SEEDbodies: fusion proteins based on strand-exchange engineered domain (SEED) CH3 heterodimers in an Fc analogue platform for asymmetric binders or immunofusions and bispecific antibodies

Improving target cell specificity using a novel monovalent bispecific IgG design

Monovalent bispecific IgGs cater to a distinct set of mechanisms of action but are difficult to engineer and manufacture because of complexities associated with correct heavy and light chain pairing. We have created a novel design, “DuetMab,” for efficient production of these molecules. The platform uses knobs-into-holes (KIH) technology for heterodimerization of 2 distinct heavy chains and increases the efficiency of cognate heavy and light chain pairing by replacing the native disulfide bond in one of the C_H1-C_L interfaces with an engineered disulfide bond. Using two pairs of antibodies, cetuximab (anti-EGFR) and trastuzumab (anti-HER2), and anti-CD40 and anti-CD70 antibodies, we demonstrate that DuetMab antibodies can be produced in a highly purified and active form, and show for the first time that monovalent bispecific IgGs can concurrently bind both antigens on the same cell. This last property compensates for the loss of avidity brought about by monovalency and improves selectivity toward the target cell.

Production of bispecific antibodies in "knobs-into-holes" using a cell-free expression system

Bispecific antibodies have emerged in recent years as a promising field of research for therapies in oncology, inflammable diseases, and infectious diseases. Their capability of dual target recognition allows for novel therapeutic hypothesis to be tested, where traditional mono-specific antibodies would lack the needed mode of target engagement. Among extremely diverse architectures of bispecific antibodies, knobs-into-holes (KIHs) technology, which involves engineering C_H3 domains to create either a “knob” or a “hole” in each heavy chain to promote heterodimerization, has been widely applied. Here, we describe the use of a cell-free expression system (Xpress CF) to produce KIH bispecific antibodies in multiple scaffolds, including 2-armed heterodimeric scFv-KIH and one-armed asymmetric BiTE-KIH with tandem scFv. Efficient KIH production can be achieved by manipulating the plasmid ratio between knob and hole, and further improved by addition of prefabricated knob or hole. These studies demonstrate the versatility of Xpress CF in KIH production and provide valuable insights into KIH construct design for better assembly and expression titer.

Self-Assembled Hybrid Aptamer-Fc Conjugates for Targeted Delivery: A Modular Chemoenzymatic Approach

Over the past decade, DNA and RNA aptamers have attracted keen research interest due to their ability to specifically bind targets of therapeutic relevance. However, their application is often hampered by a short serum half-life and missing effector functions. Conjugation of aptamers to antibody Fc fragments could improve pharmacokinetics, enable immune effector mechanisms, and provide an option for the introduction of desired payloads (e.g., toxins or fluorescent dyes). We developed a modular scaffold-supported system based on human IgG1 Fc fragments, which allows for its dual functionalization with moieties of interest. In our approach, two bioorthogonal, enzyme-mediated reactions were used in combination with oxime ligation and self-assembly based on PNA-DNA base pairing. Thus, an engineered synthetic peptide nucleic acid (PNA) oligomer was coupled to the C-termini of the Fc dimer upon sequence-specific sortase A-mediated transpeptidation. Hybridization of the resulting Fc-PNA conjugate with a tailored DNA aptamer that binds cancer-related hepatocyte growth factor receptor (c-MET) led to a hybrid construct which showed strong and specific binding to c-MET and was readily internalized by c-MET-overexpressing cells. To install an additional orthogonally addressable site, aldehyde tag technology was applied followed by oxime ligation with an aminooxy-bearing fluorescent dye as model cargo. Delivery of fluorescent probe specifically to c-MET-overexpressing cells was confirmed by flow cytometry. Our approach can provide access to engineered aptamer-Fc conjugates with desired target specificity and cytotoxic payloads.

Advances in Antibody Design

The use of monoclonal antibodies as therapeutics requires optimizing several of their key attributes. These include binding affinity and specificity, folding stability, solubility, pharmacokinetics, effector functions, and compatibility with the attachment of additional antibody domains (bispecific antibodies) and cytotoxic drugs (antibody-drug conjugates). Addressing these and other challenges requires the use of systematic design methods that complement powerful immunization and in vitro screening methods. We review advances in designing the binding loops, scaffolds, domain interfaces, constant regions, post-translational and chemical modifications, and bispecific architectures of antibodies and fragments thereof to improve their bioactivity. We also highlight unmet challenges in antibody design that must be overcome to generate potent antibody therapeutics.

Homogeneously modified immunoglobulin domains for therapeutic application

The field of therapeutic antibodies has been revolutionized over the past decade, led by the development of novel antibody-modification technologies. Besides the huge success achieved by therapeutic monoclonal antibodies, a diversity of antibody derivatives have emerged with hope to outperform their parental antibodies. Here we review the recent development of methodologies to modify immunoglobulin domains and their therapeutic applications. The innovative genetic and chemical approaches enable novel and controllable modifications on immunoglobulin domains, producing homogeneous therapeutics with new functionalities or enhanced therapeutic profiles. Such therapeutics, including antibody-drug conjugates, bispecific antibodies, and antibody/Fc fusion proteins, have demonstrated great prospects in the treatment of cancer, auto-immune diseases, infectious diseases, and many other disorders.

