A fermentation process for the manufacture of virus like particles (VLPs) in Escherichia coli (E. coli) was transferred from an external collaborator and rapidly implemented in Pfizer's cGMP pilot plant. Challenges faced in the transfer were meeting the high oxygen demand of the original process, and attempting high density cultivation of E. coli in a bioreactor system primarily designed as a seed tank for larger-scale mammalian and microbial culture. These concerns were overcome by an approach that combined process and equipment characterization, allowing suitable adaptation of the process to fit the pilot facility.
Initially, the VLP fermentation process was transferred and implemented at the laboratory scale, which allowed further process characterization data to be collected. Following this, equipment characterization was undertaken to assess the capabilities of the cGMP pilot plant bioreactor system and compare it with the laboratory-scale system. Based on this information, required process adaptations were reviewed with our collaborators, and the process was successfully implemented in the cGMP pilot plant.
CELL LINE, CULTURE MEDIUM, AND ANALYTICAL METHODS
Strain and expression system
The strain used for production of the Q-beta VLP was an E. coli K-12 derivative that constitutively overexpresses the lacI repressor. The target protein, Q-beta monomer, was expressed under the control of the hybrid tac promoter induced by lactose or isopropyl beta-D-1-thiogalactopyranoside (IPTG) in a high copy-number plasmid. Plasmid selection pressure was maintained by a kanamycin resistance gene (6). The E. coli production strain was transferred to Pfizer as vials from a cGMP working cell bank (WCB) containing a low-density cell suspension in culture medium with cryoprotectant.
Samples were collected throughout the fermentation to assess culture growth and determine Q-beta monomer titer. Cell growth was examined by measuring absorbance at 600 nm (OD600), as well as determining dry cell weight with a 1 mL cell broth sample. Titer was determined by quantifying the amount of Q-beta monomer present in the soluble fraction of a lysed cell pellet using reverse phase-high performance liquid chromatography (RP-HPLC). Any fully or partially formed VLP present in the extract was first reduced to its monomer form by incubation with a reducing agent. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis was also used to ascertain monomer identity based on comparison with a standard. Cell-free spent fermentation media samples were used to measure glycerol and lactose concentrations. Glycerol content was estimated using a Nova Bioprofile 300 Analyzer (Nova Biomedical, Waltham, MA) and lactose was measured using a YSI 2700 Biochemistry Analyzer (YSI Inc., Dayton, OH). Off-gas analysis was carried out during the fed-batch fermentation runs using Tandem off-gas analyzers by Magellan Instruments (Limpenhoe, UK) to obtain an estimate of the oxygen uptake rate (OUR). After subsequent processing and purification, the quality of the intact Q-beta VLP was assessed by several methods, including analytical size exclusion chromatography (SEC). The titer and product quality assays were transferred from our collaborator or developed in house.