Zonal rate model for axial and radial flow membrane chromatography,part II: Model‐based scale‐up

Membrane chromatography (MC) systems are finding increasing use in downstream processing trains for therapeutic proteins due to the unique mass‐transfer characteristics they provide. As a result, there is increased need for model‐based methods to scale‐up MC units using data collected on a scaled‐down unit. Here, a strategy is presented for MC unit scale‐up using the zonal rate model (ZRM). The ZRM partitions an MC unit into virtual flow zones to account for deviations from ideal plug‐flow behavior. To permit scale‐up, it is first configured for the specific device geometry and flow profiles within the scaled‐down unit so as to achieve decoupling of flow and binding related non‐idealities. The ZRM is then configured for the preparative‐scale unit, which typically utilizes markedly different flow manifolds and membrane architecture. Breakthrough is first analyzed in both units under non‐binding conditions using an inexpensive tracer to independently determine unit geometry related parameters of the ZRM. Binding related parameters are then determined from breakthrough data on the scaled‐down MC capsule to minimize sample requirements. Model‐based scale‐up may then be performed to predict band broadening and breakthrough curves on the preparative‐scale unit. Here, the approach is shown to be valid when the Pall XT140 and XT5 capsules serve as the preparative and scaled‐down units, respectively. In this case, scale‐up is facilitated by our finding that the distribution of linear velocities through the membrane in the XT140 capsule is independent of the feed flow rate and the type of protein transmitted. Introduction of this finding into the ZRM permits quantitative predictions of breakthrough over a range of industrially relevant operating conditions. Biotechnol. Bioeng. 2014;111: 1587–1594. © 2014 Wiley Periodicals, Inc.     

Zonal rate model for stacked membrane chromatography part II: characterizing ion-exchange membrane chromatography under protein retention conditions

The Zonal Rate Model (ZRM) has previously been shown to accurately account for contributions to elution band broadening, including external flow nonidealities and radial concentration gradients, in ion-exchange membrane (IEXM) chromatography systems operated under nonbinding conditions. Here, we extend the ZRM to analyze and model the behavior of retained proteins by introducing terms for intra-column mass transfer resistances and intrinsic binding kinetics. Breakthrough curve (BTC) data from a scaled-down anion-exchange membrane chromatography module using ovalbumin as a model protein were collected at flow rates ranging from 1.5 to 20 mL min(-1). Through its careful accounting of transport nonidealities within and external to the membrane stack, the ZRM is shown to provide a useful framework for characterizing putative protein binding mechanisms and models, for predicting BTCs and complex elution behavior, including the common observation that the dynamic binding capacity can increase with linear velocity in IEXM systems, and for simulating and scaling separations using IEXM chromatography. Global fitting of model parameters is used to evaluate the performance of the Langmuir, bi-Langmuir, steric mass action (SMA), and spreading-type protein binding models in either correlating or fundamentally describing BTC data. When combined with the ZRM, the bi-Langmuir, and SMA models match the chromatography data, but require physically unrealistic regressed model parameters to do so. In contrast, for this system a spreading-type model is shown to accurately predict column performance while also providing a realistic fundamental explanation for observed trends, including an observed increase in dynamic binding capacity with flow rate.     

Investigating the effects of membrane deformability on artificial capsule adhesion to the functionalized surface

