Bispecific engineered antibody domains (nanoantibodies) that interact noncompetitively with an HIV-1 neutralizing epitope and FcRn

Chimerism of avian IgY-scFv and truncated IgG-Fc: A novel strategy in cross-species antibody generation and enhancement

Chicken single-chain fragment variable (IgY-scFv) is a functional fragment and an emerging development in genetically engineered antibodies with a wide range of biomedical applications. However, scFvs have considerably shorter serum half-life due to the absence of antibody Fc region compared with the full-length antibody, and usually requires continuous intravenous administration for efficacy. A promising approach to overcome this limitation is to fuse scFv with immunoglobulin G (IgG) Fc region, for better recognition and mediation by the neonatal Fc receptor (FcRn) in the host. In this study, engineered mammalian ΔFc domains (CH2, CH3, and intact Fc region) were fused with anti-canine parvovirus-like particles avian IgY-scFv to produce chimeric antibodies and expressed in the HEK293 cell expression system. The obtained scFv-CH2, scFv-CH3, and scFv-Fc can bind with antigen specifically and dose-dependently. Surface plasmon resonance investigation confirmed that scFv-CH2, scFv-CH3, and scFv-Fc had different degrees of binding to FcRn, with scFv-Fc showing the highest affinity. scFv-Fc had a significantly longer half-life in mice compared with the unfused scFv. The identified ΔFcs are promising for the development of engineered Fc-based therapeutic antibodies and proteins with longer half-lives. The avian IgY-scFv-mammalian IgG Fc region opens up new avenues for antibody engineering, and it is a novel strategy to enhance the rapid development and screening of functional antibodies in veterinary and human medicine.

Immunotherapy targeting mesothelin in acute myeloid leukemia

Mesothelin (MSLN) is an emerging target that exists in soluble and membrane-associated forms. It is usually used for the diagnosis and treatment of MSLN-positive solid tumors. Interestingly, recent studies have shown that MSLN is highly expressed in 36% of acute myeloid leukemia (AML) patients and barely expressed in normal hematopoietic cells, which makes MSLN a promising target for the treatment of AML. It has been shown that MSLN is detectable as a diagnostic marker in its soluble form. Although the mechanism of action is unclear, MSLN remains a promising target for immunotherapy. Most MSLN research has been conducted in solid tumors, and less research has been conducted in hematopoietic tumors. Increasing research on MSLN is underway in AML, a hematopoietic neoplasm. For example, MSLN is related to extramedullary disease, minimal residual disease, and relapse in AML patients. Decreasing the expression of MSLN reduces the severity of the disease course. This information suggests that MSLN may be an ideal target for the treatment of many AML-related diseases to improve the prognosis and survival rate. At present, there are a few immunotherapies targeting MSLN in AML in preclinical and clinical trials, such as antibody-drug conjugates, bispecific T-cell engagers, and chimeric antigen receptor-T cells, which opens new room for the treatment of MSLN-related AML.

Identification and Characterization of a Novel Single Domain Antibody Against Ebola Virus

Ebola virus (EBOV) belongs to the Filoviridae family and causes severe illnesses such as hemorrhagic fever with a high mortality rate up to 90%. Now two antibody drugs termed Inmazeb and Ebanga have been approved for treating EBOV infection. However, clinical studies have demonstrated that the mortality rate of the patients who received these two antibody drugs remains above 30%. Therefore, novel therapeutics with better efficacy is still desired. The isolated human IgG1 constant domain 2 (CH2 domain) has been proposed as a scaffold for the development of C-based single domain antibodies (C-sdAbs) as therapeutic candidates against viral infections and other diseases. Here, we screened and identified a novel C-sdAb termed M24 that targets EBOV glycoprotein (GP) from a C-sdAb phage display library. M24 neutralizes the pseudotype EBOV with IC₅₀ of 0.8 nmol/L (12 ng/mL) and has modest neutralizing activity against authentic EBOV. Epitope determination, including molecular docking and site mutation analysis, discloses that M24 binds to the internal fusion loop (IFL) within GP2, a transmembrane subunit of GP. Interestingly, we found that the binding of M24 to GP at pH 5.5 has dramatically decreased compared to the binding at pH 7.5, which may lead to weak efficacy in the neutralization of authentic EBOV. Since no sdAb against EBOV infection has been reported to date, our results not only give a proof of concept that sdAbs could be utilized for the development of potential therapeutic candidates against EBOV infection, but also provide useful information for the discovery and improvement of anti-EBOV agents.

