Enzyme-mediated site-specific bioconjugation of metal complexes to proteins: sortase-mediated coupling of copper-64 to a single-chain antibody

Design of a recombinant asparaginyl ligase for site-specific modification using efficient recognition and nucleophile motifs

Asparaginyl ligases have been extensively utilized as valuable tools for site-specific bioconjugation or surface-modification. However, the application is hindered by the laborious and poorly reproducible preparation processes, unstable activity and ambiguous substrate requirements. To address these limitations, this study employed a structure-based rational approach to obtain a high-yield and high-activity protein ligase called OaAEP1-C247A-aa55-351. It was observed that OaAEP1-C247A-aa55-351 exhibits appreciable catalytic activities across a wide pH range, and the addition of the Fe3+ metal ion effectively enhances the catalytic power. Importantly, this study provides insight into the recognition and nucleophile peptide profiles of OaAEP1-C247A-aa55-351. The ligase demonstrates a higher recognition ability for the "Asn-Ala-Leu" motif and an N-terminus "Arg-Leu" as nucleophiles, which significantly increases the reaction yield. Consequently, the catalytic activity of OaAEP1-C247A-aa55-351 with highly efficient recognition and nucleophile motif, "Asn-Ala-Leu" and "Arg-Leu" under the buffer containing Fe3+ is 70-fold and 2-fold higher than previously reported OaAEP1-C247A and the most efficient butelase-1, respectively. Thus, the designed OaAEP1-C247A-aa55-351, with its highly efficient recognition and alternative nucleophile options, holds promising potential for applications in protein engineering, chemo-enzymatic modification, and the development of drugs.

Theranostic Nuclear Medicine with Gallium-68, Lutetium-177, Copper-64/67, Actinium-225, and Lead-212/203 Radionuclides

Molecular changes in malignant tissue can lead to an increase in the expression levels of various proteins or receptors that can be used to target the disease. In oncology, diagnostic imaging and radiotherapy of tumors is possible by attaching an appropriate radionuclide to molecules that selectively bind to these target proteins. The term "theranostics" describes the use of a diagnostic tool to predict the efficacy of a therapeutic option. Molecules radiolabeled with γ-emitting or β+-emitting radionuclides can be used for diagnostic imaging using single photon emission computed tomography or positron emission tomography. Radionuclide therapy of disease sites is possible with either α-, β-, or Auger-emitting radionuclides that induce irreversible damage to DNA. This Focus Review centers on the chemistry of theranostic approaches using metal radionuclides for imaging and therapy. The use of tracers that contain β+-emitting gallium-68 and β-emitting lutetium-177 will be discussed in the context of agents in clinical use for the diagnostic imaging and therapy of neuroendocrine tumors and prostate cancer. A particular emphasis is then placed on the chemistry involved in the development of theranostic approaches that use copper-64 for imaging and copper-67 for therapy with functionalized sarcophagine cage amine ligands. Targeted therapy with radionuclides that emit α particles has potential to be of particular use in late-stage disease where there are limited options, and the role of actinium-225 and lead-212 in this area is also discussed. Finally, we highlight the challenges that impede further adoption of radiotheranostic concepts while highlighting exciting opportunities and prospects.

Site-Specific 68Ga Radiolabeling of Trastuzumab Fab via Methionine for ImmunoPET Imaging

Bioconjugates of antibodies and their derivatives radiolabeled with β+-emitting radionuclides can be utilized for diagnostic PET imaging. Site-specific attachment of radioactive cargo to antibody delivery vectors provides homogeneous, well-defined immunoconjugates. Recent studies have demonstrated the utility of oxaziridine chemistry for site-specific labeling of methionine residues. Herein, we applied this approach to site-specifically radiolabel trastuzumab-derived Fab immunoconjugates with 68Ga, which can be used for in vivo PET imaging of HER2-positive breast cancer tumors. Initially, a reactive azide was introduced to a single solvent-accessible methionine residue in both the wild-type Fab and an engineered derivative containing methionine residue M74, utilizing the principles of oxaziridine chemistry. Subsequently, these conjugates were functionalized with a modified DFO chelator incorporating dibenzocyclooctyne. The resulting DFO-WT and DFO-M74 conjugates were radiolabeled with generator-produced [68Ga]Ga3+, to yield the novel PET radiotracers, [68Ga]Ga-DFO-WT and [68Ga]Ga-DFO-M74. In vitro and in vivo studies demonstrated that [68Ga]Ga-DFO-M74 exhibited a higher affinity for HER2 receptors. Biodistribution studies in mice bearing orthotopic HER2-positive breast tumors revealed a higher uptake of [68Ga]Ga-DFO-M74 in the tumor tissue, accompanied by rapid renal clearance, enabling clear delineation of tumors using PET imaging. Conversely, [68Ga]Ga-DFO-WT exhibited lower uptake and inferior image contrast compared to [68Ga]Ga-DFO-M74. Overall, the results demonstrate that the highly facile methionine-oxaziridine modification approach can be simply applied to the synthesis of stable and site-specifically modified radiolabeled antibody-chelator conjugates with favorable pharmacokinetics for PET imaging.

