Methods to measure the binding of therapeutic monoclonal antibodies to the human Fc receptor FcγRIII (CD16) using real time kinetic analysis and flow cytometry

Flow cytometry: A powerful analytical technique for characterizing the biological function of biotherapeutics and biosimilars

Biotherapeutics are complex molecules with therapeutic activity produced through biotechnology and/or genetic engineering. These medicines have clinical applications in diagnostic procedures and therapies for many disorders, including cancer, autoimmunity, and chronic degenerative diseases. Most biotherapeutics are expensive and sometimes unaffordable for low-income patients suffering from cancer or chronic illness. Biosimilars emerged in the 2000 s after patents of many innovative biotherapeutic products expired. The Biosimilar market is growing fast and demands reliable technologies for analyzing the physicochemical properties and bioactivity of products. A big challenge for biosimilar development is to prove comparable bioactivity, safety, efficacy, and toxicity profile as the innovator product. Bioactivity assessment can utilize different analytical techniques such as ELISA, flow cytometry, and surface plasmon resonance. Flow cytometry is a versatile analytical tool that can be used for the development of quantitative, reproducible, and accurate protocols suitable for routine evaluation of bioactivity in-vitro. Nevertheless, flow cytometry has been very scarcely used in comparability evaluation between biosimilar versus an originator product. Here, we review potential applications of flow cytometry to carry out functional bioassays of biotherapeutics or biosimilars.

Beyond conventional dose-response curves: Sensorgram comparison in SPR allows single concentration activity and similarity assessment

Previously we have introduced two SPR-based assay principles (dual-binding assay and bridging assay), which allow the determination of two out of three possible interaction parameters for bispecific molecules within one assay setup: two individual interactions to both targets, and/or one simultaneous/overall interaction, which potentially reflects the inter-dependency of both individual binding events. However, activity and similarity are determined by comparing report points over a concentration range, which also mirrors the way data is generated by conventional ELISA-based methods So far, binding kinetics have not been specifically considered in generic approaches for activity assessment. Here, we introduce an improved slope-ratio model which, together with a sensorgram comparison based similarity assessment, allows the development of a detailed, USP-conformal ligand binding assay using only a single sample concentration. We compare this novel analysis method to the usual concentration-range approach for both SPR-based assay principles and discuss its impact on data quality and increased sample throughput.

Surrogate potency assays: Comparison of binding profiles complements dose response curves for unambiguous assessment of relative potencies

Surface plasmon resonance (SPR) systems are widely used for detailed characterization of antibody activities including antigen and Fc-receptor binding. During the later stages of development, where the focus is to ensure that established critical quality attributes (CQAs) are maintained during cell culture, purification and formulation processes, analysis is simplified, and relative potencies are often determined. Here, simulation of binding data revealed that relative potency values, determined via parallel line analysis (PLA) and half maximal effective concentration (EC50) analysis accurately reflect changes in active concentration only if binding kinetics remain unchanged. Changes in the association rate constant shifted dose response curves, and therefore relative potencies, in the same way as changes in analyte concentration do. However, for interactions characterized by stable binding, changes in the dissociation rate constant did not result in any shift, suggesting that this type of change may go unnoticed in the dose response curve. Thus, EC50 and PLA analyses of dose response curves obtained with an anti-TNF-α antibody were complemented with the Biacore functionality for sensorgram comparison analysis, whereby changes in antigen and Fc-receptor binding profiles could be detected. Next, analysis of temperature stressed TNF-α antibody revealed that calibration free concentration analysis (CFCA) data correlated perfectly with relative potency values. Together, these results demonstrate that combinations of SPR based dose response curves, sensorgram comparison and CFCA can be used to strengthen the confidence in relative potency assessments, and suggest that SPR can potentially be used as a surrogate potency assay in the quality control of biotherapeutic medicines.

Rapid screening of IgG quality attributes - effects on Fc receptor binding

The interactions of therapeutic antibodies with fragment crystallizable γ (Fcγ) receptors and neonatal Fc receptors (FcRn) are measured in vitro as indicators of antibody functional performance. Antibodies are anchored to immune cells through the Fc tail, and these interactions are important for the efficacy and safety of therapeutic antibodies. High‐throughput binding studies on each of the human Fcγ receptor classes (FcγRI, FcγRIIa, FcγRIIb, FcγRIIIa, and FcγRIIIb) as well as FcRn have been developed and performed with human IgG after stress‐induced modifications to identify potential impact in vivo. Interestingly, we found that asparagine deamidation (D‐N) reduced the binding of IgG to the low‐affinity Fcγ receptors (FcγRIIa, FcγRIIb, FcγRIIIa, and FcγRIIIb), while FcγRI and FcRn binding was not impacted. Deglycosylation completely inhibited binding to all Fcγ receptors, but showed no impact on binding to FcRn. On the other hand, afucosylation only impacted binding to FcγRIIIa and FcγRIIIb. Methionine oxidation at levels below 7%, multiple freeze/thaw cycles and short‐term thermal/shake stress did not influence binding to any of the Fc receptors. The presence of high molecular weight species, or aggregates, disturbed measurements in these binding assays; up to 5% of aggregates in IgG samples changed the binding and kinetics to each of the Fc receptors. Overall, the screening assays described in this manuscript prove that rapid and multiplexed binding assays may be a valuable tool for lead optimization, process development, in‐process controls, and biosimilarity assessment of IgGs during development and manufacturing of therapeutic IgGs.

