Characterisation of cisplatin coordination sites in cellular Escherichia coli DNA-binding proteins by combined biphasic liquid chromatography and ESI tandem mass spectrometry

First-in-class metallo-PROTAC as an effective degrader of select Pt-binding proteins

We report the development of the first metallo-PROTAC, specifically a Pt-PROTAC, that can effectively degrade select Pt(II)-binding proteins. The Pt-PROTAC prototype successfully degraded thioredoxin-1 and thioredoxin reductase-1 in multiple myeloma cancer cell lines. Metallo-PROTACs will have important applications in the identification of metal binding proteins and as chemotherapeutic agents.

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.

Protein Folding Stability Profiling of Colorectal Cancer Chemoresistance Identifies Functionally Relevant Biomarkers

Reported here is the application of three protein folding stability profiling techniques (including the stability of proteins from rates of oxidation, thermal protein profiling, and limited proteolysis approaches) to identify differentially stabilized proteins in six patient-derived colorectal cancer (CRC) cell lines with different oxaliplatin sensitivities and eight CRC patient-derived xenografts (PDXs) derived from two of the patient derived cell lines with different oxaliplatin sensitivities. Compared to conventional protein expression level analyses, which were also performed here, the stability profiling techniques identified both unique and novel proteins and cellular components that differentiated the sensitive and resistant samples including 36 proteins that were differentially stabilized in at least two techniques in both the cell line and PDX studies of oxaliplatin resistance. These 36 differentially stabilized proteins included 10 proteins previously connected to cancer chemoresistance. Two differentially stabilized proteins, fatty acid synthase and elongation factor 2, were functionally validated in vitro and found to be druggable protein targets with biological functions that can be modulated to improve the efficacy of CRC chemotherapy. These results add to our understanding of CRC oxaliplatin resistance, suggest biomarker candidates for predicting oxaliplatin sensitivity in CRC, and inform new strategies for overcoming chemoresistance in CRC.

Systematic Identification of Proteins Binding with Cisplatin in Blood by Affinity Chromatography and a Four-Dimensional Proteomic Method

Cisplatin is widely used for the treatment of various solid tumors. It is mainly administered by intravenous injection, and a substantial amount of the drug will bind to plasma proteins, a feature that is closely related to its pharmacokinetics, activity, toxicity, and side effects. However, due to the unique properties of platinum complexes and the complexity of the blood proteome, existing methods cannot systematically identify the binding proteome of cisplatin in blood. In this study, high-abundance protein separation and an ion mobility mass spectrometry-based 4D proteomic method were combined to systematically and comprehensively identify the binding proteins of cisplatin in blood. The characteristic isotope patterns of platinated peptides and a similarity algorithm were utilized to eliminate false-positive identification. Finally, 39 proteins were found to be platinated. Bioinformatics analysis showed that the identified proteins were mainly involved in the complement and coagulation cascade pathways. The binding ratio of some peptides with cisplatin was measured based on the area ratio of the free peptide using the parallel reaction monitoring method. This study provides a new method for systematically identifying binding proteins of metal drugs in blood, and the identified proteins might be helpful for understanding the toxicity of platinum anticancer drugs.

The Protein-Binding Behavior of Platinum Anticancer Drugs in Blood Revealed by Mass Spectrometry

Cisplatin and its analogues are widely used as chemotherapeutic agents in clinical practice. After being intravenously administrated, a substantial amount of platinum will bind with proteins in the blood. This binding is vital for the transport, distribution, and metabolism of drugs; however, toxicity can also occur from the irreversible binding between biologically active proteins and platinum drugs. Therefore, it is very important to study the protein-binding behavior of platinum drugs in blood. This review summarizes mass spectrometry-based strategies to identify and quantitate the proteins binding with platinum anticancer drugs in blood, such as offline high-performance liquid chromatography/inductively coupled plasma mass spectrometry (HPLC–ICP-MS) combined with electrospray ionization mass spectrometry (ESI-MS/MS) and multidimensional LC–ESI-MS/MS. The identification of in vivo targets in blood cannot be accomplished without first studying the protein-binding behavior of platinum drugs in vitro; therefore, relevant studies are also summarized. This knowledge will further our understanding of the pharmacokinetics and toxicity of platinum anticancer drugs, and it will be beneficial for the rational design of metal-based anticancer drugs.