Enzymatic Bioconjugation: A Perspective from the Pharmaceutical Industry

Enzymes have firmly established themselves as bespoke catalysts for small molecule transformations in the pharmaceutical industry, from early research and development stages to large-scale production. In principle, their exquisite selectivity and rate acceleration can also be leveraged for modifying macromolecules to form bioconjugates. However, available catalysts face stiff competition from other bioorthogonal chemistries. In this Perspective, we seek to illuminate applications of enzymatic bioconjugation in the face of an expanding palette of new drug modalities. With these applications, we wish to highlight some examples of current successes and pitfalls of using enzymes for bioconjugation along the pipeline and try to illustrate opportunities for further development.

Recombinant protein polymer-antibody conjugates for applications in nanotechnology and biomedicine

Currently, there are over 100 antibody-based therapeutics on the market for the treatment of various diseases. The increasing importance of antibody treatment is further highlighted by the recent FDA emergency use authorization of certain antibody therapies for COVID-19 treatment. Protein-based materials have gained momentum for antibody delivery due to their biocompatibility, tunable chemistry, monodispersity, and straightforward synthesis and purification. In this review, we discuss progress in engineering the molecular features of protein-based biomaterials, in particular recombinant protein polymers, for introducing novel functionalities and enhancing the delivery properties of antibodies and related binding protein domains.

Lysine-Directed Site-Selective Bioconjugation for the Creation of Radioimmunoconjugates

The synthesis of radioimmunoconjugates via the stochastic attachment of bifunctional chelators to lysines can yield heterogeneous products with suboptimal in vitro and in vivo behavior. In response to this, several site-selective approaches to bioconjugation have been developed, yet each has intrinsic drawbacks, such as the need for expensive reagents or the complexity of incorporating unnatural amino acids into IgGs. Herein, we describe the use of a simple and facile approach to lysine-directed site-selective bioconjugation for the generation of radioimmunoconjugates. This strategy relies upon on the selective modification of single lysine residues within each light chain of the monoclonal antibody (mAb) with a branched azide-bearing perfluorophenyl ester (PFP-bisN₃) followed by the ligation of dibenzocyclooctyne (DBCO)-bearing payloads to these bioorthogonal handles via the strain-promoted azide-alkyne cycloaddition. This methodology was used to create [⁸⁹Zr]Zr-^SSKDFO-pertuzumab, a radioimmunoconjugate of the HER2-targeting mAb pertuzumab labeled with desferrioxamine (DFO) and the positron-emitting radiometal zirconium-89 (⁸⁹Zr). [⁸⁹Zr]Zr-^SSKDFO-pertuzumab was compared to a pair of analogous probes: one synthesized via random lysine modification ([⁸⁹Zr]Zr-DFO-pertuzumab) and another via thiol-maleimide chemistry ([⁸⁹Zr]Zr-^malDFO-pertuzumab). The bioconjugation strategy was assessed using ESI mass spectrometry, SDS-PAGE, and autoradiography. All three immunoconjugates demonstrated comparable binding to HER2 via flow cytometry and surface plasmon resonance (SPR), and ⁸⁹Zr-labeled variants of each were synthesized in >99% radiochemical yield and molar activities of up to ∼55.5 GBq/μmol (10 mCi/mg). Subsequently, the in vivo behavior of this trio of ⁸⁹Zr-immunoPET probes was interrogated in athymic nude mice bearing subcutaneous HER2-expressing BT-474 human breast cancer xenografts. [⁸⁹Zr]Zr-^SSKDFO-pertuzumab, [⁸⁹Zr]Zr-^malDFO-pertuzumab, and [⁸⁹Zr]Zr-DFO-pertuzumab produced positron emission tomography (PET) images with high tumoral uptake and high tumor-to-healthy organ activity concentration ratios. A terminal biodistribution study complemented the PET results, revealing tumoral activity concentrations of 126.9 ± 50.3%ID/g, 86.9 ± 53.2%ID/g, and 92.5 ± 27.2%ID/g at 144 h post-injection for [⁸⁹Zr]Zr-^SSKDFO-pertuzumab, [⁸⁹Zr]Zr-^malDFO-pertuzumab, and [⁸⁹Zr]Zr-DFO-pertuzumab, respectively. Taken together, the data clearly illustrate that this highly modular and facile approach to site-selective bioconjugation produces radioimmunoconjugates that are better-defined and more homogeneous than stochastically modified constructs and also exhibit excellent in vitro and in vivo performance. Furthermore, we contend that this lysine-directed strategy holds several key advantages over extant approaches to site-selective bioconjugation, especially in the context of production for the clinic.

18F Site-Specific Labelling of a Single-Chain Antibody against Activated Platelets for the Detection of Acute Thrombosis in Positron Emission Tomography

