Interactions of cisplatin and transplatin with proteins. Comparison of binding kinetics, binding sites and reactivity of the Pt-protein adducts of cisplatin and transplatin towards biological nucleophiles

Not All Binding Sites Are Equal: Site Determination and Folding State Analysis of Gas-Phase Protein-Metallodrug Adducts

Modern approaches in metallodrug research focus on compounds that bind protein targets rather than DNA. However, the identification of protein targets and binding sites is challenging. Using intact mass spectrometry and proteomics, we investigated the binding of the antimetastatic agent RAPTA-C to the model proteins ubiquitin, cytochrome c, lysozyme, and myoglobin. Binding to cytochrome c and lysozyme was negligible. However, ubiquitin bound up to three Ru moieties, two of which were localized at Met1 and His68 as [Ru(cym)], and [Ru(cym)] or [Ru(cym)(PTA)] adducts, respectively. Myoglobin bound up to four [Ru(cym)(PTA)] moieties and five sites were identified at His24, His36, His64, His81/82 and His113. Collision-induced unfolding (CIU) studies via ion-mobility mass spectrometry allowed measuring protein folding as a function of collisional activation. CIU of protein-RAPTA-C adducts showed binding of [Ru(cym)] to Met1 caused a significant compaction of ubiquitin, likely from N-terminal S-Ru-N chelation, while binding of [Ru(cym)(PTA)] to His residues of ubiquitin or myoglobin induced a smaller effect. Interestingly, the folded state of ubiquitin formed by His functionalization was more stable than Met1 metalation. The data suggests that selective metalation of amino acids at different positions on the protein impacts the conformation and potentially the biological activity of anticancer compounds.

Evidence of direct interaction between cisplatin and the caspase-cleaved prostate apoptosis response-4 tumor suppressor

Prostate apoptosis response-4 (Par-4) tumor suppressor protein has gained attention as a potential therapeutic target owing to its unique ability to selectively induce apoptosis in cancer cells, sensitize them to chemotherapy and radiotherapy, and mitigate drug resistance. It has recently been reported that Par-4 interacts synergistically with cisplatin, a widely used anticancer drug. However, the mechanistic details underlying this relationship remain elusive. In this investigation, we employed an array of biophysical techniques, including circular dichroism spectroscopy, dynamic light scattering, and UV-vis absorption spectroscopy, to characterize the interaction between the active caspase-cleaved Par-4 (cl-Par-4) fragment and cisplatin. Additionally, elemental analysis was conducted to quantitatively assess the binding of cisplatin to the protein, utilizing inductively coupled plasma-optical emission spectroscopy and atomic absorption spectroscopy. Our findings provide evidence of direct interaction between cl-Par-4 and cisplatin, and reveal a binding stoichiometry of 1:1. This result provides insights that could be useful in enhancing the efficacy of cisplatin-based and tumor suppressor-based cancer therapies.

AdductHunter: identifying protein-metal complex adducts in mass spectra

Mass spectrometry (MS) is an analytical technique for molecule identification that can be used for investigating protein-metal complex interactions. Once the MS data is collected, the mass spectra are usually interpreted manually to identify the adducts formed as a result of the interactions between proteins and metal-based species. However, with increasing resolution, dataset size, and species complexity, the time required to identify adducts and the error-prone nature of manual assignment have become limiting factors in MS analysis. AdductHunter is a open-source web-based analysis tool that  automates the peak identification process using constraint integer optimization to find feasible combinations of protein and fragments, and dynamic time warping to calculate the dissimilarity between the theoretical isotope pattern of a species and its experimental isotope peak distribution. Empirical evaluation on a collection of 22 unique MS datasetsshows fast and accurate identification of protein-metal complex adducts in deconvoluted mass spectra.

