[1H, 15N] Heteronuclear Single Quantum Coherence NMR Study of the Mechanism of Aquation of Platinum(IV) Ammine Complexes

Promising Anticancer Prodrugs Based on Pt(IV) Complexes with Bis-organosilane Ligands in Axial Positions

We report two novel prodrug Pt(IV) complexes with bis-organosilane ligands in axial positions: cis-dichloro(diamine)-trans-[3-(triethoxysilyl)propylcarbamate]platinum(IV) (Pt(IV)-biSi-1) and cis-dichloro(diisopropylamine)-trans-[3-(triethoxysilyl) propyl carbamate]platinum(IV) (Pt(IV)-biSi-2). Pt(IV)-biSi-2 demonstrated enhanced in vitro cytotoxicity against colon cancer cells (HCT 116 and HT-29) compared with cisplatin and Pt(IV)-biSi-1. Notably, Pt(IV)-biSi-2 exhibited higher cytotoxicity toward cancer cells and lower toxicity on nontumorigenic intestinal cells (HIEC6). In preclinical mouse models of colorectal cancer, Pt(IV)-biSi-2 outperformed cisplatin in reducing tumor growth at lower concentrations, with reduced side effects. Mechanistically, Pt(IV)-biSi-2 induced permanent DNA damage independent of p53 levels. DNA damage such as double-strand breaks marked by histone gH2Ax was permanent after treatment with Pt(IV)-biSi-2, in contrast to cisplatin's transient effects. Pt(IV)-biSi-2's faster reduction to Pt(II) species upon exposure to biological reductants supports its superior biological response. These findings unveil a novel strategy for designing Pt(IV) anticancer prodrugs with enhanced activity and specificity, offering therapeutic opportunities beyond conventional Pt drugs.

Emerging platinum(IV) prodrug nanotherapeutics: A new epoch for platinum-based cancer therapy

Owing to the unique DNA damaging cytotoxicity, platinum (Pt)-based chemotherapy has long been the first-line choice for clinical oncology. Unfortunately, Pt drugs are restricted by the severe dose-dependent toxicity and drug resistance. Correspondingly, Pt(IV) prodrugs are developed with the aim to improve the antitumor performance of Pt drugs. However, as "free" molecules, Pt(IV) prodrugs are still subject to unsatisfactory in vivo destiny and antitumor efficacy. Recently, Pt(IV) prodrug nanotherapeutics, inheriting both the merits of Pt(IV) prodrugs and nanotherapeutics, have emerged and demonstrated the promise to address the underexploited dilemma of Pt-based cancer therapy. Herein, we summarize the latest fronts of emerging Pt(IV) prodrug nanotherapeutics. First, the basic outlines of Pt(IV) prodrug nanotherapeutics are overviewed. Afterwards, how versatile Pt(IV) prodrug nanotherapeutics overcome the multiple biological barriers of antitumor drug delivery is introduced in detail. Moreover, advanced combination therapies based on multimodal Pt(IV) prodrug nanotherapeutics are discussed with special emphasis on the synergistic mechanisms. Finally, prospects and challenges of Pt(IV) prodrug nanotherapeutics for future clinical translation are spotlighted.

The Strange Case: The Unsymmetric Cisplatin-Based Pt(IV) Prodrug [Pt(CH3COO)Cl2(NH3)2(OH)] Exhibits Higher Cytotoxic Activity with respect to Its Symmetric Congeners due to Carrier-Mediated Cellular Uptake

The biological behavior of the axially unsymmetric antitumor prodrug (OC-6-44)-acetatodiamminedichloridohydroxidoplatinum(IV), 2, was deeply investigated and compared with that of analogous symmetric Pt(IV) complexes, namely, dihydroxido 1 and diacetato 3, which have a similar structure. The complexes were tested on a panel of human tumor cell lines. Complex 2 showed an anomalous higher cytotoxicity (similar to that of cisplatin) with respect to their analogues 1 and 3. Their reduction potentials, reduction kinetics, lipophilicity, and membrane affinity are compared. Cellular uptake and DNA platination of Pt(IV) complexes were deeply investigated in the sensitive A2780 human ovarian cancer cell line and in the corresponding resistant A2780cisR subline. The unexpected activity of 2 appears to be related to its peculiar cellular accumulation and not to a different rate of reduction or a different efficacy in DNA platination and/or efficiency in apoptosis induction. Although the exact mechanism of cell uptake is not fully deciphered, a series of naïve experiments indicates an energy-dependent, carrier-mediated transport: the organic cation transporters (OCTs) are the likely proteins involved.

