Photobiology of porphyrins

Hematoporphyrin derivative (HpD) photoradiation therapy (PRT) continues to show promise as a modality for the treatment of certain solid tumors. However, there are still numerous photobiological parameters pertaining to HpD PRT which are not fully defined. This presentation describes preclinical studies performed to examine the efficacy of HpD PRT for the treatment of ocular tumors. In addition, results of photobiological experiments designed to define the action spectrum characteristics, light dose rate properties and mutagenic potential of HpD PRT are discussed. Preclinical studies have examined the pharmacology of HpD and the toxicology of HpD PRT in both normal and malignant structures in the eye of pigmented rabbits. Distribution of HpD was measured by an acid extraction procedure. Vascular structures of the eye such as the choroid, iris and the experimental tumor retained significant levels of HpD following I.V. administration. Non-vascular structures such as cornea and lens did not take up HpD. Toxicity studies demonstrated that HpD administration (1-10 mg/kg) followed 48 hr later with a transpupil irradiation of red light (630 nm, 40-90 J/cm/sup 2/) resulted in well defined areas of retinal damage within the treatment field. The lenses of 18 eyes have remained clear during a 24 month follow-up. Tumor response following HpD PRT is rapid and is characterized by massive vascular disruption and tumor tissue necrosis