Understanding, manipulating and controlling cellular adhesion processes can be critical in developing biomedical technologies. Adhesive mechanisms can be used to the target, pattern and separate cells such as leukocytes from whole blood for biomedical applications. The deformability response of the cell directly affects the rolling and adhesion behavior under viscous linear shear flow conditions. To that end, the primary objective of the present study was to investigate numerically the influence of capsule membrane’s nonlinear material behavior (i.e. elastic-plastic to strain hardening) on the rolling and adhesion behavior of representative artificial capsules. Specifically, spherical capsules with radius of \(3.75\, \upmu \hbox {m}\) were represented using an elastic membrane governed by a Mooney–Rivlin strain energy functions. The surfaces of the capsules were coated with P-selectin glycoprotein-ligand-1 to initiate binding interaction with P-selectin-coated planar surface with density of \(150\,\upmu \hbox {m}^{-2}\) under linear shear flow varying from 100 to \(400\,\hbox {s}^{-1}\). The numerical model is based on the Immersed Boundary Method for rolling of deformable capsule in shear flow coupled with Monte Carlo simulation for receptor/ligand interaction modeled using Bell model. The results reveal that the mechanical properties of the capsule play an important role in the rolling behavior and the binding kinetics between the capsule contact surface and the substrate. The rolling behavior of the strain hardening capsules is relatively smoother and slower compared to the elastic-plastic capsules. The strain hardening capsules exhibits higher contact area at any given shear rate compared to elastic-plastic capsules. The increase in contact area leads to decrease in rolling velocity. The capsule contact surface is not in complete contact with the substrate because of thin lubrication film that is trapped between the capsule and substrate. This creates a concave shape on the bottom surface of the capsule that is referred to as a dimple. In addition, the present study demonstrates that the average total bond force from the capsules lifetime increases by 37 % for the strain hardening capsules compared to elastic-plastic capsules at shear rate of \(400\,\hbox {s}^{-1}\). Finally, the model demonstrates the effect of finite membrane deformation on the coupling between hydrodynamic and receptor/ligand interaction.     

Protein A immunoaffinity hollow fiber membranes for immunoglobulin G purification: experimental characterization

Immunoaffinity adsorption is increasingly used for protein purification and medical applications. Synthetic membranes have advantages as support matrices in comparison to conventional bead supports because they are not compressible and they eliminate internal diffusion limitations. The goal of this study was to explore in detail the performance of microporous hollow fibers composed of modified polysulfone to which protein A was immobilized for adsorption of human IgG. The internal matrix was characterized by scanning electron microscopy. The binding equilibrium constant was measured using both static and dynamic methods. Break-through curves up to ligand saturation were measured and used to study the effects of IgG concentration, presence of contaminant albumin, flow direction, flow mode, and especially filtrate flow rate and maximum IgG binding capacity. The highest binding capacities studied were comparable with that attainable with bead matrices. All of the breakthrough curves could be represented on a single figure when plotted versus the dimensionless relative throughput (the mass of IgG loaded on the membrane divided by the mass that would be bound when the entire fiber is in equilibrium with the feed concentration), and the effect of operating variables on the position and shape of the individual breakthrough curves could be understood in terms of a dimensional performance parameter (the product of membrane volume and maximum binding capacity divided by the filtrate flow rate). The best breakthrough curves were obtained with the highest values of the performance parameter. Based on the results, membranes as solid supports for immunoadsorption can be a useful alternative to the use of traditional columns for protein separations. (c) 1995 John Wiley & Sons, Inc.     

AGEs in human lens capsule promote the TGFβ2‐mediated EMT of lens epithelial cells: implications for age‐associated fibrosis

Proteins in basement membrane (BM) are long‐lived and accumulate chemical modifications during aging; advanced glycation endproduct (AGE) formation is one such modification. The human lens capsule is a BM secreted by lens epithelial cells. In this study, we have investigated the effect of aging and cataracts on the AGE levels in the human lens capsule and determined their role in the epithelial‐to‐mesenchymal transition (EMT) of lens epithelial cells. EMT occurs during posterior capsule opacification (PCO), also known as secondary cataract formation. We found age‐dependent increases in several AGEs and significantly higher levels in cataractous lens capsules than in normal lens capsules measured by LC‐MS/MS. The TGFβ2‐mediated upregulation of the mRNA levels (by qPCR) of EMT‐associated proteins was significantly enhanced in cells cultured on AGE‐modified BM and human lens capsule compared with those on unmodified proteins. Such responses were also observed for TGFβ1. In the human capsular bag model of PCO, the AGE content of the capsule proteins was correlated with the synthesis of TGFβ2‐mediated α‐smooth muscle actin (αSMA). Taken together, our data imply that AGEs in the lens capsule promote the TGFβ2‐mediated fibrosis of lens epithelial cells during PCO and suggest that AGEs in BMs could have a broader role in aging and diabetes‐associated fibrosis.     