Crystal structures of immunoglobulin Fc heterodimers reveal the molecular basis for heterodimer formation

We determined the X-ray crystal structure of an immunoglobulin fragment crystallizable (Fc) heterodimer, EW-RVT, at a resolution of 2.5Å and found that the designed asymmetric interaction residues located in the heterodimeric CH3 interface favor Fc heterodimer formation. We further generated an inter-CH3 disulfide-bonded heterodimeric Fc variant, EW-RVT(S-S), which exhibited improved heterodimer formation and thermodynamic stability compared with the parent EW-RVT variant. The crystal structure of EW-RVTS-S superimposed very closely with the wild-type Fc structure. Our results provide the detailed structure of heterodimeric Fc scaffolds, which will be useful for the generation of immunoglobulin G (IgG)-like bispecific antibodies.

Bispecific antibodies

Bispecific antibodies (bsAbs) combine specificities of two antibodies and simultaneously address different antigens or epitopes. BsAbs with 'two-target' functionality can interfere with multiple surface receptors or ligands associated, for example with cancer, proliferation or inflammatory processes. BsAbs can also place targets into close proximity, either to support protein complex formation on one cell, or to trigger contacts between cells. Examples of 'forced-connection' functionalities are bsAbs that support protein complexation in the clotting cascade, or tumor-targeted immune cell recruiters and/or activators. Following years of research and development (R&D), the first bsAb was approved in 2009. Another bsAb entered the market in December 2014 and several more are in clinical trials. Here, we describe the potentials of bsAbs to become the next wave of antibody-based therapies, focusing on molecules in clinical development.

A novel antibody engineering strategy for making monovalent bispecific heterodimeric IgG antibodies by electrostatic steering mechanism

Producing pure and well behaved bispecific antibodies (bsAbs) on a large scale for preclinical and clinical testing is a challenging task. Here, we describe a new strategy for making monovalent bispecific heterodimeric IgG antibodies in mammalian cells. We applied an electrostatic steering mechanism to engineer antibody light chain-heavy chain (LC-HC) interface residues in such a way that each LC strongly favors its cognate HC when two different HCs and two different LCs are co-expressed in the same cell to assemble a functional bispecific antibody. We produced heterodimeric IgGs from transiently and stably transfected mammalian cells. The engineered heterodimeric IgG molecules maintain the overall IgG structure with correct LC-HC pairings, bind to two different antigens with comparable affinity when compared with their parental antibodies, and retain the functionality of parental antibodies in biological assays. In addition, the bispecific heterodimeric IgG derived from anti-HER2 and anti-EGF receptor (EGFR) antibody was shown to induce a higher level of receptor internalization than the combination of two parental antibodies. Mouse xenograft BxPC-3, Panc-1, and Calu-3 human tumor models showed that the heterodimeric IgGs strongly inhibited tumor growth. The described approach can be used to generate tools from two pre-existent antibodies and explore the potential of bispecific antibodies. The asymmetrically engineered Fc variants for antibody-dependent cellular cytotoxicity enhancement could be embedded in monovalent bispecific heterodimeric IgG to make best-in-class therapeutic antibodies.

Bispecific antibody generated with sortase and click chemistry has broad antiinfluenza virus activity

Bispecific antibodies have therapeutic potential by expanding the functions of conventional antibodies. Many different formats of bispecific antibodies have meanwhile been developed. Most are genetic modifications of the antibody backbone to facilitate incorporation of two different variable domains into a single molecule. Here, we present a bispecific format where we have fused two full-sized IgG antibodies via their C termini using sortase transpeptidation and click chemistry to create a covalently linked IgG antibody heterodimer. By linking two potent anti-influenza A antibodies together, we have generated a full antibody dimer with bispecific activity that retains the activity and stability of the two fusion partners.

A dual-targeting triplebody mediates preferential redirected lysis of antigen double-positive over single-positive leukemic cells

The single-chain triplebody HLA-ds16-hu19 consists of three single-chain Fv (scFv) antibody fragments connected in a single polypeptide chain. This protein with dual-targeting capacity mediated preferential lysis of antigen double positive(dp) over single-positive (sp) leukemic cells by recruitment of natural killer (NK) cells as effectors. The two distal scFv modules were specific for the histocompatibility protein HLA-DR and the lymphoid antigen CD19, the central one for the Fc gamma receptor CD16. In antibody-dependent cellular cytotoxicity (ADCC) experiments with a mixture of leukemic target cells comprising both HLA-DR sp HuT-78 or Kasumi-1 cells and (HLA-DR plus CD19) dp SEM cells, the triplebody mediated preferential lysis of the dp cells even when the sp cells were present in ≤ 20-fold numerical excess.The triplebody promoted equal lysis of SEM cells at 2.5-fold and 19.5-fold lower concentrations than the parental antibodies specific for HLA-DR and CD19, respectively. Finally, the triplebody also eliminated primary leukemic cells at lower concentrations than an equimolar mixture of bispecific single-chain Fv fragments (bsscFvs) separately addressing each target antigen (hu19-ds16 and HLA-ds16). The increased selectivity of targeting and the preferential lysis of dp over sp cells achieved by dual-targeting open attractive new perspectives for the use of dual-targeting agents in cancer therapy.

