Platinum(IV) antitumour compounds: their bioinorganic chemistry

Self-Assembled Metal Complexes in Biomedical Research

Cisplatin is widely used in clinical cancer treatment; however, its application is often hindered by severe side effects, particularly inherent or acquired resistance of target cells. To address these challenges, an effective strategy is to modify the metal core of the complex and introduce alternative coordination modes or valence states, leading to the development of a series of metal complexes, such as platinum (IV) prodrugs and cyclometalated complexes. Recent advances in nanotechnology have facilitated the development of multifunctional nanomaterials that can selectively deliver drugs to tumor cells, thereby overcoming the pharmacological limitations of metal-based drugs. This review first explores the self-assembly of metal complexes into spherical, linear, and irregular nanoparticles in the context of biomedical applications. The mechanisms underlying the self-assembly of metal complexes into nanoparticles are subsequently analyzed, followed by a discussion of their applications in biomedical fields, including detection, imaging, and antitumor research.

Zn(II)-Curcumin Complexes-Based Anticancer Agents

There is a great deal of research interest in the design of alternative metallodrugs to Pt(II)-derivatives for cancer treatment. The low solubility of such drugs in biological mediums leading to poor bioavailability is the major hurdle of several metal-based anticancer agents. These issues have recently been addressed by designing bio-active ligands based on metal-containing anticancer agents. Conjugating with bioactive ligands has significantly improved the bioavailability of the metallodrugs and their cancer cell targeting ability. One such naturally available bioactive ligand is curcumin. Until recently, several curcumin-based anticancer metallodrugs have been developed and successfully demonstrated for their anticancer studies. In this article, we aim to highlight, the synthesis, structure, and anticancer properties of various Zn(II)-curcumin-based coordination complexes. The effect of introducing different functional groups, targeting ligands, and photo-active ligands on the anticancer potential of such complexes has been mentioned in detail. The current status and future perspective on curcumin-based metallodrugs for cancer treatment have also been stated.

Nanostructured mesoporous silica carriers for platinum-based conjugates with anti-inflammatory agents

This review discusses the relationship between inflammation and cancer initiation and progression, which has prompted research into anti-inflammatory approaches for cancer prevention and treatment. Specifically, it focuses on the use of inflammation-reducing agents to enhance the effectiveness of tumor treatment methods. These agents are combined with platinum(II)-based antitumor drugs to create multifunctional platinum(IV) prodrugs, allowing for simultaneous delivery to tumor cells in a specific ratio. Once inside the cells and subjected to intracellular reduction, both components can act in parallel through distinct pathways. Motivated by the objective of reducing the systemic toxicity associated with contemporary chemotherapy, and with the aim of leveraging the passive enhanced permeability and retention effect exhibited by nanostructured materials to improve their accumulation within tumor tissues, the platinum(IV) complexes have been efficiently loaded into mesoporous silica SBA-15 material. The resulting nanostructured materials are capable of providing controlled release of the conjugates when subjected to simulated plasma conditions. This feature suggests the potential for extended circulation within the body in vivo, with minimal premature release of the drug before reaching the intended target site. The primary emphasis of this review is on research that integrates these two approaches to develop chemotherapeutic treatments that are both more efficient and less harmful.

Cyclometalated C^C* Platinum(IV) NHC Complexes

We present a previously unknown series of cyclometalated platinum(IV) NHC complexes, obtained by oxidative addition of iodine, iodomethane, and different benzyl bromides to platinum(II) complexes with cyclometalated C^C* dibenzofuran, dibenzothiophene, and phenylimidazole ligands together with an acetylacetonate ligand. All compounds were characterized by standard techniques (1H, 13C, and 195Pt NMR and elemental analysis), and for three complexes, solid-state structures could be obtained. DFT calculations (B3LYP(d3)/def2-TZVPP including dispersion interactions in dichloromethane and acetone) were performed to investigate the mechanism of the oxidation and explain the exclusive formation of the observed trans products.

NIR Light and GSH Dual-Responsive Upconversion Nanoparticles Loaded with Multifunctional Platinum(IV) Prodrug and RGD Peptide for Precise Cancer Therapy

Platinum(II) drugs as a first-line anticancer reagent are limited by side effects and drug resistance. Stimuli-responsive nanosystems hold promise for precise spatiotemporal manipulation of drug delivery, with the aim to promote bioavailability and minimize side effects. Herein, a multitargeting octahedral platinum(IV) prodrug with octadecyl aliphatic chain and histone deacetylase inhibitor (phenylbutyric acid, PHB) at axial positions to improve the therapeutic effect of cisplatin was loaded on the upconversion nanoparticles (UCNPs) through hydrophobic interaction. Followed attachment of DSPE-PEG2000 and arginine-glycine-aspartic (RGD) peptide endowed the nanovehicles with high biocompatibility and tumor specificity. The fabricated nanoparticles (UCNP/Pt(IV)-RGD) can be triggered by upconversion luminescence (UCL) irradiation and glutathione (GSH) reduction to controllably release Pt(II) species and PHB, inducing profound cytotoxicity. Both in vitro and in vivo experiments demonstrated that UCNP/Pt(IV)-RGD exhibited remarkable antitumor efficiency, high tumor-targeting specificity, and real-time UCL imaging capacity, presenting an intelligent platinum(IV) prodrug-loaded nanovehicle for UCL-guided dual-stimuli-responsive combination therapy.

Exploring the effect of the axial ligands on the anticancer activity of [C,N,N'] Pt(IV) cyclometallated compounds

The synthesis of three novel [C,N,N'] Pt(IV) cyclometallated compounds containing hydroxo, dichloroacetato or trifluoroacetato axial ligands is reported. Compound [PtCl(OH)2{(CH3)2N(CH2)2NCH(4-FC6H3)}] (3) was prepared by the oxidative addition of hydrogen peroxide to [C,N,N'] Pt(II) cyclometallated compound [PtCl{(CH3)2N(CH2)2NCH(4-FC6H3)}] (1) and further the reaction of compound 3 with dichloroacetate or trifluoroacetate anhydrides led to the formation of the corresponding compounds [PtCl(CHCl2COO)2{(CH3)2N(CH2)2NCH(4-FC6H3)}] (4) and [PtCl(CF3COO)2{(CH3)2N(CH2)2NCH(4-FC6H3)}] (5). The properties of the new compounds along with those of the compound [PtCl3{(CH3)2N(CH2)2NCH(4-FC6H3)}] (2), including stability in aqueous media, reduction potential using cyclic voltammetry, cytotoxic activity against the HCT116 CRC cell line, DNA interaction, topoisomerase I and cathepsin inhibition, and computational studies involving reduction of the Pt(IV) compounds and molecular docking studies, are presented. Interestingly, the antiproliferative activity of these compounds against the HCT116 CRC cell line, which is in all cases higher than that of cisplatin, follows the same trend as the reduction potentials so that the most easily reduced compound 2 is the most potent. In contrast, according to the electrophoretic mobility and molecular docking studies, the efficacy of these compounds in binding to DNA is not related to their cytotoxicity. The most active compound 2 does not modify the DNA electrophoretic mobility while the less potent compound 3 is the most efficient in binding to DNA. Although compounds 2 and 3 have only a slight effect on cell cycle distribution and apoptosis induction, generation of ROS to a higher extent for the most easily reduced compound 2 was observed.

