Novel hypoxia-targeting Pt(iv) prodrugs

Advancements in platinum-based anticancer drug development: A comprehensive review of strategies, discoveries, and future perspectives

Platinum-based anticancer drugs have been at the forefront of cancer chemotherapy, with cisplatin emerging as a pioneer in the treatment of various malignancies. This review article provides a comprehensive overview of the evolution of platinum-based anticancer therapeutics, focusing on the development of cisplatin, platinum(IV) prodrugs, and the integration of photodynamic therapy (PDT) for enhanced cancer treatment results. The first section of the review delves into the historical context and molecular mechanisms underlying the success of cisplatin, highlighting its DNA binding properties and subsequent interference with cellular processes. Despite its clinical efficacy, the inherent limitations, including dose-dependent toxicities and acquired resistance, accelerated the exploration of novel platinum derivatives. This led to the emergence of platinum(IV) prodrugs, designed to overcome resistance mechanisms and enhance selectivity through targeted drug delivery. The subsequent section provides an in-depth analysis of the principles of design and structural modifications employed in the development of platinum(IV) prodrugs. The transitions to the incorporation of photodynamic therapy (PDT) stands out as a synergistic approach to platinum-based anticancer treatment. The photophysical properties of platinum complexes are discussed in the context of their potential application in PDT, emphasizing on combined cytotoxic effects of platinum-based drugs and light-induced reactive oxygen species generation. This dual-action approach holds great promise for overcoming the limitations of traditional chemotherapy as well as producing superior therapeutic outcomes. Overall, the present report explores the latest developments in the development and use of platinum complexes, highlighting novel strategies such combination treatments, targeted delivery methods, and the generation of multifunctional complexes. It also provides a comprehensive overview of the current landscape while proposing future directions for the development of next-generation platinum-based anticancer therapeutics.

Pt(IV) Complexes in the Search for Novel Platinum Prodrugs with Promising Activity

The kinetically inert, six coordinated, octahedral Pt(IV) complexes are termed dual-, triple-, or multi-action prodrugs based on the nature of the axially substituted ligands. These ligands are either inert or biologically active, where the nature of these axial ligands provides additional stability, synergistic biological activity or cell-targeting ability. There are many literature reports from each of these classes, mentioning the varied nature of these axial ligands. The ligands comprise drug molecules such as chlorambucil, doxorubicin, valproic acid, ethacrynic acid, biologically active chalcone, coumarin, combretastatin, non-steroidal anti-inflammatory drugs (NSAIDs) and many more, potentiating the anti-proliferative profile or reducing the side effects associated with cisplatin therapy. The targeting and non-targeting nature of these moieties exert additive or synergistic effects on the anti-cancer activity of Pt(II) moieties. Herein, we discuss the effects of these axially oriented ligands and the changes in the non-leaving am(m)ine groups and in the leaving groups on the biological activity. In this review, we have presented the latest developments in the field of Pt(IV) complexes that display promising activity with a reduced resistance profile. We have discussed the structure activity relationship (SAR) and the effects of the ligands on the biological activity of Pt(IV) complexes with cisplatin, oxaliplatin, carboplatin and the Pt core other than approved drugs. This literature work will help researchers to get an idea about Pt(IV) complexes that have been classified based on the aspects of their biological activity.

Multifunctional small-molecule theranostic agents for tumor-specific imaging and targeted chemotherapy

Cancer is one of the leading causes of death worldwide. Although great progress has been achieved in cancer diagnosis and treatment, novel therapies are still urgently needed to increase the efficacy and reduce the side effects of conventional therapies. Personalized medicine involves administering patients drugs that are specific to the characteristics of their tumors, and has significantly reduced side effects and increased overall survival rates. Multifunctional theranostic drugs are designed to combine diagnostic and therapeutic functions into a single molecule, which reduces the number of drugs administered to patients and increases patient compliance, and have shown great potential in propelling personalized medicine. This review focuses on multifunctional small-molecule theranostic agents for tumor-specific imaging and targeted chemotherapy, with a particular emphasis placed on highlighting design strategies and application in vitro or in vivo. The challenges and future perspectives of multifunctional small molecules are also discussed.