Tumor-antigen-binding bispecific antibodies for cancer treatment

Bi- and multispecific antibody derivatives (bsAbs) can be considered as the next generation of targeted biologics for cancer therapy. The general concept of bsAbs is a physical connection of recombinant antibody-derived entities with at least two binding specificities. This generates bsAbs that bind at least two antigens or epitopes, thus altering their binding functionalities and specificities in comparison to "normal" antibodies. Most bsAbs are produced as recombinant proteins, either as large IgG-like proteins that contain Fc regions, or as smaller entities with multiple antigen-binding regions but without Fc. Application of bsAbs in experimental cancer therapy currently includes molecules that bind different cell surface proteins to achieve more complete blockage of proliferative or angiogenesis-associated pathways. This approach of blocking more than one pathway component, or to simultaneously hit complementing pathways, also may limit potential escape mechanisms of cancer cells. BsAbs also are applied in the clinic as vehicles to deliver immune effector cells and/or cytokines to tumors.

Antiparallel conformation of knob and hole aglycosylated half-antibody homodimers is mediated by a CH2-CH3 hydrophobic interaction

Bispecific antibody and antibody-like molecules are of wide interest as potential therapeutics that can recognize two distinct targets. Among the variety of ways such molecules have been engineered is by creating "knob" and "hole" heterodimerization sites in the CH3 domains of two antibody heavy chains. The molecules produced in this manner maintain their biological activities while differing very little from the native human IgG sequence. To better understand the knob-into-hole interface, the molecular mechanism of heterodimerization, and to engineer Fc domains that could improve the assembly and purity of heterodimeric reaction products, we sought crystal structures of aglycosylated heterodimeric and homodimeric "knob" and "hole" Fc fragments derived from bacterial expression. The structure of the knob-into-hole Fc was determined at 2.64 Å. Except for the sites of mutation, the structure is very similar to that of the native human IgG1 Fc, consistent with a heterodimer interaction kinetic K(D) of <1 nM. Homodimers of the "knob" and "hole" mutants were also obtained, and their X-ray structures were determined at resolutions 2.5 Å and 2.1 Å, respectively. Both kinds of homodimers adopt a head-to-tail quaternary structure and thus do not contain direct knob/knob or hole/hole CH3 interactions. The head-to-tail arrangement was disfavored by adding site-directed mutations at F241 and F243 in the CH2 domains, leading to increases in both rate and efficiency of bispecific (heterodimer) assembly.

Antibody Engineering

Advanced molecular biology techniques developed during the past few decades have allowed the industry to exploit and commercialize the natural defense mechanisms that antibodies provide. This review discusses the latest advances in antibody-engineering technologies to enhance clinical efficacy and outcomes. For the constant regions, the choice of the antibody class and isotype has to be made carefully to suit the therapeutic applications. Engineering of the Fc region, either by direct targeted mutagenesis or by modifying the nature of its N -glycan, has played an important role in recent years in increasing half-life or controlling effector functions. The variable regions of the antibody are responsible for binding affinity and exquisite specificity to the target molecule, which together with the Fc determine the drug's efficacy and influence the drug dose required to obtain the desired effectiveness. A key requirement during antibody development is therefore to affinity mature the variable regions when necessary, so that they bind the therapeutic target with sufficiently high affinity to guarantee effective occupancy over prolonged periods. If the antibody was obtained from a non-human source, such as rodents, a humanization process has to be applied to minimize immunogenicity while maintaining the desired binding affinity and selectivity. Finally, we discuss the next next-generation antibodies, such as antibody-drug conjugates, bispecific antibodies, and immunocytokines, which are being developed to meet future challenges.

Therapeutic antibody engineering by high efficiency cell screening

In recent years, several cell-based screening technologies for the isolation of antibodies with prescribed properties emerged. They rely on the multi-copy display of antibodies or antibody fragments on a cell surface in functional form followed by high through put screening and isolation of cell clones that carry an antibody variant with the desired affinity, specificity, and stability. Particularly yeast surface display in combination with high-throughput fluorescence-activated cell sorting has proven successful in the last fifteen years as a very powerful technology that has some advantages over classical generation of monoclonals using the hybridoma technology or bacteriophage-based antibody display and screening. Cell-based screening harbours the benefit of single-cell online and real-time analysis and characterisation of individual library candidates. Moreover, when using eukaryotic expression hosts, intrinsic quality control machineries for proper protein folding and stability exist that allow for co-selection of high-level expression and stability simultaneously to the binding functionality. Recently, promising technologies emerged that directly rely on antibody display on higher eukaryotic cell lines using lentiviral transfection or direct screening on B-cells. The combination of immunisation, B-cell screening and next generation sequencing may open new avenues for the isolation of therapeutic antibodies with prescribed physicochemical and functional characteristics.