Positron emission tomography is the imaging modality of choice when it comes to the high sensitivity detection of key markers of thrombosis and inflammation, such as activated platelets. We, previously, generated a fluorine-18 labelled single-chain antibody (scFv) against ligand-induced binding sites (LIBS) on activated platelets, binding it to the highly abundant platelet glycoprotein integrin receptor IIb/IIIa. We used a non-site-specific bio conjugation approach with N-succinimidyl-4-[¹⁸F]fluorobenzoate (S[¹⁸F]FB), leading to a mixture of products with reduced antigen binding. In the present study, we have developed and characterised a novel fluorine-18 PET radiotracer, based on this antibody, using site-specific bio conjugation to engineer cysteine residues with N-[2-(4-[¹⁸F]fluorobenzamido)ethyl]maleimide ([¹⁸F]FBEM). ScFv_anti-LIBS and control antibody mut-scFv, with engineered C-terminal cysteine, were reduced, and then, they reacted with N-[2-(4-[¹⁸F]fluorobenzamido)ethyl]maleimide ([¹⁸F]FBEM). Radiolabelled scFv was injected into mice with FeCl₃-induced thrombus in the left carotid artery. Clots were imaged in a PET MR imaging system, and the amount of radioactivity in major organs was measured using an ionisation chamber and image analysis. Assessment of vessel injury, as well as the biodistribution of the radiolabelled scFv, was studied. In the in vivo experiments, we found uptake of the targeted tracer in the injured vessel, compared with the non-injured vessel, as well as a high uptake of both tracers in the kidney, lung, and muscle. As expected, both tracers cleared rapidly via the kidney. Surprisingly, a large quantity of both tracers was taken up by organs with a high glutathione content, such as the muscle and lung, due to the instability of the maleimide cysteine bond in vivo, which warrants further investigations. This limits the ability of the novel antibody radiotracer ¹⁸F-scFv_anti-LIBS to bind to the target in vivo and, therefore, as a useful agent for the sensitive detection of activated platelets. We describe the first fluorine-18 variant of the scFv_anti-LIBS against activated platelets using site-specific bio conjugation.

Molecular cages for biological applications

Artificial receptors able to recognise biologically relevant molecules or ions have gained interest in the chemical community because they offer a plethora of posibilities. Molecular cage compounds are polycyclic compounds with a cavity designed for the encapsulation of guest species. Once inside the host cavity, the substrate can be transported through membranes and protected from the action of enzymes or other reactive species, thus offering the possibility of interfering with biological systems. Commonly, enzymes have been an inspiration for chemists in the search and design of defined cavities for different purposes. However, the chemical preparation of molecular cages has struggled with many synthetic challenges but this effort is worthwhile as they are a very promising tool for many applications ranging from sensing, delivery, purification or even promotion of/prevention from chemical modifications. Since the early reports at the end of the 60s, this field has experienced a growing interest; this review summarises the progress in the preparation and study of cage-like compounds highlighting their importance in biological applications.

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.

An efficient method for the site-specific 99mTc labeling of nanobody

Recently, there has been a lot of interest by using nanobodies (heavy chain-only antibodies produced naturally from the Camelidae) as targeting molecules for molecular imaging, especially for the nuclear medicine imaging. A radiolabeled method that generates a homogeneous product is of utmost importance in radiotracer development for the nuclear medicine imaging. The conventional method for the radiolabeling of nanobodies is non-specifically, which conjugates the radioisotope chelating group to the side chain ɛ-amine group of lysine or sulfhydryl of cysteine of nanobodies, with a shortcoming of produce of the heterogeneous radiotracer. Here we describe a method for the site-specific radioisotope ^99mTc labeling of nanobodies by transpeptidase Sortase A. The radiolabeling process includes two steps: first step, NH₂-GGGGK(HYNIC)-COOH peptide (GGGGK = NH₂-Gly-Gly-Gly-Gly-Lys-COOH, HYNIC = 6-hydrazinonicotinyl) was labeled with ^99mTc to obtain GGGGK-HYNIC-^99mTc; second step, Sortase A catalyzes the formation of a new peptide bond between the peptide motif LPETG (NH₂-Leu-Pro-Glu-Thr-Gly-COOH) expressed C-terminally on the nanobody and the N-terminal of GGGGK-HYNIC-^99mTc. After a simple purification process, homogeneous single-conjugated and stable ^99mTc-labeled nanobodies were obtained in >50% yield. This approach demonstrates that the Sortase A-mediated conjugation is a valuable strategy for the development of site-specifically ^99mTc-labeled nanobodies.

New Bifunctional Chelators Incorporating Dibromomaleimide Groups for Radiolabeling of Antibodies with Positron Emission Tomography Imaging Radioisotopes

Positron Emission Tomography (PET) imaging with antibody-based contrast agents frequently uses the radioisotopes [64Cu]Cu2+ and [89Zr]Zr4+. The macrobicyclic chelator commonly known as sarcophagine (sar) is ideal for labeling receptor-targeted biomolecules with [64Cu]Cu2+. The siderophore chelator, desferrioxamine-B (dfo), has been widely used to incorporate [89Zr]Zr4+ into antibodies. Here, we describe new bifunctional chelators of sar and dfo: these chelators have been functionalized with dibromomaleimides (dbm), that enable site-specific and highly stable attachment of molecular cargoes to reduced, solvent-accessible, interstrand native disulfide groups. The new sar-dbm and dfo-dbm derivatives can be easily conjugated with the IgG antibody trastuzumab via reaction with reduced interstrand disulfide groups to give site-specifically modified dithiomaleamic acid (dtm) conjugates, sar-dtm-trastuzumab and dfo-dtm-trastuzumab, in which interstrand disulfides are rebridged covalently with a small molecule linker. Both sar- and dfo-dtm-trastuzumab conjugates have been radiolabeled with [64Cu]Cu2+ and [89Zr]Zr4+, respectively, in near quantitative radiochemical yield (>99%). Serum stability studies, in vivo PET imaging, and biodistribution analyses using these radiolabeled immunoconjugates demonstrate that both [64Cu]Cu-sar-dtm-trastuzumab and [89Zr]Zr-dfo-dtm-trastuzumab possess high stability in biological milieu. Dibromomaleimide technology can be easily applied to enable stable, site-specific attachment of radiolabeled chelators, such as sar and dfo, to native interstrand disulfide regions of antibodies, enabling tracking of antibodies with PET imaging.

