Protein adsorption and transport in agarose and dextran-grafted agarose media for ion exchange chromatography: Effect of ionic strength and protein characteristics

Extraction, Modification and Biomedical Application of Agarose Hydrogels: A Review

Numerous compounds present in the ocean are contributing to the development of the biomedical field. Agarose, a polysaccharide derived from marine red algae, plays a vital role in biomedical applications because of its reversible temperature-sensitive gelling behavior, excellent mechanical properties, and high biological activity. Natural agarose hydrogel has a single structural composition that prevents it from adapting to complex biological environments. Therefore, agarose can be developed into different forms through physical, biological, and chemical modifications, enabling it to perform optimally in different environments. Agarose biomaterials are being increasingly used for isolation, purification, drug delivery, and tissue engineering, but most are still far from clinical approval. This review classifies and discusses the preparation, modification, and biomedical applications of agarose, focusing on its applications in isolation and purification, wound dressings, drug delivery, tissue engineering, and 3D printing. In addition, it attempts to address the opportunities and challenges associated with the future development of agarose-based biomaterials in the biomedical field. It should help to rationalize the selection of the most suitable functionalized agarose hydrogels for specific applications in the biomedical industry.

Separation of closely related monoclonal antibody charge variant impurities using poly(ethylenimine)-grafted cation-exchange chromatography resin

The removal of protein charge variants due to complex chemical and enzymatic modifications like glycosylation, fragmentation and deamidation presents a significant challenge in the purification of monoclonal antibodies (mAb) and complicates downstream processing. These protein modifications occur either in vivo or during fermentation and downstream processing. The presence of charge variants can lead to diminished biological activity, differences in pharmacokinetics, pharmacodynamics, stability and efficacy. Therefore, these different product variants should be appropriately controlled for the consistency of product quality and to ensure patient safety. This investigation focuses on the development of a chromatography step for the removal of the charge variants from a recombinant single-chain variable antibody fragment (scFv-Fc-Ab). Poly(ethyleneimine)-grafted cation-exchange resins (Poly CSX and Poly ABX) were evaluated and compared to traditional macroporous cation-exchange and tentacle cation-exchange resins. Linear salt gradient experiments were conducted to study the separation efficiency of scFv-Fc-Ab variants using different resins. A classical thermodynamic model was used to develop a mechanistic understanding of the differences in charge variant retention behaviour of different resins. High selectivity in separation of scFv-Fc-Ab charge variants is obtained in the Poly CSX resin.

Negatively Charged Composite Nanofibrous Hydrogel Membranes for High-Performance Protein Adsorption

Nanofibrous materials are considered as promising candidates for fabricating high-efficiency chromatography media, which are urgently needed in protein pharmaceuticals purification and biological research, yet still face several bottlenecks. Herein, novel negatively charged composite nanofibrous hydrogel membranes (NHMs) are obtained by a facile combination of electrospinning and surface coating modification. The resulting NHMs exhibit controllable morphologies and chemical structures. Benefitting from the combined effect of the stable framework of silicon dioxide (SiO₂) nanofiber and the function layer of negatively charged hydrogel, as well as good pore connectivity among nanofibers, NHMs exhibit a high protein adsorption capacity of around 1000 mg g^-1, and are superior to the commercial cellulose fibrous adsorbent (Sartobind^®) and the reported nanofibrous membranous adsorbents. Moreover, due to their relatively stable physicochemical and mechanical properties, NHMs possess comprehensive adsorption performance, favorable resistance to acid and solvents, good selectivity, and excellent regenerability. The designed NHMs composite adsorbents are expected to supply a new protein chromatography platform for effective protein purification in biopharmaceuticals and biochemical reagents.

