Photodynamic therapy and cancer

Photodynamic therapy uses visible light, molecular oxygen (normally present in most tissues at adequate concentration), and an otherwise innocuous drug to destroy tumours.1 All three components are needed for the cytotoxic effect. Photodynamic therapy is licensed for the treatment of several cancers1 and age related macular degeneration2; it is also being developed for the treatment of infections.3 Photosensitising drugs may be given systemically or locally, but because the primary targeting is through precise direction of visible light from a laser or other source, the ultimate effect is local rather than systemic. This is important because, although it means that photodynamic therapy cannot cure disseminated disease, it underpins the selective targeting of this treatment and lack of generalised toxicity.

The photosensitising drug is given first, and after a time interval of between a few hours and four days—to allow the maximum concentration differential to develop between the tumour and surrounding healthy tissue—light is applied either from a laser for internal treatment or a light emitting diode or similar source for topical treatment. The tumour localised photosensitiser absorbs light of the appropriate wavelength and transfers the light derived energy …