Achieving red-light anticancer photodynamic therapy under hypoxia using Ir(<sc>iii</sc>)-COUPY conjugates

Despite the potential of photodynamic therapy (PDT), this oxygen-dependent oncological treatment is greatly restricted in the clinic by the well-known hypoxic feature of solid tumors. Here we provide new insights into the development of PDT agents based on conjugates between COUPY fluorophores and cyclometalated iridium(iii) complexes with the aim of overcoming this limitation. The structural modifications carried out within the metal core of Ir(iii)-COUPY conjugates, based on the incorporation of trifluorobenzyl groups at the cyclometalating ligands, enabled efficient exploitation of type I PDT mechanisms while retaining operativity under long-wavelength visible light, which facilitated deeper tissue penetration compared with short wavelengths. Photobiological evaluation revealed that Ir(iii)-COUPY conjugate 3c achieved potent photocytotoxicity towards cisplatin-resistant ovarian (A2780cis) and mammary (EO771) cancer cell lines, efficiently photogenerated type I and type II ROS, and photoinduced apoptotic cell death using red light irradiation (620 nm). Importantly, this Ir(iii)-COUPY conjugate retained such potent photoactivity under low-oxygen environment conditions (2% O2), delivering equipotent photocytotoxicity towards normoxic and hypoxic adherent cancer cells. Compound 3c was found to be highly phototoxic against EO771 multicellular tumor spheroids and showed no signs of toxicity or adverse effects in mice, which could facilitate in vivo phototherapeutic applications. Taken together, this study demonstrates that the conjugation between COUPY dyes and rationally designed Ir(iii) complexes is a strategy at the frontier of the development of new red light-activated photosensitizers capable of operating under hypoxia, showing the promise of achieving satisfactory anticancer PDT effects.