Iridium( <scp>iii</scp> )/rhenium( <scp>i</scp> ) complexes bearing NLG919 as the ligand for enhanced triple-negative breast cancer therapy

Growing evidence has indicated that chemoimmunotherapy has become the most promising approach for treating malignant tumors. Here, we obtained four metal complexes, two iridium(III) complexes [Ir(C^N)₂(NLG919)₂]PF₆ (C^N = 2-(2-thienyl)pyridine (thpy, Ir-NLG-1) and 2-(2,4-difluorophenyl)pyridine (dfppy, Ir-NLG-2)) and two rhenium(I) complexes [Re(N^N)(NLG919)]PF₆ (N^N) = 4,7-diphenyl-1,10-phenanthroline (DIP, Re-NLG-1) and 1,10-phenanthroline (phen, Re-NLG-2), by coupling an indoleamine 2,3-dioxygenase (IDO) inhibitor NLG919 with Ir(III) and Re(I) metals, in order to achieve the synergistic efficacy of chemotherapy and immunotherapy. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test data indicate that the three complexes, Ir-NLG-1, Ir-NLG-2 and Re-NLG-1, exhibit superior cytotoxicity against mouse breast cancer 4T1 cells. The ability to inhibit the IDO enzyme activity of Ir-NLG-1-2/Re-NLG-1 has been confirmed. Further mechanistic studies have shown that these complexes can target mitochondria to cause a decrease in the mitochondrial membrane potential (MMP) and an increase in reactive oxygen species (ROS), disrupting the function of mitochondria, and ultimately triggering cell apoptosis through a mitochondria-mediated Bcl-2/Bax/caspase 3 pathway. The wound healing and colony assays also prove that these complexes significantly inhibit the metastasis of 4T1 cells. Furthermore, while inducing apoptosis, these complexes can also promote the release of damage-related molecular patterns (calreticulin (CRT), high mobility group protein b1 (HMGB1) and adenosine triphosphate (ATP)), thereby inducing immunogenic cell death (ICD).