Trans-[bis(benzimidazol-2-ylidene)dichlorido]platinum(II) complexes with peculiar modes of action and activity against cisplatin-resistant cancer cells

Three series of cis- and trans-[bis(benzimidazol-2-ylidene)dichlorido]platinum(II) and cis-[(benzimidazol-2-ylidene)(DMSO)dichlorido]platinum(II) complexes were synthesised and screened for cytotoxicity against six human cancer cell lines. Depending on their N-alkyl and 5-alkoxycarbonyl substituents, two-digit nanomolar to single-digit micromolar IC₅₀ values against cancer cell lines intrinsically resistant to or ill-responding to cisplatin were reached by both cis- and trans-configured complexes. The stability of the complexes under aqueous biotest conditions was shown via ¹H and ¹⁹⁵Pt NMR monitoring to be dependent on their configuration and their N-substituents. Localisation studies employing click reactions with 1-alkyne- or cyclopropene-tagged derivatives revealed that the cis-complexes accumulated in the cell nuclei and the trans-complexes in the mitochondria. While the most active cis-complexes showed modes of action akin to those of cisplatin, the most active trans-complexes differed from cisplatin by much lower rates of cellular uptake and ROS production, and by their non-interaction with the cell cycle and the DNA of cancer cells. Thus, we identified structural key elements for the synthesis of optimised trans-configured NHC platinum(II) complexes with high activity also against cisplatin-refractory cancer cells.

Structural Characterization of Cis– and Trans–Pt(NH3)2Cl2 Conjugations with Chitosan Nanoparticles

The conjugation of chitosan 15 and 100 KD with anticancer drugs cis– and trans–Pt (NH₃)₂Cl₂ (abbreviated cis–Pt and trans–Pt) were studied at pH 5–6. Using multiple spectroscopic methods and thermodynamic analysis to characterize the nature of drug–chitosan interactions and the potential application of chitosan nanoparticles in drug delivery. Analysis showed that both hydrophobic and hydrophilic contacts are involved in drug–polymer interactions, while chitosan size and charge play a major role in the stability of drug–polymer complexes. The overall binding constants are K_{ch–15–cis–Pt} = 1.44 (±0.6) × 10⁵ M⁻¹, K_{ch–100–cis–Pt} = 1.89 (±0.9) × 10⁵ M⁻¹ and K_{ch–15–trans–Pt} = 9.84 (±0.5) × 10⁴ M⁻¹, and K_{ch–100–trans–Pt} = 1.15 (±0.6) × 10⁵ M⁻¹. More stable complexes were formed with cis–Pt than with trans–Pt–chitosan adducts, while stronger binding was observed for chitosan 100 in comparison to chitosan 15 KD. This study indicates that polymer chitosan 100 is a stronger drug carrier than chitosan 15 KD in vitro.

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.

Geometry-enhanced molecular representation learning for property prediction

Effective molecular representation learning is of great importance to facilitate molecular property prediction. Recent advances for molecular representation learning have shown great promise in applying graph neural networks to model molecules. Moreover, a few recent studies design self-supervised learning methods for molecular representation to address insufficient labelled molecules; however, these self-supervised frameworks treat the molecules as topological graphs without fully utilizing the molecular geometry information. The molecular geometry, also known as the three-dimensional spatial structure of a molecule, is critical for determining molecular properties. To this end, we propose a novel geometry-enhanced molecular representation learning method (GEM). The proposed GEM has a specially designed geometry-based graph neural network architecture as well as several dedicated geometry-level self-supervised learning strategies to learn the molecular geometry knowledge. We compare GEM with various state-of-the-art baselines on different benchmarks and show that it can considerably outperform them all, demonstrating the superiority of the proposed method.

