A Deep‐Red‐Absorbing Osmium(II) Complex as a Photosensitizer for Photodynamic Therapy Inducing Immunogenic Cell Death

ABSTRACT Immunogenic cell death (ICD), which converts tumor cells into their own vaccine, plays a pivotal role in the development of novel anticancer therapies. Here, a small series of osmium(II) polypyridyl complexes was synthesized, and their biological activity in the dark and upon light irradiation against various cancer cell lines was studied. The compound Os2 (bearing two 4,7‐diphenyl‐1,10‐phenanthrolines and one substituted bipyridine ligand) was discovered to be the most effective photosensitizer (PS) for photodynamic therapy (PDT) of this series through the photogeneration of 1 O 2 and •OH. In addition, Os2 was found to exhibit promising toxicity upon deep‐red irradiation under both normoxia and hypoxia. These observations indicate that this PS is working through a mixture of Type‐I and Type‐II mechanisms. More interestingly, upon 740 nm irradiation, Os2 can stimulate a strong ICD response on CT26 and MCA205 cells both in vitro and in vivo. A comprehensive immune analysis showed that mice vaccinated with Os2 ‐treated CT26‐luc cells boosted the systemic specific adaptive immune responses, including the activation of CD8 + T cells and reprogramming of macrophages, leading to effective inhibition of tumor growth. Os2 is, to the best of our knowledge, the first photoactive osmium‐based complex inducing ICD.