Plasmid DNA–Based Vaccines: Combating Infectious Diseases - A comparison of plasmid DNA-based vaccines with vaccines developed from egg and cell-based methods - BioPharm International

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Plasmid DNA–Based Vaccines: Combating Infectious Diseases
A comparison of plasmid DNA-based vaccines with vaccines developed from egg and cell-based methods


BioPharm International


The plasmid DNA production process is a readily scalable process which can be scaled up to meet worldwide manufacturing demands for vaccines. Recent process improvements to plasmid DNA production have significantly shortened the manufacturing timeline. New high-cell-density fermentation methods have dramatically increased plasmid DNA productivity and shortened the overall production timeline. These new methods are able to achieve overall plasmid yields of greater than 1 gram of purified plasmid DNA per liter of fermentation. Scaled-up downstream lysis and purification activities also have greatly reduced the overall production timeline. Large-scale purification techniques have incorporated the use of a single chromatography column, which results in higher overall yield and consistently pure plasmid DNA. It is possible to produce a de novo plasmid DNA vaccine through manufacture, filling, testing, and release in less than three months. Stability studies performed on numerous formulated plasmid DNA lots have shown that they are rock stable when properly formulated and can be stored for extended periods of time at controlled temperatures, which makes plasmid DNA a good candidate for vaccine use in fighting significant public health diseases, where delivery and stockpiling of vaccines is a serious concern. Lyophilization and formulation optimization are now being used to develop DNA vaccines, which will be stable without requiring cold storage.

Technologies for Plasmid DNA Delivery

Though a plasmid DNA vaccine has not yet been approved for commercial use in humans, there are numerous clinical trials currently being conducted using these vaccines. New technologies for plasmid DNA delivery such as electroporation are currently being used in clinical trials to enhance the immune response of the patient. Other methods of vaccine delivery include the "prime boost" method, in which the patient is first primed with the plasmid DNA vaccine and then a protein subunit vaccine is delivered to the patient in order to boost the antibody or cellular immune response.2 These new methods of plasmid DNA vaccine delivery have shown promising results in preclinical studies and clinical trials.

Early in the development of DNA vaccines, there were a number of concerns regarding the manufacturability, stability, and safety of such products. Safety concerns focused on two primary questions: would the introduced DNA remain at the site of injection, or could it migrate through the body to the reproduction organs and be incorporated into gamete cells (egg or sperm)? The second question was: could the introduced gene incorporate itself into the target cell's chromosome where it could potentially activate an oncogenic gene or inactivate a tumor suppressor? These two phenomena have been observed at a very low frequency with viral vectors; however, they do not appear to occur in the case of DNA vaccines. New constructs are still evaluated in good laboratory practices (GLP) controlled biodistribution and integration studies prior to use in clinical trials as a safety precaution.

Conclusion

Though plasmid DNA vaccines for human use are still in the stages of clinical development, the vaccines have a number of potential advantages over traditional vaccines. The plasmid DNA vectors can be designed and produced quickly. The overall plasmid DNA production, from cell bank production and testing, through fermentation, purification, filling, and release testing of final product can be completed in a very short timeframe. The process scalability and rapid production time ensures that plasmid DNA vaccines could be developed and produced to meet aggressive timelines if there were a pandemic outbreak of disease. The immune responses achieved with plasmid DNA and the use of delivery techniques, such as electroporation and the prime boost method have shown to have long-lasting results and provide disease protection.

While DNA vaccines have been approved for veterinary applications, some significant challenges remain in terms of optimizing the targeted immune responses in humans. Various strategies have shown great promise in resolving this last hurdle; scientists are now using novel adjuvants and electroporation to enhance immunogenicity and plasmid delivery to target cells. While the field of DNA vaccines is still in its infancy, and real challenges remain, the early concerns about safety and the ability to produce large quantities of DNA in a scalable, regulatory compliant manner have been resolved. The manufacturing process, which in the early days employed cesium gradients and animal-derived enzymes, such as RNAse, and a tremendous amount of labor to produce a few milligrams has evolved to the point where hundreds of grams of DNA can be produced within as short as 10 days. As science progresses forward in the treatment of diseases, it is certain that plasmid DNA will play a role in future vaccines for use in humans.

Richard B. Hancock is the executive vice president and chief operating officer at Althea Technologies, San Diego, CA, 858.882.0123,
. At the same company, Melissa M. Rosness is the director of contract management.

References

1. Okonek B, Morganstein L. Development of polio vaccines. Available from: http://www.accessexcellence.org/AE/AEC/CC/polio.html.


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