Characterization of low affinity Fcγ receptor biotinylation under controlled reaction conditions by mass spectrometry and ligand binding analysis

Chemical protein biotinylation and streptavidin or anti-biotin-based capture is regularly used for proteins as a more controlled alternative to direct coupling of the protein on a biosensor surface. On biotinylation an interaction site of interest may be blocked by the biotin groups, diminishing apparent activity of the protein. Minimal biotinylation can circumvent the loss of apparent activity, but still a binding site of interest can be blocked when labeling an amino acid involved in the binding. Here, we describe reaction condition optimization studies for minimal labeling. We have chosen low affinity Fcγ receptors as model compounds as these proteins contain many lysines in their active binding site and as such provide an interesting system for a minimal labeling approach. We were able to identify the most critical parameters (protein:biotin ratio and incubation pH) for a minimal labeling approach in which the proteins of choice remain most active toward analyte binding. Localization of biotinylation by mass spectrometric peptide mapping on minimally labeled material was correlated to protein activity in binding assays. We show that only aiming at minimal labeling is not sufficient to maintain an active protein. Careful fine-tuning of critical parameters is important to reduce biotinylation in a protein binding site.

Comparison of surface plasmon resonance binding curves for characterization of protein interactions and analysis of screening data

Label-free technologies, such as surface plasmon resonance, are typically used for characterization of protein interactions and in screening for selection of antibodies or small molecules with preferred binding properties. In characterization, complete binding curves are normally fitted to defined interaction models to provide affinity and rate constants, whereas report points indicative of binding and stability of binding are often used for analysis of screening data. As an alternative to these procedures, here we describe how the analysis, in certain cases, can be simplified by comparison with upper and lower limit binding curves that represent expected or wanted binding profiles. The use of such profiles is applied to the analysis of kinetically complex IgG-Fc receptor interactions and for selection of antibody candidates. The comparison procedure described may be particularly useful in batch-to-batch comparisons and in comparability and biosimilar studies of biotherapeutic medicines. In screening, more informed selections may become possible as entire binding profiles and not a few report points are used in the analysis and as each new sample is directly compared with a predefined outcome.

Potential antitumor therapeutic application of Grimontia hollisae thermostable direct hemolysin mutants

We report on the preparation of a new type of immunotoxin by conjugation of an epidermal growth factor receptor (EGFR)-binding peptide and an R46E mutation of thermostable direct hemolysin from Grimontia hollisae, (Gh-TDH^R46E/EB). The hybrid immunotoxin was purified to homogeneity and showed a single band with slight slower mobility than that of Gh-TDH^R46E. Cytotoxicity assay of Gh-TDH^R46E/EB on EGFR highly, moderately, low, and non-expressed cells, A431, MDA-MB-231, HeLa, and HEK293 cells, respectively, showed apparent cytotoxicity on A431 and MDA-MB-231 cells but not on HeLa or HEK293 cells. In contrast, no cytotoxicity was observed for these cells treated with either Gh-TDH^R46E or EB alone, indicating enhanced cytotoxic efficacy of Gh-TDH^R46E by the EGFR binding moiety. Further antitumor activity assay of Gh-TDH^R46E/EB in a xenograft model of athymic nude mice showed obvious shrinkage of tumor size and degeneration, necrosis, and lesions of tumor tissues compared to the normal tissues. Therefore, the combination of Gh-TDH^R46E with target affinity agents opens new possibilities for pharmacological treatment of cancers and potentiates the anticancer drug's effect.

A flow cytometry-based assay for the evaluation of antibody-dependent cell-mediated cytotoxicity (ADCC) in cancer cells

The development of new therapeutic monoclonal antibodies (mAbs) for cancer therapy will rise in the following years. The evaluation of biological activity of mAbs is required during drug development and during drug production as quality control test. Antibody-dependent cell-mediated cytotoxicity (ADCC) is a desirable activity of anticancer mAbs. Here, we describe a flow cytometry-based method to quantify ADCC that combines the staining of cancer cells, effector cells, and dead cells, with specific dyes. This method is inexpensive, has low background, and avoids the use of radioisotopes.