pH dependent aggregation and conformation changes of rituximab using SAXS and its comparison with the standard regulatory approach of biophysical characterization

Antibody-ligand interactions on a high-capacity staphylococcal protein A resin

Staphylococcal protein-A affinity chromatography has been optimized for antibody purification, achieving a current capacity of up to 90 mg/ml in packed bed. The morphology of the particles, the number of antibodies bound per ligand and the spatial arrangement of the ligands were assessed by in-situ Small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) combined with measurement of adsorption isotherms. We employed SAXS measurements to probe the nanoscale structure of the chromatographic resin. From scanning electron microcopy, the morphology and area of the beads were obtained. The adsorption isotherm revealed a bi-Langmuirian behavior where the association constant varied with the critical bulk concentration, indicating multilayer adsorption. Determining the antibody-ligand stoichiometry was crucial for understanding the adsorption mechanism, which was estimated to be 4 at lower concentrations and 4.5 at higher concentrations, suggestive of reversible protein-protein interactions. The same results were reached from the in-situ small angle X-ray scattering measurements. A stoichiometry of 6 cannot be achieved since the two protein A monomers are anchored to the stationary phase and thus sterically hindered. Normalization through ellipsoids facilitated SAXS analysis, enabling the determination of distances between ligands and antibody-ligand complexes. Density fluctuations were examined by subtracting the elliptical fit, providing insights into ligand density distribution. The dense ligand packing of TOYOPEARL® AF-rProtein A HC was confirmed, making further increases in ligand density impractical. Additionally, SAXS analysis revealed structural rearrangements of the antibody-ligand complex with increasing antibody surface load, suggesting reversible association of antibodies.

Statistically-aided development of protein A affinity chromatography for enhancing recovery and controlling quality of a monoclonal antibody

Protein A chromatography is widely used for isolation of monoclonal antibodies (mAbs) from cell culture components. In this study, the effect of different process parameters of the Protein A purification namely, binding pH, elution pH, flow rate, neutralization pH and tween concentration, on the concentration and quality of the purified mAb were evaluated. Using design of experiments approach, the critical process parameters of protein A chromatography were identified and experimentally optimized. Their impact on quality attributes, such as size variants and charge variants, of the mAb was studied. Multivariate data analysis was subsequently performed using multiple linear regression and partial least squares regression methods. It was observed that the elution pH primarily governed the concentration of the purified mAb and the content of monomers and aggregates, while the tween concentration primarily influenced the main peak of the charge variants. This is the first study that evaluates the impact of tween concentration in buffers on the protein A chromatography purification step. These studies helped in identifying the design space and defining the target robust and optimal setpoints of the responses, which were subsequently verified experimentally. These setpoints not only passed the target criteria but also resulted in the highest recoveries during the investigation. Through this statistically-aided approach, an optimized and robust protein A chromatography process was rationally developed for purification of mAbs, while achieving the desired product quality. This study highlights the influence of multiple parameters of the protein A purification process on critical quality attributes of mAbs, such as the size and charge variants, which has been a very scarcely explored area.

Raman Spectroscopic Analysis of Highly-Concentrated Antibodies under the Acid-Treated Conditions

PURPOSE

Antibody drugs are usually formulated as highly-concentrated solutions, which would easily generate aggregates, resulting in loss of efficacy. Although low pH increases the colloidal dispersion of antibodies, acid denaturation can be an issue. Therefore, knowing the physical properties at low pH under high concentration conditions is important.

METHODS

Raman spectroscopy was used to investigate pH-induced conformational changes of antibodies at 50 mg/ml. Experiments in pH 3 to 7 were performed for human serum IgG and recombinant rituximab.

RESULTS

We detected the evident changes at pH 3 in Tyr and Trp bands, which are the sensitive markers of intermolecular interactions. Thermal transition analysis over the pH range demonstrated that the thermal transition temperature (Tm) was highest at pH 3. Acid-treated and neutralized one showed higher Tm than that of pH 7, indicating that their extent of intermolecular interactions correlated with the Tm values. Onset temperature was clearly different between concentrated and diluted samples. Colloidal analyses confirmed the findings of the Raman analysis.

CONCLUSION

Our studies demonstrated the positive correlation between Raman analysis and colloidal information, validating as a method for evaluating antibody conformation associated with aggregation propensities.