Enzyme mediated incorporation of zirconium-89 or copper-64 into a fragment antibody for same day imaging of epidermal growth factor receptor

Identification of tumors which over-express Epidermal Growth Factor Receptor (EGFR) is important in selecting patients for anti-EGFR therapies. Enzymatic bioconjugation was used to introduce positron-emitting radionuclides (89Zr, 64Cu) into an anti-EGFR antibody fragment for Positron Emission Tomography (PET) imaging the same day as injection. A monovalent antibody fragment with high affinity for EGFR was engineered to include a sequence that is recognized by the transpeptidase sortase A. Two different metal chelators, one for 89ZrIV and one for 64CuII, were modified with a N-terminal glycine to enable them to act as substrates in sortase A mediated bioconjugation to the antibody fragment. Both fragments provided high-quality PET images of EGFR positive tumors in a mouse model at 3 hours post-injection, a significant advantage when compared to radiolabeled full antibodies that require several days between injection of the tracer and imaging. The use of enzymatic bioconjugation gives reproducible homogeneous products with the metal complexes selectively installed on the C-terminus of the antibody potentially simplifying regulatory approval.

Enzymatic Methods for the Site-Specific Radiolabeling of Targeting Proteins

Site-specific conjugation of proteins is currently required to produce homogenous derivatives for medicine applications. Proteins derivatized at specific positions of the polypeptide chain can actually show higher stability, superior pharmacokinetics, and activity in vivo, as compared with conjugates modified at heterogeneous sites. Moreover, they can be better characterized regarding the composition of the derivatization sites as well as the conformational and activity properties. To this aim, several site-specific derivatization approaches have been developed. Among these, enzymes are powerful tools that efficiently allow the generation of homogenous protein-drug conjugates under physiological conditions, thus preserving their native structure and activity. This review will summarize the progress made over the last decade on the use of enzymatic-based methodologies for the production of site-specific labeled immunoconjugates of interest for nuclear medicine. Enzymes used in this field, including microbial transglutaminase, sortase, galactosyltransferase, and lipoic acid ligase, will be overviewed and their recent applications in the radiopharmaceutical field will be described. Since nuclear medicine can benefit greatly from the production of homogenous derivatives, we hope that this review will aid the use of enzymes for the development of better radio-conjugates for diagnostic and therapeutic purposes.

Engineered peptide ligases for cell signaling and bioconjugation

Enzymes that catalyze peptide ligation are powerful tools for site-specific protein bioconjugation and the study of cellular signaling. Peptide ligases can be divided into two classes: proteases that have been engineered to favor peptide ligation, and protease-related enzymes with naturally evolved peptide ligation activity. Here, we provide a review of key natural peptide ligases and proteases engineered to favor peptide ligation activity. We cover the protein engineering approaches used to generate and improve these tools, along with recent biological applications, advantages, and limitations associated with each enzyme. Finally, we address future challenges and opportunities for further development of peptide ligases as tools for biological research.

ImmunoPET: Concept, Design, and Applications

Immuno-positron emission tomography (immunoPET) is a paradigm-shifting molecular imaging modality combining the superior targeting specificity of monoclonal antibody (mAb) and the inherent sensitivity of PET technique. A variety of radionuclides and mAbs have been exploited to develop immunoPET probes, which has been driven by the development and optimization of radiochemistry and conjugation strategies. In addition, tumor-targeting vectors with a short circulation time (e.g., Nanobody) or with an enhanced binding affinity (e.g., bispecific antibody) are being used to design novel immunoPET probes. Accordingly, several immunoPET probes, such as ⁸⁹Zr-Df-pertuzumab and ⁸⁹Zr-atezolizumab, have been successfully translated for clinical use. By noninvasively and dynamically revealing the expression of heterogeneous tumor antigens, immunoPET imaging is gradually changing the theranostic landscape of several types of malignancies. ImmunoPET is the method of choice for imaging specific tumor markers, immune cells, immune checkpoints, and inflammatory processes. Furthermore, the integration of immunoPET imaging in antibody drug development is of substantial significance because it provides pivotal information regarding antibody targeting abilities and distribution profiles. Herein, we present the latest immunoPET imaging strategies and their preclinical and clinical applications. We also emphasize current conjugation strategies that can be leveraged to develop next-generation immunoPET probes. Lastly, we discuss practical considerations to tune the development and translation of immunoPET imaging strategies.

Biological Assembly of Modular Protein Building Blocks as Sensing, Delivery, and Therapeutic Agents

Nature has evolved a wide range of strategies to create self-assembled protein nanostructures with structurally defined architectures that serve a myriad of highly specialized biological functions. With the advent of biological tools for site-specific protein modifications and de novo protein design, a wide range of customized protein nanocarriers have been created using both natural and synthetic biological building blocks to mimic these native designs for targeted biomedical applications. In this review, different design frameworks and synthetic decoration strategies for achieving these functional protein nanostructures are summarized. Key attributes of these designer protein nanostructures, their unique functions, and their impact on biosensing and therapeutic applications are discussed.

