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

Picoplatin binding to proteins: X-ray structures and mass spectrometry data on the adducts with lysozyme and ribonuclease A

The reactivity of the anticancer drug picoplatin (cis-amminedichlorido(2-methylpyridine)platinum(II) complex) with the model proteins hen egg white lysozyme (HEWL) and bovine pancreatic ribonuclease (RNase A) was investigated by electrospray ionisation mass spectrometry (ESI MS) and X-ray crystallography. The data were compared with those previously obtained for the adducts of these proteins with cisplatin, carboplatin and oxaliplatin under the same experimental conditions. ESI-MS data show binding of Pt to both proteins, with fragments retaining the 2-methylpyridine ligand and, possibly, a chloride ion. X-ray crystallography identifies different binding sites on the two proteins, highlighting a different behaviour of picoplatin in the absence or presence of dimethyl sulfoxide (DMSO). Metal-containing fragments bind to HEWL close to the side chains of His15, Asp18, Asp119 and both Lys1 and Glu7, whereas they bind to RNase A on the side chain of His12, Met29, His48, Asp53, Met79, His105 and His119. The data suggest that the presence of DMSO favours the loss of 2-methylpyridine and alters the ability of the Pt compound to bind to the two proteins. With both proteins, picoplatin appears to behave similarly to cisplatin and carboplatin when dissolved in DMSO, whereas it behaves more like oxaliplatin in the absence of the coordinating solvent. This study provides important insights into the pharmacological profile of picoplatin and supports the conclusion that coordinating solvents should not be used to evaluate the biological activities of Pt-based drugs.

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.

IRMPD spectroscopy and quantum-chemical simulations of the reaction products of cisplatin with the dipeptide CysGly

The inorganic antineoplastic drug cisplatin was made to react in solution with the dipeptide cysteinylglycine (CysGly), chosen as a functional model of glutathione, and the reaction products were analyzed using electrospray ionization mass spectrometry (ESI-MS). Selected complexes, i.e., the primary substitution product cis-[PtCl(NH3)2(CysGly)]+ and the chelate cis-[PtCl(NH3)(CysGly)]+, were submitted to IR multiple photon dissociation (IRMPD) spectroscopy obtaining their vibrational features. The experimental IR ion spectra were compared with the calculated IR absorptions of different plausible isomeric families, finding CysGly to bind preferentially platinum(II) via its deprotonated thiolic group in the monovalent complex, cis-[PtCl(NH3)2(CysGly)]+, and to evolve in the S,N-bound chelate structure cis-[PtCl(NH3)(CysGly)]+ through the SH and NH2 functionality of the cysteine residue. Moreover, our findings indicate that the platination reaction does not affect the CysGly peptide bond, which remains in its trans configuration. These results provide additional insights into the reactivity of Pt(II)-complexes with glutathione which is involved in cellular cisplatin resistance.

Discovery of cisplatin-binding proteins by competitive cysteinome profiling

Cisplatin is a widely used cancer metallodrug that induces cytotoxicity by targeting DNA and chelating cysteines in proteins. Here we applied a competitive activity-based protein profiling strategy to identify cisplatin-binding cysteines in cancer proteomes. A novel cisplatin target, MetAP1, was identified and functionally validated to contribute to cisplatin's cytotoxicity.

Cisplatin Binding to Human Serum Transferrin: A Crystallographic Study

The molecular mechanism of how human serum transferrin (hTF) recognizes cisplatin at the atomic level is still unclear. Here, we report the molecular structure of the adduct formed upon the reaction of hTF with cisplatin. Pt binds the side chain of Met256 (at the N-lobe), without altering the protein overall conformation.