A new large‐scale manufacturing platform for complex biopharmaceuticals

Complex biopharmaceuticals, such as recombinant blood coagulation factors, are addressing critical medical needs and represent a growing multibillion‐dollar market. For commercial manufacturing of such, sometimes inherently unstable, molecules it is important to minimize product residence time in non‐ideal milieu in order to obtain acceptable yields and consistently high product quality. Continuous perfusion cell culture allows minimization of residence time in the bioreactor, but also brings unique challenges in product recovery, which requires innovative solutions. In order to maximize yield, process efficiency, facility and equipment utilization, we have developed, scaled‐up and successfully implemented a new integrated manufacturing platform in commercial scale. This platform consists of a (semi‐)continuous cell separation process based on a disposable flow path and integrated with the upstream perfusion operation, followed by membrane chromatography on large‐scale adsorber capsules in rapid cycling mode. Implementation of the platform at commercial scale for a new product candidate led to a yield improvement of 40% compared to the conventional process technology, while product quality has been shown to be more consistently high. Over 1,000,000 L of cell culture harvest have been processed with 100% success rate to date, demonstrating the robustness of the new platform process in GMP manufacturing. While membrane chromatography is well established for polishing in flow‐through mode, this is its first commercial‐scale application for bind/elute chromatography in the biopharmaceutical industry and demonstrates its potential in particular for manufacturing of potent, low‐dose biopharmaceuticals. Biotechnol. Bioeng. 2012; 109: 3049–3058. © 2012 Wiley Periodicals, Inc.     

Whole-tree water transport scales with sapwood capacitance in tropical forest canopy trees

The present study examines the manner in which several whole‐tree water transport properties scale with species‐specific variation in sapwood water storage capacity. The hypothesis that constraints on relationships between sapwood capacitance and other water relations characteristics lead to predictable scaling relationships between intrinsic capacitance and whole‐tree behaviour was investigated. Samples of sapwood from four tropical forest canopy tree species selected to represent a range of wood density, tree size and architecture, and taxonomic diversity were used to generate moisture release curves in thermocouple psychrometer chambers, from which species‐specific values of sapwood capacitance were calculated. Sapwood capacitance was then used to scale several whole‐tree water transport properties determined from measurements of upper branch and basal sap flow, branch water potential, and axial and radial movement of deuterated water (D₂O) injected into the base of the trunk as a tracer. Sapwood capacitance ranged from 83 to 416 kg m^?3 MPa^?1 among the four species studied and was strongly correlated with minimum branch water potential, soil‐to‐branch hydraulic conductance, daily utilization of stored water, and axial and radial movement of D₂O. The species‐independent scaling of several whole‐tree water transport properties with sapwood capacitance indicated that substantial convergence in plant function at multiple levels of biological organization was revealed by a simple variable related to a biophysical property of water transport tissue.     

Gelation conditions and transport properties of hollow calcium alginate capsules

The diameter, membrane thickness, and compression intensity of hollow Ca-alginate capsules were measured at different gelation conditions, such as the reactant concentration, dropping velocity, and gelation time. The optimum operation conditions for preparing capsules were determined at 100 g/L CaCl(2), 10 g/L sodium alginate (Na-alginate), a dropping velocity of 150 droplets/min, and a gelation time of 10 min. Diffusion of some saccharide and amino acid from bulk solution into capsules was investigated, and the diffusion coefficients were calculated by the developed mathematical model. All the tested substances can diffuse easily into the capsules. The combined diffusion coefficients of the capsule D(m) are 92-99% as large as their diffusion coefficients in pure water, while the diffusion coefficients in the capsule membrane D(1) are 60-95% as large as those. By employing polyethylene glycol (PEG) and bovine serum albumin (fraction V) (BSA(V)), the molecular weight cut-off of the capsule was determined. For linear macromolecules, hollow Ca-alginate capsules have a molecular weight cut-off of 4000. No diffusion of BSA(V) into the capsules was observed.     

