Deglycosylation of mAb by EndoS for improved molecular imaging

Impact of site-specific conjugation strategies on the pharmacokinetics of antibody conjugated radiotherapeutics

Antibody radionuclide conjugates are an emerging modality for targeted imaging and potent therapy of disseminated disease. Coupling of radionuclides to monoclonal antibodies (mAbs) is typically achieved by applying non-site-specific labelling techniques. With the ambition of reducing variability, increasing labelling efficacy and stability, several site-specific conjugation strategies have been developed in recent years for toxin- and fluorophore-mAb conjugates. In this study, we studied two site-specific labelling strategies for the conjugation of the macrocyclic chelating agent, DOTA, to the anti-Leucine Rich Repeat Containing 15 (LRRC15) mAb DUNP19. Specifically, one approach utilized a DOTA-bearing peptide (FcIII) with a strong affinity for the fragment crystallizable (Fc) domain of the human IgG1 of DUNP19 (DUNP19LF-FcIII-DOTASS), while the other leveraged a chemo-enzymatic technique to substitute the N-linked bi-antennary oligosaccharides in the human IgG1 Fc domain with DOTA (DUNP19LF-gly-DOTASS). To assess if these methods impact the antibody's binding properties and targeting efficacy, comparative in vitro and in vivo studies of the generated DUNP19-conjugates were performed. While the LRRC15 binding of both radioimmunoconjugates remained intact, the conjugation methods had different impacts on their abilities to interact with FcRn and FcγRs. In vitro assessments of DUNP19LF-FcIII-DOTASS and DUNP19LF-gly-DOTASS demonstrated markedly decreased affinity for FcRn and FcγRIIIa (CD16), respectively. DUNP19LF-FcIII-DOTASS demonstrated increased blood and tissue kinetics in vivo, confirming loss of FcRn binding. While the ablated FcγR interaction of DUNP19LF-gly-DOTASS had no immediate impact on in vivo biodistribution, reduced immunotherapeutic effect can be expected in future studies as a result of reduced NK-cells interaction. In conclusion, our findings underscore the necessity for meticulous consideration and evaluation of mAb labelling strategies, extending beyond mere conjugation efficiency and radiolabeling yields. Notably, site-specific labelling methods were found to significantly influence the immunological impact of Fc interactions. Therefore, it is of paramount importance to consider the intended diagnostic or therapeutic application of the construct and to adopt conjugation strategies that ensure the preservation of critical pharmacological properties and functionality of the antibody in use.

Site-Specific Conjugation of Native Antibody: Transglutaminase-Mediated Modification of a Conserved Glutamine While Maintaining the Primary Sequence and Core Fc Glycan via Trimming with an Endoglycosidase

A versatile chemo-enzymatic tool to site-specifically modify native (nonengineered) antibodies is using transglutaminase (TGase, E.C. 2.3.2.13). With various amines as cosubstrates, this enzyme converts the unsubstituted side chain amide of glutamine (Gln or Q) in peptides and proteins into substituted amides (i.e., conjugates). A pleasant surprise is that only a single conserved glutamine (Gln295) in the Fc region of IgG is modified by microbial TGase (mTGase, EC 2.3.2.13), thereby providing a highly specific and generally applicable conjugation method. However, prior to the transamidation (access to the glutamine residue by mTGase), the steric hindrance from the nearby conserved N-glycan (Asn297 in IgG1) must be reduced. In previous approaches, amidase (PNGase F, EC 3.5.1.52) was used to completely remove the N-glycan. However, PNGase F also converts a net neutral asparagine (Asn297) to a negatively charged aspartic acid (Asp297). This charge alteration may markedly change the structure, function, and immunogenicity of an IgG antibody. In contrast, in our new method presented herein, the N-glycan is trimmed by an endoglycosidase (EndoS2, EC 3.2.1.96), hence retaining both the core N-acetylglucosamine (GlcNAc) moiety and the neutral asparaginyl amide. The trimmed glycan also reduces or abolishes Fc receptor-mediated functions, which results in better imaging agents by decreasing nonspecific binding to other cells (e.g., immune cells). Moreover, the remaining core glycan allows further derivatization such as glycan remodeling and dual conjugation. Practical and robust, our method generates conjugates in near quantitative yields, and both enzymes are commercially available.