Synthesis and Characterization of an Epidermal Growth Factor Receptor-Selective RuII Polypyridyl-Nanobody Conjugate as a Photosensitizer for Photodynamic Therapy

There is a current surge of interest in the development of novel photosensitizers (PSs) for photodynamic therapy (PDT), as those currently approved are not completely ideal. Among the tested compounds, we have previously investigated the use of RuII polypyridyl complexes with a [Ru(bipy)2 (dppz)]2+ and [Ru(phen)2 (dppz)]2+ scaffold (bipy=2,2'-bipyridine; dppz=dipyrido[3,2-a:2',3'-c]phenazine; phen=1,10-phenanthroline). These complexes selectively target DNA. However, because DNA is ubiquitous, it would be of great interest to increase the selectivity of our PDT PSs by linking them to a targeting vector in view of targeted PDT. Herein, we present the synthesis, characterization, and in-depth photophysical evaluation of a nanobody-containing RuII polypyridyl conjugate selective for the epidermal growth factor receptor (EGFR) in view of targeted PDT. Using ICP-MS and confocal microscopy, we could demonstrate that our conjugate has high selectivity for the EGFR receptor, which is a crucial oncological target because it is overexpressed and/or deregulated in a variety of solid tumors. However, in contrast to expectations, this conjugate was found to not produce reactive oxygen species (ROS) in cancer cells and is therefore not phototoxic.

Current outlook on radionuclide delivery systems: from design consideration to translation into clinics

Radiopharmaceuticals have proven to be effective agents, since they can be successfully applied for both diagnostics and therapy. Effective application of relevant radionuclides in pre-clinical and clinical studies depends on the choice of a sufficient delivery platform. Herein, we provide a comprehensive review on the most relevant aspects in radionuclide delivery using the most employed carrier systems, including, (i) monoclonal antibodies and their fragments, (ii) organic and (iii) inorganic nanoparticles, and (iv) microspheres. This review offers an extensive analysis of radionuclide delivery systems, the approaches of their modification and radiolabeling strategies with the further prospects of their implementation in multimodal imaging and disease curing. Finally, the comparative outlook on the carriers and radionuclide choice, as well as on the targeting efficiency of the developed systems is discussed.

Chelators and metal complex stability for radiopharmaceutical applications

Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

Characterization of Antibody Products Obtained through Enzymatic and Nonenzymatic Glycosylation Reactions with a Glycan Oxazoline and Preparation of a Homogeneous Antibody-Drug Conjugate via Fc N-Glycan

Glycan engineering of antibodies has received considerable attention. Although various endo-β- N-acetylglucosaminidase mutants have been developed for glycan remodeling, a side reaction has been reported between glycan oxazoline and amino groups. In this study, we performed a detailed characterization for antibody products obtained through enzymatic and nonenzymatic reactions with the aim of maximizing the efficiency of the glycosylation reaction with fewer side products. The reactions were monitored by an ultraperformance liquid chromatography system using an amide-based wide-pore column. The products were characterized by liquid chromatography coupled with tandem mass spectrometry. The side reactions were suppressed by adding glycan oxazoline in a stepwise manner under slightly acidic conditions. Through a combination of an azide-carrying glycan transfer reaction under optimized conditions and a bio-orthogonal reaction, a potent cytotoxic agent monomethyl auristatin E was site-specifically conjugated at N-glycosylated Asn297 with a drug-to-antibody ratio of 4. The prepared antibody-drug conjugate exhibited cytotoxicity against HER2-expressing cells.

Broadening the scope of sortagging

Sortases are enzymes occurring in the cell wall of Gram-positive bacteria. Sortase A (SrtA), the best studied sortase class, plays a key role in anchoring surface proteins with the recognition sequence LPXTG covalently to oligoglycine units of the bacterial cell wall. This unique transpeptidase activity renders SrtA attractive for various purposes and motivated researchers to study multiple in vivo and in vitro ligations in the last decades. This ligation technique is known as sortase-mediated ligation (SML) or sortagging and developed to a frequently used method in basic research. The advantages are manifold: extremely high substrate specificity, simple access to substrates and enzyme, robust nature and easy handling of sortase A. In addition to the ligation of two proteins or peptides, early studies already included at least one artificial (peptide equipped) substrate into sortagging reactions - which demonstrates the versatility and broad applicability of SML. Thus, SML is not only a biology-related technique, but has found prominence as a major interdisciplinary research tool. In this review, we provide an overview about the use of sortase A in interdisciplinary research, mainly for protein modification, synthesis of protein-polymer conjugates and immobilization of proteins on surfaces.