Fluorescence-Guided Spatial Drug Screening in 3D Colorectal Cancer Spheroids

The limited recapitulation of critical cancer features in 2D cultures causes poor translatability of preclinical results from in vitro assays to in vivo tumor models. This contributes to slow drug development with a low success rate. 3D cultures better recapitulate the tumor microenvironment, enabling more accurate predictions when screening drug candidates and improving the development of chemotherapeutics. Platinum (Pt) (IV) compounds are promising prodrugs designed to reduce the severe systemic toxicity of widely used Food and Drug Administration (FDA)-approved Pt(II) drugs such as cisplatin. Here, this work presents spatiotemporal evaluations in 3D colorectal cancer (CRC) spheroids of mitochondria-targeting Pt(IV) complexes. CRC spheroids provide a greater pathophysiological recapitulation of in vivo tumors than 2D cultures by a marked upregulation of the ABCG2 chemoresistance marker expression. Furthermore, new 3D-staining protocols are introduced to evaluate the real-time decrease in mitochondria membrane potential (ΔΨ) in CRC spheroids, and a Pt-sensing dye to quantify the Pt mitochondrial accumulation. Finally, this work demonstrates a correlation between in vitro results and the efficacy of the compounds in vivo. Overall, the CRC spheroids represent a fast and cost-effective model to assess the behavior of Pt compounds in vitro and predict their translational potential in CRC treatment.

Chemotherapeutic Potential of Chlorambucil-Platinum(IV) Prodrugs against Cisplatin-Resistant Colorectal Cancer Cells

Chlorambucil-platinum(IV) prodrugs exhibit multi-mechanistic chemotherapeutic activity with promising anticancer potential. The platinum(II) precursors of the prodrugs have been previously found to induce changes in the microtubule cytoskeleton, specifically actin and tubulin of HT29 colon cells, while chlorambucil alkylates the DNA. These prodrugs demonstrate significant anticancer activity in 2D cell and 3D spheroid viability assays. A notable production of reactive oxygen species has been observed in HT29 cells 72 h post treatment with prodrugs of this type, while the mitochondrial membrane potential was substantially reduced. The cellular uptake of the chlorambucil-platinum(IV) prodrugs, assessed by ICP-MS, confirmed that active transport was the primary uptake mechanism, with platinum localisation identified primarily in the cytoskeletal fraction. Apoptosis and necrosis were observed at 72 h of treatment as demonstrated by Annexin V-FITC/PI assay using flow cytometry. Immunofluorescence measured via confocal microscopy showed significant changes in actin and tubulin intensity and in architecture. Western blot analysis of intrinsic and extrinsic pathway apoptotic markers, microtubule cytoskeleton markers, cell proliferation markers, as well as autophagy markers were studied post 72 h of treatment. The proteomic profile was also studied with a total of 1859 HT29 proteins quantified by mass spectroscopy, with several dysregulated proteins. Network analysis revealed dysregulation in transcription, MAPK markers, microtubule-associated proteins and mitochondrial transport dysfunction. This study confirms that chlorambucil-platinum(IV) prodrugs are candidates with promising anticancer potential that act as multi-mechanistic chemotherapeutics.

Recent Development of Transition Metal Complexes as Chemotherapeutic Hypoxia Activated Prodrug (HAP)

Hypoxia is a state characterized by low concentration of Oxygen. Hypoxic state is often found in the central region of solid tumors. Hypoxia is associated with abnormal neovascularization resulted in poor blood flow in tissues and increased proliferation of tumor cells, imbalance between O2 supply and O2 consumption in tumor cells, high concentration of proton and strong reducibility. And, these abnormalities enhance the survival potency of the hypoxic tumours and increase the resistance towards chemotherapy and radiotherapy. One of the approach for treating hypoxic region of tumour is to use reducing environment of hypoxic tumours for reducing a molecule (hypoxia activated prodrug, HAP) and as a result the active drug will be released in hypoxic region in a controlled manner from the prodrug and kill the hypoxic tumour. Co(III) and Pt(IV) complexes with monodentate active drug molecule in the axial position can be reduced to Co(II) and Pt(II) moieties and as a result, the axial ligands (active drug) could come out from the metal center and could show its anticancer activity. In this review we have highlighted the research articles where transition metal-based complexes are used as chemotherapeutic hypoxia activated prodrug molecules which are reported in last 5 years.

Anticancer Activity of Metallodrugs and Metallizing Host Defense Peptides-Current Developments in Structure-Activity Relationship

This article provides an overview of the development, structure and activity of various metal complexes with anti-cancer activity. Chemical researchers continue to work on the development and synthesis of new molecules that could act as anti-tumor drugs to achieve more favorable therapies. It is therefore important to have information about the various chemotherapeutic substances and their mode of action. This review focuses on metallodrugs that contain a metal as a key structural fragment, with cisplatin paving the way for their chemotherapeutic application. The text also looks at ruthenium complexes, including the therapeutic applications of phosphorescent ruthenium(II) complexes, emphasizing their dual role in therapy and diagnostics. In addition, the antitumor activities of titanium and gold derivatives, their side effects, and ongoing research to improve their efficacy and reduce adverse effects are discussed. Metallization of host defense peptides (HDPs) with various metal ions is also highlighted as a strategy that significantly enhances their anticancer activity by broadening their mechanisms of action.

Supramolecular cage-mediated cargo transport

A steady stream of material transport based on carriers and channels in living systems plays an extremely important role in normal life activities. Inspired by nature, researchers have extensively applied supramolecular cages in cargo transport because of their unique three-dimensional structures and excellent physicochemical properties. In this review, we will focus on the development of supramolecular cages as carriers and channels for cargo transport in abiotic and biological systems over the past fifteen years. In addition, we will discuss future challenges and potential applications of supramolecular cages in substance transport.

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.