Advances in antitumor research of HIF-1α inhibitor YC-1 and its derivatives

Generally, hypoxia-inducible factor-1α (HIF-1α) is highly expressed in solid tumors, it plays a key role in the occurrence and development of tumors, hindering cancer treatment in various ways. The antitumor activity and pharmacological mechanism of YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1‑benzyl indazole], an HIF-1α inhibitor, and the design and synthesis of its derivatives have attracted tremendous attention in the field of antitumor research. YC-1 is a potential drug candidate and a lead compound for tumor therapy. Hence, the multifaceted mechanism of action of YC-1 and the structure activity relationship (SAR) of its derivatives are important factors to be considered for the development of HIF-1α inhibitors. Therefore, this review aimed to provide a comprehensive overview of the various antitumor mechanisms of YC-1 in antitumor research and an in-depth summary of the SAR for the development of its derivatives. A full understanding and discussion of these aspects are expected to provide potential ideas for developing novel HIF-1α inhibitors and antitumor drugs belonging to the YC-1 class. The review also highlighted the application prospects of the YC-1 class of potential antitumor candidates, and provided some unique insights about these antitumor agents.

Older but Stronger: Development of Platinum-Based Antitumor Agents and Research Advances in Tumor Immunity

Platinum (Pt) drugs have developed rapidly in clinical applications because of their broad and highly effective antitumor effects. In recent years, with the rapid development of immunotherapy, Pt-based antitumor agents have gained new challenges and opportunities. Since the discovery of their pharmacological effects in immunotherapy and tumor microenvironment regulation, research into Pt drugs has progressed to multi-ligand and multi-functional Pt precursors and their own shortcomings have been further highlighted. With the development of antitumor immunotherapy and the rise of combination therapy, the development of Pt-based drugs has started to move in the direction of multi-targeting, nanocarrier modification, immunotherapy and photodynamic therapy. In this paper, we first overview the recent applications of Pt-based drugs in antitumor inorganic chemistry, with a focus on summarizing the application of Pt-based drugs and their precursors in the anticancer immune response. The paper also provides a reasonable outlook on the future development of Pt-based drugs from the chemical and immunological perspectives, relying on the existing content and problems of Pt-based drug development. On the basis of the gathered information, joint multidisciplinary programs on implementing comprehensive immune analyses for the future development of novel anticancer metal compounds should be initiated.

Electrochemical Detection of a Novel Pt(IV) Prodrug with the Metronidazole Axial Ligand in the Hypoxic Area

We report herein a Pt(IV) prodrug with metronidazole in axial positions Pt-Mnz. The nitroaromatic axial ligand was conjugated with a cisplatin scaffold to irreversibly reduce under hypoxic conditions, thereby retaining the Pt(IV) prodrug in the area of hypoxia. X-ray near-edge adsorption spectroscopy (XANES) on dried drug-preincubated tumor cell samples revealed a gradual release of cisplatin from the Pt-Mnz prodrug instead of rapid intracellular degradation. The ability of the prodrug to penetrate into three-dimensional (3D) spheroid cellular cultures was evaluated by a novel electrochemical assay via a platinum-coated carbon nanoelectrode, capable of single-cell measurements. Using a unique technique of electrochemical measurements in single tumor spheroids, we were able to both detect the real-time response of the axial ligand to hypoxia and establish the depth of penetration of the drug into the tumor model.

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.

