High-throughput in silico workflow for optimization and characterization of multimodal chromatographic processes

While high-throughput (HT) experimentation and mechanistic modeling have long been employed in chromatographic process development, it remains unclear how these techniques should be used in concert within development workflows. In this work, a process development workflow based on HT experiments and mechanistic modeling was constructed. The integration of HT and modeling approaches offers improved workflow efficiency and speed. This high-throughput in silico (HT-IS) workflow was employed to develop a Capto MMC polishing step for mAb aggregate removal. High-throughput batch isotherm data was first generated over a range of mobile phase conditions and a suite of analytics were employed. Parameters for the extended steric mass action (SMA) isotherm were regressed for the multicomponent system. Model validation was performed using the extended SMA isotherm in concert with the general rate model of chromatography using the CADET modeling software. Here, step elution profiles were predicted for eight RoboColumn runs across a range of ionic strength, pH, and load density. Optimized processes were generated through minimization of a complex objective function based on key process metrics. Processes were evaluated at lab-scale using two feedstocks, differing in composition. The results confirmed that both processes obtained high monomer yield (>85%) and removed∼ 50 % $$ \sim 50\% $$ of aggregate species. Column simulations were then carried out to determine sensitivity to a wide range of process inputs. Elution buffer pH was found to be the most critical process parameter, followed by resin ionic capacity. Overall, this study demonstrated the utility of the HT-IS workflow for rapid process development and characterization.

Mechanistic modeling and simulation of a complex low and high loading elution behavior of a polypeptide in cation exchange chromatography

The mechanistic modeling of preparative liquid chromatography is still a challenging task. Non-ideal thermodynamic conditions may require activity coefficients for the mechanistic description of preparative chromatography. In this work, a chromatographic cation exchange step with a polypeptide having a complex elution behavior in low and high loading situations is modeled. Model calibration in the linear range of the isotherm is done by applying counterion-induced linear gradient elution experiments between pH 3.3 and pH 4.3. Inverse fitting with column loads up to 25 mg/mL_CV is performed for parameter estimation in the non-linear range. The polypeptide elution peak shows an anti-Langmuirian behavior with fronting under low loading conditions and a switch to a Langmuirian behavior with increasing load. This unusual elution behavior could be described with an extended version of the sigmoidal Self-Association isotherm, including two activity coefficients for the polypeptide and counterion in solution. The activity coefficient of the solute polypeptide shows a strong influence on the model parameters and is crucial in the linear and non-linear range of the isotherm. The modeling procedure results in a unique and robust model parameter set that is sufficient to describe the complex elution behavior and allows modeling over the full isotherm range. This article is protected by copyright. All rights reserved.

Characterization of a macroporous epoxy-polymer based resin for the ion-exchange chromatography of therapeutic proteins

This study investigated the adsorption capacity and mass transfer properties of a novel macroporous epoxy-polymer-based anion-exchanger, MPR Q, for the efficient separation of therapeutic proteins. MPR Q resin was prepared by phase separation based on spinodal decomposition followed by dextran grafting and ligand conjugation. Under static conditions, MPR Q exhibited a binding capacity of 49.8 mg-IgG/cm³-resin at pH 10, whereas the fastest adsorption was observed among the anion-exchanger resins tested. Inverse size-exclusion chromatography (iSEC) experiments revealed that the apparent pore diameter of MPR Q was approximately 90 nm, which was sufficiently large for the penetration of human IgG and bovine IgM. Moreover, the reduced height equivalent to a theoretical plate, h, of human IgG, determined using the linear gradient elution method was 65.8 and was not significantly changed in the range of linear velocities from 20.37 to 50.93 cm/min. The dynamic binding capacity at 10% breakthrough of MPR Q, determined by frontal analysis, exhibited a capacity of 43.8 mg/cm³ at 5.09 cm/min and 58% of DBC_10% was maintained even though the linear velocity was increased to 50.93 cm/min. Furthermore, a resolution for separation of IgG and BSA by MPR Q was 1.06 at 5.09 cm/min, while it was higher than that for the conventional resin at all linear velocities from 5.09 cm/min to 50.93 cm/min. Thus, it was suggested that the MPR Q developed in this study is a promising resin that can efficiently separate large biomacromolecules such as human IgG at higher velocities.