Primary adipocytes as targetable drug depot to prevent post-surgical cancer recurrence

Surgery followed by adjuvant chemotherapy or radiation therapy remains the mainstream treatment for breast cancer in the clinic. However, cancer recurrence post surgery is still common. In view of the clinical practice that autologous fat tissue grafting is often used to facilitate breast reconstruction after lumpectomy, here we develop an in vivo targetable adipocyte-based drug depot for the prevention of post-surgical cancer recurrence. We show that primary adipocytes can be metabolically labeled with clickable chemical tags (e.g., azido groups), for subsequent conjugation of dibenzocyclooctyne (DBCO)-bearing cargo via efficient click chemistry. The conjugated cargo can retain well on the adipocyte membrane. By incorporating a cleavable linker between DBCO and cargo, the conjugated cargo can be gradually released from the surface of adipocytes to effect on neighboring cells. In the context of breast cancer surgery, azido-labeled adipocytes grafted to the surgical site can capture circulating DBCO-drugs for improved prevention of 4T1 triple-negative breast cancer (TNBC) recurrence and metastasis. This targetable and refillable adipocyte-based drug depot holds great promise for drug delivery, transplantation, and other applications.

Oxaliplatin(IV) Prodrugs Functionalized with Gemcitabine and Capecitabine Induce Blockage of Colorectal Cancer Cell Growth-An Investigation of the Activation Mechanism and Their Nanoformulation

The use of platinum-based anticancer drugs, such as cisplatin, oxaliplatin, and carboplatin, is a common frontline option in cancer management, but they have debilitating side effects and can lead to drug resistance. Combination therapy with other chemotherapeutic agents, such as capecitabine and gemcitabine, has been explored. One approach to overcome these limitations is the modification of traditional Pt(II) drugs to obtain new molecules with an improved pharmacological profile, such as Pt(IV) prodrugs. The design, synthesis, and characterization of two novel Pt(IV) prodrugs based on oxaliplatin bearing the anticancer drugs gemcitabine or capecitabine in the axial positions have been reported. These complexes were able to dissociate into their constituents to promote cell death and induce apoptosis and cell cycle blockade in a representative colorectal cancer cell model. Specifically, the complex bearing gemcitabine resulted in being the most active on the HCT116 colorectal cancer cell line with an IC50 value of 0.49 ± 0.04. A pilot study on the encapsulation of these complexes in biocompatible PLGA-PEG nanoparticles is also included to confirm the retention of the pharmacological properties and cellular drug uptake, opening up to the possible delivery of the studied complexes through their nanoformulation.

Stepwise optimization of tumor-targeted dual-action platinum(iv)-gemcitabine prodrugs

While platinum-based chemotherapeutic agents have established themselves as indispensable components of anticancer therapy, they are accompanied by a variety of side effects and the rapid occurrence of drug resistance. A promising strategy to address these challenges is the use of platinum(iv) prodrugs, which remain inert until they reach the tumor tissue, thereby mitigating detrimental effects on healthy cells. Typically, platinum drugs are part of combination therapy settings. Consequently, a very elegant strategy is the development of platinum(iv) prodrugs bearing a second, clinically relevant therapeutic in axial position. In the present study, we focused on gemcitabine as an approved antimetabolite, which is highly synergistic with platinum drugs. In addition, to increase plasma half-life and facilitate tumor-specific accumulation, an albumin-binding maleimide moiety was attached. Our investigations revealed that maleimide-cisplatin(iv)-gemcitabine complexes cannot carry sufficient amounts of gemcitabine to induce a significant effect in vivo. Consequently, we designed a carboplatin(iv) analog, that can be applied at much higher doses. Remarkably, this novel analog demonstrated impressive in vivo results, characterized by significant improvements in overall survival. Notably, these encouraging results could also be transferred to an in vivo xenograft model with acquired gemcitabine resistance, indicating the high potential of this approach.

Einbau von (Bioaktiven) Äquatorialen Liganden in Platin(IV)-Komplexe

Platin(IV)‐Prodrugs sind aufgrund ihrer erhöhten Tumorselektivität und geringeren Nebenwirkungen äußerst interessante Alternativen zu Platin(II)‐Antitumortherapeutika. Im Gegensatz zur gängigen Theorie haben wir kürzlich beobachtet, dass äquatoriale Liganden von z. B. Oxaliplatin(IV)‐Komplexen unter Bildung von [(DACH)Pt(OHeq)2(OAcax)2] hydrolysiert werden können. In der hier vorgestellten Arbeit untersuchten wir die Reaktivität und synthetische Verwendbarkeit dieses Komplexes, als Vorstufe für die Entwicklung neuartiger Platin(IV)‐Komplexe, welche mit herkömmlichen Methoden nicht zugänglich sind. Tatsächlich war es möglich die äquatorialen Hydroxidoliganden z. B. durch ein oder zwei monodentate Biotin‐Liganden, die unter Standardmethoden oxidiert werden würden, zu ersetzen. Die gebildeten Komplexe erwiesen sich als sehr stabil und zeigten auch nach der Reduktion eine langsame Ligandenfreisetzung, eine ideale Eigenschaft für lang zirkulierende zielgerichtete Strategien. Daraufhin wurden zwei Platin(IV)‐Komplexe mit äquatorialen Maleimiden, für die Bindung an Serumalbumin als natürlichen Nanocarrier, synthetisiert. Die Komplexe zeigten im Vergleich zu Oxaliplatin eine stark verlängerte Plasmahalbwertszeit und eine deutlich verbesserte Antitumoraktivität in vivo. Zusammenfassend ermöglicht diese neu entwickelte Syntheseplattform den einfachen und gezielten Einbau äquatorialer Liganden in Platin(IV)‐Komplexe. Des Weiteren können verschiedene (bioaktive) Einheiten koordiniert werden, wodurch sogar zielgerichtete dreifach‐wirksame Platin(IV)‐Prodrugs mit nur einem Platinzentrum möglich wären.

Insertion of (Bioactive) Equatorial Ligands into Platinum(IV) Complexes

Platinum(IV) prodrugs are highly interesting alternatives to platinum(II) anticancer therapeutics due to their increased tumor selectivity and reduced side effects. In contrast to the established theory, we recently observed that the equatorial ligand(s) of e.g. oxaliplatin(IV) complexes can be hydrolyzed with formation of [(DACH)Pt(OHeq )2 (OAcax )2 ]. In the work presented here, we investigated the reactivity and synthetic usability of this complex to be exploited as a precursor for the development of novel platinum(IV) complexes, not able to be synthesized by conventional protocols. Indeed, we could substitute the equatorial hydroxido ligand(s) e.g. by one or two monodentate biotin ligands (which would be oxidized under standard methods). The formed complexes turned out to be very stable with slow ligand release after reduction, ideal for long-circulating tumor-targeting strategies. Therefore, two platinum(IV) complexes with equatorial maleimides, capable of exploiting serum albumin as a natural nanocarrier, were synthesized as well. The complexes showed massively prolonged plasma half-life and distinctly improved anticancer activity in vivo compared to oxaliplatin. Taken together, the newly developed synthetic platform allows the simple and specific insertion of equatorial ligands into platinum(IV) complexes. This will enable the attachment of three different (bioactive) moieties generating targeted triple-action platinum(IV) prodrugs within one single platinum complex.

