DNA Delivery Background
What are the alternative methods of DNA vaccine delivery?
DNA vaccines have tremendous potential as preventive or therapeutic agents against cancers and infectious diseases. One of the obstacles to the successful development of DNA vaccines is the limitations associated with most DNA delivery systems. A successful DNA delivery system must be able to safely and effectively deliver the DNA vaccine into cells of targeted tissue where the DNA can subsequently achieve gene expression of the encoded protein at desired levels and for the desired duration of time. To be commercially compelling, the delivery method must be readily reproducible and low cost.
One method that has shown a compelling balance of safety, efficiency, and economy in pre-clinical and initial human data is Inovio's electroporation-based DNA delivery solution.
The following table compares the main DNA carrier or delivery alternatives:
What are the challenges to DNA vaccine delivery?
Viral vectors (carriers) have been the most studied approach to intracellular DNA delivery. Leveraging the natural ability of viruses to insert and express a "genetic payload" in human cells, scientists modify a virus to carry beneficial genes such as a DNA vaccine. These viral vectors, containing their payload, are then injected by a syringe/needle in target tissue such as muscle.
There are multiple issues with viral vectors that have plagued DNA vaccine developers using this approach:
- Viral vectors may insert their genes randomly into the target cell's chromosomes, risking disruption of genetic regulatory machinery and/or causing mutations that could lead to cancer. [1]
- There are size constraints on the genetic payload that can be delivered. [2]
- They may induce an unwanted immune response against themselves, making the patient resistant to subsequent vaccinations using the same viral vector. While these viral vectors may effectively deliver their payload during the prime or first vaccination, if booster (additional) vaccinations are required, the immune system might attack and remove the viral vector before it can deliver the vaccine it is carrying. [3]
- Viral vectors can be difficult and expensive to develop and to manufacture in a controlled manner, and obtaining regulatory approval may be challenging.[4]
The industry continues to invest significant resources to achieve the therapeutic profile, i.e. the right balance of safety and efficacy, necessary to enable viral vectors to play a role in facilitating DNA vaccine delivery.
New therapeutic strategies are also being evaluated in which an alternative delivery method is used for prime applications of a vaccine and then a viral vector-based vaccine is used for the booster application of the vaccine. For example, Merck is conducting a clinical study to evaluate this prime-boost strategy with an electroporation-viral vector combination.
Liposome or lipid vectors are also modified to carry a DNA vaccine payload and are injected by needle into selected tissue. They are not highly effective in delivering their payload, are challenging to manufacture in a consistent and cost effective manner, and have shown toxicity in vivo. [5]
The gene gun or biolistic gun, which blasts microscopic DNA-vaccine-coated gold particles into the patient's skin, was thought to be an efficient method for administering DNA vaccines. "However, its use in humans was generally associated with severe local pain, erythema lasting 2-4 weeks postdelivery, and skin discoloration lasting up to 6 months. In some cases, skin necrosis was reported." [6]
The challenge of these delivery methods is that the "carrier," i.e. virus, lipid, or gold particle, they use to transport the vaccine may introduce its own unique challenges with respect to safety, utility, or manufacturability.
A DNA delivery method widely considered by scientists to be safe is the injection of naked DNA or DNA plasmids into muscle. Scientists manufacture small circular pieces of DNA, called plasmids, containing the DNA fragment encoding the desired protein relating to a targeted disease. Plasmids can be designed to code almost any desired protein (antigen) and typically do not integrate into chromosomes. Plasmid-based vaccines can be inexpensively produced in large quantities in bacteria. Unfortunately, despite their early promise, plasmids injected into muscle without any other method to enhance their level of cellular uptake typically do not achieve sufficient gene expression to induce a clinically relevant immune response.
The emergence of a promising alternative: electroporation-based DNA delivery
One alternative method of DNA delivery is attracting attention and endorsements from scientists. Called electroporation, this method uses electrical pulses to create pores in cells of muscle or skin and enhances intracellular delivery of DNA plasmids by 1,000 times or more. This method appears to provide a desirable balance of safety, efficiency, and cost effectiveness.
Inovio Pharmaceuticals is a leader in developing human applications of electroporation and possesses a significant patent estate relating to the use of electroporation for gene therapies and DNA vaccines.
1. Howe, SJ et al., 2008. Insertional mutagenesis combined with acquired somatic mutations causes leukemogenesis following gene therapy of SCID-X1 patients. J. Clin Invest. 118:3143-3150/
2. McCarty, DM. 2008. Self-complementary AAV vectors; advances and applications. Mol Ther. 16:1648-56.
3. Pantaleo, G., 2007. HIV-1 T-cell vaccines: evaluating the next step. The Lancet 8:82-83.
4. Kamen A and Henry O. 2004. Development and optimization of an adenovirus production process. J Gene Med S1:S184-92.
5. Tong, AW., 2009. Systemic therapeutic gene delivery for cancer: crafting Paris' arrow. Curr Gene Ther. 9:45-60.
6. Fuller, DH et al., 2006. Preclinical and clinical progress of particle-mediated DNA vaccines for infectious diseases. Methods 40:86-97
Next section: How does electroporation work and why is it so effective?