Goldman, Merlin Hesper; (1997) The effect of process variables on the glycosylation of gamma-interferon produced in CHO (Chinese hamster ovary) cells. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

As regulatory requirements of human therapeutics become more stringent the ability to monitor product authenticity is given greater importance. The demands for batch-to-batch consistency must now be demonstrated in ever greater detail. Industry must develop the techniques to rapidly monitor the quality of their product to meet these strict guidelines. In addition, the ability to influence the quality of a product could offer significant improvements to the efficacy of a new drug. To investigate these proposals a common industrial cell line, which had been genetically engineered to express a human protein of therapeutic use, was chosen as a suitable model system. The Chinese hamster ovary cells (CHO 320) produced recombinant human interferon-gamma (IFN-γ-γ), a glycoprotein with two N-glycosylation sites at low titres. About half of all therapeutic proteins are glycosylated, a state which confers critical implications to the activity of a drug. This additional factor offered a further degree of detail required to fully appreciate the influence of process factors on cell physiology, product titre and quality. The first improvements in product monitoring involved studies which demonstrated the separation of purified IFN-γ from culture supernatant into its three site-occupancy variants (glycoforms): doubly, singly and non-glycosylated. A micellar electrokinetic capillary electrophoresis (MECE) technique was used to separate these glycoforms in a rapid and quantitative manner not previously achieved by conventional SDS-PAGE. The first process parameter investigated was time of culture using a typical small scale industrial bioreactor system. The use capillary electrophoresis demonstrated that during batch culture, the most common industrial culture system, there was a decrease in the proportion of doubly glycosylated species with culture time. The CHO 320 cell line was batch cultured in a 15 l suspension bioreactor under controlled environmental conditions for the first time in this laboratory. It was demonstrated that by sustaining a constant power to volume ratio during culture a stable culture environment was maintained with typical CHO 320 growth cycles observed. By using mass spectrometry in combination with exoglycosidase sequencing the detailed mapping of recombinant human IFN-γ was monitored during culture for the first time. It was observed that a biantennary structure was the dominant glycosylation variant and that the proportion of high mannose and truncated IFN-γ glycoforms increased with time of culture. In addition, by the quantitative labelling of released sialylated glycans the sialylation of the molecule could be monitored for the first time as well. Using the fluorescent 2-aminobenzamide (2-AB) label there was a suggested decrease in the amount of sialylated species. Analysis of IFN-γ's C-terminal by mass spectrometry indicated that the IFN-γ polypeptide secreted by the CHO cell line was truncated by at least ten amino acids and additional proteolytic cleavage occurred in the latter part of culture. The second process parameter investigated was that of bioreactor system itself. The second bioreactor system was a perfused 2 l fluidised bed reactor. These systems are the only credible and challenging alternative to the simple batch suspension systems used in industry. Using this system a 4-fold increase in cell density was achieved as well as an improved cellular productivity. As a result a high product titre perfusate was achieved. Recombinant IFN-γ was analysed by MECE and demonstrated a constant distribution of site-occupancy variants over the greatly extended culture time. The same biantennary structure was observed but no increase in truncated or high mannose structures was observed. However, 2-AB labelling of released N-glycans demonstrated an increase in tri- and tetra-sialylated species after 210 hours. A novel capillary electrophoresis technique was used in the determination and quantitation of the isoelectric point variants of IFN- γ. Capillary isoelectrofocusing demonstrated at least 11 pI variants of IFN-γ with an increase in the number of the acidic species after 210 hours. This technique was shown to superior to conventional gel isoelectrofocusing in both terms of time and quantitation. This study shows that by the utilisation of advanced analytical techniques rapid monitoring of a model protein can be achieved. By these techniques it was demonstrated that both culture time and a novel bioreactor can influence the production of a model therapeutic protein to a level of detail not available before.

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