Recent Advances in the Molecular Design and Applications of Multispecific Biotherapeutics

Recombinant protein-based biotherapeutics drugs have transformed clinical pipelines of the biopharmaceutical industry since the launch of recombinant insulin nearly four decades ago. These biologic drugs are structurally more complex than small molecules, and yet share a similar principle for rational drug discovery and development: That is to start with a pre-defined target and follow with the functional modulation with a therapeutic agent. Despite these tremendous successes, this "one target one drug" paradigm has been challenged by complex disease mechanisms that involve multiple pathways and demand new therapeutic routes. A rapidly evolving wave of multispecific biotherapeutics is coming into focus. These new therapeutic drugs are able to engage two or more protein targets via distinct binding interfaces with or without the chemical conjugation to large or small molecules. They possess the potential to not only address disease intricacy but also exploit new therapeutic mechanisms and assess undruggable targets for conventional monospecific biologics. This review focuses on the recent advances in molecular design and applications of major classes of multispecific biotherapeutics drugs, which include immune cells engagers, antibody-drug conjugates, multispecific tetherbodies, biologic matchmakers, and small-scaffold multispecific modalities. Challenges posed by the multispecific biotherapeutics drugs and their future outlooks are also discussed.

An engineered human IgG1 CH2 domain with decreased aggregation and nonspecific binding

The immunoglobulin (Ig) CH2 domain is a promising scaffold for the development of candidate therapeutics. We have previously shown that the stability of isolated CH2 could be increased by the introduction of an additional disulfide bond and removal of seven N-terminal residues (m01s). However, both isolated CH2 and m01s aggregate, likely due to the existence of aggregation-prone regions (APRs) that we identified by using computational methods. This knowledge was used to generate a phage display library of mutants. The library was incubated at high temperature to remove aggregating CH2 domains, and then panned against a mouse anti-human CH2 monoclonal antibody targeting a conformational epitope to remove misfolded CH2s. After two rounds of panning, one clone, m01s5, with smaller APRs, was identified. After additional mutagenesis one clone, m01s5.4, which aggregated much less than m01s as measured by a turbidity assay and dynamic light scattering, was identified. m01s5.4 also exhibited much lower nonspecific binding than m01s. Engineering of a previously identified m01s-based tumor antigen-specific binder led to a dramatic reduction of its aggregation without affecting its binding. In summary, we describe a new approach for reducing aggregation based on a combination of computational and phage display methodologies, and show that aggregation of CH2-based scaffolds can be significantly reduced by the newly identified mutants, which can improve the developability of potential CH2-based therapeutics.

Engineering of Fc Fragments with Optimized Physicochemical Properties Implying Improvement of Clinical Potentials for Fc-Based Therapeutics

Therapeutic monoclonal antibodies and Fc-fusion proteins are successfully used in treatment of various diseases mainly including cancer, immune disease, and viral infection, which belong to the Fc-based therapeutics. In recent years, engineered Fc-derived antibody domains have also shown potential for Fc-based therapeutics. To increase the druggability of Fc-based therapeutic candidates, many efforts have been made in optimizing physicochemical properties and functions mediated by Fc fragment. The desired result is that we can simultaneously obtain Fc variants with increased physicochemical properties in vitro and capacity of mediating appropriate functions in vivo. However, changes of physicochemical properties of Fc may result in alternation of Fc-mediated functions and vice versa, which leads to undesired outcomes for further development of Fc-based therapeutics. Therefore, whether modified Fc fragments are suitable for achievement of expected clinical results or not needs to be seriously considered. Now, this question comes to be noticed and should be figured out to make better translation from the results of laboratory into clinical applications. In this review, we summarize different strategies on engineering physicochemical properties of Fc, and preliminarily elucidate the relationships between modified Fc in vitro and the subsequent therapeutic influence in vivo.