Simultaneous mass spectrometry analysis of cisplatin with oligonucleotide-peptide mixtures: implications for the mechanism of action

Although genomic DNA is the primary target of anticancer platinum-based drugs, interactions with proteins also play a significant role in their overall activity. In this study, competitive binding of cisplatin with an oligonucleotide and two peptides corresponding to segments of H2A and H2B histone proteins was investigated by mass spectrometry. Following the determination of the cisplatin binding sites on the oligonucleotide and peptides by tandem mass spectrometry, competitive binding was studied and transfer of platinum fragments from the platinated peptides to the oligonucleotide explored. In conjunction with previous studies on the nucleosome, the results suggest that all four of the abundant histone proteins serve as a platinum drug reservoir in the cell nucleus, providing an adduct pool that can be ultimately transferred to the DNA.

Graphical abstract

A cyclic manipulation of cage isomers via anion exchange and thermal isomerism

A cyclic manipulation of peanut cage isomers has been achieved via anion exchange and unusual cage isomerism.

Recent advances in protein metalation: structural studies

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

Protein binding studies with human serum albumin, molecular docking and in vitro cytotoxicity studies using HeLa cervical carcinoma cells of Cu(ii)/Zn(ii) complexes containing a carbohydrazone ligand

The interaction of two binuclear mixed ligand Cu(ii) complexes [Cu(o-phen)LCu(OAc)] (1) and [Cu(o-phen)LCu(o-phen)](OAc) (2) (H3L = o-HOC6H4C(H)[double bond, length as m-dash]N-NH-C(OH)[double bond, length as m-dash]N-N[double bond, length as m-dash]C(H)-C6H4OH-o) and a new mononuclear Zn(ii) complex [Zn(HL)(o-phen)(H2O)](OAc)·H2O (3) (H2L = o-HOC6H4-C(H)[double bond, length as m-dash]N-NH-C([double bond, length as m-dash]O)-NH-N[double bond, length as m-dash]C(H)-C6H4OH-o, o-phen = 1,10-phenanthroline, and OAc = CH3COO-) with human serum albumin (HSA) was studied using fluorescence quenching, synchronous and 3D fluorescence measurements and UV-vis spectroscopy. 3D fluorescence studies showed that the HSA structure was altered at the secondary and tertiary levels upon binding with the complexes. This was further supported by the electronic absorption spectral studies of HSA in the absence and presence of the compounds. The average binding distance (r) between HSA and the complexes was obtained by Förster's resonance energy transfer theory. Complex 3 was structurally characterized by X-ray crystallography. Molecular docking studies indicated that all three complexes primarily bind to HSA in subdomain IIA with amino acid residues such as Arg218 and Lys199 which are located at the entrance of Sudlow's site I. The in vitro cytotoxicities of complexes 1-3 against HeLa cells showed promising anticancer activity (IC50 = 3.5, 3.9 and 16.9 μM for 1, 2 and 3, respectively). Live cell time lapse imaging for 1 was done to capture the dynamic behavior of the cells upon treatment with the complex. Cell cycle analysis by flow cytometry with HeLa cells indicated that 1 and 2 induced cell cycle arrest in the G2/M phase while 3 induced arrest in the G0/G1 phase leading to cell death. Compounds 1 and 2 but not 3 induced apoptosis through the mitochondrial pathway as suggested from the relative p53, caspase3 and bcl2 mRNA levels measured by real-time quantitative PCR analysis.

Polyphosphate Functions In Vivo as an Iron Chelator and Fenton Reaction Inhibitor

How do organisms deal with free iron? On the one hand, iron is an essential metal that plays crucial structural and functional roles in many organisms. On the other hand, free iron is extremely toxic, particularly under aerobic conditions, where iron rapidly undergoes the Fenton reaction and produces highly reactive hydroxyl radicals. Our study now demonstrates that we have discovered one of the first physiologically relevant nonproteinaceous iron chelators and Fenton inhibitors. We found that polyphosphate, a highly conserved and ubiquitous inorganic polyanion, chelates iron and, through its multivalency, prevents the interaction of iron with peroxide and therefore the formation of hydroxyl radicals. We show that polyP provides a crucial iron reservoir for metalloproteins under nonstress conditions and effectively chelates free iron during iron stress. Importantly, polyP is present in all cells and organisms and hence is likely to take on this crucial function in both prokaryotic and eukaryotic cells.