Application of the anthraquinone drug rhein as an axial ligand in bifunctional Pt(IV) complexes to obtain antiproliferative agents against human glioblastoma cells

Octahedral Pt(IV) prodrugs are an effective way to combine cisplatin-like moieties and a second drug to obtain selective and stimuli responsive bifunctional antiproliferative compounds. Recently, two bifunctional Pt(IV) complexes have shown interesting in vitro and in vivo effects in glioblastoma, the most aggressive primary brain tumor. An interesting observation indicates that 4,5-dihydroxy-9,10-dioxo-9,10-dihydroanthracene-2-carboxylic acid (rhein) can inhibit in vivo glioma tumor progression. Furthermore, a prodrug in which cisplatin was combined with two molecules of rhein showed a potency higher than that of cisplatin toward cisplatin-resistant lung carcinoma cells. However, the high lipophilicity of this type of complex affects their solubility and bioavailability. To overcome these limits, in the present work, three Pt(IV) derivatives were obtained by differently linking one molecule of rhein and one acetato ligand at the axial position to a cisplatin core. The complexes proved to be similar to or more potent than the parent cisplatin and rhein, and the reference drug temozolomide on two human glioblastoma cell lines (U87-MG and T98G). They retained their activity under hypoxia and caused a significant reduction in the motility of both cell lines, which can be related to their ability to inhibit MMP2 and MMP9 matrix metalloproteinases. Finally, physicochemical and computational studies indicated that these Pt(IV) derivatives are more prone than rhein to cross the blood-brain barrier.

Insights into the κ-P,N Coordination of 1,3,5-Triaza-7-phosphaadamantane and Derivatives: κ-P,N-Heterometallic Complexes and a 15N Nuclear Magnetic Resonance Survey

Complexes {[(PTA)₂CpRu-μ-CN-1κC:2κ²N-RuCp(PTA)₂-ZnCl₃]}·2DMSO (13) {[ZnCl₂(H₂O)]-(PTA-1κP:2κ²N)(PTA)CpRu-μ-CN-1κC:2κ²N-RuCp(PTA)(PTA-1κP:2κ²N)-[ZnCl₂(H₂O)]}Cl (14), [RuCp(HdmoPTA)(PPh₃)(PTA)](CF₃SO₃)₂ (20), [RuCp(HdmoPTA)(HPTA)(PPh₃)](CF₃SO₃)₃ (21), and [RuCp(dmoPTA)(PPh₃)(PTA)](CF₃SO₃) (22) were obtained and characterized, and their crystal structure together with that of the previously published complex 18 is reported. The behavior of the 1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]decane (PTA) and 3,7-dimethyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (dmoPTA) ligands against protonation and κN-coordination is discussed, on the basis of ¹⁵N nuclear magnetic resonance data collected on 22 different compounds, including PTA (1), HdmoPTA (7H), and some common derivatives as free ligands (2–6 and 8), along with mono- and polymetallic complexes containing PTA and/or HdmoPTA (9–22). ¹⁵N detection via ¹H–¹⁵N heteronuclear multiple bond correlation allowed the construction of a small library of ¹⁵N chemical shifts that shed light on important features regarding κN-coordination in PTA and its derivatives.

To shed light on the behavior of the triazaphosphines 1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]decane (PTA) and 3,7-dimethyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (dmoPTA) upon κN coordination and N protonation, the ¹⁵N chemical shifts of 22 compounds, including PTA and a representative variety of its derivatives, piano-stool complexes, were collected by ¹H−¹⁵N heteronuclear multiple bond correlation nuclear magnetic resonance. New heterometallic complexes containing PTA were also synthesized and fully characterized.

