Fluorine-18 Labelled Radioligands for PET Imaging of Cyclooxygenase-2

Molecular imaging probes enable the early and accurate detection of disease-specific biomarkers and facilitate personalized treatment of many chronic diseases, including cancer. Among current clinically used functional imaging modalities, positron emission tomography (PET) plays a significant role in cancer detection and in monitoring the response to therapeutic interventions. Several preclinical and clinical studies have demonstrated the crucial involvement of cyclooxygenase-2 (COX-2) isozyme in cancer development and progression, making COX-2 a promising cancer biomarker. A variety of COX-2-targeting PET radioligands has been developed based on anti-inflammatory drugs and selective COX-2 inhibitors. However, many of those suffer from non-specific binding and insufficient metabolic stability. This article highlights examples of COX-2-targeting PET radioligands labelled with the short-lived positron emitter ¹⁸F, including radiosynthesis and PET imaging studies published in the last decade (2012–2021).

BODIPY-Pyridylhydrazone Probe for Fluorescence Turn-On Detection of Fe3+ and Its Bioimaging Application

A novel pyridylhydrazone-tethered BODIPY (BODIPY-PH) was synthesized, fully characterized via nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopic (FTIR), and single-crystal X-ray diffraction (SC-XRD) techniques, and developed for the selective detection of Fe3+ through fluorescent enhancement process. This derivative showed 1:1 binding with Fe3+ in an acetonitrile-water mixture (1:9 v/v) with the binding constant (K) of 5.4 × 104 M−1 and the limit of detection of 0.58 µM. The Fe3+ complexation reaction has been proved to be a reversible process and could be effectively repeated up to three cycles. The electronic properties of BODIPY-PH and its Fe3+ complex modeled by the density functional theory (DFT) method suggested the presence of chelation-enhanced fluorescence (CHEF) effect in the Fe3+ binding reaction. The X-ray absorption spectroscopy (XAS) probed at Fe K-edge confirmed the complex formation between BODIPY-PH and the Fe3+ in an octahedral geometry. Finally, bioimaging against human embryonic kidney (Hek293) cell, through confocal fluorescence microscopic technique indicated that the BODIPY-PH displayed good permeability and low toxicity toward the tested cell lines and showed enhanced fluorescent signal in the cells incubated with Fe3+ proving its capability for Fe3+ analysis in cellular matrix.

Aza-BODIPY probe for selective visualization of cyclooxygenase-2 in cancer cells

AZB-IMC₂ was developed as a COX-2 specific probe that exhibited a brighter fluorescence signal in cancer cells that overexpress COX-2 compared to normal cells. Oxidative stress agent-treated inflamed cell lines inducing high COX-2 levels revealed an enhanced fluorescence signal. Inhibitory studies showed a markedly reduced fluorescence intensity in cancer cells. The results suggested that AZB-IMC₂ could be developed as a promising molecular tool for imaging guiding during surgery.

A bivalent indomethacin/Aza-BODIPY conjugate can selectively visualize the COX-2 enzyme in cancer and inflamed cells confirming its potential as a COX-2-specific biomarker in clinical applications.

Coumarin Probe for Selective Detection of Fluoride Ions in Aqueous Solution and Its Bioimaging in Live Cells

We have synthesized novel coumarin-based fluorescent chemosensors for detection of fluoride ions in aqueous solution. The detection mechanism relied on a fluoride-mediated desilylation triggering fluorogenic reaction and a strong interaction between fluoride and the silicon center. In this work, the hydroxyl-decorated coumarins containing oxysilyl moiety have been synthesized through the aldehyde-functionalized coumarins. The optical responses toward fluoride, as well as aqueous stability studies of both aldehyde and hydroxyl functionalized coumarins, have been investigated. Due to the highest fluorescence enhancement upon the addition of fluoride and good stability in aqueous solution, the hydroxyl-decorated coumarin connected with the bulky tert-butyldiphenyloxysilyl group (-OSitBuPh₂) has been selected for further investigation of its potential as a fluoride sensor. This hydroxyl-decorated coumarin can selectively sense fluoride ions in aqueous media (contain 0.8% MeCN) with desirable response times (40 min). The limit of detection of this compound was determined as 0.043 ppm, satisfying the standard fluoride level (0.7 ppm) in drinking water recommended by U.S. Department of Health and Human Services. The application of this silyl-capped coumarin derivative for fluoride analysis in collected water samples displayed satisfactory analytical accuracy (<5% error). Finally, this compound was successfully employed in fluorescence bioimaging of fluoride ions in human liver cancer cells, indicating its excellent cell permeability, ability to retain inside the living cells, and good stability under physiological conditions.