Maintaining cellular iron homeostasis is critical for organismal survival. Whereas iron depletion negatively affects the many metabolic pathways that depend on the activity of iron-containing enzymes, any excess of iron can cause the rapid formation of highly toxic reactive oxygen species (ROS) through Fenton chemistry. Although several cellular iron chelators have been identified, little is known about if and how organisms can prevent the Fenton reaction. By studying the effects of cisplatin, a commonly used anticancer drug and effective antimicrobial, we discovered that cisplatin elicits severe iron stress and oxidative DNA damage in bacteria. We found that both of these effects are successfully prevented by polyphosphate (polyP), an abundant polymer consisting solely of covalently linked inorganic phosphates. Subsequent in vitro and in vivo studies revealed that polyP provides a crucial iron reservoir under nonstress conditions and effectively complexes free iron and blocks ROS formation during iron stress. These results demonstrate that polyP, a universally conserved biomolecule, plays a hitherto unrecognized role as an iron chelator and an inhibitor of the Fenton reaction.

ESI-MS Study of the Interaction of Potential Oxidovanadium(IV) Drugs and Amavadin with Model Proteins

In this study, the binding to lysozyme (Lyz) of four important VIV compounds with antidiabetic and/or anticancer activity, [VIVO(pic)2(H2O)], [VIVO(ma)2], [VIVO(dhp)2], and [VIVO(acac)2], where pic-, ma-, dhp-, and acac- are picolinate, maltolate, 1,2-dimethyl-3-hydroxy-4(1H)-pyridinonate, and acetylacetonate anions, and of the vanadium-containing natural product amavadin ([VIV(hidpa)2]2-, with hidpa3- N-hydroxyimino-2,2'-diisopropionate) was investigated by ElectroSpray Ionization-Mass Spectrometry (ESI-MS). Moreover, the interaction of [VIVO(pic)2(H2O)], chosen as a representative VIVO2+ complex, was examined with two additional proteins, myoglobin (Mb) and ubiquitin (Ub), to compare the data. The examined vanadium concentration was in the range 15-150 μM, i.e., very close to that found under physiological conditions. With pic-, dhp-, and hidpa3-, the formation of adducts n[VIVOL2]-Lyz or n[VIVL2]-Lyz is favored, while with ma- and acac- the species n[VIVOL]-Lyz are detected, with n dependent on the experimental VIV/protein ratio. The behavior of the systems with [VIVO(pic)2(H2O)] and Mb or Ub is very similar to that of Lyz. The results suggested that under physiological conditions, the moiety cis-VIVOL2 (L = pic-, dhp-) is bound by only one accessible side-chain protein residue that can be Asp, Glu, or His, while VIVOL+ (L = ma-, acac-) can interact with the two equatorial and axial sites. If the VIV complex is thermodynamically stable and does not have available coordination positions, such as amavadin, the protein cannot interact with it through the formation of coordination bonds and, in such cases, noncovalent interactions are predicted. The formation of the adducts is dependent on the thermodynamic stability and geometry in aqueous solution of the VIVO2+ complex and affects the transport, uptake, and mechanism of action of potential V drugs.

Complexation of cis-Pt and trans-Pt(NH3)2Cl2 with serum proteins: A potential application for drug delivery

AbbreviationsHAShuman serum albuminBSAbovine serum albuminβ-LGbeta-lactoglobulincis-Pt and trans-PtPt(NH₃)₂Cl₂FTIRFourier transform infraredCommunicated by Ramaswamy H. Sarma.

