Chromatography of proteins on charge-variant ion exchangers and implications for optimizing protein uptake rates

Predictive mechanistic modeling of loading and elution in protein A chromatography

Protein A chromatography is an enabling technology in current manufacturing processes of monoclonal antibodies (mAbs) and mAb derivatives, largely due to its ability to reduce the levels of process-related impurities by several orders of magnitude. Despite its widespread application, the use of mathematical modeling capable of accurately predicting the full protein A chromatographic process, including loading, post-loading wash and elution stages, has been limited. This work describes a mechanistic modeling approach utilizing the general rate model (GRM), the capabilities of which are explored and optimized using two isotherm models. Isotherm parameters were estimated by inverse-fitting simulated breakthrough curves to experimental data at various pH values. The parameter values so obtained were interpolated across the relevant pH range using a best-fit curve, thus enabling their use in predictive modeling, including of elution over a range of pH. The model provides accurate predictions (< 3% mean error in 10% dynamic binding capacity predictions and ∼ 5% mean error in elution mass and pool volume predictions, both on scale-up) for various residence times, buffer conditions and elution schemes and its effectiveness for use in scale-up and process development is shown by applying the same parameters to larger columns and a wider range of residence times.

Hindered diffusion of proteins in mixture adsorption on porous anion exchangers and impact on flow-through purification of large proteins

Complex adsorption kinetics behaviors of proteins in mixtures hampers chromatographic process development and complicates model-based prediction of separation. We investigated the adsorption characteristics of mixtures comprised of a larger protein (secretory immunoglobulins or thyroglobulin) and a smaller protein (serum albumin or green fluorescence protein) on the small-pore anion exchanger Q Sepharose FF. Confocal laser scanning microscopy measurements revealed that binding of the large protein was extremely slow and eventually stopped completely after the adsorption front penetrated just a few μm into the particle. Binding capacities after 24 h of incubation were nevertheless around 35 mg/mL of particle which is relatively high when considering that only a fraction of the particle was saturated, suggesting that locally-high bound protein concentrations are attained in a layer close to the particle surface. During mixture adsorption, the bound protein layer also significantly hindered diffusion of the smaller proteins into the particles resulting in about three times slower adsorption kinetics compared to single component adsorption. The combined effects of restricted diffusion and protein binding explain why flow-through purification of these mixtures with the small-pore resin Q Sepharose FF is effective under practical conditions. In this resin, diffusion of secretory immunoglobulins (or thyroglobulin) is restricted in the small pores so that despite their intrinsically greater affinity for the resin, much less binds compared to small proteins. Using the large-pore resin POROS 50 HQ results in faster transport, but also in more binding of secretory immunoglobulins (or thyroglobulin) compared to smaller protein impurities, preventing effective flow-through purification.

Origins of concentration gradients for diffusiophoresis

Fluid transport that is driven by gradients of pressure, gravity, or electro-magnetic potential is well-known and studied in many fields. A subtler type of transport, called diffusiophoresis, occurs in a gradient of chemical concentration, either electrolyte or non-electrolyte. Diffusiophoresis works by driving a slip velocity at the fluid-solid interface. Although the mechanism is well-known, the diffusiophoresis mechanism is often considered to be an esoteric laboratory phenomenon. However, in this article we show that concentration gradients can develop in a surprisingly wide variety of physical phenomena - imposed gradients, asymmetric reactions, dissolution, crystallization, evaporation, mixing, sedimentation, and others - so that diffusiophoresis is in fact a very common transport mechanism, in both natural and artificial systems. We anticipate that in georeservoir extractions, physiological systems, drying operations, laboratory and industrial separations, crystallization operations, membrane processes, and many other situations, diffusiophoresis is already occurring - often without being recognized - and that opportunities exist for designing this transport to great advantage.

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.

Confocal Raman microscopy of protein adsorbed in chromatographic particles

Confocal Raman microscopy is a nondestructive analytical technique that combines the chemical information from vibrational spectroscopy with the spatial resolution of confocal microscopy. It was applied, for the first time, to measure conformation and distribution of protein adsorbed in wetted chromatographic particles. Monoclonal antibody was loaded into the Fractogel EMD SO(3) (M) cation exchanger at 2 mS/cm or 10 mS/cm. Amide I and III frequencies in the Raman spectrum of the adsorbed protein suggest that there are no detectable changes of the original β-sheet conformation in the chromatographic particles. Protein depth profile measurements indicate that, when the conductivity is increased from 2 mS/cm to 10 mS/cm, there is a change in mass transport mechanism for protein adsorption, from the shrinking-core model to the homogeneous-diffusion model. In this study, the use of confocal Raman microscopy to measure protein distribution in chromatographic particles fundamentally agrees with previous confocal laser scanning microscopic investigations, but confocal Raman spectroscopy enjoys additional advantages: use of unlabeled protein to eliminate fluorescent labeling, ability for characterization of protein secondary structure, and ability for spectral normalization to provide a nondestructive experimental approach to correct light attenuation effects caused by refractive index (RI) mismatching in semiopaque chromatographic particles.