BODIPY-dipicolylamine complexes of platinum(II): X-ray structure, cellular imaging and organelle-specific near-IR light type-II PDT

Dipicolylamine (dpa) based platinum(II) complexes [Pt(L1-3)Cl]Cl (1-3), where L2 and L3 are green and red light BODIPY-tagged dpa ligands and L1 is a benzyl derivative of dpa, were synthesized and characterized and their in vitro cytotoxicity was studied. The perchlorate salt of complex 2 was structurally characterized. It showed a PtN₃Cl core with a deformed square-planar geometry. At pH 7.2, complexes 2 and 3 showed strong absorption bands at 500 nm (ε ∼6.8 × 10⁴ dm³ mol^-1 cm^-1) and 653 nm (ε ∼1.0 × 10⁵ dm³ mol^-1 cm^-1) in a 1 : 1 (v/v) mixture of dimethyl sulfoxide and Dulbecco's phosphate-buffered saline (DMSO/DPBS), respectively. They displayed respective emission bands at 515 and 677 nm having fluorescence quantum yield values of 0.36 and 0.25. Complex 3 generated singlet oxygen, as evidenced from the 1,3-diphenylisobenzofuran titration experiments and mechanistic DNA photocleavage study. It showed high photocytotoxicity in red light (600-720 nm) with half-maximal inhibitory concentration (IC₅₀) values of 1.73 and 2.67 μM in HeLa and A549 cells. The complexes showed significantly reduced chemo-PDT activity in a non-cancerous HPL1D cell line and in the dark. The 2',7'-dichlorofluorescein diacetate assay revealed reactive oxygen species-mediated type-II photodynamic therapy (PDT) activity. Cellular imaging of A549 cancer cells using complexes 2 and 3 revealed their preferential localization in mitochondria and endoplasmic reticulum. The annexin V-FITC/PI assay confirmed apoptotic cell damage. Cell cycle analysis indicated arrest in the G1 phase upon red light irradiation. Pt-DNA adduct formation was proposed from a DNA binding experiment with green light active complex 2 and 9-ethylguanine as a nucleobase from the mass spectral study.

Inhibition of the HIF-1 Survival Pathway as a Strategy to Augment Photodynamic Therapy Efficacy

Photodynamic therapy (PDT) is a non-to-minimally invasive treatment modality that utilizes photoactivatable drugs called photosensitizers to disrupt tumors with locally photoproduced reactive oxygen species (ROS). Photosensitizer activation by light results in hyperoxidative stress and subsequent tumor cell death, vascular shutdown and hypoxia, and an antitumor immune response. However, sublethally afflicted tumor cells initiate several survival mechanisms that account for decreased PDT efficacy. The hypoxia inducible factor 1 (HIF-1) pathway is one of the most effective cell survival pathways that contributes to cell recovery from PDT-induced damage. Several hundred target genes of the HIF-1 heterodimeric complex collectively mediate processes that are involved in tumor cell survival directly and indirectly (e.g., vascularization, glucose metabolism, proliferation, and metastasis). The broad spectrum of biological ramifications culminating from the activation of HIF-1 target genes reflects the importance of HIF-1 in the context of therapeutic recalcitrance. This chapter elaborates on the involvement of HIF-1 in cancer biology, the hypoxic response mechanisms, and the role of HIF-1 in PDT. An overview of inhibitors that either directly or indirectly impede HIF-1-mediated survival signaling is provided. The inhibitors may be used as pharmacological adjuvants in combination with PDT to augment therapeutic efficacy.

A gemcitabine-based conjugate with enhanced antitumor efficacy by suppressing HIF-1α expression under hypoxia

Hypoxia is one of the unique features of tumor physiology. Hypoxia inducible factor (HIF-1α), as a major transcription factor in response to hypoxia, has been considered as a promising tumor-specific target for anticancer therapy. The formation of a hypoxic microenvironment in tumors can decrease the curative effect of cytotoxic chemotherapeutic drugs. To promote the antitumor efficacy of chemotherapy by suppressing hypoxia, we designed and prepared a novel gemcitabine-based drug conjugate (GEM-5) containing a HIF-1α inhibitor (YC-1). As expected, GEM-5 showed excellent antiproliferative activity (IC50 = 0.03 μΜ under hypoxia) and remarkably induced the apoptosis of A2780 cells in vitro. Additionally, western blot analysis demonstrated that GEM-5 significantly down-regulated the expression of HIF-1α and up-regulated the expression of tumor suppressor p53. More importantly, GEM-5 effectively inhibited tumor growth in the A2780 xenograft mouse model and significantly ameliorated tumor hypoxia in vivo. This novel, simple, and effective strategy for overcoming tumor hypoxia and enhancing the antitumor effect of chemotherapeutic drugs has great potential in cancer therapy.