Elusive intermediates in cisplatin reaction with target amino acids: Platinum(II)-cysteine complexes assayed by IR ion spectroscopy and DFT calculations

The reactivity of a widely used metal based antineoplastic drug, cisplatin, cis-PtCl₂(NH₃)₂, with L-cysteine (Cys) has been investigated using a combination of electrospray ionization mass spectrometry (ESI-MS), IRMPD gas phase ion spectroscopy and DFT calculations. The cysteine lateral chain represents one of the main platination sites in proteins, which is believed to be related to the resistance mechanisms to cisplatin. The vibrational features of the mass-selected substitution product cis-[PtCl(NH₃)₂(Cys)]⁺ and the intercepted cis-[PtCl(NH₃)₂(H₂O)(Cys)]⁺ intermediate complex were compared to calculated IR spectra, enabling the assessment of the sampled ions structures. In cis-[PtCl(NH₃)₂(Cys)]⁺, cysteine was found to bind platinum through the sulfur atom as a thiolate zwitterion, highlighting the enhanced acidity of the cysteine thiol group upon metal coordination. The cis-[PtCl(NH₃)₂(H₂O)(Cys)]⁺ structure complies with the non-covalent encounter complex, formed by cis-[PtCl(NH₃)₂(H₂O)]⁺ and neutral cysteine. This species is able to undergo the substitution process to produce cis-[PtCl(NH₃)₂(Cys)]⁺ when activated as a mass-isolated ion suggesting its participation in the reaction mechanism of cisplatin with cysteine in solution. Finally, the DFT-calculated energy profile for the substitution reaction was correlated with the peculiar gas-phase reactivity of this non-covalent complex, resulting to be 10-fold less reactive toward substitution than the corresponding methionine complex.

Cisplatin causes covalent inhibition of protein-tyrosine phosphatase 1B (PTP1B) through reaction with its active site cysteine: Molecular, cellular and in vivo mice studies

Protein tyrosine phosphatase 1B (PTP1B) is a critical regulator of different signalling cascades such as the EGFR pathway. The biological importance of PTP1B is further evidenced by knockout mice studies and the identification of recurrent mutations/deletions in PTP1B linked to metabolic and oncogenic alterations. Cisplatin is among the most widely used anticancer drug. The biological effects of cisplatin are thought to arise primarily from DNA damaging events involving cisplatin-DNA adducts. However, increasing evidence indicate that the biological properties of cisplatin could also rely on the perturbation of other processes such as cell signalling through direct interaction with certain cysteine residues in proteins. Here, we provide molecular, cellular and in vivo evidence suggesting that PTP1B is a target of cisplatin. Mechanistic studies indicate that cisplatin inhibited PTP1B in an irreversible manner and binds covalently to the catalytic cysteine residue of the enzyme. Accordingly, experiments conducted in cells and mice exposed to cisplatin showed inhibition of endogenous PTP1B and concomitant increase in tyrosine phosphorylation of EGFR. These findings are consistent with previous studies showing tyrosine phosphorylation-dependent activation of the EGFR pathway by cisplatin and with recent studies suggesting PTP1B inhibition by cisplatin and other platinum complexes. Importantly, our work provides novel mechanistic evidence that PTP1B is a protein target of cisplatin and is inhibited by this drug at molecular, cellular and in vivo levels. In addition, our work may contribute to the understanding of the pathways undergoing modulation upon cisplatin administration beyond of the established genotoxic effect of cisplatin.

Reactions of Arsenoplatin-1 with Protein Targets: A Combined Experimental and Theoretical Study

Arsenoplatin-1 (AP-1) is a dual-action anticancer metallodrug with a promising pharmacological profile that features the simultaneous presence of a cisplatin-like center and an arsenite center. We investigated its interactions with proteins through a joint experimental and theoretical approach. The reactivity of AP-1 with a variety of proteins, including carbonic anhydrase (CA), superoxide dismutase (SOD), myoglobin (Mb), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and human serum albumin (HSA), was analyzed by means of electrospray ionization mass spectrometry (ESI MS) measurements. In accordance with previous observations, ESI MS experiments revealed that the obtained metallodrug–protein adducts originated from the binding of the [(AP-1)-Cl]⁺ fragment to accessible protein residues. Remarkably, in two cases, i.e., Mb and GAPDH, the formation of a bound metallic fragment that lacked the arsenic center was highlighted. The reactions of AP-1 with various nucleophiles side chains of neutral histidine, methionine, cysteine, and selenocysteine, in neutral form as well as cysteine and selenocysteine in anionic form, were subsequently analyzed through a computational approach. We found that the aquation of AP-1 is energetically disfavored, with a reaction free energy of +19.2 kcal/mol demonstrating that AP-1 presumably attacks its biological targets through the exchange of the chloride ligand. The theoretical analysis of thermodynamics and kinetics for the ligand-exchange processes of AP-1 with His, Met, Cys, Sec, Cys^–, and Sec^– side chain models unveils that only neutral histidine and deprotonated cysteine and selenocysteine are able to effectively replace the chloride ligand in AP-1.

