Protein separations with induced pH gradients using cation-exchange chromatographic columns containing weak acid groups

Rapid, Direct, Noninvasive Method to Determine the Amount of Immobilized Protein

Protein immobilization is of utmost importance in many areas, where various proteins are used for selective detection of target compounds. Despite the importance given to determine the amount of immobilized protein, there is no simple method that allows direct, noninvasive detection. In this work, a method based on pH transition, occurring during change of solution ionic strength, was developed. The method utilized the ionic character of the immobilized protein while implementing biologically compatible buffers. Five different proteins, namely, glucose oxidase, horseradish peroxidase, bovine serum albumin, lysozyme, and protein A, were immobilized in different amounts on a porous polymeric matrix, and their pH transition was measured using lactate buffer of various concentrations and pH values. A linear correlation was found between the amount of immobilized protein and the amplitude of the pH transition, allowing the detection down to 2 nmol of immobilized protein. By changing the buffer concentration and pH, the sensitivity of the method could be tailored. Criteria based on the symmetry of the pH transition peak have been developed to determine if a particular measurement is within a linear range. In addition, a mathematical model was developed enabling prediction of pH transition profiles based solely on the protein amino acid sequence, the buffer pK_a value(s), and the amount of immobilized protein.Hence, it can be used to design pH transition method experiments to achieve the required sensitivity for a target sample. Since the proposed method is noninvasive, it can be routinely applied during optimization of the immobilization protocol, for quality control, and also as an in-process monitoring tool.

Separation of charge variants of a monoclonal antibody by overloaded ion exchange chromatography

A procedure for adjusting the content of charge variants of monoclonal antibody by ion exchange chromatography has been developed. The band splitting phenomenon was utilized to split the protein load into two parts, i.e., the flowthrough and bound fractions, which were either enriched or depleted with some of variants. The phenomenon was triggered by thermodynamic effects resulting from oversaturation of the resin binding sites at high column loadings as well as from kinetic effects arising from limited rates of mass transport. Cation exchange chromatography (CEX) and anion exchange chromatography (AEX) separations were examined, with the reverse order of the variant elution: acidic, main, basic in CEX, and basic, main, acidic in AEX, and the corresponding reverse enrichment tendency in the collected fractions. The separations were performed by pH gradient, whose course was simplified to two stages: isocratic loading and washing at mild pH to load and partly elute the protein, followed by a rapid pH change towards non-binding conditions to desorb the remains of the protein load. To improve yield of the operation, possibility of recycling of waste fractions was considered. To predict the process performance, a dynamic model was developed, which accounted for both adsorption kinetics and thermodynamics.

Purification of Modified Therapeutic Proteins Available on the Market: An Analysis of Chromatography-Based Strategies

Proteins, which have inherent biorecognition properties, have long been used as therapeutic agents for the treatment of a wide variety of clinical indications. Protein modification through covalent attachment to different moieties improves the therapeutic's pharmacokinetic properties, affinity, stability, confers protection against proteolytic degradation, and increases circulation half-life. Nowadays, several modified therapeutic proteins, including PEGylated, Fc-fused, lipidated, albumin-fused, and glycosylated proteins have obtained regulatory approval for commercialization. During its manufacturing, the purification steps of the therapeutic agent are decisive to ensure the quality, effectiveness, potency, and safety of the final product. Due to the robustness, selectivity, and high resolution of chromatographic methods, these are recognized as the gold standard in the downstream processing of therapeutic proteins. Moreover, depending on the modification strategy, the protein will suffer different physicochemical changes, which must be considered to define a purification approach. This review aims to deeply analyze the purification methods employed for modified therapeutic proteins that are currently available on the market, to understand why the selected strategies were successful. Emphasis is placed on chromatographic methods since they govern the purification processes within the pharmaceutical industry. Furthermore, to discuss how the modification type strongly influences the purification strategy, the purification processes of three different modified versions of coagulation factor IX are contrasted.

A case study of the mechanism of unfolding and aggregation of a monoclonal antibody in ion exchange chromatography

A mechanistic model for describing unfolding of a monoclonal antibody (mAb) in ion exchange chromatography has been developed. The model reproduced retention behavior characteristic for conformational changes of antibodies upon adsorption, including: multi-peak elution, aggregate formation, and recovery reduction. Two competitive paths in the adsorption mechanism of the unfolded protein were assumed: refolding in the adsorbed phase to the native form followed by its desorption, or direct desorption followed by instantaneous aggregation in the liquid phase. The reduction in recovery of the eluted protein was attributed to spreading of the unfolded protein on the adsorbent surface, which enhanced the binding affinity. The model was formulated based on the analysis of retention behavior of a model mAb that was eluted in pH gradients on a strong cation exchange resin. The pH profile was found to be distorted in the presence of the protein, which was ascribed to dissociation of ionizable groups of the protein in the adsorbed phase. Since the protein retention was strongly pH dependent, that phenomenon was also accounted for in mathematical modeling. A series of independent experiments was designed to evaluate the model parameters that quantified the process thermodynamics and kinetics: the Henry constants of the native, unfolded, spread and aggregated forms of the protein along with underlying kinetic coefficients. The model was efficient in reproducing the retention pattern of the protein and the aggregate content in eluting band profiles. After proper calibration, the model can potentially be used to quantify protein unfolding and elution in other ion exchange systems.

Solvent modulated linear pH gradient elution for the purification of conventional and bispecific antibodies: Modeling and application

Classical ion-exchange chromatography using a linear salt gradient to elute the adsorbed protein at fixed pH is the most common method to separate product-related impurities during downstream processing of biopharmaceuticals. Linear pH gradient elution provides a useful alternative by separating proteins in a linear pH gradient at fixed salt concentration. Although linear pH gradient elution provides excellent selectivity, it is rarely encountered in industrial purification processes. Here, a stoichiometric displacement model is used to characterize pH gradient elution based on simple linear gradient elution experiments. Protein retention behavior is described with respect to the pH dependencies of the characteristic binding charge and the equilibrium constant of the ion exchange reaction. Furthermore, the influence of solvent composition using PEG as a mobile phase modifier is investigated. Validity and applicability of the model are demonstrated for the purification of a conventional monoclonal antibody from soluble aggregates and for a novel bispecific antibody format containing a unique product-related impurity profile. pH step elution protocols are derived from model calculations without further optimization experiments necessary.