Site-Specific Chemical Conjugation of Antibodies by Using Affinity Peptide for the Development of Therapeutic Antibody Format

Transglutaminase-mediated proximity labeling of a specific Lys residue in a native IgG antibody

The fusion protein of an engineered zymogen of microbial transglutaminase (EzMTG) with a protein G variant, EzMTG-pG, enabled the proximity-based, tag-free labeling of Lys65 in the heavy chain of a native IgG antibody (trastuzumab) with a Gln-donor peptidyl substrate functionalized with a fluorescent molecule.

Biospecific Chemistry for Covalent Linking of Biomacromolecules

Interactions among biomacromolecules, predominantly noncovalent, underpin biological processes. However, recent advancements in biospecific chemistry have enabled the creation of specific covalent bonds between biomolecules, both in vitro and in vivo. This Review traces the evolution of biospecific chemistry in proteins, emphasizing the role of genetically encoded latent bioreactive amino acids. These amino acids react selectively with adjacent natural groups through proximity-enabled bioreactivity, enabling targeted covalent linkages. We explore various latent bioreactive amino acids designed to target different protein residues, ribonucleic acids, and carbohydrates. We then discuss how these novel covalent linkages can drive challenging protein properties and capture transient protein-protein and protein-RNA interactions in vivo. Additionally, we examine the application of covalent peptides as potential therapeutic agents and site-specific conjugates for native antibodies, highlighting their capacity to form stable linkages with target molecules. A significant focus is placed on proximity-enabled reactive therapeutics (PERx), a pioneering technology in covalent protein therapeutics. We detail its wide-ranging applications in immunotherapy, viral neutralization, and targeted radionuclide therapy. Finally, we present a perspective on the existing challenges within biospecific chemistry and discuss the potential avenues for future exploration and advancement in this rapidly evolving field.

Site-selective antibody-lipid conjugates for surface functionalization of red blood cells and targeted drug delivery

Displaying antibodies on carrier surfaces facilitates precise targeting and delivery of drugs to diseased cells. Here, we report the synthesis of antibody-lipid conjugates (ALCs) through site-selective acetylation of Lys 248 in human Immunoglobulin G (IgG) and the development of antibody-functionalized red blood cells (immunoRBC) for targeted drug delivery. ImmunoRBC with the HER2-selective antibody trastuzumab displayed on the surface (called Tras-RBC) was constructed following a three-step procedure. First, a peptide-guided, proximity-induced reaction transferred an azidoacetyl group to the ε-amino group of Lys 248 in the Fc domain. Second, the azide-modified IgG was subsequently conjugated with dibenzocyclooctyne (DBCO)-functionalized lipids via strain-promoted azide-alkyne cycloaddition (SPAAC) to result in ALCs. Third, the lipid portion of ALCs was then inserted into the cell membranes, and IgGs were displayed on red blood cells (RBCs) to construct immunoRBCs. We then loaded Tras-RBC with a photosensitizer (PS), Zinc phthalocyanine (ZnPc), to selectively target HER2-overexpressing cells, release ZnPc into cancer cells following photolysis, and induce photodynamic cytotoxicity in the cancer cells. This work showcases assembling immunoRBCs following site-selective lipid conjugation on therapeutic antibodies and the targeted introduction of PS into cancer cells. This method could apply to the surface functionalization of other membrane-bound vesicles or lipid nanoparticles for antibody-directed drug delivery.

Peptide-Directed Attachment of Hydroxylamines to Specific Lysines of IgG Antibodies for Bioconjugations with Acylboronates

The role of monoclonal antibodies as vehicles to deliver payloads has evolved as a powerful tool in cancer therapy in recent years. The clinical development of therapeutic antibody conjugates with precise payloads holds great promise for targeted therapeutic interventions. The use of affinity-peptide mediated functionalization of native off-the-shelf antibodies offers an effective approach to selectively modify IgG antibodies with a drug-antibody ratio (DAR) of 2. Here, we report the traceless, peptide-directed attachment of two hydroxylamines to native IgGs followed by chemoselective potassium acyltrifluoroborate (KAT) ligation with quinolinium acyltrifluoroborates (QATs), which provide enhanced ligation rates with hydroxylamines under physiological conditions. By applying KAT ligation to the modified antibodies, conjugation of small molecules, proteins, and oligonucleotides to off-the-shelf IgGs proceeds efficiently, in good yields, and with simultaneous cleavage of the affinity peptide-directing moiety.

Advancing protein therapeutics through proximity-induced chemistry

Recent years have seen a remarkable growth in the field of protein-based medical treatments. Nevertheless, concerns have arisen regarding the cytotoxicity limitations, low affinity, potential immunogenicity, low stability, and challenges to modify these proteins. To overcome these obstacles, proximity-induced chemistry has emerged as a next-generation strategy for advancing protein therapeutics. This method allows site-specific modification of proteins with therapeutic agents, improving their effectiveness without extensive engineering. In addition, this innovative approach enables spatial control of the reaction based on proximity, facilitating the formation of irreversible covalent bonds between therapeutic proteins and their targets. This capability becomes particularly valuable in addressing challenges such as the low affinity frequently encountered between therapeutic proteins and their targets, as well as the limited availability of small molecules for specific protein targets. As a result, proximity-induced chemistry is reshaping the field of protein drug preparation and propelling the revolution in novel protein therapeutics.