Pt(IV) antitumor prodrugs: dogmas, paradigms, and realities

Platinum(II)-based drugs are widely used for the treatment of solid tumors, especially in combination protocols. Severe side effects and occurrence of resistance are the major limitations to their clinical use. To overcome these drawbacks, a plethora of Pt(IV) derivatives, acting as anticancer prodrugs, have been designed, synthesized and preclinically (often only in vitro) tested. Here, we summarize the recent progress in the development and understanding of the chemical properties and biochemical features of these Pt(IV) prodrugs, especially those containing bioactive molecules as axial ligands, acting as multi-functional agents. Even though no such prodrugs have been yet approved for clinical use, many show encouraging pharmacological profiles. Thus, a better understanding of their features is a promising approach towards improving the available Pt-based anticancer agents.

Cis,cis,trans-[PtIVCl2(NH3)2(perillato)2], a dual-action prodrug with excellent cytotoxic and antimetastatic activity

Two Pt(iv) conjugates containing one or two molecules of perillic acid (4-isopropenylcyclohexene-1-carboxylic acid), an active metabolite of limonene, were synthesized both with traditional and microwave-assisted methods and characterized. Their antiproliferative activity was tested on a panel of human tumor cell lines. In particular, cis,cis,trans-[Pt^IVCl₂(NH₃)₂(perillato)₂] exhibited excellent antiproliferative and antimetastatic activity on A-549 lung tumor cells at nanomolar concentrations. A number of in vitro biological tests were performed to decipher some aspects of its mechanism of action, including transwell migration and invasion as well as wound healing assay.

On the hydrolytic stability of unsymmetric platinum(iv) anticancer prodrugs containing axial halogens

The hydrolytic stability of Pt(iv) complexes is determined by all the six ligands that coordinate to the Pt(iv) center. By appropriately choosing all the ligands during the design of Pt(iv) prodrugs, the stability of Pt(iv) prodrugs can be improved.

A multi-methodological inquiry of the behavior of cisplatin-based Pt(IV) derivatives in the presence of bioreductants with a focus on the isolated encounter complexes

The study of Pt(IV) antitumor prodrugs able to circumvent some drawbacks of the conventional Pt(II) chemotherapeutics is the focus of a lot of attention. This paper reports a thorough study based on experimental methods (reduction kinetics, electrochemistry, tandem mass spectrometry and IR ion spectroscopy) and quantum-mechanical DFT calculations on the reduction mechanism of cisplatin-based Pt(IV) derivatives having two hydroxido (1), one hydroxido and one acetato (2), or two acetato ligands (3) in axial position. The biological reductants glutathione and ascorbic acid were taken into consideration. The presence of a hydroxido ligand resulted to play an important role in the chemical reduction with ascorbic acid, as verified by ¹⁵N-NMR kinetic analysis using ¹⁵N-enriched complexes. The reactivity trend (1 > 2 > 3) does not reflect the respective reduction peak potentials (1 < 2 < 3), an inverse relationship already documented in similar systems. Turning to a simplified environment, the Pt(IV) complexes associated with a single reductant molecule (corresponding to the encounter complex occurring along the reaction coordinate in bimolecular reactions in solution) were characterized by IR ion spectroscopy and sampled for their reactivity under collision-induced dissociation (CID) conditions. The complexes display a comparable reduction reactivity ordering as that observed in solution. DFT calculations of the free energy pathways for the observed fragmentation reactions provide theoretical support for the CID patterns and the mechanistic hypotheses on the reduction process are corroborated by the observed reaction paths. The bulk of these data offers a clue of the intricate pathways occurring in solution.Graphic abstract.

Antiproliferative Activity of Pt(IV) Conjugates Containing the Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) Ketoprofen and Naproxen †

Cisplatin and several non-steroidal anti-inflammatory drugs (NSAIDs) have been proven to act synergistically or at least additively on several tumor cell lines. Dual-action cisplatin-based Pt(IV) combos containing ketoprofen and naproxen offer good antiproliferative performance on a panel of human tumor cell lines, including a malignant pleural mesothelioma (MPM) one, a very chemoresistant tumor. The main reason of the increased activity relies on the enhanced lipophilicity of these Pt(IV) conjugates that in turn promotes increased cellular accumulation. A quick Pt(IV)→Pt(II) reduction generates the active cisplatin metabolite. The NSAID adjuvant action seems to be almost independent from cyclooxygenase-2 (COX-2) expression in the tumor cells under investigation (lung A-549, colon HT-29, HCT 116, SW480, ovarian A2780, and biphasic MPM MSTO-211H), but it seems to rely (at least in part) on the activation of the NSAID activated gene, NAG-1 (a member of the transforming growth factor beta, TGF-β, superfamily), which has been suggested to be involved in NSAID antiproliferative activity.