Hypoxia-Activated Prodrug Enabling Synchronous Chemotherapy and HIF-1α Downregulation for Tumor Treatment

The overexpression of HIF-1α in solid tumors due to hypoxia is closely related to drug resistance and consequent treatment failure. Herein, we constructed a hypoxia-activated prodrug named as YC-Dox. This prodrug could be activated under hypoxic conditions and undergo self-immolation to release doxorubicin (Dox) and YC-1 hemisuccinate (YCH-1), which could execute chemotherapy and result in HIF-1α downregulation, respectively. This prodrug is capable of specifically releasing Dox and YCH-1 in response to hypoxia, leading to a substantial synergistic potency and a remarkable cytotoxic selectivity (>8-fold) for hypoxic cancer cells over normoxic healthy cells. The in vivo experiments reveal that this prodrug can selectively aim at hypoxic cancer cells and avoid undesired targeting of normal cells, leading to elevated therapeutic efficacy for tumor treatment and minimized adverse effects on normal tissues.

The design of cyclometalated iridium(iii)-metformin complexes for hypoxic cancer treatment

Modulating the hypoxic microenvironment is the priority for tumor treatment. Cytometalated iridium(iii)-metformin conjugates were synthesized for treating hypoxic cancer cells for the first time, which alleviate hypoxia via mitochondria respiration inhibition, thus displaying 10-fold higher cytotoxicity, emerging anti-metastasis and anti-inflammatory activities than a metformin-free Ir(iii) complex and cisplatin against hypoxic cancer cells.

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.

CAIXplatins: Highly Potent Platinum(IV) Prodrugs Selective Against Carbonic Anhydrase IX for the Treatment of Hypoxic Tumors

Hypoxia and the acidic microenvironment play a vital role in tumor metastasis and angiogenesis, generally compromising the chemotherapeutic efficacy. This provides a tantalizing angle for the design of platinum(IV) prodrugs for the effective and selective killing of solid tumors. Herein, two carbonic anhydrase IX (CAIX)-targeting platinum(IV) prodrugs have been developed, named as CAIXplatins. Based on their strong affinity for and inhibition of CAIX, CAIXplatins can not only overcome hypoxia and the acidic microenvironment, but also inhibit metabolic pathways of hypoxic cancer cells, resulting in a significantly enhanced therapeutic effect on hypoxic MDA-MB-231 tumors both in vitro and in vivo compared with cisplatin/oxaliplatin, accompanied with excellent anti-metastasis and anti-angiogenesis activities. Furthermore, the cancer selectivity indexes of CAIXplatins are 70-90 times higher than those of cisplatin/oxaliplatin with effectively alleviated side-effects.

Multifunctional Pt(iv) complexes containing a glutathione S-transferase inhibitor lead to enhancing anticancer activity and preventing metastasis of osteosarcoma cells

Cisplatin has been clinically applied in the treatment of osteosarcoma (OS), but its efficacy is severely limited due to drug resistance and metastasis. One of the chief culprits is the overexpression of glutathione S-transferases (GSTs) in cancer cells, which can accelerate the interaction of glutathione (GSH) with cisplatin, reducing its biological effects. In this study, three Pt(iv) complexes derived from cisplatin conjugated with a GST inhibitor (NBDHEX) were designed and synthesized. The stabilities and releasing capabilities of these complexes, as well as their abilities to inhibit GSTs, were investigated together with their in vitro anticancer activities toward osteosarcoma cells. Among them, complex 2, bearing one NBDHEX derivative and a hydroxyl group at the axial positions, could markedly kill human OS cells due to its suitable stability and prominent ability to inhibit GSTs. Meanwhile, it can prevent the metastasis of OS via down-regulating Akt. Thus, complex 2 has the potential for further research for the treatment of OS.

