Plasmid DNA purification

The demand for efficient production methods of plasmid DNA (pDNA) has increased vastly in response to rapid advances in the use of pDNA in gene therapy and in vaccines since the advantageous safety concerns associated with non-viral over viral vectors.A prerequisite for the success of plasmid-based therapies is the development of cost-effective and generic production processes of pDNA. However, to satisfy strict regulatory guidelines, the material must be available as highly purified, homogeneous preparations of supercoiled circular covalently closed (ccc) pDNA. Large-scale production of pDNA for therapeutic use is a relatively new field in bioprocessing. The shift from small-scale plasmid production for cell transfection to large-scale production sets new constraints on the bacterial fermentation, processing of bacterial lysate and final purification and formulation of the plasmid DNA. The choice of bacterial strain used for plasmid cultivation affects the plasmid yield, the proportion of different isoforms and the amount of endotoxins in the starting material. The choice of bacterial strain will be greatly influenced by the production and purification procedures of pDNA. Master and working cell banks need to be characterised and established. Alkaline lysis of the bacteria damages the pDNA, resulting in a reduced recovery of ccc pDNA and an increase in partially denaturated ccc pDNA and open circular (oc) forms. Shear stress in these processes needs to be tightly controlled, and buffer composition and pH need to be optimised. To obtain a homogeneous plasmid DNA preparation, different pDNA purification strategies aim at capturing ccc pDNA and eliminating the oc isoform. A highly purified final product corresponding to the stringent recommendations set forth by health and regulatory authorities can be achieved by (i). different chromatography techniques integrated with ultra/diafiltration to achieve optimal purification results; (ii). the formulation of the final pDNA product, that requires a detailed study of the plasmid structure; and (iii). the development of sensitive analytical methods to detect different impurities (proteins, RNA, chromosomal DNA, and endotoxins). We present here a revue of the whole process to obtain such a plasmid DNA, and report an example of RNAse-free purification of ccc pDNA that could be used for gene therapy.