Lysosome-targeted cyclometalated iridium(III) complexes: JMJD inhibition, dual induction of apoptosis and autophagy

A series of cyclometalated iridium(III) complexes with the formula [Ir(C^N)2 L](PF6) (C^N = 2-phenylpyridine (ppy, in Ir-1), 2-(2-thienyl)pyridine (thpy, in Ir-2), 2-(2,4-difluorophenyl)pyridine (dfppy, in Ir-3), L = 2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)quinolin-8-ol) were designed and synthesized, which utilize 8-hydroxyquinoline derivative as N^N ligands to chelate the cofactor Fe2+ of the Jumonji domain-containing protein (JMJD) histone demethylase. As expected, the results of UV/Vis titration analysis confirm the chelating capabilities of Ir-1-3 for Fe2+, and molecular docking studies also show that Ir-1-3 can interact with the active pocket of JMJD protein, and treatment of cells with Ir-1-3 results in significant upregulation of trimethylated histone 3 lysine 9 (H3K9Me3), indicating the inhibition of JMJD activity. Meanwhile, Ir-1-3 exhibit much higher cytotoxicity against the tested tumor cell lines compared with the clinical chemotherapeutic agent cisplatin. And Ir-1-3 can block the cell cycle at G2/M phase and inhibit cell migration and colony formation. Further studies show that Ir-1-3 can specifically accumulate in lysosomes, damage the integrity of lysosomes, and induce apoptosis and autophagy. Reduction of mitochondrial membrane potential (MMP) and elevation of reactive oxygen species (ROS) also contribute to the antitumor effects of Ir-1-3. Finally, Ir-1 can inhibit tumor growth effectively in vivo and increase the expression of H3K9Me3 in tumor tissues. Our study demonstrates that these iridium(III) complexes are promising anticancer agents with multiple functions, including the inhibition of JMJD and induction of apoptosis and autophagy.

Post-complexation Functionalization of Cyclometalated Iridium(III) Complexes and Applications to Biomedical and Material Sciences

Cyclometalated iridium(III) (Ir(III)) complexes exhibit excellent photophysical properties that include large Stokes shift, high emission quantum yields, and microsecond-order emission lifetimes, due to low-lying metal-to-ligand charge transfer (spin-forbidden singlet-triplet (³MLCT) transition). As a result, analogs have been applied for research not only in the material sciences, such as the development of organic light-emitting diodes (OLEDs), but also for photocatalysts, bioimaging probes, and anticancer reagents. Although a variety of methods for the synthesis and the applications of functionalized cyclometalated iridium complexes have been reported, functional groups are generally introduced to the ligands prior to the complexation with Ir salts. Therefore, it is difficult to introduce thermally unstable functional groups such as peptides and sugars due to the harsh reaction conditions such as the high temperatures used in the complexation with Ir salts. In this review, the functionalization of Ir complexes after the formation of cyclometalated Ir complexes and their biological and material applications are described. These methods are referred to as "post-complexation functionalization (PCF)." In this review, applications of PCF to the design and synthesis of Ir(III) complexes that exhibit blue -red and white color emissions, luminescence pH probes, luminescent probes of cancer cells, compounds that induce cell death in cancer cells, and luminescent complexes that have long emission lifetimes are summarized.

Design and synthesis of a luminescent iridium complex-peptide hybrid (IPH) that detects cancer cells and induces their apoptosis

Tumor necrosis factor related apoptosis inducing ligand (TRAIL) triggers the cell-extrinsic apoptosis pathway by complexation with its signaling receptors such as death receptors (DR4 and DR5). TRAIL is a C₃-symmetric type II transmembrane protein, consists of three monomeric units. Cyclometalated iridium(III) complexes such as fac-Ir(tpy)₃ (tpy = 2-(4-tolyl)pyridine) also possess a C₃-symmetric structure and are known to have excellent luminescence properties. In this study, we report on the design and synthesis of a C₃-symmetric and luminescent Ir complex-peptide hybrid (IPH), which contains a cyclic peptide that had been reported to bind to death receptor (DR5). The results of MTT assay of Jurkat, K562 and Molt-4 cells with IPH and co-staining experiments with IPH and an anti-DR5 antibody indicate that IPH binds to DR5 and induces apoptosis in a manner parallel to the DR5 expression level. Mechanistic studies of cell death suggest that apoptosis and necrosis-like cell death are differentiated by the position of the hydrophilic part that connects Ir complex and the peptide units. These findings suggest that IPHs could be a promising tool for controlling apoptosis and necrosis by activation of the extra-and intracellular cell death pathway and to develop new anticancer drugs that detect cancer cells and induce their cell death.