Gene therapy in cystic fibrosis

Theoretically, cystic fibrosis transmembrane conductance regulator (CFTR) g ene replacement during the neonatal period can decrease morbidity and morta lity from cystic fibrosis (CF). In vivo gene transfers have been accomplish ed in CF patients. Choice of vector, mode of delivery to airways, transloca tion of genetic information, and sufficient expression level of the normali zed CFTR gene are issues that currently are being addressed in the field. T he advantages ana limitations of viral vectors are a function of the parent virus. Viral vectors used in this setting include adenovirus (Ad) and aden o-associated virus (AAV). Initial studies with Ad vectors resulted in a vec tor that was efficient for gene transfer with dose-limiting inflammatory ef fects due to the large amount of viral protein delivered. The next generati on of Ad vectors, with more viral coding sequence deletions, has a longer d uration of activity and elicits a lesser degree of cell-mediated immunity i n mice. A more recent generation of Ad vectors has no viral genes remaining . Despite these changes, the problem of humoral immunity remains with Ad ve ctors. A variety of strategies such as vector systems requiring single, or widely spaced, administrations, pharmacologic immunosuppression at administ ration, creation of a stealth vector, modification of immunogenic epitopes, or tolerance induction are being considered to circumvent humoral immunity . AAV vectors have been studied in animal and human models. They do not app ear to induce inflammatory changes over a wide range of doses. The level of CFTR messenger RNA expression is difficult to ascertain with AAV vectors s ince the small size of the vector relative to the CFTR gene leaves no space for vector-specific sequences on which to base assays to distinguish endog enous from vector-expressed messenger RNA. In general, AAV vectors appear t o be safe and have superior duration profiles. Cationic liposomes are lipid -DNA complexes. These vectors generally have been less efficient than viral vectors but do not stimulate inflammatory and immunologic responses. Anoth er challenge to the development of clinically feasible gene therapy is deli very mode. Early pulmonary delivery systems relied on the direct instillati on of aerosolized vectors, which can result in the induction of adverse rea ctions because vector is delivered into the lung parenchyma. More recent st udies have examined the potential for using spray technologies to target ae rosolized AAV vectors to the larger central airways, thereby avoiding alveo lar exposure and adverse effects. Comparisons of lung deposition with nebul ized delivery of aerosol and spray delivery indicate that spraying results in a more localized deposition pattern (predominantly in the proximal airwa ys) and significantly higher, deposition fractions than nebulization. These findings could lead to more efficient and targeted lung delivery of aeroso lized gene vectors in the future.