Robust mechanistic modeling of protein ion-exchange chromatography

Mechanistic models for ion-exchange chromatography of proteins are well-established and a broad consensus exists on most aspects of the detailed mathematical and physical description. A variety of specializations of these models can typically capture the general locations of elution peaks, but discrepancies are often observed in peak position and shape, especially if the column load level is in the non-linear range. These discrepancies may prevent the use of models for high-fidelity predictive applications such as process characterization and development of high-purity and -productivity process steps. Our objective is to develop a sufficiently robust mechanistic framework to make both conventional and anomalous phenomena more readily predictable using model parameters that can be evaluated based on independent measurements or well-accepted correlations. This work demonstrates the implementation of this approach for industry-relevant case studies using both a model protein, lysozyme, and biopharmaceutical product monoclonal antibodies, using cation-exchange resins with a variety of architectures (SP Sepharose FF, Fractogel EMD SO₃^-, Capto S and Toyopearl SP650M). The modeling employs the general rate model with the extension of the surface diffusivity to be variable, as a function of ionic strength or binding affinity. A colloidal isotherm that accounts for protein-surface and protein-protein interactions independently was used, with each characterized by a parameter determined as a function of ionic strength and pH. Both of these isotherm parameters, along with the variable surface diffusivity, were successfully estimated using breakthrough data at different ionic strengths and pH. The model developed was used to predict overloads and elution curves with high accuracy for a wide variety of gradients and different flow rates and protein loads. The in-silico methodology used in this work for parameter estimation, along with a minimal amount of experimental data, can help the industry adopt model-based optimization and control of preparative ion-exchange chromatography with high accuracy.

Increasing immunoglobulin G adsorption in dextran-grafted protein A gels

The formation of a stable spatial arrangement of protein A ligands is a great challenge for the development of high-capacity polymer-grafted protein A adsorbents due to the complexity in interplay between coupled ligands and polymer chain. In this work, carboxymethyl dextrans (CMDs) with different molecular weight were introduced to provide stable spatial ligand arrangement in CMD-grafted protein A gels to improve IgG adsorption. The result showed that coupling of protein A ligand in CMD-grafted layer had no marked influence on pore size and dextran layers coupled with the ligands were stable in experimental range of salt concentrations. The result of IgG adsorption revealed that carboxymethyl dextran T10, a short CMD, was more suitable as a scaffold for the synthesis of high-capacity protein A gels. Moreover, the maximal adsorption capacity for IgG was obtained to be 96.4 mg/g gel at ionic capacities of 300-350 mmol/L and a ligand density of 15.2 mg/g gel. Dynamic binding capacity for IgG exhibited a higher capacity utilization in CMD-grafted protein A gels than non-grafted protein A gel. The research presented a tactics to establish a stable dextran layer coupled with protein A ligands and demonstrated its importance to improve binding capacity for IgG.

Experimental investigation and mass transfer modelling of 3D printed monolithic cation exchangers

3D printing has recently found application in chromatography as a means to create ordered stationary phases with improved separation efficiency. Currently, 3D printed stationary phases are limited by the lack of 3D printing materials suitable for chromatographic applications, and require a strict compromise in terms of desired resolution, model size and the associated print time. Modelling of mass transfer in 3D printed monoliths is also fundamental to understand and further optimise separation performance of 3D printed stationary phases. In this work, a novel 3D printing material was formulated and employed to fabricate monolithic cation exchangers (CEXs) with carboxyl functionalities. CEXs were printed with ligand densities of 0.7, 1.4, 2.1 and 2.8 mmol/g and used in batch adsorption experiments with lysozyme as model protein. All CEXs demonstrated high binding strength towards lysozyme, with maximum binding capacities of up to 108 mg/mL. The experimental results were described using mass transfer models based on lumped pore diffusion and lumped solid diffusion mechanisms adapted to reflect the complex geometry of the 3D printed monoliths. An exact 3D model as well as less computationally demanding 1D and 2D approximations were evaluated in terms of their quality to capture the experimental trend of batch adsorption kinetic data. Overall, the model results indicate that mass transfer in the fabricated CEXs is mostly controlled by pore diffusion at high protein concentrations in the mobile phase, with solid diffusion becoming important at low protein concentrations. Also, the kinetic data were approximated equally well by both the full 3D model as well as the 2D approximation, indicating leaner mathematical models of lower dimensionality can be employed to describe mass transfer in complex three dimensional geometries. We believe this work will help spur the development of 3D printable materials for separations and aid in the development of quantitative platforms to evaluate and optimise the performance of 3D printed monoliths.

