Bispecific single-chain diabody-mediated killing of endoglin-positive endothelial cells by cytotoxic T lymphocytes

Fc-comprising scDb-based trivalent, bispecific T-cell engagers for selective killing of HER3-expressing cancer cells independent of cytokine release

Background

Bispecific T-cell engagers are an established therapeutic strategy for the treatment of hematologic malignancies but face several challenges when it comes to their application for the treatment of solid tumors, including on-target off-tumor adverse events. Employing an avidity-mediated specificity gain by introducing an additional binding moiety for the tumor-associated antigen can be achieved using formats with a 2+1 stoichiometry.

Methods

Besides biochemical characterization and validation of target cell binding to cancer cells with different HER3 expression, we used in vitro co-culture assays with human peripheral blood mononuclear cells (PBMCs) and HER3-expressing target cells to determine T-cell activation, T-cell proliferation and PBMC-mediated cancer cell lysis of HER3-positive cell lines by the trivalent, bispecific antibodies.

Results

In this study, we developed trivalent, bispecific antibodies comprising a silenced Fc region for T-cell retargeting to HER3-expressing tumor cells, combining a bivalent single-chain diabody (scDb) fused to a first heterodimerizing Fc chain with either an Fab or scFv fused to a second heterodimerizing Fc chain. All these HER3-targeting T-cell engagers comprising two binding sites for HER3 and one binding site for CD3 mediated target cell killing. However, format and orientation of binding sites influenced efficacy of target cell binding, target cell-dependent T-cell activation and T-cell-mediated target cell killing. Beneficial effects were seen when the CD3 binding site was located in the scDb moiety. These molecules showed efficient killing of medium HER3-expressing cancer cells with very low induction of cytokine release, while sparing target cells with low or undetectable HER3 expression.

Conclusion

Our study demonstrates that these trivalent, bispecific antibodies represent formats with superior interdomain spacing resulting in efficient target cell killing and a potential advantageous safety profile due to very low cytokine release.

A scDb-based trivalent bispecific antibody for T-cell-mediated killing of HER3-expressing cancer cells

HER3 is a member of the EGF receptor family and elevated expression is associated with cancer progression and therapy resistance. HER3-specific T-cell engagers might be a suitable treatment option to circumvent the limited efficacy observed for HER3-blocking antibodies in clinical trials. In this study, we developed bispecific antibodies for T-cell retargeting to HER3-expressing tumor cells, utilizing either a single-chain diabody format (scDb) with one binding site for HER3 and one for CD3 on T-cells or a trivalent bispecific scDb-scFv fusion protein exhibiting an additional binding site for HER3. The scDb-scFv showed increased binding to HER3-expressing cancer cell lines compared to the scDb and consequently more effective T-cell activation and T-cell proliferation. Furthermore, the bivalent binding mode of the scDb-scFv for HER3 translated into more potent T-cell mediated cancer cell killing, and allowed to discriminate between moderate and low HER3-expressing target cells. Thus, our study demonstrated the applicability of HER3 for T-cell retargeting with bispecific antibodies, even at moderate expression levels, and the increased potency of an avidity-mediated specificity gain, potentially resulting in a wider safety window of bispecific T-cell engaging antibodies targeting HER3.

The making of bispecific antibodies

During the past two decades we have seen a phenomenal evolution of bispecific antibodies for therapeutic applications. The 'zoo' of bispecific antibodies is populated by many different species, comprising around 100 different formats, including small molecules composed solely of the antigen-binding sites of two antibodies, molecules with an IgG structure, and large complex molecules composed of different antigen-binding moieties often combined with dimerization modules. The application of sophisticated molecular design and genetic engineering has solved many of the technical problems associated with the formation of bispecific antibodies such as stability, solubility and other parameters that confer drug properties. These parameters may be summarized under the term 'developability'. In addition, different 'target product profiles', i.e., desired features of the bispecific antibody to be generated, mandates the need for access to a diverse panel of formats. These may vary in size, arrangement, valencies, flexibility and geometry of their binding modules, as well as in their distribution and pharmacokinetic properties. There is not 'one best format' for generating bispecific antibodies, and no single format is suitable for all, or even most of, the desired applications. Instead, the bispecific formats collectively serve as a valuable source of diversity that can be applied to the development of therapeutics for various indications. Here, a comprehensive overview of the different bispecific antibody formats is provided.

