Mikhail Popkov

Rabbit Immune Repertoires as Sources for Therapeutic Monoclonal Antibodies: The Impact of Kappa Allotype-correlated Variation in Cysteine Content on Antibody Libraries Selected by Phage Display

The rabbit immune repertoire has long been a rich source of diagnostic polyclonal antibodies. Now it also holds great promise as a source of therapeutic monoclonal antibodies. On the basis of phage display technology, we recently reported the first humanization of a rabbit monoclonal antibody. The allotypic diversity of rabbit immunoglobulins prompted us to compare different rabbit immune repertoires for the generation and humanization of monoclonal antibodies that bind with strong affinity to antigens involved in tumor angiogenesis. In particular, we evaluated the diversity of unselected and selected chimeric rabbit/human Fab libraries that were derived from different kappa light chain allotypes. Most rabbit light chains have an extra disulfide bridge that links the variable and constant domains in addition to the two intrachain disulfide bridges shared with mouse and human kappa light chains. Here we evaluate the impact of this increased disulfide bridge complexity on the generation and selection of chimeric rabbit/human Fab libraries. We demonstrate that rabbits with mutant bas and wild-type parental b9 allotypes are excellent sources for therapeutic monoclonal antibodies. Featured among the selected clones with b9 allotype is a rabbit/human Fab that binds with a dissociation constant of 1nM to both human and mouse Tie-2, which will facilitate its evaluation in mouse models of human cancer. Examination of 228 new rabbit antibody sequences allowed for a comprehensive comparison of the LCDR3 and HCDR3 length diversity in rabbits. This study revealed that rabbits exhibit an HCDR3 length distribution more closely related to human antibodies than mouse antibodies.

Intradiabodies, Bispecific, Tetravalent Antibodies for the Simultaneous Functional Knockout of Two Cell Surface Receptors

The specific and high affinity binding properties of intracellular antibodies (intrabodies), combined with their ability to be stably expressed in defined organelles, provides powerful tools with a wide range of applications in the field of functional genomics and gene therapy. Intrabodies have been used to specifically target intracellular proteins, manipulate biological processes, and contribute to the understanding of their functions as well as for the generation of phenotypic knockouts in vivo by surface depletion of extracellular or transmembrane proteins. In order to study the biological consequences of knocking down two receptor-tyrosine kinases, we developed a novel intrabody-based strategy. Here we describe the design, engineering, and characterization of a bispecific, tetravalent endoplasmic reticulum (ER)-targeted intradiabody for simultaneous surface depletion of two endothelial transmembrane receptors, Tie-2 and vascular endothelial growth factor receptor 2 (VEGF-R2). Comparison of the ER-targeted intradiabody with the corresponding conventional ER-targeted single-chain antibody fragment (scFv) intrabodies demonstrated that the intradiabody is significantly more efficient with respect to efficiency and duration of surface depletion of Tie-2 and VEGF-R2. In vitro endothelial cell tube formation assays suggest that the bispecific intradiabody exhibits strong antiangiogenic activity, whereas the effect of the monospecific scFv intrabodies was weaker. These findings suggest that simultaneous interference with the VEGF and the Tie-2 receptor pathways results in at least additive antiangiogenic effects, which may have implications for future drug developments. In conclusion, we have identified a highly effective ER-targeted intrabody format for the simultaneous functional knockout of two cell surface receptors.

Chemically programmed monoclonal antibodies for cancer therapy: Adaptor immunotherapy based on a covalent antibody catalyst

Proposing that a blend of the chemical diversity of small synthetic molecules with the immunological characteristics of the antibody molecule will lead to therapeutic agents with superior properties, we here present a device that equips small synthetic molecules with both effector function and long serum half-life of a generic antibody molecule. As a prototype, we developed a targeting device that is based on the formation of a covalent bond of defined stoichiometry between a 1,3-diketone derivative of an integrin αvβ3 and αvβ5 targeting Arg-Gly-Asp peptidomimetic and the reactive lysine of aldolase antibody 38C2. The resulting complex was shown to (i) spontaneously assemble in vitro and in vivo, (ii) selectively retarget antibody 38C2 to the surface of cells expressing integrins αvβ3 and αvβ5, (iii) dramatically increase the circulatory half-life of the Arg-Gly-Asp peptidomimetic, and (iv) effectively reduce tumor growth in animal models of human Kaposis sarcoma and colon cancer. This immunotherapeutic has the potential to target a variety of human cancers, acting on both the vasculature that supports tumor growth as well as the tumor cells themselves. Further, by use of a generic antibody molecule that forms a covalent bond with a 1,3-diketone functionality, essentially any compound can be turned into an immunotherapeutic agent thereby not only increasing the diversity space that can be accessed but also multiplying the therapeutic effect.

