Novel Hybrid Compound of a Plinabulin Prodrug with an IgG Binding Peptide for Generating a Tumor Selective Noncovalent-Type Antibody-Drug Conjugate

Structure Derivatization of IgG-Binding Peptides and Analysis of Their Secondary Structure by Circular Dichroism Spectroscopy

Mid-sized cyclic peptides are a promising modality for modern drug discovery. Their larger interaction area coupled with an appropriate secondary structure is more suitable than small molecules for binding to the target protein. In this study, we conducted a structure derivatization of an immunoglobulin G (IgG)-binding peptide (15-IgBP), a β-hairpin-like cyclic peptide with a twisted β-strand and assessed the effect of the secondary structure on IgG-binding activity using circular dichroism (CD) spectra analysis. As a result, derivatization at the Ala5 and Gly9 positions affected the secondary structure of 15-IgBP, in particular the appearance of a small positive peak in the 220-240 nm region characteristic of 15-IgBP in the CD spectrum. Maintaining this peak at a moderate level may be important for the expression of IgG binding activity. We found the small methyl group at Ala5 to be crucial for retaining the preferred secondary structure; we also found Gly9 could be replaced by D-amino acids. By integrating these findings with previous results of the structure-activity relationship, we obtained four potent affinity peptides for IgG binding (Kd = 4.24-5.85 nM). Furthermore, we found the Gly9 position can be substituted for D-Lys. This is a new potential site for attaching functional units for conjugation with IgG for the preparation of homogeneous antibody-drug conjugates.

Natural killer cell-mimic nanoparticles can actively target and kill acute myeloid leukemia cells

Natural killer (NK) cells play a crucial role in recognizing and killing emerging tumor cells. However, tumor cells develop mechanisms to inactivate NK cells or hide from them. Here, we engineered a modular nanoplatform that acts as NK cells (NK cell-mimics), carrying the tumor-recognition and death ligand-mediated tumor-killing properties of an NK cell, yet without being subject to tumor-mediated inactivation. NK cell mimic nanoparticles (NK.NPs) incorporate two key features of activated NK cells: cytotoxic activity via the death ligand, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), and an adjustable tumor cell recognition feature based on functionalization with the NK cell Fc-binding receptor (CD16, FCGR3A) peptide, enabling the NK.NPs to bind antibodies targeting tumor antigens. NK.NPs showed potent in vitro cytotoxicity against a broad panel of cancer cell lines. Upon functionalizing the NK.NPs with an anti-CD38 antibody (Daratumumab), NK.NPs effectively targeted and eliminated CD38-positive patient-derived acute myeloid leukemia (AML) blasts ex vivo and were able to target and kill CD38-positive AML cells in vivo, in a disseminated AML xenograft system and reduced AML burden in the bone marrow compared to non-targeted, TRAIL-functionalized liposomes. Taken together, NK.NPs are able to mimicking key antitumorigenic functions of NK cells and warrant their development into nano-immunotherapeutic tools.

IgG Fc Affinity Ligands and Their Applications in Antibody-Involved Drug Delivery: A Brief Review

Antibodies are not only an important class of biotherapeutic drugs, but also are targeting moieties for achieving active targeting drug delivery. Meanwhile, the rapidly increasing application of antibodies and Fc-fusion proteins has inspired the emerging development of downstream processing technologies. Thus, IgG Fc affinity ligands have come into being and have been widely exploited in antibody purification strategies. Given the high binding affinity and specificity to IgGs, binding stability in physiological medium conditions, and favorable toxicity and immunogenicity profiles, Fc affinity ligands are gradually applied to antibody delivery, non-covalent antibody-drug conjugates or antibody-mediated active-targeted drug delivery systems. In this review, we will briefly introduce IgG affinity ligands that are widely used at present and summarize their diverse applications in the field of antibody-involved drug delivery. The challenges and outlook of these systems are also discussed.