Single-Domain Antibodies As Therapeutics against Human Viral Diseases

In full-size formats, monoclonal antibodies have been highly successful as therapeutics against cancer and immune diseases. However, their large size leads to inaccessibility of some epitopes and relatively high production costs. As an alternative, single-domain antibodies (sdAbs) offer special advantages compared to full-size antibodies, including smaller size, larger number of accessible epitopes, relatively low production costs and improved robustness. Currently, sdAbs are being developed against a number of viruses, including human immunodeficiency virus-1 (HIV-1), influenza viruses, hepatitis C virus (HCV), respiratory syncytial virus (RSV), and enteric viruses. Although sdAbs are very potent inhibitors of viral infections, no sdAbs have been approved for clinical use against virial infection or any other diseases. In this review, we discuss the current state of research on sdAbs against viruses and their potential as therapeutics against human viral diseases.

Systemic and topical use of monoclonal antibodies to prevent the sexual transmission of HIV

: Passive immunization, the transfer of antibodies to a nonimmune individual to provide immunological protection, has been used for over 100 years to prevent and treat human infectious diseases. The introduction of techniques to produce human mAbs has revolutionized the field, and a large number of human mAbs have been licensed for the treatment of cancer, autoimmune and inflammatory diseases. With the recent discovery and production of highly potent broadly neutralizing and other multifunctional antibodies to HIV, mAbs are now being considered for HIV therapy and prophylaxis. In this review, we briefly present recent advances in the anti-HIV mAb field and outline strategies for the selection, engineering and production of human mAbs, including the modification of their structure for optimized stability and function. We also describe results from nonhuman primate studies and phase 1 clinical trials that have tested the safety, tolerability, pharmacokinetics, and efficacy of mAb-based HIV prevention strategies, and discuss the future of parenteral and topical mAb administration for the prevention of HIV transmission.

Engineered antibody CH2 domains binding to nucleolin: Isolation, characterization and improvement of aggregation

Smaller recombinant antibody fragments are now emerging as alternatives of conventional antibodies. Especially, immunoglobulin (Ig) constant CH2 domain and engineered CH2 with improved stability are promising as scaffolds for selection of specific binders to various antigens. We constructed a yeast display library based on an engineered human IgG1 CH2 scaffold with diversified loop regions. A group of CH2 binders were isolated from this yeast display library by panning against nucleolin, which is a tumor-associated antigen involved in cell proliferation, tumor cell growth and angiogenesis. Out of 20 mutants, we selected 3 clones exhibiting relatively high affinities to nucleolin on yeasts. However, recombinant CH2 mutants aggregated when they were expressed. To find the mechanism of the aggregation, we employed computational prediction approaches through structural homology models of CH2 binders. The analysis of potential aggregation prone regions (APRs) and solvent accessible surface areas (ASAs) indicated two hydrophobic residues, Val264 and Leu309, in the β-sheet, in which replacement of both charged residues led to significant decrease of the protein aggregation. The newly identified CH2 binders could be improved to use as candidate therapeutics or research reagents in the future.

Fc Engineering for Developing Therapeutic Bispecific Antibodies and Novel Scaffolds

Therapeutic monoclonal antibodies have become molecules of choice to treat autoimmune disorders, inflammatory diseases, and cancer. Moreover, bispecific/multispecific antibodies that target more than one antigen or epitope on a target cell or recruit effector cells (T cell, natural killer cell, or macrophage cell) toward target cells have shown great potential to maximize the benefits of antibody therapy. In the past decade, many novel concepts to generate bispecific and multispecific antibodies have evolved successfully into a range of formats from full bispecific immunoglobulin gammas to antibody fragments. Impressively, antibody fragments such as bispecific T-cell engager, bispecific killer cell engager, trispecific killer cell engager, tandem diabody, and dual-affinity-retargeting are showing exciting results in terms of recruiting and activating self-immune effector cells to target and lyse tumor cells. Promisingly, crystallizable fragment (Fc) antigen-binding fragment and monomeric antibody or half antibody may be particularly advantageous to target solid tumors owing to their small size and thus good tissue penetration potential while, on the other hand, keeping Fc-related effector functions such as antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity, antibody-dependent cell-mediated phagocytosis, and extended serum half-life via interaction with neonatal Fc receptor. This review, therefore, focuses on the progress of Fc engineering in generating bispecific molecules and on the use of small antibody fragment as scaffolds for therapeutic development.