Cisplatin: The first metal based anticancer drug

Cisplatin or (SP-4-2)-diamminedichloridoplatinum(II) is one of the most potential and widely used drugs for the treatment of various solid cancers such as testicular, ovarian, head and neck, bladder, lung, cervical cancer, melanoma, lymphomas and several others. Cisplatin exerts anticancer activity via multiple mechanisms but its most acceptable mechanism involves generation of DNA lesions by interacting with purine bases on DNA followed by activation of several signal transduction pathways which finally lead to apoptosis. However, side effects and drug resistance are the two inherent challenges of cisplatin which limit its application and effectiveness. Reduction of drug accumulation inside cancer cells, inactivation of drug by reacting with glutathione and metallothioneins and faster repairing of DNA lesions are responsible for cisplatin resistance. To minimize cisplatin side effects and resistance, combination therapies are used and have proven more effective to defect cancers. This article highlights a systematic description on cisplatin which includes a brief history, synthesis, action mechanism, resistance, uses, side effects and modulation of side effects. It also briefly describes development of platinum drugs from very small cisplatin complex to very large next generation nanocarriers conjugated platinum complexes.

Hybrid inorganic (nonporous silica)/organic (alginate) core-shell platform for targeting a cisplatin-based Pt(IV) anticancer prodrug

Nonporous silica nanoparticles with an external shell containing the 3‑aminopropyl arm (SiNP) were further decorated with alginic acid (SiNP-ALG) as a potential biocompatible delivery system for Pt antitumor agents. Such particles were coupled with the prodrug (OC‑6‑44)‑acetato(β‑alaninato)diamminedichloridoplatinum(IV), 1, through the formation of amide bonds between the pendant carboxylate groups on SiNP-ALG and the free amino group of the complex. Cytosol extracted from tumor cells was able to quickly and efficiently reduce the Pt(IV) prodrug, and produces the active metabolite cisplatin. SiNP-ALG-Pt conjugate was more active than both cisplatin and 1, due to its more efficient cell uptake, whereas the SiNP-ALG unplatinated nanoparticles were deprived of any nonspecific toxicity.

An unsymmetric cisplatin-based Pt(iv) derivative containing 2-(2-propynyl)octanoate: a very efficient multi-action antitumor prodrug candidate

The design, synthesis, characterization and biological properties of a Pt(iv) complex containing the very active inhibitor of histone deacetylase (2-propynyl)octanoic acid, POA, as an axial ligand are reported here. The title complex, namely (OC-6-44)-acetatodiamminedichlorido(2-(2-propynyl)octanoato)platinum(iv), 1, containing POA in racemic or in enantiomeric forms, was one/two orders of magnitude more active than cisplatin, depending on the chemo-sensitivity of the cancer cell lines. Moreover, 1 exhibited similar or even better antiproliferative activity than (OC-6-33)-diamminedichloridobis(2-propylpentanoato)platinum(iv), 2, containing two molecules of the well-known histone deacetylase inhibitor 2-propylpentanoic (valproic) acid. The high potency of 1 is likely due to its high cellular accumulation and to the synergism between the DNA-damaging cisplatin and the histone deacetylase inhibitor POA, both released upon the intracellular reduction of 1. Prodrug 1, after oral administration, caused an impressive reduction of the tumor mass (94%) in a model of solid tumor (murine Lewis lung carcinoma), compared to that of the control, whereas (intraperitoneal) cisplatin induced a tumor regression of 75% only. A good accumulation of 1 was observed in the tumor mass. The time course of the body weight attested that cisplatin induced elevated anorexia, whereas treatment with 1 did not induce significant body weight loss throughout the therapeutic experiment.

The cisplatin-based Pt(iv)-diclorofibrato multi-action anticancer prodrug exhibits excellent performances also under hypoxic conditions

The cisplatin/clofibrato combos are multi-action Pt(iv) complexes active on a panel of human tumor cell lines, also under hypoxic conditions.

