Delivery of protein therapeutics affords very fast but transient effects due to inevitable degradation. Selective and safe transfection of endothelial cells with genes encoding therapeutic proteins may help to overcome this problem and bring about new means for treatment of diverse disease conditions, acute (thrombosis, respiratory distress syndrome, stroke, inflammation) and chronic (tumor growth, systemic and pulmonary hypertension, atherosclerosis, diabetes). However, endothelial cells are difficult to transfect even in cell cultures; the challenges of implementation of this goal in vivo are formidable [3, 10, 12, 15].
Nonviral vehicles for gene delivery (e.g., cationic liposomes) are generally viewed as safer than viral means, yet the latter offer more effective and prolonged transgene expression. However, both nonviral and viral vehicles have no tropism to endothelium, whereas nontarget cells may take up circulating liposomes and/or express high levels of receptors for viruses (e.g., hepatic cells). Thus most transgene proteins are produced predominantly in liver and other tissues, which are at risk of toxicity due to inadvertent transduction. Targeting of either nonviral or viral carriers of genetic materials to endothelial determinants, for example using monoclonal antibodies to ACE, PECAM-1, or selectins conjugated to the liposomes or viral particles, might enhance effectiveness of transfection of endothelial cells and help to reduce side effects [9, 12]. Coupling of artificial affinity moieties (antibodies or peptides) to rationally selected viral determinants also blocks their function of receptor-mediated docking to nontarget cells, thus altering viral tropism (viral retargeting) and permitting partially reduced side effects .
Identification and utilization of promoters for PECAM-1, Flt-1 (a receptor for vascular endothelial growth factor), and other endothelium-specific proteins also enhances specificity of transfection of vascular endothelium . Loading genetic materials driven by endothelial-specific promoters into vehicles utilizing affinity carriers directed to endothelial surface determinants synergistically combines advantages of transductional and transcriptional targeting . For example, successful specific gene expression in the pulmonary endothelium was achieved using ACE antibody-conjugated adenoviruses driven by an endothelium-specific Flt-1 promoter (Figure 1).
Viral retargeting can be attained either by chemical conjugation of targeting antibodies or peptides to the surface of viral capsid or by genetic incorporation of these affinity moieties into the capsid structure. Genetic retargeting approaches have to a large extent been limited by the structural constraints of viral capsid proteins: Only a few relatively small ligands have been inserted without disruption of the viability of the virus. Efforts are currently underway to incorporate bigger single-chain fragments of antibodies (scFv) directed against identified vascular targets into viral capsid and/or to use phage panning in vivo to define unique vascular addresses for specific vascular beds .
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