Amino acid-linked platinum(II) compounds: non-canonical nucleoside preferences and influence on glycosidic bond stabilities

Abstract

Nucleobases serve as ideal targets where drugs bind and exert their anticancer activities. Cisplatin (cisPt) preferentially coordinates to 2′-deoxyguanosine (dGuo) residues within DNA. The dGuo adducts that are formed alter the DNA structure, contributing to inhibition of function and ultimately cancer cell death. Despite its success as an anticancer drug, cisPt has a number of drawbacks that reduce its efficacy, including repair of adducts and drug resistance. Some approaches to overcome this problem involve development of compounds that coordinate to other purine nucleobases, including those found in RNA. In this work, amino acid-linked platinum(II) (AAPt) compounds of alanine and ornithine (AlaPt and OrnPt, respectively) were studied. Their reactivity preferences for DNA and RNA purine nucleosides (i.e., 2′-deoxyadenosine (dAdo), adenosine (Ado), dGuo, and guanosine (Guo)) were determined. The chosen compounds form predominantly monofunctional adducts by reacting at the N1, N3, or N7 positions of purine nucleobases. In addition, features of AAPt compounds that impact the glycosidic bond stability of Ado residues were explored. The glycosidic bond cleavage is activated differentially for AlaPt-Ado and OrnPt-Ado isomers. Formation of unique adducts at non-canonical residues and subsequent destabilization of the glycosidic bonds are important features that could circumvent platinum-based drug resistance.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s00775-019-01693-y) contains supplementary material, which is available to authorized users.

Positive and negative nano-electrospray mass spectrometry of ruthenated serum albumin supported by docking studies: an integrated approach towards defining metallodrug binding sites on proteins

Negative mode nanoLC/nano ESI MS was used for determing Ru(ii) binding sites on protein.

Platinum Binds Proteins in the Endoplasmic Reticulum of S. cerevisiae and Induces Endoplasmic Reticulum Stress

Pt(II)-based anticancer drugs are widely used in the treatment of a variety of cancers, but their clinical efficacy is hindered by undesirable side effects and resistance. While much research has focused on Pt(II) drug interactions with DNA, there is increasing interest in proteins as alternative targets and contributors to cytotoxic and resistance mechanisms. Here, we describe a chemical proteomic method for isolation and identification of cellular protein targets of platinum compounds using Pt(II) reagents that have been modified for participation in the 1,3 dipolar cycloaddition "click" reaction. Using this method to visualize and enrich for targets, we identified 152 proteins in Pt(II)-treated Saccharomyces cerevisiae. Of interest was the identification of multiple proteins involved in the endoplasmic reticulum (ER) stress response, which has been proposed to be an important cytoplasmic mediator of apoptosis in response to cisplatin treatment. Consistent with possible direct targeting of this pathway, the ER stress response was confirmed to be induced in Pt(II)-treated yeast along with in vitro Pt(II)-inhibition of one of the identified proteins, protein disulfide isomerase.

The biological activity of Zeise's salt and its derivatives

With the aim to design new biologically active bioinorganic drugs of aspirin, whose mode of action is based on the inhibition of the cyclooxygenase(COX) enzymes, derivatives of Zeise's salt were synthesized in this structure-activity relationship study. Surprisingly, not only these Zeise-aspirin compounds but also Zeise's salt itself showed high inhibitory potency against COX enzymes in in vitro assays. In contrast, potassium tetrachloroplatinate and cisplatin did not influence the enzyme activity at equimolar concentrations. It was demonstrated by LC-ESI tandem-mass spectrometry that Zeise's salt platinates the essential amino acids Tyr385 (active site of the enzyme) and Ser516 (will be acetylated by aspirin) of COX-1, thereby strongly impairing the function of the enzyme. This finding demonstrates for the first time that Zeise's salt is pharmacologically active and is a potent enzyme inhibitor.

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.