Comparison between carbohydrate and salt-based macromolecular crowders for cell preservation at higher temperatures

In this investigation, the macromolecular crowding effect of a carbohydrate-based polymer, pullulan, and a salt-based polymer, poly-(4-styrenesulfonic-acid) sodium salt (PSS) was compared for the storage of A549 lung carcinoma cells, at temperatures greater than that of liquid nitrogen storage tanks. A DoE-CCD response surface model was used to optimise medium compositions comprising DMSO and a macromolecular crowder (MMC; pullulan, PSS and their combinations). The effect of adding MMCs was evaluated in terms of post-preservation viability, apoptotic population and growth curve analysis. The optimised medium consisting of 10% DMSO and 3% pullulan in the basal medium (BM) could facilitate long-term cell preservation for 90 days at - 80 °C, resulting in cell viability of ∼83%. The results also showed a significant decrease in the apoptotic population at all time points for the optimised composition of the freezing medium. These results indicated that adding 3% pullulan to the freezing medium improved the post-thaw viability and reduced the apoptotic cell population.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03571-6.

Potential of Good's buffers to inhibit denaturation of myofibrillar protein upon freezing

The current systematic study sought to examine the potential use of three Good's buffers (MES, MOPS and HEPES) in inhibiting myofibrillar protein (MFP) denaturation induced by acidity changes. The highest degree of acidity variation was found in the center and bottom of large bottles due to the freeze-concentration effect. Good's buffer tended to basify during freezing, and it could prevent the crystallization of sodium phosphate (Na-P) buffer. Acidification upon freezing Na-P disrupted the natural conformation of MFP and induced the formation of large proteins aggregates with tight packing. The 15 mM MES, 20 mM MOPS, and 30 mM HEPES were respectively added to neutralize the strong acidity drop induced by freezing 20 mM Na-P, and all of them significantly improved the stability of the MFP conformation (P < 0.05). This work is not only critical to meet the growing demand for protein, but also groundbreaking for broadening the applicability of Good's buffers in the food industry.

Structural Characterization of the Full-Length Anti-CD20 Antibody Rituximab

Rituximab, a murine–human chimera, is the first monoclonal antibody (mAb) developed as a therapeutic agent to target CD20 protein. Its Fab domain and its interaction with CD20 have been extensively studied and high-resolution atomic models obtained by X-ray diffraction or cryo-electron microscopy are available. However, the structure of the full-length antibody is still missing as the inherent protein flexibility hampers the formation of well-diffracting crystals and the reconstruction of 3D microscope images. The global structure of rituximab from its dilute solution is here elucidated by small-angle X-ray scattering (SAXS). The limited data resolution achievable by this technique has been compensated by intensive computational modelling that led to develop a new and effective procedure to characterize the average mAb conformation as well as that of the single domains. SAXS data indicated that rituximab adopts an asymmetric average conformation in solution, with a radius of gyration and a maximum linear dimension of 52 Å and 197 Å, respectively. The asymmetry is mainly due to an uneven arrangement of the two Fab units with respect to the central stem (the Fc domain) and reflects in a different conformation of the individual units. As a result, the Fab elbow angle, which is a crucial determinant for antigen recognition and binding, was found to be larger (169°) in the more distant Fab unit than that in the less distant one (143°). The whole flexibility of the antibody has been found to strongly depend on the relative inter-domain orientations, with one of the Fab arms playing a major role. The average structure and the amount of flexibility has been studied in the presence of different buffers and additives, and monitored at increasing temperature, up to the complete unfolding of the antibody. Overall, the structural characterization of rituximab can help in designing next-generation anti-CD20 antibodies and finding more efficient routes for rituximab production at industrial level.