Pharmacokinetic properties of IgG and various Fc fusion proteins in mice

Fusion to an IgG Fc region is an established strategy to extend the half-life of therapeutic proteins. Most Fc fusion proteins, however, do not achieve the long half-life of IgGs. Based on findings that scFv-Fc fusion proteins exhibit a shorter half-life than the corresponding IgG molecules, we performed a comparative study of different antibody-derived Fc fusion proteins. We could confirm that fusion of single-chain Fv (scFv) and single-chain diabody (scDb) molecules to an Fc region yields in fusion proteins with substantially extended half-lives compared with the single-chain versions. However, even fusion proteins with a size similar to that of IgG, e.g., scDb-Fc, did not have a half-life as long as an IgG molecule. Binding to the neonatal Fc receptor (FcRn) under acidic and neutral conditions was similar for IgG and all Fc fusion proteins. However, we observed differences between IgG and the Fc fusion proteins for dissociation of FcRn-bound proteins induced by shifting from acidic to neutral pH, reflecting the physiological release mechanism, further supporting a contribution of the kinetics of pH-dependent release from FcRn to the pharmacokinetic properties of IgG and Fc fusion proteins.

A tale of two specificities: bispecific antibodies for therapeutic and diagnostic applications

•

Recombinant DNA technologies are leading the rapid expansion of bispecific antibody formats.

•

The therapeutic potential of bispecific antibodies is being realized through creative design.

•

Bispecific antibodies are potentially underutilized reagents for diagnostics.

Artificial manipulation of antibody genes has facilitated the production of several unique recombinant antibody formats, which have highly important therapeutic and biotechnological applications. Although bispecific antibodies (bsAbs) are not new, they are coming to the forefront as our knowledge of the potential efficacy of antibody-based therapeutics expands. The next generation of bsAbs is developing due to significant improvements in recombinant antibody technologies. This review focuses on recent advances with a particular focus on improvements in format and design that are contributing to the resurgence of bsAbs, and in particular, on innovative structures applicable to next generation point-of-care (POC) devices with applicability to low resource environments.

Positron emission tomography imaging of CD105 expression during tumor angiogenesis

PURPOSE

Overexpression of CD105 (endoglin) correlates with poor prognosis in many solid tumor types. Tumor microvessel density (MVD) assessed by CD105 staining is the current gold standard for evaluating tumor angiogenesis in the clinic. The goal of this study was to develop a positron emission tomography (PET) tracer for imaging CD105 expression.

METHODS

TRC105, a chimeric anti-CD105 monoclonal antibody, was conjugated to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and labeled with (64)Cu. FACS analysis and microscopy studies were performed to compare the CD105 binding affinity of TRC105 and DOTA-TRC105. PET imaging, biodistribution, blocking, and ex vivo histology studies were performed on 4T1 murine breast tumor-bearing mice to evaluate the ability of (64)Cu-DOTA-TRC105 to target tumor angiogenesis. Another chimeric antibody, cetuximab, was used as an isotype-matched control.

RESULTS

FACS analysis of human umbilical vein endothelial cells (HUVECs) revealed no difference in CD105 binding affinity between TRC105 and DOTA-TRC105, which was further validated by fluorescence microscopy. (64)Cu labeling was achieved with high yield and specific activity. Serial PET imaging revealed that the 4T1 tumor uptake of the tracer was 8.0 ± 0.5, 10.4 ± 2.8, and 9.7 ± 1.8%ID/g at 4, 24, and 48 h post-injection, respectively (n = 3), higher than most organs at late time points which provided excellent tumor contrast. Biodistribution data as measured by gamma counting were consistent with the PET findings. Blocking experiments, control studies with (64)Cu-DOTA-cetuximab, as well as ex vivo histology all confirmed the in vivo target specificity of (64)Cu-DOTA-TRC105.