Protein engineering and the use of molecular modeling and simulation: the case of heterodimeric Fc engineering

Computational and structure guided methods can make significant contributions to the development of solutions for difficult protein engineering problems, including the optimization of next generation of engineered antibodies. In this paper, we describe a contemporary industrial antibody engineering program, based on hypothesis-driven in silico protein optimization method. The foundational concepts and methods of computational protein engineering are discussed, and an example of a computational modeling and structure-guided protein engineering workflow is provided for the design of best-in-class heterodimeric Fc with high purity and favorable biophysical properties. We present the engineering rationale as well as structural and functional characterization data on these engineered designs.

A heterodimeric Fc-based bispecific antibody simultaneously targeting VEGFR-2 and Met exhibits potent antitumor activity

Heterodimeric Fc designed by engineering the CH3 homodimeric interface of immunoglobulin G1 serves as an attractive scaffold for the generation of bispecific antibodies (bsAb) due to the favorable properties of the Fc region. In this study, we describe a heterodimeric Fc generated by substituting the conserved electrostatic interactions at the CH3 core interface with asymmetric hydrophobic interactions and introducing asymmetric, long-range electrostatic interactions at the rim of the CH3 interface. Coexpression of Fc proteins carrying the combined CH3 variant pairs in HEK293F cells produced the heterodimer, which was purified with more than 90% yield, and retained wild-type Fc biophysical properties. The heterodimeric Fc was exploited to generate a bsAb simultaneously targeting both the Met receptor tyrosine kinase and the VEGF receptor 2 (VEGFR-2), with two respective antigen-specific, single-chain variable fragments (scFv) into the N-terminus. The Met × VEGFR-2 bsAb bound concurrently to the two target antigens, efficiently inhibited the downstream signaling and tube formation stimulated by the two receptors in human endothelial cells, and exhibited more potent antitumor efficacy in MKN45 human gastric cancer xenograft models than both the parent monospecific antibody alone. Collectively, based on the newly designed heterodimeric Fc-based bsAb, our results provide the therapeutic potential of bsAb targeting both Met and VEGFR-2 simultaneously for the treatment of human cancers.

Production of stable bispecific IgG1 by controlled Fab-arm exchange: scalability from bench to large-scale manufacturing by application of standard approaches

The manufacturing of bispecific antibodies can be challenging for a variety of reasons. For example, protein expression problems, stability issues, or the use of non-standard approaches for manufacturing can result in poor yield or poor facility fit. In this paper, we demonstrate the use of standard antibody platforms for large-scale manufacturing of bispecific IgG1 by controlled Fab-arm exchange. Two parental antibodies that each contain a single matched point mutation in the CH3 region were separately expressed in Chinese hamster ovary cells and manufactured at 1000 L scale using a platform fed-batch and purification process that was designed for standard antibody production. The bispecific antibody was generated by mixing the two parental molecules under controlled reducing conditions, resulting in efficient Fab-arm exchange of >95% at kg scale. The reductant was removed via diafiltration, resulting in spontaneous reoxidation of interchain disulfide bonds. Aside from the bispecific nature of the molecule, extensive characterization demonstrated that the IgG1 structural integrity was maintained, including function and stability. These results demonstrate the suitability of this bispecific IgG1 format for commercial-scale manufacturing using standard antibody manufacturing techniques.

Development of a human IgG4 bispecific antibody for dual targeting of interleukin-4 (IL-4) and interleukin-13 (IL-13) cytokines

Human bispecific antibodies have great potential for the treatment of human diseases. Although human IgG1 bispecific antibodies have been generated, few attempts have been reported in the scientific literature that extend bispecific antibodies to other human antibody isotypes. In this paper, we report our work expanding the knobs-into-holes bispecific antibody technology to the human IgG4 isotype. We apply this approach to generate a bispecific antibody that targets IL-4 and IL-13, two cytokines that play roles in type 2 inflammation. We show that IgG4 bispecific antibodies can be generated in large quantities with equivalent efficiency and quality and have comparable pharmacokinetic properties and lung partitioning, compared with the IgG1 isotype. This work broadens the range of published therapeutic bispecific antibodies with natural surface architecture and provides additional options for the generation of bispecific antibodies with differing effector functions through the use of different antibody isotypes.

Bispecific antibodies and trispecific immunocytokines for targeting the immune system against cancer: preparing for the future

Monoclonal anti-tumor antibodies (mAbs) that are clinically effective usually recruit, via their constant fragment (Fc) domain, Fc receptor (FcR)-positive accessory cells of the immune system and engage these additionally against the tumor. Since T cells are FcR negative, these important cells are not getting involved. In contrast to mAbs, bispecific antibodies (bsAbs) can be designed in such a way that they involve T cells. bsAbs are artificially designed molecules that bind simultaneously to two different antigens, one on the tumor cell, the other one on an immune effector cell such as CD3 on T cells. Such dual antibody constructs can cross-link tumor cells and T cells. Many such bsAb molecules at the surface of tumor cells can thus build a bridge to T cells and aggregate their CD3 molecules, thereby activating them for cytotoxic activity. BsAbs can also contain a third binding site, for instance a Fc domain or a cytokine that would bind to its respective cytokine receptor. The present review discusses the pros and cons for the use of the Fc fragment during the development of bsAbs using either cell-fusion or recombinant DNA technologies. The recombinant antibody technology allows the generation of very efficient bsAbs containing no Fc domain such as the bi-specific T-cell engager (BiTE). The strong antitumor activity of these molecules makes them very interesting new cancer therapeutics. Over the last decade, we have developed another concept, namely to combine bsAbs and multivalent immunocytokines with a tumor cell vaccine. The latter are patient-derived tumor cells modified by infection with a virus. The virus-Newcastle Disease Virus (NDV)-introduces, at the surface of the tumor cells, viral molecules that can serve as general anchors for the bsAbs. Our strategy aims at redirecting, in an Fc-independent fashion, activities of T cells and accessory cells against autologous tumor antigens. It creates very promising perspectives for a new generation of efficient and safe cancer therapeutics that should confer long-lasting anti-tumor immunity.