On enzymatic remodeling of IgG glycosylation; unique tools with broad applications

The importance of IgG glycosylation has been known for many years not only by scientists in glycobiology but also by human pathogens that have evolved specific enzymes to modify these glycans with fundamental impact on IgG function. The rise of IgG as a major therapeutic scaffold for many cancer and immunological indications combined with the availability of unique enzymes acting specifically on IgG Fc-glycans have spurred a range of applications to study this important post-translational modification on IgG. This review article introduces why the IgG glycans are of distinguished interest, gives a background on the unique enzymatic tools available to study the IgG glycans and finally presents an overview of applications utilizing these enzymes for various modifications of the IgG glycans. The applications covered include site-specific glycan transglycosylation and conjugation, analytical workflows for monoclonal antibodies and serum diagnostics. Additionally, the review looks ahead and discusses the importance of O-glycosylation for IgG3, Fc-fusion proteins and other new formats of biopharmaceuticals.

Cross-reactivity of mouse IgG subclasses to human Fc gamma receptors: Antibody deglycosylation only eliminates IgG2b binding

Immunoglobulin G (IgG) antibodies are important for protection against pathogens and exert effector functions through binding to IgG-Fc receptors (FcγRs) on myeloid and natural killer cells, resulting in destruction of opsonized target cells. Despite interspecies differences, IgG subclasses and FcγRs show substantial similarities and functional conservation between mammals. Accordingly, binding of human IgG (hIgG) to mouse FcγRs (mFcγRs) has been utilized to study effector functions of hIgG in mice. In other applications, such as immunostaining with mouse IgG monoclonal antibodies (mAbs), these cross-reactivities are undesired and prone to misinterpretation. Despite this drawback, the binding of mouse IgG (mIgG) subclasses to human FcγR (hFcγR) classes has never been fully documented. Here, we report detailed and quantifiable characterization of binding affinities for all mIgG subclasses to hFcγRs, including functional polymorphic variants. mIgG subclasses show the strongest binding to hFcγRIa, with relative affinities mIgG2a = mIgG2c > mIgG3 >> mIgG2b, and no binding by mIgG1. hFcγRIIa/b showed general low reactivities to all mIgG (mIgG1> mIgG2a/c > mIgG2b), with no reactivity to mIgG3. A particularly high affinity was observed for mIgG1 to the hFcγRIIa-R131 polymorphic variant. hFcγRIIIa showed lower binding (mIgG2a/c > mIgG3), slightly favouring binding to the hFcγRIIIa-V158 over the F158 polymorphic variant. No binding was observed of mIgG to hFcγRIIIb. Deglycosylation of mIgG1 did not abrogate binding to hFcγRIIa-R131, nor did deglycosylation of mIgG2a/c and mIgG3 prevent hFcγRIa binding. Importantly, deglycosylation of the least cross-reactive mIgG subclass, mIgG2b, abrogated reactivity to all hFcγRs. Together, these data document for the first time the full spectrum of cross-reactivities of mouse IgG to human FcγRs.

Antibody Conjugations via Glycosyl Remodeling

The antibody Fc-glycans are interesting targets for conjugation of cytotoxic compounds due to their localization and their chemical composition. In striving to obtain site-specific conjugates, the antibody Fc-glycans have been explored in numerous ways. Here we present a two-step enzymatic methodology coupled to click-chemistry for conjugation at the core GlcNAc of the Fc-glycan resulting in ADCs that are homogenous with a DAR 2.0, retain antigen binding, and display cytotoxic anti-tumor effects both in vitro and in vivo.

