals, under the same experimental conditions. Additional details on the binding mode of these metallodrugs toward RNase A are provided by electrospray ionization mass spectrometry (ESI MS) measurements, thus offering insight on the occurring metal-protein interactions. Notably, while carboplatin and cisplatin mainly bind the side chain of Met29, oxaliplatin also binds the side chains of Asp14, of catalytically important His119 and, to a lesser extent, of His105. On the basis of the available data, a likely mechanism for oxaliplatin hydrolysis and binding to the protein is proposed. These results are potentially useful for a better understanding of the biological chemistry, toxicity and side effects of this important class of antitumor agents.

Interactions of gold-based drugs with proteins: the structure and stability of the adduct formed in the reaction between lysozyme and the cytotoxic gold(III) compound Auoxo3

The structure and stability of the adduct formed in the reaction between Auoxo3, a dinuclear gold(iii) compound, and the model protein hen egg white lysozyme (HEWL) are investigated by X-ray crystallography, UV-Vis absorption spectroscopy and circular dichroism (CD). It is found that Auoxo3 breaks down completely, undergoes reduction and produces reactive gold(i) species able to bind the protein and form stable derivatives. The behaviour of Auoxo3 is compared with that of two analogous gold(iii) complexes previously studied: a few significant differences are highlighted. The general implications of these new results for the mode of action of cytotoxic gold complexes are discussed.

Protein Recognition of Gold-Based Drugs: 3D Structure of the Complex Formed When Lysozyme Reacts with Aubipy(c.)

The structure of the adduct formed in the reaction between Aubipy(c), a cytotoxic organogold(III) compound, and the model protein hen egg white lysozyme (HEWL) has been solved by X-ray crystallography. It emerges that Aubipy(c), after interaction with HEWL, undergoes reduction of the gold(III) center followed by detaching of the cyclometalated ligand; the resulting naked gold(I) ion is found bound to the protein at Gln121. A direct comparison between the present structure and those previously solved for the lysozyme adducts with other gold(III) compounds demonstrates that coordinated ligands play a key role in the protein-metallodrug recognition process. Structural data support the view that gold(III)-based antitumor prodrugs are activated through metal reduction.

Carboplatin binding to histidine

Carboplatin is a second-generation platinum anticancer agent used for the treatment of a variety of cancers. Previous X-ray crystallographic studies of carboplatin binding to histidine (in hen egg-white lysozyme; HEWL) showed the partial conversion of carboplatin to cisplatin owing to the high NaCl concentration used in the crystallization conditions. HEWL co-crystallizations with carboplatin in NaBr conditions have now been carried out to confirm whether carboplatin converts to the bromine form and whether this takes place in a similar way to the partial conversion of carboplatin to cisplatin observed previously in NaCl conditions. Here, it is reported that a partial chemical transformation takes place but to a transplatin form. Thus, to attempt to resolve purely carboplatin binding at histidine, this study utilized co-crystallization of HEWL with carboplatin without NaCl to eliminate the partial chemical conversion of carboplatin. Tetragonal HEWL crystals co-crystallized with carboplatin were successfully obtained in four different conditions, each at a different pH value. The structural results obtained show carboplatin bound to either one or both of the N atoms of His15 of HEWL, and this particular variation was dependent on the concentration of anions in the crystallization mixture and the elapsed time, as well as the pH used. The structural details of the bound carboplatin molecule also differed between them. Overall, the most detailed crystal structure showed the majority of the carboplatin atoms bound to the platinum centre; however, the four-carbon ring structure of the cyclobutanedicarboxylate moiety (CBDC) remained elusive. The potential impact of the results for the administration of carboplatin as an anticancer agent are described.

Chemical conversion of cisplatin and carboplatin with histidine in a model protein crystallized under sodium iodide conditions

Crystals of HEWL with cisplatin and HEWL with carboplatin grown in sodium iodide conditions both show a partial chemical transformation of cisplatin or carboplatin to a transiodoplatin (PtI₂X₂) form. The binding is only at the N^δ atom of His15. A further Pt species (PtI₃X) is also seen, in both cases bound in a crevice between symmetry-related protein molecules.