Development of Prodrug-Payloads for Targeted Therapeutic Applications of Platinum-Acridine Anticancer Agents

A synthetic platform has been developed that provides access to platinum(IV) prodrugs of highly cytotoxic platinum-acridine anticancer agents and allows them to be incorporated into conjugation-ready prodrug-payloads (PPLs). The PPLs can be conveniently assembled in highly efficient microscale reactions utilizing strain-promoted azide-alkyne cycloaddition chemistry. Model reactions were performed to study the stability of the PPLs in buffers and media and to assess their compatibility with cysteine-maleimide Michael addition chemistry. Amide coupling was a successful strategy to generate a conjugate containing integrin-targeted cyclo[RGDfK] peptide. Reactions with ascorbate were performed to mimic the reductive activation of the PPLs and the latter conjugate, and a cyanine (Cy5) fluorophore-labeled PPL was used to probe the reduction of platinum(IV) in cancer cells by confocal microscopy. The PPL concept introduced here should be evaluated for treating solid tumors with PAs using cancer-targeting vehicles, such as antibody-drug conjugates.

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.

Oxidation of the Platinum(II) Anticancer Agent [Pt{(p-BrC6F4)NCH2CH2NEt2}Cl(py)] to Platinum(IV) Complexes by Hydrogen Peroxide

PtIV coordination complexes are of interest as prodrugs of PtII anticancer agents, as they can avoid deactivation pathways owing to their inert nature. Here, we report the oxidation of the antitumor agent [PtII(p-BrC6F4)NCH2CH2NEt2}Cl(py)], 1 (py = pyridine) to dihydroxidoplatinum(IV) solvate complexes [PtIV{(p-BrC6F4)NCH2CH2NEt2}Cl(OH)2(py)].H2O, 2·H2O with hydrogen peroxide (H2O2) at room temperature. To optimize the yield, 1 was oxidized in the presence of added lithium chloride with H2O2 in a 1:2 ratio of Pt: H2O2, in CH2Cl2 producing complex 2·H2O in higher yields in both gold and red forms. Despite the color difference, red and yellow 2·H2O have the same structure as determined by single-crystal and X-ray powder diffraction, namely, an octahedral ligand array with a chelating organoamide, pyridine and chloride ligands in the equatorial plane, and axial hydroxido ligands. When tetrabutylammonium chloride was used as a chloride source, in CH2Cl2, another solvate, [PtIV{(p-BrC6F4)NCH2CH2NEt2}Cl(OH)2(py)].0.5CH2Cl2,3·0.5CH2Cl2, was obtained. These PtIV compounds show reductive dehydration into PtII [Pt{(p-BrC6F4)NCH=CHNEt2}Cl(py)], 1H over time in the solid state, as determined by X-ray powder diffraction, and in solution, as determined by 1H and 19F NMR spectroscopy and mass spectrometry. 1H contains an oxidized coordinating ligand and was previously obtained by oxidation of 1 under more vigorous conditions. Experimental data suggest that oxidation of the ligand is favored in the presence of excess H2O2 and elevated temperatures. In contrast, a smaller amount (1Pt:2H2O2) of H2O2 at room temperature favors the oxidation of the metal and yields platinum(IV) complexes.

Visible light-activatable platinum(IV) prodrugs harnessing CD36 for ovarian cancer therapy

We hereby engineered photoactivatable Pt(IV) metallodrugs that harness CD36 to target ovarian cancer cells. Pt(IV) compounds mimic the structure of fatty acids and take advantage of CD36 as a "Trojan horse" to gain entry into the cells. We confirmed that CD36-dependent entry occurs using graphite furnace atomic absorption spectroscopy with ovarian cancer cells expressing different levels of CD36 and a CD36 inhibitor, SSO. Once the Pt(IV) metallodrugs enter the cancer cells, they can be activated to form Pt(II) with characteristics of cisplatin under visible light (490 nm) irradiation, promoting photoinduced electron transfer from the attached fluorophore to the metal center. This light-induced activation can increase the cytotoxicity of the Pt(IV) metallodrugs by up to 20 times toward ovarian cancer cells, inducing DNA damage and enabling efficient elimination of drug-resistant cancer cells.

Platinum(IV) Derivatives of [Pt(1S,2S-diaminocyclohexane)(5,6-dimethyl-1,10-phenanthroline)] with Diclofenac Ligands in the Axial Positions: A New Class of Potent Multi-action Agents Exhibiting Selectivity to Cancer Cells

The platinum(II) complex [Pt(1S,2S-diaminocyclohexane)(5,6-dimethyl-1,10-phenanthroline)]²⁺ (Pt^II56MeSS, 1) exhibits high potency across numerous cancer cell lines acting by a multimodal mechanism. However, 1 also displays side toxicity and in vivo activity; all details of its mechanism of action are not entirely clear. Here, we describe the synthesis and biological properties of new platinum(IV) prodrugs that combine 1 with one or two axially coordinated molecules of diclofenac (DCF), a non-steroidal anti-inflammatory cancer-selective drug. The results suggest that these Pt(IV) complexes exhibit mechanisms of action typical for Pt(II) complex 1 and DCF, simultaneously. The presence of DCF ligand(s) in the Pt(IV) complexes promotes the antiproliferative activity and selectivity of 1 by inhibiting lactate transporters, resulting in blockage of the glycolytic process and impairment of mitochondrial potential. Additionally, the investigated Pt(IV) complexes selectively induce cell death in cancer cells, and the Pt(IV) complexes containing DCF ligands induce hallmarks of immunogenic cell death in cancer cells.

Effective monitoring of Platinum-DNA adducts formation under simulated physiological conditions by CE-ICP-MS/MS

The leading Pt(II)-based anticancer drugs have been used for decades; however, chemotherapy with their application is burdened with severe side effects. The administration of compounds capable of DNA platination in the form of prodrugs has the potential to overcome the drawbacks associated with their use. Progress toward their clinical application depends on establishing proper methodologies that would allow assessing their ability to bind to DNA in the biological environment. Herein, we propose implementing the approach based on the hyphenation of capillary electrophoresis with inductively coupled plasma tandem mass spectrometry (CE-ICP-MS/MS) for studying Pt-DNA adduct formation. The presented methodology opens the possibility to employ the multielement monitoring for studying the differences in the behavior of Pt(II) and Pt(IV) complexes and, interestingly, revealed the formation of various adducts with DNA and cytosol components for the latter one.