Binding mechanisms of half-sandwich Rh(III) and Ru(II) arene complexes on human serum albumin: a comparative study

Abstract

Various half-sandwich ruthenium(II) arene complexes and rhodium(III) arene complexes have been intensively investigated due to their prominent anticancer activity. The interaction of the organometallic complexes of Ru(η⁶-p-cymene) and Rh(η⁵-C₅Me₅) with human serum albumin (HSA) was studied in detail by a combination of various methods such as ultrafiltration, capillary electrophoresis, ¹H NMR spectroscopy, fluorometry and UV–visible spectrophotometry in the presence of 100 mM chloride ions. Binding characteristics of the organometallic ions and their complexes with deferiprone, 2-picolinic acid, maltol, 6-methyl-2-picolinic acid and 2-quinaldic acid were evaluated. Kinetic aspects and reversibility of the albumin binding are also discussed. The effect of low-molecular-mass blood components on the protein binding was studied in addition to the interaction of organorhodium complexes with cell culture medium components. The organometallic ions were found to bind to HSA to a high extent via a coordination bond. Release of the bound metal ions was kinetically hindered and could not be induced by the denaturation of the protein. Binding of the Ru(η⁶-p-cymene) triaqua cation was much slower (ca. 24 h) compared to the rhodium congener (few min), while their complexes interacted with the protein relatively fast (1–2 h). The studied complexes were bound to HSA coordinatively. The highly stable and kinetically inert 2-picolinate Ru(η⁶-p-cymene) complex bound in an associative manner preserving its original entity, while lower stability complexes decomposed partly or completely upon binding to HSA. Fast, non-specific and high-affinity binding of the complexes on HSA highlights their coordinative interaction with various types of proteins possibly decreasing effective drug concentration.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s00775-019-01683-0) contains supplementary material, which is available to authorized users.

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.

Fate of cisplatin and its main hydrolysed forms in the presence of thiolates: a comprehensive computational and experimental study

Thiolations and bidentations drive the chemical fate of cisplatin compounds in intracellular medium.

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.

Pt-induced crosslinks promote target enrichment and protection from serum nucleases

Identifying the interactions of small molecules with biomolecules in complex cellular environments is a significant challenge. As one important example, despite being widely used for decades, much is still not understood regarding the cellular targets of Pt(II)-based anticancer drugs. In this study we introduce a novel method for isolation of Pt(II)-bound biomolecules using a DNA hybridization pull-down approach. Using a modified Pt reagent, click-ligation of a DNA oligonucleotide to both a Pt(II)-bound DNA hairpin and bovine serum albumin (BSA) are demonstrated. Subsequent hybridization to a biotin-labeled oligonucleotide allows for efficient isolation of Pt(II)-bound species by streptavidin pulldown. We also find that platinated bovine serum albumin readily crosslinks to DNA in the absence of click ligation, and that a fraction of BSA-bound Pt(II) can transfer to DNA over time. Interestingly, in in vitro studies, fragmented mammalian DNA that is crosslinked to BSA through Pt(II) exhibits significantly increased protection from degradation by serum nucleases.

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.

Platinated porphyrin as a new organelle and nucleus dual-targeted photosensitizer for photodynamic therapy

Organelle and nucleus dual-targeted anticancer drugs are being increasingly used for efficient cancer therapy as they can attack the double vital sites of tumor cells. In this work, we synthesized and characterized two new porphyrin compounds Pt-Por-RB and Me-Por-RB. The spectral titration results suggest that both Pt-Por-RB and Me-Por-RB bind to DNA efficiently in an intercalation binding mode. Upon irradiation, Pt-Por-RB with low dark-cytotoxicity can rapidly generate singlet oxygen to damage the tumor cells through the process of photodynamic therapy. Compared with Me-Por-RB, Pt-Por-RB was not only internalized in the organelles, but also in the nuclei of HeLa cells, probably due to the presence of platinum complexes, as analyzed using the confocal laser scanning microscope. Thus, with the combination of organelle and nucleus dual-targeting property and high efficiency of singlet oxygen generation, Pt-Por-RB showed a significant therapeutic activity against tumor cells.