The Influence of Glycans-Specific Bioconjugation on the FcγRI Binding and In vivo Performance of 89Zr-DFO-Pertuzumab

Rationale: The overwhelming majority of radioimmunoconjugates are produced via random conjugation methods predicated on attaching bifunctional chelators to the lysines of antibodies. However, this approach inevitably produces poorly defined and heterogeneous immunoconjugates because antibodies have several lysines distributed throughout their structure. To circumvent this issue, we have previously developed a chemoenzymatic bioconjugation strategy that site-specifically appends cargoes to the biantennary heavy chain glycans attached to C_H2 domains of the immunoglobulin's Fc region. In the study at hand, we explore the effects of this approach to site-specific bioconjugation on the Fc receptor binding and in vivo behavior of radioimmunoconjugates.

Methods: We synthesized three desferrioxamine (DFO)-labeled immunoconjugates based on the HER2-targeting antibody pertuzumab: one using random bioconjugation methods (DFO-^nsspertuzumab) and two using variants of our chemoenzymatic protocol (DFO-^sspertuzumab-EndoS and DFO-^sspertuzumab-βGal). Subsequently, we characterized these constructs and evaluated their ability to bind HER2, human FcγRI (huFcγRI), and mouse FcγRI (muFcγRI). After radiolabeling the immunoconjugates with zirconium-89, we conducted PET imaging and biodistribution studies in two different mouse models of HER2-expressing breast cancer.

Results: MALDI-ToF and SDS-PAGE analysis confirmed the site-specific nature of the bioconjugation, and flow cytometry and surface plasmon resonance (SPR) revealed that all three immunoconjugates bind HER2 as effectively as native pertuzumab. Critically, however, SPR experiments also illuminated that DFO-^sspertuzumab-EndoS possesses an attenuated binding affinity for huFcγRI (17.4 ± 0.3 nM) compared to native pertuzumab (4.7 ± 0.2 nM), DFO-^nsspertuzumab (4.1 ± 0.1 nM), and DFO-^sspertuzumab-βGal (4.7 ± 0.2 nM). ImmunoPET and biodistribution experiments in athymic nude mice bearing HER2-expressing BT474 human breast cancer xenografts yielded no significant differences in the in vivo behavior of the radioimmunoconjugates. Yet experiments in tumor-bearing humanized NSG mice revealed that ⁸⁹Zr-DFO-^sspertuzumab-EndoS produces higher activity concentrations in the tumor (111.8 ± 39.9 %ID/g) and lower activity concentrations in the liver and spleen (4.7 ± 0.8 %ID/g and 13.1 ± 4.0 %ID/g, respectively) than its non-site-specifically labeled cousin, a phenomenon we believe stems from the altered binding of the former to huFcγRI.

Conclusion: These data underscore that this approach to site-specific bioconjugation not only produces more homogeneous and well-defined radioimmunoconjugates than traditional methods but may also improve their in vivo performance in mouse models by reducing binding to FcγRI.

New Developments in Dual-Labeled Molecular Imaging Agents

Intraoperative detection of tumors has had a profound impact on how cancer surgery is performed and addresses critical unmet needs in surgical oncology. Tumor deposits, margins, and residual cancer can be imaged through the use of fluorescent contrast agents during surgical procedures to complement visual and tactile guidance. The combination of fluorescent and nuclear contrast into a multimodality agent builds on these capabilities by adding quantitative, noninvasive nuclear imaging capabilities to intraoperative imaging. This review focuses on new strategies for the development and evaluation of targeted dual-labeled molecular imaging agents while highlighting the successful first-in-human application of this technique.

