Half-sandwich IridiumIII N-heterocyclic carbene antitumor complexes and biological applications

A promising future of metal-N-heterocyclic carbene complexes in medicinal chemistry: The emerging bioorganometallic antitumor agents

Metal complexes based on N-heterocyclic carbene (NHC) ligands have emerged as promising broad-spectrum antitumor agents in bioorganometallic medicinal chemistry. In recent decades, studies on cytotoxic metal-NHC complexes have yielded numerous compounds exhibiting superior cytotoxicity compared to cisplatin. Although the molecular mechanisms of these anticancer complexes are not fully understood, some potential targets and modes of action have been identified. However, a comprehensive review of their biological mechanisms is currently absent. In general, apoptosis caused by metal-NHCs is common in tumor cells. They can cause a series of changes after entering cells, such as mitochondrial membrane potential (MMP) variation, reactive oxygen species (ROS) generation, cytochrome c (cyt c) release, endoplasmic reticulum (ER) stress, lysosome damage, and caspase activation, ultimately leading to apoptosis. Therefore, a detailed understanding of the influence of metal-NHCs on cancer cell apoptosis is crucial. In this review, we provide a comprehensive summary of recent advances in metal-NHC complexes that trigger apoptotic cell death via different apoptosis-related targets or signaling pathways, including B-cell lymphoma 2 (Bcl-2 family), p53, cyt c, ER stress, lysosome damage, thioredoxin reductase (TrxR) inhibition, and so forth. We also discuss the challenges, limitations, and future directions of metal-NHC complexes to elucidate their emerging application in medicinal chemistry.

Recent Advancement in the Synthesis of Ir-Based Complexes

Cancer is a devastating disease with over 100 types, including lung and breast cancer. Cisplatin and metal-based drugs are limited due to their drug resistance and side effects. Iridium-based compounds have emerged as promising candidates due to their unique chemical properties and resemblance to platinum compounds. The objective of this study is to investigate the synthesis and categorization of iridium complexes, with a particular emphasis on their potential use as anticancer agents. The major focus of this research is to examine the synthesis of these complexes and their relevance to the field of cancer treatment. The negligible side effects and flexibility of cyclometalated iridium(III) complexes have garnered significant interest. Organometallic half-sandwich Ir(III) complexes have notable benefits in cancer research and treatment. The review places significant emphasis on categorizing iridium complexes according to their ligand environment, afterward considering the ligand density and coordination number. This study primarily focuses on several methods for synthesizing cyclometalated and half-sandwich Ir complexes, divided into subgroups based on ligand denticity. The coordination number of iridium complexes determines the number of ligands coordinated to the central iridium atom, which impacts their stability and reactivity. Understanding these complexes is crucial for designing compounds with desired properties and investigating their potential as anticancer agents. Cyclometalated iridium(III) complexes, which contain a meta-cycle with the E-M-C order σ bond, were synthesized in 1999. These complexes have high quantum yields, significant stock shifts, luminescence qualities, cell permeability, and strong photostability. They have been promising in biosensing, bioimaging, and phosphorescence of heavy metal complexes.

Mitochondrial targeting half-sandwich iridium(III) and ruthenium(II) dppf complexes and in vitro anticancer assay

Considering the potential application of half-sandwich and ferrocenyl-containing organometallic complexes in the area of anticancer, four half-sandwich iridium(III) (Ir^III) and ruthenium(II) (Ru^II) diphenylphosphino ferrocene (dppf) complexes were prepared in this study. Complexes showed favorable anti-proliferation activity towards A549 cell lines compared to cisplatin, meanwhile, which could effectively inhibit cell migration. These complexes followed an energy dependence uptake mechanism, effectively accumulated in mitochondria with a Pearson's Colocalization Coefficient (PCC) of 0.77, decreased the mitochondrial membrane potential, induced a surge of reactive oxygen species, disturbed cell cycle, and eventually led to apoptosis. Western blot assay further confirmed that these complexes induced apoptosis following a mitochondrial pathway. Above all, half-sandwich Ir^III and Ru^II dppf complexes show the prospect of becoming a new multifunctional therapeutic platform for mitochondrial targeted imaging and anticancer drugs.

