Chemically modified antibodies as diagnostic imaging agents

Pretargeting: A Path Forward for Radioimmunotherapy

Pretargeted radioimmunodiagnosis and radioimmunotherapy aim to efficiently combine antitumor antibodies and medicinal radioisotopes for high-contrast imaging and high-therapeutic-index (TI) tumor targeting, respectively. As opposed to conventional radioimmunoconjugates, pretargeted approaches separate the tumor-targeting step from the payload step, thereby amplifying tumor uptake while reducing normal-tissue exposure. Alongside contrast and TI, critical parameters include antibody immunogenicity and specificity, availability of radioisotopes, and ease of use in the clinic. Each of the steps can be optimized separately; as modular systems, they can find broad applications irrespective of tumor target, tumor type, or radioisotopes. Although this versatility presents enormous opportunity, pretargeting is complex and presents unique challenges for clinical translation and optimal use in patients. The purpose of this article is to provide a brief historical perspective on the origins and development of pretargeting strategies in nuclear medicine, emphasizing 2 protein delivery systems that have been extensively evaluated (i.e., biotin-streptavidin and hapten-bispecific monoclonal antibodies), as well as radiohaptens and radioisotopes. We also highlight recent innovations, including pretargeting with bioorthogonal chemistry and novel protein vectors (such as self-assembling and disassembling proteins and Affibody molecules). We caution the reader that this is by no means a comprehensive review of the past 3 decades of pretargeted radioimmunodiagnosis and pretargeted radioimmunotherapy. But we do aim to highlight major developmental milestones and to identify benchmarks for success with regard to TI and toxicity in preclinical models and clinically. We believe this approach will lead to the identification of key obstacles to clinical success, revive interest in the utility of radiotheranostics applications, and guide development of the next generation of pretargeted theranostics.

Simple, Rapid Chemical Labeling and Screening of Antibodies with Luminescent Peptides

Sensitive and selective detection assays are essential for the accurate measurement of analytes in both clinical and research laboratories. Immunoassays that rely on nonoverlapping antibodies directed against the same target analyte (e.g., sandwich enzyme-linked immunosorbent assays (ELISAs)) are commonly used molecular detection technologies. Use of split enzyme reporters has simplified the workflow for these traditionally complex assays. However, identifying functional antibody pairs for a given target analyte can be cumbersome, as it generally involves generating and screening panels of antibodies conjugated to reporters. Accordingly, we sought a faster and easier reporter conjugation strategy to streamline antibody screening. We describe here the development of such a method that is based on an optimized ternary NanoLuc luciferase. This bioluminescence complementation system is comprised of a reagent-based thermally stable polypeptide (LgTrip) and two small peptide tags (β9 and β10) with lysine-reactive handles for direct conjugation onto antibodies. These reagents enable fast, single-step, wash-free antibody labeling and sensitive functional screening. Simplicity, speed, and utility of the one-pot labeling technology are demonstrated in screening antibody pairs for the analyte interleukin-4. The screen resulted in the rapid development of a sensitive homogeneous immunoassay for this clinically relevant cytokine.

Enabling the controlled assembly of antibody conjugates with a loading of two modules without antibody engineering

The generation of antibody conjugates with a loading of two modules is desirable for a host of reasons. Whilst certain antibody engineering approaches have been useful in the preparation of such constructs, a reliable method based on a native antibody scaffold without the use of enzymes or harsh oxidative conditions has hitherto not been achieved. The use of native antibodies has several advantages in terms of cost, practicality, accessibility, time and overall efficiency. Herein we present a novel, reliable method of furnishing antibody conjugates with a loading of two modules starting from a native antibody scaffold.

SNAP-Tag Technology: A Promising Tool for Ex Vivo Immunophenotyping

BACKGROUND

SNAP-tag, a self-labeling protein tag, is commonly used for in vitro and in vivo analysis of bound target proteins. We report the first evidence that SNAP-tag could be used for ex vivo detection of enriched biological markers.

METHODS

Proof of concept was established for target c-kit receptor, a pathological and diagnostic marker for a variety of cancers. SNAP-tag conjugates with stem-cell factor (SCF) fusion proteins were designed and their binding and specificity was validated in vitro using flow cytometry and immunostaining.