Site-selective template-directed synthesis of antibody Fc conjugates with concomitant ligand release

Template-directed methods are emerging as some of the most effective means to conjugate payloads at selective sites of monoclonal antibodies (mAbs). We have previously reported a method based on an engineered Fc-III reactive peptide to conjugate a radionuclide chelator to K317 of antibodies with the concomitant release of the Fc-III peptide ligand. Here, our method was redesigned to target two lysines proximal to the Fc-III binding site, K248 and K439. Using energy minimization predictions and a semi-combinatorial synthesis approach, we sampled multiple Fc-III amino acid substituents of A3, H5, L6 and E8, which were then converted into Fc-III reactive conjugates. Middle-down MS/MS subunit analysis of the resulting trastuzumab conjugates revealed that K248 and K439 can be selectively targeted using the Fc-III reactive variants L6Dap, L6Orn, L6Y and A3K or A3hK, respectively. Across all variants tested, L6Orn-carbonate appeared to be the best candidate, yielding a degree and yield of conjugation of almost 2 and 100% for a broad array of payloads including radionuclide chelators, fluorescent dyes, click-chemistry reagents, pre-targeted imaging reagents, and some cytotoxic small molecules. Furthermore, L6Orn carbonate appeared to yield similar conjugation results across multiple IgG subtypes. In vivo proof of concept was achieved by conjugation of NODAGA to the PD1/PD-L1 immune checkpoint inhibitor antibody atezolizumab, followed by PET imaging of PD-L1 expression in mice bearing PD-L1 expressing tumor xenograft using radiolabeled [64Cu]Cu-atezolizumab.

Chemical technology principles for selective bioconjugation of proteins and antibodies

Proteins are multifunctional large organic compounds that constitute an essential component of a living system. Hence, control over their bioconjugation impacts science at the chemistry-biology-medicine interface. A chemical toolbox for their precision engineering can boost healthcare and open a gateway for directed or precision therapeutics. Such a chemical toolbox remained elusive for a long time due to the complexity presented by the large pool of functional groups. The precise single-site modification of a protein requires a method to address a combination of selectivity attributes. This review focuses on guiding principles that can segregate them to simplify the task for a chemical method. Such a disintegration systematically employs a multi-step chemical transformation to deconvolute the selectivity challenges. It constitutes a disintegrate (DIN) theory that offers additional control parameters for tuning precision in protein bioconjugation. This review outlines the selectivity hurdles faced by chemical methods. It elaborates on the developments in the perspective of DIN theory to demonstrate simultaneous regulation of reactivity, chemoselectivity, site-selectivity, modularity, residue specificity, and protein specificity. It discusses the progress of such methods to construct protein and antibody conjugates for biologics, including antibody-fluorophore and antibody-drug conjugates (AFCs and ADCs). It also briefs how this knowledge can assist in developing small molecule-based covalent inhibitors. In the process, it highlights an opportunity for hypothesis-driven routes to accelerate discoveries of selective methods and establish new targetome in the precision engineering of proteins and antibodies.

Bioconjugation Techniques for Enhancing Stability and Targeting Efficiency of Protein and Peptide Therapeutics

Bioconjugation techniques have emerged as powerful tools for enhancing the stability and targeting efficiency of protein and peptide therapeutics. This review provides a comprehensive analysis of the various bioconjugation strategies employed in the field. The introduction highlights the significance of bioconjugation techniques in addressing stability and targeting challenges associated with protein and peptide-based drugs. Chemical and enzymatic bioconjugation methods are discussed, along with crosslinking strategies for covalent attachment and site-specific conjugation approaches. The role of bioconjugation in improving stability profiles is explored, showcasing case studies that demonstrate successful stability enhancement. Furthermore, bioconjugation techniques for ligand attachment and targeting are presented, accompanied by examples of targeted protein and peptide therapeutics. The review also covers bioconjugation approaches for prolonging circulation and controlled release, focusing on strategies to extend half-life, reduce clearance, and design-controlled release systems. Analytical characterization techniques for bioconjugates, including the evaluation of conjugation efficiency, stability, and assessment of biological activity and targeting efficiency, are thoroughly examined. In vivo considerations and clinical applications of bioconjugated protein and peptide therapeutics, including pharmacokinetic and pharmacodynamic considerations, as well as preclinical and clinical developments, are discussed. Finally, the review concludes with an overview of future perspectives, emphasizing the potential for novel conjugation methods and advanced targeting strategies to further enhance the stability and targeting efficiency of protein and peptide therapeutics.

Epitope-Based Specific Antibody Modifications

Modified antibodies have essential roles in analytic, diagnostic, and therapeutic uses, and thus, these antibodies are required to have optimal physical and biological properties. Consequently, the development of methods for site-selective antibody modification is crucial. Herein, we used epitope-based affinity labeling to introduce a Fab region-selective antibody modification method. Although labeling that exploits the high affinity between an antibody and its epitope may appear straightforward, it remains challenging probably because of the loss of target affinity caused by modification around the epitope-binding site. By thoroughly screening the modifying agent structure, reaction conditions, and purification methods, we developed an efficient method for the selective modification of the Fab region of the antibody while maintaining the high affinity for the epitope.

Specific peptide conjugation to a therapeutic antibody leads to enhanced therapeutic potency and thermal stability by reduced Fc dynamics

Antibody-drug conjugates are powerful tools for combatting a wide array of cancers. Drug conjugation to a therapeutic antibody often alters molecular characteristics, such as hydrophobicity and effector function, resulting in quality deterioration. To develop a drug conjugation methodology that maintains the molecular characteristics of the antibody, we engineered a specific peptide for conjugation to the Fc region. We used trastuzumab and the chelator (DOTA) as model antibody and payload, respectively. Interestingly, peptide/DOTA-conjugated trastuzumab exhibited enhanced antibody-dependent cellular cytotoxicity (ADCC) and increased thermal stability. Detailed structural and thermodynamic analysis clarified that the conjugated peptide blocks the Fc dynamics like a "wedge." We revealed that (1) decreased molecular entropy results in enhanced ADCC, and (2) blockade of Fc denaturation results in increased thermal stability. Thus, we believe that our methodology is superior not only for drug conjugation but also as for reinforcing therapeutic antibodies to enhance ADCC and thermal stability.

