TRANSDERMAL MICRONEEDLES AND ELECTROPORATION
Transdermal DNA immunization involves the use of arrays of microneedles, each a few hundred microns long, to pierce the barrier
of the stratum corneum (i.e., the skin's outer layer, typically 10–20 µm thick) and deliver the vaccine (7). The skin has
a high concentration of APCs called Langerhans cells, which makes it an attractive target. The advantages of microneedles
are that they are easy to administer and cause less pain at the injection site than does a conventional needle. There are
various strategies to achieve vaccination using microneedles. The simplest involves arrays of microneedles that are used to
pierce or scarify the stratum corneum, thus increasing permeability to a topically applied DNA solution. This approach is
suitable for preclinical studies, but is not scalable. Alternatively, the microneedles can be coated with the dried vaccine
which dissolves following administration. Hollow microneedles can be used to inject solutions containing the vaccine. These
can be submillimeter arrays or scaled-down needles in the range of 1–2 mm. An interesting alternative to fixed, disposable
microneedles is a solid, soluble microneedle array that is either formulated or coated with the vaccine. This array is inserted
into the skin, where the microneedles dissolve or degrade, leaving only the backing to be disposed of and thus eliminating
contaminated sharps.
Smallpox DNA vaccination of mice was demonstrated using the Easy Vax device supplied by Cellectis therapeutics (Paris), and
was the first example of microneedle-mediated electroporation (8). The gun-shaped Easy Vax combines the electroporation and
microneedle approaches, with cutaneous Langerhans cells as the targeted APCs. The plasmid DNA was dried onto the tips of the
eighty-microneedle array and inserted into the skin to enable the DNA to dissolve, followed by the delivery of six electric
pulses.
While the microneedle approach is gaining interest for delivery of therapeutic and antigenic recombinant proteins, only a
few studies in mice have shown immune protection against microneedle-administered DNA vaccines. An additional problem when
translating animal results to humans relates to the amount of DNA that can be loaded onto a microneedle array. A typical human
clinical trial injected dose of 2–4 mg DNA is already relatively small compared with the murine equivalent from which it is
extrapolated, yet this would be prohibitively high to expect to dissolve intradermally from a dried formulation on a microneedle
array, which leaves liquid DNA solution injection as the only viable strategy.
Cellectis therapeutics also supplies the DermaVax electroporator (see Figure 1c), which requires a DNA solution to be injected
sub-dermally using a hypodermic syringe, followed by electroporation through a series of pulses from 2 mm needle electrodes
attached to the bench-top DermaVax apparatus. The DermaVax system induces a 100- to 1000-fold greater increase in gene expression
over injection alone and is being used for the delivery of DNA vaccines against HIV-1, as well as vaccines against prostate
and colorectal cancers in human clinical trials (9).
Inovio Pharmaceuticals has developed a transdermal electroporation device called CELLECTRA-3P. In pilot studies in nonhuman
primates, Inovio demonstrated that intradermal administration of a smallpox vaccine or an H5N1 vaccine successfully protected
animals from a lethal challenge of monkeypox or an H5N1 infection, respectively (10, 11). The device is now in Phase I clinical
trials for a universal influenza vaccine. Inovio has also developed a prototype minimally invasive electroporation device
that has been demonstrated for influenza DNA vaccine delivery.
PRESSURE INJECTORS
Gene guns have been used since the advent of DNA vaccination for high-pressure transdermal delivery of microbeads coated with
DNA (biolistics). PowderMed (Oxford, UK, acquired by Pfizer) developed a handheld particle-mediated epidermal delivery (PMED)
device that uses a high-pressure helium minicylinder to fire DNA-coated gold microbeads into the epidermis. Because the gold
beads enter the cell cytoplasm and target more APCs than does needle injection, greater CTL responses can be generated for
the same amount of DNA (11). In one example, PMED was used to deliver 2 µg of two plasmids to rhesus macaques: one with an
H1N1 hemagglutinin gene, the other expressing the adjuvant GM–CSF. This method generated good antibody and CTL responses (12).
However, limitation to the microbead approach is the amount of DNA that can be coated onto beads, which could restrict its
ability to be scaled up for human applications.
Merial (Duluth, GA) developed the pioneering canine melanoma DNA vaccine Oncept, which was fully approved in 2010. It contains
the human tyrosinase gene, delivered (following surgical tumor removal) in four doses at two-week intervals, followed by a
single dose every six months. Oncept is administered using a needle-free Canine Transdermal Device developed by Bioject Medical
Technologies (Portland, OR) (13). Bioject specializes in the development of needle-free injection systems that use high-pressure
injection to fire a stream of the DNA vaccine solution through the epidermis. Their latest device, ZetaJet, is small and spring-activated
for ease of use in the field, and fires up to 0.5 mL into skin or muscle tissue.
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