Integrin αvβ3 targeted therapy for Kaposi's sarcoma with an in vitro evolved antibody

Here, we define integrin αvβ3 as a molecular target for antibody therapy for Kaposis sarcoma (KS). We previously reported, using a new phage display strategy based on designed combinatorial V gene libraries, the humanization of mouse monoclonal antibody LM609 directed to human integrin αvβ3. In the present study, we describe the in vitro affinity maturation of humanized LM609 by using a phage display strategy for the sequential and parallel optimization of three complementarity determining regions of the antibody molecule. The evolved Fab had an affinity of 150 pM and was converted into IgG1 by use of a new mammalian expression vector. The resulting whole antibody, designated JC‐7U IgG1, was found to selectively target human KS in a nude mouse model and inhibit tumor growth at a therapeutically relevant dose. Because of its high affinity and its high degree of humanization, JC‐7U IgG1 is an excellent drug candidate for therapeutic applications that involve integrin αvβ3 as the molecular target. Of particular interest is therapy for KS, breast cancer, melanoma, and other cancers in which integrin αvβ3 is expressed on both angiogenic endothelial cells and tumor cells, which would allow a dual antiangiogenic and antitumor strike with a single drug.

Small molecule drug activity in melanoma models may be dramatically enhanced with an antibody effector

Monoclonal antibody (mAb) 38C2 belongs to a group of catalytic antibodies that were generated by reactive immunization and contains a reactive lysine. 38C2 catalyzes aldol and retro‐aldol reactions, using an enamine mechanism, and mechanistically mimics natural aldolase enzymes. In addition, mAb 38C2 can be redirected to target integrins αvβ3 and αvβ5 through the formation of a covalent bond between a β‐diketone derivative of an arginine–glycine–aspartic acid (RGD) peptidomimetic and the reactive lysine residue in the antibody combining site to provide the chemically programmed mAb cp38C2. In this study, we investigated the potential of enhancing the activity of receptor‐binding small molecule drug (SCS‐873) through antibody conjugation. Using a M21 human melanoma xenograft model in nude mice, cp38C2 inhibited the growth of the tumor by ˜81%. The chemically programmed antibody was shown to be highly active at a low concentration while SCS‐873 alone was ineffective even at dosages ˜1,000‐fold higher than those used for the chemically programmed antibody. In vitro programming of the catalytic antibody was shown to be as effective as in vivo programming. In an experimental metastasis assay, treatment with mAb cp38C2 significantly prolonged overall survival of tumor‐bearing severe combined immuno‐deficient (SCID) mice when compared to treatment with unprogrammed mAb 38C2, SCS‐873 alone or the integrin‐specific monoclonal antibody LM609. In vitro, cp38C2 inhibited human and mouse endothelial and human melanoma cell adhesion, migration and invasion. Additionally, cp38C2 inhibited human and mouse endothelial cell proliferation and was active in complement‐dependent cytotoxicity assays. These studies establish the potential of chemically programmed monoclonal antibodies as a novel and effective class of immunotherapeutics that combine the merits of traditional small molecule drug design with immunotherapy.

Instant immunity through chemically programmable vaccination and covalent self-assembly

The ability to instantly create a state of immunity as achieved in the passive transfer of hyperimmune globulin has had a tremendous impact on public health. Unlike passive immunization, active immunization, which is the foundation of vaccinology, is an anticipatory strategy with inherent limitations. Here we show that elements of active and passive immunization can be combined to create an effective chemistry-driven approach to vaccinology. Reactive immunization was used to create a reservoir of covalent polyclonal antibodies in 3 mouse strains that were subsequently engrafted with syngeneic CT26 colon or B16F10 melanoma tumors. Upon administration of designed integrin αvβ3 and αvβ5 adapter ligands, the induced covalent polyclonal antibodies self-assembled with the adapter ligands and the animals mounted an instant, chemically programmed, polyclonal response against the implanted tumors. Significant therapeutic responses were observed without recourse to adjuvant therapy. The chemically programmed immune responses were driven by antibody-dependent cellular cytotoxicity and complement-directed cytotoxicity. We suggest that this type of chemistry-driven approach to vaccinology is underexplored and may provide routes to vaccines to protect against diseases that have proven intractable to biology-driven vaccine approaches.

