Large scale plasmid production

Abstract

Plasmid DNA was originally produced in small amounts for genetic modification for protein production purposes. However since the rapid advances in gene therapy and DNA vaccines, plasmid DNA became a therapeutic product. The use of non-viral vectors for safety purposes implied the necessity of producing large amounts at high purity level. The established techniques for small scale were unsuitable for the large-scale requirements. We therefore designed a plasmid purification process specially aimed for the large scale. Few studies have been published for producing high concentration of plasmid DNA in culture. The common feature for high plasmid production was the use of conditions of culture far from optimal for bacteria growth. We investigated therefore several cultivation techniques. It was tried to produce high amounts of plasmid by optimising the culture media, by growing bacteria in conditions of pH or temperature far from optimal bacterial growth. It was also tested to produce plasmids by growing bacteria in bioreactors by batch culture in different conditions. It was finally also tested to grow bacteria by fed batch techniques, both by linear or controlled feed. Highest plasmid amounts could be obtained with the fed batch using a controlled feed that finally produced 240 mg/L of plasmid DNA. A new method for harvesting bacteria has been developed. The aim was to harvest bacteria by body feed filtration. However no efficient bacteria capture in industrial compatible conditions could be obtained with conventional filter aids, either cellulose or diatomaceous earth. The filtration was therefore proposed to be done by using cross-linking agents for interacting the bacteria to the filter aids. Those methods yielded much better recoveries, but leaded to filter cakes that were of poor quality. The filtration could be finally be produced in industrial compatible conditions by the addition of a mineral (bentonite) to the bacteria prior to filtration with diatomaceous earth. This method yielded 100% recovery of the bacteria in industrial compatible conditions. The critical step for plasmid purification is its extraction from the bacteria cell, the lysis step. The lysis will define in what conditions the plasmid will have to be dealt with for further purification steps. Previous method were efficient however difficult to apply at large scale. They would also produce high amounts of contaminants that are difficult to remove at large scale. Therefore a new method for lysis at high temperature was designed. A range of conditions was obtained for selectively precipitate the plasmid during the lysis while other bacterial contaminants remain in solution. Then plasmid can be redissolved in a following step. Several conditions have proven being efficient. Finally the different steps of the process were linked together. The filtration method was adapted for a complex culture provided by the fed batch technique. The lysis steps were applied by pumping the different lysis buffers through the filter cake for releasing the plasmid from the filter while the cell debris remain trapped. Several conditions were elaborated for precipitating or binding the plasmid to the filter cake in order to be able to wash out first the contaminants while the plasmid can be eluted in the following step in a clarified lysate compatible with chromatography requirements.