Joseph, J; (2008) Engineering tools to support the design and operation of initial capture chromatography. Doctoral thesis , UCL (University College London).

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Abstract

Monoclonal antibodies are the largest and most rapidly expanding category of biopharmaceuticals today with applications across a wide range of diseases. While Protein A affinity chromatography has been almost universally applied as the primary purification step for process-scale production of these molecules, it suffers from disadvantages of high cost, limited throughput and the need for extensive methods development. These issues are magnified by dramatic increases in cell culture titres, which have shifted the burden to downstream processing in the race for increased manufacturing productivity and decreased cost of goods (COG/g). The aim of this thesis is the development of a decisional framework with which to address these concerns allowing for the rapid visual representation and comparison of process performance and key process trade-offs that result from the adoption of various column-based processing strategies. An empirical framework has been developed which significantly reduces the number of small scale studies required to predict flow conditions at increasing scale. Predictions were found to be within 10% of published data for columns at production-scale. Column hydrodynamics are closely related to the rigidity of the chromatographic resin used. Given the inherent physicochemical nature of chromatographic separations, techniques are developed with which to investigate the effect that pH changes have on resin rigidity during multi-cycle operation. Results demonstrate the importance of considering the impact on resin stability when operating over long campaigns. Trials with a range of existing and prototype resins are used to both verify the approach and to illustrate the nature of the process insights which may be gained. For example, it was found that increasing back-bone resin rigidity by 7 fold resulted in a 6 fold increase in compressive strength under high pH mobile phase conditions. Knowledge of column behaviour at large scales provides a more realistic basis for the prediction of the throughput limits of chromatographic operation. This is accomplished through an integrated graphical framework, based on a verified mathematical model for the design and analysis of a Protein A affinity chromatography step. A case study is used to illustrate the methodology and investigates the effect of different column sizes and linear flow rates on operational costs and scheduling, so as to identify the optimum column size and operation for a given process duty. The study also considers resin choice and the use of ultrafiltration pre-concentration prior to the chromatographic step. Results show that conditions of maximal column productivity are not equivalent to conditions of minimum COG/g. Cost minimisation was found to occur when the number of cycles of production were minimised by maximising resin binding capacity. The framework was further extended to investigate optimal operation of the chromatographic cycle with particular reference to the post-load washing stage. Studies show the interaction between the washing and loading strategy and demonstrate that wash and adsorption stages should be considered as a single integrated stage within the chromatographic cycle. Alternative washing strategies for various resin materials are compared and evaluated. The strategies developed in this thesis enable rapid development of optimal process-scale chromatographic design and also bear significant implications for the economics of antibody production. Moreover, the approaches and techniques developed are generic and should be broadly applicable to most packed bed chromatographic separations.

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