Protein adsorption and transport in polymer-functionalized ion-exchangers

A wide variety of stationary phases is available for use in preparative chromatography of proteins, covering different base matrices, pore structures and modes of chromatography. There has recently been significant growth in the number of such materials in which the base matrix is derivatized to add a covalently attached or grafted polymer layer or, in some cases, a hydrogel that fills the pore space. This review summarizes the main structural and functional features of ion exchangers of this kind, which represent the largest class of such materials. Although the adsorption and transport properties may generally be used operationally and modeled phenomenologically using the same methods as are used for proteins in conventional media, there are noteworthy mechanistic differences in protein behavior in these adsorbents. A fundamental difference in protein retention is that it may be portrayed as partitioning into a three-dimensional polymer phase rather than adsorption at an extended two-dimensional surface, as applies in more conventional media. Beyond this partitioning behavior, however, the polymer-functionalized media often display rapid intraparticle transport that, while qualitatively comparable to that in conventional media, is sufficiently rapid quantitatively under certain conditions that it can lead to clear benefits in key measures of performance such as the dynamic binding capacity. Although possible mechanistic bases for the retention and transport properties are discussed, appreciable areas of uncertainty make detailed mechanistic modeling very challenging, and more detailed experimental characterization is likely to be more productive.

Protein adsorption and transport in dextran-modified ion-exchange media. III. Effects of resin charge density and dextran content on adsorption and intraparticle uptake

Custom-synthesized variants of the commercial Capto S resin were used to examine the effects of resin charge density and dextran content on protein adsorption and intraparticle uptake. For the small protein lysozyme, resin charge density had the greatest effect on equilibrium capacity, consistent with calculations suggesting that lysozyme capacity should be limited by the available charge on the resin. Isocratic retention data and confocal microscopy imaging for this protein revealed a consistent ordering of the resins linking stronger protein-resin interactions with higher static capacities but slower intraparticle uptake rates over the range of properties studied. For the larger protein lactoferrin, it was found that increasing dextran content led to increased protein exclusion from the dextran layer, but that increasing resin charge density helped overcome the exclusion, presumably due to the increased electrostatic attraction between the resin and protein. Collectively examining the lysozyme and lactoferrin data along with information from previous studies suggests that a trade-off in maximizing dynamic capacities should exist between static capacities that increase to a finite extent with increased resin charge density and uptake rates that decrease with increased charge density. Column breakthrough data for lysozyme and lactoferrin appear to support the hypothesis, though it appears that whether a resin charge density is low or high must be considered in relation to the protein charge density. Using these trends, this work could be useful in guiding resin selection or design.

Cation-exchange chromatography of monoclonal antibodies: characterisation of a novel stationary phase designed for production-scale purification

A novel cation-exchange resin, Eshmuno™ S, was compared to Fractogel® SO3(-) (M) and Toyopearl GigaCap S-650M. The stationary phases have different base matrices, and carry specific types of polymeric surface modifications. Three monoclonal antibodies (mAbs) were used as model proteins to characterize these chromatographic resins. Results from gradient elutions, stirred batch adsorptions and confocal laser scanning microscopic investigations were used to elucidate binding behaviour of mAbs onto Eshmuno™ S and Fractogel® SO3(-) and the corresponding transport mechanisms on these two resins. The number of charges involved in mAb binding for Eshmuno™ S is lower than for Fractogel® SO3(-), indicating a slightly weaker electrostatic interaction. Kinetics from batch uptake experiments are compared to kinetic data obtained from confocal laser scanning microscopy images. Both experimental approaches show an accelerated protein adsorption for the novel stationary phase. The influence of pH, salt concentrations and residence times on dynamic binding capacities was determined. A higher dynamic binding capacity for Eshmuno™ S over a wider range of pH values and residence times was found compared to Fractogel® SO3(-) and Toyopearl GigaCap S-650M. The capture of antibodies from cell culture supernatant, as well as post-protein A eluates, were analyzed with respect to their host cell protein (hcp) removal capabilities. Comparable or even better hcp clearance was observed at much higher protein loading for Eshmuno™ S than Fractogel® SO3(-) or Toyopearl GigaCap S-650M.

Cation-exchange chromatography of monoclonal antibodies: characterisation of a novel stationary phase designed for production-scale purification

A novel cation-exchange resin, Eshmuno™ S, was compared to Fractogel® SO3(-) (M) and Toyopearl GigaCap S-650M. The stationary phases have different base matrices, and carry specific types of polymeric surface modifications. Three monoclonal antibodies (mAbs) were used as model proteins to characterize these chromatographic resins. Results from gradient elutions, stirred batch adsorptions and confocal laser scanning microscopic investigations were used to elucidate binding behaviour of mAbs onto Eshmuno™ S and Fractogel® SO3(-) and the corresponding transport mechanisms on these two resins. The number of charges involved in mAb binding for Eshmuno™ S is lower than for Fractogel® SO3(-), indicating a slightly weaker electrostatic interaction. Kinetics from batch uptake experiments are compared to kinetic data obtained from confocal laser scanning microscopy images. Both experimental approaches show an accelerated protein adsorption for the novel stationary phase. The influence of pH, salt concentrations and residence times on dynamic binding capacities was determined. A higher dynamic binding capacity for Eshmuno™ S over a wider range of pH values and residence times was found compared to Fractogel® SO3(-) and Toyopearl GigaCap S-650M. The capture of antibodies from cell culture supernatant, as well as post-protein A eluates, were analyzed with respect to their host cell protein (hcp) removal capabilities. Comparable or even better hcp clearance was observed at much higher protein loading for Eshmuno™ S than Fractogel® SO3(-) or Toyopearl GigaCap S-650M.

Multiscale modeling of protein uptake patterns in chromatographic particles

A parallel diffusion model is presented to explain an apparent transition in uptake mechanism seen in experimental observations of protein uptake into porous adsorbents. While such models have been invoked previously, this mesoscopic description is augmented here by microscopic models for representing surface diffusion by "hopping" and adsorption within a Gibbs surface excess formulation. These contributions lead to a relation for the apparent protein diffusivity as a function of adsorption conditions, which can be used predictively with knowledge of a few readily measured physical quantities. The approach can be useful in seeking optimal conditions for preparative protein chromatography separations.