Gene Delivery Using Physical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 83

into two categories: (1) the use of biological vectors and (2) techniques employing
either chemical or physical approaches. The biological methods utilize viral and
bacterial vectors and are the method of choice due to their high transfection effi-
ciency. These vectors target to the cell receptor and the biological entry mechanism
for achieving the cell transfection. Retroviruses and adenoviruses are the most com-
monly used vectors under clinical trials. Some bacterial vectors are delivered through
the gastrointestinal (GI) tract by transfecting the cells at mucosal surfaces. However,
difficulties with viral vectors in formulation and side effects like immunogenec-
ity, allergic reactions, host rejection, mutagenecity, and oncogenecity restrict them
to use in clinical practice. Thus, long-term safety studies of these vectors for each
application are necessary before these products can be successfully marketed [3] .
Manufacturing quality controls, such as stringent quality control standards, higher
price, and proper cold storage conditions are impediments in the use of biological
vectors for routine clinical use.
The nonviral vectors, that is, the nonbiological techniques of gene delivery,
involve treatment of cells by chemical and physical means. The chemical methods
are comprised of DNA delivery using various chemical agents such as cationic lip-
ids and polymers. Moreover, these agents can be modified to improve cell targeting
and nuclear localization. Chemical methods can reliably and reproducibly transfect
the mammalian cell lines in vitro , but for systemic administration the transfection
becomes more challenging because of the extracellular and intracellular gene deliv-
ery barriers as well as the need for a large volume of tissue to be transfected for clin-
ically beneficial therapy. Chemical vectors are easy to scale up and reproduce with
minimum host immune interaction, and are suitable for selected organs such as the
lung or airway with mucosal tissue as the target site, or for fairly localized tissues,
such as intratumoral inoculation; however, the transfection efficiency is compro-
mised compared to viral vectors [4] . Further, the challenge of preparing a consistent
formulation with DNA stably mixed in the delivery media must be met for consistent
and reproducible gene transfection.
In 1990, Wolff et al. [5] demonstrated high-level DNA expression of reporter
genes in mouse skeletal muscle after the mice were injected with purified “naked”
DNA plasmids. This opened a new era for local delivery of nucleic acids for disease
treatment and vaccination. A naked DNA injection, without any carrier, into local
tissue or into the systemic circulation is probably the simplest and safest physical/
mechanical approach of gene delivery. However, the expression level and the area
of tissue treated at each dose after a naked DNA injection are highly limited due
to rapid degradation by serum and cytoplasmic nucleases and fast clearance by the
mononuclear phagocyte system and the reticular endothelial system (RES) system
[6,7] . Hence, various physical manipulations have been used to enhance the effi-
ciency (rate and extent) of gene delivery to the desired defective cells, especially in
the in vivo conditions. These physical methods for gene transfer have also demon-
strated the ability to circumvent various extra- and intracellular barriers, which sig-
nificantly compromise the efficiency of gene delivery, including massive dilution of
DNA upon injection, accessibility of the target site, and entry into the cell and the
nucleus, thus enhancing gene expression in the desired cells for the desired duration