DMAP‐BODIPY Alkynes: A Convenient Tool for Labeling Biomolecules for Bimodal PET–Optical Imaging

Several new boron dipyrromethene/N,N‐dimethylaminopyridine (BODIPY‐DMAP) assemblies were synthesized as precursors for bimodal imaging probes (optical imaging, OI/positron emission tomography, PET). The photophysical properties of the new compounds were also studied. The first proof‐of‐concept was obtained with the preparation of several new BODIPY‐labeled bombesins and evaluation of the affinity for bombesin receptors by using a competition binding assay. Fluorination reactions were investigated on DMAP‐BODIPY precursors as well as on DMAP‐BODIPY‐labeled bombesins. Chemical modifications on the BODIPY core were also performed to obtain luminescent dyes emitting in the therapeutic window (650–900 nm), suitable for in vivo imaging, making these compounds promising precursors for PET/optical dual‐modality imaging agents.     

Development of a 18F‐Labeled Tetrazine with Favorable Pharmacokinetics for Bioorthogonal PET Imaging

A low‐molecular‐weight ¹⁸F‐labeled tetrazine derivative was developed as a highly versatile tool for bioorthogonal PET imaging. Prosthetic groups and undesired carrying of ¹⁸F through additional steps were evaded by direct ¹⁸F‐fluorination of an appropriate tetrazine precursor. Reaction kinetics of the cycloaddition with trans‐cyclooctenes were investigated by applying quantum chemical calculations and stopped‐flow measurements in human plasma; the results indicated that the labeled tetrazine is suitable as a bioorthogonal probe for the imaging of dienophile‐tagged (bio)molecules. In vitro and in vivo investigations revealed high stability and PET/MRI in mice showed fast homogeneous biodistribution of the ¹⁸F‐labeled tetrazine that also passes the blood–brain barrier. An in vivo click experiment confirmed the bioorthogonal behavior of this novel tetrazine probe. Due to favorable chemical and pharmacokinetic properties this bioorthogonal agent should find application in bioimaging and biomedical research.     

In Cellulo Generation of Fluorescent Probes for Live-Cell Imaging of Cylooxygenase-2

Live-cell imaging with fluorescent probes is an essential tool in chemical biology to visualize the dynamics of biological processes in real-time. Intracellular disease biomarker imaging remains a formidable challenge due to the intrinsic limitations of conventional fluorescent probes and the complex nature of cells. This work reports the in cellulo assembly of a fluorescent probe to image cyclooxygenase-2 (COX-2). We developed celecoxib-azide derivative 14 , possessing favorable biophysical properties and excellent COX-2 selectivity profile. In cellulo strain-promoted fluorogenic click chemistry of COX-2-engaged compound 14 with non/weakly-fluorescent compounds 11 and 17 formed fluorescent probes 15 and 18 for the detection of COX-2 in living cells. Competitive binding studies, biophysical, and comprehensive computational analyses were used to describe protein-ligand interactions. The reported new chemical toolbox enables precise visualization and tracking of COX-2 in live cells with superior sensitivity in the visible range.     

Design and Synthesis of a γ1β8‐Cyclodextrin Oligomer: A New Platform with Potential Application as a Dendrimeric Multicarrier

We report the synthesis and characterization of a water‐soluble, star‐shaped macromolecular platform consisting of eight β‐cyclodextrin (β‐CD) units anchored to the narrower rim of a γ‐CD core through bis(triazolyl)alkyl spacers. The efficient synthetic protocol is based on the microwave (MW)‐promoted Cu‐catalyzed 1,3‐dipolar cycloaddition of CD monoazides to CD monoacetylenes. The ligand‐hosting capability of the construct has been assessed by relaxometric titration and nuclear magnetic relaxation dispersion (NMRD) profiling, which showed it to be good, and this was supported by molecular dynamics simulations. To demonstrate the feasibility of obtaining supramolecular structures with high hosting ability, we designed a dimeric platform, formed by joining two nonamers through the γ‐CD cores through a bis(lithocholic acid) linker. With a view to the potential biological applications, cytotoxicity and extent of binding to human serum albumin were assessed. The properties of this dendrimeric multicarrier make it suitable for pharmaceutical and diagnostic purposes, ranging from targeted drug delivery to molecular imaging.     