Versatile Platinum(IV) Prodrugs of Naproxen and Acemetacin as Chemo-Anti-Inflammatory Agents

Developing new and versatile platinum(IV) complexes that incorporate bioactive moieties is a rapidly evolving research strategy for cancer drug discovery. In this study, six platinum(IV) complexes (1-6) that are mono-substituted in the axial position with a non-steroidal anti-inflammatory molecule, naproxen or acemetacin, were synthesised. A combination of spectroscopic and spectrometric techniques confirmed the composition and homogeneity of 1-6. The antitumour potential of the resultant complexes was assessed on multiple cell lines and proved to be significantly improved compared with cisplatin, oxaliplatin and carboplatin. The platinum(IV) derivatives conjugated with acemetacin (5 and 6) were determined to be the most biologically potent, demonstrating GI₅₀ values ranging between 0.22 and 250 nM. Remarkably, in the Du145 prostate cell line, 6 elicited a GI₅₀ value of 0.22 nM, which is 5450-fold more potent than cisplatin. A progressive decrease in reactive oxygen species and mitochondrial activity was observed for 1-6 in the HT29 colon cell line, up to 72 h. The inhibition of the cyclooxygenase-2 enzyme was also demonstrated by the complexes, confirming that these platinum(IV) complexes may reduce COX-2-dependent inflammation and cancer cell resistance to chemotherapy.

Cytotoxicity and mechanism of action of metal complexes: An overview

After the discovery of cisplatin, many metal compounds were investigated for the therapy of diseases, especially cancer. The high therapeutic potential of metal-based compounds is related to the special properties of these compounds, such as their redox activity and ability to target vital biological sites. The overproduction of ROS and the consequent destruction of the membrane potential of mitochondria and/or the DNA helix is one of the known pathways leading to the induction of apoptosis by metal complexes. The apoptosis process can occur via the death receptor pathway and/or the mitochondrial pathway. The expression of Bcl2 proteins and the caspase family play critical roles in these pathways. In addition to apoptosis, autophagy is another process that regulates the suppression or promotion of various cancers through a dual action. On the other hand, the ability to interact with DNA is an important property found in several metal complexes with potent antiproliferative effects against cancer cells. These interactions were classified into two important categories: covalent/coordinated or subtle, and non-coordinated interactions. The anticancer activity of metal complexes is sometimes achieved by the simultaneous combination of several mechanisms. In this review, the anticancer effect of metal complexes is mechanistically discussed by different pathways, and some effective agents on their antiproliferative properties are explained.

Platinum-Based TREM2 Inhibitor Suppresses Tumors by Remodeling the Immunosuppressive Microenvironment

Triggering receptor expressed on myeloid cells-2 (TREM2) is a key pro-tumorigenic marker of tumor-infiltrating macrophages, showing potent immunosuppressive activity in tumor microenvironment. A platinum(IV) complex OPA derived from oxaliplatin (OP) and artesunate (ART) exhibited direct cytotoxicity against human colon cancer cells and immunomodulatory activity to inhibit TREM2 on macrophages in vitro and vivo. Furthermore, OPA deterred the tumor growth in mouse models bearing MC38 colorectal tumor by reducing the number of CD206⁺ and CX₃ CR1⁺ immunosuppressive macrophages; it also promoted the expansion and infiltration of immunostimulatory dendritic, cytotoxic T, and natural killer cells. OPA is the first small-molecular TREM2 inhibitor capable of relieving immunosuppressive tumor microenvironment and enhancing chemical anticancer efficiency of a platinum drug, thus showing typical characteristics of a chemoimmunotherapeutic agent.

Platinum(IV) compounds as potential drugs: a quantitative structure-activity relationship study

Introduction

Machine learning methods, coupled with a tremendous increase in computer power in recent years, are promising tools in modern drug design and drug repurposing.

Methods

Machine learning predictive models, publicly available at chemosophia.com, were used to predict the bioactivity of recently synthesized platinum(IV) complexes against different kinds of diseases and medical conditions. Two novel QSAR models based on the BiS algorithm are developed and validated, capable to predict activities against the SARS-CoV virus and its RNA dependent RNA polymerase.

Results

The internal predictive power of the QSAR models was tested by 10-fold cross-validation, giving cross-R2 from 0.863 to 0.903. 38 different activities, ranging from antioxidant, antibacterial, and antiviral activities, to potential anti-inflammatory, anti-arrhythmic and anti-malarial activity were predicted for a series of eighteen platinum(IV) complexes.

Conclusion

Complexes 1, 3 and 13 have high generalized optimality criteria and are predicted as potential SARS-CoV RNA dependent RNA polymerase inhibitors.

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.

Potent Platinum(IV) Prodrugs That Incorporate a Biotin Moiety to Selectively Target Cancer Cells

Four platinum(IV) prodrugs incorporating a biotin moiety to selectively target cancer cells were synthesised, characterised, and their biological activity assessed. All complexes exhibited exceptional in vitro cytotoxicity against a panel of cancer cell lines, with [Pt(5,6-dimethyl-1,10-phenanthroline)(1S,2S-diaminocyclohexane)(biotin)(hydroxido)](NO₃)₂, (2) exhibiting the lowest GI₅₀ of 4 nM in the prostate Du145 cancer cell line. Each complex displayed significantly enhanced activity compared to cisplatin, with 2 being 1000-fold more active in the HT29 colon cancer cell line. Against the MCF-7 breast cancer cell line, in which high levels of biotin receptors are expressed, 2, [Pt(4,7-dimethoxy-1,10-phenanthroline)(1S,2S-diaminocyclohexane)(biotin)(hydroxido)](NO₃)₂, (3), and [Pt(5-methyl-1,10-phenanthroline)(1S,2S-diaminocyclohexane)(biotin)(hydroxido)](NO₃)₂, (4) exhibited enhanced activity compared to their platinum(II) cores, with 4 being 6-fold more active than its platinum(II) precursor. Furthermore, 3 exhibited 3-fold greater selectivity towards MCF-7 breast cancer cells compared to MCF10A breast healthy cells, and this was further confirmed by platinum uptake studies, which showed 3 to have almost 3-fold greater uptake in MCF-7 cells, compared to MCF10A cells. The results show that lipophilicity and selectivity both contributed to the cellular uptake of 1-4; however, this was not always translated to the observed cytotoxicity.

