Photodynamic therapy in the treatment of cancer: current state of the art.

Photodynamic therapy (PDT) is a treatment modality using a photosensitising drug and light to kill cells. The clinical use of PDT requires the presence of a photosensitising agent, oxygen and light of a specific wavelength which matches the absorption characteristics of the photosensitiser. When the photosensitiser is activated by the appropriate wavelength of light, it interacts with molecular oxygen to form a toxic, short-lived species known as singlet oxygen, which is thought to mediate cellular death. The appeal of PDT in oncology is that the photosensitiser tends to be retained in tumour tissues for a longer period of time as compared with normal tissues resulting in a large therapeutic index. This potential for minimal normal tissue toxicity has prompted an interest in studying PDT as a cancer treatment. Furthermore, the use of PDT is not precluded by prior radiotherapy, chemotherapy or surgery. The development of PDT has been hampered by the limitations of the older photosensitisers, namely limited depth of tissue penetration, and extended skin phototoxicity which limits the number of applications during a course of treatment. However, newer photosensitisers are being developed which allow greater depth of tissue penetration and have minimal skin phototoxicity allowing for multiple fractionated treatments. With such advancements, PDT has great potential to become an integral part of cancer treatment in the future.