Transferrin serves as a mediator to deliver organometallic ruthenium(II) anticancer complexes into cells

We report herein a systematic study on interactions of organometallic ruthenium(II) anticancer complex [(η(6)-arene)Ru(en)Cl](+) (arene = p-cymene (1) or biphenyl (2), en = ethylenediamine) with human transferrin (hTf) and the effects of the hTf-ligation on the bioavailability of these complexes with cisplatin as a reference. Incubated with a 5-fold excess of complex 1, 2, or cisplatin, 1 mol of diferric hTf (holo-hTf) attached 0.62 mol of 1, 1.01 mol of 2, or 2.14 mol of cisplatin. Mass spectrometry revealed that both ruthenium complexes coordinated to N-donors His242, His273, His578, and His606, whereas cisplatin bound to O donors Tyr136 and Tyr317 and S-donor Met256 in addition to His273 and His578 on the surface of both apo- and holo-hTf. Moreover, cisplatin could bind to Thr457 within the C-lobe iron binding cleft of apo-hTf. Neither ruthenium nor platinum binding interfered with the recognition of holo-hTf by the transferrin receptor (TfR). The ruthenated/platinated holo-hTf complexes could be internalized via TfR-mediated endocytosis at a similar rate to that of holo-hTf itself. Moreover, the binding to holo-hTf well preserved the bioavailability of the ruthenium complexes, and the hTf-bound 1 and 2 showed a similar cytotoxicity toward the human breast cancer cell line MCF-7 to those of the complexes themselves. However, the conjugation with holo-hTf significantly reduced the cellular uptake of cisplatin and the amount of platinated DNA adducts formed intracellularly, leading to dramatic reduction of cisplatin cytotoxicity toward MCF-7. These findings suggest that hTf can serve as a mediator for the targeting delivery of Ru(arene) anticancer complexes while deactivating cisplatin.

Anticancer metallodrug research analytically painting the "omics" picture--current developments and future trends

Anticancer metallodrug development has for a long time been characterised by the similarity of new drug candidates to cisplatin and DNA as the primary target. Recent advances in bioanalytical techniques with high sensitivity and selectivity have revealed that metal-based drugs can undergo a wide range of biomolecular interactions beyond DNA and have generated interest in proteins as possible targets for metallodrugs. In fact, implementation of metallomics approaches that are able to reveal the fate of the compounds in biological systems can help to move drug development towards more targeted and rational design of novel metallodrugs. Additionally, proteomic screening and gene expression analysis can provide insight into physiological response to drug treatment and identify the reasons for drug resistance. Herein, we review selected applications which led to a better understanding of the mode of action of clinically established metal-based anticancer agents and novel metallodrug candidates.

Transferrin-cisplatin specifically deliver cisplatin to HepG2 cells in vitro and enhance cisplatin cytotoxicity

Cisplatin is a major broad-spectrum chemotherapeutic agent, however, its dose-dependent side effects limit the administration of large doses. Presently, developing a drug targeted delivery system is suggested as one of the most promising approaches to minimize the side effects of cisplatin. Here, we found that each human serum transferrin (HTf) has the potential to bind with over 22 cisplatins, and the complex of apo-HTf-cisplatin can specifically deliver cisplatin to HepG2 cells (human hepatocellular liver carcinoma cell line) in vitro, and facilitate HepG2 cells to apoptosis. Moreover, proteomics methods revealed that the abundances of 23 proteins in HepG2 cells were remarkably altered in response to cisplatin/apo-HTf-cisplatin exposure, and Realtime-PCR revealed that a number of important genes related to chemotherapeutic cytotoxicity and chemotherapeutic resistance are differentially transcribed between the HepG2 cells of cisplatin exposed and HTf-cisplatin exposed. The pathway analysis of the differentially expressed proteins and gene transcriptions indicated that those regulated proteins and gene transcriptions are involved in apoptosis regulation, transcription, cell cycle control, protein biosynthesis, energy metabolism, signal transduction, protein binding and other functions. It indicated that the cisplatin toxicity in HepG2 cell is diverse, the transport process has an effect on the cisplatin cytotoxicity, and the mechanism of the apoptosis of HepG2 cells induced by apo-HTf-cisplatin is different from that of cisplatin.

