Cellular And Preclinical Examination Of Photodynamic Therapy

Cellular and preclinical examination of photodynamic therapyCharles J. Gomer, Angela Ferrario, Natalie Rucker, Sam Wong, Elisa Morinelli,Grace Liu and Bernard C. SzirthClayton Ocular Oncology Center, Children Hospital of Los Angelesand Departments of Pediatrics and Radiation Oncology, USC School of Medicine4650 Sunset Boulevard, Los Angeles, CA 90027ABSTRACTIn -vitro studies have been performed to compare the mode of action of porphyrin induced photodynamictherapy and hyperthermia. Parent and temperature resistant clones of Chinese hamster and mousefibrosarcoma cells were treated with either PDT or hyperthermia. Quantitative survival curves obtainedfrom these experiments indicate that temperature resistant cells are not cross resistant to PDT even thoughboth treatments induce the synthesis of stress proteins.In -vivo studies were performed to evaluate the photosensitizing efficiency to mono -l- aspartyl chlorin e6 (NPe6). Tumor response and normal skin response were determined using a murine tumor system. Thestudies indicate that NPe6 is an extremely effective tumor photosensitizer with rapid in -vivo clearanceproperties which may eliminate the side effects of prolonged and systemic skin photosensitivity.1. INTRODUCTIONPhotodynamic therapy (PDT) is the treatment of malignant lesions with visible light following thesystemic administration of a tumor localizing photosensitizer (1,2,3). Hematoporphyrin derivative (HpD)and a purified component of HpD called Photofrin II are the photosensitizers currently used in clinical PDT.The procedure exploits both the tumor localizing and photosensitizing properties of these compounds.These drugs are preferentially retained in tumor tissue when compared to surrounding normal tissue (4).The generation of cytotoxic singlet oxygen via a Type II photochemical reaction following absorption ofvisible light by these porphyrins is believed to play a major role in cytotoxicity (5). Advancements in theavailability of lasers and fiber optic light delivery systems have enhanced the selectively in which tumorlocalizing porphyrins can be activated. While there have been anecdotal cases of clinical PDT for almostevery anatomical site and histological type of solid tumor, the use of PDT appears to be most encouragingfor solid tumors of the bronchus, esophagus, bladder, head and neck, skin, and eye (6 -11).Numerous studies have demonstrated that PDT is extremely effective in generating cytotoxic damage tosubcellular organelles and biomolecules (12). Photodegradation of lipids, proteins and nucleic acids canroutinely be observed following porphyrin photosensitization. Membrane damage can lead to inhibition oftransport of amino acids and nucleocides, increased permeability and rupture of liposomes as well as markedcontraction and rupture of mitochondria (13). Enzymes bound to hydrophobic regions of mitochondria(membrane bound) are extremely sensitive to PDT (14). At the level of the nucleus, PDT can producesingle strand breaks in alkali labile sites in DNA, sister chromatid exchanges and chromosome aberrations(15,16). However, even though a large spectrum of specific types of subcellular damage have beendocumented following PDT, the actual target site(s) for cytotoxicity has not been satisfactorily identified.There is also considerable interest in the synthesis of new photosensitizers for use in PDT. The