Editorial: photodynamic therapy and detection.

This special issue on photodynamic therapy (PDT) and detection has been assembled by the guest editor by soliciting individual contributions from specialists in the field throughout the world. Photodynamic therapy is a combination of light with photosensitizing agents and oxygen present in a tumor, leading to photochemical and photobiological reactions that result in irreversible photodamage to the tumor.1 During this process a photosensitizer absorbs energy from light. The absorbed energy can be transferred to molecular oxygen to produce singlet oxygen (1O2) and oxygen radicals, which are highly toxic and react further with cellular components to cause cell death. The absorbed energy can also be passed off by fluorescence, which can be utilized for tumor photodetection (PD).1 Although the light was used for the treatment of vitiligo 3000 years ago by the Indians, the Egyptians, and the Chinese,2 the real era of PDT began with the discovery just over one hundred years ago by Oscar Raab, a medical student working in the group of Hermann von Tappeiner in Munich, who accidentally found that illumination of a thunderstorm could kill paramecia in the presence of acridine.3 This finding was followed in the same group by clinical PDT treatment of some skin non-malignant and malignant diseases with topical and intratumoral administration of photosensitizing compounds.4 The current era of PDT and PD probably began with studies by Lipson and Schwartz at the Mayo Clinic in the 1960s, who found fluorescence in tumor tissues following injection of crude hematoporphyrin (HP). Modifications of the HP led to the development of hematoporphyrin derivative (HpD, a porphyrin mixture), which was a better tumor localizer than the crude HP.5-7 The properties of HpD as a tumor localizer and phototherapeutic agent were systematically studied by the Dougherty’s group in the 1970s and 1980s at the Roswell Park Cancer Institute in Buffalo, New York.8,9 The purified version of HpD with monomers removed was made by the same group and named Photofrin, which was approved for clinical use by regulatory health agencies worldwide.8-10 Thanks to the pioneering work of this group. Photofrin is probably the most widely used sensitizer in the clinic. However, Photofrin is a complex mixture of porphyrins with widely differing properties and weak light absorption at wavelengths above 600 nm where light penetration into tissue is optimal. Moreover, the clinical application of Photofrin is limited by prolonged cutaneous photosensitivity resulting from its slow plasma clearance. These shortcomings stimulated the search for pure compounds with rapid plasma and tissue elimination, enhanced tumor selectivity and strong light