Site-Specific Antibody Conjugation Using Modified Bisected N-Glycans: Method Development and Potential toward Tunable Effector Function

Antibody-drug conjugates (ADCs) have garnered worldwide attention for disease treatment, as they possess high target specificity, a long half-life, and outstanding potency to kill or modulate the functions of targets. FDA approval of multiple ADCs for cancer therapy has generated a strong desire for novel conjugation strategies with high biocompatibility and controllable bioproperties. Herein, we present a bisecting glycan-bridged conjugation strategy that enables site-specific conjugation without the need for the oligosaccharide synthesis and genetic engineering of antibodies. Application of this method is demonstrated by conjugation of anti-HER2 human and mouse IgGs with a cytotoxic drug, monomethyl auristatin E. The glycan bridge showed outstanding stability, and the resulting ADCs eliminated HER2-expressing cancer cells effectively. Moreover, our strategy preserves the feasibility of glycan structure remodeling to fine-tune the immunogenicity and pharmacokinetic properties of ADCs through glycoengineering.

The Dawn of a New Era: Targeting the "Undruggables" with Antibody-Based Therapeutics

The high selectivity and affinity of antibodies toward their antigens have made them a highly valuable tool in disease therapy, diagnosis, and basic research. A plethora of chemical and genetic approaches have been devised to make antibodies accessible to more "undruggable" targets and equipped with new functions of illustrating or regulating biological processes more precisely. In this Review, in addition to introducing how naked antibodies and various antibody conjugates (such as antibody-drug conjugates, antibody-oligonucleotide conjugates, antibody-enzyme conjugates, etc.) work in therapeutic applications, special attention has been paid to how chemistry tools have helped to optimize the therapeutic outcome (i.e., with enhanced efficacy and reduced side effects) or facilitate the multifunctionalization of antibodies, with a focus on emerging fields such as targeted protein degradation, real-time live-cell imaging, catalytic labeling or decaging with spatiotemporal control as well as the engagement of antibodies inside cells. With advances in modern chemistry and biotechnology, well-designed antibodies and their derivatives via size miniaturization or multifunctionalization together with efficient delivery systems have emerged, which have gradually improved our understanding of important biological processes and paved the way to pursue novel targets for potential treatments of various diseases.

Characterization of human anti-EpCAM antibodies for developing an antibody-drug conjugate

We previously generated fully human antibody-producing TC-mAb mice for obtaining potential therapeutic monoclonal antibodies (mAbs). In this study, we investigated 377 clones of fully human mAbs against a tumor antigen, epithelial cell adhesion molecule (EpCAM), to determine their antigen binding properties. We revealed that a wide variety of mAbs against EpCAM can be obtained from TC-mAb mice by the combination of epitope mapping analysis of mAbs to EpCAM and native conformational recognition analysis. Analysis of 72 mAbs reacting with the native form of EpCAM indicated that the EpCL region (amino acids 24-80) is more antigenic than the EpRE region (81-265), consistent with numerous previous studies. To evaluate the potential of mAbs against antibody-drug conjugates, mAbs were directly labeled with DM1, a maytansine derivative, using an affinity peptide-based chemical conjugation (CCAP) method. The cytotoxicity of the conjugates against a human colon cancer cell line could be clearly detected with high-affinity as well as low-affinity mAbs by the CCAP method, suggesting the advantage of this method. Thus, this study demonstrated that TC-mAb mice can provide a wide variety of antibodies and revealed an effective way of identifying candidates for fully human ADC therapeutics.

AJICAP Second Generation: Improved Chemical Site-Specific Conjugation Technology for Antibody-Drug Conjugate Production

The site-directed chemical conjugation of antibodies remains an area of great interest and active efforts within the antibody-drug conjugate (ADC) community. We previously reported a unique site modification using a class of immunoglobulin-G (IgG) Fc-affinity reagents to establish a versatile, streamlined, and site-selective conjugation of native antibodies to enhance the therapeutic index of the resultant ADCs. This methodology, termed "AJICAP", successfully modified Lys248 of native antibodies to produce site-specific ADC with a wider therapeutic index than the Food and Drug Administration-approved ADC, Kadcyla. However, the long reaction sequences, including the reduction-oxidation (redox) treatment, increased the aggregation level. In this manuscript, we aimed to present an updated Fc-affinity-mediated site-specific conjugation technology named "AJICAP second generation" without redox treatment utilizing a "one-pot" antibody modification reaction. The stability of Fc affinity reagents was improved owing to structural optimization, enabling the production of various ADCs without aggregation. In addition to Lys248 conjugation, Lys288 conjugated ADCs with homogeneous drug-to-antibody ratio of 2 were produced using different Fc affinity peptide reagent possessing a proper spacer linkage. These two conjugation technologies were used to produce over 20 ADCs from several combinations of antibodies and drug linkers. The in vivo profile of Lys248 and Lys288 conjugated ADCs was also compared. Furthermore, nontraditional ADC production, such as antibody-protein conjugates and antibody-oligonucleotide conjugates, were achieved. These results strongly indicate that this Fc affinity conjugation approach is a promising strategy for manufacturing site-specific antibody conjugates without antibody engineering.

