5-Aminolevulinic acid-based photochemical internalization of the immunotoxin MOC31-gelonin generates synergistic cytotoxic effects in vitro

Photochemical internalization (PCI) is a novel method for the endosomal or lysosomal release of membrane-impermeable molecules into the cytosol of tar get cells. This novel technology is based on the photodynamically induced r upture of endocytic vesicles preloaded with molecules of therapeutic intere st. PCI of the ribosome-inactivating plant toxin gelonin and the immunotoxi n monoclonal antibody 31 (MOC31)-gelonin has been performed previously by t he use of the endocytic vesicle-localizing photosensitizers TPPS2a and AlPc S2a and light, demonstrating synergistic toxicity against the more than 20 different cell lines tested, most of them of neoplastic origin. In this stu dy we demonstrate that 5-aminolevulinic acid (5-ALA)-induced protoporphyrin IX (PpIX) is also capable of inducing PCI of MOC31-gelonin in the human co lon adenocarcinoma cell line WiDr. The cells were incubated with 1 mM 5-ALA for up to 8 h in serum-free medium and from 24 to 96 h in serum-containing medium. Fluorescence microscopical studies indicate a partial plasma membr ane localization of PpIX when 5-ALA was applied under serum-free conditions . This plasma membrane localization was not seen when 5-ALA was given in th e presence of serum. There was a granular component of the PpIX localizatio n in addition to a diffuse cytoplasmic localization. The granular component resembled the localization of the fluorescent dye conjugate Alexa-gelonin and the lysosomal localizing dye acridine orange. Our present results provi de evidence for an endocytic vesicle-associated fraction of PpIX after 5-AL A incubation of the WiDr cells. We demonstrate that PCI, by combining 5-ALA , MOC31-gelonin and light, induces a synergistic cytotoxic effect against t he WiDr cells.