Mitochondria\nLocalized Anticancer Iridium(III) Prodrugs\nfor Targeted Delivery of Myeloid Cell Leukemia‑1 (Mcl-1) Inhibitors\nand Cytotoxic Iridium(III) Complex

Myeloid cell leukemia-1 (Mcl-1) is an antiapoptotic oncoprotein\noverexpressed in several malignancies and acts as one of the promising\ntherapeutic targets for cancer. Even though there are several small\nmolecule based Mcl-1 inhibitors reported, the delivery of Mcl-1 inhibitor\nat the target site is quite challenging. In this regard, we developed\na series of mitochondria targeting luminescent cyclometalated iridium(III)\nprodrugs bearing Mcl-1 inhibitors via ester linkage due to the presence\nof Mcl-1 protein in the outer mitochondrial membrane. Among the synthesized\nprodrugs, IrThpy@L2 was found to exhibit the potent cytotoxicity (IC<sub>50</sub> = 30.93 nM) against HCT116 cell line when compared with\nbare Mcl-1 inhibitors (IC<sub>50</sub> > 100 μM). Mechanistic\nstudies further revealed that IrThpy@L2 quickly gets internalized\ninside the mitochondria of HCT116 cells and undergoes activation in\nthe presence of overexpressed esterase which leads to the release\nof two cytotoxic species i.e. Mcl-1 inhibitors (I-2) and cytotoxic\niridium(III) complex (IrThpy@OH). The improved cytotoxicity of IrThpy@L2\nis due to the mitochondria targeting ability of iridium(III) prodrug,\nsubsequent esterase activated release of I-2 to inhibit Mcl-1 protein\nand IrThpy@OH to generate reactive oxygen species (ROS). After prodrug\nactivation, the released cytotoxic species cause mitochondrial membrane\ndepolarization, activate a cascade of mitochondria-mediated cell death\nevents, and arrest the cell cycle in S-phase which leads to apoptosis.\nThe potent anticancer activity of IrThpy@L2 was further evident from\nthe drastic morphological changes, size reduction in the solid tumor\nmimicking 3D multicellular tumor spheroids (MCTS) of HCT116.