The majority of the non-AF508 mutations are rare, some occurring in only a few CF 
patients, and they result in a range of disease severity from severe to mild^. 
Although treatment for patients with CF is improving, the median survival age of CF 
patients is only 29 years. Treatment typically involves a combination of physio-therapy, 
antibiotic treatment, and pancreatic enzyme supplements. Recently, aerosol DNase has been 
shown to improve pulmonary function and decrease the rate of respiratory tract 
exacerbation. Although all of the current therapies address symptoms of the disease, none 
impact on the primary defect in Cl" transport. Gene therapy holds the promise of targeting 
the primary defect by endowing airway cells with ability to reconstitute proper CT transport 
in vivo. 
3.0 RATIONALE 
3 . 1 Genetic Therapy Approaches in the Treatment of CF 
Correction of the basic defect in cAMP-stimulated chloride conductance by transfer of the 
CFTR cDNA into epithelial cells derived from CF patients^ has led to the hypothesis that a 
similar genetic delivery in an intact lung could prevent the development of CF lung disease. 
The anatomy and histologic organization of the respiratory tract dictates that this can only 
be accomplished with a gene transfer vector which will function in vivo to deliver a normal 
CFTR cDNA directly to the airway epithelial cells. The ability of a vector to target airway 
epithelial cells is, therefore of central importance. 
A number of vector systems have been described for potential use in gene therapy. For 
CF, retroviral systems are of limited utility because most of the respiratory epithelium is 
comprised of non-dividing cells. Several non- viral systems have been proposed such as 
receptor-mediated endocytosis methods and liposome-mediated delivery. Neither of these 
systems has the intrinsic ability to generate long term expression. 
One logical approach to this problem is to use recombinant virus vectors derived from 
isolates which have evolved mechanisms for infection and gene transfer into the human 
respiratory tract. One such vector, adenovirus type 5 (Ad5), has been used for gene 
transfer in the cotton rat model^TO. The use of recombinant adenoviruses for CF gene 
therapy in humans has recently been approved by the Recombinant DNA Advisory 
Committee of the National Institutes of Health, and several clinical trials using Ad-CFTR 
vectors are currently in progress. However, data in both non-human primates ^ and 
humans 1 3 indicate the pulmonary inflammation occurs at high adenovirus vector dose 
required for CFTR expression . 
Alternative vector systems that combine the benefits of direct in vivo delivery with the 
potential to persist stably for prolonged periods are necessary for prevention of the 
pulmonary complications of CF. One such system that fits these criteria, without evidence 
to date of toxicity, uses an adeno-associated virus vector. 
3.2 Adeno-Associated Virus (AAV) Vectors 
Adeno-associated virus (AAV) vectors are among a small number of recombinant virus 
vector systems which have been shown to have utility as in vivo gene transfer agents 14,15 
and thus are potentially of great importance for human gene therapy. AAV vectors are 
capable of high-frequency stable transduction and expression in a variety of cells including 
CF bronchial and nasal epithelial cells5>16-18 
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Recombinant DNA Research, Volume 20 
