II. Cationic lipid-formulated plasmid DNA-based Bacillus anthracis vaccine: evaluation of plasmid DNA persistence and integration potential.

Several formulated plasmid DNA (pDNA)-based vaccines are being evaluated for safety and efficacy in healthy human subjects. A safety concern for any vaccine that contains genetic material, be it whole organism, live-attenuated, or gene-based, is the potential for integration into genomic DNA (gDNA). To address this concern, a preclinical pDNA persistence/integration study was conducted in rabbits to determine the level of pDNA in muscle 2, 28, and 64 days after intramuscular injection of DMRIE:DOPE-formulated pDNAs encoding Bacillus anthracis detoxified LF and PA proteins (VCL-AB01 vaccine). Total DNA was extracted from day 64 muscle tissue and fractionated by column agarose gel electrophoresis (CAGE). Plasmid copy number (PCN) in muscle 64 days after injection (geometric mean, 2808 PCN/microg of total DNA or 150,000 diploid genomes) was determined by quantitative polymerase chain reaction. Analysis of total DNA from five VCLAB01- injected rabbits revealed that two of five samples had no detectable PCN in the high molecular weight fraction after one round of CAGE, two samples had PCN under the lower limit of quantitation, and the remaining sample had 123 PCN/microg. All PCN in the latter sample cleared after an additional round of CAGE. It appears, therefore, that persisting PCN fractionate as low molecular weight material and are most likely not integrated into gDNA. Even if the worst-case assumption is made that the highest PCN found associated with gDNA represented covalently integrated pDNA inserts, the frequency of mutation would still be 500-fold lower than the autosomal spontaneous mutation rate.