Temperature and pH-Dependent Behaviors of mAb Drugs: A Case Study for Trastuzumab

The distortions in the high-order structure of therapeutic monoclonal antibodies (mAbs) under different environmental conditions acutely affect mAb stability, resulting in altered safety, efficacy, and shelf-life profiles. The overall stability of mAbs depends on many factors, and it requires complementary techniques for an in-depth analysis. The stability of mAbs can be characterized by differential centrifugal sedimentation (DCS), differential scanning calorimetry (DSC), differential scanning fluorimetry (DSF), and size exclusion chromatography (SEC) techniques. In this report, temperature-ramped dynamic light scattering (DLS), and circular dichroism (CD) spectroscopy were employed as complementary tools to show how temperature and pH affect the aggregation of a model mAb, trastuzumab, in solution. The results showed that the aggregation onset temperature of trastuzumab defined by DLS was 75 °C, which decreases the amount of β-sheets and causes a slight increase in helix structures. Moreover, the melting temperature of trastuzumab was determined to be between 80–83 °C by temperature-ramped CD spectrophotometry, which is in line with the Tm of trastuzumab’s Fab region tested with DSC. Thus, unfolding and aggregation of trastuzumab start simultaneously at 75 °C, and unfolding triggers the aggregation. The temperature-ramped CD and DLS methods are robust tools to determine the thermal behavior of biosimilars in various solution conditions. Their complementary usage provides solid scientific background for regulatory applications and a better understanding of mAb instability and its relationship with structural changes.

Analytical Similarity Assessment of Biosimilars: Global Regulatory Landscape, Recent Studies and Major Advancements in Orthogonal Platforms

Biopharmaceuticals are one of the fastest-growing sectors in the biotechnology industry. Within the umbrella of biopharmaceuticals, the biosimilar segment is expanding with currently over 200 approved biosimilars, globally. The key step towards achieving a successful biosimilar approval is to establish analytical and clinical biosimilarity with the innovator. The objective of an analytical biosimilarity study is to demonstrate a highly similar profile with respect to variations in critical quality attributes (CQAs) of the biosimilar product, and these variations must lie within the range set by the innovator. This comprises a detailed comparative structural and functional characterization using appropriate, validated analytical methods to fingerprint the molecule and helps reduce the economic burden towards regulatory requirement of extensive preclinical/clinical similarity data, thus making biotechnological drugs more affordable. In the last decade, biosimilar manufacturing and associated regulations have become more established, leading to numerous approvals. Biosimilarity assessment exercises conducted towards approval are also published more frequently in the public domain. Consequently, some technical advancements in analytical sciences have also percolated to applications in analytical biosimilarity assessment. Keeping this in mind, this review aims at providing a holistic view of progresses in biosimilar analysis and approval. In this review, we have summarized the major developments in the global regulatory landscape with respect to biosimilar approvals and also catalogued biosimilarity assessment studies for recombinant DNA products available in the public domain. We have also covered recent advancements in analytical methods, orthogonal techniques, and platforms for biosimilar characterization, since 2015. The review specifically aims to serve as a comprehensive catalog for published biosimilarity assessment studies with details on analytical platform used and critical quality attributes (CQAs) covered for multiple biotherapeutic products. Through this compilation, the emergent evolution of techniques with respect to each CQA has also been charted and discussed. Lastly, the information resource of published biosimilarity assessment studies, created during literature search is anticipated to serve as a helpful reference for biopharmaceutical scientists and biosimilar developers.

Sedimentation Velocity Analytical Ultracentrifugation Analysis of Marketed Rituximab Drug Product Size Distribution

PURPOSE

Analytical methods suitable for intact drug products are often necessary to evaluate the equivalence in physicochemical properties between two drug products (DP) containing the same drug substance (DS), e.g., an innovator biologic drug and its proposed biosimilar. Analytical Ultracentrifugation (AUC) is a biophysics technique applied to the analysis of size and shape of biomolecules. However, the application of AUC to formulated monoclonal antibody (mAb) DP at high concentration has not been reported.

METHODS

A sedimentation velocity (SV) AUC procedure with a short-pathlength centerpiece was applied to two marketed rituximab DPs, Rituxan® (US) and Reditux® (India), without any buffer exchange or dilution. Detailed precision analysis was performed.

RESULTS

Highly reproducible sedimentation coefficient values (S) and peak areas were obtained for the dominant (> 84%) monomeric rituximab peak. The minor mAb fragment peaks had large variation in both S values and peak areas (3-12%). The identification of oligomer peaks was only reproducible once the abundance was higher than 2%.

CONCLUSIONS

SV-AUC provides an orthogonal characterization tool for protein size distribution, composition and assay, which could be informative for biosimilar drug developers who mostly only have access to formulated mAb. However, AUC needs thorough validation on its accuracy, precision and sensitivity.