Hypoxia-activated nanomedicines for effective cancer therapy

Hypoxia, a common characteristic in solid tumors, is found in phenotypically aggressive cancers that display resistance to typical cancer interventions. Due to its important role in tumor progression, tumor hypoxia has been considered as a primary target for cancer diagnosis and treatment. An advantage of hypoxia-activated nanomedicines is that they are inactive in normoxic cells. In hypoxic tumor tissues and cells, these nanomedicines undergo reduction by activated enzymes (usually through 1 or 2 electron oxidoreductases) to produce cytotoxic substances. In this review, we will focus on approaches to design nanomedicines that take advantage of tumor hypoxia. These approaches include: i) inhibitors of hypoxia-associated signaling pathways; ii) prodrugs activated by hypoxia; iii) nanocarriers responsive to hypoxia, and iv) bacteria mediated hypoxia targeting therapy. These strategies have guided and will continue to guide nanoparticle design in the near future. These strategies have the potential to overcome tumor heterogeneity to improve the efficiency of radiotherapy, chemotherapy and diagnosis.

Niacin-ligated platinum(iv)-ruthenium(ii) chimeric complexes synergistically suppress tumor metastasis and growth with potentially reduced toxicity in vivo

Niacin-ligated platinum(iv)-ruthenium(ii) chimeric complexes (PtRu 1-4) have been synthesized and evaluated for their antitumor performance. Using the optimal complex, PtRu-1, we show that this water-soluble chimeric prodrug not only potently inhibits the metastasis and proliferation of tumor cells but also has an unexpectedly higher safety margin in animals compared with the traditionally-used, clinically approved drug cisplatin.

Interfering in apoptosis and DNA repair of cancer cells to conquer cisplatin resistance by platinum(iv) prodrugs

Platinum(iv) prodrugs targeting the DNA repair mechanism downregulate myeloid cell leukemia-1 (Mcl-1) and homologous recombination proteins (RAD51, BRCA2), thereby enhancing cytotoxicity against cisplatin-resistant cancer cells.

Approaches to combat hypoxia in cancer therapy and the potential for in silico models in their evaluation

AIM

The negative impact of tumour hypoxia on cancer treatment outcome has been long-known, yet there has been little success combating it. This paper investigates the potential role of in silico modelling to help test emerging hypoxia-targeting treatments in cancer therapy.

METHODS

A Medline search was undertaken on the current landscape of in silico models that simulate cancer therapy and evaluate their ability to test hypoxia-targeting treatments. Techniques and treatments to combat tumour hypoxia and their current challenges are also presented.

RESULTS

Hypoxia-targeting treatments include tumour reoxygenation, hypoxic cell radiosensitization with nitroimidazoles, hypoxia-activated prodrugs and molecular targeting. Their main challenges are toxicity and not achieving adequate delivery to hypoxic regions of the tumour. There is promising research toward combining two or more of these techniques. Different types of in silico therapy models have been developed ranging from temporal to spatial and from stochastic to deterministic models. Numerous models have compared the effectiveness of different radiotherapy fractionation schedules for controlling hypoxic tumours. Similarly, models could help identify and optimize new treatments for overcoming hypoxia that utilize novel hypoxia-targeting technology.

CONCLUSION

Current therapy models should attempt to incorporate more sophisticated modelling of tumour angiogenesis/vasculature and vessel perfusion in order to become more useful for testing hypoxia-targeting treatments, which typically rely upon the tumour vasculature for delivery of additional oxygen, (pro)drugs and nanoparticles.

Emerging platinum(iv) prodrugs to combat cisplatin resistance: from isolated cancer cells to tumor microenvironment

