Although it is generally thought that gene therapy has a great clinical potential, so far a major hurdle for its clinical use has been the lack of methods for specific and efficient in vivo gene delivery. One of the main biological barriers for gene delivery is that for most gene delivery systems the therapeutic gene enters the cell by endocytosis, ending up in endosomes or lysosomes. In order to be expressed the therapeutic gene must escape from these organelles before being degraded by lysosomal enzymes. Therefore most efficient gene therapy vector systems incorporate means for inducing endosomal escape. However, as our own results and reports from the literature have shown, for many gene therapy vectors endosomal escape can still be substantially improved. Thus, in vitro we have shown that PCI can increase transfection by polylysine-complexed plasmids >100-fold, with >70% of the cells being transfected.
Ribosome-inactivating proteins (RIPs)
The ribosome-inactivating plant protein toxins (RIPs) are taken up into cells by endocytosis and a small fraction (<5%) is relocated via the Golgi apparatus to the cytosol where these toxins can inhibit protein synthesis and induce cell death. Type II RIPs contains 2 polypeptide chains, A and B, where B binds to the cell surface, while A has a cytotoxic effect. Type I RIPs lack the B-chain, are taken up and relocated to the cytosol only to a minor extent, and therefore exert low cytotoxicity. We have recently documented that PCI potentiates the cytotoxic effect of the type I RIP gelonin (and other similar toxins) several hundred-fold in vitro.
Photosensitizers are often preferentially located in tumor tissues with a tumor-to-normal surrounding tissue ratio of approximately 3:1. Since the cytotoxic effect of the photosen-sitizers is exerted only in areas exposed to light, the specificity of the photodynamic treatment for tumor tissue is relatively high. However, although this has not been shown gelonin is not expected to have any inherent specificity for neoplastic lesions. In an attempt to improve the specificity of gelonin uptake, gelonin has been conjugated to an antibody (MOC31, towards the antigen EGP-2 found on most carcinomas), and we have shown in vitro that the immunotoxin MOC31-gelonin increases the specificity of the treatment for the target cells.