Current Gene Therapy - Volume 3, Issue 2, 2003

Numerous gene therapy vectors, both viral and non-viral, are taken into the cell by endocytosis, and for efficient gene delivery the therapeutic genes carried by such vectors have to escape from endocytic vesicles so that the genes can further be translocated to the nucleus. Since endosomal escape is often an inefficient process, release of the transgene from endosomes represents one of the most important barriers for gene transfer by many such vectors. To improve endosomal escape we have developed a new technology, named photochemical internalisation (PCI). In this technology photochemical reactions are initiated by photosensitising compounds localised in endocytic vesicles, inducing rupture of these vesicles upon light exposure. The technology constitutes an efficient light-inducible gene transfer method in vitro, where light-induced increases in transfection or viral transduction of more than 100 and 30 times can be observed, respectively. The method can potentially be developed into a sitespecific method for gene delivery in vivo.This article will review the background for the PCI technology, and several aspects of PCI induced gene delivery with synthetic and viral vectors will be discussed. Among these are: (i) The efficiency of the technology with different gene therapy vectors, (ii) use of PCI with targeted vectors, (iii) the timing of DNA delivery relative to the photochemical treatment.The prospects of using the technology for site-specific gene delivery in vivo will be thoroughly discussed, with special emphasis on the possibilities for clinical use. In this context our in vivo experience with the PCI technology as well as the clinical experience with photodynamic therapy will be treated, as this is highly relevant for the clinical use of PCI-mediated gene delivery.The use of photochemical treatments as a tool for understanding the more general mechanisms of transfection will also be discussed.

The biological therapy of tumors using live viruses was first proposed a century ago but was abandoned due to potential virulence of wild-type strains. Thanks to advances in recombinant technology, replication-restricted strains have been genetically engineered, which replicate selectively within tumor cells. Examples include replication-competent mutants of herpes simplex virus (HSV), adenovirus, vesicular stomatitis virus, reovirus and measles virus. Replication-restricted oncolytic viruses are able to propagate selectively within solid tumor nodules exerting direct antitumor activity by killing infected tumor cells at the completion of a replicative cycle. In the process, they generate an intratumoral inflammatory response, which under the appropriate circumstances, may trigger the activation of an adaptive antitumor immune response, a process that has been named in situ tumor vaccination. Recombinant HSV may offer distinct advantages in oncolytic therapy of epithelial tumors. HSV is highly infectious to tumors of epithelial origin, resulting in high efficacy, there is considerable redundancy in HSV receptors, which makes the loss of HSV receptors by tumors due to mutations less likely and potent anti-herpetic drugs are commercially available, which may be used clinically to control undesired side effects. Herewith we describe the use of oncolytic viral therapy against intraperitoneal malignancies with special emphasis on oncolytic herpes simplex virus. We review the preclinical evidence on the efficacy and safety of intraperitoneal applications of HSV and discuss the rationale for its use for oncolytic therapy and in situ tumor vaccination of intraperitoneal tumors.

Targeted gene therapy aims at achieving the expression of therapeutic transgenes in specific and restricted cell populations, thus sparing all other cells of the unwanted effects of the gene product. This strategy is particularly appealing for therapy of the central nervous system (CNS), where many different cell types exist, and where the inappropriate expression of a molecule can produce grave consequences. To accomplish the objectives of targeted gene therapy, two different approaches have been developed. The first one consists in creating vectors that will deliver the transgene exclusively to the selected cells, that is manipulating the transductional capacities of the vector, and the second one is based on the transcriptional properties of the transgene, so that it will only be expressed in cells where the appropriate transcriptional machinery is present. Reaching the goals of targeted gene expression will greatly increase the specificity and safety of gene therapy, thus getting us closer to the fulfillment of the expectations generated by this new branch of molecular medicine.

Preclinical studies in animal models and human clinical trials have evaluated the safety and efficacy of adenoviral vectors for cancer gene therapy. These studies have indicated that gene delivery via adenoviral vectors, including p53 gene therapy, represents a promising therapeutic modality for many types of human cancers. This review focuses on novel strategies to induce apoptosis in glioma cells by transduction with adenoviral vectors carrying a variety of apoptosis-related genes, including Fas ligand, Fas, FADD, caspase-8, p53, p33ING1, p73α, Bax, Apaf-1, caspase-9, IκBdN, caspase-3, Bcl-2, and Bcl-XL. We conclude that adenoviral vector-mediated delivery of apoptosis-related genes other than p53 is a potentially useful gene therapy approach toward the treatment of human brain tumors.

The past decade has been marked by significant advances in the application of gene transfer into living cells of animals and humans, with the resulting catapulting of preclinical and basic scientific concepts into therapeutic trials. A variety of virus-mediated gene delivery techniques have proved to be superior to other methodologies. This article concisely reviews the current status of gene and tumor therapy, focusing on virus-based technologies, describes the molecular biology of neurotropic herpesviruses and the application of herpes simplex virus, a relative of pseudorabies virus (PRV) in gene transfer and cancer therapy protocols. Finally, it addresses the issue of whether PRV, a nonhuman pathogen, could serve as a suitable research and therapeutic tool as concerns genetic and tumor therapy.