RESULTS

Ex vivo diagnostic application of the fusion protein was demonstrated in comparison with anti-c-kit antibody for peripheral blood samples from leukemia patients and colorectal tissue specimens.

Antibody Labeling with Fluorescent Dyes Using Magnetic Protein A and Protein G Beads

Antibodies labeled with small molecules like fluorescent dyes, cytotoxic drugs, and radioactive tracers are essential tools in biomedical research, immunodiagnostics and more recently as therapeutic agents. Traditional methods for labeling antibodies with small molecules require purified antibodies at relatively high concentration, involve multiple dialysis steps and have limited throughput. However, several applications, including the field of Antibody Drug Conjugates (ADCs), will benefit from new methods that will allow labeling of antibodies directly from cell media. Such methods may allow antibodies to be screened in biologically relevant assays, for example, the receptor-mediated antibody internalization assay in the case of ADCs. Here, we describe a method (on-bead method) that enables labeling of small amounts of antibodies directly from cell media. This approach utilizes high capacity magnetic Protein A and Protein G affinity beads to capture antibodies from the cell media followed by labeling with small molecules using either amine or thiol chemistry and subsequent elution of the labeled antibodies. Taking fluorescent dyes as surrogates for small molecules, we demonstrate the on-bead labeling of three different mouse antibodies directly from cell media using both amine and thiol labeling chemistry. The high binding affinity of antibodies to Protein A and Protein G ensures high recoveries as well as high purity of the labeled antibodies. In addition, use of magnetic beads allows multiple samples to be handled manually, thereby significantly improving labeling throughput.

Immunochemical Methods Applied to Art-Historical Materials: Identification and Localization of Proteins by ELISA and IFM

Despite the large diffusion of natural organic substances in art-historical materials, their characterization presents many challenges due to the chemical complexity and instability with respect to degradation processes. Among natural products, proteins have been largely used in the past as binders but also as adhesives or additives in coating layers. Nevertheless, biological identification of proteins in art-historical objects is one of the most recent achievements obtained in heritage science thanks to the development of specifically tailored bio-analytical strategies. In the context of this active emerging discipline, immunological methods stand out for sensitivity, specificity and versatility for both protein recognition and localization in micro-samples. Furthermore, the growing use of immunological techniques for advanced diagnostics and clinical applications ensures continuous improvement in their analytical performance. Considering such, this review provides an overview of the most recent applications of enzyme linked immunosorbent assay and immunofluorescence microscopy techniques in the field of heritage materials. Specifically, the main strengths and potentials of the two techniques as well as their limits and drawbacks are presented and discussed herein.

A mild TCEP-based para-azidobenzyl cleavage strategy to transform reversible cysteine thiol labelling reagents into irreversible conjugates

It has recently emerged that the succinimide linkage of a maleimide thiol addition product is fragile, which is a major issue in fields where thiol functionalisation needs to be robust. Herein we deliver a strategy that generates selective cysteine thiol labelling reagents, which are stable to hydrolysis and thiol exchange.

Next-generation disulfide stapling: reduction and functional re-bridging all in one

Herein we present a significant step towards next-generation disulfide stapling reagents. A novel class of reagent has been designed to effect both disulfide reduction and functional re-bridging. The strategy has been applied to great success across various peptides and proteins. Moreover, application to a multi-disulfide system resulted in functional re-bridging without disulfide scrambling.

Optimization and expansion of a site-selective N-methylpyridinium-4-carboxaldehyde-mediated transamination for bacterially expressed proteins

Site-selective bioconjugation methods are valuable because of their ability to confer new properties to proteins by the chemical attachment of specific functional groups. Well-defined bioconjugates obtained through these methods have found utility for the study of protein function and the creation of protein-based materials. We have previously reported a protein modification strategy to modify the N-terminus of peptides and proteins using N-methylpyridinium-4-carboxaldehyde benzenesulfonate (Rapoport's salt, RS) as a transamination reagent, which oxidizes the N-terminal amino group to provide a uniquely reactive aldehyde or ketone. This functional handle can subsequently be modified with an alkoxyamine reagent of choice. Previous work had found glutamate terminal sequences to be highly reactive toward RS-mediated transamination. However, proteins of interest are often recombinantly expressed in E. coli, where the expression of a glutamate-terminal protein is rendered difficult because the N-terminal methionine derived from the start codon is not cleaved when Glu is in the second position. In this work, we describe a way to overcome this difficulty via the insertion of a Factor Xa proteolytic cleavage site to acquire the optimal glutamate residue at the N-terminus. Additionally, we present studies on alternative high-yielding sequences containing N-terminal residues that can be expressed directly. We have used site-directed mutagenesis to validate these findings on a model cellulase enzyme, an endoglucanase from the thermophilic Pyrococcus horikoshii. Activity assays performed with these mutants show that RS transamination and subsequent modification with alkoxyamines have no negative impact on cellulolytic ability.