[Development of Synthetic Methodology for Mid-size Peptide Based on Disulfide Bond Formation]

Disulfide bonds in peptides contribute to the immobilization and rigidity of their structures, leading to the expression of biological activity and resistance to metabolic enzymes. In addition, disulfide bonds are important in the construction of conjugates comprising two bioactive molecules such as peptides, sugars and drugs. Therefore, new methods of disulfide bond formation contribute to a more efficient construction of disulfide products. This article reviews studies on development of synthetic methodology for disulfide bond formation by using 3-nitro-2-pyridinesulfenyl (Npys) compounds. We have developed a one-pot solid-phase disulfide ligation (SPDSL) method by using an Npys resin, which can easily afford an asymmetric disulfide bond that is generated using two types of thiol-containing components such as peptides and small molecules. The disulfide-linked conjugation between a hydrophobic molecule and a hydrophilic peptide can be easily prepared. Based on the SPDSL strategy, we also developed a disulfide-driven cyclic peptide synthesis, which represents a new strategy to prepare cyclic peptides from two different fragments. By generating a disulfide bond between two fragments, the entropically favorable intramolecular amide bond formation can be achieved, resulting in the reduction of racemization at the coupling site. We found that methyl 3-nitro-2-pyridinesulfenate (Npys-OMe) functions as a disulfide bond-forming reagent possessing mildly oxidative activity. This reagent enhances intramolecular disulfide bond formation between two thiols for the synthesis of cyclic peptides under mildly acidic conditions. As the applications of Npys-OMe, we demonstrated the disulfide bond formation on thiols-containing peptidyl resin.

A self-activating nanoized vascular disrupting agent for selective anti-tumor therapy

Vascular disrupting agents (VDAs) have great potential in antitumor therapy, while the efficiency is limited by cardiovascular toxicity. In this study, a self-activating nanoized plinabulin (poly (_l-glutamic acid) grafted Azo-Plinabulin, AzoP-NP) was constructed. The AzoP-NPs can selectively be activated to an amino derivative of plinabulin (AmP) by intrinsic tumor hypoxia, disrupting tumor vessels and amplifying hypoxia, whilst be activated by self-amplified tumor hypoxia, then selectively inhibit tumor growth. In 4T1 tumor model, the AzoP-NPs had a selective biodistribution in tumor, as the free AmP in tumors at 24 h after AzoP-NPs treatment was 18.6 fold of that after AmP treatment and significantly higher than that in other tissues. Accordingly, AzoP-NPs resulted in no obvious acute cardiovascular toxicity (plasma von Willebrand factor in PBS, AzoP-NPs and AmP group: 143.1, 184.0 and 477.6 ng/mL) and a significantly stronger tumor inhibition than AmP. And the sustained release of drug in AzoP-NPs led to a higher maximum tolerated dose (MTD) (MTD of AzoP-NPs and AmP: > 80 vs 20 mg/kg). In addition, AzoP-NPs amplified tumor hypoxic, and synergized the anti-tumor effect of Tirapazamine (TPZ), a hypoxia-activated drug in clinical trials, with an inhibition rate of 97.7% and Q value of 1.89. Therefore, our findings provide new insights into next generation VDAs and their application in tumor therapy.

Cyclic Dipeptide: A Privileged Molecular Scaffold to Derive Structural Diversity and Functional Utility

Cyclic dipeptides (CDPs) are the simplest form of cyclic peptides with a wide range of applications from therapeutics to biomaterials. CDP is a versatile molecular platform endowed with unique properties such as conformational rigidity, intermolecular interactions, structural diversification through chemical synthesis, bioavailability and biocompatibility. A variety of natural products with the CDP core exhibit anticancer, antifungal, antibacterial, and antiviral activities. The inherent bioactivities have inspired the development of synthetic analogues as drug candidates and drug delivery systems. CDP plays a crucial role as conformation and molecular assembly directing core in the design of molecular receptors, peptidomimetics and fabrication of functional material architectures. In recent years, CDP has rapidly become a privileged scaffold for the design of advanced drug candidates, drug delivery agents, bioimaging, and biomaterials to mitigate numerous disease conditions. This review describes the structural diversification and multifarious biomedical applications of the CDP scaffold, discusses challenges, and provides future directions for the emerging field.