Adsorption of a Humanized Monoclonal Antibody onto Thermoresponsive Copolymer-Grafted Sepharose Fast Flow Sorbents

The batch adsorption behavior of a humanized monoclonal antibody (hIgG2 mAb) with thermoresponsive polymer (TRP)-modified Sepharose Fast Flow sorbents with different compositions of grafted copolymers is described. At high protein loadings, the adsorption with negatively charged copolymer-modified sorbents exhibited S-shaped isotherms in most cases, indicative of unrestricted multilayer adsorption. The adsorption capacity of the negatively charged copolymer-modified sorbents increased with an increase in the applied environmental temperature due to increased protein-sorbent surface hydrophobic and electrostatic interactions. The affinity of the hIgG2 mAb for a positively charged copolymer-grafted sorbent was much lower than that found for the negatively charged copolymer-grafted sorbents at both 20 and 50 °C due to electrostatic repulsive effects. This study has documented that the molecular functionalities of the grafted copolymer can significantly affect the adsorption behavior of this humanized mAb at both 20 and 50 °C with the isothermal dependencies revealing subtle effects due to copolymer composition.

Effect of pore structure on protein adsorption mechanism on ion exchange media: A preliminary study using low field nuclear magnetic resonance

The adsorption process of bovine serum albumin (BSA), ovalbumin (OVA) and human immunoglobulin G (IgG) on agarose ion-exchange media Q Sepharose FF and two dextran-grafted agarose media including Q Sepharose XL and Capto Q were studied using low field nuclear magnetic resonance (NMR). The T₂ relaxation time was found directly proportional to the pore size and diminished after protein adsorbed, therefore, a theoretical model describing the relationship between protein binding amount and T₂ relaxation signals was established. The model parameters, a, which reflects the contact area between the adsorbed protein and media surface, and the δ, which defined as the ratio of the protein volume to the pore volume after adsorption, were found to describe the pore occupation states of proteins in media with different pore structures very well. For small proteins, such as BSA and OVA, monolayer adsorption occurred on Q Sepharose FF, which has no dextran chains. Therefore, the adsorbed protein only occupied 49.05% of the pore volume for BSA and 25.51% for OVA, and contact area of each protein on the media were also low, suggesting mostly monolayer adsorption occurred. In the contrast, their adsorption to Q Sepharose XL and Capto Q with dextran chains tended to form multilayer adsorption, thus higher contact area was obtained and the pore volumes were almost 100% occupied. For large protein, such as IgG, the adsorption to all these three media was similar and about 30% of the pore volume were occupied, probably due to the similar restriction for IgG to entering the media pore. Results of this study will help to elucidate the relationship between protein adsorption and pore size variation, which present the significance of low field NMR in understanding protein adsorption mechanism.

Recent advances in protein chromatography with polymer-grafted media

The strategy of using polymer-grafted media is effective to create protein chromatography of high capacity and uptake rate, giving rise to an excellent performance in high-throughput protein separation due to its high dynamic binding capacity. Taking the scientific development and technological innovation of protein chromatography as the objective, this review is devoted to an overview of polymer-grafted media reported in the last five years, including their fabrication routes, protein adsorption and chromatography, mechanisms behind the adsorption behaviors, limitations of polymer-grafted media and chromatographic operation strategies. Particular emphasis is placed on the elaboration and discussion on the behaviors of ion-exchange chromatography (IEC) with polymer-grafted media because IEC is the most suitable chromatographic mode for this kind of media. Recent advances in both the theoretical and experimental investigations on polymer-grafted media are discussed by focusing on their implications to the rational design of novel chromatographic media and mobile phase conditions for the development of high-performance protein chromatography. It is concluded that polymer-grafted media are suitable for development of IEC and mixed-mode chromatography with charged and low hydrophobic ligands, but not for hydrophobic interaction chromatography with high hydrophobic ligands and affinity chromatography with ligands that have single binding site on the protein.