Advances in molecular imaging for diagnosis of digestive tract cancers

Digestive tract cancers are common cancer types and have high incidence and mortality. Currently available diagnostic methods have some limitations that make an early and accurate diagnosis and prompt treatment difficult. Molecular imaging, which has been formally defined as visualization, characterization and measurement at the molecular level instead of the anatomic level, significantly increases the sensitivity and specificity of cancer detection. Several modalities have been utilized for molecular imaging in digestive tract cancers, such as endoscopy, scintigraphy (PET/SPECT), magnetic resonance imaging (MRI), and ultrasound (US). Antibodies, peptides, and aptamers are classes of molecular probes that have been extensively used as affinity ligands. After being conjugated with various labels such as radioisotopes, fluorophore, supermagnetic or paramagnetic metals and microbubbles, the probes can specifically target tumor cells and stroma and are used with imaging modalities to detect cancers. Molecular imaging is a methodology for not only the early detection of cancer, but also the judgment of tumor staging and the guidance of therapy. With the development of new instrument and probes, as well as multi-modal platforms, molecular imaging has been gradually perfected and taken from bench to bedside, bringing opportunities for early, accurate and comprehensive diagnosis of digestive tract cancers.

The radiolabeling of proteins by the [18F]AlF method

A new ([(18)F]AlF)(2+)-binding ligand that contains 1,4,7-triazacyclononane-1,4-diacetate (NODA) attached to a methyl phenylacetic acid group (MPA) was conjugated to N-(2-aminoethyl)maleimide (EM) to form NODA-MPAEM. The NODA-MPAEM was labeled with ([(18)F]AlF)(2+) at 105°C in 49-82% yield and conjugated at room temperature to an antibody Fab' fragment in 69-80% yield (total time ∼50min) and with retention of immunoreactivity. These data indicate that the rapid and simple [(18)F]AlF-labeling method can be easily adapted for preparing heat-sensitive compounds with (18)F quickly and in high yields.

Chemoselective surface immobilization of proteins through a cleavable peptide

Surface immobilization of biomolecules is a fundamental step in several experimental techniques such as surface plasmon resonance analysis and microarrays. Oxime ligation allows reaching chemoselective protein immobilization with the retention of native-like conformation by proteins. Beside the need for chemoselective ligation of molecules to surface/particle, equally important is the controlled release of the immobilized molecules, even after a specific binding event. For this purpose, we have designed and assessed in an SPR experiment a peptide linker able to (i) anchor a given protein (enzymes, receptors, or antibodies) to a surface in a precise orientation and (ii) release the immobilized protein after selective enzymatic cleavage. These results open up the possibility to anchor to a surface a protein probe leaving bioactive sites free for interaction with substrates, ligands, antigens, or drugs and successively remove the probe-ligand complex by enzymatic cleavage. This peptide linker can be considered both an improvement of SPR analysis for macromolecular interaction and a novel strategy for drug delivery and biomaterial developments.

New covalent capture probes for imaging and therapy, based on a combination of binding affinity and disulfide bond formation

We describe the synthesis and development of new reactive DOTA-metal complexes for covalently targeting engineered receptors in vivo, which have superior tumor uptake and clearance properties for biomedical applications. These probes are found to clear efficiently through the kidneys and minimally through other routes, but bind persistently in the tumor target. We also explore the new technique of Cerenkov luminescence imaging to optically monitor radiolabeled probe distribution and kinetics in vivo. Cerenkov luminescence imaging uniquely enables sensitive noninvasive in vivo imaging of a β(-) emitter such as (90)Y with an optical imager.