Development of a High-Affinity Antibody-Binding Peptide for Site-Specific Modification

Immunoglobulin G (IgG)-binding peptides such as 15-IgBP are convenient tools for the site-specific modification of antibodies and the preparation of homogeneous antibody-drug conjugates. A peptide such as 15-IgBP can be selectively crosslinked to the fragment crystallizable region of human IgG in an affinity-dependent manner via the ϵ-amino group of Lys8. Previously, we found that the peptide 15-Lys8Leu has a high affinity (K_d =8.19 nM) due to the presence of the γ-dimethyl group in Leu8. The primary amino group required for the crosslinking to the antibodies has, however, been lost. Here, we report the design and synthesis of a novel unnatural amino acid, 4-(2-aminoethylcarbamoyl)leucine (Aecl), which possesses both the γ-dimethyl fragment and a primary amino group. A peptide containing Aecl8 (15-Lys8Aecl) was synthesized and showed a binding affinity ten times higher (K_d =24.3 nM) than that of 15-IgBP (K_d =267 nM). Fluorescein isothiocyanate (FITC)-labeled 15-Lys8Aecl with an N-hydroxy succinimide ester at the side chain of Aecl8 (FITC-15-Lys8Aecl(OSu)) successfully labeled an antibody (trastuzumab, Herceptin^® ) with the fluorophore. This peptide scaffold has both strong binding affinity and crosslinking capability, and could be a useful tool for the selective chemical modification of antibodies with molecules of interest such as drugs.

Synthesis and biological evaluation of a monocyclic Fc-binding antibody-recruiting molecule for cancer immunotherapy

Antibody-recruiting molecules (ARMs) are bispecific molecules composed of an antibody-binding motif and a target-binding motif that redirect endogenous antibodies to target cells to elicit immune responses. To enhance the translational potential of ARMs, it is crucial to design antibody/target-binding motifs that have strong affinity and are easy to synthesize. Here, we synthesized a novel Fc-binding ARM (Fc-ARM) that targets folate receptor (FR)-positive cancer cells, Reo-3, using a recently developed monocyclic peptide 15-Lys8Leu, which binds strongly to the Fc region of an antibody. Reo-3 bound to the Fc region of the antibody with a K d of 5.8 nM, and recruited a clinically used antibody mixture to attack FR-positive IGROV-1 cells as efficiently as Fc-ARM2, in which a bicyclic Fc-binding peptide was used. These results indicate that 15-Lys8Leu, which can be synthesized readily, is suitable for various applications including the development of Fc-ARMs.

An overview of process development for antibody-drug conjugates produced by chemical conjugation technology

Introduction: We discuss chemical conjugation strategies for antibody-drug conjugates (ADCs) from an industrial perspective and compare three promising chemical conjugation technologies to produce site-specific ADCs.Areas covered: Currently, nine ADCs are commercially approved and all are produced by chemical conjugation technology. However, seven of these ADCs contain a relatively broad drug distribution, potentially limiting their therapeutic indices. In 2019, the first site-specific ADC was launched on the market by Daiichi-Sankyo. This achievement, and an analysis of clinical trials over the last decade, indicates that current industrial interest in the ADC field is shifting toward site-specific conjugation technologies. From an industrial point of view, we aim to provide guidance regarding established conjugation methodologies that have already been applied to scale-up stages. With an emphasis on highly productive, scalable, and synthetic process robustness, conjugation methodologies for ADC production is discussed herein.Expert opinion: All three chemical conjugation technologies described in this review have various advantages and disadvantages, therefore drug developers can utilize these depending on their biological and/or protein targets. The future landscape of the ADC field is also discussed.