Engineered IgG1-Fc--one fragment to bind them all

The crystallizable fragment (Fc) of the immunoglobulin class G (IgG) is a very attractive scaffold for the design of novel therapeutics due to its quality of uniting all essential antibody functions. This article reviews the functionalization of this homodimeric glycoprotein by diversification of structural loops of CH3 domains for the design of Fcabs, i.e. antigen-binding Fc proteins. It reports the design of libraries for the selection of nanomolar binders with wildtype-like in vivo half-life and correlation of Fc receptor binding and ADCC. The in vitro and preclinical biological activity of selected Fcabs is compared with that of clinically approved antibodies. Recently, the great potential of the scaffold for the development of therapeutics for clinical use has been shown when the HER2-binding Fcab FS102 entered clinical phase I. Furthermore, methods for the engineering of biophysical properties of Fcabs applicable to proteins in general are presented as well as the different approaches in the design of heterodimeric Fc-based scaffolds used in the generation of bispecific monoclonal antibodies. Finally, this work critically analyzes and compares the various efforts in the design of highly diverse and functional libraries that have been made in the engineering of IgG1-Fc and structurally similar scaffolds.

Engineered antibody domains with significantly increased transcytosis and half-life in macaques mediated by FcRn

Engineered antibody domains (eAds) are promising candidate therapeutics but their half-life is relatively short partly due to weak or absent binding to the neonatal Fc receptor (FcRn). We developed a novel approach to increase the eAd binding to FcRn based on a combination of structure-based design, computational modeling and phage display methodologies. By using this approach, we identified 2 IgG1 CH2-derived eAds fused to a short FcRn-binding motif derived from IgG1 CH3 that exhibited greatly enhanced FcRn binding with strict pH dependency. Importantly, the increased affinity resulted in significantly enhanced FcRn-mediated epithelial transcytosis and prolonged elimination half-life (mean 44.1 hours) in cynomolgus macaques. These results demonstrate for the first time that the half-life of isolated eAds can be prolonged (optimized) by increasing their binding to FcRn while maintaining their small size, which has important implications for development of therapeutics, including eAd-drug conjugates with enhanced penetration in solid tissues.

High Affinity Binders to EphA2 Isolated from Abdurin Scaffold Libraries; Characterization, Binding and Tumor Targeting

Abdurins are a novel antibody-like scaffold derived from the engineering of a single isolated CH2 domain of human IgG. Previous studies established the prolonged serum half-life of Abdurins, the result of a retained FcRn binding motif. Here we present data on the construction of large, diverse, phage-display and cell-free DNA display libraries and the isolation of high affinity binders to the cancer target, membrane-bound ephrin receptor tyrosine kinase class A2 (EphA2). Antigen binding regions were created by designing combinatorial libraries into the structural loops and Abdurins were selected using phage display methods. Initial binders were reformatted into new maturation libraries and low nanomolar binders were isolated using cell-free DNA display, CIS display. Further characterization confirmed binding of the Abdurins to both human and murine EphA2 proteins and exclusively to cell lines that expressed EphA2, followed by rapid internalization. Two different EphA2 binders were labeled with ⁶⁴Cu, using a bifunctional MeCOSar chelator, and administered to mice bearing tumors from transplanted human prostate cancer cells, followed by PET/CT imaging. The anti-EphA2 Abdurins localized in the tumors as early as 4 hours after injection and continued to accumulate up to 48 hours when the imaging was completed. These data demonstrate the ability to isolate high affinity binders from the engineered Abdurin scaffold, which retain a long serum half-life, and specifically target tumors in a xenograft model.