The interaction of arsenoplatin-1 (AP-1), a promising dual-action anticancer metallodrug featuring the simultaneous presence of a cisplatin-like center and an arsenite center, with proteins was studied via a synergetic experimental and theoretical approach. The reactivity of AP-1 with a variety of proteins was analyzed through ESI MS measurements and, at the same time, the reactions of AP-1 with molecular models of the side chains of histidine, methionine, cysteine, and selenocysteine were analyzed through a computational approach.

Spectroscopic/Computational Characterization and the X-ray Structure of the Adduct of the VIVO-Picolinato Complex with RNase A

The structure, stability, and enzymatic activity of the adduct formed upon the reaction of the V-picolinato (pic) complex [VIVO(pic)2(H2O)], with an octahedral geometry and the water ligand in cis to the V═O group, with the bovine pancreatic ribonuclease (RNase A) were studied. While electrospray ionization-mass spectrometry, circular dichroism, and ultraviolet-visible absorption spectroscopy substantiate the interaction between the metal moiety and RNase A, electron paramagnetic resonance (EPR) allows us to determine that a carboxylate group, stemming from Asp or Glu residues, and imidazole nitrogen from His residues are involved in the V binding at acidic and physiological pH, respectively. Crystallographic data demonstrate that the VIVO(pic)2 moiety coordinates the side chain of Glu111 of RNase A, by substituting the equatorial water molecule at acidic pH. Computational methods confirm that Glu111 is the most affine residue and interacts favorably with the OC-6-23-Δ enantiomer establishing an extended network of hydrogen bonds and van der Waals stabilizations. By increasing the pH around neutrality, with the deprotonation of histidine side chains, the binding of the V complex to His105 and His119 could occur, with that to His105 which should be preferred when compared to that to the catalytically important His119. The binding of the V compound affects the enzymatic activity of RNase A, but it does not alter its overall structure and stability.

From Preassociation to Chelation: A Survey of Cisplatin Interaction with Methionine at Molecular Level by IR Ion Spectroscopy and Computations

Methionine (Met) plays an important role in the metabolism of cisplatin anticancer drug. Yet, methionine platination in aqueous solution presents a highly complex pattern of interconnected paths and intermediates. This study reports on the reaction of methionine with the active aqua form of cisplatin, cis-[PtCl(NH₃)₂(H₂O)]⁺, isolating the encounter complex of the reactant pair, {cis-[PtCl(NH₃)₂(H₂O)]⁺·Met}, by electrospray ionization. In the unsolvated state, charged intermediates are characterized for their structure and photofragmentation behavior by IR ion spectroscopy combined with quantum-chemical calculations, obtaining an outline of the cisplatin-methionine reaction at a molecular level. To summarize the major findings: (i) the {cis-[PtCl(NH₃)₂(H₂O)]⁺·Met} encounter complex, lying on the reaction coordinate of the Eigen-Wilkins preassociation mechanism for ligand substitution, is delivered in the gas phase and characterized by IR ion spectroscopy; (ii) upon vibrational excitation, ligand exchange occurs within {cis-[PtCl(NH₃)₂(H₂O)]⁺·Met}, releasing water and cis-[PtCl(NH₃)₂(Met)]⁺, along the calculated energy profile; (iii) activated cis-[PtCl(NH₃)₂(Met)]⁺ ions undergo NH₃ departure, forming a chelate complex, [PtCl(NH₃)(Met)]⁺, whose structure is congruent with overwhelming S-Met ligation as the primary coordination step. The latter process involving ammonia loss marks a difference with the prevailing chloride replacement in protic solvent, pointing to the effect of a low-polarity environment.

