Photosensitizers for Photodynamic Therapy: Photochemistry in the Service of Oncology

The combined use of a photosensitizing agent, light and dioxygen for the treatment of diseases has become known as photodynamic therapy (PDT) and was first discovered more than one hundred years ago. Over the years, PDT has proven its potential for the treatment of malignant and non-malignant lesions in addition to classical cancer therapy in numerous clinical studies, but application as a routine method is still limited. In this review, the development of modern PDT since the beginning of the twentieth century is briefly portrayed. The underlying mechanisms of phototoxicity are explained, and the requirements for ideal photosensitizers (PS) are underlined. Selected PS are introduced and examples of some of the many attempts made at the optimization of PDT by developing new PS with improved chemical and phototoxic properties, are reviewed. Promising 3rd generation PS are introduced and newer approaches to increasing tumor selectivity and efficacy of PDT, such as with nanoparticles, are discussed.