Cisplatin plays a pivotal role in the treatment of various malignant tumors, but its therapeutic effects are hampered by drug resistance. Pt(iv) prodrugs represent a promising class of "non-conventional" platinum-based anticancer agents to circumvent drug resistance, which can be easily functionalized with other bioactive ligands. One strategy is to build "dual-action" and "multi-action" Pt(iv) prodrugs that not only damage DNA but also perturb other pathways related to cisplatin resistance to achieve combinatorial therapeutic effects. Another way to overcome the shortcomings of cisplatin is to deliver Pt(iv) prodrugs via nanocarriers. Most studies in this area have focused on designing prodrugs based on the mechanism of cisplatin resistance within isolated cancer cells. Recent findings, however, reveal that the tumor microenvironment also plays important roles in the development of cisplatin resistance. This perspective focuses on various types of novel cisplatin-based Pt(iv) complexes, including Pt-loaded nanostructures, to overcome cisplatin resistance. Special attention will be devoted to complexes that target the tumor microenvironment, which is a new area for the development of effective Pt(iv) prodrugs. Our summary and outlook may have a hope to help researchers in the field generate new ideas and strategies to develop more potent Pt(iv) prodrugs to combat cisplatin resistance.

Fuplatin: An Efficient and Low-Toxic Dual-Prodrug

As FDA-approved chemotherapeutic agents, cisplatin, oxaliplatin, and 5-fluorouracil are widely used in clinic but limited by severe side-effects. To ameliorate their respective defects, a series of "dual-prodrug" by linking oxoplatin and 5-FU were designed and synthesized. The assembled compounds 10-17, named Fuplatin, exhibited much higher cytotoxicity against the tested cancer cells while lower cytotoxicity toward the human normal lung cells than free drugs or their combinations. Among them, 14 enhanced cellular accumulation with 62- and 825-fold amount of oxaliplatin and 8 at 9 h, respectively, significantly induced DNA damage and cell apoptosis, and inhibited migration and invasion in HCT-116 cells. Compound 14 arrested the cell cycle at S and G2 phases and up-regulated thymidylate synthase and p53, consistent with the results of the combination, suggesting 14 adopted a collaborative mode of 5-FU and oxaliplatin to kill cancer cells. In vivo, compound 14 showed high antitumor effect and no observable toxicity in NOD/SCID mice bearing HCT-116 tumors.

A Pt(iv)-mediated polymer architecture for facile and stimuli-responsive intracellular gene silencing with chemotherapy

Conventional chemotherapy has been impeded by the inherent characteristics of cancer including fast mutagenesis and drug resistance; thus a combination therapy consisting of multiple therapeutic strategies has attracted much attention. However, the loading processes of multiple therapeutic molecules affect each other; thus the development of a nanocarrier that enables independent loading of the cargo molecules has been demanded. Herein, we report an ingeniously designed Pt(iv)-mediated polymeric architecture (Pt-PA) for combinatorial gene and chemotherapy to address the issue, prepared by crosslinking a cationic polymer (polyethylenimine, PEI) with a Pt(iv) prodrug. Therapeutic siRNA (anti-BCL2) was simply loaded by electrostatic interaction to form a stable nanocomplex. In the cellular study, the simultaneous release of both the active Pt(ii) drug and siRNA was monitored under the intracellular reducing environment, driven by dissociation of the polymer architecture due to an inherent characteristic of the Pt(iv) crosslinker. Therefore, an enhanced gene silencing effect and an anticancer effect were observed. Furthermore, in the animal study, an improved therapeutic effect of the nanocomplex was observed, which can be explained by tumor targeting via the EPR effect, and enhanced drug and siRNA release at the intracellular environment simultaneously. Taken together, the overall results from in vitro and in vivo studies strongly suggest the therapeutic potential of our precisely designed Pt(iv)-mediated polymer architecture.

Toward Multi-Targeted Platinum and Ruthenium Drugs-A New Paradigm in Cancer Drug Treatment Regimens?