Use of top-down and bottom-up Fourier transform ion cyclotron resonance mass spectrometry for mapping calmodulin sites modified by platinum anticancer drugs

Calmodulin (CaM) is a highly conserved, ubiquitous, calcium-binding protein; it binds to and regulates many different protein targets, thereby functioning as a calcium sensor and signal transducer. CaM contains 9 methionine (Met), 1 histidine (His), 17 aspartic acid (Asp), and 23 glutamine acid (Glu) residues, all of which can potentially react with platinum compounds; thus, one-third of the CaM sequence is a possible binding target of platinum anticancer drugs, which represents a major challenge for identification of specific platinum modification sites. Here, top-down electron capture dissociation (ECD) was used to elucidate the transition metal-platinum(II) modification sites. By using a combination of top-down and bottom-up mass spectrometric (MS) approaches, 10 specific binding sites for mononuclear complexes, cisplatin and [Pt(dien)Cl]Cl, and dinuclear complex [{cis-PtCl(2)(NH(3))}(2)(μ-NH(2)(CH(2))(4)NH(2))] on CaM were identified. High resolution MS of cisplatin-modified CaM revealed that cisplatin mainly targets Met residues in solution at low molar ratios of cisplatin-CaM (2:1), by cross-linking Met residues. At a high molar ratio of cisplatin:CaM (8:1), up to 10 platinum(II) bind to Met, Asp, and Glu residues. [{cis-PtCl(2)(NH(3))}(2)(μ-NH(2)(CH(2))(4)NH(2))] forms mononuclear adducts with CaM. The alkanediamine linker between the two platinum centers dissociates due to a trans-labilization effect. [Pt(dien)Cl]Cl forms {Pt(dien)}(2+) adducts with CaM, and the preferential binding sites were identified as Met51, Met71, Met72, His107, Met109, Met124, Met144, Met145, Glu45 or Glu47, and Asp122 or Glu123. The binding of these complexes to CaM, particularly when binding involves loss of all four original ligands, is largely irreversible which could result in their failure to reach the target DNA or be responsible for unwanted side-effects during chemotherapy. Additionally, the cross-linking of cisplatin to CaM might lead to the loss of the biological function of CaM or CaM-Ca(2+) due to limiting the flexibility of the CaM or CaM-Ca(2+) complex to recognize target proteins or blocking the binding region of target proteins to CaM.

Mass spectrometry evidence for cisplatin as a protein cross-linking reagent

Cisplatin is a potent anticancer drug, which functions by cross-linking adjacent DNA guanine residues. However within 1 day of injection, 65-98% of the platinum in the blood plasma is protein-bound. It is generally accepted that cisplatin binds to methionine and histidine residues, but what is often underappreciated is that platinum from cisplatin has a 2+ charge and can form up to four bonds. Thus, it has the potential to function as a cross-linker. In this report, the cross-linking ability of cisplatin is demonstrated by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) with the use of standard peptides, the 16.8 kDa protein calmodulin (CaM), but was unsuccessful for the 64 kDa protein hemoglobin. The high resolution and mass accuracy of FTICR MS along with the high degree of fragmentation of large peptides afforded by collisionally activated dissociation (CAD) and electron capture dissociation (ECD) are shown to be a valuable means of characterizing cross-linking sites. Cisplatin is different from current cross-linking reagents by targeting new functional groups, thioethers, and imidazoles groups, which provides complementarity with existing cross-linkers. In addition, platinum(II) inherently has two positive charges which enhance the detection of cross-linked products. Higher charge states not only promote the detection of cross-linking products with less purification but result in more comprehensive MS/MS fragmentation and can assist in the assignment of modification sites. Moreover, the unique isotopic pattern of platinum flags cross-linking products and modification sites by mass spectrometry.

Inductive properties of polypyridyl ruthenium complexes significantly regulate various protein distributions in Escherichia coli

Ruthenium complexes with similar octahedral structures but different intrinsic inductive properties significantly influence the total cellular protein distributions, which may affect different metabolic pathways. A systematic study of the relationship between ruthenium complexes and Escherichia coli was undertaken, using two-dimensional gel electrophoresis analysis and the identification of various proteins by mass data mining. Based on the low similarities (<40%) between the total protein distributions, the inductive properties of the ruthenium complexes are relevant to the formation of the protein-Ru interaction in addition to the Ru-DNA interaction. Two major protein functions in E. coli BL21 that were reduced by compound 1 were oxidoreductases and transporters, corresponding to 29% and 25% of the 24 down-regulated proteins. The main biological processes of the proteins down-regulated by compound 1 were related to carbohydrate reactions, including in transport, tricarboxylic acid (TCA) cycle, glycolysis, and gluconeogenesis. All four ruthenium complexes shared similar up-regulated proteins, including clpB and kpyk1, and down-regulated similar proteins, including ompA and ybbN. This result supports that the presence of Ru-protein interactions is a major factor affecting bacteria growth, and particularly transport and carbohydrate-related reactions.