Current status of iridium-based complexes against lung cancer

Lung cancer is one of the most common malignant tumors, with the highest mortality rate in the world, and its incidence is second only to breast cancer. It has posed a serious threat to human health. Cisplatin, a metal-based drug, is one of the most widely used chemotherapeutic agents for the treatment of various cancers. However, its clinical efficacy is seriously limited by numerous side effects and drug resistance. This has led to the exploration and development of other transition metal complexes for the treatment of malignant tumors. In recent years, iridium-based complexes have attracted extensive attention due to their potent anticancer activities, limited side effects, unique antitumor mechanisms, and rich optical properties, and are expected to be potential antitumor drugs. In this review, we summarize the recent progress of iridium complexes against lung cancer and introduce their anti-tumor mechanisms, including apoptosis, cycle arrest, inhibition of lung cancer cell migration, induction of immunogenic cell death, etc.

A Therapeutic Journey of Pyridine-Based Heterocyclic Compounds as Potent Anticancer Agents: A Review (From 2017 to 2021)

Pyridine derivatives are the most common and significant heterocyclic compounds, which show their fundamental characteristics to various pharmaceutical agents and natural products. Pyridine derivatives possess several pharmacological properties and a broad degree of structural diversity that is considered most valuable to explore the novel therapeutic agents. These compounds have an extensive range of biological activities such as antifungal, antibacterial, anticancer, anti-obesity, anti-inflammatory, antitubercular, antihypertensive, antineuropathic, antihistaminic, antiviral activities, and antiparasitic. The potent therapeutic properties of pyridine derivatives allow medicinal chemists to synthesize novel and effective chemotherapeutic agents. Consequently, the imperative objective of this comprehensive review is to summarize and investigate the literature regarding recent advancements in pyridine-based heterocycles to treat several kinds of cancer. Furthermore, the performances of pyridine derivatives were compared with some standard drugs including etoposide, sorafenib, cisplatin, and triclosan against different cancer cell lines. We hope this study will support the new thoughts to pursue the most active and less toxic rational designs.

Triazolyl-Functionalized N-Heterocyclic Carbene Half-Sandwich Compounds: Coordination Mode, Reactivity and in vitro Anticancer Activity

We report investigations on the anticancer activity of organometallic [M^II/III (η⁶ -p-cymene/η⁵ -pentamethylcyclopentadienyl)] (M=Ru, Os, Rh, and Ir) complexes of N-heterocyclic carbenes (NHCs) substituted with a triazolyl moiety. Depending on the precursors, the NHC ligands displayed either mono- or bidentate coordination via the NHC carbon atom or as N,C-donors. The metal complexes were investigated for their stability in aqueous solution, with the interpretation supported by density functional theory calculations, and reactivity to biomolecules. In vitro cytotoxicity studies suggested that the nature of both the metal center and the lipophilicity of the ligand determine the biological properties of this class of compounds. The Ir^III complex 5 d bearing a benzimidazole-derived ligand was the most cytotoxic with an IC₅₀ value of 10 μM against NCI-H460 non-small cell lung carcinoma cells. Cell uptake and distribution studies using X-ray fluorescence microscopy revealed localization of 5 d in the cytoplasm of cancer cells.

Synthesis, characterization and catalytic properties of cationic N-heterocyclic carbene silver complexes

Three new dibenzimidazolium salts bridged by 2-methylenepropane-1,3-diyl group were synthesized. Their dinuclear N-heterocyclic carbene Ag(I) complexes were prepared by the reactions of these salts with Ag2O. The structures of the synthesized compounds were defined by nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FT-IR), elemental analysis, and LC-MSMS (for complexes) techniques. Stability of the silver complexes was confirmed by 1H NMR spectroscopy. Catalytic activities of Ag(I) compounds were tested for three-component coupling reaction of some aldehydes, amines, and phenylacetylene.

