Intraocular Drug DelIvery - part 5 pptx

138 Kim and Miller PDT as a therapeutic modality Antibody-based targeting is one method currently under investigation In vitro studies using various tumor cell lines have shown that photosensitizers conjugated to monoclonal antibodies can achieve a higher phototoxic effect at lower doses than with drug or antibody alone (44,45) In vivo work in a mouse rhabomyosarcoma model yielded similar results (46) While direct attachment of photosensitizing drugs to monoclonal antibodies is possible, the number of molecules that can be bound to one antibody is limited due to loss of or alterations in antigenic specificity The use of spacers such as dextran, polyglutamic acid, or polyvinyl alcohol (PVA) has been proposed to address these issues (47) This method of conjugation allows high molar ratios of drug to antibody while conferring water-solubility to the final compound Jiang et al (44) linked BPD to 5E8, a monoclonal antibody against a cell-surface glycoprotein, using PVA and demonstrated 15-fold higher phototoxicity with the conjugate than with BPD alone For current ocular applications, the intended target for photosensitizer delivery is the neovascular endothelium One strategy is to bind the photosensitizer to a molecule directed at binding sites on the CNV endothelium, such as VEGF receptors or integrins Work in our laboratory focused on a peptide ATWLPPR, which has been shown to bind specifically to the VEGFR2 receptor also known as KDR or FLK-1 This peptide completely inhibits VEGF binding to VEGFR2 (48) We produced a targeted photosensitizer by binding verteporfin to a PVA linker and then to the homing peptide ATWLPPR (49) For controls we used verteporfin–PVA, which is a large but untargeted molecule, and also commercially available verteporfin In vivo experiments were carried out in the laser-injury model of CNV in the rat for which dosimetry for verteporfin PDT has been optimized (50) We found that PDT using both targeted verteporfin and verteporfin–PVA were effective in CNV closure One day following treatment with targeted verteporfin, fluorescein angiography demonstrated no perfusion or leakage from CNV Both large molecules were more efficient than unbound verteporfin in achieving CNV closure PDT was also performed to normal retina and choroid to assess selectivity No angiographic changes were seen day after PDT using VEGFR2-targeted PDT Histologically, the eye treated with VEGFR2-targeted verteporfin showed preserved retina and very minimal changes to RPE In contrast, treatment of normal retina and choroid using the verteporfin–PVA control showed hyperfluorescence on angiography and retinal damage on light microscopy In addition to tissue-specific targeting, increasing knowledge regarding the importance of the subcellular localization of photosensitizers has raised the potential for intracellular drug targeting There is evidence that PDT using drugs such as BPD which localize in mitochondria results in a rapid release of cytochrome c into the cytosol which initiates the apoptotic cascade (51) Photosensitizers, such as NPe6, which localize to lysosomes can induce apoptosis or necrosis, and those which accumulate in the plasma membrane can activate pathways that either lead to cell rescue or cell death (51,52) Some have suggested that targeting drug to the cell nucleus, which is particularly sensitive to damage from reactive oxygen species, could increase the efficiency of PDT (53) A better understanding of the cellular mechanisms involved in the response to PDT will allow for identification of specific intracellular targets for photosensitizer delivery as well as combination therapies directed toward modulation of signaling pathways such as those leading to apoptosis Such advances in the delivery and design of drugs used in PDT hold the promise of better visual outcomes for a greater number of patients Photodynamic Therapy 139 REFERENCES Raab O Uber die Wirkung fluoreszierender Stoffe auf Infusorien Z Biol 1900; 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9:102–108 Rosenkranz AA, Jans DA, Sobolev AS Targeted intracellular delivery of photosensitizers to enhance photodynamic efficiency Immunol Cell Biol 2000; 78:452–464 10 Thermal-Sensitive Liposomes Sanjay Asrani Duke University Eye Center, Durham, North Carolina, U.S.A Morton F Goldberg and Ran Zeimer Wilmer Ophthalmological Institute, Johns Hopkins University, Baltimore, Maryland, U.S.A INTRODUCTION Liposomes are microscopic lipid bubbles designed to entrap drugs They have been used locally as well as systemically for targeting of drugs to specific organs or for prolonging drug effect The encapsulation of drugs in liposomes has been shown to reduce the toxicity, provide solubility in plasma, and enhance permeability through tissue barriers Some applications related to cancer and infectious diseases have reached clinical use, while others are currently in Phase I–III human clinical trials A method has been developed to target drugs locally in the eye via a lightbased mechanism The method, called laser-targeted delivery (LTD) (1–3), consists of encapsulating a drug in heat-sensitive liposomes, injecting them intravenously, and releasing their content at the site of choice by noninvasively warming up the targeted tissue with a laser pulse directed through the pupil of the eye The specific temperature needed for the phase transition is 41 C (105.8 F), which causes the liposomes to release their contents in the blood in