Plasmid vectors have been widely used for DNA vaccines and gene therapy. Following intramuscular injection, the plasmid that persists is extrachromosomal and integration into host DNA, if it occurs at all, is negligible. However, new technologies for improving DNA delivery could increase the frequency of integration. In the present study, we tested the effect of electroporation on plasmid uptake and potential integration following intramuscular injection in mice, using a plasmid containing the mouse erythropoietin gene. Electroporation increased plasmid tissue levels by approximately six-to 34-fold. Using a quantitative gel-purification assay for integration, electroporation was found to markedly increase the level of plasmid associated with high-molecular-weight genomic DNA. To confirm integration and identify the insertion sites, we developed a new assay -referred to as repeat-anchored integration capture (RAIC) PCR -that is capable of detecting rare integration events in a complex mixture in vivo. Using this assay, we identified four independent integration events. Sequencing of the insertion sites suggested a random integration process, but with short segments of homology between the vector breakpoint and the insertion site in three of the four cases. This is the first definitive demonstration of integration of plasmid DNA into genomic DNA following injection in vivo.

The primary safety concern for DNA vaccines is their potential to integrate into the host cell genome. We describe an integration assay based on purification of high-molecular-weight genomic DNA away from free plasmid using gel electrophoresis, such that the genomic DNA can then be assayed for integrated plasmid using a sensitive PCR method. The assay sensitivity was approximately 1 plasmid copy/µg DNA (representing ∼150,000 diploid cells). Using this assay, we carried out integration studies of three different plasmid DNA vaccines, containing either the influenza hemagglutinin, influenza matrix or HIV gag gene. Six weeks after intramuscular injection, free plasmid was detected in treated muscle at levels ranging from approximately 1,000 to 4,000 copies/µg DNA. At 6 months, the plasmid levels ranged between 200 and 800 copies/µg DNA. Gel purification of genomic DNA revealed that essentially all of the detectable plasmid in treated quadriceps was extrachromosomal. If integration had occurred, the frequency was ≤1–8 integrations per 150,000 diploid cells, which would be at least three orders of magnitude below the spontaneous mutation rate. Our results suggest that the risk of mutation due to integration of plasmid DNA vaccines following intramuscular injection is negligible.

Studies have been designed to examine the potential integration of DNA vaccines into the host cell genome. This is of concern because of the possibility of insertional mutagenesis resulting in the inactivation of tumor suppressor genes or the activation of oncogenes. The requirements for adequate testing were determined to be (1) a method to purify host cell genomic DNA from nonintegrated free plasmid, (2) a sensitive method to detect integrated plasmid in the purified genomic DNA, and (3) stringent methods to avoid contamination. These requirements were fulfilled by agarose-gel electrophoresis, the polymerase chain reaction, and separation of each activity with stringent handling procedures, respectively. An exploratory experiment was carried out in which mice were injected with 100 micrograms of vaccine plasmid DNA in each quadriceps. Examination of quadriceps and 12 other tissues at several time points failed to reveal any evidence of integration at a sensitivity level that could detect 1 to 7.5 integrations in 150,000 nuclei. A worst-case scenario determined that this would be at least 3 orders of magnitude below the spontaneous mutation frequency.

A variety of factors could affect the frequency of integration of plasmid DNA vaccines into host cellular DNA, including DNA sequences within the plasmid, the expressed gene product (antigen), the formulation, delivery method, route of administration, and the type of cells exposed to the plasmid. In this report, we examined the tissue distribution and potential integration of plasmid DNA vaccines following intramuscular administration in mice and guinea pigs. We compared needle versus Biojector (needleless jet) delivery, examined the effect of aluminum phosphate adjuvants, compared the results of different plasmid DNA vaccines, and tested a gene (the human papilloma virus E7 gene) whose protein product is known to increase integration frequency in vitro. Six weeks following intramuscular injection, the vast majority of the plasmid was detected in the muscle and skin near the injection site; lower levels of plasmid were also detected in the draining lymph nodes. At early time points (1–7 days) after injection, a low level of systemic exposure could be detected. Occasionally, plasmid was detected in gonads, but it dissipated rapidly and was extrachromosomal – indicating a low risk of germline transmission. Aluminum phosphate adjuvant had no effect on the tissue distribution and did not result in a detectable increase in integration frequency. Biojector delivery, compared with needle injection, greatly increased the uptake of plasmid (particularly in skin at the injection site), but did not result in a detectable increase in integration frequency. Finally, injection of a plasmid DNA vaccine containing the human papilloma virus type 16 E7 gene, known to increase integration in vitro, did not result in detectable integration in mice. These results suggest that the risk of integration following intramuscular injection of plasmid DNA is low under a variety of experimental conditions.

Increased expression of cyclooxygenase-2 (COX-2), the rate-limiting enzyme in prostaglandin synthesis, has been associated with growth regulation and carcinogenesis in several systems. COX-2 is known to be induced by cytokines and the skin tumor promoter 12-tetradecanoylphorbol-13-myristate (TPA). In the present study, we investigated the effects of several non-TPA-type tumor promoters on COX-2 expression in immortalized mouse liver cells. Specifically, we tested peroxisome proliferators (PPs), which are rodent liver tumor promoters that cause gross alterations in cellular lipid metabolism, the rodent liver tumor promoter phenobarbital, and the skin tumor promoters okadaic acid and thapsigargin. The PPs Wy-14643, mono-ethylhexyl phthalate, clofibrate, ciprofibrate ethyl ester, and eicosatetraynoic acid each caused large increases in COX-2 mRNA and protein, with maximal expression seen approximately 10 h after treatment of quiescent cells. COX-2 expression was also induced by thapsigargin, okadaic acid, and calcium ionophore A23187, but not by phenobarbital or the steroid PP dehydroepiandrosterone sulfate. Induction of COX-2 expression generally resulted in increased synthesis of prostaglandin E 2 (PGE 2 ). However, the PPs caused little or no increase in PGE 2 levels, and they inhibited serum-induced PGE 2 synthesis. Unlike nonsteroidal anti-inflammatory drugs, the PPs do not directly inhibit cyclooxygenase enzyme activity in vitro. Thus, PPs regulate prostaglandin metabolism via both positive (COX-2 induction) and inhibitory mechanisms. In summary, the strong induction of COX-2 expression by PPs, thapsigargin, and okadaic acid suggests a possible role for COX-2 in the growth regulatory activity of these non-TPA-type tumor promoters.