“DNA vaccines offer a host of advantages over conventional vaccine technology,” noted Henry Hebel, Ph.D., COO of VGXI, a CMO. “Because they can be produced rapidly, they are ideal for combating emerging diseases. Moreover, they are safe to produce and administer, and do not constitute a threat to the production team.”
But as other participants in the conference noted, plasmid manufacturing poses a number of critical challenges. The E. coli host is renowned for the large quantities of endotoxins that it produces, and plasmids are notoriously hard to work with.
“Ease of manufacture” is a relative term, and what may be easy on the benchtop may be devilishly difficult when scale-up time arrives. The field is also subject to the “intellectual property thicket syndrome,” in which patents crowd out one another, raising expenses and confusing legal issues. “Plan early and build twice as much product as you think you’ll need,” Dr. Hebel advised.
The VGXI team searches for the most straightforward solution, weighing relative risks and benefits, while taking into account intellectual property. The purification procedure that it selected consists of anion exchange membranes, followed by hydrophilic interaction chromatography, which is then followed by thiophilic interaction chromatography for polishing.
“We consider cell lysis the critical step in the purification process,” said Dr. Hebel, “At this stage we avoid any procedure that could shear the plasmids. We also look for alternatives to traditional purification chromatography during the subsequent steps.”
The plasmid concentration step is another difficult issue, as mishandling can cause shearing and other destructive changes to the preparation. “As we optimize our protocol, we look to remove roadblocks,” he continued. “We need to justify the designation of contamination levels, degree of acceptable supercoiling, limits of cleaning, and the amount of plasmid DNA required.”