ABSTRACT
In 1904, Von Tappeiner discovered photodynamic therapy, using photosensitization followed by irradiation with visible light, to cause tissue destruction.1 The following year Von Tappeiner and Jesionek discovered that human skin malignancies improved after the application of a 5% solution of eosin dye and subsequent exposure to lamps or sunlight.2 Auler and Banzer, in 1947, were the first to use systemic PDT with intravenous hematoporphyrin derivatives (HpD) of human and animal tumors as photosensitizers.3 In 1978, Dougherty et al4 performed the first study using intravenous HpD on human
cutaneous photosensitivity.5 Consequently, research efforts were expanded to test different photosensitizing agents that could be applied topically. Kennedy et al,6 in 1990, were the first to publish a study that yielded positive results in the treatment of basal cell carcinoma using topical ALA-PDT. Psoriasis has also been sucessfully treated using PDT.7,8
BASIC SCIENCE
The utility of PDT in the treatment of warts is rooted in the destructive effects of light that is focused on a specific cell type, while avoiding damage to surrounding structures. The photodynamic reaction excites photosensitizers (mainly porphyrins) with visible light in the presence of oxygen, yielding free radicals.9 Cytotoxic species induce targeted cellular destruction.9 This effect is produced by using sensitizing molecules via intravenous administration, or direct topical or intralesional application; these molecules selectively enter targeted cells. Most sensitizing molecules are not endogenous to the human body. HpD has been the focus of most of the published work but, as a result of its lack of selectivity for skin tumors and long-lasting photosensitivity, its utility has proved limited. In contrast, topical administration of ‘endogenous’ sensitizers such as ALA induce the synthesis of photosensitizers (protoporphyrin IX) in target tissues.10