While medicinal inorganic chemistry has been practised for over 5000 years, it was not until the late 1800s when Alfred Werner published his ground-breaking research on coordination chemistry that we began to truly understand the nature of the coordination bond and the structures and stereochemistries of metal complexes. We can now readily manipulate and fine-tune their properties. This had led to a multitude of complexes with wide-ranging biomedical applications. This review will focus on the use and potential of metal complexes as important therapeutic agents for the treatment of cancer. With major advances in technologies and a deeper understanding of the human genome, we are now in a strong position to more fully understand carcinogenesis at a molecular level. We can now also rationally design and develop drug molecules that can either selectively enhance or disrupt key biological processes and, in doing so, optimize their therapeutic potential. This has heralded a new era in drug design in which we are moving from a single- toward a multitargeted approach. This approach lies at the very heart of medicinal inorganic chemistry. In this review, we have endeavored to showcase how a "multitargeted" approach to drug design has led to new families of metallodrugs which may not only reduce systemic toxicities associated with modern day chemotherapeutics but also address resistance issues that are plaguing many chemotherapeutic regimens. We have focused our attention on metallodrugs incorporating platinum and ruthenium ions given that complexes containing these metal ions are already in clinical use or have advanced to clinical trials as anticancer agents. The "multitargeted" complexes described herein not only target DNA but also contain either vectors to enable them to target cancer cells selectively and/or moieties that target enzymes, peptides, and intracellular proteins. Multitargeted complexes which have been designed to target the mitochondria or complexes inspired by natural product activity are also described. A summary of advances in this field over the past decade or so will be provided.

Promoting antitumor efficacy by suppressing hypoxia via nano self-assembly of two irinotecan-based dual drug conjugates having a HIF-1α inhibitor

Conjugated a hydrophobic HIF-1α inhibitor (YC-1) with a hydrophilic anticancer drug, irinotecan (Ir), into one molecular entity, provided a new insight into the design of smart nanodrug delivery systems to hypoxic tumor tissue.

Squaramide-Based Pt(II) Complexes as Potential Oxygen-Regulated Light-Triggered Photocages

Two new squaramide-based platinum(II) complexes C1 and C2 have been synthesized and fully characterized. Their photoresponse has been assessed and is discussed. A remarkable enhancement in the DNA binding activity has been observed for both complexes, up on irradiation. For C2, the release of Pt(II) provoked by its irradiation has been studied. The response of C2 has been found to be regulated by the presence of oxygen. In vitro cytotoxicity tests show an enhancement in the activity of complex C2 after selective irradiation under hypoxic conditions. Resulting Pt(II) species have been isolated and characterized by various analytical methods establishing this type of squaramido-based complexes as a proof of concept for new Pt(II) photocages.

Current Developments in Pt(IV) Prodrugs Conjugated with Bioactive Ligands

To overcome the side effects of and resistance to cisplatin, a variety of Pt(IV) prodrugs were designed and synthesized via different modifications including combination with lipid chains to increase hydrophobicity, conjugation with short peptide chains or nanoparticles to improve drug delivery, or addition of bioactive ligands to the axial positions of Pt(IV) complexes to exert dual-function effects. This review summarizes the recent progress in the development of Pt(IV) prodrugs conjugated with bioactive-targeting ligands, including histone deacetylase inhibitors, p53 agonists, alkylating agents, and nonsteroidal anti-inflammatory agents. Although Pt(IV) complexes that conjugated with bioactive ligands show satisfactory anticancer effects, none has been approved for clinical use. Therefore, we hope that this review will contribute to further study and development of Pt(IV) complexes conjugated with bioactive and other ligands.

Hypoxia-Activated Anticancer Prodrug for Bioimaging, Tracking Drug Release, and Anticancer Application

A novel anticancer theranostic prodrug, FDU-DB-NO₂, specifically activated by hypoxia for selective two-photon imaging hypoxia status, real-time tracking drug release, and solid tumor therapy was designed. The devised prodrug consists of an anticancer drug floxuridine (FDU), a fluorescence dye precursor 4'-(diethylamino)-1,1'-biphenyl-2-carboxylate (DB), and a hypoxic trigger 4-nitrobenzyl group. In normal cells, FDU-DB-NO₂ is "locked". Whereas in tumor cells, the prodrug is "unlocked" by hypoxia and results in fluorescent dye 7-(diethylamino)coumarin (CM) generation along with FDU release. The amounts and rates of CM formation and FDU release were controlled by hypoxic status and increased with the decreasing of the O₂ concentration. The hypoxic status, distribution of oxygen, and amount of FDU release in tumor cells, spheroids, and tumor tissue could be visualized by fluorescence. FDU-DB-NO₂ showed high cytotoxicity against hypoxic MCF-7 and MCG-803 cell lines and no cytotoxicity against normoxic BRL-3A cells and exhibited effective inhibition on tumor growth of MCF-7-cell-inoculated xenograft nude mice. This strategy may provide a promising platform for selective two-photon imaging hypoxia status, real-time tracking drug release, and personalized solid tumor treatment.