Protein adsorption to poly(2-aminoethyl methacrylate)-grafted Sepharose gel: Effects of chain length and charge density

Grafting functional polymer chains onto porous resins has been found to drastically increase both adsorption capacity and uptake rate in protein chromatography. In this work, 2-aminoethyl methacrylate (AEM) was used for grafting onto Sepharose FF gel, and six anion-exchangers of different polyAEM (pAEM) chain lengths (ionic capacities, ICs), FF-pAEM, were obtained for protein adsorption and chromatography. It was found that protein adsorption capacity (q_m) increased with increasing pAEM chain length, but the uptake rate, represented by the ratio of effective pore diffusivity to the free solution diffusivity (D_e/D₀), showed an up-down trend, reaching a peak value (D_e/D₀=0.55) at an IC of 313 mmol/L. Partial charge neutralization of the AEM-grafted resin of the highest IC (513 mmol/L) by reaction with sodium acetate produced three charge-reduced resins, FF-pAEM513-R. With reducing the charge density, the adsorption capacity kept unchanged and then decreased, but the uptake rate monotonically increased, reaching a maximum (about 2-fold increase) at a residual IC of 263 mmol/L. It is notable that, at the same IC, the charge-reduced resin (FF-pAEM513-R) presented similar or even higher values of q_m and D_e/D₀ than its FF-pAEM counterpart. Particularly, at the same IC of 263 mmol/L, a ~50% enhancement of D_e/D₀ was observed. Both adsorption capacity and uptake rate in the charge-reduced resin with a residual IC of 339 mmo/L (FF-pAEM513-R339) decreased more sharply with increasing NaCl concentration by comparison with FF-pAEM513, indicating its increased salt-sensitivity than FF-pAEM513. That is, charge reduction on the AEM-grafted resin could accelerate protein uptake at 0 mmol/L NaCl but decrease salt tolerance. Column breakthrough experiments showed that FF-pAEM513-R339 was favorable for high flow rate protein chromatography at low NaCl concentration (0 mmol/L), whereas FF-pAEM513 was a good choice in a wide range of salt concentrations at low flow rate. This research proved the excellent protein chromatography performance of the AEM-based anion-exchangers.

Estimating and leveraging protein diffusion on ion-exchange resin surfaces

Protein mobility at solid-liquid interfaces can affect the performance of applications such as bioseparations and biosensors by facilitating reorganization of adsorbed protein, accelerating molecular recognition, and informing the fundamentals of adsorption. In the case of ion-exchange chromatographic beads with small, tortuous pores, where the existence of surface diffusion is often not recognized, slow mass transfer can result in lower resin capacity utilization. We demonstrate that accounting for and exploiting protein surface diffusion can alleviate the mass-transfer limitations on multiple significant length scales. Although the surface diffusivity has previously been shown to correlate with ionic strength (IS) and binding affinity, we show that the dependence is solely on the binding affinity, irrespective of pH, IS, and resin ligand density. Different surface diffusivities give rise to different protein distributions within the resin, as characterized using confocal microscopy and small-angle neutron scattering (length scales of micrometer and nanometer, respectively). The binding dependence of surface diffusion inspired a protein-loading approach in which the binding affinity, and hence the surface diffusivity, is modulated by varying IS. Such gradient loading increased the protein uptake efficiency by up to 43%, corroborating the importance of protein surface diffusion in protein transport in ion-exchange chromatography.