QSPR study on Hydrophobicity of Pt(II) complexes with surface electrostatic potential-based descriptors

Pt(II) complexes play an important role in bioinorganic chemistry due to their antitumor activities. In the present study, we focused on building predictive models for the hydrophobicity of Pt(II) complexes. A five-parameter model, integrating frontier orbital energies (E_HOMO, E_LUMO) and descriptors derived from electrostatic potentials on molecular surface, was firstly constructed by using multiple linear regression (MLR) method. Mechanistic interpretations of the introduced descriptors were elucidated in terms of intermolecular interactions in the n-octanol/water partition system. Then, four up-to-date modeling methods, including support vector machine (SVM), least-squares support vector machine (LSSVM), random forest (RF) and Gaussian process (GP), were utilized to build the nonlinear models. Systematical validations including leave-one-out cross-validation, the validation for test set, as well as a very rigorous Monte Carlo cross-validation (MCCV) were performed to verify the reliability of the constructed models. The peak, median and integralR_ext² values of the best GP model are 0.88, 0.86 and 0.84, respectively. The root mean squared errors for the test set (RMSEP) of the MLR, SVM, LSSVM and GP models fall in the range of 0.62-0.71. Although they are not superior to prior models built with the use of a number of descriptors, the results are satisfactory. Applicability domain of the model was evaluated.

Bioactive Platinum(IV) Complexes Incorporating Halogenated Phenylacetates

A new series of cytotoxic platinum(IV) complexes (1-8) incorporating halogenated phenylacetic acid derivatives (4-chlorophenylacetic acid, 4-fluorophenylacetic acid, 4-bromophenylacetic acid and 4-iodophenylacetic acid) were synthesised and characterised using spectroscopic and spectrometric techniques. Complexes 1-8 were assessed on a panel of cell lines including HT29 colon, U87 glioblastoma, MCF-7 breast, A2780 ovarian, H460 lung, A431 skin, Du145 prostate, BE2-C neuroblastoma, SJ-G2 glioblastoma, MIA pancreas, the ADDP-resistant ovarian variant, and the non-tumour-derived MCF10A breast line. The in vitro cytotoxicity results confirmed the superior biological activity of the studied complexes, especially those containing 4-fluorophenylacetic acid and 4-bromophenylacetic acid ligands, namely 4 and 6, eliciting an average GI₅₀ value of 20 nM over the range of cell lines tested. In the Du145 prostate cell line, 4 exhibited the highest degree of potency amongst the derivatives, displaying a GI₅₀ value of 0.7 nM, which makes it 1700-fold more potent than cisplatin (1200 nM) and nearly 7-fold more potent than our lead complex, 56MESS (4.6 nM) in this cell line. Notably, in the ADDP-resistant ovarian variant cell line, 4 (6 nM) was found to be almost 4700-fold more potent than cisplatin. Reduction reaction experiments were also undertaken, along with studies aimed at determining the complexes' solubility, stability, lipophilicity, and reactive oxygen species production.

Stimuli-responsive platinum and ruthenium complexes for lung cancer therapy

Lung cancer is the most common cause of cancer-related deaths worldwide. More efficient treatments are desperately needed. For decades, the success of platinum-based anticancer drugs has promoted the exploration of metal-based agents. Four ruthenium-based complexes have also entered clinical trials as candidates of anticancer metallodrugs. However, systemic toxicity, severe side effects and drug-resistance impeded their applications and efficacy. Stimuli-responsiveness of Pt- and Ru-based complexes provide a great chance to weaken the side effects and strengthen the clinical efficacy in drug design. This review provides an overview on the stimuli-responsive Pt- and Ru-based metallic anticancer drugs for lung cancer. They are categorized as endo-stimuli-responsive, exo-stimuli-responsive, and dual-stimuli-responsive prodrugs based on the nature of stimuli. We describe various representative examples of structure, response mechanism, and potential medical applications in lung cancer. In the end, we discuss the future opportunities and challenges in this field.

Platinum(IV) Complexes of the 1,3,5-Triamino Analogue of the Biomolecule Cis-Inositol Designed as Innovative Antineoplastic Drug Candidates

Metal complexes occupy a special place in the field of treatment and diagnostics. Their main advantages stem from the possibility of fine-tuning their thermodynamic properties and kinetic behavior in the biological milieu by applying different approaches such as properly constructed inner coordination sphere, appropriate choice of ligands, metal oxidation state, redox potential, etc., which are specific to these compounds. Here we discuss the design and synthesis of two octahedral cationic Pt(IV) complexes of the tridentate ligand all-cis-2,4,6-triaminocyclohexane-1,3,5-triol (taci) with composition, fac-[Pt(taci)I3]+, 1 and bis-[Pt(taci)2]4+, 2 as well as the potential for their application as antineoplastic agents. The complexes have been isolated in a solid state as: fac-[Pt(taci)I3]I·3H2O (1A), fac-[Pt(taci)I3]I (1B), fac-[Pt(taci)I3]I·2DMF (1C), bis-[Pt(taci)2](CO3)2·6H2O (2A) by changing the acidity of the reaction systems, the molar ratios of the reagents and the counterions, and by re-crystallization. The ligand taci is coordinated through the NH2-groups, each molecule occupying three coordination places in the inner coordination sphere of Pt(IV). Monitoring of the hydrolysis processes of 1A and 2A at different acidity showed that while 2A remained stable over the study period, the I--ions in 1A were successively substituted, with the main product under physiologically mimetic conditions being fac,cis-[Pt(taci)I(OH)2]+ (h2). The antiproliferative tests involved eight cancer cell models, among which chemosensitive (derived from leukemias and solid tumors) and chemoresistant human Acute myeloid leukemia lines (HL-60/Dox, HL-60/CDDP), as well as the non-malignant kidney' cells HEK-293T showed that the complexes 1A and 2A are characterized by a fundamentally different profile of chemosensitivity and spectrum of cytotoxic activity compared to cisplatin. The new Pt(IV) complexes were shown to be more effective in selectively inhibiting the proliferation of human malignant cells compared to cisplatin. Remarkable activity was recorded for 1A, which showed an effect (IC50 = 8.9 ± 2.4) at more than 16-fold lower concentration than cisplatin (IC50 = 144.4 ± 9.8) against the resistant cell line HL-60/CDDP. In parallel, 1A exhibited virtually the same cytotoxic effect against the parental HL-60 cells (IC50 = 9.0 ± 1.2), where cisplatin displays comparable chemosensitivity (IC50 = 8.3 ± 0.8). The determined resistance indices (RI~1) show unequivocally that the resistant lines are sensitive to both compounds tested; therefore, they are capable of overcoming the mechanisms of cisplatin resistance. The structural features of these compounds and their promising pharmacological properties justify their inclusion in the group of "non-classical metal-based antitumor compounds" and are a prerequisite for the admission of alternative mechanisms of action.