Change of charge variant composition of trastuzumab upon stressing at physiological conditions

Cation-exchange chromatography is a widely used approach to study charge heterogeneity of monoclonal antibodies. Heterogeneity may arise both in vitro and in vivo because of the susceptibility of monoclonal antibodies to undergo chemical modifications. Modifications may adversely affect the potency of the drug, induce immunogenicity or affect pharmacokinetics. In this study, we evaluated the application of optimized pH gradient systems for the separation of charge variants of trastuzumab after forced degradation study. pH gradient-based elution resulted in high-resolution separation of some 20 charge variants after 3 weeks at 37°C under physiological conditions. The charge variants were further characterized by LC-MS-based peptide mapping. There was no significant difference in the binding properties to HER2 or a range of Fcγ receptors between non-stressed and stressed trastuzumab.

Analysis of complex protein elution behavior in preparative ion exchange processes using a colloidal particle adsorption model

A fundamental understanding of the protein retention mechanism in preparative ion exchange (IEX) chromatography columns is essential for a model-based process development approach. For the past three decades, the mechanistic description of protein retention has been based predominantly on the steric mass action (SMA) model. In recent years, however, retention profiles of proteins have been reported more frequently for preparative processes that are not consistent with the mechanistic understanding relying on the SMA model. In this work, complex elution behavior of proteins in preparative IEX processes is analyzed using a colloidal particle adsorption (CPA) model. The CPA model is found to be capable of reproducing elution profiles that cannot be described by the traditional SMA model. According to the CPA model, the reported complex behavior can be ascribed to a strong compression and concentration of the elution front in the lower unsaturated part of the chromatography column. As the unsaturated part of the column decreases with increasing protein load density, exceeding a critical load density can lead to the formation of a shoulder in the peak front. The general applicability of the model in describing preparative IEX processes is demonstrated using several industrial case studies including multiple monoclonal antibodies on different IEX adsorber systems. In this context, the work covers both salt controlled and pH-controlled protein elution.

Protein adsorption on ion exchange adsorbers: A comparison of a stoichiometric and non-stoichiometric modeling approach

For mechanistic modeling of ion exchange (IEX) processes, a profound understanding of the adsorption mechanism is important. While the description of protein adsorption in IEX processes has been dominated by stoichiometric models like the steric mass action (SMA) model, discrepancies between experimental data and model results suggest that the conceptually simple stoichiometric description of protein adsorption provides not always an accurate representation of nonlinear adsorption behavior. In this work an alternative colloidal particle adsorption (CPA) model is introduced. Based on the colloidal nature of proteins, the CPA model provides a non-stoichiometric description of electrostatic interactions within IEX columns. Steric hindrance at the adsorber surface is considered by hard-body interactions between proteins using the scaled-particle theory. The model's capability of describing nonlinear protein adsorption is demonstrated by simulating adsorption isotherms of a monoclonal antibody (mAb) over a wide range of ionic strength and pH. A comparison of the CPA model with the SMA model shows comparable model results in the linear adsorption range, but significant differences in the nonlinear adsorption range due to the different mechanistic interpretation of steric hindrance in both models. The results suggest that nonlinear adsorption effects can be overestimated by the stoichiometric formalism of the SMA model and are generally better reproduced by the CPA model.

Separation of monoclonal antibody monomer-dimer mixtures by gradient elution with ceramic hydroxyapatite

Modeling the chromatographic separations of proteins at manufacturing scale is important since downstream processing costs are often dominant. At such scales, the columns are highly overloaded heightening the challenge of predicting performance. In this work, the separation of a monoclonal antibody monomer-dimer mixture is conducted by gradient elution chromatography with ceramic hydroxyapatite (CHT) columns Type I and Type II under overloaded conditions. Phosphate gradients are shown to be preferable over sodium chloride gradients since the latter result in undesirable pH transitions generated within the column itself. Using sodium phosphate gradients separation is obtained with both CHT types, achieving approximately 90% recovery at 99% monomer purity starting with a mixture containing 30% dimer at total protein loads up to 30 mg/mL. Because of its higher binding capacity, even higher loadings can be obtained with CHT Type I without monomer breakthrough. A hybrid model is developed to describe the separation. The model, based on an empirical description of two-component, competitive isotherms at low sodium phosphate concentration coupled with the stoichiometric displacement model at higher sodium phosphate concentrations, is in good agreement with the experiments using the linear driving force (LDF) approximation to describe adsorption/desorption kinetics. The same LDF rate coefficient predicts the separation at loadings between 0.8 and 30 mg/mL. The model developed in this work can be used as a general tool to optimize operating conditions, understand what factors limit performance, and compare different operating modes.

