Cyclometalated iridium(III)-lonidamine conjugates: Mitochondrial targeting and pyroptosis induction

A series of cyclometalated Ir(III)-lonidamine (LND) complexes (Ir-LND-1-6) with the formula [Ir(C^N)₂bpy(4-CH₃-4'-CH₂OLND)](PF₆) (Ir-LND-1-3) and [Ir(C^N)₂bpy(4-CH₂OLND-4'-CH₂OLND)](PF₆) (Ir-LND-4-6) (C^N = 2-phenylpyridine (ppy, in Ir-LND-1 and Ir-LND-4), 2-(2-thienyl) pyridine (thpy, in Ir-LND-2 and Ir-LND-5) and 2-(2,4-difluorophenyl) pyridine (dfppy, in Ir-LND-3 and Ir-LND-6)), were designed and synthesized. 3-(4,5-dimethylthiazol-2-yl)-2,5-biphenyltetrazolium bromide (MTT) assay data showed that the cytotoxicity of Ir-LND-1-3 carry one LND moiety was superior to that of Ir-LND-4-6 with two LND moieties. Therefore, we selected Ir-LND-1-3 as model compounds to investigate the anti-tumor mechanism of the Ir(III)-LND system. The results showed that Ir-LND-1-3 could inhibit cancer cell migration and colony formation. In addition, Ir-LND-1-3 could penetrate into HeLa cells and localized to mitochondria, further disrupting mitochondrial membrane potential (MMP), increasing intracellular reactive oxygen species (ROS), and reducing intracellular adenosine triphosphate (ATP). Further exploration of anti-tumor mechanisms showed that pyroptosis was the main mode of Ir-LND-1-3 induced cell death, manifested as membrane perforation and swelling, activation of caspase-3 and cleavage of Gasdermin E (GSDME), as well as release of lactic dehydrogenase (LDH) and ATP. The pyroptosis induced by Ir-LND-1-3 also initiated immunogenic cell death (ICD) by triggering the release of calreticulin (CRT) and high mobility group protein b1 (HMGB1) on the cell surface.