Integrin αIIbβ3-specific synthetic human monoclonal antibodies and hcdr3 peptides that potently inhibit platelet aggregation1

The interaction of fibrinogen with integrin αIIbβ3 (GPIIb/IIIa), in part mediated by an RGD tripeptide motif, is an essential step in platelet aggregation. Based on their inhibition of platelet aggregation, three integrin αIIbβ3 inhibitors are clinically approved. The clinically most widely used integrin αIIbβ3 inhibitor abciximab is a chimeric mouse/human antibody that induces thrombocytopenia, often severe, in 1–2% of patients due to a human anti‐mouse antibody (HAMA) response. In addition, unlike other ligands mimicking small molecular drugs, abciximab cross‐reacts with integrin αvβ3 and αMβ2. Here we used phage display to select monoclonal antibodies specific to integrin αIIbβ3 from a synthetic human antibody library based on the randomized HCDR3 sequence VGXXXRADXXXYAMDV. The selected antibodies revealed a strong consensus in HCDR3 (V(V/W)CRAD(K/R)RC) and high specificity toward integrin αIIbβ3 but not to other RGD binding integrins such as αvβ3, αvβ5, and α5β1. The selected antibodies as well as three synthetic peptides (VWCRADRRC, VWCRADKRC, and VVCRADRRC) whose sequences were derived from the HCDR3 sequences of the selected antibodies strongly inhibited the interaction between integrin αIIbβ3 and fibrinogen and platelet aggregation ex vivo. To our knowledge, these are the first fully human monoclonal antibodies that are specific to integrin αIIbβ3 and can potently inhibit platelet aggregation.

Zinc Finger Transcription Factors Designed for Bispecific Coregulation of ErbB2 and ErbB3 Receptors: Insights into ErbB Receptor Biology

ABSTRACT Signaling through the ErbB family of tyrosine kinase receptors in normal and cancer-derived cell lines contributes to cell growth and differentiation. In this work, we altered the levels of ErbB2 and ErbB3 receptors, individually and in combination, by using 6-finger and 12-finger synthetic zinc finger protein artificial transcription factors (ATFs) in an epidermoid squamous cell carcinoma line, A431. We successfully designed 12-finger ATFs capable of coregulating ErbB3 and ICAM-1 or ErbB2 and ErbB3. With ATFs, the effects of changes in ErbB2 and ErbB3 receptor levels were evaluated by using cell proliferation, cell migration, and cell signaling assays. Cell proliferation was increased when ErbB2 and ErbB3 were both overexpressed. Cell migration on collagen was decreased when ErbB2 was down-regulated, yet migration on laminin was significantly increased with ErbB3 overexpression. ErbB2 and ErbB3 overexpression also stimulated the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways. Our ATF approach has elucidated differences in ErbB receptor-mediated proliferation, migration, and intracellular signaling that cannot be explained merely by the presence or absence of particular ErbB receptors and emphasizes the dynamic nature of the ErbB signaling system. The transcription factor approach developed here provides a gene-economical route to the regulation of multiple genes and may be important for complex gene therapies.

Rapid monoclonal antibody generation via dendritic cell targeting in vivo.

Dendritic cells (DC) are the professional antigen-presenting cells of the immune system. Previous studies have demonstrated that targeting foreign antigens to DC leads to enhanced antigen (Ag)-specific responses in vivo. However, the utility of this strategy for the generation of MAbs has not been investigated. To address this question we immunized mice with IgG-peptide conjugates prepared with the hamster anti-murine CD11c MAb N418. Synthetic peptides corresponding to two different exposed regions of DC-specific ICAM-3 grabbing nonintegrin (DC-SIGN), a human C-type lectin, were conjugated to N418 using thiol-based chemistry. The N418 MAb served as the targeting molecule and synthetic peptides as the Ag (MAb-Ag). A rapid and peptide specific serum IgG response was produced by Day 7 when the synthetic peptides were linked to the N418 MAb, compared to peptide co-delivered with the N418 without linkage. Spleen cells from N418-peptide immunized mice were fused on Day 10, and three IgG1/k monoclonal antibodies (MAbs) were selected to one of the peptide epitopes (MID-peptide). One of the MAbs, Novik 2, bound to two forms of recombinant DC-SIGN protein in enzyme-linked immunosorbent assay (ELISA), and was specifically inhibited by the MID-peptide in solution. Two of these MAbs show specific binding to DC-SIGN expressed by cultured human primary DC. We conclude that in vivo DC targeting enhances the immunogenicity of synthetic peptides and is an effective method for the rapid generation of MAbs to predetermined epitopes.

Recent Innovations and Analytical Applications of Phage Display Libraries

Abstract Filamentous phages are used widely for research and analytical technology development. This is due to the unique biology of the f-phages including its single stranded DNA genome and ability to display proteins on the surface of the intact phage particle, which have advanced the fields of recombinant DNA technology and molecular diversity technologies, respectively. Herein we review recent trends in phage display technologies that allow the evolution of selected binders into optimized molecules. Using this technology large polypeptide libraries are presented on the surface of filamentous phages. The selection of peptides and antibodies from phage display libraries has proven significant for routine isolation of peptides and antibodies for diagnostic and therapeutic applications.