approaches. Each general approach has its own virtues and is discussed separately. 
The specific rationale for adenovirus is then developed. 
II.B.3.a. Nonintegrative Approaches 
In transient expression approaches, the cDNA coding for CFTR is 
introduced into affected cells by vectors that do not require chromosomal 
integration for transcription. Typically, expression of the foreign protein by 
nonintegrative approaches will last from days to weeks. One approach envisioned is 
to utilize a cDNA cloned into a plasmid vector encapsulated into a liposome that 
can be delivered to the airway cell via inhalation. Liposomes can carry a variety of 
plasmid vectors, including vectors that can promote nuclear targeting of foreign 
DNA. Liposomal technologies for the efficient transfer of genes have improved 
rapidly, but at present they probably do not approach the degree of efficacy required 
to reach a significant fraction of airway epithelial cells (34). It is also not yet known 
whether there will be a requirement for selective targeting of a liposome to one 
epithelial cell type or another. However, it is clear that cells facing the airway 
lumen will require correction. 
Nonintegrative therapy can also be delivered to the airway epithelial cells by 
a variety of viral vectors. For example, it is possible that vectors that are continually 
maintained episomally [e.g., Epstein-Barr virus vectors (35) can be considered. The 
vaccinia viral system has been utilized to express CFTR transiently in airway 
epithelial cells and correct the Cl' channel defect (36); however, the fact that these 
viruses are ultimately lytic make them unlikely candidates for gene therapy 
purposes. 
II.B.3.b. Integrative Approaches 
The alternative strategy in gene therapy is to utilize vectors that promote 
integration of the foreign cDNA into the host genome. The obvious advantage of 
this approach is that the corrective gene can be maintained indefinitely in 
progenitor cells and can ultimately be viewed as effecting a ’cure’ of cystic fibrosis 
airways disease. The most efficient vector presently available for this purpose is the 
retrovirus. Retroviruses deliver RNA into the host cell, which is reverse transcribed 
into DNA and, via retroviral integrase activities, incorporates the foreign DNA into 
the host genome. Recent studies have indicated that retrovirally delivered normal 
CFTR cDNAs correct the cystic fibrosis apical membrane Cl" channel in polarized 
airway epithelial cells (37). This result, plus the observation that correction can be 
maintained in progenitor cells, which can differentiate into transporting epithelial 
cells in vitro for up to 6 months, indicates that this may be an efficient means of 
treating cystic fibrosis lung disease. The low proliferative rate of airway epithelial 
cells, however, makes it likely that this vector will not be sufficiently efficient to be 
practically useful for treating adult CF lung disease. 
II.B.3.C. Adenovirus 
The development of effective gene therapies for CF lung disease requires the 
creation of new approaches that are capable of targeting genes to airway epithelial 
cells by direct administration in the airway. Furthermore, gene transfer must be 
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Recombinant DNA Research, Volume 17 
