Exposing "bright" metals : promising advances in photoactivated anticancer transition metal complexes

Photodynamic therapy (PDT) is an increasingly prominent field in anticancer research. PDT agents are typically nontoxic in the absence of light and can be stimulated with non-ionising irradiation to "activate" their cytotoxic effect. Photosensitizers are not classified as chemotherapy drugs although it is advantageous to control the toxicity of a drug through localised irradiation allowing for selective treatment. Transition metals are an extremely versatile class of compounds with various unique properties such as oxidation state, coordination number, redox potential and molecular geometry that can be tailored for specific uses. This makes them excellent PDT candidates as their properties can be manipulated to absorb a specific range of light wavelengths, cross cellular membranes or target specific sites in vitro. This article reviews recent advances in transition metal PDT agents, with a focus on structural scaffolds from which several metal complexes in a series are synthesised, as well as their in vitro cytotoxicity in the presence or absence of irradiation.