Site-Specific Albumin-Selective Ligation to Human Serum Albumin under Physiological Conditions

Human serum albumin (HSA) is the most abundant protein in human blood plasma. It plays a critical role in the native transportation of numerous drugs, metabolites, nutrients, and small molecules. HSA has been successfully used clinically as a noncovalent carrier for insulin (e.g., Levemir), GLP-1 (e.g., Liraglutide), and paclitaxel (e.g., Abraxane). Site-specific bioconjugation strategies for HSA only would greatly expand its role as the biocompatible, non-toxic platform for theranostics purposes. Using the enabling one-bead one-compound (OBOC) technology, we generated combinatorial peptide libraries containing myristic acid, a well-known binder to HSA at Sudlow I and II binding pockets, and an acrylamide. We then used HSA as a probe to screen the OBOC myristylated peptide libraries for reactive affinity elements (RAEs) that can specifically and covalently ligate to the lysine residue at the proximity of these pockets. Several RAEs have been identified and confirmed to be able to conjugate to HSA covalently. The conjugation can occur at physiological pH and proceed with a high yield within 1 h at room temperature. Tryptic peptide profiling of derivatized HSA has revealed two lysine residues (K225 and K414) as the conjugation sites, which is much more specific than the conventional lysine labeling strategy with N-hydroxysuccinimide ester. The RAE-driven site-specific ligation to HSA was found to occur even in the presence of other prevalent blood proteins such as immunoglobulin or whole serum. Furthermore, these RAEs are orthogonal to the maleimide-based conjugation strategy for Cys34 of HSA. Together, these attributes make the RAEs the promising leads to further develop in vitro and in vivo HSA bioconjugation strategies for numerous biomedical applications.

Site-Selective Lysine Acetylation of Human Immunoglobulin G for Immunoliposomes and Bispecific Antibody Complexes

Site-selective acetylation of a single lysine residue in a protein that reaches a lysine acetyltransferase's accuracy, precision, and reliability is challenging. Here, we report a peptide-guided, proximity-driven group transfer reaction that acetylates a single lysine residue, Lys 248, of the fragment crystallizable region (Fc region) in the heavy chain of the human Immunoglobulin G (IgG). An Fc-interacting peptide bound with the Fc domain and positioned a phenolic ester close to Lys 248, which induced a nucleophilic reaction and resulted in the transfer of an acetyl group to Lys 248. The acetylation reaction proceeded to a decent yield under the physiological condition without the need for deglycosylation, unnatural amino acids, or catalysts. Along with acetylation, functional moieties such as azide, alkyne, fluorescent molecules, or biotin could also be site-selectively installed on Lys 248, allowing IgG's further derivatization. We then synthesized an antibody-lipid conjugate and constructed antibody-conjugated liposomes (immunoliposomes), targeting HER2-positive (HER2+) cancer cells. We also built a bispecific antibody complex (bsAbC) covalently linking an anti-HER2 antibody and an anti-CD3 antibody. The bsAbC showed in vitro effector-cell-mediated cytotoxicity at nanomolar concentrations. Compared with bispecific antibodies (bsAbs), bsAbCs are constructed based on native IgGs and contain two antigen-binding sites to each antigen, twice that of bsAbs. Altogether, this work reports a method of site-selective acetylation of native antibodies, highlights a facile way of site-selective IgG functionalization, and underscores the potential of bsAbCs in cancer immunotherapy.

Antibody-Incorporated Nanomedicines for Cancer Therapy

Antibody-based cancer therapy, one of the most significant therapeutic strategies, has achieved considerable success and progress over the past decades. Nevertheless, obstacles including limited tumor penetration, short circulation half-lives, undesired immunogenicity, and off-target side effects remain to be overcome for the antibody-based cancer treatment. Owing to the rapid development of nanotechnology, antibody-containing nanomedicines that have been extensively explored to overcome these obstacles have already demonstrated enhanced anticancer efficacy and clinical translation potential. This review intends to offer an overview of the advancements of antibody-incorporated nanoparticulate systems in cancer treatment, together with the nontrivial challenges faced by these next-generation nanomedicines. Diverse strategies of antibody immobilization, formats of antibodies, types of cancer-associated antigens, and anticancer mechanisms of antibody-containing nanomedicines are provided and discussed in this review, with an emphasis on the latest applications. The current limitations and future research directions on antibody-containing nanomedicines are also discussed from different perspectives to provide new insights into the construction of anticancer nanomedicines.

Radiation dosimetry and efficacy of an 89Zr/225Ac-labeled humanized anti-MUC5AC antibody

INTRODUCTION

Theranostic applications are currently difficult to achieve owing to the limited evaluation of suitable chelators for therapeutic nuclides, such as 225Ac and 227Th. With a focus on targeted α therapy and theranostics using human IgG as a drug-delivery system (i.e., combining highly cytotoxic α-particle emitter radiation with efficient tumor targeting), we developed a recombinant humanized Nd2 (hNd2) as an anti-MUC5AC antibody since MUC5AC is highly expressed in patients with pancreatic cancer. Therefore, we aimed to evaluate the performance of 89Zr- (for diagnosis) and 225Ac- (for therapy) labeling of these antibodies using well-controlled radioisotope (RI)-labeling technology in pancreatic cancer mouse models.

METHODS

89Zr-labeled hNd2 (NMK89) and 225Ac-labeled hNd2 (NMT25) were manufactured by chemical conjugation using affinity peptides. A binding assay and the evaluation of plasma stability were performed in vitro to confirm the properties of NMK89 and NMT25. In vivo, we evaluated biodistribution, positron emission tomography (PET)/computed tomography (CT) imaging, antitumor effects, and toxicity. Moreover, the exposure dose in humans was estimated based on the biodistribution evaluation in normal mice.

RESULTS

NMK89 and NMT25 showed binding specificity to MUC5AC and stability with radiochemical purity ≥90% in mice and human plasma following incubation for 168 h. NMK89 showed high accumulation in tumors and low non-specific accumulation in normal tissues. The antitumor effect of NMT25 was dose-dependent and significantly suppressed tumor growth in the NMT25 treatment groups compared with the control group (p < 0.05). NMK89 and NMT25 showed similar pharmacokinetics and biodistribution characteristics. Additionally, the human estimated exposure dose of NMK89 and NMT25 was confirmed, and the effective dose of NMK89 and NMT25 was 0.33 mSv/MBq and 177.5 mSv/MBq, respectively.