SpoIIIE mechanism of directional translocation involves target search coupled to sequence‐dependent motor stimulation

SpoIIIE/FtsK are membrane‐anchored, ATP‐fuelled, directional motors responsible for chromosomal segregation in bacteria. Directionality in these motors is governed by interactions between specialized sequence‐recognition modules (SpoIIIE‐γ/FtsK‐γ) and highly skewed chromosomal sequences (SRS/KOPS). Using a new combination of ensemble and single‐molecule methods, we dissect the series of steps required for SRS localization and motor activation. First, we demonstrate that SpoIIIE/DNA association kinetics are sequence independent, with binding specificity being uniquely determined by dissociation. Next, we show by single‐molecule and modelling methods that hexameric SpoIIIE binds DNA non‐specifically and finds SRS by an ATP‐independent target search mechanism, with ensuing oligomerization and binding of SpoIIIE‐γ to SRS triggering motor stimulation. Finally, we propose a new model that provides an entirely new interpretation of previous observations for the origin of SRS/KOPS‐directed translocation by SpoIIIE/FtsK.     

A model of diffusion and mass flow of water in cylindrical membrane systems with application to plant roots

Summary Measurements of the radial diffusion of tritiated water, combined with axial and radial flow in an artificial cylindrical membrane, are examined with the aid of a mathematical model. The results are used to assess how far measurement of diffusion of labeled water in plant roots may throw light on pathways of movement of water and on barriers to flow.     

Chromatography of human prothrombin from Nitschmann fraction III on Q Sepharose Fast Flow using axial and radial flow column

An axial column (Hitrap Q 5 ml, 2.5 2 1.6 cm) and a radial flow column (3.5 2 5 cm) packed with Q Sepharose Fast Flow media had been evaluated for the separation of human prothrombin. Nitschmann fraction III dissolved in buffered saline (0.10 M sodium chloride buffered with 0.06 M Tris/HCl to pH 7.5) was the starting material. Effects of sample flow rate of the two columns were screened. Under radial flow conditions using the radial column, sample flow rate up to 15 ml/min (i.e. 18 bed volumes/h) was achieved and the operating pressure was below 0.2 MPa eventhough the elution velocity was 30 ml/min. Breakthrough capacity was determined by analyzing the total protein and prothrombin activity of the target protein-containing fraction under subsaturating conditions and both columns had almost the same breakthrough capacity per ml media, indicating that the sample loading was independent of radial column geometry. It was concluded that the radial column is an attractive alternate to traditional axial packed bed column, exhibiting very good potential for use in the separation of human prothrombin.     

Simulation of the effect of mixing,scale-up and pH-value regulation during glutamic acid fermentation

A mixing model is coupled with fermentation kinetics in order to simulate a fermentation as a function of mixing conditions and scale-up. The mixing model for a batch stirred tank with three stirrers consists of three regions, each of them characterized by an ideally mixed compartment around the stirrer and two macromixers, i.e. cascades of tank-in-series, describing the recirculation flow. The model contains four parameters — radial and axial circulation time, volume of the ideally mixed stirrer compartment and the number of tanks in each cascade. These values, determined by Mayr et al. in function of the operational conditions and scale-up, were choosen to simulate the fermentation of glutamic acid to show the pH-fluctuation at different control and scale conditions. By choosing optimal regulation properties, such as input flow rate and/or concentration of the base, regulation span, position of the pH-electrode and base input location, etc., fluctuations of the pH-value in the bio-reactor can be minimized. However, the negative effect of insufficient mixing conditions can be reduced only by an increasing number of the base input places. In large scale fermentors, the axial circulation time is rather high, about 5–10 times larger than the radial one. This might result in a large amplitude of the pH-fluctuation. As it is shown, using an input place for base in each stirrer region, the negative impact of the insufficient axial mixing on the fermentation can be diminished perfectly. In this case ammonia should be fed into the reactor as an aqueous solution.     

