"AquaF" Building Blocks for Water-Compatible SN2 18F-Fluorination of Small-Molecule Radiotracers.

Despite the widespread use of hydrophilic building blocks to incorporate 18F and improve tracer pharmacokinetics, achieving effective leaving group-mediated nucleophilic 18F-fluorination in water (excluding 18F/19F-exchange) remains a formidable challenge. Here, we present a water-compatible SN2 leaving group-mediated 18F-fluorination method employing preconjugated "AquaF" (phosphonamidic fluorides) building blocks. Among 19 compact tetracoordinated pentavalent P(V)-F candidates, the "AquaF" building blocks exhibit superior water solubility, sufficient capacity for 18F-fluorination in water, and excellent in vivo metabolic properties. Two nitropyridinol leaving groups, identified from a pool of leaving group candidates that further enhance the precursor water solubility, enable 18F-fluorination in water with a 10-2 M-1 s-1 level reaction rate constant (surpassing the 18F/19F-exchange) at room temperature. With the exergonic concerted SN2 18F-fluorination mechanism confirmed, this 18F-fluorination method achieves ∼90% radiochemical conversions and reaches a molar activity of 175 ± 40 GBq/µmol (using 12.2 GBq initial activity) in saline for 12 "AquaF"-modified proof-of-concept functional substrates and small-molecule 18F-tracers. [18F]AquaF-Flurpiridaz demonstrates significantly improved radiochemical yield and molar activity compared to 18F-Flurpiridaz, alongside enhanced cardiac uptake and heart/liver ratio in targeted myocardial perfusion imaging, providing a comprehensive illustration of "AquaF" building blocks-assisted water-compatible SN2 18F-fluorination of small-molecule radiotracers.

Cu(II)-Mediated direct 18F-dehydrofluorination of phosphine oxides in high molar activity.

BACKGROUND: The 18F/19F-isotope exchange method employing P(V)-centered prosthetic groups demonstrates advantages in addressing mild one-step aqueous 18F-labeling of peptides and proteins. However, the molar activity (Am) achieved through isotope exchange remains relatively low, unless employing a high initial activity of [18F]F-. To overcome this drawback, our work introduces a novel approach through a Cu-mediated direct 18F-dehydrofluorination of phosphine oxides. This method leverages the straightforward separation of the 18F-labeled product from the phosphine oxide precursors, aiming to primarily increase Am. RESULTS: Through a 19F-dehydrofluorination efficiency test, Cu(OAc)2 was identified as the optimal oxidative metal salt, exhibiting a remarkable 100% conversion within one hour. Leveraging the straightforward separation of phosphine oxide precursors and phosphinic fluoride products, the Am of an activated ester, [18F]4, sees an impressive nearly 15-fold increase compared to the 18F/19F-isotope exchange, with the same initial activity of [18F]F-. Furthermore, this Cu(II)-mediated 18F-dehydrofluorination approach demonstrates tolerance up to 20% solvent water content, which enables the practical radiosynthesis of 18F-labeled water-soluble molecules under non-drying conditions. CONCLUSIONS: The direct 18F-dehydrofluorination of phosphine oxide prosthetic groups has been successfully accomplished, achieving a high Am via Cu(II)-mediated oxidative addition and reductive elimination.

Surfactants Accelerate Isotope Exchange-Based 18F-Fluorination in Water.

Radiochemical yields (RCYs) of isotope exchange-based 18F-fluorination of non-carbon-centered substrates in water are rationally enhanced by adding surfactants, which increases both the rate constant k and local reactant concentrations. Among 12 surfactants, the cationic surfactant cetrimonium bromide (CTAB) and two nonionic surfactants (Tween 20 and Tween 80) were selected for their superior catalytic effects, namely, electrostatic effects or solubilization effects. For a model substrate, bis(4-methoxyphenyl)phosphinic fluoride, the 18F-fluorination rate constant (k) increased up to 7-fold, while its saturation concentration rose up to 15-fold due to micelle formation, encapsulating 70-94% of the substrate. With 30.0 mmol/L CTAB, the required 18F-labeling temperature of a typical organofluorosilicon prosthesis ([18F]SiFA) decreased from 95 °C to room temperature, achieving an RCY of 22%. For an E[c(RGDyK)]2-derived peptide tracer with an organofluorophosphine prosthesis, the RCY in water at 90 °C achieved 25%, correspondingly increasing the molar activity (Am). After high-performance liquid chromatography (HPLC) or solid-phase purification, the residual selected surfactant concentrations in the tracer injections were well below the FDA DII (Inactive Ingredient Database) limits or the LD50 in mice.

18 F-Labeling Chemistry in Aqueous Media.

18 F-Labeled molecular tracers and subsequent positron emission tomography are indispensable molecular imaging tools in medical diagnosis and research. The preparation of 18 F-labeled molecular tracers involves critical steps such as the 18 F-labeling reaction, work-up, and 18 F-product purification, which are governed by 18 F-labeling chemistry. Since direct incorporation of 18 F in aqueous media exhibits many advantages in practice, this Review summarizes the existing 18 F-labeling methods in aqueous media, which are sorted by atoms forming chemical covalent bonds with F. The Review is focused on the respective reaction mechanism, the water effect and the applications of these methods for the development of 18 F-radiopharmaceuticals. The research progress on aqueous nucleophilic labeling methods using [18 F]F- as the 18 F source has been mainly discussed.

Isotope Exchange-Based 18F-Labeling Methods.