CONCLUSION

NMK89 showed specific accumulation in the MUC5AC-expressing tumors, while NMT25 showed strong antitumor effects. These results suggest NMK89 and NMT25 as promising theranostic agents for pancreatic cancer.

Enhanced transglycosylation activity of an Endo-F3 mutant by ligand-directed localization

At present, numerous studies have been reported to remodel the N-glycans of therapeutic antibodies for the gain of functions. Among the ways of remodeling antibody N-glycans, the chemoenzymatic glycoengineering approach by endoglycosidase (ENGase) has been deeply investigated and provided a significant tool for IgG glycoengineering. Among these cases, the transglycosylation activity of Endo-F3, compared to Endo-S and S2, is insufficient and limits its power in remodeling IgG glycosylation. Herein, we chemically conjugated the Endo-F3 mutant D165A with an Fc binding peptide (FcBP), aiming to improve the affinity of Endo-F3 D165A to IgGs, and therefore enhance the transglycosylation activity of D165A. In this report, we investigated the conjugation site of FcBP to D165A and the linkers between them and found that the conjugation indeed significantly increases the transglycosylation activity of D165A. Meanwhile, we optimized the FcBP-D165A catalyzed transglycosylation process, including the enzyme quantity, oxazoline concentration, and so on. Finally, by this method, we remodeled the N-glycans of rituximab and trastuzumab into homogeneous S2G2F, G2F, GN2M3, and M3 types with decreased enzyme quantity, oxazoline ratio, and catalyzing time. This method not only provides an enhanced ENGase for IgG glycoengineering but also suggests that ligand-directed localization of enzymes is a potential strategy to enhance the activity of enzymes towards the targeted substrate.

Site-Selective Antibody-Drug Conjugation by a Proximity-Driven S to N Acyl Transfer Reaction on a Therapeutic Antibody

Immunoglobulin Gs (IgGs) contain many Lys and Cys residues, which results in an unwanted complex product mixture with conventional drug conjugation methods. We selectively acylated the ε-NH₂ of K248 on trastuzumab using an IgG Fc-binding peptide (FcBP) equipped with a 5-norbornene-2-carboxylic acid thioester (AbClick-1). AbClick-1 locates its thioester close to the ε-NH₂ of K248 while binding to trastuzumab. Consequently, the thioester underwent proximity-driven selective acylation of ε-NH₂ through an S to N acyl transfer reaction. Furthermore, N-tert-butyl maleimide accelerated the cross-linking reaction with an approximately 95% yield of the desired product by scavenging the byproduct (FcBP-SH). Only K248 was modified selectively with the 5-norbornene-2-carbonyl group, which was further modified by click reaction to afford an antibody-drug conjugate (ADC) with two drugs per antibody. The resulting ADCs showed remarkable in vitro and in vivo anticancer activity. Our results demonstrate that a thioester is a promising chemical entity for proximity-driven site-selective conjugation of antibodies.

Designing of a bispecific antibody against SARS-CoV-2 spike glycoprotein targeting human entry receptors DPP4 and ACE2

COVID-19 originated in Wuhan city, China, in 2019 erupted a global pandemic that had put down nearly 3 million lives and hampered the socio-economic conditions of all nations. Despite the available treatments, this disease is not being controlled totally and spreading swiftly. The deadly virus commences infection by hACE2 receptor and its co-receptors (DPP4) engagement with the viral spike protein in the lung alveolar epithelial cells, indicating a primary therapeutic target. The current research attempts to design an in-silico Bispecific antibody (BsAb) against viral spike glycoprotein and DPP4 receptors. Regdanvimab and Begelomab were identified to block the D614G mutated spike glycoprotein of SARS-CoV-2 and host DPP4 receptor, respectively. The designed BsAb was modified by using KIH (Knobs into Holes) and CrossMAb techniques to prevent heavy chain and light chain mispairings. Following the modifications, the site-specific molecular docking studies were performed, revealing a relatively higher binding affinity of BsAb with spike glycoprotein and DPP4 co-receptor than control BsAb. Also, for blocking the primary entry receptor, hACE2, an anti-viral peptide was linked to the Fc region of BsAb that blocks the hACE2 receptor by linker cleavage inside the infected host. Thus, the designed BsAb and anti-viral peptide therapy could be a promising triumvirate way to obstruct the viral entry by blocking the receptor engagement.

A versatile platform for the tumor-targeted delivery of immune checkpoint-blocking immunoglobin G

Immunotherapies based on immune checkpoint-blocking antibodies have been considered the most attractive cancer treatments in recent years. However, the systemic administration of immune checkpoint-blocking antibodies is limited by low response rates and high risk of inducing immune-related adverse events (irAEs), which might be overcome by the tumor-targeted delivery of these antibodies. To achieve tumor-targeted delivery, immune checkpoint-blocking antibodies are usually modified with tumor-homing ligands through difficult genetic fusion or chemical conjugation. As most immune checkpoint-blocking antibodies are immunoglobin G (IgG) antibodies, we hypothesize that these IgG antibodies might be noncovalently modified with a tumor-homing ligand fused to an IgG-binding domain (IgBD). To test this hypothesis, the tumor-homing Z_PDGFRβ affibody, which targets platelet-derived growth factor receptor β (PDGFRβ), was fused to the Fab-selective IgBD in a trimeric format. After mixing Z_PDGFRβ fused to the IgBD with immune checkpoint-blocking IgG against programmed death-ligand 1 (αPD-L1), a novel homogenous complex was formed, indicating that αPD-L1 had been successfully modified with Z_PDGFRβ fused to the IgBD. Z_PDGFRβ-modified αPD-L1 bound to both PDGFRβ and PD-L1, thus leading to greater tumor uptake and antitumor effects in mice bearing PDGFRβ⁺PD-L1⁺ tumor grafts. In addition, due to the broad spectrum of IgBD for IgG, immune checkpoint-blocking IgG antibodies against cytotoxic T-lymphocyte-associated protein 4 (αCTLA-4) and signal regulatory protein alpha (αSIRPα) were also modified with Z_PDGFRβ fused to the IgBD. These results demonstrated that a tumor-homing ligand fused to the IgBD might be developed as a versatile platform for the modification of immune checkpoint-blocking IgG antibodies to achieve tumor-targeted delivery.

