Mixed-ligand iridium(iii) complexes as photodynamic anticancer agents

Area of Science:

• Coordination Chemistry
• Photodynamic Therapy (PDT)
• Cancer Research

Background:

• Phosphorescent iridium complexes are promising candidates for photodynamic therapy (PDT) due to their ability to generate reactive oxygen species.
• Developing novel metal-based agents with targeted cellular uptake and specific organelle localization is crucial for enhancing PDT efficacy.
• Understanding the mechanisms of action, including reactive oxygen species (ROS) generation and cellular pathway activation, is key for optimizing anticancer strategies.

Purpose of the Study:

• To synthesize and characterize a series of mixed-ligand phosphorescent iridium complexes for potential use as photodynamic therapeutic agents.
• To evaluate the cellular uptake, photophysical properties, and anticancer efficacy of these iridium complexes in human lung adenocarcinoma A549 cells.
• To investigate the organelle-specific targeting (lysosomes and mitochondria) and the underlying mechanisms of apoptosis induction by the most promising complexes.

Main Methods:

• Synthesis and characterization of four mixed-ligand iridium complexes (Ir1-Ir4) with varying ligands.
• Assessment of luminescence quantum yields, phosphorescence lifetimes, and hydrolytic stability in aqueous solutions.
• Evaluation of cellular uptake, organelle-specific imaging (lysosomes, mitochondria), singlet oxygen generation, and phototoxicity in A549 cells using 425 nm irradiation.

Main Results:

• All synthesized iridium complexes exhibited high luminescence, long phosphorescence lifetimes, and effective singlet oxygen production.
• Complexes Ir2 and Ir4 demonstrated superior cellular uptake in A549 cells, with Ir2 targeting lysosomes and Ir4 targeting mitochondria.
• Both Ir2 and Ir4 showed potent photodynamic therapy (PDT) effects, inducing apoptosis via ROS generation and caspase activation; Ir2 exhibited a phototoxicity index (PI) > 54 in A549 cells.

Conclusions:

• Mixed-ligand iridium complexes Ir2 and Ir4 are effective photodynamic therapeutic agents with distinct organelle-targeting capabilities and potent anticancer activity.
• The pH-sensitive emission of Ir2 allows for effective lysosomal damage, while Ir4's mitochondrial targeting impairs cellular function.
• While promising, the application of these complexes for PDT might be limited to superficial tumors due to the short excitation wavelength (425 nm), highlighting areas for future development.