CONCLUSION

This is the first successful PET imaging study of CD105 expression. Fast, prominent, persistent, and CD105-specific uptake of the tracer in the 4T1 tumor was observed. Further studies are warranted and currently underway.

Bispecific antibodies for cancer therapy

Bispecific antibodies, in contrast to conventional monoclonal antibodies, can bind simultaneously two different antigens. Taking advantage of this virtue, they are mostly designed for immune effector cell redirection to tumors and for radionuclide pretargeting to tumors. Bispecific antibodies of the first generation were produced by chemical cross-linking or cell-fusion technologies. More recently, the application of genetic engineering technologies gave rise to numerous formats of bispecific antibody fragments and whole IgG molecules. Because bispecific antibodies enable therapeutic strategies that are not possible with conventional monoclonal antibodies, they attract strong interest. Several bispecific antibody formats have already shown clinical efficacy in cancer patients, catalyzing efforts to translate the imaginative bispecific antibody concepts into effective therapies.

The antigenome: from protein subunit vaccines to antibody treatments of bacterial infections?

New strategies are needed to master infectious diseases. The so-called "passive vaccination", i.e., prevention and treatment with specific antibodies, has a proven record and potential in the management of infections and entered the medical arena more than 100 years ago. Progress in the identification of specific antigens has become the hallmark in the development of novel subunit vaccines that often contain only a single immunogen, frequently proteins, derived from the microbe in order to induce protective immunity. On the other hand, the monoclonal antibody technology has enabled biotechnology to produce antibody species in unlimited quantities and at reasonable costs that are more or less identical to their human counterparts and bind with high affinity to only one specific site of a given antigen. Although, this technology has provided a robust platform for launching novel and successful treatments against a variety of devastating diseases, it is up till now only exceptionally employed in therapy of infectious diseases. Monoclonal antibodies engaged in the treatment of specific cancers seem to work by a dual mode; they mark the cancerous cells for decontamination by the immune system, but also block a function that intervenes with cell growth. The availability of the entire genome sequence of pathogens has strongly facilitated the identification of highly specific protein antigens that are suitable targets for neutralizing antibodies, but also often seem to play an important role in the microbe's life cycle. Thus, the growing repertoire of well-characterized protein antigens will open the perspective to develop monoclonal antibodies against bacterial infections, at least as last resort treatment, when vaccination and antibiotics are no options for prevention or therapy. In the following chapter we describe and compare various technologies regarding the identification of suitable target antigens and the foundation of cognate monoclonal antibodies and discuss their possible applications in the treatment of bacterial infections together with an overview of current efforts.

Targeting cancer vasculature via endoglin/CD105: a novel antibody-based diagnostic and therapeutic strategy in solid tumours

Endoglin/CD105 is well acknowledged as being the most reliable marker of proliferation of endothelial cells, and it is overexpressed on tumour neovasculature. Our current knowledge of its structure, physiological role, and tissue distribution suggests that targeting of endoglin/CD105 is a novel and powerful diagnostic and therapeutic strategy in human malignancies, through the imaging of tumour-associated angiogenesis and the inhibition of endothelial cell functions related to tumour angiogenesis. Among biotherapeutic agents, monoclonal antibodies have shown a major impact on the clinical course of human malignancies of different histotypes. Along this line, the potential efficacy of anti-endoglin/CD105 antibodies and their derivatives for clinical purposes in cancer is supported by a large body of available pre-clinical in vitro and in vivo data. In this review, the main findings supporting the translation of antibody-based endoglin/CD105 targeting from pre-clinical studies to clinical applications in human cancer are summarized and discussed.