Chemical Site-Specific Conjugation Platform to Improve the Pharmacokinetics and Therapeutic Index of Antibody-Drug Conjugates

To overcome a lack of selectivity during the chemical modification of native non-engineered antibodies, we have developed a technology platform termed "AJICAP" for the site-specific chemical conjugation of antibodies through the use of a class of IgG Fc-affinity reagents. To date, a limited number of antibody-drug conjugates (ADCs) have been synthesized via this approach, and no toxicological study was reported. Herein, we describe the compatibility and robustness of AJICAP technology, which enabled the synthesis of a wide variety of ADCs. A stability assessment of a thiol-modified antibody synthesized by AJICAP technology indicated no appreciable increase in aggregation or decomposition upon prolonged storage, indicating that the unexpectedly stable thiol intermediate has a great potential intermediate for payload or linker screening or large-scale manufacturing. Payload conjugation with this stable thiol intermediate generated several AJICAP-ADCs. In vivo xenograft studies indicated that the AJICAP-ADCs displayed significant tumor inhibition comparable to benchmark ADC Kadcyla. Furthermore, a rat pharmacokinetic analysis and toxicology study indicated an increase in the maximum tolerated dose, demonstrating an expansion of the AJICAP-ADC therapeutic index, compared with stochastic conjugation technology. This is the first report of the therapeutic index estimation of site-specific ADCs produced by utilizing Fc affinity reagent conjugation. The described site-specific conjugation technology is a powerful platform to enable next-generation ADCs through reduced heterogeneity and enhanced therapeutic index.

Site-selective lysine conjugation methods and applications towards antibody-drug conjugates

Site-selective protein modification is of significant interest in chemical biology research, with lysine residues representing a particularly challenging target. Whilst lysines are popular for bioconjugation, due to their nucleophilicity, solvent accessibility and the stability of the resultant conjugates, their high abundance means site-selectivity is very difficult to achieve. Antibody-drug conjugates (ADCs) present a powerful therapeutic application of protein modification, and have often relied extensively upon lysine bioconjugation for their synthesis. Here we discuss advances in methodologies for achieving site-selective lysine modification, particularly within the context of antibody conjugate construction, including the cysteine-to-lysine transfer (CLT) protocol which we have recently reported.

Precision Modification of Native Antibodies

Antibodies, particularly of the immunoglobulin G (IgG) isotype, are a group of biomolecules that are extensively used as affinity reagents for many applications in research, disease diagnostics, and therapy. Most of these applications require antibodies to be modified with specific functional moieties, including fluorophores, drugs, and proteins. Thus, a variety of methodologies have been developed for the covalent labeling of antibodies. The most common methods stably attach functional molecules to lysine or cysteine residues, which unavoidably results in heterogeneous products that cannot be further purified. In an effort to prepare homogeneous antibody conjugates, bioorthogonal handles have been site-specifically introduced via enzymatic treatment, genetic code expansion, or genetically encoded tagging, followed by functionalization using bioorthogonal conjugation reactions. The resulting homogeneous products have proven superior to their heterogeneous counterparts for both in vitro and in vivo usage. Nevertheless, additional chemical treatment or protein engineering of antibodies is required for incorporation of the bioorthogonal handles, processes that often affect antibody folding, stability, and/or production yield and cost. Accordingly, concurrent with advances in the fields of bioorthogonal chemistry and protein engineering, there is growing interest in site-specifically labeling native (nonengineered) antibodies without chemical or enzymatic treatments. In this review, we highlight recent strategies for producing site-specific native antibody conjugates and provide a comprehensive summary of the merits and disadvantages of these strategies.

Affinity-Based Methods for Site-Specific Conjugation of Antibodies

Conjugation of various reagents to antibodies has long been an elegant way to combine the superior binding features of the antibody with other desired but non-natural functions. Applications range from labels for detection in different analytical assays to the creation of new drugs by conjugation to molecules which improves the pharmaceutical effect. In many of these applications, it has been proven advantageous to control both the site and the stoichiometry of the conjugation to achieve a homogeneous product with predictable, and often also improved, characteristics. For this purpose, many research groups have, during the latest decade, reported novel methods and techniques, based on small molecules, peptides, and proteins with inherent affinity for the antibody, for site-specific conjugation of antibodies. This review provides a comprehensive overview of these methods and their applications and also describes a historical perspective of the field.

Recent Advances in the Delivery Carriers and Chemical Conjugation Strategies for Nucleic Acid Drugs

With the development of new anticancer medicines, novel modalities are being explored for cancer treatment. For many years, conventional modalities, such as small chemical drugs and antibody drugs, have worked by "inhibiting the function" of target proteins. In recent years, however, nucleic acid drugs, such as ASOs and siRNAs, have attracted attention as a new modality for cancer treatment because nucleic acid drugs can directly promote the "loss of function" of target genes. Recently, nucleic acid drugs for use in cancer therapy have been extensively developed and some of them have currently been under investigation in clinical trials. To develop novel nucleic acid drugs for cancer treatment, it is imperative that cancer researchers, including ourselves, cover and understand those latest findings. In this review, we introduce and provide an overview of various DDSs and ligand modification technologies that are being employed to improve the success and development of nucleic acid drugs, then we also discuss the future of nucleic acid drug developments for cancer therapy. It is our belief this review will increase the awareness of nucleic acid drugs worldwide and build momentum for the future development of new cancer-targeted versions of these drugs.

