Adapting virus filtration to continuous processing: Effects of product and process variability on filtration performance.

Virus filtration (VF) is an important unit operation in the manufacture of biotherapeutics that provides robust removal of potential virus contaminants. Small virus removal can be impacted by the low operating pressures and potential depressurization events that are often associated with continuous operations where increased operational flexibility for higher loading at low flux and low pressure is required. In this study, we evaluated the impact of low flux (7 LMH) and pressure interruptions on minute virus of mice (MVM) removal. We used long-term filtrations conducted to a target throughput of 1000 L/m2 with four different monoclonal antibodies on small-scale hollow fiber virus filters with a hydrophilic modified polyvinylidene fluoride membrane. These conditions are certainly challenging for any VF operation and ensuring robust viral clearance under such conditions is critical to the design and implementation of continuous VF. Planova BioEX filters effectively removed MVM at 4 log or greater when run continuously for up to 6 days. Interestingly, pressure increases associated with filter fouling over the duration of long-term filtrations were shown to be reflective of load material variability and could be remediated by implementation of an inline prefilter. Pressure interruptions had minimal impact on overall MVM logarithmic reduction value. Effective virus removal was achieved with pressure increases being largely product-specific, which demonstrates the capability of the virus filter to remove virus independent of pressure increases that are expected to occur with increased protein load.

Determination of protein concentration in downstream biomanufacturing processes by in-line index of refraction.

Protein concentration determination is a necessary in-process control for the downstream operations within biomanufacturing. As production transitions from batch mode to an integrated continuous bioprocess paradigm, there is a growing need to move protein concentration quantitation from off-line to in-line analysis. One solution to fulfill this process analytical technology need is an in-line index of refraction (IoR) sensor to measure protein concentration in real time. Here the performance of an IoR sensor is evaluated through a series of experiments to assess linear response, buffer matrix effects, dynamic range, sensor-to-sensor variability, and the limits of detection and quantitation. The performance of the sensor was also tested in two bioprocessing scenarios, ultrafiltration and capture chromatography. The implementation of this in-line IoR sensor for real-time protein concentration analysis and monitoring has the potential to improve continuous bioprocess manufacturing.

Multi-angle light scattering as a process analytical technology measuring real-time molecular weight for downstream process control.

For many protein therapeutics including monoclonal antibodies, aggregate removal process can be complex and challenging. We evaluated two different process analytical technology (PAT) applications that couple a purification unit performing preparative hydrophobic interaction chromatography (HIC) to a multi-angle light scattering (MALS) system. Using first principle measurements, the MALS detector calculates weight-average molar mass, Mw and can control aggregate levels in purification. The first application uses an in-line MALS to send start/stop fractionation trigger signals directly to the purification unit when preset Mw criteria are met or unmet. This occurs in real-time and eliminates the need for analysis after purification. The second application uses on-line ultra-high performance size-exclusion liquid chromatography to sample from the purification stream, separating the mAb species and confirming their Mw using a µMALS detector. The percent dimer (1.5%) determined by the on-line method is in agreement with the data from the in-line application (Mw increase of approximately 2750 Da). The novel HIC-MALS systems demonstrated here can be used as a powerful tool for real-time aggregate monitoring and control during biologics purification enabling future real time release of biotherapeutics.

On-Line Ion Exchange Liquid Chromatography as a Process Analytical Technology for Monoclonal Antibody Characterization in Continuous Bioprocessing.

Combining process analytical technology (PAT) with continuous production provides a powerful tool to observe and control monoclonal antibody (mAb) fermentation and purification processes. This work demonstrates on-line liquid chromatography (on-line LC) as a PAT tool for monitoring a continuous biologics process and forced degradation studies. Specifically, this work focused on ion exchange chromatography (IEX), which is a critical separation technique to detect charge variants. Product-related impurities, including charge variants, that impact function are classified as critical quality attributes (CQAs). First, we confirmed no significant differences were observed in the charge heterogeneity profile of a mAb through both at-line and on-line sampling and that the on-line method has the ability to rapidly detect changes in protein quality over time. The robustness and versatility of the PAT methods were tested by sampling from two purification locations in a continuous mAb process. The PAT IEX methods used with on-line LC were a weak cation exchange (WCX) separation and a newly developed shorter strong cation exchange (SCX) assay. Both methods provided similar results with the distribution of percent acidic, main, and basic species remaining unchanged over a 2 week period. Second, a forced degradation study showed an increase in acidic species and a decrease in basic species when sampled on-line over 7 days. These applications further strengthen the use of on-line LC to monitor CQAs of a mAb continuously with various PAT IEX analytical methods. Implementation of on-line IEX will enable faster decision making during process development and could potentially be applied to control in biomanufacturing.