Chlorambucil-conjugated platinum(IV) prodrugs to treat triple-negative breast cancer in vitro and in vivo

Modification of platinum (II) into lipophilic platinum (IV) compounds by introducing biologically active molecules were widely employed to develop new platinum-based prodrugs in the past decade. In this paper, two chlorambucil platinum (IV) complexes, CLB-Pt and CLB-Pt-CLB, were synthesized and displayed very potent antiproliferative activity against all the tested cancer cell lines, such as A549, HeLa and MCF-7, especially to treat the well-known refractory triple-negative breast cancer. CLB-Pt-CLB significantly improved cell-killing effect in triple-negative subtype MDA-MB-231 cells, and showed much stronger cytotoxicity than either monotherapy or combination of cisplatin and chlorambucil. CLB-Pt-CLB prodrug entered cells in dramatically increased amount compared with cisplatin and enhanced DNA damage, inducing cancer cell apoptosis. It exhibited high anticancer activity and no observable toxicity in BALB/c nude mice bearing MDA-MB-231 tumors. The chlorambucil moiety not only greatly assisted the passive diffusion of CLB-Pt-CLB into cells, but also produced the synergism with cisplatin in targeting DNA.

Combination of 7-hydroxycoumarin in a platinum(IV) complex derived from cisplatin enhanced cytotoxicity with multiple mechanisms of action

A novel compound, Cou-platin, composed of 7-hydroxycoumarin and a platinum(IV) moiety derived from cisplatin was designed and synthesized. Significantly, Cou-platin exhibited more potent in vitro antitumor activity against all tested cancer cell lines than that of cisplatin, which was mainly attributed to the liberation of cisplatin and 7-hydroxycoumarin upon reduction with a biomolecular agent. Besides, cellular accumulation of Cou-platin was dramatically increased among several cancer cells in contrast to cisplatin. Flow cytometry study revealed that Cou-platin arrested cell cycle at G2 phase and induced cell apoptosis. Western blots results indicated that it not only activated cell apoptosis pathway, but also inhibited extracellular regulated protein kinases/mitogen-activated protein kinase pathway. In vivo tests showed that Cou-platin, at equimolar dose to cisplatin, could inhibit tumor growth in nude mouse HCT116 tumor xenograft models almost as cisplatin and oxaliplatin, but with less toxicity.

Reduction of ormaplatin by an extended series of thiols unravels a remarkable correlation

Reduction of the Pt(iv) anticancer active prodrug ormaplatin by an extended series of thiols has been studied, revealing a remarkable linear free-energy correlation.

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.

Platinum(IV) prodrugs multiply targeting genomic DNA, histone deacetylases and PARP-1

Several Pt(IV) prodrugs containing SAA, a histone deacetylases inhibitor, were designed and prepared for multiply targeting genomic DNA, histone deacetylases and PARP-1. The resulting Pt(IV) prodrug had significantly strong antiproliferative activity against the tested cancer cell lines, especially SAA1, derived from the conjugation of cisplatin and SAA, had potent ability to overcome cisplatin resistance. Under the combined action of DNA platination and inhibition of HDACs and PARP-1 activity, the cytotoxic activity of SAA1 was 174-fold higher than cisplatin against cisplatin-resistant SGC7901/CDDP cancer cells. The mechanism of action of SAA1 was preliminarily investigated, in which cellular uptake, cell apoptosis and cell cycle arrest as well as western blot analysis were made by treating SAA1 with SGC7901/CDDP cells. Besides, HDACs inhibition activity and PARP-1 enzyme inhibition of SAA1 were also studied.