A traditional approach for inserting DNA into microbial and animal cells in culture has been by the application of an electric
pulse, which creates transient pores in the cell membrane and allows entry of the negatively charged DNA molecules. Electroporation
has been adapted from its in vitro single-cell origins to enhance in vivo delivery to tissues following intramuscular injection, and enables a significant increase in transfection efficiency and
immune response over injection alone. The disadvantages include pain at the injection site due to the electric pulse, and
the requirement for specialized electroporation devices. The most common approaches use a disposable grid consisting of injection
and electrode needles attached to the device that provides the electric pulse. The DNA solution is injected, needle electrodes
are inserted through the skin and into muscle tissue, and an electric field applied to facilitate DNA entry into dendritic
cells and myocytes.
Ichor Medical Systems (San Diego, CA) has developed the TriGrid Delivery System (TriGrid) a hand-held electroporation device
that has been demonstrated to increase DNA vaccine delivery efficiency by as much as 1000-fold over injection alone. Four
electrodes are arranged in a diamond shape around the central needle, which is a conventional single-use hypodermic syringe
that is inserted into the device, and an electric pulse is administered upon injection (see Figure 1a). Ichor has demonstrated
greatly enhanced potency of DNA vaccines delivered with the TriGrid in several animal models using DNA vaccines that encode
antigens such as anthrax, hepatitis B virus, and tumor-associated antigens. The device is being evaluated in several clinical
trials, and results demonstrating enhanced responses to a prophylactic HIV DNA vaccine delivered with TriGrid compared with
conventional needle injection have been published recently (4). In addition, Ichor has shown in animal models that delivery
of DNA encoding the protein therapeutics erythropoietin and interferon-β may be a viable alternative to injection of the recombinant
Figure 1: Devices for plasmid DNA delivery indluding (a) TriGrid and (b) CELLECTRA with applicator (for intramuscular electroporation),
(c) DermaVax (for transdermal electroporation) and (d) ZetaJet (for subcutaneous or intramuscular high-pressure injection).
A therapeutic plasmid DNA application (although not a vaccine) that was licensed for pigs in 2008 is LifeTide SW, developed
by VGX Animal Health (The Woodlands, TX) (5). The plasmid produces growth hormone-releasing hormone and is administered to
sows of breeding age to increase the number of piglets weaned, and requires only a single treatment. Plasmid delivery is by
direct injection into skeletal muscle, followed by electroporation using a portable electrokinetic device. The device, called
CELLECTRA, was developed by Inovio (see Figure 1b) and uses a sterile, disposable electroporation needle array. Inovio has
also demonstrated 1000-fold greater DNA delivery compared with injection alone. The device is equally suited to DNA vaccine
applications, and users of CELLECTRA have targeted a range of pathogens including HIV, Clostridium difficile, hepatitis C virus, and malaria parasites. Inovio has an ongoing Phase II clinical trial for cervical-cancer therapy (human
papilloma virus therapeutic vaccine) using the device. Additional ongoing human clinical trials include Phase I trials for
prophylactic vaccines for influenza (i.e., avian and seasonal) and HIV, and a Phase I clinical trial for HIV therapy.
While ample evidence indicates electroporation enhances the delivery and immunogenicity of injected DNA vaccines, it has the
drawback of increased discomfort at the administration site due to the electrical pulses. However, initial human clinical-trial
data with intramuscular delivery indicates that the electroporation procedure is tolerable for therapeutic and even prophylactic
applications. Furthermore, continued improvements to device design are likely to result in increased acceptability of the
procedure. A novel approach to delivering electrical pulses within target tissue that may increase tolerability is the generation
of electrical fields by piezoelectricity, as used in a contactless, noninvasive device developed by Inovio.
Another interesting alternative to penetrative electrodes has been developed by MagneGene (Lake Forest, CA), which uses contactless
magnetic electroporation. This technique relies on a strong, rapidly changing magnetic field generated by a paddle-shaped
device to induce rotating electric fields in the host tissue. This device has been evaluated in guinea pigs following injection
with reporter plasmid, where enhanced in vivo gene expression was demonstrated. Although the magnetic field causes muscle spasms, these subside immediately after use of
the device, and there is no pain following the procedure (6).