Immunotoxin Screening System: A Rapid and Direct Approach to Obtain Functional Antibodies with Internalization Capacities

Toxins, while harmful and potentially lethal, have been engineered to develop potent therapeutics including cytotoxins and immunotoxins (ITs), which are modalities with highly selective targeting capabilities. Currently, three cytotoxins and IT are FDA-approved for treatment of multiple forms of hematological cancer, and additional ITs are tested in the clinical trials or at the preclinical level. For next generation of ITs, as well as antibody-mediated drug delivery systems, specific targeting by monoclonal antibodies is critical to enhance efficacies and reduce side effects, and this methodological field remains open to discover potent therapeutic monoclonal antibodies. Here, we describe our application of engineered toxin termed a cell-based IT screening system. This unique screening strategy offers the following advantages: (1) identification of monoclonal antibodies that recognize cell-surface molecules, (2) selection of the antibodies that are internalized into the cells, (3) selection of the antibodies that induce cytotoxicity since they are linked with toxins, and (4) determination of state-specific activities of the antibodies by differential screening under multiple experimental conditions. Since the functional monoclonal antibodies with internalization capacities have been identified successfully, we have pursued their subsequent modifications beyond antibody drug conjugates, resulting in development of immunoliposomes. Collectively, this screening system by using engineered toxin is a versatile platform, which enables straight-forward and rapid selection for discovery of novel functional antibodies.

Antibody-Based Receptor Targeting Using an Fc-Binding Peptide-Dodecaborate Conjugate and Macropinocytosis Induction for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a radiation method used for cancer therapy. Cellular uptake of boron-10 (¹⁰B) atoms induces cancer cell death by the generation of alpha particles and recoiling lithium-7 (⁷Li) nuclei when the cells are irradiated with low-energy thermal neutrons. Current BNCT technology shows effective therapeutic benefits in refractory cancers such as brain tumors and head and neck cancers. However, improvements to cancer targeting and the cellular uptake efficacy of the boron compounds and the expansion of the diseases treatable by BNCT are highly desirable. In this research, we aimed to develop an antibody-based drug delivery method for BNCT through the use of the Z33 peptide, which shows specific recognition of and interaction with the Fc domain of human IgG, for on-demand receptor targeting. In addition, we determined with an in vitro assay that macropinocytosis induction during antibody-based drug delivery is crucial for the biological activity of BNCT.

The Chemical Methods of Disulfide Bond Formation and Their Applications to Drug Conjugates

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The disulfide bond possesses unique chemical and biophysical properties which distinguish it as one of the key structural elements of bioactive proteins and peptides, important drugs and other materials. The chemo-selective synthesis of these structures and the exploration of their function have been of longstanding interest to the chemistry community. The past decades have witnessed significant progress in both areas. This review will summarize the historically established and recently developed chemical methods in disulfide bond formation. The discussion will also be extended to the use of the disulfide linkers in small molecules, and peptide- and protein-drug conjugates. It is hoped that the combined overview of the fundamental chemistries and applications to drug discovery will inspire creative thinking and stimulate future novel uses of these versatile chemistries.

Synthesis and structure-activity relationship studies of IgG-binding peptides focused on the C-terminal histidine residue

Currently, IgG-binding peptides are widely utilized as a research tool, as molecules that guide substrates to the Fc site for site-selective antibody modification, leading to preparation of a homogeneous antibody-drug conjugate. In this study, a structure-activity relationship study of an IgG-binding peptide, 15-IgBP, that is focused on its C-terminal His residue was performed in an attempt to create more potent peptides. A peptide with a substitution of His17 by 2-pyridylalanine (2-Pya) showed a good binding affinity (15-His17(2-Pya), K d = 75.7 nM). In combination with a previous result, we obtained 15-Lys8Leu/His17(2-Pya)-OH that showed a potent binding affinity (K d = 2.48 nM) and avoided three synthetic problems concerning the p-hydroxybenzyl amidation at the C-terminus, the difficulty associated with coupling at the His7 position and the racemization of 2-Pya.

