Submicron Aggregation of Chemically Denatured Monoclonal Antibody

Fluorescence-Based Protein Stability Monitoring-A Review

Proteins are large biomolecules with a specific structure that is composed of one or more long amino acid chains. Correct protein structures are directly linked to their correct function, and many environmental factors can have either positive or negative effects on this structure. Thus, there is a clear need for methods enabling the study of proteins, their correct folding, and components affecting protein stability. There is a significant number of label-free methods to study protein stability. In this review, we provide a general overview of these methods, but the main focus is on fluorescence-based low-instrument and -expertise-demand techniques. Different aspects related to thermal shift assays (TSAs), also called differential scanning fluorimetry (DSF) or ThermoFluor, are introduced and compared to isothermal chemical denaturation (ICD). Finally, we discuss the challenges and comparative aspects related to these methods, as well as future opportunities and assay development directions.

In Situ Monitoring of Protein Unfolding/Structural States under Cold High-Pressure Stress

Biopharmaceutical formulations may be compromised by freezing, which has been attributed to protein conformational changes at a low temperature, and adsorption to ice-liquid interfaces. However, direct measurements of unfolding/conformational changes in sub-0 °C environments are limited because at ambient pressure, freezing of water can occur, which limits the applicability of otherwise commonly used analytical techniques without specifically tailored instrumentation. In this report, small-angle neutron scattering (SANS) and intrinsic fluorescence (FL) were used to provide in situ analysis of protein tertiary structure/folding at temperatures as low as -15 °C utilizing a high-pressure (HP) environment (up to 3 kbar) that prevents water from freezing. The results show that the α-chymotrypsinogen A (aCgn) structure is reasonably maintained under acidic pH (and corresponding pD) for all conditions of pressure and temperature tested. On the other hand, reversible structural changes and formation of oligomeric species were detected near -10 °C via HP-SANS for ovalbumin under neutral pD conditions. This was found to be related to the proximity of the temperature of cold denaturation of ovalbumin (T_CD ∼ -17 °C; calculated via isothermal chemical denaturation and Gibbs-Helmholtz extrapolation) rather than a pressure effect. Significant structural changes were also observed for a monoclonal antibody, anti-streptavidin IgG1 (AS-IgG1), under acidic conditions near -5 °C and a pressure of ∼2 kbar. The conformational perturbation detected for AS-IgG1 is proposed to be consistent with the formation of unfolding intermediates such as molten globule states. Overall, the in situ approaches described here offer a means to characterize the conformational stability of biopharmaceuticals and proteins more generally under cold-temperature stress by the assessment of structural alteration, self-association, and reversibility of each process. This offers an alternative to current ex situ methods that are based on higher temperatures and subsequent extrapolation of the data and interpretations to the cold-temperature regime.

Challenges for design of aggregation-resistant variants of granulocyte colony-stimulating factor

Non-native protein aggregation is a long-standing issue in pharmaceutical biotechnology. A rational design approach was used in order to identify variants of recombinant human granulocyte colony-stimulating factor (rhG-CSF) with lower aggregation propensity at solution conditions that are typical of commercial formulation. The approach used aggregation-prone-region (APR) predictors to select single amino acid substitutions that were predicted to decrease intrinsic aggregation propensity (IAP). The results of static light scattering temperature-ramps and chemical unfolding experiments demonstrated that none of the selected variants exhibited improved aggregation resistance, and the apparent conformational stability of each variant was lower than that of WT. Aggregation studies under partly denaturing conditions suggested that the IAP of at least one variant remained unaltered. Overall, this study highlights a general challenge in designing aggregation resistance for proteins, due to the need to accurately predict both APRs and conformational stability.

Combining Unfolding Reversibility Studies and Molecular Dynamics Simulations to Select Aggregation-Resistant Antibodies

The efficient development of new therapeutic antibodies relies on developability assessment with biophysical and computational methods to find molecules with drug-like properties such as resistance to aggregation. Despite the many novel approaches to select well-behaved proteins, antibody aggregation during storage is still challenging to predict. For this reason, there is a high demand for methods that can identify aggregation-resistant antibodies. Here, we show that three straightforward techniques can select the aggregation-resistant antibodies from a dataset with 13 molecules. The ReFOLD assay provided information about the ability of the antibodies to refold to monomers after unfolding with chemical denaturants. Modulated scanning fluorimetry (MSF) yielded the temperatures that start causing irreversible unfolding of the proteins. Aggregation was the main reason for poor unfolding reversibility in both ReFOLD and MSF experiments. We therefore performed temperature ramps in molecular dynamics (MD) simulations to obtain partially unfolded antibody domains in silico and used CamSol to assess their aggregation potential. We compared the information from ReFOLD, MSF, and MD to size-exclusion chromatography (SEC) data that shows whether the antibodies aggregated during storage at 4, 25, and 40 °C. Contrary to the aggregation-prone molecules, the antibodies that were resistant to aggregation during storage at 40 °C shared three common features: (i) higher tendency to refold to monomers after unfolding with chemical denaturants, (ii) higher onset temperature of nonreversible unfolding, and (iii) unfolding of regions containing aggregation-prone sequences at higher temperatures in MD simulations.

