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Perfusion dynamics play a vital role in delivering essential nutrients and oxygen to tissues while removing metabolic waste products. Imaging techniques such as magnetic resonance imaging (MRI), computed tomography (CT), and positron emission tomography (PET) use contrast agents to visualize perfusion and clearance patterns; however, each technique has specific limitations. Hybrid PET/MRI combines the quantitative power and sensitivity of PET with the high functional and anatomical detail of MRI and holds great promise for precision in molecular imaging. However, the development of dual PET/MRI probes has been hampered by challenging synthesis and radiolabeling. Here, we present a novel PET/MRI probe, [ 18 F][Gd(FL 1 )], which exhibits excellent stability comparable to macrocyclic MRI contrast agents used in clinical practice. The unique molecular design of [ 18 F][Gd(FL 1 )] allows selective and expeditious radiolabeling of the gadolinium chelate in the final synthetic step. Leveraging the strengths of MRI and PET signals, the probe enables quantitative in vivo mapping of perfusion and excretion dynamics through an innovative voxel-based analysis. The diagnostic capabilities of [ 18 F][Gd(FL 1 )] were demonstrated in a pilot study on healthy mice, successfully detecting early cases of unilateral renal dysfunction, a condition that is typically challenging to diagnose. This study introduces a new approach for PET/MRI and emphasizes a streamlined probe design for practical synthesis and improved diagnostic accuracy., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

MerTK PET imaging holds potential as a promising approach for assessing tumor aggressiveness and monitoring treatment response. In this study, we synthesized a series of 18 F- and 68 Ga-labeled tracers derived from MerTK inhibitors for detection of MerTK expression. Among the synthesized agents, the dimeric compounds [ 68 Ga] 10 and [ 68 Ga] 12 demonstrated good in vivo and in vitro stability, high affinities to the MerTK receptor, and good MerTK-targeting specificity. Notably, [ 68 Ga] 10 exhibited a tumor uptake of 2.6 ± 0.2%ID/g at 1 h p. i. in B16F10 tumor-bearing mice, nearly tripling the uptake of its monomeric counterpart [ 68 Ga] 3 . A similar enhancement was observed with [ 68 Ga] 12 compared to its monomeric analog [ 68 Ga] 6 . Additionally, [ 18 F] 14 achieved a tumor uptake of 7.6 ± 0.5%ID/g at 2 h p. i., outperforming the previously reported [ 18 F] 15 . Biodistribution analysis further validated the results, highlighting their potential for clinical investigation.

The orexin 1 receptor (OX1R) has been suggested to be involved in the reward and autonomic nervous systems. Positron emission tomography (PET) of OX1R contributes to elucidating its role and developing new drugs. However, there are no useful PET probes for in vivo imaging of OX1R. Here, we newly designed and synthesized 18 F-labeled 1,4-diazepane derivatives and evaluated their utilities as OX1R PET probes. In particular, BTF showed high and selective binding affinity for OX1R. In a biodistribution study using normal mice, [ 18 F]BTF exhibited brain uptake, and radioactivity in the brain was significantly decreased by preinjection of unlabeled BTF. In a PET/CT study, it was suggested that [ 18 F]BTF has the potential to visualize high-expression regions of OX1R in the normal mouse brain. Collectively, [ 18 F]BTF has the fundamental features of an OX1R PET probe, and further studies may lead to the development of more useful probes.

Serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter involved in many physiological and pathological mechanisms through its numerous receptors. Among these, the 5-HT 2B receptor is known to play a key role in multiple brain disorders but remains poorly understood. Positron emission tomography (PET) can contribute to a better understanding of pathophysiological mechanisms regulated by the 5-HT 2B receptor. To develop the first PET radiotracer for the 5-HT 2B receptor, RS-127445, a well-known 5-HT 2B receptor antagonist, was labeled with fluorine-18. [ 18 F]RS-127445 was synthesized in a high radiochemical purity and with a good molar activity and radiochemical yield. Preliminary PET scans in rats showed good brain penetration of [ 18 F]RS-127445. However, competition experiments and in vitro autoradiography showed high non-specific binding, especially to brain white matter., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Due to the fast reaction rate, 18 F-labeled tetrazines have been widely applied in positron emission tomography (PET) imaging in cancer research and drug discovery. In this work, several functional 18 F-triazolyl-tetrazines were rapidly obtained through an optimized copper-catalyzed alkyene-azide cycloaddition reaction system in >99% radiochemical conversions. Notably, the commonly used 18 F-labeled azides were isolated through cartridges and directly used for cycloadditions, which greatly simplified the labeling procedure. The assembled triazolyl-tetrazines demonstrated high in vitro stability and reaction kinetics, exhibiting considerable potential for the development of PET agents.