Potent Chlorambucil-Platinum(IV) Prodrugs

The DNA-alkylating derivative chlorambucil was coordinated in the axial position to atypical cytotoxic, heterocyclic, and non-DNA coordinating platinum(IV) complexes of type, [Pt^IV(H_L)(A_L)(OH)₂](NO₃)₂ (where H_L is 1,10-phenanthroline, 5-methyl-1,10-phenanthroline or 5,6-dimethyl-1,10-phenanthroline, A_L is 1S,2S-diaminocyclohexane). The resultant platinum(IV)-chlorambucil prodrugs, PCLB, 5CLB, and 56CLB, were characterized using high-performance liquid chromatography, nuclear magnetic resonance, ultraviolet-visible, circular dichroism spectroscopy, and electrospray ionization mass spectrometry. The prodrugs displayed remarkable antitumor potential across multiple human cancer cell lines compared to chlorambucil, cisplatin, oxaliplatin, and carboplatin, as well as their platinum(II) precursors, PHENSS, 5MESS, and 56MESS. Notably, 56CLB was exceptionally potent in HT29 colon, Du145 prostate, MCF10A breast, MIA pancreas, H460 lung, A2780, and ADDP ovarian cell lines, with GI₅₀ values ranging between 2.7 and 21 nM. Moreover, significant production of reactive oxygen species was detected in HT29 cells after treatment with PCLB, 5CLB, and 56CLB up to 72 h compared to chlorambucil and the platinum(II) and (IV) precursors.

Near-Infrared-II Light Induced Mild Hyperthermia Activate Cisplatin-Artemisinin Nanoparticle for Enhanced Chemo/Chemodynamic Therapy and Immunotherapy

Chemodynamic therapy (CDT) is an effective cancer treatment that uses Fenton reaction to induce cancer cell death. Current clinical applications of CDT are limited by the dependency of external supply of metal ions as well as low catalytic efficiency. Here, a highly efficient metal-free CDT by using endoperoxide bridge-containing artesunate as free radical-generating substance is developed. A Pt(IV) prodrug (A-Pt) containing two artesunate molecules in the axial direction is synthesized, which can be decomposed into cisplatin and artesunate under reducing intracellular environment in tumor cells. To improve the catalytic efficiency for Fenton reaction, a near-infrared-II (NIR-II) photothermal agent IR1048 is incorporated to achieve a mild hyperthermia effect. By encapsulating the A-Pt and IR1048 with human serum albumin, A-Pt-IR NP are formulated for efficient drug delivery in 4T1 tumor-bearing mice. NIR-II light irradiation of A-Pt-IR NP treated mice show accelerated Fenton reaction. In addition, A-Pt-IR NP could also induce strong immunogenic cell death, which effectively reverses the immunosuppressive tumor microenvironment, and augments antitumor immunity. This study demonstrates that A-Pt-IR NP are potent biodegradable NIR-II active chemotherapy/CDT nanomedicine for clinical translation.

Computational Exploration of the Synergistic Anticancer Effect of a Multi-Action Ru(II)-Pt(IV) Conjugate

An in-depth computational study of the ability of a recently proposed multi-action Ru(II)–Pt(IV) conjugate to act as a photosensitizer in photodynamic therapy (PDT) and chemotherapeutic drugs is presented here. The investigated complex is characterized by a polypyridyl Ru(II) chromophore linked to a Pt(IV) complex that, acting as a prodrug, should be activated by reduction releasing the Ru-based chromophore that can absorb light of proper wavelength to be used in PDT. The reaction mechanism for active species formation has been fully elucidated by means of density functional theory and its time-dependent extension. The reduction mechanism, assisted by ascorbate, of the Pt(IV) prodrug to the Pt(II) active species has been explored, taking into consideration all the possible modes of attack of the reductant for releasing the axial ligands and affording active cisplatin. Given the similarity in the photophysical properties of the chromophore linked or not to the Pt(IV) complex, both the Ru(II)–Pt(IV) conjugate precursor and the Ru(II) chromophore should be able to act as PDT photosensitizers according to type I and type II photoprocesses. In particular, they are able to generate singlet oxygen cytotoxic species as well as auto-ionize to form highly reactive O₂^–• species.

A computational study on the ability of a multi-action Ru(II)−Pt(IV) conjugate to act as a photosensitizer in photodynamic therapy (PDT) and chemotherapeutic drugs is presented here. The reduction mechanism of the Pt(IV) complex along with the photophysical properties of both the prodrug Ru(II)−Pt(IV) conjugate and Ru(II) complex is provided. The mechanism of action has been fully elucidated by means of density functional theory and its time-dependent extension.

Heavy-Metal Trojan Horse: Enterobactin-Directed Delivery of Platinum(IV) Prodrugs to Escherichia coli

The global crisis of untreatable microbial infections necessitates the design of new antibiotics. Drug repurposing is a promising strategy for expanding the antibiotic repertoire. In this study, we repurpose the clinically approved anticancer agent cisplatin into a targeted antibiotic by conjugating its Pt(IV) prodrug to enterobactin (Ent), a triscatecholate siderophore employed by Enterobacteriaceae for iron (Fe) acquisition. The l-Ent-Pt(IV) conjugate (l-EP) exhibits antibacterial activity against Escherichia coli K12 and the uropathogenic isolate E. coli CFT073. Similar to cisplatin, l-EP causes a filamentous morphology in E. coli and initiates lysis in lysogenic bacteria. Studies with E. coli mutants defective in Ent transport proteins show that Ent mediates the delivery of l-EP into the E. coli cytoplasm, where reduction of the Pt(IV) prodrug releases the cisplatin warhead, causing growth inhibition and filamentation of E. coli. Substitution of Ent with its enantiomer affords the d-Ent-Pt(IV) conjugate (d-EP), which displays enhanced antibacterial activity, presumably because d-Ent cannot be hydrolyzed by Ent esterases and thus Fe cannot be released from this conjugate. E. coli treated with l/d-EP accumulate ≥10-fold more Pt as compared to cisplatin treatment. By contrast, human embryonic kidney cells (HEK293T) accumulate cisplatin but show negligible Pt uptake after treatment with either conjugate. Overall, this work demonstrates that the attachment of a siderophore repurposes a Pt anticancer agent into a targeted antibiotic that is recognized and transported by siderophore uptake machinery, providing a design strategy for drug repurposing by siderophore modification and heavy-metal "trojan-horse" antibiotics.

