Mono- and Dinuclear Phosphorescent Rhenium(I) Complexes: Impact of Subcellular Localization on Anticancer Mechanisms.

Elucidation of relationship among chemical structure, cellular uptake, localization, and biological activity of anticancer metal complexes is important for the understanding of their mechanisms of action. Organometallic rhenium(I) tricarbonyl compounds have emerged as potential multifunctional anticancer drug candidates that can integrate therapeutic and imaging capabilities in a single molecule. Herein, two mononuclear phosphorescent rhenium(I) complexes (Re1 and Re2), along with their corresponding dinuclear complexes (Re3 and Re4), were designed and synthesized as potent anticancer agents. The subcellular accumulation of Re1-Re4 was conveniently analyzed by confocal microscopy in situ in live cells by utilizing their intrinsic phosphorescence. We found that increased lipophilicity of the bidentate ligands could enhance their cellular uptake, leading to improved anticancer efficacy. The dinuclear complexes were more potent than the mononuclear counterparts. The molecular anticancer mechanisms of action evoked by Re3 and Re4 were explored in detail. Re3 with a lower lipophilicity localizes to lysosomes and induces caspase-independent apoptosis, whereas Re4 with higher lipophilicity specially accumulates in mitochondria and induces caspase-independent paraptosis in cancer cells. Our study demonstrates that subcellular localization is crucial for the anticancer mechanisms of these phosphorescent rhenium(I) complexes.

Anticancer Cyclometalated Iridium(III) Complexes with Planar Ligands: Mitochondrial DNA Damage and Metabolism Disturbance.

Emerging studies have shown that mitochondrial DNA (mtDNA) is a potential target for cancer therapy. Herein, six cyclometalated Ir(III) complexes Ir1-Ir6 containing a series of extended planar diimine ligands have been designed and assessed for their efficacy as anticancer agents. Ir1-Ir6 show much higher cytotoxicity than cisplatin and they can effectively localize to mitochondria. Among them, complexes Ir3 and Ir4 with dipyrido[3,2- a:2',3'- c]phenazine (dppz) ligands can bind to DNA tightly in vitro, intercalate to mtDNA in situ, and induce mtDNA damage. Ir3- and Ir4-impaired mitochondria exhibit decline of mitochondrial membrane potential, disability of adenosine triphosphate generation, disruption of mitochondrial energetic and metabolic status, which subsequently cause protective mitophagy, G0/G1 phase cell cycle arrest, and apoptosis. In vivo antitumor evaluations also show that Ir4 can inhibit tumor xenograft growth effectively. Overall, our work proves that targeting the mitochondrial genome may present an effective strategy to develop metal-based anticancer agents to overcome cisplatin resistance.

Inhibition of autophagic flux by cyclometalated iridium(iii) complexes through anion transportation.

Synthetic anion transporters that can interfere with the intracellular pH homeostasis are gaining increasing attention for tumor therapy, however, the biological mechanism of anion transporters remains to be explored. In this work, two phosphorescent cyclometalated Ir(iii) complexes containing 2-phenylpyridine (ppy) as the cyclometalated ligand, and 2,2'-biimidazole (H2biim, Ir1) or 2-(1H-imidazol-2-yl)pyridine (Hpyim, Ir2) as the ancillary ligands have been synthesized and characterized. Due to the protonation and deprotonation process of the N-H groups on H2biim and Hpyim, Ir1 and Ir2 display pH-dependent phosphorescence and can specifically image lysosomes. Both Ir1 and Ir2 can act as anion transporters mainly through the anion exchange mechanism with higher potency observed for Ir1. Mechanism investigation shows that Ir1 and Ir2 can induce caspase-independent cell death through reactive oxygen species (ROS) elevation. As Ir1 and Ir2 can alkalinize lysosomes through anion disturbance, they can inhibit autophagic flux. Our work provides a novel anticancer mechanism of metal complexes, which gives insights into the innovative structure-based design of new metallo-anticancer agents.

Light-Up Mitophagy in Live Cells with Dual-Functional Theranostic Phosphorescent Iridium(III) Complexes.

Phosphorescent Ir(III) complexes are expected to be new multifunctional theranostic platforms that enable the integration of imaging capabilities and anticancer properties. Mitophagy is an important selective autophagic process that degrades dysfunctional mitochondria. Until now, the regulation of mitophagy is still poorly understood. Herein, we present two phosphorescent cyclometalated iridium(III) complexes (Ir1 and Ir2) that can accumulate in mitochondria and induce mitophagy. Because of their intrinsic phosphorescence, they can specially image mitochondria and track mitochondrial morphological alterations. Mechanism studies show that Ir1 and Ir2 induce mitophagy by depolarization of mitochondrial membrane potential, depletion of cellular ATP, perturbation in mitochondrial metabolic status, and induction of oxidative stress. Moreover, no sign of apoptosis is observed in Ir1- and Ir2-treated cells under the same conditions that an obvious mitophagic response is initiated. We demonstrate that Ir1 is a promising theranostic agent that can induce mitophagy and visualize changes in mitochondrial morphology simultaneously.

Tumor-targeted supramolecular nanoparticles self-assembled from a ruthenium-ß-cyclodextrin complex and an adamantane-functionalized peptide.

We report here a supramolecular strategy to assemble a cyclodextrin-functionalized anticancer Ru(ii) complex with an adamantane-appended tumor-targeting peptide into discrete and stable phosphorescent nanostructures that can induce cell death in integrin αvß3-rich tumor cells with high selectivity. This strategy presents new opportunities for the construction of tumor-targeting metallo-anticancer therapeutics.