Exploring the Hydrolytic Behavior of the Platinum(IV) Complexes with Axial Acetato Ligands

Platinum(IV) complexes are generally thought to be kinetically inert, and are expected to be stable enough to resist premature aquation before entering the cancer cells. Nevertheless, in this work, complex 2 with axial acetato ligands can hydrolyze relatively quickly under biologically relevant conditions with a half-life of 91.7 min, resulting in the loss of the equatorial chlorido ligand. Further study indicated that the fast hydrolysis of complex 2 may be attributed to the strong σ-donor ability of N-isopropyl-1R,2R-diaminocyclohexane, and an increasing σ-donor ability of the amine group can promote the hydrolysis rate of the corresponding platinum(IV) complex. The experiment results were proven by the corresponding DFT calculation. Our study can help to re-evaluate the aqueous properties of the platinum(IV) complexes with axial acetate, which may be less inert to hydrolysis than expected under biologically relevant conditions.

Antiproliferative activity of a series of cisplatin-based Pt(IV)-acetylamido/carboxylato prodrugs

We report studies of a novel series of Pt(IV) complexes exhibiting an asymmetric combination of acetylamido and carboxylato ligands in the axial positions. We demonstrate efficient synthesis of a series of analogues, differing in the alkyl chain length and hence lipophilicity, from a stable acetylamido/hydroxido complex formed by reaction of cisplatin with peroxyacetimidic acid (PAIA). NMR spectroscopy and X-ray crystallography confirm the identity of the resulting complexes, and highlight subtle differences in the structure and stability of acetylamido complexes compared to the equivalent acetato complexes. Reduction of acetylamido complexes, whether achieved chemically or electro-chemically, is significantly more difficult than that of acetate complexes, resulting in lower antiproliferative activity for shorter-chain complexes. For those with longer chains and hence greater cell uptake, this difference is negated and acetylamido complexes are as active as acetato analogues, both exhibiting antiproliferative potency (1/IC50) against A2780 ovarian cancer cells similar to that of cisplatin.

May glutamine addiction drive the delivery of antitumor cisplatin-based Pt(IV) prodrugs?

A small series of Pt(IV) prodrugs containing Gln-like (Gln=glutamine) axial ligands has been designed with the aim to take advantage of the increased demand of Gln showed by some cancer cells (glutamine addiction). In complex 4 the Gln, linked through the α-carboxylic group is recognized by the Gln transporters, in particular by the solute carrier transporter SLC1A5. All compounds showed cellular accumulation, as well as antiproliferative activity, related to their lipophilicity, as already demonstrated for the majority of Pt(IV) prodrugs, that enter cells mainly by passive diffusion. On the contrary, when the Gln concentration in cell medium is near or lower to the physiological value, complex 4 acts as a Trojan horse: it enters SLC1A5-overexpressing cells, where, upon reduction, it releases the active metabolite cisplatin and the Gln-containing ligand, thus preventing any possible extrusion by the L-type amino acid transporter LAT1. This selective mechanism could decrease off-target accumulation of 4 and, consequently, Pt-associated side-effects.

Investigation of the Inertness to Hydrolysis of Platinum(IV) Prodrugs

Platinum(IV) complexes are an important class of compounds that can act as prodrugs, and due to their inertness, if correctly designed, they could have low toxicity outside the cancer cell and improve the pharmacological properties of the platinum(II) anticancer agents that are currently used in the clinic. Because of the efforts that are concentrated on the use of axial ligands able to control the reduction potentials, lipophilicity, charge, selectivity, targeting, and cell uptake of the Pt(IV) complexes, we considered to be of interest to probe the inertness of such complexes that is assumed to be a fulfilled prerequisite. To this aim, a density functional theory computational analysis of the hydrolysis mechanism and the corresponding energy profiles for a series of Pt(IV) derivatives of cisplatin, carboplatin, and oxaliplatin with acetato, haloacetato, and chlorido ligands was performed to probe their stability in biological fluids. The heights of the barriers calculated along the hydrolysis pathways for the associative displacement of ligands both in axial and equatorial positions confirm that Pt(IV) complexes are, in general, more inert than the corresponding Pt(II) drugs even if inertness is lower than expected. Some exceptions exist, such as derivatives of oxaliplatin for the hydrolysis in equatorial position. The nature of the axial ligands influences the course of the hydrolysis reaction even if a decisive role is played by the ligands in equatorial positions. The mechanism of the aquation in axial position of cisplatin Pt(IV) derivative with two chlorido axial ligands assisted by Pt(II) cisplatin was elucidated, and the calculated activation energy confirms the catalytic role played by the Pt(II) complex.