Murine endoglin-specific single-chain Fv fragments for the analysis of vascular targeting strategies in mice

Endoglin has been identified as a promising cell surface antigen for vascular targeting approaches in cancer therapy, e.g. employing antibody molecules as targeting moieties. However, in vivo analysis of such strategies in mouse models requires antibodies recognizing endoglin on mouse endothelial cells. Here we describe the isolation of single-chain Fv fragments (scFvs) from phage display libraries, which bind to the extracellular region of mouse endoglin. One of these clones, scFv mE12, showed strong (K(d)=11 nM) and selective binding to purified endoglin and also to the endoglin-expressing mouse endothelioma cell line eEnd.2. This antibody recognized a linear epitope located in the N-terminal region (aa 27-361) of endoglin. Cell binding was further increased by generating a bivalent scFv-Fc fusion protein composed of scFv mE12 and the human gamma1 Fc part. Moreover, scFv mE12 was endowed with an additional cysteine residue in the linker region and applied for the generation of anti-endoglin scFv immunoliposomes capable of selectively binding to endoglin-expressing cells. Thus, anti-mouse endoglin scFv mE12 should be useful to analyze vascular targeting strategies in mice.

Anti-cancer therapies targeting the tumor stroma

For anti-tumor therapy different strategies have been employed, e.g., radiotherapy, chemotherapy, or immunotherapy. Notably, these approaches do not only address the tumor cells themselves, but also the tumor stroma cells, e.g., endothelial cells, fibroblasts, and macrophages. This is of advantage, since these cells actively contribute to the proliferative and invasive behavior of the tumor cells via secretion of growth factors, angiogenic factors, cytokines, and proteolytic enzymes. In addition, tumor stroma cells take part in immune evasion mechanisms of cancer. Thus, approaches targeting the tumor stroma attract increasing attention as anti-cancer therapy. Several molecules including growth factors (e.g., VEGF, CTGF), growth factor receptors (CD105, VEGFRs), adhesion molecules (alphavbeta3 integrin), and enzymes (CAIX, FAPalpha, MMPs, PSMA, uPA) are induced or upregulated in the tumor microenvironment which are otherwise characterized by a restricted expression pattern in differentiated tissues. Consequently, these molecules can be targeted by inhibitors as well as by active and passive immunotherapy to treat cancer. Here we discuss the results of these approaches tested in preclinical models and clinical trials.

Bispecific Antibodies: Molecules That Enable Novel Therapeutic Strategies

Bispecific antibodies are unique in the sense that they can bind simultaneously two different antigens. This property enables the development of therapeutic strategies that are not possible with conventional monoclonal antibodies. The large panel of imaginative bispecific antibody formats that has been developed reflects the strong interest for these molecules. Although in many cases the manufacturing of clinical grade material remains challenging, several bispecific antibody formats are currently in clinical trials.

Antibody-based antiangiogenic cancer therapy

Monoclonal antibodies are beginning to fulfil their potential as therapeutics in different pathological conditions, with a special focus on cancer. At the same time, antiangiogenic therapy has evolved into one of the most active fields in cancer research. The logical combination of both strategies has produced a growing number of antibodies aimed to interfere with tumour angiogenesis at different steps of the angiogenic process. This development has taken advantage of recombinant technologies that have revolutionised the selection and production of monoclonal antibodies, and gene therapy approaches that achieve sustained and effective concentrations of therapeutic antibodies in vivo. Here, the current knowledge about these antibody-based antiangiogenic strategies is summarised and the authors propose a novel therapeutic approach based on the blocking of crucial binding sites present in the extracellular matrix.

