Metal organic frameworks in photodynamic therapy of cancer: functional roles of active metal centres and integrated and loaded photosensitizers in the framework

Abstract

Cancer is a major health challenge that is accountable for a large percentage of disease-related deaths worldwide. Annually, new cancer diagnoses reach over 10 million with a record of about 9.6 million deaths. Owing to the high prevalence of novel cases and low survival rate of cancer, studies on its diagnosis and treatment are important. Current methods used for the treatment of cancer include surgery, radiotherapy, molecular-targeted therapy, immunotherapy and chemotherapy; each with its own limitations. Photodynamic therapy (PDT) is another method developed for the treatment of cancer from conventional chemotherapy, radiotherapy and surgery. PDT is better than other therapies owing to its non-invasiveness and fast cure process. The challenges of PDT are poor biocompatibility, acidic environment and complex synthetic procedures. To overcome these barriers, metal organic frameworks (MOFs) and their nanoscale metal organic framework (NMOF) counterparts have been incorporated into PDT. MOFs and NMOFs are assemblies of organic ligands and inorganic metal nodes fashioned into ordered networks applied as photosensitizers (PS) or carriers of the same in PDT. The benefits of MOF-based platforms for PDT include their hydrophilic property, high stability, high reactive oxygen species (ROS) utilization and efficient renal clearance. Several MOFs synthesized from hydrothermal, solvothermal, microwave-assisted, sonochemical, ball and liquid-assisted, and mechanochemical milling procedures have been applied for PDT with positive results. However, future research is expected to focus on the design of innovative MOFs and NMOFs that are suitable for various conditions. This review focuses on photodynamic therapy using metal organic frameworks. It discusses the roles and activities of active metal ions/clusters, PS integrated into MOF scaffolds, PS carried in MOF backbone, MOFs integrated with upconversion nanoparticles (UCNP), recent accomplishments, and the future direction of MOFs in photosensitization for photodynamic cancer therapy.