Roles of interstitial fraction and load conditions on the dynamic binding capacity of proteins on capillary‐channeled polymer fiber columns

Capillary‐channeled polymer (C‐CP) fibers are used as a stationary phase for ion‐exchange chromatography of proteins. Collinear packing of the fibers permits operation at high linear velocities (U_o > 100 mm s^?1) and low backpressure (<2,000 psi) on analytical‐scale columns. Rapid solvent transport is matched with very efficient solute mass transfer as fibers are virtually non‐porous with respect to the size of the target protein molecules. Lack of porosity of course limits the equilibrium binding capacity of stationary phases. Breakthrough curves and frontal analysis are used to better understand trade‐offs between the kinetic and thermodynamic properties as C‐CP fibers are applied in preparative situations. Fiber columns packed to different interstitial fraction values affect both the total fiber surface area (e.g., equilibrium binding capacity [EBC]) and the permittivity to flow and mass transport characteristics (e.g., dynamic binding capacity [DBC]). The EBC of the nylon 6 C‐CP fibers was found to be 1.30 mg g^?1, with isotherms that were best matched by a Moreau model, showing linearity up to solute concentrations of ～0.4 mg mL^?1. Isotherms generated under flow conditions were equally well approximated using Langmuir, Freundlich, and Moreau isotherm models. Fairly linear responses were seen up to the maximum load concentration of 1.2 mg mL^?1. Counterintuitively, dynamic studies revealed that conditions of high column porosity yielded a DBC that is ～70% higher than the EBC. These findings point to potential advantages in terms downstream processing applications, where protein throughput and yield are critical metrics. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 31:97–109, 2015     

A beta-1,2-xylosyltransferase from Cryptococcus neoformans defines a new family of glycosyltransferases

Cryptococcus neoformans is an opportunistic fungal pathogen characterized by a prominent polysaccharide capsule that envelops the cell. Although this capsule is dispensable for in vitro growth, its presence is essential for virulence. The capsule is primarily made of two xylose-containing polysaccharides, glucuronoxylomannan and galactoxylomannan. There are likely to be multiple xylosyltransferases (XTs) involved in capsule synthesis, and the activities of these enzymes are potentially important for cryptococcal virulence. A beta-1,2-xylosyltransferase with specificity appropriate for capsule synthesis was purified approximately 3000-fold from C. neoformans, and the corresponding gene was identified and cloned. This sequence conferred XT activity when expressed in Saccharomyces cerevisiae, which lacks endogenous XT activity. The gene, termed CXT1 for cryptococcal xylosyltransferase 1, encodes a 79-kDa type II membrane protein with an N-linked glycosylation site and two DXD motifs. These latter motifs are believed to coordinate divalent cation binding in the activity of glycosyltransferases. Site-directed mutagenesis of one DXD motif abolished Cxt1p activity, even though this activity does not depend on the addition of a divalent cation. This may indicate a novel catalytic mechanism for glycosyl transfer. Five homologs of Cxt1p were found in the genome sequence of C. neoformans and 34 within the sequences of other fungi, although none were found in other organisms. Many of the homologous proteins are similar in size to Cxt1p, and all are conserved with respect to the essential DXD motif. These proteins represent a new family of glycosyltransferases, found exclusively within the fungal kingdom.     

Breakthrough performance of plasmid DNA on ion-exchange membrane columns

Breakthrough performance of plasmid DNA adsorption on ion-exchange membrane columns was theoretically and experimentally investigated using batch and fixed-bed systems. System dispersion curves showed the absence of flow non-idealities in the experimental arrangement. Breakthrough curves (BTC) were significantly affected by inlet flow rate and solute concentration. In the theoretical analysis, a model was integrated by the serial coupling of the membrane transport model and the system dispersion model. A transport model that considers finite kinetic rate and column dispersed flow was used in the study. A simplex optimization routine, coupled to the solution of the partial differential model equations, was employed to estimate the maximum adsorption capacity constant, the equilibrium desorption constant, and the forward interaction rate constant, which are the parameters of the membrane transport model. The analysis shows that as inlet concentration or flow rate increases, the deviation of the model from the experimental behavior decreases. The BTCs displacement as inlet concentration increases was explained in terms of a greater degree of column saturation reached and more efficient operation accomplished. The degree of column saturation was not influenced by inlet flow rate. It was necessary to consider in the column model the slight variation in the BTC produced by the axial dispersion, in order to accomplish the experimental curve dispersion. Consequently, the design criteria that for Pe > 40 the column axial dispersion can be neglected should be taken with precaution.     