Mechanisms of cell uptake and toxicity of the anticancer drug cisplatin

Two major issues which hamper the use of the anticancer drug cisplatin are the development of cancer cell resistance and its nephrotoxicity. One possible mechanism by which resistance is reported to develop is a reduction in drug uptake across the cell membrane. While the passive uptake of cisplatin has long been cited as an important contribution, far greater attention has been given to active modes of uptake, particularly in recent research. Using unilamellar lipid vesicles together with the stopped-flow kinetic method we show here that the permeability coefficient of cisplatin increases significantly with the chloride concentration of the medium. This supports the hypothesis that cisplatin can enter cells via passive permeation through the lipid phase of the membrane, but becomes trapped within the cytoplasm because dissociation of chloride ligands yields a membrane-impermeant positively-charged aqua derivative. This is important evidence for a major role of passive membrane diffusion in the uptake of cisplatin, and suggests that reduced cell uptake is unlikely to be a significant mechanism leading to the development of drug resistance. Studies of rubidium ion uptake into the cytoplasm of Xenopus oocytes via the Na(+),K(+)-ATPase show significant inhibition of this ion pump when cisplatin is present in the cytoplasm. Because Na(+),K(+)-ATPase activity is essential to the survival of all animal cells, e.g. via maintenance of cell volume, and the Na(+),K(+)-ATPase is expressed at particularly high levels within the membranes of kidney tubules where it plays a crucial role in nutrient reabsorption, these results suggest that cisplatin-induced inhibition of the Na(+),K(+)-ATPase is a likely contributing cause for the nephrotoxicity of cisplatin.

A new entry to asymmetric platinum(IV) complexes via oxidative chlorination

Pt(IV) complexes are usually prepared by oxidation of the corresponding Pt(II) counterparts, typically using hydrogen peroxide or chlorine. A different way to synthesize asymmetrical Pt(IV) compounds is the oxidative chlorination of Pt(II) counterparts with N-chlorosuccinimide. The reaction between cisplatin cis-[PtCl2(NH3)2], carboplatin, cis-[PtCl2(dach)] and cis-[Pt(cbdc)(dach)] (cbdc = cyclobutane-1,1'-dicarboxylato; dach = cyclohexane-1R,2R-diamine) with N-chlorosuccinimide in ethane-1,2-diol was optimized to produce the asymmetric Pt(IV) octahedral complexes [PtA2Cl(glyc)X2] (A2 = 2 NH3 or dach; glyc = 2-hydroxyethanolato; X2 = 2 Cl or cbdc) in high yield and purity. The X-ray crystal structure of the [Pt(cbdc)Cl(dach)(glyc)] complex is also reported. Moreover, the oxidation method proved to be versatile enough to produce other mixed Pt(IV) derivatives varying the reaction medium. The two trichlorido complexes easily undergo a pH-dependent hydrolysis reaction, whereas the dicarboxylato compounds are stable enough to allow further coupling reactions for drug targeting and delivery via the glyc reactive pendant. Therefore, the coupling reaction between the [Pt(cbdc)Cl(dach)(glyc)] and a model carboxylic acid, a model amine, and selectively protected amino acids is reported.

Platinum(IV) prodrugs with haloacetato ligands in the axial positions can undergo hydrolysis under biologically relevant conditions

Losing ligands rapidly: Pt(IV) complexes with haloacetato ligands can hydrolyze rapidly under biological conditions (pH 7 and 37 °C, see scheme) and the rate increases with increasing pH value. Possible mechanisms for this hydrolysis are examined using H2(18)O and ESI-MS analysis.

