Gene Delivery Using Physical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 104

is delivered using a transducer to conduct sound through the culture medium into the
cells. Mammalian cells differ in their tolerance to ultrasound energy. Some cells are
quite robust, whereas others are sensitive. It is best to conduct a small pilot study to
determine the sensitivity of the cell line being transfected. The cells are then grown
normally and assayed as appropriate to the characteristics of the delivered gene. The
expression of CAT protein after applying ultrasound energy to various cell lines like
He�a, C-127, and NIH/3T3 results in enhanced transfection efficiency [237] .
However, even with the use of ultrasound contrast reagents, most studies only
show an average of 10- to 15-fold increase in reporter gene expression, which is con-
siderably below that which can be achieved with EP. A combination of EP and ultra-
sound (electrosonoporation) has been described by Yamashita et al. In their study,
electrosonoporation was superior to either EP or ultrasound alone [238] . However,
it is difficult to compare this study to others in the field due to insufficient method-
ological details for the EP component.
3.8 Iontophoresis
Iontophoresis involves the application of a low-density electric current for enhancing
the penetration of preferably charged molecules through pre-existing cellular path-
ways in cells and tissues [239,240] . This method is used to enhance the penetration
of nucleic acids noninvasively. The ease of application, minimization of systemic side
effects, and increased drug penetration directly into the target region have resulted in
an extensive clinical use of iontophoresis mainly in the transdermal field. In 1908,
the German scientist Wirtz first investigated ocular iontophoresis for the treatment of
corneal ulcers, keratitis, and episcleritis, by passing an electric current through elec-
trolyte-saturated cotton sponges placed over the globe [241] . In subsequent decades,
iontophoresis has been extensively investigated for delivering ophthalmic drugs,
including dyes, antibacterial, antiviral, antifungal, steroids, antimetabolites, and even
genes. However, despite its widespread use and study during the first 60 years of the
twentieth century, iontophoresis has never been used as standard procedure because
of the lack of carefully controlled clinical trials and the paucity of toxicity data.
The mechanisms of iontophoretic transport across a membrane include direct
interactions of the electric field with the charge of an ionic compound (electropho-
resis or electromigration), convective solvent flow affecting the transport of both
neutral and ionic compounds (electro-osmosis), and electric field-induced pore for-
mation in the membrane (EP or electropermeabilization) [242] . Currently, iontopho-
resis is believed to be carried out through paracellular transport [243,244] . Negatively
charged human skin at neutral pH also enhances the electro-osmotic flow from anode
to cathode when current is applied. Hence, enhanced tissue penetration of neutral
molecules by anodal iontophoresis and improved penetration of cations through this
supplementary driving force are achieved [239,245] . The iontophoretic efficiency has
been affected by several factors, including ion and molecule permeability, environ-
mental pH [243] , current density (constant or alternating), shape of the electrodes,
duration of treatment, drug properties, and concentration [246] .