Analysis of mass transport models for protein adsorption to cation exchanger by visualization with confocal laser scanning microscopy

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.

Modeling competitive cytokine adsorption dynamics within hemoadsorption beads used to treat sepsis

Extracorporeal blood purification is a promising therapeutic modality for sepsis, a potentially fatal, dysfunctional immunologic state caused by infection. Removal of inflammatory mediators such as cytokines from the blood may help attenuate hyper-inflammatory signaling during sepsis and improve patient outcomes. We are developing a hemoadsorption device to remove cytokines from the circulating blood using biocompatible, porous sorbent beads. In this work, we investigated whether competitive adsorption of serum solutes affects cytokine removal dynamics within the hemoadsorption beads. Confocal laser scanning microscopy (CLSM) was used to quantify intraparticle adsorption profiles of fluorescently labeled IL-6 in horse serum, and results were compared to predictions of a two component competitive adsorption model. Supraphysiologic IL-6 concentrations were necessary to obtain adequate CLSM signal, therefore unknown model parameters were fit to CLSM data at high IL-6 concentrations, and the fitted model was used to simulate cytokine adsorption behavior at physiologically relevant levels which were below the microscopy detection threshold. CLSM intraparticle IL-6 adsorption profiles agreed with predictions of the competitive adsorption model, indicating displacement of cytokine by high affinity serum solutes. However, competitive adsorption effects were predicted using the model to be negligible at physiologic cytokine concentrations associated with hemoadsorption therapy.

IL-6 adsorption dynamics in hemoadsorption beads studied using confocal laser scanning microscopy

Sepsis is characterized by a systemic inflammatory response caused by infection, and can result in organ failure and death. Removal of inflammatory mediators such as cytokines from the circulating blood is a promising treatment for severe sepsis. We are developing an extracorporeal hemoadsorption device to remove cytokines from the blood using biocompatible, polymer sorbent beads. In this study, we used confocal laser scanning microscopy (CLSM) to directly examine adsorption dynamics of a cytokine (IL-6) within hemoadsorption beads. Fluorescently labeled IL-6 was incubated with sorbent particles, and CLSM was used to quantify spatial adsorption profiles of IL-6 within the sorbent matrix. IL-6 adsorption was limited to the outer 15 microm of the sorbent particle over a relevant clinical time period, and intraparticle adsorption dynamics was modeled using classical adsorption/diffusion mechanisms. A single model parameter, alpha = q(max) K/D, was estimated by fitting CLSM intensity profiles to our mathematical model, where q(max) and K are Langmuir adsorption isotherm parameters, and D is the effective diffusion coefficient of IL-6 within the sorbent matrix. Given the large diameter of our sorbent beads (450 microm), less than 20% of available sorbent surface area participates in cytokine adsorption. Development of smaller beads may accelerate cytokine adsorption by maximizing available surface area per bead mass.

Approaches to high-performance preparative chromatography of proteins

Preparative liquid chromatography is widely used for the purification of chemical and biological substances. Different from high-performance liquid chromatography for the analysis of many different components at minimized sample loading, high-performance preparative chromatography is of much larger scale and should be of high resolution and high capacity at high operation speed and low to moderate pressure drop. There are various approaches to this end. For biochemical engineers, the traditional way is to model and optimize a purification process to make it exert its maximum capability. For high-performance separations, however, we need to improve chromatographic technology itself. We herein discuss four approaches in this review, mainly based on the recent studies in our group. The first is the development of high-performance matrices, because packing material is the central component of chromatography. Progress in the fabrication of superporous materials in both beaded and monolithic forms are reviewed. The second topic is the discovery and design of affinity ligands for proteins. In most chromatographic methods, proteins are separated based on their interactions with the ligands attached to the surface of porous media. A target-specific ligand can offer selective purification of desired proteins. Third, electrochromatography is discussed. An electric field applied to a chromatographic column can induce additional separation mechanisms besides chromatography, and result in electrokinetic transport of protein molecules and/or the fluid inside pores, thus leading to high-performance separations. Finally, expanded-bed adsorption is described for process integration to reduce separation steps and process time.

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.

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

Intraparticle transport of proteins usually represents the principal resistance controlling their uptake in preparative separations. In ion-exchange chromatography two limiting models are commonly used to describe such uptake: pore diffusion, in which only free protein in the pore lumen contributes to transport, and homogeneous diffusion, in which the transport flux is determined by the gradient in the total protein concentration, free or adsorbed. Several studies have noted a transition from pore to homogeneous diffusion with increasing ionic strength in some systems, and here we investigate the mechanistic basis for this transition. The studies were performed on a set of custom-synthesized methacrylate-based strong cation exchangers differing in ligand density into which uptake of two proteins was examined using confocal microscopy and frontal loading experiments. We find that the transition in uptake mechanism occurs in all cases studied, and generally coincides with an optimum in the dynamic binding capacity at moderately high flow rates. The transition appears to occur when protein-surface attraction is weakened sufficiently, and this is correlated with the isocratic retention factor k' for the system of interest: the transition occurs in the vicinity of k' approximately 3000. This result, which may indicate that adsorption is sufficiently weak to allow the protein to diffuse along or near the surface, provides a predictive basis for optimizing preparative separations using only isocratic retention data.

Ion exchange chromatography of antibody fragments

Effects of pH and conductivity on the ion exchange chromatographic purification of an antigen-binding antibody fragment (Fab) of pI 8.0 were investigated. Normal sulfopropyl (SP) group modified agarose particles (SP Sepharosetrade mark Fast Flow) and dextran modified particles (SP Sepharose XL) were studied. Chromatographic measurements including adsorption isotherms and dynamic breakthrough binding capacities, were complemented with laser scanning confocal microscopy. As expected static equilibrium and dynamic binding capacities were generally reduced by increasing mobile phase conductivity (1-25 mS/cm). However at pH 4 on SP Sepharose XL, Fab dynamic binding capacity increased from 130 to 160 (mg/mL media) as mobile phase conductivity changed from 1 to 5 mS/cm. Decreasing protein net charge by increasing pH from 4 to 5 at 1.3 mS/cm caused dynamic binding capacity to increase from 130 to 180 mg/mL. Confocal scanning laser microscopy studies indicate such increases were due to faster intra-particle mass transport and hence greater utilization of the media's available binding capacity. Such results are in agreement with recent studies related to ion exchange of whole antibody molecules under similar conditions.

Modeling and simulation of protein uptake in cation exchanger visualized by confocal laser scanning microscopy

Confocal laser scanning microscopy (CLSM) has been extensively applied in the area of protein chromatography to investigate the uptake mechanism of protein in adsorbents. However, due to the light attenuation in the deeper layers of a specimen, quantitative analysis using CLSM data is still far from reality. In this work, an attenuation equation for describing the darkening of the CLSM image in the deeper scanning layers was developed. Bovine serum albumin (BSA) adsorption to SP Sepharose FF was performed by batch adsorption and micro-column chromatography on which protein concentration in single absorbents were visualized by CLSM. The parameters in the equation were estimated by fitting it to the fluorescence intensity profiles obtained at adsorption equilibrium, and then the equation was used to simulate the effect caused by the light scattering and absorption. CLSM analysis demonstrated that BSA adsorption to SP Sepharose FF followed the shrinking core pattern and was predicted reasonably well by the pore diffusion model in combination with the attenuation equation. By comparison of the CLSM data with the simulations, it shows that the attenuation equation was useful to demonstrate the validity of an intraparticle mass transport model for the estimation of intraparticle protein concentration profiles.