EX VIVO VERSUS IN VIVO GENE THERAPY
Gene therapy is a means of treatment for diseases that have limited or no therapeutic options. Once a disease has been identified as a possible candidate, the gene necessary for treatment must be identified and cloned. Enough must be understood about the disease and the gene product in order to ensure that the therapeutic component is delivered to the appropriate cellular compartment responsible for its processing and sub-sequent biological activity. The necessity for long-term gene expression or requirements that expression be timed with other biological processes will influence the design of the vector component of the therapy. The target tissue/organ must be readily accessible and a defined, measurable endpoint must be identified for assessment of therapeutic efficacy. Several strategies can be used for gene transfer. Direct injection of vector/ DNA complexes into the bloodstream is often char-acterized by low levels of gene expression, making large amounts of vector necessary for therapeutic efficacy. Broad distribution of the vector may adversely affect the function and health of normal tissues and cause adverse reactions. This method also has limited utility for the treatment of tissues with restricted blood supplies such as muscle and tumors. Other delivery methods employed for in vivo gene transfer include intratumoral, intraperitoneal subcutaneous, and intra-muscular injection. These are crude, invasive and, in some cases, require specialized surgical skills, making efficient tissue-targeted delivery systems essential for successful gene transfer.
Ex vivo gene transfer strategies involve isolation and culture of cellular targets. Gene transfer is achieved by direct application of the vector (virus, plasmid) for efficient gene expression (Fig. 1). Cells are then studied after treatment and only healthy cells expressing the therapeutic gene collected and given to the patient. Initially, this approach was limited to disorders in which the relevant cell population (i.e., bone marrow, hepatocytes) could be removed from the affected individual, modified and then replaced. Today, cells can be administered by local or systemic injection, delivered in encapsulated form or as tissues comprised of cells seeded on scaffolds. Patient safety is often improved with ex vivo gene transfer since the host immune response to the vector or toxic effects associated with transfection reagents are eliminated.