A mechanistic examination of salting out in protein-polymer membrane interactions

Developing a mechanistic understanding of protein dynamics and conformational changes at polymer interfaces is critical for a range of processes including industrial protein separations. Salting out is one example of a procedure that is ubiquitous in protein separations yet is optimized empirically because there is no mechanistic description of the underlying interactions that would allow predictive modeling. Here, we investigate peak narrowing in a model transferrin-nylon system under salting out conditions using a combination of single-molecule tracking and ensemble separations. Distinct surface transport modes and protein conformational changes at the negatively charged nylon interface are quantified as a function of salt concentration. Single-molecule kinetics relate macroscale improvements in chromatographic peak broadening with microscale distributions of surface interaction mechanisms such as continuous-time random walks and simple adsorption-desorption. Monte Carlo simulations underpinned by the stochastic theory of chromatography are performed using kinetic data extracted from single-molecule observations. Simulations agree with experiment, revealing a decrease in peak broadening as the salt concentration increases. The results suggest that chemical modifications to membranes that decrease the probability of surface random walks could reduce peak broadening in full-scale protein separations. More broadly, this work represents a proof of concept for combining single-molecule experiments and a mechanistic theory to improve costly and time-consuming empirical methods of optimization.

Emerging biomaterials for downstream manufacturing of therapeutic proteins

Downstream processing is considered one of the most challenging phases of industrial manufacturing of therapeutic proteins, accounting for a large portion of the total production costs. The growing demand for therapeutic proteins in the biopharmaceutical market in addition to a significant rise in upstream titers have placed an increasing burden on the downstream purification process, which is often limited by high cost and insufficient capacities. To achieve efficient production and reduced costs, a variety of biomaterials have been exploited to improve the current techniques and also to develop superior alternatives. In this work, we discuss the significance of utilizing traditional biomaterials in downstream processing and review the recent progress in the development of new biomaterials for use in protein separation and purification. Several representative methods will be highlighted and discussed in detail, including affinity chromatography, non-affinity chromatography, membrane separations, magnetic separations, and precipitation/phase separations. STATEMENT OF SIGNIFICANCE: Nowadays, downstream processing of therapeutic proteins is facing great challenges created by the rapid increase of the market size and upstream titers, starving for significant improvements or innovations in current downstream unit operations. Biomaterials have been widely used in downstream manufacturing of proteins and efforts have been continuously devoted to developing more advanced biomaterials for the implementation of more efficient and economical purification methods. This review covers recent advances in the development and application of biomaterials specifically exploited for various chromatographic and non-chromatographic techniques, highlighting several promising alternative strategies.

Selective removal of closely related clipped protein impurities using poly(ethylenimine)- grafted anion-exchange chromatography resin

Proteolytic degradation is a serious problem that complicates downstream processing during production of recombinant therapeutic proteins. It can lead to decreased product yield, diminished biological activity, and suboptimal product quality. Proteolytic degradation or protein truncation is observed in various expression hosts and is mostly attributed to the activity of proteases released by host cells. Since these clipped proteins can impact pharmacokinetics and immunogenicity in addition to potency, they need to be appropriately controlled to ensure consistency of product quality and patient safety. A chromatography step for the selective removal of clipped proteins from an intact protein was developed in this study. Poly(ethylenimine)-grafted anion- exchange resins (PolyQUAT and PolyPEI) were evaluated and compared to traditional macroporous anion-exchange and tentacled anion-exchange resins. Isocratic retention experiments were conducted to determine the retention factors (k') and charge factors (Z) were determined through the classical stoichiometric displacement model. High selectivity in separation of closely related clipped proteins was obtained with the PolyQUAT resin. A robust design space was established for the PolyQUAT chromatography through Design-Of-Experiments (DoE) based process optimization. Results showed a product recovery of up to 63% with purity levels >99.0%. Approximately, one-log clearance of host cell protein and two-logs clearance of host cell DNA were also obtained. The newly developed PolyQUAT process was compared with an existing process and shown to be superior with respect to the number of process steps, process time, process yield, and product quality.