Improving biophysical properties of a bispecific antibody scaffold to aid developability: quality by molecular design

While the concept of Quality-by-Design is addressed at the upstream and downstream process development stages, we questioned whether there are advantages to addressing the issues of biologics quality early in the design of the molecule based on fundamental biophysical characterization, and thereby reduce complexities in the product development stages. Although limited number of bispecific therapeutics are in clinic, these developments have been plagued with difficulty in producing materials of sufficient quality and quantity for both preclinical and clinical studies. The engineered heterodimeric Fc is an industry-wide favorite scaffold for the design of bispecific protein therapeutics because of its structural, and potentially pharmacokinetic, similarity to the natural antibody. Development of molecules based on this concept, however, is challenged by the presence of potential homodimer contamination and stability loss relative to the natural Fc. We engineered a heterodimeric Fc with high heterodimeric specificity that also retains natural Fc-like biophysical properties, and demonstrate here that use of engineered Fc domains that mirror the natural system translates into an efficient and robust upstream stable cell line selection process as a first step toward a more developable therapeutic.

Efficient generation of stable bispecific IgG1 by controlled Fab-arm exchange

The promise of bispecific antibodies (bsAbs) to yield more effective therapeutics is well recognized; however, the generation of bsAbs in a practical and cost-effective manner has been a formidable challenge. Here we present a technology for the efficient generation of bsAbs with normal IgG structures that is amenable to both antibody drug discovery and development. The process involves separate expression of two parental antibodies, each containing single matched point mutations in the CH3 domains. The parental antibodies are mixed and subjected to controlled reducing conditions in vitro that separate the antibodies into HL half-molecules and allow reassembly and reoxidation to form highly pure bsAbs. The technology is compatible with standard large-scale antibody manufacturing and ensures bsAbs with Fc-mediated effector functions and in vivo stability typical of IgG1 antibodies. Proof-of-concept studies with HER2×CD3 (T-cell recruitment) and HER2×HER2 (dual epitope targeting) bsAbs demonstrate superior in vivo activity compared with parental antibody pairs.

LUZ-Y, a novel platform for the mammalian cell production of full-length IgG-bispecific antibodies

The ability of bispecific antibodies to simultaneously bind two unique antigens has great clinical potential. However, most approaches utilized to generate bispecific antibodies yield antibody-like structures that diverge significantly from the structure of archetype human IgG, and those that do approach structural similarity to native antibodies are often challenging to engineer and manufacture. Here, we present a novel platform for the mammalian cell production of bispecific antibodies that differ from their parental mAbs by only a single point mutation per heavy chain. Central to this platform is the addition of a leucine zipper to the C terminus of the C(H)3 domain of the antibody that is sufficient to drive the heterodimeric assembly of antibody heavy chains and can be readily removed post-purification. Using this approach, we developed various antibody constructs including one-armed Abs, bispecific antibodies that utilize a common light chain, and bispecific antibodies that pair light chains to their cognate heavy chains via peptide tethers. We have applied this technology to various antibody pairings and will demonstrate the engineering, purification, and biological activity of these antibodies herein.

Progress in overcoming the chain association issue in bispecific heterodimeric IgG antibodies

The development of bispecific antibodies has attracted substantial interest, and many different formats have been described. Those specifically containing an Fc part are mostly tetravalent, such as stabilized IgG-scFv fusions or dual-variable domain (DVD) IgGs. However, although they exhibit IgG-like properties and technical developability, these formats differ in size and geometry from classical IgG antibodies. Thus, considerable efforts focus on bispecific heterodimeric IgG antibodies that more closely mimic natural IgG molecules. The inherent chain association problem encountered when producing bispecific heterodimeric IgG antibodies can be overcome by several methods. While technologies like knobs-into-holes (KiH) combined with a common light chain or the CrossMab technology enforce the correct chain association, other approaches, e.g., the dual-acting Fab (DAF) IgGs, do not rely on a heterodimeric Fc part. This review discusses the state of the art in bispecific heterodimeric IgG antibodies, with an emphasis on recent progress.

Fc-fusion proteins: new developments and future perspectives

Since the first description in 1989 of CD4-Fc-fusion antagonists that inhibit human immune deficiency virus entry into T cells, Fc-fusion proteins have been intensely investigated for their effectiveness to curb a range of pathologies, with several notable recent successes coming to market. These promising outcomes have stimulated the development of novel approaches to improve their efficacy and safety, while also broadening their clinical remit to other uses such as vaccines and intravenous immunoglobulin therapy. This increased attention has also led to non-clinical applications of Fc-fusions, such as affinity reagents in microarray devices. Here we discuss recent results and more generally applicable strategies to improve Fc-fusion proteins for each application, with particular attention to the newer, less charted areas.