A novel site-specific chemical conjugation of IgG antibodies by affinity peptide for immunoassays

Recently, there has been an increasing interest in site-specific modifications of antibodies used in immunoassays for disease diagnosis and as antibody therapeutics, such as antibody-drug conjugates. Previously, we established a site-specific chemical conjugation system using an IgG-Fc binding chemical conjugation affinity peptide (CCAP). CCAP could be used only for the modification of human IgG owing to the lack of affinity of CCAP to rodent IgG molecules. In this study, novel CCAP reagents are proposed, which can be used for both human and mouse IgG, based on the Staphylococcus aureus protein A domain-derived affinity peptides Z34C and Z33. Compared with the activity of a conventional randomly modified antibody, mouse IgG modified using this method had favourable features in two immunoassays, demonstrating the advantages of the proposed CCAP method in preserving antibody functionality during conjugation.

Chromatographic analysis of site-specific antibody-drug conjugates produced by AJICAP first-generation technology using a recombinant FcγIIIa receptor-ligand affinity column

Commercially approved conventional antibody-drug conjugates (ADCs) are produced as heterogeneous mixtures containing a stochastic distribution of payloads decorating the antibody molecules resulting in decreased efficacy and thus lowering their therapeutic index. Control of the DAR and conjugation site in the development of next-generation ADCs is believed to assist in increasing the therapeutic index of these targeted biologics leading to overall enhanced clinical efficacy and reduced toxicity. A chemical site-specific conjugation technology termed AJICAP® allows ADC developers to control both the location and quantity of the payload conjugation to an antibody. Furthermore, this simplified ADC composition enables a streamlined chemical analysis. Here we report the chromatographic separation of site-specific ADCs produced by AJICAP® technology using an analytical affinity chromatography HPLC column containing a recombinant FcγIIIa receptor-ligand immobilized on a non-porous polymer resin (NPR). These HPLC analyses provided visually clear chromatogram results reflecting the heterogeneity of each ADC. The affinity strength was also measured by biolayer interferometry (BLI) and predicted by molecular structure analysis. The results indicate that AJICAP® technology is a promising solution to link hydrophobic payloads to antibodies without compromising antibody receptor function. This study also shows that FcγIIIa-NPR column can be used to characterize site-specific conjugated ADCs compared to ADCs synthesized using conventional methods.

Proximity Histidine Labeling by Umpolung Strategy Using Singlet Oxygen

While electrophilic reagents for histidine labeling have been developed, we report an umpolung strategy for histidine functionalization. A nucleophilic small molecule, 1-methyl-4-arylurazole, selectively labeled histidine under singlet oxygen (¹O₂) generation conditions. Rapid histidine labeling can be applied for instant protein labeling. Utilizing the short diffusion distance of ¹O₂ and a technique to localize the ¹O₂ generator, a photocatalyst in close proximity to the ligand-binding site, we demonstrated antibody Fc-selective labeling on magnetic beads functionalized with a ruthenium photocatalyst and Fc ligand, ApA. Three histidine residues located around the ApA binding site were identified as labeling sites by liquid chromatography-mass spectrometry analysis. This result suggests that ¹O₂-mediated histidine labeling can be applied to a proximity labeling reaction on the nanometer scale.

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.

Assay concept for detecting anti-drug IgM in human serum samples by using a novel recombinant human IgM positive control

Aim: Development and qualification of an easy-to-use ELISA for detection of IgM anti-drug antibodies (ADA) and its use in a clinical Phase I trial. Results & methodology: During the assay development two positive control (PC) approaches, the preparation of a chemically conjugated and a recombinant PC, were pursued. With both PCs, the assay was developed and successfully qualified considering the regulatory guidelines. For a case study, the IgM ADA isotyping assay with the recombinant PC was selected. Different courses and intensities of immune response regarding IgM signals were demonstrated. Conclusion: The easy-to-use ELISA allowed IgM-ADA detection in clinical samples. Conjugated and recombinant IgM PCs were comparable regarding assay sensitivity, precision and suitability.

Site-selective modification strategies in antibody-drug conjugates

Antibody-drug conjugates (ADCs) harness the highly specific targeting capabilities of an antibody to deliver a cytotoxic payload to specific cell types. They have garnered widespread interest in drug discovery, particularly in oncology, as discrimination between healthy and malignant tissues or cells can be achieved. Nine ADCs have received approval from the US Food and Drug Administration and more than 80 others are currently undergoing clinical investigations for a range of solid tumours and haematological malignancies. Extensive research over the past decade has highlighted the critical nature of the linkage strategy adopted to attach the payload to the antibody. Whilst early generation ADCs were primarily synthesised as heterogeneous mixtures, these were found to have sub-optimal pharmacokinetics, stability, tolerability and/or efficacy. Efforts have now shifted towards generating homogeneous constructs with precise drug loading and predetermined, controlled sites of attachment. Homogeneous ADCs have repeatedly demonstrated superior overall pharmacological profiles compared to their heterogeneous counterparts. A wide range of methods have been developed in the pursuit of homogeneity, comprising chemical or enzymatic methods or a combination thereof to afford precise modification of specific amino acid or sugar residues. In this review, we discuss advances in chemical and enzymatic methods for site-specific antibody modification that result in the generation of homogeneous ADCs.

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.

Photoconjugation of an Fc-Specific Peptide Enables Efficient DAR 2 Antibody-Drug Conjugate Formation

Antibody-drug conjugates (ADCs) in current clinical usage have been constructed via the nonspecific conjugation of drugs to antibodies, rendering the manufacturing processes difficult. In this study, a high-affinity IgG Fc-binding peptide equipped with a photoreactive amino acid was developed and successfully conjugated to Glu-382 of trastuzumab in a site-specific manner. The resulting conjugate was employed to generate a DAR 2 ADC product using click chemistry.