A Versatile Approach for the Site‐Specific Modification of Recombinant Antibodies Using a Combination of Enzyme‐Mediated Bioconjugation and Click Chemistry

A unique two‐step modular system for site‐specific antibody modification and conjugation is reported. The first step of this approach uses enzymatic bioconjugation with the transpeptidase Sortase A for incorporation of strained cyclooctyne functional groups. The second step of this modular approach involves the azide–alkyne cycloaddition click reaction. The versatility of the two‐step approach has been exemplified by the selective incorporation of fluorescent dyes and a positron‐emitting copper‐64 radiotracer for fluorescence and positron‐emission tomography imaging of activated platelets, platelet aggregates, and thrombi, respectively. This flexible and versatile approach could be readily adapted to incorporate a large array of tailor‐made functional groups using reliable click chemistry whilst preserving the activity of the antibody or other sensitive biological macromolecules.     

Click‐Assembled,Oxygen‐Sensing Nanoconjugates for Depth‐Resolved,Near‐Infrared Imaging in a 3 D Cancer Model

Hypoxia is an important contributing factor to the development of drug‐resistant cancer, yet few nonperturbative tools exist for studying oxygenation in tissues. While progress has been made in the development of chemical probes for optical oxygen mapping, penetration of such molecules into poorly perfused or avascular tumor regions remains problematic. A click‐assembled oxygen‐sensing (CAOS) nanoconjugate is reported and its properties demonstrated in an in vitro 3D spheroid cancer model. The synthesis relies on the sequential click‐based ligation of poly(amidoamine)‐like subunits for rapid assembly. Near‐infrared confocal phosphorescence microscopy was used to demonstrate the ability of the CAOS nanoconjugates to penetrate hundreds of micrometers into spheroids within hours and to show their sensitivity to oxygen changes throughout the nodule. This proof‐of‐concept study demonstrates a modular approach that is readily extensible to a wide variety of oxygen and cellular sensors for depth‐resolved imaging in tissue and tissue models.     

Diketopyrrolopyrrole Bis-Phosphonate Conjugate: A New Fluorescent Probe for In Vitro Bone Imaging

The synthesis of a conjugate molecule between an unusual red-fluorescent diketopyrrolopyrrole (DPP) unit and a bis-phosphonate (BP) precursor by a click-chemistry strategy to target bone tissue and monitor the interaction is reported. After thorough investigation, conjugation through a triazole unit between a γ-azido rather than a β-azido BP and an alkyne-functionalized DPP fluorophore group turned out to be the winning strategy. Visualization of the DPP-BP conjugate on osteoclasts and specific antiresorption activity were successfully demonstrated.     

Drug delivery and imaging with polydiacetylene micelles

This concept article summarizes our recent findings regarding photopolymerized micelles obtained from the self-assembly of diacetylene-containing amphiphiles. Their synthesis and characterization are presented as well as some biomedical applications, such as tumor imaging and drug delivery. Finally, ongoing studies and future challenges are briefly discussed.     

CXCR4‐Targeted and MMP‐Responsive Iron Oxide Nanoparticles for Enhanced Magnetic Resonance Imaging

MRI offers high spatial resolution with excellent tissue penetration but it has limited sensitivity and the commonly administered contrast agents lack specificity. In this study, two sets of iron oxide nanoparticles (IONPs) were synthesized that were designed to selectively undergo copper‐free click conjugation upon sensing of matrix metalloproteinase (MMP) enzymes, thereby leading to a self‐assembled superparamagnetic nanocluster network with T₂ signal enhancement properties. For this purpose, IONPs with bioorthogonal azide and alkyne surfaces masked by polyethylene glycol (PEG) layers tethered to CXCR4‐targeted peptide ligands were synthesized and characterized. The IONPs were tested in vitro and T₂ signal enhancements of around 160 % were measured when the IONPs were incubated with cells expressing MMP2/9 and CXCR4. Simultaneous systemic administration of the bioorthogonal IONPs in tumor‐bearing mice demonstrated the signal‐enhancing ability of these ‘smart’ self‐assembling nanomaterials.     

Identification of Living Legionella pneumophila Using Species‐Specific Metabolic Lipopolysaccharide Labeling

Legionella pneumophila is a pathogenic bacterium involved in regular outbreaks characterized by a relatively high fatality rate and an important societal impact. Frequent monitoring of the presence of this bacterium in environmental water samples is necessary to prevent these epidemic events, but the traditional culture‐based detection and identification method requires up to 10 days. Reported herein is a method allowing identification of Legionella pneumophila by metabolic lipopolysaccharide labeling which targets, for the first time, a precursor to monosaccharides that are specifically present within the O‐antigen of the bacterium. This new approach allows easy detection of living Legionella pneumophila, while other Legionella species are not labeled.