Conference scene: progress with promising human antibodies

Antibodies and antibody-based therapeutics have become big business, with annual sales over US$50 billion, accounting for >6% of worldwide pharmaceutical revenues. Ten molecules have blockbuster status (>US$1 billion), with six generating more than US$6 billion in sales. In excess of 300 products based on this rapidly maturing technology are in clinical trials. The generation and manufacture of human antibodies is now routine, although the cost of goods remains an issue. Optimizing combinations of antibodies with other therapeutics (e.g., chemotherapy) is a major short-term goal, while target validation and product differentiation remain significant hurdles if growth is to continue. Some of the notable highlights of the recent 16th International Conference on Human Antibodies and Hybridomas meeting in Cannes, France are described below. The conference was sponsored by the international journal Human Antibodies, in association with the Integrative Medical Sciences Association (IMSA). The Program Chairman was Professor Mark Glassy, IMSA, San Diego, CA, USA.

Engineering a CD123xCD3 bispecific scFv immunofusion for the treatment of leukemia and elimination of leukemia stem cells

Engineered bispecific antibodies that recruit cytotoxic lymphocytes to kill specific tumor cells have been showing promising clinical results. Here, we describe a bispecific single-chain Fv (scFv) immunofusion or BIf to target CD123(+) leukemia, that contains an anti-CD123 scFv fused at the N-terminus of human IgG1 hinge-C(H)2-C(H)3, and an anti-CD3 scFv fused at C-terminus. When expressed from transfected CHO-S cells, CD123xCD3 BIf forms a homodimer that provides a structure of N-terminal tumor-targeting domain that closely resembles natural antibody. The CD123xCD3 dimeric structure also provides binding affinity to CD123(+) tumor cells with a Kd of 10(-10) M, one to two orders of magnitude stronger than traditional bispecific antibody constructs. The location of the anti-CD3 scFv at C-terminus of BIf reduces the binding affinity to CD3(+) T cells by two orders, which could help to prevent non-specific T-cell activation. CD123xCD3 BIf is able to achieve T-cell-mediated target cell killing activities at low pM levels with E/T ratios as low as 2. Overall, the inclusion of human IgG1 constant region in BIf construct increases target cell-binding affinity; potentially increases serum half-life by its larger size and FcRn-mediated salvage system; and includes the abilities to activate the additional antibody-mediated cellular cytotoxicities.

World Bispecific Antibody Summit, September 27-28, 2011, Boston, MA

With more than 30 therapeutic monoclonal antibodies (mAbs) approved and annual global sales of the products at ~$50 billion in 2010, these products have proven to be successful in many ways. Nevertheless, there is room for improvement in performance, and substantial unmet medical needs remain. As a consequence, numerous organizations are devoting resources to engineering novel mAbs such as bispecific antibodies that have increased functionality compared with unmodified IgG molecules. The World Bispecific Antibody Summit, organized by Hanson Wade, drew over 100 participants to Boston to discuss engineering novel bispecific antibodies, generating lead candidates and clinical study and commercialization of the molecules. Approaches such as the trifunctional antibody (TRION), dual variable domain-Ig (Abbott), two-in-one (Genentech), dual affinity retargeting (MacroGenics), kappa-lambda body (NovImmune), bispecific T-cell engager (Micromet) and chemical generation (CovX/Pfizer) were discussed in detail. In addition, posters describing bispecific Affibody® molecules for targeting of EGFR and HER2 (Affibody), T-cell receptor based bi-specifics that target HLA-peptides (Immunocore), a novel mAb-Fv bispecific antibody format utilizing Fc region (Xencore), generation of a tetravalent bispecific antibody against IL4 and IL13 for the treatment of idiopathic pulmonary fibrosis (Sanofi), Combining Affibody® molecules and the Albumod™ technology to create long acting multispecific protein therapeutics (Royal Institute of Technology, Affibody) and COVA301 as a highly potent bispecific inhibitor of IL-17A and TNF-α (Covagen) were presented.

Generating bispecific human IgG1 and IgG2 antibodies from any antibody pair

Bispecific antibodies and antibody fragments are a new class of therapeutics increasingly utilized in the clinic for T cell recruitment (catumaxomab anti-EpCAM/CD3 and blinatumomab anti-CD19/CD3), increase in the selectivity of targeting, or simultaneous modulation of multiple cellular pathways. While the clinical potential for certain bispecific antibody formats is clear, progress has been hindered because they are often difficult to manufacture, may suffer from suboptimal pharmacokinetic properties, and may be limited due to potential immunogenicity issues. Current state-of-the-art human IgG-like bispecific technologies require co-expression of two heavy chains with a single light chain, use crossover domains to segregate light chains, or utilize scFv (single-chain fragment variable)-Fc fusion. We have engineered both human IgG1 and IgG2 subtypes, with minimal point mutations, to form full-length bispecific human antibodies with high efficiency and in high purity. In our system, the two antibodies of interest can be expressed and purified separately, mixed together under appropriate redox conditions, resulting in a formation of a stable bispecific antibody with high yields. With this approach, it is not necessary to generate new antibodies that share a common light chain, therefore allowing the immediate use of an existing antibody regardless of whether it has been generated via standard hybridoma or display methods. We demonstrate the generality of the approach and show that these bispecific antibodies have properties similar to those of wild-type IgGs, and we further demonstrate the utility of the technology with an example of a CD3/CD20 bispecific antibody that effectively depletes B cells in vitro and in vivo.