The Impact of FcγRI Binding on Immuno-PET

Antibodies are promising vectors for PET imaging. However, the high uptake of radioimmunoconjugates in nontarget tissues such as the liver and spleen hampers their performance as radiotracers. This off-target uptake can lead to suboptimal tumor-to-background activity concentration ratios, decreasing the contrast of images and reducing their diagnostic utility. A possible cause of this uptake is the sequestration of radioimmunoconjugates by immune cells bearing Fc-γ-receptors (FcγR) that bind to the Fc regions of antibodies. Methods: Since the heavy chain glycans influence the affinity of FcγR for the Fc domain, we set out to investigate whether radioimmunoconjugates with truncated glycans would exhibit altered binding to FcγRI and, in turn, improved in vivo performance. Using the HER2-targeting antibody trastuzumab, we synthesized a series of desferrioxamine-bearing immunoconjugates with differing glycosylation states and interrogated their FcγRI binding via surface plasmon resonance, enzyme-linked immunosorbent assay, and flow cytometry. Furthermore, we labeled these immunoconjugates with ⁸⁹Zr and explored their biodistribution in athymic nude, NSG, and humanized NSG mice bearing human epidermal growth factor receptor 2-expressing human breast cancer xenografts. Results: We observed a strong correlation between the impaired in vitro FcγRI binding of deglycosylated immunoconjugates and significant decreases in the in vivo off-target uptake of the corresponding ⁸⁹Zr-labeled radioimmunoconjugates (i.e., liver activity concentrations are reduced by ∼3.5-fold in humanized NSG mice). These reductions in off-target uptake were accompanied by concomitant increases in the tumoral activity concentrations of the glycoengineered radioimmunoconjugates, ultimately yielding improved tumor-to-healthy organ contrast and higher quality PET images. Conclusion: Our findings suggest that the deglycosylation of antibodies represents a facile strategy for improving the quality of immuno-PET in animal models as well as in certain patient populations.

Streptococcal Endo-β-N-Acetylglucosaminidase Suppresses Antibody-Mediated Inflammation In Vivo

Endo-β-N-acetylglucosaminidase (EndoS) is a family 18 glycosyl hydrolase secreted by Streptococcus pyogenes. Recombinant EndoS hydrolyzes the β-1,4-di-N-acetylchitobiose core of the N-linked complex type glycan on the asparagine 297 of the γ-chains of IgG. Here, we report that EndoS and IgG hydrolyzed by EndoS induced suppression of local immune complex (IC)-mediated arthritis. A small amount (1 µg given i.v. to a mouse) of EndoS was sufficient to inhibit IgG-mediated arthritis in mice. The presence of EndoS disturbed larger IC lattice formation both in vitro and in vivo, as visualized with anti-C3b staining. Neither complement binding in vitro nor antigen-antibody binding per se were affected. Thus, EndoS could potentially be used for treating patients with IC-mediated pathology.

Evaluation of Anti-LGR5 Antibodies by ImmunoPET for Imaging Colorectal Tumors and Development of Antibody-Drug Conjugates

Leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5) is highly expressed in colorectal tumors and marks colon cancer stem cells that drive tumor growth and metastasis. Recently, we showed that LGR5 is a promising target for antibody-drug conjugate (ADC) therapy. However, it is important to identify LGR5-positive tumors that would respond to ADC treatment. Prior to drug conjugation, we evaluated two different anti-LGR5 monoclonal antibodies (mAbs), 8F2 and 9G5, using ⁸⁹Zr-immunoPET to select the optimal mAb for ADC development and tumor imaging. Binding, specificity, and internalization were compared, and mAbs were prescreened as ADC candidates against colon cancer cells using secondary ADCs. Both mAbs demonstrated strong, specific binding in 293T-LGR5 cells but not 293T-vector cells. In DLD-1 colorectal cancer cells, which express high levels of LGR5, the mAbs rapidly internalized into lysosomes and promoted ADC-induced cytotoxicity, with 8F2 exhibiting slightly higher potency. No binding was detected in DLD-1-shLGR5 (LGR5 knockdown) cells. ⁸⁹Zr-DFO-LGR5 mAbs were generated and shown to retain high affinity and LGR5-dependent uptake in vitro. PET/CT imaging of DLD-1 tumors was performed 5 days postinjection of ⁸⁹Zr-DFO-LGR5 mAbs, and findings were consistent with biodistribution data, which showed significantly higher tumor uptake (%ID/g) for ⁸⁹Zr-DFO-8F2 (17.9 ± 2.2) compared to ⁸⁹Zr-DFO-9G5 (5.5 ± 1.2) and ⁸⁹Zr-DFO-IgG (3.8 ± 1.0). No significant uptake was observed in DLD-1-shLGR5 tumors. This study identifies 8F2 as the optimal candidate for ADC development and provides initial evidence that ⁸⁹Zr-DFO-LGR5 mAbs may be utilized to stratify tumors which would respond best to LGR5-targeted ADC therapy.

