What is
PDT?
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.
It is important to note that the dependence of the
activation of the drug on the light of specific wavelength
which is the basic step in the mechanisms of photodynamic
reaction makes PDT one of the most target orientated
therapies in existence.