Theoretical investigation into the mechanism of 3'-dGMP oxidation by [Pt(IV)Cl4(dach)]

The mechanism for the oxidation of 3'-dGMP by [PtCl(4)(dach)] (dach = diaminocyclohexane) in the presence of [PtCl(2)(dach)] has been investigated using density functional theory. We find that the initial complexation, i.e., the formation of [PtCl(3)(dach)(3'-dGMP)], is greatly assisted by the reaction of the encounter pair [PtCl(2)(dach)···3'-dGMP] with [PtCl(4)(dach)], leading to migration of an axial chlorine ligand from platinum(IV) to platinum(II). A dinuclear platinum(II)/platinum(IV) intermediate could not be found, but the reaction is predicted to pass through a platinum(III)/platinum(III) transition structure. A cyclization process, i.e., C8-O bond formation, from [PtCl(3)(dach)(3'-dGMP)] occurs through an intriguing phosphate-water-assisted deprotonation reaction, analogous to the opposite of a proton shuttle mechanism. Followed by this, the guanine moiety is oxidized via dissociation of the Pt(IV)-Cl(ax) bond, and the cyclic ether product is finally formed after deprotonation. We have provided rationalizations, including molecular orbital explanations, for the key steps in the process. Our results help to explain the effect of [PtCl(4)(dach)] on the complexation step and the effect of a strong hydroxide base on the cyclization reaction. The overall reaction cycle is intricate and involves autocatalysis by a platinum(II) species.

Quantitative measurement of the reduction of platinum(IV) complexes using X-ray absorption near-edge spectroscopy (XANES)

The platinum(II) drugs cisplatin, carboplatin and oxaliplatin are usefully employed against a range of malignancies, but toxicities and resistance have spurred the search for improved analogs. This has included investigation of the platinum(IV) oxidation state, which provides greater kinetic inertness. It is generally accepted that Pt(IV) complexes must be reduced to Pt(II) for activation. As such, the ability to monitor reduction of Pt(IV) complexes is critical to guiding the design of candidates, and providing mechanistic understanding. Here we report in full that the white line height of X-ray absorption near-edge spectra (XANES) of Pt complexes, normalized to the post-edge minima, can be used to quantitatively determine the proportion of each oxidation state in a mixture. A series of Pt(IV) complexes based on the Pt(II) complexes cisplatin and transplatin were prepared with chlorido, acetato or hydroxido axial ligands, and studies into their reduction potential and cytotoxicity against A2780 human ovarian cancer cells were performed, demonstrating the relationship between reduction potential and cytotoxicity. Analysis of white line height demonstrated a clear and consistent difference between Pt(II) (1.52 ± 0.05) and Pt(IV) (2.43 ± 0.19) complexes. Reduction of Pt(IV) complexes over time in cell growth media and A2780 cells was observed by XANES, and shown to correspond with their reduction potentials and cytotoxicities. We propose that this method is useful for monitoring reduction of metal-based drug candidates in complex biological systems.

Insights into the acid-base properties of Pt(IV)-diazidodiam(m)inedihyroxido complexes from multinuclear NMR spectroscopy

Platinum(IV) am(m)ine complexes are of interest as potential anticancer pro-drugs, but there are few reports of their acid-base properties. We have studied the acid-base properties of three photoactivatable anticancer platinum(IV)-diazidodiam(m)ine complexes (cis,trans,cis-[Pt(IV)(N(3))(2)(OH)(2)(NH(3))(2)], trans,trans,trans-[Pt(IV)(N(3))(2)(OH)(2)(NH(3))(2)], and cis,trans-[Pt(IV)(N(3))(2)(OH)(2)(en)]) using multinuclear NMR methods and potentiometry. In particular, the combination of both direct and indirect techniques for the detection of (15)N signals has allowed changes of the chemical shifts to be followed over the pH range 1-11; complementary (14)N NMR studies have been also carried out. A distinct pK(a) value of approximately 3.4 was determined for all the investigated complexes, involving protonation/deprotonation reactions of one of the axial hydroxido groups, whereas a second pH-dependent change for the three complexes at approximately pH 7.5 appears not to be associated with a loss of an am(m)ine or hydroxido proton from the complex. Our findings are discussed in comparison with the limited data available in the literature on related complexes.