Reactivity of anticancer metallodrugs with serum proteins: new insights from size exclusion chromatography-ICP-MS and ESI-MS

A method based on the coupling of high resolution size-exclusion liquid chromatography using a polymer stationary phase with inductively coupled plasma mass spectrometry was developed to study the interactions of two metallodrugs - cisplatin and RAPTA-T - with the serum proteins albumin and transferrin. In contrast to previous approaches, the technique allowed the total recovery of the metals from the column and was able to discriminate between the different species of the metallodrugs and their complexes with the proteins at femtomolar detection levels. Metal binding was found to be dependent on the protein concentration and on the incubation time of the sample. Cisplatin was found to bind the serum proteins to the same extent, whereas RAPTA-T showed marked preference for transferrin. The affinity of the ruthenium complex for holo-transferrin was higher than for the apo-form suggesting a cooperative iron-mediated metal binding mechanism. RAPTA-T binding to holo-transferrin was further investigated by electrospray mass spectrometry using both the intact protein and a model peptide mimicking the iron-binding pocket.

Analytical methodologies for metallomics studies of antitumor Pt-containing drugs

Pt-containing drugs are nowadays essential components in cancer chemotherapy. However, drug resistance and side effects limit the efficiency of the treatments. In order to improve the response to Pt-based drugs, different administration strategies or new Pt-compounds have been developed with little success. The reason for this failure could be that the mechanism of action of these drugs is not completely understood. In this way, metallomics studies may contribute to clarify the interactions of Pt-containing drugs within the organism. This review is mainly focused on the role of Analytical Chemistry on the study of the interactions between Pt-based drugs and biomolecules. A summary of the analytical techniques and the most common sample treatment procedures currently used in metallomics studies of these drugs is presented. Both are of paramount importance to study these complex samples preserving the drug-biomolecule interaction. Separation and detection techniques must be carefully selected in order to achieve the intended goals. The use of multidimensional hyphenated techniques is usually necessary for a better understanding of the Pt-based drugs interactions in the organism. An overview of Pt-drugs biological interactions is presented, considering the different sample matrices and the drugs course through the organism. Samples analysed in the included studies are blood, urine, cell cytosol, DNA as well as the drugs themselves and their derivatives. However, most of these works are based on in vitro experiments or incubations of standards, leading in some cases to contradictory results depending on the experimental conditions used. Though in vivo experiments represent a great challenge due to the high complexity and the low concentrations of the Pt-adducts in real samples, these studies must be undertaken to get a deeper understanding of the real interactions concerning Pt-containing drugs.

Is glutathione the major cellular target of cisplatin? A study of the interactions of cisplatin with cancer cell extracts

Cisplatin is an anticancer drug whose efficacy is limited because tumors develop resistance to the drug. Resistant cells often have elevated levels of cellular glutathione (GSH), believed to be the major cellular target of cisplatin that inactivates the drug by binding to it irreversibly, forming [Pt(SG)(2)] adducts. We show by [(1)H,(15)N] HSQC that the half-life of (15)N labeled cisplatin in whole cell extracts is approximately 75 min, but no Pt-GSH adducts were observed. When the low molecular mass fraction (<3 kDa) of the extracts was incubated with cisplatin, binding to GSH was observed probably due to removal of high molecular mass platinophiles. Two-thirds of the Pt adducts formed in whole cell extracts, had a molecular mass >3 kDa. [Pt(SG)(2)] cannot account for more than 20% of the Pt adducts. The concentration of reduced thiols in the high molecular mass fraction of the extracts is six times higher than in the low molecular mass fraction.