Structures of multidomain proteins adsorbed on hydrophobic interaction chromatography surfaces.

In hydrophobic interaction chromatography (HIC), interactions between buried hydrophobic residues and HIC surfaces can cause conformational changes that interfere with separations and cause yield losses. This paper extends our previous investigations of protein unfolding in HIC chromatography by identifying protein structures on HIC surfaces under denaturing conditions and relating them to solution behavior. The thermal unfolding of three model multidomain proteins on three HIC surfaces of differing hydrophobicities was investigated with hydrogen exchange mass spectrometry (HXMS). The data were analyzed to obtain unfolding rates and Gibbs free energies for unfolding of adsorbed proteins. The melting temperatures of the proteins were lowered, but by different amounts, on the different surfaces. In addition, the structures of the proteins on the chromatographic surfaces were similar to the partially unfolded structures produced in the absence of a surface by temperature as well as by chemical denaturants. Finally, it was found that patterns of residue exposure to solvent on different surfaces at different temperatures can be largely superimposed. These findings suggest that protein unfolding on various HIC surfaces might be quantitatively related to protein unfolding in solution and that details of surface unfolding behavior might be generalized.

Unfolding of a model protein on ion exchange and mixed mode chromatography surfaces.

Recent studies with proteins indicate that conformational changes and aggregation can occur during ion exchange chromatography (IEC). Such behavior is not usually expected, but could lead to decreased yield and product degradation from both IEC and multi mode chromatography (MMC) that has ligands of both hydrophobic and charged functionalities. In this study, we used hydrogen exchange mass spectrometry to investigate unfolding of the model protein BSA on IEC and MMC surfaces under different solution conditions at 25°C. Increased solvent exposure, indicating greater unfolding relative to that in solution, was found for protein adsorbed on cationic IEC and MMC surfaces in the pH range of 3.0 to 4.5, where BSA has decreased stability in solution. There was no effect of anionic surfaces at pH values in the range from 6.0 to 9.0. Differences of solvent exposure of whole molecules when adsorbed and in solution suggest that adsorbed BSA unfolds at lower pH values and may show aggregation, depending upon pH and the surface type. Measurements on digested peptides showed that classifications of stability can be made for various regions; these are generally retained as pH is changed. When salt was added to MMC systems, where electrostatic interactions would be minimized, less solvent exposure was seen, implying that it is the cationic moieties, rather than the hydrophobic ligands, which cause greater surface unfolding at low salt concentrations. These results suggest that proteins of lower stability may exhibit unfolding and aggregation during IEC and MMC separations, as they can with hydrophobic interaction chromatography.

Protein stability and structure in HIC: hydrogen exchange experiments and COREX calculations.

Hydrogen exchange mass spectrometry (HXMS) coupled to proteolytic digestion has been used to probe the conformation of bovine ß-lactoglobulin (BLG), bovine α-lactalbumin (BLA), and human serum albumin (HSA) in solution and while adsorbed to the hydrophobic interaction chromatography media Phenyl Sepharose 6FF. All three proteins show evidence of EX1 exchange kinetics, indicating a loss of stability on the surface. HX protection patterns for all three proteins also indicate that the unfolded form is only partially solvent exposed. The hydrogen-deuterium exchange patterns of BLG and BLA on the surface suggest a structure that resembles each protein's respective solution phase molten globule state. The low stability of Domain II of HSA observed on Phenyl Sepharose 6FF also suggests a link to solution stability because Domain II is frequently cited as the least stable domain in solution unfolding pathways. COREX, an algorithm used to compute protein folding stabilities, correctly predicts solution hydrogen-deuterium exchange patterns for BLG and offers insight into its adsorbed phase stabilities but is unreliable for BLA predictions. The results of this work demonstrate a link between solution-phase local stability patterns and the nature of partially unfolded states that proteins can adopt on HIC surfaces.