318. Tumor Growth Inhibition from Tumor Targeted Delivery of Diphtheria Toxin Gene

Abstract

Recent advances in the nucleic acid delivery technology have renewed interest in gene therapy. These advances can benefit not only gene therapy but also nucleic acid therapeutic approaches based on RNA interference, antisense etc. To realize the full potential of nucleic acid therapy, efficient vector systems that are tissue targeted and administered systemically need to be developed. Developing viral systems for systemic delivery is still challenging due to the immunogenecity and safety concerns. Non-viral delivery systems are more amenable to systemic delivery since they can be created from less toxic molecules. First generation non-viral systems were proven to be ineffective for systemic delivery due to their poor bio-availability and pharmacology. Some of the problems of the first generation systems can be overcome by providing the targeting capability and inhibiting non-specific interactions. There have been a number of efforts to achieve this by developing molecular conjugates where molecules capable of providing different functions required for nucleic acid delivery are linked together to form single molecules possessing multiple delivery functions. We have developed a molecular conjugate based on synthetic polymers that has some of the essential properties of a gene delivery molecule. This is based on a polyethyleneimine (PEI) polymer conjugate where a fraction of PEI amine is conjugated with polyethyleneglycol (PEG) and an integrin targeted peptide (RGD) ligand is attached to the end of PEG molecules. This polymer conjugate when combined with nucleic acid such as plasmid or short RNA, form nanoparticles with condensed nucleic acid core, a steric surface coat and a layer of ligands on the surface. This ligand surface is expected to target the nanoparticle to the endothelial cells in the neo-vasculature. In vivo experiments using reporter gene plasmid combined with this polymer conjugate in tumor bearing mice showed selective gene expression in tumor tissue with minimal expression in other tissues. Expression appears to be in the tumor blood vessels indicating neo-vascular targeting by the RGD peptide. We further examined if the level of expression obtained in tumor is sufficient to show tumor growth inhibition. Tumor bearing mice were treated with a plasmid encoding the A-chain of Diphtheria Toxin (DT-A) combined with the polymer conjugate by tail vein injection, once every three days. This resulted in significant tumor growth inhibition indicating that the tumor delivery achieved by this polymer conjugate may be sufficient for therapeutic efficacy.