Conjugation Ratio, Light Dose, and pH Affect the Stability of Panitumumab-IR700 for Near-Infrared Photoimmunotherapy

Near-infrared photoimmunotherapy (NIR-PIT), a newly developed cancer-cell-specific therapy, relies on a monoclonal antibody-photoabsorber conjugate (APC) and is based on a photoinduced ligand release reaction. Local exposure of the tumor to NIR light induces rapid immunogenic necrotic cell death. The molecular properties of APCs, including their stability and aggregation properties, have important implications for the long-term stability and shelf life. In this study, panitumumab was conjugated with IRDye700DX (IR700) as a model for other NIR-PIT agents. Higher IR700-to-mAb conjugation ratios correlated with increased in vitro cell death up to a ratio of 2.5 dye molecules per antibody. Conjugation ratios higher than 2.5 did not improve cell killing activity. APC aggregation was induced in a light-dose-dependent manner. A near-room-level light dose was sufficient to induce aggregation of APCs. Solvent pH lower than 4 induced aggregation, but higher pH did not induce aggregation. The IR700-to-mAb conjugation ratio, light irradiation dose, and solvent pH affect the APC stability and efficacy.

Relationship of PEG-induced precipitation with protein-protein interactions and aggregation rates of high concentration mAb formulations at 5 °C

Native protein-protein interactions can play an important role in determining the tendency of monoclonal antibodies (mAbs) to aggregate under storage conditions. In this context, phase separation of mAb solutions induced by the addition of neutral polymers such as poly(ethylene glycol) (PEG) represents a simple method to assess the tendency of proteins to self-associate in the native state. Here, we investigated their relationships between PEG-induced phase separation, protein-protein interactions and long-term aggregation rate of several formulations of four mAbs at 100 mg/mL and 5 °C over 12 weeks of storage. We observed that the location of the phase boundary correlated well with the osmotic second virial coefficient B₂₂ determined in absence of the polymer, indicating that for our solutions PEG primarily leads to depletion forces between protein molecules, which are additive to protein-protein interactions. However, limited correlation between aggregation rate at 5 °C and phase behavior was observed across different mAbs, pH values and ionic strengths, indicating that colloidal stability is not the only determinant of aggregation even at such low temperature and high protein concentration. Our results contribute to the growing realization that aggregation propensity in the context of antibody developability is a complex feature, which depends on a variety of biophysical properties rather than one single parameter.

Inhibiting the fibrillation of a GLP-1-like peptide

Aggregation, including the formation of fibrils, poses significant challenges for the development of therapeutic peptides. To prepare stable peptide formulations, some understanding of the mechanisms underpinning the fibrillation process is required. A thioflavin T fluorescence assay was first used to determine the fibrillation profile of a GLP-1-like peptide (G48) at conditions being considered to formulate the peptide. G48 concentrations ranged from 0 to 600 µM and three pH values (pH 3.7, 7.4 and 8.5) were evaluated. Kinetic data demonstrate that G48 displays a pH-dependent aggregation profile. At pH 3.7, which is below the isoelectric point of G48 (pI ~ 5), kinetics representative of amorphous aggregates forming via a nucleation-independent mechanism were seen. At pH 7.4 and 8.5 (pH > pI) typical nucleation-dependent aggregation kinetics were observed. The weight concentration of β-sheet rich aggregates (FL_max) correlated inversely with net charge, so lower FL_max values were observed at pH 3.7 and 8.5 than at pH 7.4. Incorporation of a non-ionic surfactant (polysorbate 80) into the peptide solution suppressed the fibrillation of G48 at all pH values and maintained the native peptide conformation, whereas a phenolic co-formulant (ferulic acid) had minimal effects on fibril growth. Peptide fibrillation, which can occur within a range of formulation concentrations and pH values, can hence be inhibited by the judicious use of excipients.

Understanding mAb aggregation during low pH viral inactivation and subsequent neutralization

Monoclonal antibodies (mAbs) and related recombinant proteins continue to gain importance in the treatment of a great variety of diseases. Despite significant advances, their manufacturing can still present challenges owing to their molecular complexity and stringent regulations with respect to product purity, stability, safety, and so forth. In this context, protein aggregates are of particular concern due to their immunogenic potential. During manufacturing, mAbs routinely undergo acidic treatment to inactivate viral contamination, which can lead to their aggregation and thereby to product loss. To better understand the underlying mechanism so as to propose strategies to mitigate the issue, we systematically investigated the denaturation and aggregation of two mAbs at low pH as well as after neutralization. We observed that at low pH and low ionic strength, mAb surface hydrophobicity increased whereas molecular size remained constant. After neutralization of acidic mAb solutions, the fraction of monomeric mAb started to decrease accompanied by an increase on average mAb size. This indicates that electrostatic repulsion prevents denatured mAb molecules from aggregation under acidic pH and low ionic strength, whereas neutralization reduces this repulsion and coagulation initiates. Limiting denaturation at low pH by d-sorbitol addition or temperature reduction effectively improved monomer recovery after neutralization. Our findings might be used to develop innovative viral inactivation procedures during mAb manufacturing that result in higher product yields.

Light-scattering detection within the difficult size range of protein particle measurement using flow cytometry

The phenomenon of protein aggregation is a prominent challenge that impacts biopharmaceutical development at every stage. It may have a number of deleterious effects on protein drugs, including the loss of efficacy, induction of immunogenicity, altered pharmacokinetics and reduced shelf life. At present, multiple methods are available for counting and sizing particles over a broad range of sizes. However, there remains a conundrum in the measurement of particles in the submicrometer range, from 100 nm to 2 μm. In this study, the capability of our new laboratory built FCM system to detect model polystyrene (PS) and silica (SiO₂) submicrometer microspheres was evaluated and benchmarked against flow field-flow fractionation (FFF). The FCM system showed its advantages on sensitivity, selectivity, reproducibility and speed. The laboratory-built FCM system can readily analyze model PS and SiO₂ microspheres down to 200 nm, covering much of the difficult range from 100 nm to 2 μm. Our data also showed that this machine was able to monitor the distribution of antibody aggregates ranged between 200 nm and 10 μm, suggesting its usability for characterizing protein aggregation in future.