Engineering Antibodies with C-Terminal Sortase-Mediated Modification for Targeted Nanomedicine

The current advances in nanoengineered materials coupled with the precise targeting capability of recombinant antibodies can create nanoscale diagnostics and therapeutics which show enhanced accumulation and extended retention at a target tissue. Smaller antibodies such as single-chain variable fragments (scFv) preserve the selective and strong binding of their parent antibody to their antigen with the benefits of low immunogenicity, more efficient tissue penetration and easy introduction of functional residues suitable for site-specific conjugation. This is of high importance as nonspecific antibody modification often involves attachment to free cysteine or lysine amino acids which may reside in the active site, leading to reduced antigen binding.In this chapter, we outline a facile and versatile chemoenzymatic approach for production of targeted nanocarrier scFv conjugates using the bacterial trans-peptidase Sortase A (Srt A). Srt A efficiently mediates sequence-specific peptide ligation under mild conditions and has few undesirable side reactions. We first describe the production, purification and characterization of Srt A enzyme and a scFv construct which targets activated platelets, called scFv_{anti-GPIIb/IIIa}. Following this, our protocol illustrates the chemoenzymatic modification of the antibody at the C-terminus with an orthogonal click chemistry linker. This avoids any random attachment to the biologically active antigen binding site of the antibody. Finally, we describe the modification of a nanoparticle surface with scFv attachment via two methods: (1) direct Sortase-mediated conjugation; or (2) a two-step system which consists of scFv Sortase-mediated conjugation followed by strain promoted azide-alkyne cycloaddition. Finally, methodology is described to assess the successful assembly of targeted particles.

Dynamic control of hydrogel crosslinking via sortase-mediated reversible transpeptidation

Cell-laden hydrogels whose crosslinking density can be dynamically and reversibly tuned are highly sought-after for studying pathophysiological cellular fate processes, including embryogenesis, fibrosis, and tumorigenesis. Special efforts have focused on controlling network crosslinking in poly(ethylene glycol) (PEG) based hydrogels to evaluate the impact of matrix mechanics on cell proliferation, morphogenesis, and differentiation. In this study, we sought to design dynamic PEG-peptide hydrogels that permit cyclic/reversible stiffening and softening. This was achieved by utilizing reversible enzymatic reactions that afford specificity, biorthogonality, and predictable reaction kinetics. To that end, we prepared PEG-peptide conjugates to enable sortase A (SrtA) induced tunable hydrogel crosslinking independent of macromer contents. Uniquely, these hydrogels can be completely degraded by the same enzymatic reactions and the degradation rate can be tuned from hours to days. We further synthesized SrtA-sensitive peptide linker (i.e., KCLPRTGCK) for crosslinking with 8-arm PEG-norbornene (PEG8NB) via thiol-norbornene photocrosslinking. These hydrogels afford diverse softening paradigms through control of network structures during crosslinking or by adjusting enzymatic parameters during on-demand softening. Importantly, user-controlled hydrogel softening promoted spreading of human mesenchymal stem cells (hMSCs) in 3D. Finally, we designed a bis-cysteine-bearing linear peptide flanked with SrtA substrates at the peptide's N- and C-termini (i.e., NH2-GGGCKGGGKCLPRTG-CONH2) to enable cyclic/reversible hydrogel stiffening/softening. We show that matrix stiffening and softening play a crucial role in growth and chemoresistance in pancreatic cancer cells. These results represent the first dynamic hydrogel platform that affords cyclic gel stiffening/softening based on reversible enzymatic reactions. More importantly, the chemical motifs that affords such reversible crosslinking were built-in on the linear peptide crosslinker without any post-synthesis modification. STATEMENT OF SIGNIFICANCE: Cell-laden 'dynamic' hydrogels are typically designed to enable externally stimulated stiffening or softening of the hydrogel network. However, no enzymatic reaction has been used to reversibly control matrix crosslinking. The application of SrtA-mediated transpeptidation in crosslinking and post-gelation modification of biomimetic hydrogels is innovative because of the specificity of the reaction and reversible tunability of crosslinking kinetics. While SrtA has been previously used to crosslink and fully degrade hydrogels, matrix softening and reversible stiffening of cell-laden hydrogels has not been reported. By designing simple peptide substrates, this unique enzymatic reaction can be employed to form a primary network, to gradually soften hydrogels, or to reversibly stiffen hydrogels. As a result, this dynamic hydrogel platform can be used to answer important matrix-related biological questions that are otherwise difficult to address.

Enzymatic On-Resin Peptide Cleavage and in Situ Cyclization One-Pot Strategy for the Synthesis of Cyclopeptide and Cyclotide

A one-pot strategy combining sortase A mediated on-resin peptide cleavage and in situ cyclization was developed for the synthesis of cyclic peptides. This strategy was applied to synthesize head-to-tail cyclic antibacterial bovine lactoferricin peptide LFcinB_20-35 in a yield of 67%. The one-pot strategy was compatible with an oxidative folding reaction, and complex cyclotides containing one or two disulfide bonds, such as sunflower trypsin inhibitors-1 and α-conotoxin MII, were successfully synthesized in one pot in a yield of 77% and 61%, respectively.

Site-specific chelator-antibody conjugation for PET and SPECT imaging with radiometals

Antibodies and their derivatives radiolabelled with positron- and gamma-emitting radiometals enable sensitive and quantitative molecular Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) imaging of antibody distribution in vivo. Chelators that are covalently attached to antibodies allow radiolabelling with metallic PET and SPECT radioisotopes. Conventional strategies for chelator-protein conjugation generate heterogeneous mixtures of bioconjugates that can exhibit reduced affinity for their receptor targets, and undesirable biodistribution and pharmacokinetics. Recent advances in bioconjugation technology enable site-specific modification to generate well-defined constructs with superior properties. Herein we survey existing site-specific chelator-protein conjugation methods. These include chelator attachment to cysteines/disulfide bonds or the glycan region of the antibody, enzyme-mediated chelator conjugation, and incorporation of sequences of amino acids that chelate the radiometal. Such technology will allow better use of PET and SPECT imaging in the development of antibody-based therapies.