Effects of polymer graft properties on protein adsorption and transport in ion exchange chromatography: a multiscale modeling study

Multiscale simulation is used to study the adsorption of lysozyme onto ion exchangers obtained by grafting charged polymers into a porous matrix, in systems with various polymer properties and strengths of electrostatic interaction. Molecular dynamics simulations show that protein partitioning into the polymer-filled pore space increases with the overall charge content of the polymers, while the diffusivity in the pore space decreases. However, the combination of greatly increased partitioning and modestly decreased diffusion results in macroscopic transport rates that increase as a function of charge content, as the large concentration driving force due to enhanced pore space partitioning outweighs the reduction in the pore space diffusivity. Matrices having greater charge associated with the grafted polymers also exhibit more diffuse intraparticle concentration profiles during transient adsorption. In systems with a high charge content per polymer and a low protein loading, the polymers preferentially partition toward the surface due to favorable interactions with the surface-bound protein. These results demonstrate the potential of multiscale modeling to illuminate qualitative trends between molecular properties and the adsorption equilibria and kinetic properties observable on macroscopic scales.

High ionic strength narrows the population of sites participating in protein ion-exchange adsorption: a single-molecule study

The retention and elution of proteins in ion-exchange chromatography is routinely controlled by adjusting the mobile phase salt concentration. It has repeatedly been observed, as judged from adsorption isotherms, that the apparent heterogeneity of adsorption is lower at more-eluting, higher ionic strength. Here, we present an investigation into the mechanism of this phenomenon using a single-molecule, super-resolution imaging technique called motion-blur Points Accumulation for Imaging in Nanoscale Topography (mbPAINT). We observed that the number of functional adsorption sites was smaller at high ionic strength and that these sites had reduced desorption kinetic heterogeneity, and thus narrower predicted elution profiles, for the anion-exchange adsorption of α-lactalbumin on an agarose-supported, clustered-charge ligand stationary phase. Explanations for the narrowing of the functional population such as inter-protein interactions and protein or support structural changes were investigated through kinetic analysis, circular dichroism spectroscopy, and microscopy of agarose microbeads, respectively. The results suggest the reduction of heterogeneity is due to both electrostatic screening between the protein and ligand and tuning the steric availability within the agarose support. Overall, we have shown that single molecule spectroscopy can aid in understanding the influence of ionic strength on the population of functional adsorbent sites participating in the ion-exchange chromatographic separation of proteins.

Fluorescence recovery after photobleaching investigation of protein transport and exchange in chromatographic media

A fully-mechanistic understanding of protein transport and sorption in chromatographic materials has remained elusive despite the application of modern continuum and molecular observation techniques. While measuring overall uptake rates in proteins in chromatographic media is relatively straightforward, quantifying mechanistic contributions is much more challenging. Further, at equilibrium in fully-loaded particles, measuring rates of kinetic exchange and diffusion can be very challenging. As models of multicomponent separations rely on accurate depictions of protein displacement and elution, a straightforward method is desired to measure the mobility of bound protein in chromatographic media. We have adapted fluorescence recovery after photobleaching (FRAP) methods to study transport and exchange of protein at equilibrium in a single particle. Further, we have developed a mathematical model to capture diffusion and desorption rates governing fluorescence recovery and investigate how these rates vary as a function of protein size, binding strength and media type. An emphasis is placed on explaining differences between polymer-modified and traditional media, which in the former case is characterized by rapid uptake, slow displacement and large elution pools, differences that have been postulated to result from steric and kinetic limitations. Finally, good qualitative agreement is achieved predicting flow confocal displacement profiles in polymer-modified materials, based solely on estimates of kinetic and diffusion parameters from FRAP observations.