Enzyme-containing spin membranes for rapid digestion and characterization of single proteins

Mass spectrometry characterization of antibodies at the intact and subunit levels: from targeted to large-scale analysis

Antibodies are one of the most formidable molecular weapons available to our immune system. Their high specificity against a target (antigen) and capability of triggering different immune responses (e.g., complement system activation and antibody-dependent cell-mediated cytotoxicity) make them ideal drugs to fight many different human diseases. Currently, both monoclonal antibodies and more complex molecules based on the antibody scaffold are used as biologics. Naturally, such highly heterogeneous molecules require dedicated analytical methodologies for their accurate characterization. Mass spectrometry (MS) can define the presence and relative abundance of multiple features of antibodies, including critical quality attributes. The combination of small and large variations within a single molecule can only be determined by analyzing intact antibodies or their large (25 to 100 kDa) subunits. Hence, top-down (TD) and middle-down (MD) MS approaches have gained popularity over the last decade. In this Young Scientist Feature we discuss the evolution of TD and MD MS analysis of antibodies, including the new frontiers that go beyond biopharma applications. We will show how this field is now moving from the "quality control" analysis of a known, single antibody to the high-throughput investigation of complex antibody repertoires isolated from clinical samples, where the ultimate goal is represented by the complete gas-phase sequencing of antibody molecules without the need of any a priori knowledge.

Rapid Quantitation of Various Therapeutic Monoclonal Antibodies Using Membranes with Fc-Specific Ligands

Therapeutic monoclonal antibodies (mAbs) provide effective treatments for many diseases, including cancer, autoimmune disorders, and, lately, COVID-19. Monitoring the concentrations of mAbs is important during their production and subsequent processing. This work demonstrates a 5 min quantitation of most human immunoglobulin G (IgG) antibodies through capture of mAbs in membranes modified with ligands that bind to the fragment crystallizable (Fc) region. This enables binding and quantitation of most IgG mAbs. Layer-by-layer (LBL) adsorption of carboxylic acid-rich polyelectrolytes in glass-fiber membranes in 96-well plates allows functionalization of the membranes with Protein A or a peptide, oxidized Fc20 (oFc20), with high affinity for the Fc region of human IgG. mAb capture occurs in <1 min during the flow of solutions through modified membranes, and subsequent binding of a fluorophore-labeled secondary antibody enables quantitation of the captured mAbs using fluorescence. The intra- and inter-plate coefficients of variations (CV) are <10 and 15%, respectively, satisfying the acceptance criteria for many assays. The limit of detection (LOD) of 15 ng/mL is on the high end of commercial enzyme-linked immunosorbent assays (ELISAs) but certainly low enough for monitoring of manufacturing solutions. Importantly, the membrane-based method requires <5 minutes, whereas ELISAs typically take at least 90 min. Membranes functionalized with oFc20 show greater mAb binding and lower LODs than membranes with Protein A. Thus, the membrane-based 96-well-plate assay, which is effective in diluted fermentation broths and in mixtures with cell lysates, is suitable for near-real-time monitoring of the general class of human IgG mAbs during their production.

Analysis of Protein Glycosylation after Rapid Digestion Using Protease-Containing Membranes in Spin Columns

Glycosylation is an important protein post-translational modification that plays a pivotal role in the bioactivity of therapeutic proteins and in the infectivity of viral proteins. Liquid chromatography with tandem mass spectrometry readily identifies protein glycans with site specificity. However, the overnight incubation used in conventional in-solution proteolysis leads to high turnaround times for glycosylation analysis, particularly when sequential in-solution digestions are needed for site-specific glycan identification. Using bovine fetuin as a model glycoprotein, this work first shows that in-membrane digestion in ∼3 min yields similar glycan identification and quantitation when compared to overnight in-solution digestion. Protease-containing membranes in a spin column enable digestion of therapeutic proteins (trastuzumab and erythropoietin) and a viral protein (SARS-CoV-2 receptor binding domain) in ∼30 s. Glycan identification is similar after in-solution and in-membrane digestion, and limited in-membrane digestion enhances the identification of high-mannose glycans in trastuzumab. Finally, stacked membranes containing trypsin and chymotrypsin allow fast sequential proteolytic digestion to site-specifically identify the glycans of SARS-CoV-2 receptor binding domain. One can easily assemble the protease-containing membranes in commercial spin columns, and spinning multiple columns simultaneously will facilitate parallel analyses.

Determination of the Serum Concentrations of the Monoclonal Antibodies Bevacizumab, Rituximab, and Panitumumab Using Porous Membranes Containing Immobilized Peptide Mimotopes

Effective monoclonal antibody (mAb) therapies require a threshold mAb concentration in patient serum. Moreover, the serum concentration of the mAb Bevacizumab should reside in a specific range to avoid side effects. Methods for conveniently determining the levels of mAbs in patient sera could allow for personalized dosage schedules that lead to more successful treatments. This work utilizes microporous nylon membranes functionalized with antibody-binding peptides to capture Bevacizumab, Rituximab, or Panitumumab from diluted (25%) serum. Modification of the capture-peptide terminus is often crucial to creating the affinity necessary for effective binding. The high purity of eluted mAbs allows for their quantitation using native fluorescence, and membranes are effective in spin devices that can be used in any laboratory. The technique is effective over the therapeutic range of Bevacizumab concentrations. Future work aims at further modifications to develop rapid point-of-care devices and decrease detection limits.

Improving the throughput of immunoaffinity purification and enzymatic digestion of therapeutic proteins using membrane-immobilized reagent technology

Continued interest in protein therapeutics has motivated the development of improved bioanalytical tools to support development programs. LC-MS offers specificity, sensitivity, and multiplexing capabilities without the need for target-specific reagents, making it a valuable alternative to ligand binding assays. Immunoaffinity purification (IP) and enzymatic digestion are critical, yet extensive and time-consuming components of the "gold standard" bottom-up approach to LC-MS-based protein quantitation. In the present work, commercially available technology, based on membrane-immobilized reagents in spin column and plate format, is applied to reduce IP and digestion times from hours to minutes. For a standard monoclonal antibody, the lower limit of quantitation was 0.1 ng μL-1 compared to 0.05 ng μL-1 for the standard method. A pharmacokinetics (PK) study dosing Herceptin in rat was analyzed by both the membrane and the standard method with a total sample processing time of 4 h and 20 h, respectively. The calculated concentrations at each time point agreed within 8% between both methods, and PK values including area under the curve (AUC), half-life (T1/2), mean residence time (MRT), clearance (CL), and volume of distribution (Vdss) agreed within 6% underscoring the utility of the membrane methodology for quantitative bioanalysis workflows.