Antibody Conjugate Assembly on Ultrasound-Confined Microcarrier Particles

Bioconjugates are important next-generation drugs and imaging agents. Assembly of these increasingly complex constructs requires precise control over processing conditions, which is a challenge for conventional manual synthesis. This inadequacy has motivated the pursuit of new approaches for efficient, controlled modification of high-molecular-weight biologics such as proteins, carbohydrates, and nucleic acids. We report a novel, hands-free, semiautomated platform for synthetic manipulation of biomolecules using acoustically responsive microparticles as three-dimensional reaction substrates. The microfluidic reactor incorporates a longitudinal acoustic trap that controls the chemical reactions within a localized acoustic field. Forces generated by this field immobilize the microscale substrates against the continuous flow of participating chemical reagents. Thus, the motion of substrates and reactants is decoupled, enabling exquisite control over multistep reaction conditions and providing high-yield, high-purity products with minimal user input. We demonstrate these capabilities by conjugating clinically relevant antibodies with a small molecule. The on-bead synthesis comprises capture of the antibody, coupling of a fluorescent tag, product purification, and product release. Successful capture and modification of a fluorescently labeled antibody are confirmed via fold increases of 49 and 11 in the green (antibody)- and red (small-molecule dye)-channel median intensities determined using flow cytometry. Antibody conjugates assembled on acoustically responsive, ultrasound-confined microparticles exhibit similar quality and quantity to those prepared manually by a skilled technician.

Antibody-Drug Conjugates Using Mouse-Canine Chimeric Anti-Dog Podoplanin Antibody Exerts Antitumor Activity in a Mouse Xenograft Model

Antibody-drug conjugates (ADCs), which consist of a monoclonal antibody (mAb), a linker, and a payload, can deliver a drug to cancer tissues. We previously produced an anti-dog podoplanin (dPDPN) mAb, PMab-38, which reacts with dPDPN-expressing canine melanomas and squamous cell carcinomas (SCCs), but not with dPDPN-expressing canine type I alveolar cells or lymphatic endothelial cells, indicating that PMab-38 possesses cancer specificity. In this study, we developed an ADC, P38B-DM1, using the mouse-canine chimeric anti-dPDPN antibody, P38B as the antibody, a peptide linker, and emtansine as the payload using the chemical conjugation by affinity peptide (CCAP) method. We investigated its cytotoxicity against dPDPN-overexpressed Chinese hamster ovary (CHO/dPDPN) cells in vitro and its antitumor activity using a mouse xenograft model of CHO/dPDPN cells. P38B-DM1 showed cytotoxicity to CHO/dPDPN cells in a dose-dependent manner in vitro. Furthermore, P38B-DM1 exhibited higher antitumor activity than P38B in the mouse xenograft model. These results suggest that P38B-DM1, developed using the CCAP method, is useful for antibody therapy against dPDPN-expressing canine SCCs and melanomas.

Natural selection based on coordination chemistry: computational assessment of [4Fe-4S]-maquettes with non-coded amino acids

Cysteine is the only coded amino acid in biology that contains a thiol functional group. Deprotonated thiolate is essential for anchoring iron-sulfur ([Fe-S]) clusters, as prosthetic groups to the protein matrix. [Fe-S] metalloproteins and metalloenzymes are involved in biological electron transfer, radical chemistry, small molecule activation and signalling. These are key metabolic and regulatory processes that would likely have been present in the earliest organisms. In the context of emergence of life theories, the selection and evolution of the cysteine-specific R-CH₂-SH side chain is a fascinating question to confront. We undertook a computational [4Fe-4S]-maquette modelling approach to evaluate how side chain length can influence [Fe-S] cluster binding and stability in short 7-mer and long 16-mer peptides, which contained either thioglycine, cysteine or homocysteine. Force field-based molecular dynamics simulations for [4Fe-4S] cluster nest formation were supplemented with density functional theory calculations of a ligand-exchange reaction between a preassembled cluster and the peptide. Secondary structure analysis revealed that peptides with cysteine are found with greater frequency nested to bind preformed [4Fe-4S] clusters. Additionally, the presence of the single methylene group in cysteine ligands mitigates the steric bulk, maintains the H-bonding and dipole network, and provides covalent Fe-S(thiolate) bonds that together create the optimal electronic and geometric structural conditions for [4Fe-4S] cluster binding compared to thioglycine or homocysteine ligands. Our theoretical work forms an experimentally testable hypothesis of the natural selection of cysteine through coordination chemistry.

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.

Self-Associating Peptides for Modular Bifunctional Conjugation of Tetramer Macromolecules in Living Cells

Monitoring the assembly of macromolecules to design entities with novel properties can be achieved either chemically creating covalent bonds or by noncovalent connections using appropriate structural motifs. In this report, two self-associating peptides (named K3 and E3) that originate from p53 tetramerization domain were developed as tools for highly specific and noncovalent heterotetramerization of two biomolecules. The pairing/coupling preferences of K3 and E3 were first evaluated by molecular modeling data and confirmed using circular dichroism spectroscopy, size-exclusion chromatography, and biological assays. Regardless of the moieties fused to K3 and E3, these two peptides self-assembled into dimers of dimers to form bivalent heterotetrameric complexes that proved to be extremely stable inside living cells. The benefits of the multivalency in terms of avidity, specificity, and expanded functional activity were strikingly revealed when the proliferating cell nuclear antigen (PCNA), which is essential for DNA replication, was targeted using a heterotetramer presenting both an antibody fragment against PCNA and a specific PCNA binder peptide. In vitro heterotetramerization of these two known PCNA ligands increased their binding efficiencies to PCNA up to 80-fold compared to the best homotetramer counterpart. In cellulo, the heterotetramers were able to efficiently inhibit DNA replication and to trigger cell death. Altogether, we demonstrate that these two biselective self-assembling peptidic domains offer a versatile noncovalent conjugation method that can be easily implemented for protein engineering.