Anti-EGFR biparatopic-SEED antibody has enhanced combination-activity in a single molecule

Certain combinations of non-competitive anti-EGFR antibodies have been reported to produce new effects on cells compared to either antibody used separately. New and enhanced combination-activity includes increased inhibition of signaling, increased receptor internalization and degradation, reduced proliferation of tumor cell lines and induction of complement-dependent cytotoxicity (CDC) effector function. To test requirements and mechanisms to elicit enhanced combination-activity with different EGFR binding domains, we created an anti-EGFR biparatopic antibody. A biparatopic antibody interacts through two different antigen-binding sites to a single antigen. A heterodimeric antibody with one binding domain derived from the C225 antibody and one binding domain derived from the humanized 425 (hu425) antibody was built on the strand-exchange engineered domain (SEED) scaffold. This anti-EGFR biparatopic-SEED antibody was compared to parental antibodies used alone and in combination, and to the corresponding monovalent anti-EGFR-SEED antibodies used alone or in combination. We found that the anti-EGFR biparatopic-SEED had enhanced activity, similar to the combination of the two parental antibodies. Combinations of monovalent anti-EGFR-SEED antibodies did not produce enhanced effectiveness in cellular assays. Our results show that the anti-EGFR biparatopic antibody created using the SEED scaffold has enhanced combination-activity in a single molecule. Furthermore, these data suggest that the potential to cross-link the two different epitopes is an important requirement in the mechanism of enhanced combination-activity.

Properties of 7ND-CCL2 are modulated upon fusion to Fc

7ND, a truncated version of the chemokine MCP-1/CCL2 lacking amino acids 2-8, is a potent antagonist of CCR2. In contrast to CCL2, 7ND is an obligate monomer. Similar to other chemokines, the in vivo half-life of 7ND is very short and its use as an antagonist in disease models is thus limited. We therefore constructed a 7ND-Fc fusion protein to extend the half-life of 7ND and overcome its limitations as a potential therapeutic antagonist. When we tested the properties of the fusion molecule in vitro, we found to our surprise that 7ND-Fc, in contrast to 7ND, produced a distinct, albeit small, chemotactic response in THP-1 cells, and a robust chemotactic response in L1.2 cells stably transfected with CCR2. To test whether this unexpected observation might be due to the bivalency of 7ND-Fc stemming from the dimeric nature of Fc fusions, we produced a heterodimeric Fc fusion which displays only one 7ND moiety, using a technology called strand exchange of engineered CH3 domains (SEED). The monovalent construct had properties equivalent to the parent 7ND. Furthermore, partial agonist activity appears to depend on receptor density as well as the signaling pathway examined. However, we were able to show that 7ND-Fc, but not 7ND alone, has antagonistic activity in experimental autoimmune encephalomyelitis, a murine model of multiple sclerosis.

Dual targeting strategies with bispecific antibodies

Monoclonal antibodies are widely used for the treatment of cancer, inflammatory and infectious diseases and other disorders. Most of the marketed antibodies are monospecific and therefore capable of interacting and interfering with a single target. However, complex diseases are often multifactorial in nature, and involve redundant or synergistic action of disease mediators or upregulation of different receptors, including crosstalk between their signaling networks. Consequently, blockade of multiple, different pathological factors and pathways may result in improved therapeutic efficacy. This result can be achieved by combining different drugs, or use of the dual targeting strategies applying bispecific antibodies that have emerged as an alternative to combination therapy. This review discusses the various dual targeting strategies for which bispecific antibodies have been developed and provides an overview of the established bispecific antibody formats.

Quantitative analysis of the interaction strength and dynamics of human IgG4 half molecules by native mass spectrometry

Native mass spectrometry (MS) is a powerful technique for studying noncovalent protein-protein interactions. Here, native MS was employed to examine the noncovalent interactions involved in homodimerization of antibody half molecules (HL) in hinge-deleted human IgG4 (IgG4Δhinge). By analyzing the concentration dependence of the relative distribution of monomer HL and dimer (HL)(2) species, the apparent dissociation constant (K(D)) for this interaction was determined. In combination with site-directed mutagenesis, the relative contributions of residues at the CH3-CH3 interface to this interaction could be characterized and corresponding K(D) values quantified over a range of 10(-10)-10(-4) M. The critical importance of this noncovalent interaction in maintaining the intact dimeric structure was also proven for the full-length IgG4 backbone. Using time-resolved MS, the kinetics of the interaction could be measured, reflecting the dynamics of IgG4 HL exchange. Hence, native MS has provided a quantitative view of local structural features that define biological properties of IgG4.