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.

Structural Basis of Drug Recognition by Human Serum Albumin

Background:

Human serum albumin (HSA), the most abundant protein in plasma, is a monomeric multi-domain macromolecule with at least nine binding sites for endogenous and exogenous ligands. HSA displays an extraordinary ligand binding capacity as a depot and carrier for many compounds including most acidic drugs. Consequently, HSA has the potential to influence the pharmacokinetics and pharmacodynamics of drugs.

Objective:

In this review, the structural determinants of drug binding to the multiple sites of HSA are analyzed and discussed in detail. Moreover, insight into the allosteric and competitive mechanisms underpinning drug recognition, delivery, and efficacy are analyzed and discussed.

Conclusion:

As several factors can modulate drug binding to HSA (e.g., concurrent administration of drugs competing for the same binding site, ligand binding to allosteric-coupled clefts, genetic inherited diseases, and post-translational modifications), ligand binding to HSA is relevant not only under physiological conditions, but also in the pharmacological therapy management.

Systematic Screening and Deep Analysis of CoPt Binding Peptides Leads to Enhanced CoPt Nanoparticles Using Designed Peptides

Using protein and peptide additives to direct the crystallization of inorganic materials is a very attractive and environmentally friendly strategy to access complex and sometimes inaccessible mineral phases. CoPt is a very desirable high-magnetoanisotropic material in its L1₀ phase, but this is acquired by annealing at high temperatures which is incompatible with delicate nanomaterial assembly. Previous studies identified one peptide with high affinity to CoPt and four peptides with high affinity to FePt L1₀ phase nanoparticles (NPs) through phage display biopanning selection. While synthesis mediated by these peptides offered a small degree of L1₀ character to the NPs, they do not have the magnetoanistropy required for applications. In this study, we improve the activity of peptide directed crystallization by designing second generation peptides. We use the five literature sequences (LS) to probe the binding affinity deeper through dissection (alanine scanning), reduction (truncations), and substitution of the LS to find key amino acids and motifs. This is performed using a SPOT peptide array, importantly probing interactions at three stages of NP formation: with precursor, during synthesis, and with NPs. We found four universal features: 1) the importance of basic residues, particularly lysine flanking both ends of the sequence; 2) the importance of methionine; 3) shorter sequences show higher affinity than longer ones; and 4) acidic residues have a negative impact on binding with aspartic acid less favorable than glutamic acid. However, an acidic amino acid benefits, presumably to balance charge. The short motif KSLS had high affinity in all assays. Three sequences were selected from the screening, and three sequences were designed from the rules above. These were used to mediate a green synthesis of CoPt nanoparticles. The screened peptides mediated the formation of NPs with improved coercivity (90-110 Oe) compared to the LS (30-80 Oe), while the designed peptides facilitated formation of CoPt NPs with the highest coercivity (109 to 132 Oe), representing a massive improvement on L1₀ character. This result along with deeper insight this methodology brings offers vast potential for the future.

Cisplatin binding to β-lactoglobulin: a structural study

The first structural study on the interaction of β-lactoglobulin with the anticancer compound cisplatin is here reported by combining spectroscopic, crystallographic and mass spectrometry techniques.