Sortase-Mediated Ligation of Purely Artificial Building Blocks

Sortase A (SrtA) from Staphylococcus aureus has been often used for ligating a protein with other natural or synthetic compounds in recent years. Here we show that SrtA-mediated ligation (SML) is universally applicable for the linkage of two purely artificial building blocks. Silica nanoparticles (NPs), poly(ethylene glycol) and poly(N-isopropyl acrylamide) are chosen as synthetic building blocks. As a proof of concept, NP–polymer, NP–NP, and polymer–polymer structures are formed by SrtA catalysis. Therefore, the building blocks are equipped with the recognition sequence needed for SrtA reaction—the conserved peptide LPETG—and a pentaglycine motif. The successful formation of the reaction products is shown by means of transmission electron microscopy (TEM), matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-ToF MS), and dynamic light scattering (DLS). The sortase catalyzed linkage of artificial building blocks sets the stage for the development of a new approach to link synthetic structures in cases where their synthesis by established chemical methods is complicated.

Enzyme-Based Labeling Strategies for Antibody-Drug Conjugates and Antibody Mimetics

Strategies for site-specific modification of proteins have increased in number, complexity, and specificity over the last years. Such modifications hold the promise to broaden the use of existing biopharmaceuticals or to tailor novel proteins for therapeutic or diagnostic applications. The recent quest for next-generation antibody–drug conjugates (ADCs) sparked research into techniques with site selectivity. While purely chemical approaches often impede control of dosage or locus of derivatization, naturally occurring enzymes and proteins bear the ability of co- or post-translational protein modifications at particular residues, thus enabling unique coupling reactions or protein fusions. This review provides a general overview and focuses on chemo-enzymatic methods including enzymes such as formylglycine-generating enzyme, sortase, and transglutaminase. Applications for the conjugation of antibodies and antibody mimetics are reported.

Site-Specific Protein Labeling via Sortase-Mediated Transpeptidation

Strategies for site-specific protein modification are highly desirable for the construction of conjugates containing non-genetically-encoded functional groups. Ideally, these strategies should proceed under mild conditions, and be compatible with a wide range of protein targets and non-natural moieties. The transpeptidation reaction catalyzed by bacterial sortases is a prominent strategy for protein derivatization that possesses these features. Naturally occurring or engineered variants of sortase A from Staphylococcus aureus catalyze a ligation reaction between a five-amino-acid substrate motif (LPXTG) and oligoglycine nucleophiles. By pairing proteins and synthetic peptides that possess these ligation handles, it is possible to install modifications onto the protein N- or C-terminus in site-specific fashion. As described in this unit, the successful implementation of sortase-mediated labeling involves straightforward solid-phase synthesis and molecular biology techniques, and this method is compatible with proteins in solution or on the surface of live cells. © 2017 by John Wiley & Sons, Inc.

One-step purification and immobilization of extracellularly expressed sortase A by magnetic particles to develop a robust and recyclable biocatalyst

Sortase A (SrtA) is a transpeptidase widely used to site-specifically modify peptides and proteins and shows promise for industrial applications. In this study, a novel strategy was developed for constructing immobilized-SrtA as a robust and recyclable enzyme via direct immobilization of extracellularly expressed SrtA in the fermentation supernatant using magnetic particles. Efficient extracellular SrtA expression was achieved in Escherichia coli through molecular engineering, including manipulation of the protein transport pathway, codon optimization, and co-expression of molecular chaperones to promote expressed SrtA secretion into the medium at high levels. Subsequently, a simple one-step protocol was established for the purification and immobilization of SrtA containing a His-tag from the fermentation supernatant onto a nickel-modified magnetic particle. The immobilized SrtA was proved to retain full enzymatic activity for peptide-to-peptide ligation and protein modification, and was successfully reused for five cycles without obvious activity loss.

Site-selective orientated immobilization of antibodies and conjugates for immunodiagnostics development

Immobilized antibody systems are the key to develop efficient diagnostics and separations tools. In the last decade, developments in the field of biomolecular engineering and crosslinker chemistry have greatly influenced the development of this field. With all these new approaches at our disposal, several new immobilization methods have been created to address the main challenges associated with immobilized antibodies. Few of these challenges that we have discussed in this review are mainly associated to the site-specific immobilization, appropriate orientation, and activity retention. We have discussed the effect of antibody immobilization approaches on the parameters on the performance of an immunoassay.

Radiolabelled Polymeric Materials for Imaging and Treatment of Cancer: Quo Vadis?