Improvement of Kiteplatin Efficacy by a Benzoato Pt(IV) Prodrug Suitable for Oral Administration

Kiteplatin, [PtCl₂(cis-1,4-DACH)] (DACH = diaminocyclohexane), contains an isomeric form of the oxaliplatin diamine ligand trans-1R,2R-DACH and has been proposed as a valuable drug candidate against cisplatin- and oxaliplatin-resistant tumors, in particular, colorectal cancer. To further improve the activity of kiteplatin, it has been transformed into a Pt(IV) prodrug by the addition of two benzoato groups in the axial positions. The new compound, cis,trans,cis-[PtCl₂(OBz)₂(cis-1,4-DACH)] (1; OBz = benzoate), showed cytotoxic activity at nanomolar concentration against a wide panel of human cancer cell lines. Based on these very promising results, the investigation has been extended to the in vivo activity of compound 1 in a Lewis Lung Carcinoma (LLC) model and its suitability for oral administration. Compound 1 resulted to be remarkably stable in acidic conditions (pH 1.5 to mimic the stomach environment) undergoing a drop of the initial concentration to ~60% of the initial one only after 72 h incubation at 37 °C; thus resulting amenable for oral administration. Interestingly, in a murine model (2·10⁶ LLC cells implanted i.m. into the right hind leg of 8-week old male and female C57BL mice), a comparable reduction of tumor mass (~75%) was observed by administering compound 1 by oral gavage and the standard drug cisplatin by intraperitoneal injection, thus indicating that, indeed, there is the possibility of oral administration for this dibenzoato prodrug of kiteplatin. Moreover, since the mechanism of action of Pt(IV) prodrugs involves an initial activation by chemical reduction to cytotoxic Pt(II) species, the reduction of 1 by two bioreductants (ascorbic acid/sodium ascorbate and glutathione) was investigated resulting to be rather slow (not complete after 120 h incubation at 37 °C). Finally, the neurotoxicity of 1 was evaluated using an in vitro assay.

Enhancing Circulation and Tumor Accumulation of Carboplatin via an Erythrocyte-Anchored Prodrug Strategy

The short circulatory half-lives and low tumor accumulation of carboplatin greatly limit the drug's efficacy in vivo. Herein, we address these challenges by using a prodrug strategy and present the rational design of a novel platinum(IV) anticancer prodrug that can hitchhike on erythrocytes. This prodrug, designated as ERY1-Pt^IV , can bind to erythrocytes efficiently and stably, possessing a circulatory half-life 18.5 times longer than that of carboplatin in mice. This elongated circulatory half-life enables platinum to accumulate at levels 7.7 times higher than with carboplatin, with steady levels in the tumors. As a consequence, the ERY1-Pt^IV prodrug is proved to exhibit significantly enhanced antitumor activity and reduced side effects compared with carboplatin. Collectively, our novel approach highlights an efficient strategy to utilize intrinsic erythrocytes as auto-binding carriers to enhance the tumor accumulation and subsequent antitumor efficacy of platinum drugs.

Rapid DNA interstrand cross-linking of Pt(IV) compound

Pt(IV) anticancer compounds have been developed for several decades to overcome the drawbacks of their Pt(II) congeners, and the reduction of Pt(IV) to Pt(II) has been commonly regarded as a necessary step in the activation of Pt(IV) compounds prior to targeting DNA. However, blockage of glutathione (GSH) biosynthesis resulted in a slight effect on the cytotoxicity of oxoplatin in yeast Saccharomyces cerevisiae strains, urging us to reconsider the mechanism of actions for the "inert" Pt(IV) complexes. Using X-ray absorption near-edge spectroscopy (XANES), our data demonstrated that Pt(IV) complex oxoplatin could bind to DNA in a tetravalent state. Both alkaline denaturing agarose electrophoresis and thermal denaturation-renaturation assay revealed that oxoplatin could rapidly produce stable interstrand crosslinks (ICLs), which can further translate into a fast cell-killing process in cancer cells. Using quantitative real-time PCR and immunofluorescence analysis, we also proved that Pt(IV) complex oxoplatin could induce a quick intracellular response of the FA/BRCA pathway in cancer cells that involves the DNA interstrand crosslinking repair system, and this quick response to ICLs was independent with the intracellular GSH levels. Cell cycle analysis showed that short incubation with oxoplatin can induce a strong S phase arrest in HeLa cells, indicating that the rapid interstrand crosslinks produced by oxoplatin might stall the replication fork, result in the double-strand breaks, and eventually induce cell death. Our results implied that, besides the reduction mechanism to release the Pt(II) congeners, direct and rapid interstrand cross-linking with DNA by Pt(IV) compounds might be a unique mechanism for Pt(IV) compounds, which may provide new insight for the development of next-generation platinum-based drugs.

Understanding the Role of Axial Ligands in Modulating the Biopharmaceutical Outcomes of Cisplatin(IV) Derivatives

Cisplatin is a platinum (Pt)-based anticancer drug with broad-scale clinical utility. However, due to its hydrophilic nature and high kinetic reactivity, it offers numerous drug delivery challenges. Limitations such as severe systemic toxicities, chemoresistance, extensive cisplatin-plasma protein interaction, and limited cellular drug uptake reduce the therapeutic impact of cisplatin therapy. Cisplatin(IV) prodrug formation can effectively resolve these challenges. The selection of axial ligands could play a key role in determining the fate of cisplatin(IV) prodrugs by modulating the therapeutic and biopharmaceutical outcomes of therapy. Hereby, three cisplatin(IV) derivatives were developed utilizing valproate, tocopherol, and chlorambucil as axial ligands, and their biopharmaceutical performance was compared along with cisplatin. The impact of cisplatin(IV) derivative formation on their kinetic stability, drug-albumin interaction, cytotoxicity profile, cellular uptake pattern, self-assembling behavior, hemotoxicity, and tumor biodistribution pattern was analyzed to establish the correlation between the structural properties of cisplatin(IV) agents and their biopharmaceutical outcomes. The kinetic inertness of the designed cisplatin(IV) compounds helped in minimizing their plasma protein interactions and ensuring their stability in the blood environment. The lipophilicity enhancement due to Pt(IV) prodrug formation critically helped in enhancing the cellular drug uptake and reduced the dependence on transporters for drug uptake. The lipophilicity and activity of axial ligands were the key drivers governing the biopharmaceutical performance of the Pt(IV) derivatives. The properties of the axial ligand, such as its therapeutic activity, chemical backbone, and functional groups present in its structure, were the critical factors determining their plasma protein interaction, cellular uptake, anticancer activity, and self-assembly pattern. Cisplatin(IV) derivative formation further improved the amount of platinum accumulated in tumors after intravenous injection compared to free cisplatin therapy (2.7-5.4 folds increment) and reduced drug-erythrocyte interactions. Overall, the results highlighted the potential of cisplatin(IV) agents in resolving cisplatin drug delivery challenges and denoted the critical role of axial ligand selection in Pt(IV) prodrug designing.