Reaction of human metallothionein-3 with cisplatin and transplatin

Human metallothioneins, small cysteine- and metal-rich proteins, play an important role in the acquired resistance to platinum-based anticancer drugs. These proteins contain a M(II)4(CysS)11 cluster and a M(II)3(CysS)9 cluster localized in the alpha-domain and the beta-domain, respectively. The noninducible isoform metallothionein-3 (Zn7MT-3) is mainly expressed in the brain, but was found overexpressed in a number of cancer tissues. Since the structural properties of this isoform substantially differ from those of the ubiquitously occurring Zn7MT-1/Zn7MT-2 isoforms, the reactions of cis-diamminedichloridoplatinum(II) (cisplatin) and trans-diamminedichloridoplatinum(II) (transplatin) with human Zn7MT-3 were investigated and the products characterized. A comparison of the reaction kinetics revealed that transplatin reacts with cysteine ligands of Zn7MT-3 faster than cisplatin. In both binding processes, stoichiometric amounts of Zn(II) were released from the protein. Marked differences between the reaction rates of cisplatin and transplatin binding to Zn7MT-3 and the formation of the Pt-S bonds suggest that the binding of both Pt(II) compounds is a complex process, involving at least two subsequent binding steps. The electrospray ionization mass spectrometry characterization of the products showed that whereas all ligands in cisplatin were replaced by cysteine thiolates, transplatin retained its carrier ammine ligands. The 113Cd NMR studies of Pt1 113Cd6MT-3 revealed that cisplatin binds preferentially to the beta-domain of the protein. The rates of reaction of cisplatin and transplatin with Zn7MT-3 were much faster than those of cisplatin and transplatin with Zn7MT-2. The biological consequences of a substantially higher reactivity of cisplatin toward Zn7MT-3 than Zn7MT-2 in the acquired resistance to platinum-based drugs are discussed.

Exploring metallodrug-protein interactions by mass spectrometry: comparisons between platinum coordination complexes and an organometallic ruthenium compound

Electrospray ionisation mass spectrometry was used to analyse the reactions of metal compounds with mixtures of selected proteins. Three representative medicinally relevant compounds, cisplatin, transplatin and the organometallic ruthenium compound RAPTA-C, were reacted with a pool of three proteins, ubiquitin, cytochrome c and superoxide dismutase, and the reaction products were analysed using high-resolution mass spectrometry. Highly informative electrospray ionisation mass spectra were acquired following careful optimisation of the experimental conditions. The formation of metal-protein adducts was clearly observed for the three proteins. In addition, valuable information was obtained on the nature of the protein-bound metallofragments, on their distribution among the three different proteins and on the binding kinetics. The platinum compounds were less reactive and considerably less selective in protein binding than RAPTA-C, which showed a high affinity towards ubiquitin and cytochrome c, but not superoxide dismutase. In addition, competition studies between cisplatin and RAPTA-C showed that the two metallodrugs have affinities for the same amino acid residues on protein binding.

Trans labilization of am(m)ine ligands from platinum(II) complexes by cancer cell extracts

Cisplatin, cis-[Pt(NH(3))(2)Cl(2)], is an effective anticancer agent in wide clinical use whose efficacy is affected by cellular interactions with sulfur-containing nucleophiles. These interactions can potentially enhance the efficacy of the drug by mediating its delivery to nuclear DNA or inactivate the drug by binding to it irreversibly or by labilizing the NH(3) ligands. Despite the potential importance of trans-labilization reactions in the mechanism of action of the drug, few detailed studies on trans labilization of the ammines have been conducted. We used 2D NMR to show that some trans labilization occurs in proliferating cells and that aqueous extracts of cancer cells labilized 20% of the amine ligands of cis-[PtCl(2)((13)CH(3)NH(2))(2)] after a 12-h incubation. Both low molecular mass nucleophiles (less than 3 kDa) and high molecular mass nucleophiles (more than 3 kDa) labilize the amines with similar efficiency. Studies with model compounds show that thiols and thioethers bind to platinum(II) at similar rates, but thioethers are significantly more efficient at labilizing the am(m)ine at lower pH. N-Acetylcysteine is a more efficient trans-labilizer than glutathione, suggesting that the displacement of the amine proceeds through an associative mechanism. The lag time, the time that elapses from the formation of the Pt-S bond till the release of the amine trans to the sulfur, depends on the pH (for thiols), increasing at lower pH. Quantification of the platinum adducts obtained from incubation of cisplatin with cell extracts indicates that two thirds of the platinum is bound to cellular components with molecular mass greater than 3 kDa.