Owing to their tunable blood circulation time and suitable plasma stability, polymer-based nanomaterials hold a great potential for designing and utilising multifunctional nanocarriers for efficient imaging and effective treatment of cancer. When tagged with appropriate radionuclides, they may allow for specific detection (diagnosis) as well as the destruction of tumours (therapy) or even customization of materials, aiming to both diagnosis and therapy (theranostic approach). This review provides an overview of recent developments of radiolabelled polymeric nanomaterials (natural and synthetic polymers) for molecular imaging of cancer, specifically, applying nuclear techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT). Different approaches to radiolabel polymers are evaluated from the methodical radiochemical point of view. This includes new bifunctional chelating agents (BFCAs) for radiometals as well as novel labelling methods. Special emphasis is given to eligible strategies employed to evade the mononuclear phagocytic system (MPS) in view of efficient targeting. The discussion encompasses promising strategies currently employed as well as emerging possibilities in radionuclide-based cancer therapy. Key issues involved in the clinical translation of radiolabelled polymers and future scopes of this intriguing research field are also discussed.

Peptide oligomers from ultra-short peptides using sortase

Sortase A catalyzed ligation of ultra-short peptides leads to inter/intra-molecular transpeptidation to form either linear or cyclic oligomers dependent upon the peptide length. Cyclic peptides were the main products for peptides with more than 15aa. However, for ultra-short (<15aa) peptides, cyclic oligomers became predominant in prolonged reactions. Peptides with 1-3 aminoglycines were equally active but peptide oligomers from peptide containing more than one aminoglycine were prone to hydrolysis.

Orthogonal enzymatic reactions for rapid crosslinking and dynamic tuning of PEG–peptide hydrogels

A biocompatible PEG–peptide hydrogel with dynamically tunable stiffness was developed through sortase A-mediated crosslinking and mushroom tyrosinase-triggered stiffening.

Spontaneous Isopeptide Bond Formation as a Powerful Tool for Engineering Site-Specific Antibody-Drug Conjugates

Spontaneous isopeptide bond formation, a stabilizing posttranslational modification that can be found in gram-positive bacterial cell surface proteins, has previously been used to develop a peptide-peptide ligation technology that enables the polymerization of tagged-proteins catalyzed by SpyLigase. Here we adapted this technology to establish a novel modular antibody labeling approach which is based on isopeptide bond formation between two recognition peptides, SpyTag and KTag. Our labeling strategy allows the attachment of a reporting cargo of interest to an antibody scaffold by fusing it chemically to KTag, available via semi-automated solid-phase peptide synthesis (SPPS), while equipping the antibody with SpyTag. This strategy was successfully used to engineer site-specific antibody-drug conjugates (ADCs) that exhibit cytotoxicities in the subnanomolar range. Our approach may lead to a new class of antibody conjugates based on peptide-tags that have minimal effects on protein structure and function, thus expanding the toolbox of site-specific antibody conjugation.

Site-Specifically Labeled Immunoconjugates for Molecular Imaging--Part 2: Peptide Tags and Unnatural Amino Acids

Molecular imaging using radioisotope- or fluorophore-labeled antibodies is increasingly becoming a critical component of modern precision medicine. Yet despite this promise, the vast majority of these immunoconjugates are synthesized via the random coupling of amine-reactive bifunctional probes to lysines within the antibody, a process that can result in heterogeneous and poorly defined constructs with suboptimal pharmacological properties. In an effort to circumvent these issues, the last 5 years have played witness to a great deal of research focused on the creation of effective strategies for the site-specific attachment of payloads to antibodies. These chemoselective modification methods yield immunoconjugates that are more homogenous and better defined than constructs created using traditional synthetic approaches. Moreover, site-specifically labeled immunoconjugates have also been shown to exhibit superior in vivo behavior compared to their randomly modified cousins. The over-arching goal of this two-part review is to provide a broad yet detailed account of the various site-specific bioconjugation approaches that have been used to create immunoconjugates for positron emission tomography (PET), single photon emission computed tomography (SPECT), and fluorescence imaging. In Part 1, we covered site-specific bioconjugation techniques based on the modification of cysteine residues and the chemoenzymatic manipulation of glycans. In Part 2, we will detail two families of bioconjugation approaches that leverage biochemical tools to achieve site-specificity. First, we will discuss modification methods that employ peptide tags either as sites for enzyme-catalyzed ligations or as radiometal coordination architectures. And second, we will examine bioconjugation strategies predicated on the incorporation of unnatural or non-canonical amino acids into antibodies via genetic engineering. Finally, we will compare the advantages and disadvantages of the modification strategies covered in both parts of the review and offer a brief discussion of the overall direction of the field.

Improved variants of SrtA for site-specific conjugation on antibodies and proteins with high efficiency

Sortase mediated ligation is a highly specific platform for conjugation that relies on the specificity of the transpeptidase Sortase A (SrtA) for short peptide sequences (LPXTG and GGG). SrtA retains its specificity while accepting a wide range of potential substrates, but its broad use is limited by the wild-type enzyme’s poor kinetics, which require large amounts of SrtA and extended reaction times for efficient conjugation. Prior explorations have aimed to improve the kinetics of SrtA with limited success. Herein we describe the discovery of further improved SrtA variants with increased efficiency for the conjugation reaction, and demonstrate their robustness in labelling proteins and antibodies in a site-specific manner. Our variants require significantly lower amounts of enzyme than WT SrtA and can be used to attach small molecules to the N or C-terminus of the heavy or light chain in antibodies with excellent yields. These improved variants can also be used for highly efficient site-specific PEGylation.