Antibody Guided Molecular Imaging of Infective Endocarditis

In this protocol, we describe the application of using a high affinity monoclonal antibody generated against the major pilin protein component of the pilin structure of Enterococcus faecalis as a PET imaging agent for enterococcal endocarditis detection. The anti-pilin -mAb 64Cu conjugate was able to specifically label enterococcal endocarditis vegetation in vivo in a rodent endocarditis model. By targeting pili, a covalently linked surface antigen extending from the bacterial surface, we provided evidence that gram-positive pilin represent a logical surface antigen to define or target an infectious agent for molecularly guided imaging. Our goal in providing a detailed protocol of our efforts is to enable others to build upon this methodology to answer pertinent translational and basic research questions in the pursuit of diagnosis and treatment of infective endocarditis.

EpCAM as multi-tumour target for near-infrared fluorescence guided surgery

Background

Evaluation of resection margins during cancer surgery can be challenging, often resulting in incomplete tumour removal. Fluorescence-guided surgery (FGS) aims to aid the surgeon to visualize tumours and resection margins during surgery. FGS relies on a clinically applicable imaging system in combination with a specific tumour-targeting contrast agent. In this study EpCAM (epithelial cell adhesion molecule) is evaluated as target for FGS in combination with the novel Artemis imaging system.

Methods

The NIR fluorophore IRDye800CW was conjugated to the well-established EpCAM specific monoclonal antibody 323/A3 and an isotype IgG1 as control. The anti-EpCAM/800CW conjugate was stable in serum and showed preserved binding capacity as evaluated on EpCAM positive and negative cell lines, using flow cytometry and cell-based plate assays. Four clinically relevant orthotopic tumour models, i.e. colorectal cancer, breast cancer, head and neck cancer, and peritonitis carcinomatosa, were used to evaluate the performance of the anti-EpCAM agent with the clinically validated Artemis imaging system. The Pearl Impulse small animal imaging system was used as reference. The specificity of the NIRF signal was confirmed using bioluminescence imaging and green-fluorescent protein.

Results

All tumour types could clearly be delineated and resected 72 h after injection of the imaging agent. Using NIRF imaging millimetre sized tumour nodules were detected that were invisible for the naked eye. Fluorescence microscopy demonstrated the distribution and tumour specificity of the anti-EpCAM agent.

Conclusions

This study shows the potential of an EpCAM specific NIR-fluorescent agent in combination with a clinically validated intraoperative imaging system to visualize various tumours during surgery.

A review of performance of near-infrared fluorescence imaging devices used in clinical studies

Near-infrared fluorescence (NIRF) molecular imaging holds great promise as a new "point-of-care" medical imaging modality that can potentially provide the sensitivity of nuclear medicine techniques, but without the radioactivity that can otherwise place limitations of usage. Recently, NIRF imaging devices of a variety of designs have emerged in the market and in investigational clinical studies using indocyanine green (ICG) as a non-targeting NIRF contrast agent to demark the blood and lymphatic vasculatures both non-invasively and intraoperatively. Approved in the USA since 1956 for intravenous administration, ICG has been more recently used off label in intradermal or subcutaneous administrations for fluorescence imaging of the lymphatic vasculature and lymph nodes. Herein, we summarize the devices of a variety of designs, summarize their performance in lymphatic imaging in a tabular format and comment on necessary efforts to develop standards for device performance to compare and use these emerging devices in future, NIRF molecular imaging studies.