Mechanistic Modeling of Preparative Column Chromatography for Biotherapeutics

Chromatography has long been, and remains, the workhorse of downstream processing in the production of biopharmaceuticals. As bioprocessing has matured, there has been a growing trend toward seeking a detailed fundamental understanding of the relevant unit operations, which for some operations include the use of mechanistic modeling in a way similar to its use in the conventional chemical process industries. Mechanistic models of chromatography have been developed for almost a century, but although the essential features are generally understood, the specialization of such models to biopharmaceutical processing includes several areas that require further elucidation. This review outlines the overall approaches used in such modeling and emphasizes current needs, specifically in the context of typical uses of such models; these include selection and improvement of isotherm models and methods to estimate isotherm and transport parameters independently. Further insights are likely to be aided by molecular-level modeling, as well as by the copious amounts of empirical data available for existing processes.

Using osmotic stress to stabilize mannitol production in Synechocystis sp. PCC6803

BACKGROUND

Mannitol is a C(6) polyol that is used in the food and medical sector as a sweetener and antioxidant, respectively. The sustainable production of mannitol, especially via the direct conversion of CO₂ by photosynthetic cyanobacteria, has become increasingly appealing. However, previous work aiming to achieve mannitol production in the marine Synechococcus sp. PCC7002 via heterologous expression of mannitol-1-phosphate-5-dehydrogenase (mtlD) and mannitol-1-phosphatase (m1p, in short: a 'mannitol cassette'), proved to be genetically unstable. In this study, we aim to overcome this genetic instability by conceiving a strategy to stabilize mannitol production using Synechocystis sp. PCC6803 as a model cyanobacterium.

RESULTS

Here, we explore the stabilizing effect that mannitol production may have on cells faced with osmotic stress, in the freshwater cyanobacterium Synechocystis sp. PCC6803. We first validated that mannitol can function as a compatible solute in Synechocystis sp. PCC6803, and in derivative strains in which the ability to produce one or both of the native compatible solutes was impaired. Wild-type Synechocystis, complemented with a mannitol cassette, indeed showed increased salt tolerance, which was even more evident in Synechocystis strains in which the ability to synthesize the endogenous compatible solutes was impaired. Next we tested the genetic stability of all these strains with respect to their mannitol productivity, with and without salt stress, during prolonged turbidostat cultivations. The obtained results show that mannitol production under salt stress conditions in the Synechocystis strain that cannot synthesize its endogenous compatible solutes is remarkably stable, while the control strain completely loses this ability in only 6 days. DNA sequencing results of the control groups that lost the ability to synthesize mannitol revealed that multiple types of mutation occurred in the mtlD gene that can explain the disruption of mannitol production.

CONCLUSIONS

Mannitol production in freshwater Synechocsytis sp. PCC6803 confers it with increased salt tolerance. Under this strategy, genetically instability which was the major challenge for mannitol production in cyanobacteria is tackled. This paper marks the first report of utilization of the response to salt stress as a factor that can increase the stability of mannitol production in a cyanobacterial cell factory.

Microheterogeneity of Recombinant Antibodies: Analytics and Functional Impact

Antibodies are typical examples of biopharmaceuticals which are composed of numerous, almost infinite numbers of potential molecular entities called variants or isoforms, which constitute the microheterogeneity of these molecules. These variants are generated during biosynthesis by so-called posttranslational modification, during purification or upon storage. The variants differ in biological properties such as pharmacodynamic properties, for example, Antibody Dependent Cellular Cytotoxicity, complement activation, and pharmacokinetic properties, for example, serum half-life and safety. Recent progress in analytical technologies such as various modes of liquid chromatography and mass spectrometry has helped to elucidate the structure of a lot of these variants and their biological properties. In this review the most important modifications (glycosylation, terminal modifications, amino acid side chain modifications, glycation, disulfide bond variants and aggregation) are reviewed and an attempt is made to give an overview on the biological properties, for which the reports are often contradictory. Even though there is a deep understanding of cellular and molecular mechanism of antibody modification and their consequences, the clinical proof of the effects observed in vitro and in vivo is still not fully rendered. For some modifications such as core-fucosylation of the N-glycan and aggregation the effects are clear and should be monitored, but with others such as C-terminal lysine clipping the reports are contradictory. As a consequence it seems too early to tell if any modification can be safely ignored.