Beyond Cisplatin: Combination Therapy with Arsenic Trioxide

Platinum drugs (cisplatin, oxaliplatin, and carboplatin) and arsenic trioxide are the only commercial inorganic non-radioactive anticancer drugs approved by the US Food and Drug Administration. Numerous efforts are underway to take advantage of the synergy between the anticancer activity of cisplatin and arsenic trioxide - two drugs with strikingly different mechanisms of action. These include co-encapsulation of the two drugs in novel nanoscale delivery systems as well as the development of small molecule agents that combine the activity of these two inorganic materials. Several of these new molecular entities containing Pt-As bonds have broad anticancer activity, are robust in physiological buffer solutions, and form stable complexes with biopolymers. This review summarizes results from a number of preclinical studies involving the combination of cisplatin and As2O3, co-encapsulation and nanoformulation efforts, and the chemistry and cytotoxicity of the first member of platinum anticancer agents with an arsenous acid moiety bound to the platinum(II) center: arsenoplatins.

Mass spectrometric quantification of the binding ratio of metal-based anticancer complexes with protein thiols

RATIONALE

The binding ratio of metal complexes with cysteinyl thiols in proteins plays an important role in deciphering the mechanisms of action of therapeutic metal complexes, but its analysis is still a significant challenge. In this work, a quantitative mass spectrometry method is developed to determine the binding ratio of metal-based anticancer complexes with cysteines in human copper chaperone protein Atox1.

METHODS

A novel strategy based on a thiol-specific stable isotopic labelling reagent was developed to determine the binding ratio of metal-based anticancer complexes, namely cisplatin and organometallic ruthenium complex [(η⁶ -biphenyl)RuCl(en)]PF₆ (en = ethylenediamine), with the cysteinyl residues of Atox1.

RESULTS

Both cisplatin and the ruthenium complex were reactive not only to Cys15 and/or Cys18, the copper(I) binding site of Atox1, but also to Cys44. The binding ratios of the ruthenium complex with the cysteinyl residues were much higher than those of cisplatin. However, the addition of copper(I) could markedly increase the binding ratios of cysteinyl residues of Atox1 with cisplatin, but not with the ruthenium complex.

CONCLUSIONS

This strategy can not only precisely determine the binding ratios of metal complexes to protein thiols, but also be helpful in distinguishing thiol-binding sites from other binding sites of metal complexes in proteins. We expect wide application of this method to the research of covalent/coordinative interactions between metal complexes and protein thiols.

Platinum(II) O,S Complexes Inhibit the Aggregation of Amyloid Model Systems

Platinum(II) complexes with different cinnamic acid derivatives as ligands were investigated for their ability to inhibit the aggregation process of amyloid systems derived from Aβ, Yeast Prion Protein Sup35p and the C-terminal domain of nucleophosmin 1. Thioflavin T binding assays and circular dichroism data indicate that these compounds strongly inhibit the aggregation of investigated peptides exhibiting IC₅₀ values in the micromolar range. MS analysis confirms the formation of adducts between peptides and Pt(II) complexes that are also able to reduce amyloid cytotoxicity in human SH-SY5Y neuroblastoma cells. Overall data suggests that bidentate ligands based on β-hydroxy dithiocinnamic esters can be used to develop platinum or platinoid compounds with anti-amyloid aggregation properties.

Principles and methods used to grow and optimize crystals of protein-metallodrug adducts, to determine metal binding sites and to assign metal ligands

The characterization of the interactions between biological macromolecules (proteins and nucleic acids) and metal-based drugs is a fundamental prerequisite for understanding their mechanisms of action. X-ray crystallography enables the structural analysis of such complexes with atomic level detail. However, this approach requires the preparation of highly diffracting single crystals, the measurement of diffraction patterns and the structural analysis and interpretation of macromolecule-metal interactions from electron density maps. In this review, we describe principles and methods used to grow and optimize crystals of protein-metallodrug adducts, to determine metal binding sites and to assign and validate metal ligands. Examples from the literature and experience in our own laboratory are provided and key challenges are described, notably crystallization and molecular model refinement against the X-ray diffraction data.