Evaluation of anticancer effects in vitro of new iridium(III) complexes targeting the mitochondria

Iridium(III) complexes have the potential to serve as novel therapeutic drugs for treating tumor. In this work, three new complexes [Ir(ppy)₂(cdppz)](PF₆) (1) (ppy = 2-phenylpyridine, cdppz = 11-chlorodipyrido[3,2-a,2',3'-c]phenazine), [Ir(bzq)₂(cdppz)](PF₆) (2) (bzq = benzo[h]quinolone) and [Ir(piq)₂(cdppz)](PF₆) (3) (piq = 1-phenylisoquinoline) were prepared as well as characterized. MTT (3-(4,5-dimethylthiazole)-2,5-diphenyltetraazolium bromide) assay revealed that the complex 2 exerted potent cytotoxicity against to various cancer cells lines and particularly for SGC-7901 cells. Meanwhile, the complexes could suppress cell colonies formation and migration ability. Apoptosis assays of AO/EB staining as well as flow cytometry revealed that the synthesized complexes may cause apoptosis of SGC-7901 cells. Moreover, the decline of mitochondrial membrane potential (MMP), elevation of intracellular reactive oxygen species (ROS) levels and release of cytochrome c demonstrated the complexes could cause apoptosis mainly through the mitochondrial death pathway and arrest cell at G0/G1 phase. Additionally, the complexes have significant influence on the expression of proteins which is interrelated to cell apoptosis. In summary, our studies provided fundamental information regarding the further study of the possible anticancer mechanisms of iridium (III) complexes.

Lysosome-targeted chemotherapeutics: Anticancer mechanism of N-heterocyclic carbene iridium(III) complex

N-heterocyclic carbenes-modified half-sandwich iridium(III) complex [(η⁵-C₅Me₄C₆H₄C₆H₅)Ir(C^C)Cl]PF₆ (C1) (where C^C is a N-heterocyclic carbene ligand) can effectively prevent the proliferation of human cervical cancer cells. Here, this study aims to investigate the in-deep anticancer effects of this complex on non-small cell lung cancer cells and explore the underlying molecular mechanism. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay showed that iridium(III) complex had potent cytotoxicity studies towards non-small cell lung cancer cells (A549), human lung squamous cells (L78), human cervical cancer cells (Hela) and human bronchial epithelial cells (BEAS-2B). Colocalization and cellular uptake studies were analyzed by confocal microscopy. Notably, C1 targeted lysosomes and entered the cancer cells partially through an energy-dependent pathway, inducing the release of cathepsins and other proteins. These proteins regulated lysosomal-mitochondrial dysfunction, thus leading to the release of cytochrome c (cyt c), which amplified apoptotic signals by activating many downstream pathways such as caspase pathways to promote cell apoptosis. The results showed that the inhibitory mechanism of this organometallic iridium(III) complex may involve caspase-associated apoptosis initiated by the lysosomal-mitochondrial pathway.

Imidazole and Benzimidazole Modified Half-Sandwich IridiumIII N-Heterocyclic Carbene Complexes: Synthesis, Anticancer Application, and Organelle Targeting

Herein, we report the synthesis, characterization and anticancer activity of a series of half-sandwich iridium^III imidazole and benzimidazole N-heterocyclic carbene (NHC) anticancer complexes, and the general formula of which can be expressed as [(η⁵-Cp^x)Ir(C^∧N)Cl]Cl (Cp^x: pentamethylcyclopentadienyl (Cp^*) or biphenyl derivatives (Cp^xbiph); C^∧N: imidazole and benzimidazole NHC chelating ligands). Compared with cis-platin, these complexes showed interesting antitumor activity against A549 cells. Complexes could bind to bovine serum albumin (BSA) by means of static quenching mode, catalyze the oxidation of nicotinamide adenine dinucleotide (NADH) and increase the levels of reactive oxygen species (ROS). Meanwhile, these complexes could arrest the cell cycles of A549 cells and influence the mitochondrial membrane potential significantly. Due to the inherent luminescence property, laser confocal test show that complexes could enter cells followed an energy-dependent mechanism and effectively accumulate in lysosome (the value of Pearson's co-localization coefficient is 0.70 after 1 h), further destroy lysosome integrity and induce apoptosis.

