PDT, or photodynamic therapy, is a treatment method which has three components; a chemical compound, known as photosensitiser (drug), the light whose wavelength matches the absorption band of the drug, and oxygen. The interaction between the photosensitiser and the light in the presence of oxygen brings about the photodynamic effect, characterised by the destruction of the treated target tissue. The primers of the photodynamic effect are singlet oxygen and other reactive species which are generated from molecular oxygen through a light activated photosensitiser.
The release of such toxic species affects the cellular microstructures and its constituents directly, as well as indirectly, through tissue microvasculature leading to photodynamic effect expressed as cell death by apoptosis and necrosis. In addition to the local effects, more recently the influence of inflammatory and immune responses have also being recognised.
Nevertheless, PDT is essentially a local treatment method and thus far its therapeutic effectiveness in a general systemic disease has not been demonstrated. Although PDT is used to treat in a variety of local lesions its ‘forte’ is in oncology. Indeed it is in the cancer domain that most of the research and clinical work on PDT has been conducted.
Clinical PDT is carried out as a two phase procedure. In the first phase, the tissue to be treated (the target) is presensitised by a photosensitiser which is administered topically or systemically. Photosensitisers used in clinical practice selectively accumulate in cancer tissue with a high concentration ratio between target/normal cells after a latent period, the duration of which is specific to the photosensitiser used.
In the second phase, illumination, the pre-sensitised tissue is exposed to light of a specific wavelength. The light activated chemical affects the available oxygen in the tissue which thus initiates a chain of reactions leading to photodynamic effect and to necrosis of the targeted cells. Photodynamic response (effect) is targeted orientated meaning maximal destruction of the cancer with minimal collateral injury to the surrounding normal tissue.