Photochemical internalization of tumor-targeted protein toxins

PCI Biotech AS, 1366 Lysaker, NorwayPhotochemical internalization (PCI) is a method for intra-cellular delivery of hydrophilic macromolecular drugswith intracellular targets as well as other drugs with lim-ited ability to penetrate cellular membranes. Such drugsenter cells by means of endocytosis and are to a large ex-tent degraded by hydrolytic enzymes in the lysosomes un-less they possess a mechanism for cytosolic translocation.PCI is based on photodynamic therapy (PDT) specificallytargeting the endosomes and lysosomes of the cells, sothat the drugs in these vesicles can escape into the cytosolfrom where they can reach their targets. The preferentialretention of the photosensitizer (PS) in tumor tissue incombination with controlled light delivery makes PCI rel-atively selective for cancer tissue. The tumor specificity ofPCI can be further increased by delivery of drugs thatselectively target the tumors. Indeed, this has beenshown by PCI delivery of several targeted protein toxins.Targeted protein toxins may be regarded as ideal drugsfor PCI delivery, and may represent the clinical futurefor the PCI technology. Lasers Surg. Med. 43:721–733,2011. 2011 Wiley-Liss, Inc.Key words: photodynamic; PDT; PCI; targeting; saporin;geloninBACKGROUNDA major problem with cancer therapeutics is still selec-tivity, and treatment-limiting adverse effects are often anobstacle for patient cure. Conventional cancer therapeu-tics are in general antiproliferative and are, unfortunate-ly, affecting rapidly dividing healthy cells in addition tothe cancer cells causing major adverse effects such asnausea, bone marrow suppression, impaired wound heal-ing, and in some cases secondary cancers. New advancesin cancer therapeutics include the use of drugs based onbiomolecules such as DNA, RNA, or proteins linked to acancer-targeting moiety. Such drugs can provide highercancer selectivity compared to conventional chemothera-peutics since their targets are overexpressed in malignantcells. In addition, drugs based on such biomolecules maybe less toxic than conventional chemotherapeutics caus-ing less adverse effects. Despite the high potential of bio-logic drugs in cancer treatment, very few have obtainedmarket approval, and these approved drugs mostly exerttheir action on cell surface receptors or extracellulargrowth factors. Biological drugs with intracellular targetshave, however, with few exceptions, not shown sufficientefficacy to warrant clinical approval. This is to a large ex-tent due to limited cellular and intracellular drugdelivery.The plasma membrane is semipermeable and allowsnutrients and other essential elements to enter the cell.Small and lipophilic molecules are able to diffuse freelyacross the membrane (Fig. 1A), and the passage of largerand polar molecules such as amino acids, sugars, and ionsare transported through the plasma membrane by specificchannel or carrier proteins (Fig. 1B). Large hydrophilicmolecules use a third mechanism to get into the cell; endo-cytosis, where the plasma membrane engulfs the moleculeto be taken up and allows intracellular transport withinthe endocytic vesicle compartments [1,2] (Fig. 1C). Unlessthe internalized material has a mechanism to escape theendocytic vesicles, molecules taken into the cell by meansof endocytosis is to a large extent transported to the lyso-somes where they are degraded by hydrolytic enzymes(Fig. 2A). This is a major obstacle for biomolecule-baseddrugs having intracellular targets [3].PHOTOCHEMICAL INTERNALIZATION (PCI)Photochemical internalization (PCI) is a method for cy-tosolic release of drugs trapped in endocytic vesicles. Themethod is based on photodynamic therapy (PDT) targetedto endosomes and lysosomes which causes the membranesof these vesicles to burst so that drugs trapped on the in-side can diffuse into the cytosol from where they can reachtheir target (Fig. 2B). PCI has in vitro been shown inmore than 80 cell lines to increase the biological activityof a large variety of clinically relevant drugs that do notreadily penetrate the plasma membrane [4]. Examplesare type I ribosome-inactivating proteins (RIPs) [5,6],