Identification of cisplatin-binding sites on the large cytoplasmic loop of the Na+/K+-ATPase

Cisplatin is the most widely used chemotherapeutic drug for the treatment of various types of cancer; however, its administration brings also numerous side effects. It was demonstrated that cisplatin can inhibit the Na⁺/K⁺-ATPase (NKA), which can explain a large part of the adverse effects. In this study, we have identified five cysteinyl residues (C452, C456, C457, C577, and C656) as the cisplatin binding sites on the cytoplasmic loop connecting transmembrane helices 4 and 5 (C45), using site-directed mutagenesis and mass spectrometry experiments. The identified residues are known to be susceptible to glutathionylation indicating their involvement in a common regulatory mechanism.

Raman-markers of X-ray radiation damage of proteins

Despite their high relevance, the mechanisms of X-ray radiation damage on protein structure yet have to be completely established. Here, we used Raman microspectrophotometry to follow X-ray-induced chemical modifications on the structure of the model protein bovine pancreatic ribonuclease (RNase A). The combination of dose-dependent Raman spectra and ultrahigh resolution (eight structures solved using data collected between 0.85 and 1.17 Å resolution on the same single crystal) allowed direct observation of several radiation damage events, including covalent bond breakages and formation of radicals. Our results are relevant for analytical photodamage detection and provide implications for a detailed understanding of the mechanisms of photoproduct formation.

A Photoactive Platinum(IV) Anticancer Complex Inhibits Thioredoxin-Thioredoxin Reductase System Activity by Induced Oxidization of the Protein

Thioredoxin (Trx) is an important enzyme in the redox signaling pathway and is usually overexpressed in tumor cells. We demonstrate herein that the photoactive platinum(IV) anticancer complex trans,trans,trans-[Pt(N₃)₂(OH)₂(Py)₂] (1) can bind to His, Glu, and Gln residues of Trx upon the irradiation of blue light. More importantly, complex 1 can also induce the oxidation of Met, Trp, and the Cys catalytic sites to form disulfide bonds by generating reactive oxygen species (ROS) upon photoactivation. These eventually lead to inhibition of activity of Trx enzyme and the Trx system and further increase in the cellular ROS level. We speculate that the oxidative damage not only inhibits Trx activity but also greatly contributes to the anticancer action of complex 1.

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.

Using phosphine ligands with a biological role to modulate reactivity in novel platinum complexes

Three platinum complexes with cis and trans configuration cis-[Pt(TCEP)2Cl2], cis-[Pt(tmTCEP)2Cl2] and trans-[Pt(TCEP)2Cl2], where TCEP is tris(2-carboxyethyl)phosphine, have been synthesized and fully characterized by usual techniques including single-crystal X-ray diffraction for trans-[Pt(TCEP)2Cl2] and cis-[Pt(tmTCEP)2Cl2]. Here, we also report on an esterification process of TCEP, which takes place in the presence of alcohols, leading to a platinum complex coordinated to an ester tmTCEP (2-methoxycarbonylethyl phosphine) ligand. The stability in solution of the three compounds and their interaction with biological models such as DNA (pBR322 and calf thymus DNA) and proteins (lysozyme and RNase) have also been studied.

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.

Capturing platinum in cisplatin: kinetic reactions with recombinant human apo-metallothionein 1a

Detailed mass spectra data show the stepwise sequence of cisplatin deconstruction by apo-metallothionein.

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.

Cisplatin Binding Sites in Human H-Chain Ferritin

The aim of this work is to identify the cisplatin binding sites on human H-chain ferritin. High-resolution X-ray crystallography reveals that cisplatin binds four distinct protein sites, that is, the side chains of His136 and Lys68, the side chain of His105, the side chain of Cys90 and the side chain of Cys102. These Pt binding sites are compared with those observed for the adduct that cisplatin forms upon encapsulation within horse spleen L-chain ferritin (87% identity with human L-chain ferritin).

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.