18F-Labeling methods for the preparation of 18F-labeled molecular probes can be classified into electrophilic fluorination, nucleophilic fluorination, metal-F coordination, and 18F/19F isotope exchange. Isotope exchange-based 18F-labeling methods demonstrate mild conditions featuring water resistance and facile high-performance liquid chromatography-free purification in direct 18F-labeling of substrates. This paper systematically reviews isotope exchange-based 18F-labeling methods sorted by the adjacent atom bonding with F, i.e., carbon and noncarbon atoms (Si, B, P, S, Ga, Fe, etc.). The respective isotope exchange mechanism, radiolabeling condition, radiochemical yield, molar activity, and stability of the 18F-product are mainly discussed for each isotope exchange-based 18F-labeling method as well as the cutting-edge application of the corresponding 18F-labeled molecular probes.

Simplified one-pot 18F-labeling of biomolecules with in situ generated fluorothiophosphate synthons in high molar activity.

Rationale: Conventional 18F-labeling methods that demand substrate pre-modification or lengthy radiosynthesis procedures have impeded the visualization and translation of numerous biomolecules, as biomarkers or ligands, using modern positron emission tomography techniques in vivo. Moreover, 18F-labeled biomolecules in high molar activity (Am) that are indispensable for sensitive imaging could be only achieved under strict labeling conditions. Methods: Herein, 18F-labeled fluorothiophosphate (FTP) synthons in high Am have been generated rapidly in situ in reaction solutions with < 5% water via nucleophilic substitution by wet [18F]F-, which required minimal processing from cyclotron target water. Results: Various 18F-labeled FTP synthons have been prepared in 30 sec at room temperature with high radiochemical yields > 75% (isolated, non-decay-corrected). FTP synthons with unsaturated hydrocarbon or activated ester group can conjugate with typical small molecules, peptides, proteins, and metallic nanoparticles. 337-517 GBq µmol-1 Am has been achieved for 18F-labeled c(RGDyK) peptide using an automatic module with 37-74 GBq initial activity. Conclusion: The combination of high 18F-fluorination efficiency of FTP synthons and following mild conjugation condition provides a universal simplified one-pot 18F-labeling method for broad unmodified biomolecular substrates.

Direct 18F-Labeling of Biomolecules via Spontaneous Site-Specific Nucleophilic Substitution by F- on Phosphonate Prostheses.

We describe a high radiochemical yield late-stage direct 18F-labeling of bare biomolecules containing common active groups. Spontaneity and site-selectivity are attributed to the remarkably higher rates of nucleophilic substitution reactions on phosphonates than on other electrophiles by F- at various hydrogen bond forms. Rapid access to many medicinally significant 18F-labeled biomolecules is achieved at 21-68% radiochemical yields and 35.9-55.1 GBq µmol-1 molar activities both manually or automatically.

Small-Molecule Absorber A1094 as a Stable and Fast-Clearing NIR-II Imaging Agent.

Photoacoustic imaging (PAI) in the second near-infrared window (NIR-II) is conducive to deep-tissue imaging due to small scattering coefficients, but often requires exogenous imaging agents. At present, nanoparticle-based NIR-II imaging agents are mainly used in non-clinical studies, some basic components of which are resistant to metabolism in vivo. The aim of this study was to examine the ∼600 Da croconaine absorber A1094, absorbing lights around 1094 nm, as a rare, small-molecule NIR-II imaging agent in vivo. The clinical translational potential of A1094 injection were systematically revealed, including sufficient solubility and freeness in blood, good anti-interference ability, and favourable pH/oxidation-reduction/metabolic stabilities. After intravenous administration of A1094 injection, PAI of murine ears exhibited comparable capillaries visibility to that of PAI with popular Au nanorods. The contrasts achieved with A1094 and Au nanorods were 1.78 and 1.29 times higher than before administration, in the healthy group, and 3.25 and 1.58 times higher in the inflammation group. Notably, A1094 demonstrated a desired faster liver clearance than Au nanorods. The PAI signal of A1094 was cleared by 74.2 % after 3 h, whereas Au nanorods were only cleared by 43.1 %. The main metabolic mechanisms of A1094 were identified as N-methylation and lipid hydrolysis by murine liver microsomes in vitro. Therefore, A1094 may have promising clinical potential as a stable and fast-clearing NIR-II imaging agent.

18F-Labelled pyrrolopyrimidines reveal brain leucine-rich repeat kinase 2 expression implicated in Parkinson's disease.

18F-Labelled pyrrolopyrimidines were synthesized and evaluated as positron emission tomography (PET) probes to determine leucine-rich repeat kinase 2 (LRRK2) expression in the brain. With pyrrolopyrimidine derivative PF-06447475 as the lead compound, two in vivo-stable 18F-labelled pyrrolopyrimidines ([18F]1 and [18F]2) were synthesized automatically at radiochemical yields 8-10% (non-decay-corrected) with molar activities of 0.95 and 0.5 GBq/µmol, respectively. The measured Kd of 6.90 nM for 1 and 14.27 nM for 2 demonstrated high affinities for LRRK2. The LRRK2 G2019S mice had higher uptakes (P < 0.01) of [18F]1 in the olfactory bulb, striatum, and hippocampus than WT mice during microPET/CT imaging, consistent with immunohistology results of LRRK2 distribution. [11C]CFT microPET/CT imaging demonstrated a lower expression of dopamine transporter in LRRK2 G2019S mice. Parkinson's disease-like deficits in dopamine transporter synthesis and cognitive declines were noticed along with LRRK2 expression increase in the olfactory bulb, striatum, and hippocampus. Therefore, 18F-labelled pyrrolopyrimidines can reflect real-time LRRK2 expression changes implicated in Parkinson's disease, which paves the way for LRRK2-related neurodegenerative precise therapy.