Extensive Chemical Modifications in the Primary Protein Structure of IgG1 Subvisible Particles Are Necessary for Breaking Immune Tolerance.

A current concern with the use of therapeutic proteins is the likely presence of aggregates and submicrometer, subvisible, and visible particles. It has been proposed that aggregates and particles may lead to unwanted increases in the immune response with a possible impact on safety or efficacy. The aim of this study was thus to evaluate the ability of subvisible particles of a therapeutic antibody to break immune tolerance in an IgG1 transgenic mouse model and to understand the particle attributes that might play a role in this process. We investigated the immunogenic properties of subvisible particles (unfractionated, mixed populations, and well-defined particle size fractions) using a transgenic mouse model expressing a mini-repertoire of human IgG1 (hIgG1 tg). Immunization with proteinaceous subvisible particles generated by artificial stress conditions demonstrated that only subvisible particles bearing very extensive chemical modifications within the primary amino acid structure could break immune tolerance in the hIgG1 transgenic mouse model. Protein particles exhibiting low levels of chemical modification were not immunogenic in this model.

Comparative Evaluation of Two Methods for Preparative Fractionation of Proteinaceous Subvisible Particles--Differential Centrifugation and FACS.

PURPOSE: The goal of this study was to compare and evaluate two preparative techniques for fractionation of proteinaceous subvisible particles. This work enables future studies to address the potential biological consequences of proteinaceous subvisible particles in protein therapeutic products. METHODS: Particles were generated by heat stress and separated by size using differential centrifugation and FACS (Fluorescence-activated cell sorter). Resulting fractions were characterized by size-exclusion chromatography, light obscuration, flow imaging microscopy and resonant mass measurement. RESULTS: Here we report the optimization and comprehensive evaluation of two methods for preparative fractionation of subvisible proteinaceous particles into distinct size fractions in the range between 0.25 and 100 µm: differential centrifugation and FACS. Using these methods, well-defined size fractions were prepared and characterized in detail. Critical assessment and comparison of the two techniques demonstrated their complementarity and for the first time-their relative advantages and drawbacks. CONCLUSIONS: FACS and differential centrifugation are valuable tools to prepare well-defined size-fractions of subvisible proteinaceous particles. Both techniques possess unique and advantageous attributes and will likely find complementary application in future research on the biological consequences of proteinaceous subvisible particles.

Determination of the Density of Protein Particles Using a Suspended Microchannel Resonator.

One of the analytical tools for characterization of subvisible particles, which gained popularity over the last years because of its unique capabilities, is the resonance mass measurement technique. However, a challenge that this technique presents is the need to know the exact density of the measured particles in order to obtain accurate size calculations. The density of proteinaceous subvisible particles has not been measured experimentally yet and to date researchers have been using estimated density values. In this paper, we report for a first-time experimental measurements of the density of protein particles (0.2-5 µm in size) using particles created by stressing three different proteins using four different types of stress conditions. Interestingly, the particle density values that were measured varied between 1.28 and 1.33 g/cm(3) and were lower than previous estimates. Furthermore, it was found that although the density of proteinaceous particles was affected to a very low degree by the stress conditions used to generate them, there is relatively larger difference between particles originating from different classes of proteins (e.g., monoclonal antibody vs. bovine serum albumin).

Selective Oxidation of Methionine and Tryptophan Residues in a Therapeutic IgG1 Molecule.

Oxidation of methionine and tryptophan are common degradation pathways for monoclonal antibodies and present major analytical challenges in biotechnology. Generally, protein oxidation is detectable in stability and/or stressed samples (e.g., exposed to hydrogen peroxide, UV light, or metal ions). The induced chemical modifications may impact the biological activity of antibodies and may have biological consequences. However, these effects and the contribution of individual protein modifications are difficult to delineate as different amino acids are often oxidized simultaneously and accompanied by other degradants such as aggregates, especially in forced degradation studies. Here, we report a new method to obtain selective oxidation of methionine or tryptophan by using oxidation reagents combined with large excess of free tryptophan or methionine, correspondingly. More specifically, using hydrogen peroxide or tert-butyl hydroperoxide in combination with addition of free tryptophan allowed for selective oxidation of methionine. Conversely, the use of 2,2-azobis(2-amidinopropane) dihydrochloride in combination with free methionine resulted in selective tryptophan oxidation, whereas methionine oxidation was not significantly altered. This novel stress model system may prove to be valuable tool in future mechanistic studies of oxidative degradation of protein therapeutics.