Under-Reported Aspects of Platinum Drug Pharmacology

Platinum drugs remain the backbone of many antineoplastic regimens. Among the numerous chemical or pharmacological effects of platinum drugs, some aspects tend to be under-reported. Thus, this perspective paper intends to stress some neglected properties of platinum drugs: first, the physico-chemical characteristics (aquation reaction kinetics) that determine site-specific toxicity; second, the impact on RNA molecules. Knowledge of the ‘RNA world’ has dramatically changed our understanding of cellular and molecular biology. The inherent RNA-crosslinking properties should make platinum-based drugs interact with coding and non-coding RNAs. Third, we will discuss the impact on the immune system, which is now recognized to substantially contribute to chemotherapy efficacy. Together, platinum drugs are in fact old drugs, but are worth re-focusing on. Many aspects are still mysterious but can pave the way to new drugs or an improved application of the already existing compounds.

Covalent adducts of melphalan with free amino acids and a model peptide studied by liquid chromatography/tandem mass spectrometry

RATIONALE

Melphalan is a frequently used chemotherapeutical agent for the treatment of myeloma, breast cancer, ovarian cancer and sarcoma of soft tissue. A good knowledge of the reactivity of the drug toward the different amino acids, e.g. covalent adduct formation, is crucial for the understanding of its activity and side effects during cancer treatment.

METHODS

The reactivity of melphalan and sites of adduct formation were studied by in vitro incubation of melphalan with free amino acids and glutathione as a model peptide. The formed covalent adducts were investigated using ultra-performance liquid chromatography tandem mass spectrometry (UPLC/MS/MS) using a triple-quadrupole instrument. Accurate mass measurements for the confirmation of characteristic product ions were performed on a quadrupole time-of-flight (QTOF) mass spectrometer.

RESULTS

The incubation of melphalan with different classes of amino acids resulted in the formation of adducts on the amino and carboxyl termini, as well as adduct formation in the reactive side chains of Cys, Met, Tyr, His, Lys, Asp and Glu. All these melphalan adducts could be identified by their characteristic collision-induced dissociation (CID) product ion patterns.

CONCLUSIONS

The present study demonstrates the reactivity of melphalan towards the functional groups of amino acids. The different alkylation site products show distinctive fragmentation patterns, which enable a fast identification of the different melphalan adducts. This study is a first important step towards a better understanding of the adduct formation in more complex molecules, e.g. peptides and proteins.

Mass Spectrometry Uncovers Molecular Reactivities of Coordination and Organometallic Gold(III) Drug Candidates in Competitive Experiments That Correlate with Their Biological Effects

The reactivity of three cytotoxic organometallic gold(III) complexes with cyclometalated C,N,N and C,N ligands (either six- or five-membered metallacycles), as well as that of two representative gold(III) complexes with N-donor ligands, with biological nucleophiles has been studied by ESI-MS on ion trap and time-of-flight instruments. Specifically, the gold compounds were reacted with mixtures of nucleophiles containing l-histidine (imine), l-methionine (thioether), l-cysteine (thiol), l-glutamic acid (carboxylic acid), methylseleno-l-cysteine (selenoether), and in situ generated seleno-l-cysteine (selenol) to judge the preference of the gold compounds for binding to selenium-containing amino acid residues. Moreover, the gold compounds' reactivity was studied with proteins and nucleic acid building blocks. These experiments revealed profound differences between the coordination and organometallic families and even within the family of organometallics, which allowed insights to be gained into the compounds mechanisms of action. In particular, interactions with seleno-l-cysteine appear to reflect well the compounds' inhibition properties of the seleno-enzyme thioredoxin reductase and to a certain extent their antiproliferative effects in vitro. Therefore, mass spectrometry is successfully applied for linking the molecular reactivity and target preferences of metal-based drug candidates to their biological effects. Finally, this experimental setup is applicable to any other metallodrug that undergoes ligand substitution reactions and/or redox changes as part of its mechanism of action.