Aza-BODIPY based polymeric nanoparticles for cancer cell imaging

Near infrared (NIR) fluorescent dyes that are widely used for cancer imaging usually suffer from their hydrophobicity. To overcome this problem, a water-suspendable and biodegradable NIR-light-activating aza-BODIPY (AZB-NO₂) encapsulated in polymeric nanoparticles was prepared as a new class of deep-tissue imaging agent. AZB-NO₂ possesses an intense, broad NIR absorption band (600–800 nm) with a remarkably high fluorescent quantum yield. After being encapsulated with a biodegradable polycaprolactone (PCL) and a Kolliphor P188 surfactant by emulsification-solvent evaporation method, the AZB-NO₂ formed a spherical shape as observed in scanning electron micrographs (SEM) with a hydrodynamic average size of 201 nm (average PDI = 0.185). The results from transmission electron micrographs (TEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping indicated that the AZB-NO₂ homogeneously distributed in the polymeric shell. UV-visible-NIR and fluorescence spectra of the obtained nanoparticles, AZB-NO₂@PCL, revealed that the nanoparticles prepared by using 0.8 mg dye loading exhibited the highest fluorescence quantum yield. These nanoparticles were then applied for fluorescence imaging in human glioblastoma cell line (U-251). After the cells were exposed to AZB-NO₂@PCL, the materials appeared to be localized inside U-251 cells within 3 h and the fluorescence signal enhanced along with the increased incubation times. Moreover, 3D cell culture was used in this study to mimic in vivo tumor environments. The AZB-NO₂@PCL exhibited bright fluorescence from U-251 cells inside 3D Ca-alginate scaffolds after 24 h incubation. Our study successfully demonstrated that the encapsulation of hydrophobic aza-BODIPY dye could enhance the water-suspendability of the dye yielding biocompatible nanoparticles efficiently used in cancer cell imaging applications.

Encapsulation of hydrophobic aza-BODIPY dye could enhance its hydrophilicity yielding biocompatible nanoparticles which can be efficiently used in cancer cell imaging applications.

Radiofluorination of a NHC-PF5 adduct: toward new probes for 18F PET imaging

The radiofluorination of N-heterocyclic carbene (NHC) phosphorus(v) fluoride adducts has been investigated. The results show that the IMe-PF₅ derivative (IMe = 1,3-dimethylimidazol-2-ylidene) undergoes a Lewis acid promoted ¹⁸F-¹⁹F isotopic exchange. The resulting radiofluorinated probe is remarkably resistant to hydrolysis both in vitro as well as in vivo.

Preparation of [18F]-NHC-BF3 conjugates and their applications in PET imaging

[¹⁸F]-NHC-BF₃ functionalized bioactive molecules can be successfully synthesized by ¹⁸F–¹⁹F isotopic exchange in one labelling step affording highly stable PET probes which can visualize targeted tumours in mice.

Synthesis and Evaluation of [(18) F]-Ammonium BODIPY Dyes as Potential Positron Emission Tomography Agents for Myocardial Perfusion Imaging

Recently, we demonstrated the potential of a [(18) F]-trimethylammonium BODIPY dye for cardiac imaging. This is the first example of the use of the [(18) F]-ammonium BODIPY dye for positron emission tomography (PET) myocardial perfusion imaging (MPI). In this report, we extend our study to other ammonium BODIPY dyes with different nitrogen substituents. These novel ammonium BODIPY dyes were successfully prepared and radiolabeled by the SnCl4 -assisted (18) F-(19) F isotopic exchange method. The microPET results and the biodistribution data reveal that nitrogen substituent changes have a significant effect on the in vivo and pharmacological properties of the tracers. Of the novel [(18) F]-ammonium BODIPY dyes prepared in this work, the [(18) F]-dimethylethylammonium BODIPY is superior in terms of myocardium uptake and PET imaging contrast. These results support our hypothesis that the ammonium BODIPY dyes have a great potential for use as PET/optical dual-modality MPI probes.