Dual Functions of Cyclometalated Iridium(III) Complexes: Anti-Metastasis and Lysosome-Damaged Photodynamic Therapy.

Four phosphorescent cyclometalated iridium(III) complexes containing benzimidazole moiety have been designed and synthesized. These Ir(III) complexes can effectively inhibit several cancerous processes, including cell migration, invasion, colony formation, and angiogenesis. Interestingly, they show a much higher singlet oxygen quantum yield in an acidic solution than in a neutral solution. Upon irradiation at 425 nm with low energy (1.2 J cm-2), they can induce apoptosis through lysosomal damage, evaluation of reactive oxygen species level, and activation of caspase-3/7. The highest phototoxicity index is >476, with almost no dark cytotoxicity observed for Ir4. Ir4 can also inhibit tumor growth effectively in nude mice in vivo after photodynamic therapy. An in vitro assay against 70 kinases indicates that maternal embryonic leucine zipper kinase (MELK), PIK3CA, and AMPK are the possible molecular targets. The half maximal inhibitory concentration of Ir4 toward MELK is 1.27 µM. Our study demonstrates that these Ir(III) complexes are promising anticancer agents with dual functions, including metastasis inhibition and lysosome-damaged photodynamic therapy.

Targeting cancer cell metabolism with mitochondria-immobilized phosphorescent cyclometalated iridium(iii) complexes.

Cancer cell metabolism is reprogrammed to sustain the high metabolic demands of cell proliferation. Recently, emerging studies have shown that mitochondrial metabolism is a potential target for cancer therapy. Herein, four mitochondria-targeted phosphorescent cyclometalated iridium(iii) complexes have been designed and synthesized. Complexes 2 and 4, containing reactive chloromethyl groups for mitochondrial fixation, show much higher cytotoxicity than complexes 1 and 3 without mitochondria-immobilization properties against the cancer cells screened. Further studies show that complexes 2 and 4 induce caspase-dependent apoptosis through mitochondrial damage, cellular ATP depletion, mitochondrial respiration inhibition and reactive oxygen species (ROS) elevation. The phosphorescence of complexes 2 and 4 can be utilized to monitor the perinuclear clustering of mitochondria in real time, which provides a reliable and convenient method for in situ monitoring of the therapeutic effect and gives hints for the investigation of anticancer mechanisms. Genome-wide transcriptional analysis shows that complex 2 exerts its anticancer activity through metabolism repression and multiple cell death signalling pathways. Our work provides a strategy for the construction of highly effective anticancer agents targeting mitochondrial metabolism through rational modification of phosphorescent iridium complexes.

Ester-Modified Cyclometalated Iridium(III) Complexes as Mitochondria-Targeting Anticancer Agents.

Organometallic iridium complexes are potent anticancer candidates which act through different mechanisms from cisplatin-based chemotherapy regimens. Here, ten phosphorescent cyclometalated iridium(III) complexes containing 2,2'-bipyridine-4,4'-dicarboxylic acid and its diester derivatives as ligands are designed and synthesized. The modification by ester group, which can be hydrolysed by esterase, facilitates the adjustment of drug-like properties. The quantum yields and emission lifetimes are influenced by variation of the ester substituents on the Ir(III) complexes. The cytotoxicity of these Ir(III) complexes is correlated with the length of their ester groups. Among them, 4a and 4b are found to be highly active against a panel of cancer cells screened, including cisplatin-resistant cancer cells. Mechanism studies in vitro indicate that they undergo hydrolysis of ester bonds, accumulate in mitochondria, and induce a series of cell-death related events mediated by mitochondria. Furthermore, 4a and 4b can induce pro-death autophagy and apoptosis simultaneously. Our study indicates that ester modification is a simple and feasible strategy to enhance the anticancer potency of Ir(III) complexes.

Cyclometalated iridium(iii) complexes as lysosome-targeted photodynamic anticancer and real-time tracking agents.

Stimuli-activatable photosensitizers (PSs) are highly desirable for photodynamic therapy (PDT) to selectively demolish tumor cells. On the other hand, lysosomes are emerging as attractive anticancer targets. Herein, four cyclometalated iridium(iii)-ß-carboline complexes with pH-responsive singlet oxygen (1O2) production and lysosome-specific imaging properties have been designed and synthesized. Upon visible light (425 nm) irradiation, they show highly selective phototoxicities against cancer cells. Notably, complex 2 ([Ir(N^C)2(N^N)](PF6) in which N^C = 2-phenylpyridine and N^N = 1-(2-benzimidazolyl)-ß-carboline) displays a remarkably high phototoxicity index (PI = IC50 in the dark/IC50 in light) of >833 against human lung carcinoma A549 cells. Further studies show that 2-mediated PDT induces caspase-dependent apoptosis through lysosomal damage. The pH-responsive phosphorescence of complex 2 can be utilized to monitor the lysosomal integrity upon PDT, which provides a reliable and convenient method for in situ monitoring of therapeutic effect and real-time assessment of treatment outcome. Our work provides a strategy for the construction of highly effective multifunctional subcellular targeted photodynamic anticancer agents through rational structural modification of phosphorescent metal complexes.

Cyclometalated Ir(III) complexes as targeted theranostic anticancer therapeutics: combining HDAC inhibition with photodynamic therapy.

The successful design and anticancer mechanistic studies of a series of cyclometalated Ir(III) complexes with histone deacetylase inhibitory and photodynamic therapy (PDT) activities are reported.