Serum metallothionein in patients with testicular cancer

PURPOSE

Metallothioneins (MTs) have been disclosed as a useful diagnostic factor for tumour progression and drug resistance in a variety of malignancies. Increased levels of MT in blood serum have been found in patients with several types of cancer, but there is no available information on serum MT levels in patients with testicular germ cell tumour (TGCT). The aim of the study was to determine MT levels in serum of patients with TGCT and to evaluate the portion of platinum (Pt) that binds to MT after cisplatin administration since MTs could be involved in drug resistance.

METHODS

Concentration of total MT was determined in serum of 25 men with newly diagnosed TGCT by differential pulse voltammetry. The fractionation of serum was carried out by size exclusion high-performance liquid chromatography (SE-HPLC), while concentration of Pt in collected fractions was determined by inductively coupled plasma mass spectrometry.

RESULTS

Concentration of serum MT was significantly higher in TGCT patients than in healthy volunteers. The results of SE-HPLC analysis showed that only a small amount of Pt was bound to proteins in the area of MT elution.

CONCLUSIONS

Significant increase in MT levels in individuals with TGCT indicates certain health problem and, in combination with other commonly used diagnostic tools, could improve early diagnosis.

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.

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.

Copper chaperone Atox1 interacts with the metal-binding domain of Wilson's disease protein in cisplatin detoxification

Human copper transporters ATP7B (Wilson's disease protein) and ATP7A (Menkes' disease protein) have been implicated in tumour resistance to cisplatin, a widely used anticancer drug. Cisplatin binds to the copper-binding sites in the N-terminal domain of ATP7B, and this binding may be an essential step of cisplatin detoxification involving copper ATPases. In the present study, we demonstrate that cisplatin and a related platinum drug carboplatin produce the same adduct following reaction with MBD2 [metal-binding domain (repeat) 2], where platinum is bound to the side chains of the cysteine residues in the CxxC copper-binding motif. This suggests the same mechanism for detoxification of both drugs by ATP7B. Platinum can also be transferred to MBD2 from copper chaperone Atox1, which was shown previously to bind cisplatin. Binding of the free cisplatin and reaction with the cisplatin-loaded Atox1 produce the same protein-bound platinum intermediate. Transfer of platinum along the copper-transport pathways in the cell may serve as a mechanism of drug delivery to its target in the cell nucleus, and explain tumour-cell resistance to cisplatin associated with the overexpression of copper transporters ATP7B and ATP7A.

Dissociation kinetics of the streptavidin-biotin interaction measured using direct electrospray ionization mass spectrometry analysis

Dissociation rate constants (k (off)) for the model high affinity interaction between biotin (B) and the homotetramer of natural core streptavidin (S(4)) were measured at pH 7 and temperatures ranging from 15 to 45 °C using electrospray ionization mass spectrometry (ESI-MS). Two different approaches to data analysis were employed, one based on the initial rate of dissociation of the (S(4) + 4B) complex, the other involving nonlinear fitting of the time-dependent relative abundances of the (S(4) + iB) species. The two methods were found to yield k (off) values that are in good agreement, within a factor of two. The Arrhenius parameters for the dissociation of the biotin-streptavidin interaction in solution were established from the k (off) values determined by ESI-MS and compared with values measured using a radiolabeled biotin assay. Importantly, the dissociation activation energies determined by ESI-MS agree, within 1 kcal mol(-1), with the reported value. In addition to providing a quantitative measure of k (off), the results of the ESI-MS measurements revealed that the apparent cooperative distribution of (S(4) + iB) species observed at short reaction times is of kinetic origin and that sequential binding of B to S(4) occurs in a noncooperative fashion with the four ligand binding sites being kinetically and thermodynamically equivalent and independent.