Advances in monoclonal antibody technology: genetic engineering of mice, cells, and immunoglobulins

The ability to produce antibodies that are directed against specific antigens has played a crucial role in advancing scientific discoveries. Recombinant technologies have extended the application of antibodies beyond the research laboratory and into the clinic for the treatment of cancer and other diseases. Creative approaches using these technologies have been used to reduce the antibody to its minimal functional size, and/or make them bifunctional (immunotoxins), bispecific, or less immunoreactive (humanized). Additionally, mice that are engineered to generate antibodies of human genomic origin have been used to produce therapeutic antibodies and are being further developed. As the research and clinical demands for antibodies continue to increase, the development of improved resources (cell lines and animals) to improve production efficiency, generate larger repertoires, and deliver greater yields of antibodies is being explored, and advances in this area are discussed further in this review.

Antibody phage display technologies with special reference to angiogenesis

The presence of blood vessels is a prerequisite for normal development, tissue growth, and tissue repair. However, its abnormal occurrence or absence can also potentiate disease processes. Angiogenic therapies have been used to stimulate blood vessel growth in ischemic conditions such as severe end-stage peripheral vascular disease, ischemic heart disease and stroke and for inhibition of angiogenesis in tumors. The targeting and identification of novel endothelial cell (EC) markers that can ultimately be used in angiogenic strategies is an expanding field but is limited by the availability of reagents. For instance repeated injection of mouse monoclonal antibodies (Mabs) against angiogenic EC, can result in the production of autoantibodies. Therefore, these mouse Mabs cannot be used for therapeutic purposes. Phage display technology was employed in this context to select antibodies, proteins, and peptides against known or novel EC antigens. Furthermore, technologies have been developed that enable the specific targeting of epitopes on cells including the endothelium with high-affinity/avidity antibodies. The focus for these antibody targeting strategies are markers that are unique or up-regulated on angiogenic EC including the vascular endothelial growth factor receptor (VEGFR) KDR, endoglin (CD105), and the extracellular domain B (ED-B) domain of fibronectin (FN). These markers are reviewed herein.

Recombinant bispecific antibodies for cancer therapy

Bispecific antibodies can serve as mediators to retarget effector mechanisms to disease-associated sites. Studies over the past two decades have revealed the potentials but also the limitations of conventional bispecific antibodies. The development of recombinant antibody formats has opened up the possibility of generating bispecific molecules with improved properties. This review summarizes recent developments in the field of recombinant bispecific antibodies and discusses further requirements for clinical development.

Highlights on endoglin (CD105): from basic findings towards clinical applications in human cancer

Antibody targeting of tumor-associated vasculature is a promising therapeutic approach in human cancer; however, a specific cell membrane marker for endothelial cells of tumor vasculature has not been discovered yet. Endoglin (CD105) is a cell-surface glycoprotein most recently identified as an optimal indicator of proliferation of human endothelial cells. The finding that CD105 is over-expressed on vascular endothelium in angiogenetic tissues has prompted several pre-clinical studies designed to get a deeper understanding on the role of CD105 in angiogenesis, and to evaluate the most appropriate clinical setting(s) to utilize CD105 as a therapeutic target. In this review, the foreseeable clinical applications of CD105 in human cancer are discussed.

Isolation of endothelial cell-specific human antibodies from a novel fully synthetic scFv library

We have isolated single-chain Fv fragments directed against human endothelial cells from a novel fully synthetic human scFv library (scFv 479). This library was constructed using the variable germline segments DP47 and DPkappa9 as scaffolds. Complementarity determining regions 3 (CDR) of the variable heavy and light chain were introduced with a length of 9 amino acid residues. In total, 16 amino acid positions of all six CDRs exposed in the antigen-binding site were randomized and the library was produced from synthetic oligonucleotides encoding the entire scFv fragment. From this library endothelial-specific scFv fragments were either selected using the recombinant extracellular domain of human endoglin (CD105) or by cell selections with human dermal microvascular endothelial cells (HDMEC). These scFv fragments might be useful for the generation of vascular or tumor targeting agents in cancer therapy.