Cisplatin binds human copper chaperone Atox1 and promotes unfolding in vitro

Cisplatin (cisPt), Pt(NH(3))(2)Cl(2), is a cancer drug believed to kill cells via DNA binding and damage. Recent work has implied that the cellular copper (Cu) transport machinery may be involved in cisPt cell export and drug resistance. Normally, the Cu chaperone Atox1 binds Cu(I) via two cysteines and delivers the metal to metal-binding domains of ATP7B; the ATP7B domains then transfer the metal to the Golgi lumen for loading on cuproenzymes. Here, we use spectroscopic methods to test if cisPt interacts with purified Atox1 in solution in vitro. We find that cisPt binds to Atox1's metal-binding site regardless of the presence of Cu or not: When Cu is bound to Atox1, the near-UV circular dichroism signals indicate Cu-Pt interactions. From NMR data, it is evident that cisPt binds to the folded protein. CisPt-bound Atox1 is however not stable over time and the protein begins to unfold and aggregate. The reaction rates are limited by slow cisPt dechlorination. CisPt-induced unfolding of Atox1 is specific because this effect was not observed for two unrelated proteins that also bind cisPt. Our study demonstrates that Atox1 is a candidate for cisPt drug resistance: By binding to Atox1 in the cytoplasm, cisPt transport to DNA may be blocked. In agreement with this model, cell line studies demonstrate a correlation between Atox1 expression levels, and cisplatin resistance.

Synthesis, characterization, and cytotoxicity of platinum(IV) carbamate complexes

The synthesis, characterization, and cytotoxicity of eight new platinum(IV) complexes having the general formula cis,cis,trans-[Pt(NH(3))(2)Cl(2)(O(2)CNHR)(2)] are reported, where R = tert-butyl (4), cyclopentyl (5), cyclohexyl (6), phenyl (7), p-tolyl (8), p-anisole (9), 4-fluorophenyl (10), or 1-naphthyl (11). These compounds were synthesized by reacting organic isocyanates with the platinum(IV) complex cis,cis,trans-[Pt(NH(3))(2)Cl(2)(OH)(2)]. The electrochemistry of the compounds was investigated by cyclic voltammetry. The aryl carbamate complexes 7-11 exhibit reduction peak potentials near -720 mV vs Ag/AgCl, whereas the alkyl carbamate complexes display reduction peak potentials between -820 and -850 mV vs Ag/AgCl. The cyclic voltammograms of cis,cis,trans-[Pt(NH(3))(2)Cl(2)(O(2)CCH(3))(2)] (1), cis,cis,trans-[Pt(NH(3))(2)Cl(2)(O(2)CCF(3))(2)] (2), and cis-[Pt(NH(3))(2)Cl(4)] (3) were measured for comparison. Density functional theory studies were undertaken to investigate the electronic structures of 1-11 and to determine their adiabatic electron affinities. A linear correlation (R(2) = 0.887) between computed adiabatic electron affinities and measured reduction peak potentials was discovered. The biological activity of 4-11 and, for comparison, cisplatin was evaluated in human lung cancer A549 and normal MRC-5 cells by the MTT assay. The compounds exhibit comparable or slightly better activity than cisplatin against the A549 cells. In MRC-5 cells, all are equally or slightly less cytotoxic than cisplatin, except for 4 and 5, which are more toxic.

Studies on cellular accumulation of satraplatin and its major metabolite JM118 and their interactions with glutathione

Before the active form of a Pt drug reaches its major pharmacological target in the cell nucleus, the Pt complex has to accumulate in cells, and during its transportation into cells and inside cells, it reacts with various biomolecules. Satraplatin is the first orally administered Pt drug under active clinical investigation. The major metabolite of this Pt(IV) complex is its Pt(II) analogue (JM118), which also has significant anticancer properties. Here we report the role of active transport in cellular entry of satraplatin and JM118 and interactions of these Pt complexes with glutathione. The results reveal that the organic cation transporters may play a more important role in the mechanism of cytotoxicity of JM118 than in the cytotoxicity of cisplatin. In contrast, satraplatin is a poor substrate of these transporters. In addition, satraplatin reacts with glutathione with the rate markedly lower than JM118 and cisplatin. Interestingly, satraplatin can be activated by glutathione allowing it to react with DNA, although to a much lower extent than in the case of another Pt(IV) drug tetraplatin.