Detoxifying antitumoral drugs via nanoconjugation: the case of gold nanoparticles and cisplatin

Nanoparticles (NPs) have emerged as a potential tool to improve cancer treatment. Among the proposed uses in imaging and therapy, their use as a drug delivery scaffold has been extensively highlighted. However, there are still some controversial points which need a deeper understanding before clinical application can occur. Here the use of gold nanoparticles (AuNPs) to detoxify the antitumoral agent cisplatin, linked to a nanoparticle via a pH-sensitive coordination bond for endosomal release, is presented. The NP conjugate design has important effects on pharmacokinetics, conjugate evolution and biodistribution and results in an absence of observed toxicity. Besides, AuNPs present unique opportunities as drug delivery scaffolds due to their size and surface tunability. Here we show that cisplatin-induced toxicity is clearly reduced without affecting the therapeutic benefits in mice models. The NPs not only act as carriers, but also protect the drug from deactivation by plasma proteins until conjugates are internalized in cells and cisplatin is released. Additionally, the possibility to track the drug (Pt) and vehicle (Au) separately as a function of organ and time enables a better understanding of how nanocarriers are processed by the organism.

Serum depletion of holo-ceruloplasmin induced by silver ions in vivo reduces uptake of cisplatin

There is an emerging link between extracellular copper concentration and the uptake of cisplatin mediated by copper transporter CTR1 in cell cultures and unicellular eukaryotes. To test the link between extracellular copper level and cisplatin uptake by organs in vivo we used mice with low copper status parameters induced by AgCl-containing diet (Ag-mice). In Ag-mice, serum copper status and liver copper metabolism were characterized. It was shown that the expression level of copper transporter genes and activity of ubiquitous intracellular cuproenzymes were not affected but the level of serum holo-ceruloplasmin was not detectable. Silver was selectively absorbed by liver and accumulated in the mitochondrial matrix. Silver was present in an exchangeable form and was excreted through bile. Ag-mice model is characterized by high reproducibility, reversibility, synchronicity, and definiteness of ceruloplasmin-associated copper deficiency. After cisplatin treatment Ag-mice, as compared to control mice, demonstrated the delay in platinum uptake by organs during first 30 min. This effect was not observed at later time points probably due to cisplatin induced copper release to blood, which resulted in the recovery of copper status. These data allowed us to conclude that cisplatin uptake was coupled to copper transport in vivo.

Is the reactivity of M(II)-arene complexes of 3-hydroxy-2(1H)-pyridones to biomolecules the anticancer activity determining parameter?

Hydroxypyr(id)ones are versatile ligands for the synthesis of organometallic anticancer agents, equipping them with fine-tunable pharmacological properties. Herein, we report on the preparation, mode of action, and in vitro anticancer activity of Ru(II)- and Os(II)-arene complexes with alkoxycarbonylmethyl-3-hydroxy-2-pyridone ligands. The hydrolysis and binding to amino acids proceed quickly, as characterized by NMR spectroscopy and ESI mass spectrometry. However, the reaction with amino acids causes cleavage of the pyridone ligands from the metal center because the amino acids act as multidentate ligands. A similar behavior was also observed during the reactions with the model proteins ubiquitin and cytochrome c, yielding mainly [protein + M(eta(6)-p-cymene)] adducts (M = Ru, Os). Notably the ligand cleavage of the Os derivative was significantly slower than of its Ru analogue, which could explain its higher activity in in vitro anticancer assays. Furthermore, the reaction of the compounds to 5'-GMP was characterized and coordination to the N7 of the guanine moiety was demonstrated by (1)H NMR spectroscopy and X-ray diffraction analysis. CDK2/Cyclin A protein kinase inhibition studies revealed potent activity of the Ru and Os complexes.

Cytotoxic Profile and Peculiar Reactivity with Biomolecules of a Novel "Rule-Breaker" Iodidoplatinum(II) Complex

Novel and surprising biological properties were disclosed for the platinum(II) complex cis-diiodidodiisopropylamineplatinum(II). Remarkably, this new platinum(II) complex manifests pronounced antiproliferative properties in vitro, in some cases superior to those of cisplatin. A peculiar reactivity with the model protein cytochrome c was indeed highlighted based on the loss of amine ligands and retention of iodides.