Engineered Fc based antibody domains and fragments as novel scaffolds

Therapeutic monoclonal antibodies (mAbs) have been successful for the therapy of a number of diseases mostly cancer and immune disorders. However, the vast majority of mAbs approved for clinical use are full size, typically in IgG1 format. These mAbs may exhibit relatively poor tissue penetration and restricted epitope access due to their large size. A promising solution to this fundamental limitation is the engineering of smaller scaffolds based on the IgG1 Fc region. These scaffolds can be used for the generation of libraries of mutants from which high-affinity binders can be selected. Comprised of the CH2 and CH3 domains, the Fc region is important not only for the antibody effector function but also for its long half-life. This review focuses on engineered Fc based antibody fragments and domains including native (dimeric) Fc and monomeric Fc as well as CH2 and monomeric CH3, and their use as novel scaffolds and binders. The Fc based binders are promising candidate therapeutics with optimized half-life, enhanced tissue penetration and access to sterically restricted binding sites resulting in an increased therapeutic efficacy. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.

Discovery of novel candidate therapeutics and diagnostics based on engineered human antibody domains

The smallest independently folded antibody fragments, the domains, are emerging as promising scaffolds for candidate therapeutics and diagnostics that bind specifically targets of interest. The discovery of such binders is based on several technologies including structure-based design and generation of libraries of mutants displayed on phage or yeast, next-generation sequencing for diversity analysis, panning and screening of the libraries, affinity maturation of selected binders, and their expression, purification, and characterization for specific binding, function, and aggregation propensity. In this review, we describe these technologies as applied for the generation of engineered antibody domains (eAds), especially those derived from the human immunoglobulin heavy chain variable region (VH) and the second domain of IgG1 heavy chain constant region (CH2) as potential candidate therapeutics and diagnostics, and discuss examples of eAds against HIV-1 and cancer-related proteins.

Engineered antibody variable and constant domains as therapeutic candidates

Monoclonal antibodies have been successfully used for the therapy of various diseases. However, because of their large size (∼150 kD), many limitations have also been found during their development and manufacture. The use of antibody fragments with smaller sizes is one of the strategies to overcome these limitations. Antibody heavy chain variable domains (12∼15 kD) have already been widely used for the development of variable domain-based engineered antibody domains (termed V-based eAds) targeting different antigens. Recently, antibody second heavy chain constant domains (∼12 kD) were proposed as novel scaffolds for library construction and selection of specific binders termed constant domain-based eAds (C-based eAds) as novel candidate therapeutics, which might also confer additional crystallizable fragment functions. Both V- and C-based eAds are promising therapeutic candidates. This review summarizes progress in the development of eAds, and discusses the related patents and their potential applications.

Interactions of IgG1 CH2 and CH3 Domains with FcRn

Antibody fragments are emerging as promising biopharmaceuticals because of their relatively small-size and other unique properties. However, when compared to full-size antibodies, most of the current antibody fragments of VH or VL display greatly reduced half-lives. A promising approach to overcome this problem is through the development of novel antibody fragments based on IgG Fc region, which contributes to the long half-life of IgG through its unique pH-dependent association with the neonatal Fc receptor (FcRn). The IgG Fc region comprises two CH2 and two CH3 domains. In this report, we present a comparative study of the FcRn binding capability of the CH2 and CH3 domains. The stability and aggregation resistance of these domains were also investigated and compared. We found that monomeric CH2 and CH3 domains exhibited the pH-dependent FcRn binding while the dimeric forms of CH2 and CH3 domains did not. Although all of these domains had high serum stability, they had aggregation tendencies as measured by dynamic light scattering. By providing a better understanding of the structure-activity relationship of the Fc fragment, these results guide further approaches to generate novel Fc-based small-size antibody fragments that possess pH-dependent FcRn binding capability, desired in vivo half-lives, and other favorable biophysical properties for their druggability.

Human monoclonal antibodies as candidate therapeutics against emerging viruses and HIV-1

More than 40 monoclonal antibodies (mAbs) have been approved for a number of disease indications with only one of these (Synagis) - for a viral disease, and not for therapy but for prevention. However, in the last decade novel potent mAbs have been discovered and characterized with potential as therapeutics against viruses of major importance for public health and biosecurity including Hendra virus (HeV), Nipah virus (NiV), severe acute respiratory syndrome coronavirus (SARS-CoV), Ebola virus (EBOV), West Nile virus (WNV), influenza virus (IFV) and human immunodeficiency virus type 1 (HIV-1). Here, we review such mAbs with an emphasis on antibodies of human origin, and highlight recent results as well as technologies and mechanisms related to their potential as therapeutics.