Gene Delivery Using Viral Vectors - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 219

with liver-restricted expression of human uridine diphospho-glucuronosyl transferase
1A1. These gutless adenoviral vectors were also used in ex vivo gene delivery of the
human cystic fibrosis (CF) gene into human sweat glands, showing that the CF gene
could be efficiently delivered to and expressed by the epithelial cells of sweat glands
[43] . The data obtained suggested that the gutless Ad vector system is one of the
most potential viral vector systems for gene therapy.
Lethal acute toxicity was found when the animal received the higher dose sys-
temic injection of gutless Ad vectors into nonhuman primates. Two baboons were
injected with 5.6  10 12 or 1.1  10 13 gutless viral particles/kg [44] . The acute toxic-
ity was consistent with the activation of the innate inflammatory response, indicating
that acute toxicity is due to viral particles and independent of viral gene expression.
Production of high-quality gutless Ad vectors is more difficult than for the first- and
second-generation adenoviral vectors. Both helper viruses and gutless ad vectors are
added in cell cultures to coinfect Cre-expressing cells. Instability of the vectors may
result from recombination between the helper viral DNA and the vector DNA within
the productive cells. The inability to produce high-quality vectors in large quantity
also limits extensive investigation and application of the vector in gene therapies.
Several attempts were made to improve the gutless Ad vector system by the modi-
fication of the present Cre/ lox P recombination system or by the use of a different
recombination system, such as the FLP/frt system [40,45,46] . However, the current
most competent means of producing gutless Ad vectors is still based on the scheme
of deleting the packaging signal in the helper virus DNA. A major breakthrough in
gutless vector production is still needed before this type of vector may be ready for
clinical applications.
5.2.6 Application of Adenoviral Vectors
5.2.6.1 Liver-Targeted Gene Therapy
Liver-targeted gene therapy is being actively developed as a method for treating vari-
ous liver diseases, and it is effective on both inherited disorders and acquired condi-
tions, such as infectious and neoplastic diseases, cirrhosis of the liver, and immune
rejection of transplants. Recombinant adenoviral vectors are used extensively in liver-
directed gene therapy because they can infect nondividing cells with high efficiency
and are rapidly concentrated in the liver after systemic administration [47,48] .
Adenoviral vector-mediated gene expression therapy could replace missing gene
products that cause inherited diseases. A biological source of circulating proteins, such
as factor IX or erythropoietin, is used for gene replacement strategies of adenoviral
vectors. Ad vector that maintains high transduction efficiency in vivo with reduced tox-
icity and gene transfer to the liver can also be used to convert this organ into a fac-
tory of secreted proteins needed to treat conditions that do not affect the liver itself.
Gene delivery to the liver using first-generation vectors via lungs resulted in toxicity
and immune responses, which have limited the duration of gene expression [49,50] . In
a clinical trial for gene replacement in patients suffering from ornithine transcarbamy-
lase deficiency, a patient died because of a remarkably high dose of vector (10 13 vector