Targeted liposome entrapped iridium(III) complexes significantly increase antitumor activity in vitro and in vivo

In this study, we designed and synthesized a new ligand, FMPIP (2-(3-fluoro-2-methylphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline), and its three cyclometalated iridium(III) complexes: [Ir(ppy)₂(FMPIP)](PF₆) (Ir3a, ppy = 2-phenylpyridine), [Ir(bzq)₂(FMPIP)](PF₆) (Ir3b, bzq = benzo[h]quinoline), and [Ir(piq)₂(FMPIP)](PF₆) (Ir3c, piq = 1-phenylisoquinoline). We evaluated antiproliferative activity of Ir3a, Ir3b and Ir3c against hepatocellular carcinoma cell lines (Huh7, HepG2, SK-Hep1, HCCLM3) using 3-(4,5-dimethylthiazole-2-yl)-diphenyltetrazolium bromide (MTT) assay. Ir3a-Ir3c exhibited negligible cytotoxicity toward all tested cell lines, each with half-maximal inhibitory concentration (IC₅₀) values > 100 μM, which was caused by poor solubility of Ir3a, Ir3b and Ir3c in phosphate-buffered saline (PBS). To overcome this limitation, we encapsulated Ir3a-Ir3c into liposomes to produce Ir3alp, Ir3blp and Ir3clp. Ir3alp, Ir3blp and Ir3clp significantly enhanced antiproliferative potency (IC₅₀ < 5.0 μM). To further enhance anticancer efficacy, we used three kinds of targeted functional liposome to encapsulate Ir3a to form Ir3aT1lp, Ir3aT2lp and Ir3aT3lp. Against Huh7 cells, Ir3aT1lp, Ir3aT2lp and Ir3aT3lp showed IC₅₀ values of 1.34 ± 0.04 μM, 1.26 ± 0.01 μM, and 1.23 ± 0.07 μM, respectively. Mechanistic studies revealed that Ir3alp, Ir3aT3lp, Ir3blp and Ir3clp trigger intracellular reactive oxygen species (ROS) generation, loss of mitochondrial membrane potential and apoptosis, we further observed some markers consistent with ferroptosis. Ir3aT3lp displayed potent antitumor efficacy with a tumor growth inhibition rate of 64.90%, simultaneously, they induce an increment of CD8⁺ T cells. These results show that Ir3alp, Ir3aT3lp, Ir3blp and Ir3clp induced ferroptosis to stimulate immune response to elevate the CD8⁺ T cells to exert antitumor efficiency.