Protein-intrinsic and signaling network-based sources of resistance to EGFR- and ErbB family-targeted therapies in head and neck cancer

Agents targeting EGFR and related ErbB family proteins are valuable therapies for the treatment of many cancers. For some tumor types, including squamous cell carcinomas of the head and neck (SCCHN), antibodies targeting EGFR were the first protein-directed agents to show clinical benefit, and remain a standard component of clinical strategies for management of the disease. Nevertheless, many patients display either intrinsic or acquired resistance to these drugs; hence, major research goals are to better understand the underlying causes of resistance, and to develop new therapeutic strategies that boost the impact of EGFR/ErbB inhibitors. In this review, we first summarize current standard use of EGFR inhibitors in the context of SCCHN, and described new agents targeting EGFR currently moving through pre-clinical and clinical development. We then discuss how changes in other transmembrane receptors, including IGF1R, c-Met, and TGF-β, can confer resistance to EGFR-targeted inhibitors, and discuss new agents targeting these proteins. Moving downstream, we discuss critical EGFR-dependent effectors, including PLC-γ; PI3K and PTEN; SHC, GRB2, and RAS and the STAT proteins, as factors in resistance to EGFR-directed inhibitors and as alternative targets of therapeutic inhibition. We summarize alternative sources of resistance among cellular changes that target EGFR itself, through regulation of ligand availability, post-translational modification of EGFR, availability of EGFR partners for hetero-dimerization and control of EGFR intracellular trafficking for recycling versus degradation. Finally, we discuss new strategies to identify effective therapeutic combinations involving EGFR-targeted inhibitors, in the context of new system level data becoming available for analysis of individual tumors.

A novel bispecific antibody format enables simultaneous bivalent and monovalent co-engagement of distinct target antigens

Bispecific antibodies based on full-length antibody structures are more optimal than fragment-based formats because they benefit from the favorable properties of the Fc region. However, the homodimeric nature of Fc effectively imposes bivalent binding on all current full-length bispecific antibodies, an attribute that can result in nonspecific activation of cross-linked receptors. We engineered a novel bispecific format, referred to as mAb-Fv, that utilizes a heterodimeric Fc region to enable monovalent co-engagement of a second target antigen in a full-length context. mAb-Fv constructs co-targeting CD16 and CD3 were expressed and purified as heterodimeric species, bound selectively to their co-target antigens, and mediated potent cytotoxic activity by NK cells and T cells, respectively. The capacity to co-engage distinct target antigens simultaneously with different valencies is an improved feature for bispecific antibodies with promising therapeutic implications.

Therapeutic assessment of SEED: a new engineered antibody platform designed to generate mono- and bispecific antibodies

The strand-exchange engineered domain (SEED) platform was designed to generate asymmetric and bispecific antibody-like molecules, a capability that expands therapeutic applications of natural antibodies. This new protein engineered platform is based on exchanging structurally related sequences of immunoglobulin within the conserved CH3 domains. Alternating sequences from human IgA and IgG in the SEED CH3 domains generate two asymmetric but complementary domains, designated AG and GA. The SEED design allows efficient generation of AG/GA heterodimers, while disfavoring homodimerization of AG and GA SEED CH3 domains. Using a clinically validated antibody (C225), we tested whether Fab derivatives constructed on the SEED platform retain desirable therapeutic antibody features such as in vitro and in vivo stability, favorable pharmacokinetics, ligand binding and effector functions including antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity. In addition, we tested SEED with combinations of binder domains (scFv, VHH, Fab). Mono- and bivalent Fab-SEED fusions retain full binding affinity, have excellent biochemical and biophysical stability, and retain desirable antibody-like characteristics conferred by Fc domains. Furthermore, SEED is compatible with different combinations of Fab, scFv and VHH domains. Our assessment shows that the new SEED platform expands therapeutic applications of natural antibodies by generating heterodimeric Fc-analog proteins.

Rational engineering of antibody therapeutics targeting multiple oncogene pathways

Monoclonal antibodies have significantly advanced our ability to treat cancer, yet clinical studies have shown that many patients do not adequately respond to monospecific therapy. This is in part due to the multifactorial nature of the disease, where tumors rely on multiple and often redundant pathways for proliferation. Bi- or multi- specific antibodies capable of blocking multiple growth and survival pathways at once have a potential to better meet the challenge of blocking cancer growth, and indeed many of them are advancing in clinical development. ( 1) However, bispecific antibodies present significant design challenges mostly due to the increased number of variables to consider. In this perspective we describe an innovative integrated approach to the discovery of bispecific antibodies with optimal molecular properties, such as affinity, avidity, molecular format and stability. This approach combines simulations of potential inhibitors using mechanistic models of the disease-relevant biological system to reveal optimal inhibitor characteristics with antibody engineering techniques that yield manufacturable therapeutics with robust pharmaceutical properties. We illustrate how challenges of meeting the optimal design criteria and chemistry, manufacturing and control concerns can be addressed simultaneously in the context of an accelerated therapeutic design cycle. Finally, to demonstrate how this rational approach can be applied, we present a case study where the insights from mechanistic modeling were used to guide the engineering of an IgG-like bispecific antibody.