Potential anticancer agent for selective damage to mitochondria or lysosomes: Naphthalimide-modified fluorescent biomarker half-sandwich iridium (III) and ruthenium (II) complexes

In this work, five naphthalimide-modified half-sandwich iridium and ruthenium complexes ([(η⁵-Cp^x)Ir(NˆN)Cl]PF₆, [(η⁶-p-cym)Ru(NˆN)Cl]PF₆) have been presented. The anticancer activities of the complexes against various cancer cell lines were investigated, among them, complexes 2 and 4 showed better anticancer activity than cisplatin, and their anticancer activity is better than complex 5 without fluorophore. In addition, a series of biological tests of complex 2 were performed using flow cytometry, the results indicated that the complex could induce cell death in a variety of ways. By changing of the ligands, the complexes exhibited different photophysical properties, and the mechanism of action of the complexes entering the cell and inducing apoptosis are different. Moreover, complex 2 successfully targeted mitochondria, while complex 4 targeted lysosomes, causing mitochondrial damage and lysosomal damage to induce apoptosis. Excitingly, complex 2 has good antimetastatic ability to cancer cells. Furthermore, complexes 2 and 4 did not have a significant effect on the NADH binding reaction, but they had a moderate binding ability to BSA.

Mitochondria targeted and NADH triggered photodynamic activity of chloromethyl modified Ru(ii) complexes under hypoxic conditions

Three chloromethyl-modified Ru(ii) complexes were designed and synthesized as mitochondria targeting photosensitizers, which can generate carbon radicals in the presence of NADH under visible light irradiation, cause DNA cleavage and covalent binding in Ar-saturated solutions, and lead to apoptosis of human ovarian carcinoma SKOV-3 cells under hypoxic conditions (3% O2), demonstrating a new mode of type I mechanism to overcome the limitation of hypoxia in photodynamic therapy (PDT).

Rhodamine-modified fluorescent half-sandwich iridium and ruthenium complexes: potential application as bioimaging and anticancer agents

Most half-sandwich metal anticancer complexes are non-fluorescent, which results in an uncertain mechanism of action (MoA).

Triphenyltin(IV) acylhydrazone compounds: Synthesis, structure and bioactivity

Four new triphenyltin(IV) acylhydrazone compounds of the type Ph₃SnCH₂CH₂CONHN=R (where Ph = phenyl; R = isopropyl, isobutyl, cyclopentyl and cyclooctyl) were synthesized and characterized by elemental analysis, infrared spectrum (IR), nuclear magnetic resonance spectrum (NMR) and mass spectrum (MS). The crystal structures were determined and showed that tin atoms were four-coordinated and adopted a pseudo-tetrahedron configuration. Tin(IV) compounds show excellent bovine serum albumin (BSA) binding properties, and can oxidize nicotinamide-adenine dinucleotid (NADH) to generate reactive oxygen species (ROS), which inducing apoptosis effectively. Bioassay results indicated that tin(IV) compounds have stronger cytotoxic activity against A549 human lung cancer cells compared with cis-platin used clinically, and showing some selectivity.

Dual-functional cyclometalated iridium imine NHC complexes: highly potent anticancer and antimetastatic agents

Six cyclometalated iridium(iii) complexes have been synthesized and characterized. They showed strong anticancer and antimetastatic activity.

Highly potent half-sandwich iridium and ruthenium complexes as lysosome-targeted imaging and anticancer agents

A new class of half-sandwich Ir and Ru compounds containing P^P-chelating ligands can be developed as potential multifunctional theranostic platforms that combine bioimaging and anticancer capabilities.