Tumour photosensitizers: approaches to enhance the selectivity and efficiency of photodynamic therapy.

While Photofrin, the photosensitizer currently in clinical use for photodynamic therapy (PDT) of tumours, has been shown to be both efficacious and safe in the treatment of a variety of human cancers, its chemical heterogeneity and low absorbance in the phototherapeutically useful wavelength range (600-850 nm) make the development of new photosensitizers with improved characteristics desirable. A suitable manipulation of the molecular structure of porphyrins offers several interesting possibilities for controlling the optical and photophysical properties of the photosensitizer, as well as its biodistribution between tumour and peritumoural tissues or at the subtissular and subcellular level. The achievement of these goals may also be facilitated by the association of the photosensitizer with selected delivery systems, opening the way to a qualitative and quantitative improvement of PDT.