Protein Purification: Chromatography and Scalability in Biopharma

Chromatography plays an instrumental role in the purification of biopharmaceutical products. Biopharma companies are continuously striving to enhance manufacturing and production processes. Prof. Giorgio Carta, an expert within the field, recently presented a talk on 'Challenges and Opportunities for Improved Resins for the Chromatographic Processing of Biopharmaceuticals' during a workshop on ‘Applying Innovative Chromatography Tools for Purification and Recovery of Biomolecules’ at the Bioprocess International Conference and Exhibition 2017.

“The purity requirements of current biopharmaceuticals continue to demand efficient and effective chromatographic modalities.” Prof. Giorgio Carta, Dept. of Chemical Engineering, University of Virginia

“…pressure will increasingly mount to improve process efficiency and reduce costs while continuing to assure high product quality and manufacturability.”

We had the pleasure of speaking with Giorgio, who provided us with an extraordinary insight into the current state of play, factors to consider when 'scaling up’ separation processes, and general challenges and developments when applying chromatographic techniques to bioprocesses. More specifically, his team are exploring ways to optimize the use of chromatography, as a tool for large-scale purification.

Laura Mason (LM): Could you tell me more about your recent work on the use of novel chromatographic resins?

Giorgio Carta (GC): Our recent work is focused on fundamentally understanding the physical properties of chromatographic resins used for the purification of biopharmaceuticals and the relevant surface-biomolecule interactions. We are especially interested in higher-resolution chromatography modalities that can be used for more effective separations of large biomolecules, including antibody monomer-dimer separations, mAb charge variant separation, and the purification of virus-like-particles.

We are also focusing on the development of process design tools that integrate the power of high-throughput screening technologies with mechanistic modeling to speed-up the design and optimization of chromatography as a tool for process-scale protein purification.

Finally, we have been interested in approaches that can be used to study fouling of chromatography columns used in process operations and to validate cleaning protocols. In all cases, our goal is to utilize engineering principles and tools coupled with advanced experimental techniques to rationally describe and understand the process.

LM: Process scale-up is important to biopharma companies, what factors need to be considered when 'scaling up’ separation processes?

GC: The scalability of chromatographic processes is dependent on many factors, but principal among those are the mechanical strength, the chemical stability and the inertness of the chromatography media with respect to the bimolecular properties of the product being purified.

One aspect of scalability is the ability to predict chromatographic performance based on small scale laboratory experiments. In our work we have focused both on polymer-grafted resins and open-pore matrices. While the former often provide greater binding capacity and sometimes faster kinetics, we find that the latter tend to be more predictable and are often better suited for platform processes.

LM: What implications can protein unfolding have on separation, and what challenges does this present to biopharma companies?

GC: Protein unfolding on chromatographic surfaces as long been known to occur on hydrophobic stationary phases, such as those used in reverse-phase chromatography (RPC) and hydrophobic interaction chromatography (HIC).

Recently, however, it has been discovered that protein unfolding can also occur in ion exchange chromatography as well as with multimodal interaction media. The problem is especially serious when protein unfolding results in the on-column formation of aggregates, which has been shown to occur for a variety of monoclonal antibodies (mAbs) on certain cation exchange resins.

Aggregates are often regarded as a critical quality attribute and have to be controlled within tight limits. Their on-column formation is a particular concern since it is affected by the operating conditions, including feed load composition, elution conditions, and residence time.

In our research we have used both macroscopic and microscopic techniques to study the occurrence of unfolding and on-column aggregation.  A particularly insightful technique is hydrogen-deuterium exchange mass spectrometry (HX-MS) coupled with proteolytic fragmentation and high resolution ultra-high-pressure liquid chromatography (UPLC). The technique has been shown to yield important clues about the nature of on-column unfolding and aggregate formation and to shed light on what parts of the biomolecule undergo conformational change as a result of interactions with the stationary phase.

Comparing a large number of cation exchange (CEX) stationary phases, we have found that polymer grafted materials, resins with a hydrophobic backbone, as well as resins that contain a bimodal distribution of pores sizes, including a large fraction of small pores, tend to induce mAb conformational changes upon binding compared to hydrophilic, open-pore resins. Among the latter, a high-resolution cation exchange resin, which is based on a hydrophilic open-pore base matrix, has been shown to minimize occurrence of the unfolding phenomena.

LM: How particularly important is it for biopharma to adopt novel, advanced chromatography technologies? Are there any particular tools or technologies you feel have been particularly successful in recent years, or that you feel show promise in the future?

GC: The purity requirements of current biopharmaceuticals continue to demand efficient and effective chromatographic modalities. I expect that even greater challenges will arise as the demand increases for biopharmaceutical products that are less heterogeneous than current products. Charge variants, conformationally altered species, and glycoforms pose especially difficult separation challenges that will require resins capable of high resolution separations. Since these are large molecules, diffusion is slow and chromatographic efficiency is compromised at higher flow rates of the mobile phase.

Designing resins with smaller bead size, larger pores and adequate mechanical strength that can be used to enhance chromatographic efficiency is critical. Additionally, non-mAb biologics will require new selective ligands that incorporate multiple interaction modes to improve the intrinsic selectivity and mimic affinity adsorbents.

LM: Thinking about the evolution of chromatography, what would you consider to be the key developments?

GC: Key developments are both related to the stationary phase materials and to the processes used to achieve separation. There has been a lot of interest in continuous processing. Key to the development of continuous chromatography as a practical unit operation is the development of on-line monitors that can be interfaced with robust control strategies and systems to ensure product quality. Mechanistic understanding of biomolecule surface interactions and process modeling are also needed to improve scale-up and strengthen process understanding.

LM: What do you see as the main obstacle to the advancement of chromatography techniques and their application in the biopharma industry?

GC: Since, currently, the cost of goods is usually a small percentage of the total cost of biopharmaceuticals, the focus has been much more on robustness than on cost-effectiveness. This is not sustainable in the long run as biopharmaceuticals become more widespread and pressure will increasingly mount to improve process efficiency and reduce costs while continuing to assure high product quality and manufacturability. I expect that we will see improvements in stationary phases and in processes that are less expensive and more reliable than the existing alternatives.

Giorgio Carta was speaking to Laura Elizabeth Mason, Science Editor, Technology Networks.