Breakthrough performance of stacks of dye-cellulosic fabric in affinity chromatography of lysozyme

The breakthrough performance of stacks of dye-cellulosic fabric in affinity chromatography of lysozyme was investigated in batch and flow experiments. Breakthrough curves were significantly affected by flow rate and were not dependent on the feed solution concentration. System dispersion curves could not explain the flow-rate dependence. Breakthrough curves were analyzed by coupling the kinetic model for pore mass transfer as the only controlling resistance and a system dispersion model. From the analysis, pore film mass transfer resistance was found to be the leading rate-limiting factor when the residence time in the column is greater than 5 min. The model was used to predict the operating and design parameters needed to obtain sharp breakthrough curves. Selectivity studies using lysozyme and bovine serum albumin mixtures showed a high system selectivity for lysozyme.     

Expanded bed adsorption of protein with DEAE Spherodex M

In this article, the bed expansion behavior and the hydrodynamic and protein adsorption properties of the DEAE Spherodex M in expanded bed with mobile phases of different viscosities have been studied. The axial liquid-phase dispersion coefficient is found to be on the order of 10(-6) m(2)/s, falling into the common range from 1.0 x 10(-6) to 1.0 x 10(-5) m(2)/s observed previously in expanded bed operation. Because of the small size of the adsorbent, the high pore diffusivity of protein and the favorable column efficiency (low dispersion coefficient), the dynamic binding capacity (DBC) of bovine serum albumin (BSA) at 5% breakthrough in the expanded bed reaches over 80% that of the equilibrium adsorption density (EAD). Moreover, a theoretical model with unadjustable model parameters is used for the prediction of the breakthrough curves. Computer simulations show that the model agrees well with the experimental results at breakthrough less than about 50%. It indicates that the model is promising in the prediction of protein breakthrough behavior because breakthrough profiles at 5-50% breakthrough points are more important in practical applications.     

Characterization of a continuous supermacroporous monolithic matrix for chromatographic separation of large bioparticles

A continuous supermacroporous monolithic chromatographic matrix has been characterized using a capillary model, experimental breakthrough curves, and pressure drop experiments. The model describes the convective flow and its dispersive mixing effects, mass transfer resistance, pore size distribution, and the adsorption behavior of the monolithic matrix. It is possible to determine an effective pore size distribution by fitting the capillary model to experimental breakthrough curves and pressure drop experiments. The model is able to describe the flow rate dependence of the experimental breakthrough curves. Mass transport resistance was due to: (i) dispersive mixing effects in the convective flow in the pores; and (ii) slow diffusion in the stagnant film covering the surface within each pore, under adsorption conditions. The monolithic matrix can be described by a very narrow pore size distribution, illustrating one of the advantages of the gel. A broader pore size distribution results in increased band broadening. This can be studied easily using the model developed in this investigation.     

大孔型NaCS-PDMDAAC微胶囊固定化细胞在摇床和鼓泡塔中的培养研究

NaCS-PDMDAAC生物微胶囊囊膜较为致密,影响胶囊内外物质的交换,从而影响胶囊内细胞的生长。利用淀粉酶对致孔剂淀粉的降解作用制备了一种大孔型的纤维素硫酸钠_聚二甲基二烯丙基氯化铵（NaCS-PDMDAAC）生物微胶囊,实验表明胶囊的孔径和通透性能都有了很大的提高。将酵母和大肠杆菌作为模型细胞包埋于胶囊中分别通过摇瓶和鼓泡塔半连续培养,在鼓泡塔中胶囊内细胞的密度要高于摇床,表明氧气的传递是胶囊内好氧细胞生长的限制因素,大孔胶囊由于囊膜孔径变大,氧气的传递更为快速,在鼓泡塔中大孔型胶囊内的最大细胞密度比常规胶囊要高出20%～110%。由于对氧气的需求量的不同,大肠杆菌菌浓提高的程度要高于酵母。