Development of cell-based assay for predictively evaluating the FcγR-mediated human immune cell activation by therapeutic monoclonal antibodies

Molecular imaging of gastrointestinal stromal tumor using anti-c-KIT antibody and its fragments

BACKGROUND

Gastrointestinal stromal tumors (GISTs) are malignant subepithelial tumors, known for their poor prognosis due to distant metastasis. Because GIST is covered by a normal mucosal layer, effective tissue biopsy under conventional endoscopy is difficult, thereby leading to delayed diagnosis and a dismal prognosis. We performed molecular imaging of GIST targeting c-KIT using fluorescence-labeled anti-c-KIT antibody/fragments and fluorescent endoscopy.

METHODS

Mouse anti-human c-KIT monoclonal antibody, its F(ab')2 and Fab fragments were labeled with AF680. Two GIST cell lines (GIST-T1, GIST-882M) were used for experiments. Antibodies were intravenously administered to mice xenografted with GIST-T1 or GIST-882M, and each tumor was observed using IVIS Spectrum and self-developed simple fluorescent endoscopy.

RESULTS

The GIST-T1 cell live imaging revealed strong signals on cell membranes after 1 min incubation, and thereafter, they aggregated and internalized inside the cells within 130 min in all antibody/fragment groups. In vivo mouse experiments, AF680-labeled IgG slowly accumulated in tumors peaking at 24 h after injection. However, AF680-labeled F(ab')2 and Fab rapidly accumulated in tumors peaking at 1-2 h, and completely cleared from the body within 24 h. Fab showed the strongest fluorescence intensity in tumors. Fluorescence endoscopy could clearly detect GIST xenograft tumors 1-2 h after AF680-labeled F(ab')2 and Fab injection.

CONCLUSIONS

AF680-labeled antibody/fragments showed clear and specific fluorescence signals in GIST xenografts in mice. Particularly, AF680-labeled Fab showed the strongest signal intensity at 1-2 h post-administration and rapid clearance, suggestive of the safety. This approach may enable molecular imaging diagnosis of GIST by endoscopy in outpatient settings in the future.

Fcγ Receptor Activation by Human Monoclonal Antibody Aggregates

Protein aggregates are a potential risk factor for immunogenicity. The measurement, characterization, and control of protein aggregates in drug products are indispensable for the development of biopharmaceuticals, including therapeutic mAbs. In this study, Fcγ receptor (FcγR)-expressing reporter cell lines were used to analyze the FcγR-activation properties of mAb aggregates. Comparison of aggregates of mAbs harboring different IgG subclasses revealed that the FcγR-activation profiles of the mAb aggregates were dependent on IgG subclass. In addition, aggregates of Fc-engineered mAb with enhanced FcγR-activation properties exhibited stronger activation of FcγRs than was observed in the wild-type aggregates, whereas aggregates of Fc-engineered mAb with decreased FcγR-activation properties showed reduced activation. These results suggest that FcγR activation by mAb aggregates depends greatly on the Fc functions of the native (nonaggregated) mAbs. We also showed that aggregates of mAbs smaller than 1 μm in size have the potential to directly activate FcγRs. Unintended immune cell activation can be induced on account of FcγR activation by mAb aggregates and such FcγR activation may contribute to immunogenicity, and therefore, analysis of the FcγR-activation properties of mAb aggregates using FcγR-expressing reporter cell lines is a promising approach for the characterization of mAb aggregates.

A Cell-Based Reporter Assay Measuring the Activation of Fc Gamma Receptors Induced by Therapeutic Monoclonal Antibodies

Fc gamma receptors (FcγRs) are expressed on the surface of various immune cells, and the interactions between FcγRs and the Fc region of immunoglobulin G are involved in the activation of immune cells by antigen-bound antibodies. Fc-mediated immune-cell activations are related to both the efficacy and the safety of therapeutic monoclonal antibodies. It is indispensable to elucidate the Fc-mediated functions in the development of therapeutic monoclonal antibodies. Here, we describe a cell-based assay using FcγR-expressing reporter cell lines that can be used to evaluate the human FcγR-activation properties of therapeutic monoclonal antibodies by a rapid and simple procedure.

Development and characterization of an anti-rituximab monoclonal antibody panel

During the development of monoclonal antibodies (mAbs) and other therapeutic proteins, immunogenicity, in particular the induction of anti-drug antibodies (ADAs), is an important concern, and thus immunogenicity assessment is a requirement for their approval. Establishment of appropriate methods for detecting and characterizing ADAs is necessary for immunogenicity assessment, but the lack of commonly available reference standards makes it difficult to compare and evaluate the methods. It is also difficult to compare the data with those obtained by other methods or facilities without reference standards. Here, we developed a panel of ADAs against anti-CD20 rituximab (Rituxan®, MabThera®); the panel consisted of eight clones of recombinant human-rat chimeric mAbs that target rituximab. The anti-rituximab mAbs showed different binding properties (specificity, epitope and affinity), and different neutralization potencies for CD20 binding, complement-dependent cytotoxicity and antibody-dependent cell-mediated cytotoxicity. The molecular size of the immune complex consisting of rituximab and the anti-rituximab mAb differed among the clones, and was well correlated with their level of Fcγ-receptor activation. These results suggest that the ADAs chosen for the newly developed panel are suitable surrogates for human ADAs, which exhibit different potential to affect the efficacy and safety of rituximab. Next, we used this panel to compare several ADA-detecting assays and revealed that the assays had different abilities to detect the ADAs with different binding characteristics. We conclude that our panel of ADAs against rituximab will be useful for the future development and characterization of assays for immunogenicity assessment.