Early-Stage Incorporation Strategy for Regioselective Labeling of Peptides using the 2-Cyanobenzothiazole/1,2-Aminothiol Bioorthogonal Click Reaction

Cyanopyridoimidazole/1,2-Aminothiol Click Reaction: A Novel Bioorthogonal Reaction for Synthesis of Radiotracers

We herein described the design and synthesis of the cyanopyridoimidazoles (CPIs) as new bioorthogonal click reagents toward 1,2-aminothiol groups. Kinetic and density functional theory-based studies of the synthetic compounds revealed that incorporating an electron-withdrawing substituent into the CPI scaffold lowers its lowest unoccupied molecular orbital energy, consequently increasing reactivity. Optimized CPI 8a showed rapid reactivity and high stability in physiological conditions and has been demonstrated to be suitable for various radiotracer synthetic methods. Based on the new bioorthogonal reaction, a [67Ga]Ga-labeled prostate-specific membrane antigen-targeted probe was successfully prepared for in vivo imaging of prostate cancer in an animal model.

Recent applications of CBT-Cys click reaction in biological systems

Click chemistry is a hot topic in many research fields. A biocompatible reaction from fireflies has attracted increasing attention since 2009. Herein, we focus on the firefly-sourced click reaction between cysteine (Cys) and 2-cyanobenzothiazole (2-CBT). This reaction has many excellent properties, such as rapidity, simplicity and high selectivity, which make it successfully applied in protein labeling, molecular imaging, drug discovery and other fields. Meanwhile, its unique ability to form nanoparticles expands its applications in biological systems. We review its principle, development, and latest applications in the past 5 years and hope this review provides more profound and comprehensive insights to its further application.

Linker Molecules Convert Commercial Fluorophores into Tailored Functional Probes during Biolabelling

Many life‐science techniques and assays rely on selective labeling of biological target structures with commercial fluorophores that have specific yet invariant properties. Consequently, a fluorophore (or dye) is only useful for a limited range of applications, e.g., as a label for cellular compartments, super‐resolution imaging, DNA sequencing or for a specific biomedical assay. Modifications of fluorophores with the goal to alter their bioconjugation chemistry, photophysical or functional properties typically require complex synthesis schemes. We here introduce a general strategy that allows to customize these properties during biolabelling with the goal to introduce the fluorophore in the last step of biolabelling. For this, we present the design and synthesis of ‘linker’ compounds, that bridge biotarget, fluorophore and a functional moiety via well‐established labeling protocols. Linker molecules were synthesized via the Ugi four‐component reaction (Ugi‐4CR) which facilitates a modular design of linkers with diverse functional properties and bioconjugation‐ and fluorophore attachment moieties. To demonstrate the possibilities of different linkers experimentally, we characterized the ability of commercial fluorophores from the classes of cyanines, rhodamines, carbopyronines and silicon‐rhodamines to become functional labels on different biological targets in vitro and in vivo via thiol‐maleimide chemistry. With our strategy, we showed that the same commercial dye can become a photostable self‐healing dye or a sensor for bivalent ions subject to the linker used. Finally, we quantified the photophysical performance of different self‐healing linker–fluorophore conjugates and demonstrated their applications in super‐resolution imaging and single‐molecule spectroscopy.