Gadolinium and Platinum in Tandem: Real-time Multi-Modal Monitoring of Drug Delivery by MRI and Fluorescence Imaging

A novel dual-imaging cisplatin-carrying molecular cargo capable of performing simultaneous optical and MR imaging is reported herein. This long-lasting MRI contrast agent (r₁ relaxivity of 23.4 mM^-1s^-1 at 3T, 25 ^oC) is a photo-activated cisplatin prodrug (PtGdL) which enables real-time monitoring of anti-cancer efficacy. PtGdL is a model for monitoring the drug delivery and anti-cancer efficacy by MRI with a much longer retention time (24 hours) in several organs such as renal cortex and spleen than GdDOTA and its motif control GdL. Upon complete release of cisplatin, all PtGdL is converted to GdL enabling subsequent MRI analyses of therapy efficacy within its reasonably short clearance time of 4 hours. There is also responsive fluorescence enhancement for monitoring by photon-excitation.

Cisplatin and transplatin interaction with methionine: bonding motifs assayed by vibrational spectroscopy in the isolated ionic complexes

IRMPD spectroscopy discloses N- versus S-platination.

Ru-Based CO releasing molecules with azole ligands: interaction with proteins and the CO release mechanism disclosed by X-ray crystallography

Structural data on the adducts formed upon reaction of Ru-based CO releasing molecules containing azole ligands with model proteins are reported.

Cisplatin-Protein Interactions: Unexpected Drug Binding to N-Terminal Amine and Lysine Side Chains

Literature studies carried out by mass spectrometry and X-ray crystallography have demonstrated that cisplatin is able to bind proteins mainly close to Met, His, and free Cys side chains. To identify possible alternative modes of cisplatin binding to proteins at the molecular level, here we have solved the high-resolution X-ray structure of the adduct formed in the reaction between the drug and the model protein thaumatin, which does not contain any His and free Cys residues and possesses just one buried Met. Our data reveal unexpected cisplatin binding sites on the protein surface that could have general significance: cisplatin fragments -[Pt(NH3)2Cl](+), -[Pt(NH3)Cl2], and -[Pt(NH3)2(OH2)](2+) bind to a protein N-terminus and close to Lys and Glu side chains.

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.

Oxaliplatin vs. cisplatin: competition experiments on their binding to lysozyme

The model protein hen egg white lysozyme was challenged with oxaliplatin and cisplatin. ESI mass spectrometry, surface plasmon resonance and thermal shift analyses demonstrate the formation of a bis-platinum adduct, though in very small amounts. Crystals of the bis-platinum adduct were obtained using two different preparations and the X-ray structures were solved at 1.85 Å and 1.95 Å resolution. Overall, the obtained data point out that, under the analyzed conditions, the two Pt drugs have similar affinities for the protein, but bind on its surface at two non-overlapping sites. In other words, these two drugs manifest a significantly different reactivity with this model protein and do not compete for the same protein binding sites.

Cisplatin binding to human serum albumin: a structural study

The reaction between cisplatin and human serum albumin (HSA) was investigated by X-ray crystallography and crystal structures of the cisplatin/HSA adduct were eventually solved for the first time. Structural data unambiguously prove that cisplatin mainly binds to His105 and Met329 side chains; additional binding sites are detected at His288, Met298, and Met548 and at His535, His67 and His247.

Unusual mode of protein binding by a cytotoxic π-arene ruthenium(ii) piano-stool compound containing an O,S-chelating ligand

A new π-arene Ru(ii) piano-stool compound, showing significant cytotoxic activityin vitro, was synthesized. The X-ray structure of this compound and that of its complex with RNase A were determined.

Mapping the protein-binding sites for iridium(iii)-based CO-releasing molecules

Mass spectrometry, Raman microspectroscopy, circular dichroism and X-ray crystallography have been used to investigate the reaction of CO-releasing molecule Cs₂IrCl₅CO with the model protein RNase A.

The X-ray structure of the primary adducts formed in the reaction between cisplatin and cytochrome c

In the present study, the interactions between cisplatin and cytochrome c are investigated. Based on high-resolution X-ray diffraction data, two monometalated species, i.e. cyt c-Pt(NH3)2 and cyt c-Pt(NH3)2Cl, are found to be the main adducts that form in the reaction between the protein and the drug. Both monodentate and bidentate platinum coordination to the protein is observed, with platinum binding either to Met65 or to Met65 and Glu61, simultaneously.