Reverse Intersystem Crossing from a Triplet State of Rose Bengal

36 LLE Review, Volume 77 Photodynamic therapy is a treatment in which the combination of a dye, light, and oxygen causes photochemically induced cell death. Observations of this effect occurred at least as early as the end of the last century. In 1900, Raab reported that the dye acridine rapidly killed paramecia when exposed to light, but had no effect in the dark.1 This observation quickly inspired attempts to use this effect to treat disease. Light and the dye eosin were combined to treat skin cancer in 1903;2 however, significant progress in applying photodynamic therapy to the treatment of cancer did not occur until the 1940s and 1950s, when it was discovered that porphyrin-based photosensitive dyes preferentially accumulated in malignant tissues.3,4 A sustained series of studies into the mechanisms and applications of photodynamic therapy for the treatment of a broad range of cancers was initiated by Dougherty in the 1970s.5 This work led the U.S. Food and Drug Administration in December 1995 to approve the treatment of advanced esophageal cancer using photodynamic therapy with Photofrin®, a porphyrin-based photosensitizer. In 1998 this approval was extended to cover the treatment of early-stage lung cancer. Other countries have also approved photodynamic therapy for the treatment of bladder, gastric, and cervical cancers. Several review articles have been published that provide an overview of the clinical results as well as the open questions about this therapy that require further research.6,7