Background: 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 nonionising
irradiation to “activate” their cytotoxic effect. Photosensitzers 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.
Conclusion: The success of clinical photoactive agents such as Photofrin® has inspired the development
of thousands of potential PDT agents. Transition metal complexes in particular have demonstrated
excellent versatility and diversity when it comes to PDT for treatment of invasive cancers.
This review has highlighted some of the many recent advances of transition metal PDT agents with
high in vitro and in vivo phototoxic activity. Photoactive transition metal complexes have proven their
potential due to their inherent physicochemical variety, allowing them to fill a niche in the PDT
Keywords: Photodynamic therapy, transition metals, cancer, irradiation, light, reactive oxygen species.
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