Environment-Sensitive Fluorescent Labelling of Peptides by Luciferin Analogues

Environment-sensitive fluorophores are very valuable tools in the study of molecular and cellular processes. When used to label proteins and peptides, they allow for the monitoring of even small variations in the local microenvironment, thus acting as reporters of conformational variations and binding events. Luciferin and aminoluciferin, well known substrates of firefly luciferase, are environment-sensitive fluorophores with unusual and still-unexploited properties. Both fluorophores show strong solvatochromism. Moreover, luciferin fluorescence is influenced by pH and water abundance. These features allow to detect local variations of pH, solvent polarity and local water concentration, even when they occur simultaneously, by analyzing excitation and emission spectra. Here, we describe the characterization of (amino)luciferin-labeled derivatives of four bioactive peptides: the antimicrobial peptides GKY20 and ApoB_L, the antitumor peptide p53pAnt and the integrin-binding peptide RGD. The two probes allowed for the study of the interaction of the peptides with model membranes, SDS micelles, lipopolysaccharide micelles and Escherichia coli cells. K_d values and binding stoichiometries for lipopolysaccharide were also determined. Aminoluciferin also proved to be very well-suited to confocal laser scanning microscopy. Overall, the characterization of the labeled peptides demonstrates that luciferin and aminoluciferin are previously neglected environment-sensitive labels with widespread potential applications in the study of proteins and peptides.

N, S-Double Labeling of N-Terminal Cysteines via an Alternative Conjugation Pathway with 2-Cyanobenzothiazole

Conjugation of 2-cyanobenzothiazole (CBT) with N-terminal cysteines (NCys) typically gives a luciferin product. We herein report an alternative reaction pathway leading to an N-terminal amidine rendering the side chain thiol available for further modification. Examination of peptide sequence dependence of this amidine conjugation reveals a tripeptide tag CIS that allows facile N, S-double labeling of a protein of interest with >90% yield. This alternative reaction pathway of CBT-NCys condensation presents a significant addition to the toolbox for site-specific protein modifications.

A novel clickable MSAP agent for dual fluorescence/nuclear labeling of biovectors

Simple and efficient synthesis of dual-modality imaging agents for preoperative surgical planning and intraoperative surgical guidance.

Two bifunctional desferrioxamine chelators for bioorthogonal labeling of biovectors with zirconium-89

We report two bifunctional chelators, DFO-Cys and DFO-CBT, to label biovectors with zirconium-89 according to the 2-cyanobenzothiazole/1,2-aminothiol cycloaddition. Their features are high labeling yields, rapid and efficient bioconjugation, metabolically stable luciferin-based end products, and applicability to orthogonal two-step labeling of sensitive biomolecules.

A Flexible Synthesis of 68Ga-Labeled Carbonic Anhydrase IX (CAIX)-Targeted Molecules via CBT/1,2-Aminothiol Click Reaction

We herein describe a flexible synthesis of a small library of ⁶⁸Ga-labeled CAIX-targeted molecules via an orthogonal 2-cyanobenzothiazole (CBT)/1,2-aminothiol click reaction. Three novel CBT-functionalized chelators (1–3) were successfully synthesized and labeled with the positron emitter gallium-68. Cross-ligation between the pre-labeled bifunctional chelators (BFCs) and the 1,2-aminothiol-acetazolamide derivatives (8 and 9) yielded six new ⁶⁸Ga-labeled CAIX ligands with high radiochemical yields. The click reaction conditions were optimized to improve the reaction rate for applications with short half-life radionuclides. Overall, our methodology allows for a simple and efficient radiosynthetic route to produce a variety of ⁶⁸Ga-labeled imaging agents for tumor hypoxia.

Early-Stage Incorporation Strategy for Regioselective Labeling of Peptides using the 2-Cyanobenzothiazole/1,2-Aminothiol Bioorthogonal Click Reaction

Herein, we describe a synthetic strategy for the regioselective labeling of peptides by using a bioorthogonal click reaction between 2-cyanobenzothiazole (CBT) and a 1,2-aminothiol moiety. This methodology allows for the facile and site-specific modification of peptides with various imaging agents, including fluorophores and radioisotope-containing prosthetic groups. We investigated the feasibility of an early-stage incorporation of dipeptide 1 into targeting vectors, such as c[RGDyK(C)] and HER2 pep, during solid-phase peptide synthesis. Then, the utility of the click reaction to label bioactive peptides with a CBT-modified imaging agent (FITC-CBT, 9) was assessed. The ligation reaction was found to be highly selective and efficient under various conditions. The fluorescently labeled peptides 2 and 3 were obtained in respective yields of 88 and 82 % under optimized conditions.