Cisplatin and carboplatin are platinum anticancer agents that are used to treat a variety of cancers. Previous X-ray crystallographic studies of carboplatin binding to histidine in hen egg-white lysozyme (HEWL) showed a partial chemical conversion of carboplatin to cisplatin owing to the high sodium chloride concentration used in the crystallization conditions. Also, the co-crystallization of HEWL with carboplatin in sodium bromide conditions resulted in the partial conversion of carboplatin to the transbromoplatin form, with a portion of the cyclobutanedicarboxylate (CBDC) moiety still present. The results of the co-crystallization of HEWL with cisplatin or carboplatin in sodium iodide conditions are now reported in order to determine whether the cisplatin and carboplatin converted to the iodo form, and whether this took place in a similar way to the partial conversion of carboplatin to cisplatin in NaCl conditions or to transbromoplatin in NaBr conditions as seen previously. It is reported here that a partial chemical transformation has taken place to a transplatin form for both ligands. The NaI-grown crystals belonged to the monoclinic space group P2₁ with two molecules in the asymmetric unit. The chemically transformed cisplatin and carboplatin bind to both His15 residues, i.e. in each asymmetric unit. The binding is only at the N^δ atom of His15. A third platinum species is also seen in both conditions bound in a crevice between symmetry-related molecules. Here, the platinum is bound to three I atoms identified based on their anomalous difference electron densities and their refined occupancies, with the fourth bound atom being a Cl atom (in the cisplatin case) or a portion of the CBDC moiety (in the carboplatin case).

The binding of platinum hexahalides (Cl, Br and I) to hen egg-white lysozyme and the chemical transformation of the PtI6 octahedral complex to a PtI3 moiety bound to His15

The platinum hexahalides have an octahedral arrangement of six halogen atoms bound to a Pt centre, thus having an octahedral shape that could prove to be useful in interpreting poor electron-density maps. In a detailed characterization, PtI₆ chemically transformed to a square-planar PtI₃ complex bound to the N^δ atom of His15 of HEWL was also observed, which was not observed for PtBr₆ or PtCl₆.

This study examines the binding and chemical stability of the platinum hexahalides K₂PtCl₆, K₂PtBr₆ and K₂PtI₆ when soaked into pre-grown hen egg-white lysozyme (HEWL) crystals as the protein host. Direct comparison of the iodo complex with the chloro and bromo complexes shows that the iodo complex is partly chemically transformed to a square-planar PtI₃ complex bound to the N^δ atom of His15, a chemical behaviour that is not exhibited by the chloro or bromo complexes. Each complex does, however, bind to HEWL in its octahedral form either at one site (PtI₆) or at two sites (PtBr₆ and PtCl₆). As heavy-atom derivatives of a protein, the octahedral shape of the hexahalides could be helpful in cases of difficult-to-interpret electron-density maps as they would be recognisable ‘objects’.

Interactions between anticancer trans-platinum compounds and proteins: crystal structures and ESI-MS spectra of two protein adducts of trans-(dimethylamino)(methylamino)dichloridoplatinum(II)

The adducts formed between trans-(dimethylamino)(methylamino)dichloridoplatinum(II), [t-PtCl2(dma)(ma)], and two model proteins, i.e., hen egg white lysozyme and bovine pancreatic ribonuclease, were independently characterized by X-ray crystallography and electrospray ionization mass spectrometry. In these adducts, the Pt(II) center, upon chloride release, coordinates either to histidine or aspartic acid residues while both alkylamino ligands remain bound to the metal. Comparison with the cisplatin derivatives of the same proteins highlights for [t-PtCl2(dma)(ma)] a kind of biomolecular metalation remarkably different from that of cisplatin.

Unusual structural features in the lysozyme derivative of the tetrakis(acetato)chloridodiruthenium(II,III) complex

The reaction between the paddle-wheel tetrakis(acetato)chloridodiruthenium(II,III) complex, [Ru2(μ-O2CCH3)4Cl] and hen egg-white lysozyme (HEWL) was investigated through ESI-MS and UV/Vis spectroscopy and the formation of a stable metal-protein adduct was unambiguously demonstrated. Remarkably, the diruthenium core is conserved in the adduct while two of the four acetate ligands are released. The crystal structure of this diruthenium-protein derivative was subsequently solved through X-ray diffraction analysis to 2.1 Å resolution. The structural data are in agreement with the solution results. It was found that HEWL binds two diruthenium moieties, at Asp101 and Asp119, respectively, with the concomitant release of two acetate ligands from each diruthenium center.

Cisplatin binding to proteins: molecular structure of the ribonuclease a adduct

The crystal structure of the main adduct formed in the reaction between cisplatin and bovine pancreatic ribonuclease is reported here. Notably, in both of the protein molecules present in the asymmetric unit, platinum(II) binding takes place exclusively at the level of Met29. In one of the two molecules, the Gln28 side chain completes the platinum coordination sphere, anchoring the cisplatin fragment to the protein in a bidentate fashion. These results contain interesting implications for understanding the biological chemistry of this important drug.