Kinetics-Based Structural Requirements of Human Immunoglobulin G Binding Peptides

Currently, antibodies are widely used not only in research but also in therapy. Hence, peptides that selectively bind to the fragment crystallizable site of an antibody have been extensively utilized in various research efforts such as the preparation of antibody-drug conjugates (ADC). Consequently, appropriate peptides that bind to immunoglobulin G (IgG) with a specific K _d value and also k _on and k _off values will be useful in different applications, and these kinetic parameters have been perhaps overlooked but are key to development of peptide ligands with advantageous binding properties. We prepared structural derivatives of IgG-binding peptide 1 and evaluated the binding affinity and kinetic rates of the products by surface plasmon resonance assay and isothermal titration calorimetry to obtain novel peptides with beneficial antibody binding properties. In this way, 15-Lys8Leu with fast-binding and slow-release features was obtained through a shortened peptide 15-IgBP. On the other hand, we successfully obtained distinctive peptide, 15-Lys8Tle, with a similar K _d value but with k _on and k _off values that were as much as six-fold different from those of 15-IgBP. These new peptides are useful for the elucidation of kinetic effects on the function of IgG-binding peptides and various applications of antibody or antibody-drug interactions, such as immunoliposome, ADC, or half-life extension strategy, by using a peptide with the appropriate kinetic features.

Affinity Maturation of a Cyclic Peptide Handle for Therapeutic Antibodies Using Deep Mutational Scanning

Meditopes are cyclic peptides that bind in a specific pocket in the antigen-binding fragment of a therapeutic antibody such as cetuximab. Provided their moderate affinity can be enhanced, meditope peptides could be used as specific non-covalent and paratope-independent handles in targeted drug delivery, molecular imaging, and therapeutic drug monitoring. Here we show that the affinity of a recently reported meditope for cetuximab can be substantially enhanced using a combination of yeast display and deep mutational scanning. Deep sequencing was used to construct a fitness landscape of this protein-peptide interaction, and four mutations were identified that together improved the affinity for cetuximab 10-fold to 15 nm Importantly, the increased affinity translated into enhanced cetuximab-mediated recruitment to EGF receptor-overexpressing cancer cells. Although in silico Rosetta simulations correctly identified positions that were tolerant to mutation, modeling did not accurately predict the affinity-enhancing mutations. The experimental approach reported here should be generally applicable and could be used to develop meditope peptides with low nanomolar affinity for other therapeutic antibodies.

Fc-Binding Ligands of Immunoglobulin G: An Overview of High Affinity Proteins and Peptides

The rapidly increasing application of antibodies has inspired the development of several novel methods to isolate and target antibodies using smart biomaterials that mimic the binding of Fc-receptors to antibodies. The Fc-binding domain of antibodies is the primary binding site for e.g., effector proteins and secondary antibodies, whereas antigens bind to the Fab region. Protein A, G, and L, surface proteins expressed by pathogenic bacteria, are well known to bind immunoglobulin and have been widely exploited in antibody purification strategies. Several difficulties are encountered when bacterial proteins are used in antibody research and application. One of the major obstacles hampering the use of bacterial proteins is sample contamination with trace amounts of these proteins, which can invoke an immune response in the host. Many research groups actively develop synthetic ligands that are able to selectively and strongly bind to antibodies. Among the reported ligands, peptides that bind to the Fc-domain of antibodies are attractive tools in antibody research. Besides their use as high affinity ligands in antibody purification chromatography, Fc-binding peptides are applied e.g., to localize antibodies on nanomaterials and to increase the half-life of proteins in serum. In this review, recent developments of Fc-binding peptides are presented and their binding characteristics and diverse applications are discussed.