The development of theranostic agents for radiopharmaceuticals based on therapeutic alpha emitters marks an important clinical need. We describe a strategy for the development of theranostic agents of this type via the functionalization of the ligand with the diagnostic radionuclide fluorine-18. An analogue of macropa, an 18-membered macrocyclic chelator with high affinity for alpha therapeutic radiometals, was synthesized and its complexation properties with metal ions were determined. The new macropa-F ligand was used for quantitative radiometal complexation with lead-203 and bismuth-207, as surrogates for their alpha-emitting radioisotopes. As a diagnostic partner, a radiofluorinated macropa ligand was used for quantitative bismuth(III) and lead(II) complexation. All fluorine-18 and radiometal complexes are highly stable in human serum over several days. This study presents a new proof-of-principle approach for developing theranostic agents based on alpha-emitting radionuclides and fluorine-18.

Hepatocellular cancer (HCC) remained a life-threatening carcinoma. Agents for HCC imaging and therapy were expected to possess different intratumoral retention time. To construct an agent with different intratumoral retention time when applied for tumor imaging or therapy remained great values. A lasialoglycoprotein receptor (ASGPR) targeted lactobionic acid derivative (LABO) was constructed for fluorescent imaging and photodynamic therapy of HCC. 18 F labeled LABO ( 18 F-LABO) was developed for PET imaging of HCC. LABO and 18 F-LABO showed similar molecular structure. LABO exhibited characteristic of viscosity and concentration-induced intratumoral in-situ self-assembly to expand the intratumoral retention. LABO was non-fluorescent at free stage, but emitted NIR fluorescence and generated irradiation-induced ROS after self-assembly for fluorescent imaging and photodynamic therapy. ASGPR specificity of LABO and 18 F-LABO was confirmed using HepG2 cell. Biodistribution and fluorescent imaging confirmed the different tumor retention time of LABO and 18 F-LABO when used for photodynamic therapy and PET imaging. PET imaging and photodynamic therapy were performed on HepG2 tumor bearing mice, which revealed that 18 F-LABO/LABO could specifically accumulated in the HepG2 tumor for tumor location/inhibition. LABO/ 18 F-LABO with excellent HCC specificity but different intratumoral behaviors showed great values for the PET/NIR imaging and photodynamic therapy for HCC., (© 2024. The Author(s).)

Heat shock protein 90 (Hsp90) is a promising target for cancer therapy and imaging. Accurate detection of Hsp90 levels in tumors via noninvasive PET imaging might be beneficial for management. To achieve this, the precursor compound Dimer-Sansalvamide A (Dimer-San A) was PEGylated and modified by conjugating it with the bifunctional chelator 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). The 18 F-labeled PEGylated Dimer-SanA decapeptide ( 18 F-PEGylated San A) was completed within 30 min using a two-step process. In vitro stability and specificity were assessed, including competition studies with the Hsp90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG). MicroPET imaging was performed on PL45 tumor-bearing mice to evaluate probe accumulation and tumor-to-muscle ratios. Biodistribution studies determined the route of excretion. The probe resulted in a radiochemical yield of 23.11% with a purity exceeding 95%. In vitro , 18 F-PEGylated San A exhibited high stability and selectively accumulated in Hsp90-positive PL45 cells, with binding effectively blocked by the Hsp90 inhibitor 17AAG, confirming its specificity. MicroPET imaging of PL45 tumor-bearing mice showed significant probe accumulation in tumor tissues at 1 and 2 h postinjection (4.06 ± 0.30 and 3.72 ± 0.61%ID/g, respectively), with optimal tumor-to-muscle ratios observed at 2 h postinjection (6.09 ± 1.92). While 18 F-PEGylated San A demonstrates enhanced water solubility, as indicated by increased kidney uptake relative to liver accumulation. The study successfully incorporated PEG units to create the novel probe 18 F-PEGylated San A targeting to Hsp90 without affecting its targeting capability, aimed at improving the pharmacokinetics and PET imaging of Hsp90 expression noninvasively.

Abnormalities in the RAS-RAF signaling pathway occur in many solid tumors, leading to aberrant tumor proliferation, invasion, and metastasis. Due to the elusive pharmacology of RAS, RAF inhibitors have become the main targeted therapeutic drugs. Naporafenib (LXH-254) is a high-affinity pan-RAF inhibitor with FDA Fast Track Qualification. We sought to develop an 18 F-labeled molecular probe from LXH-254 for PET imaging of tumors overexpressing RAF to noninvasively screen patients for susceptibility to targeted RAF therapy. To reduce the lipid solubility, LXH-254 was designed with triethylene glycol di( p -toluenesulfonate) (TsO-PEG 3 -OTs) to obtain the precursor (LXH-254-OTs) and a nucleophilic substitution reaction with 18 F to obtain the tracer ([ 18 F]F-LXH-254). [ 18 F]F-LXH-254 exhibited good molar activity (7.16 ± 0.81 GBq/μmol), radiochemical purity (>95%), and stability. Micro-PET imaging revealed distinct radioactivity accumulation of [ 18 F]F-LXH-254 in tumors in the imaging groups, whereas in the blocked group, the tumor radioactivity level was consistent with the background tissue, illustrating the affinity and specificity of [ 18 F]F-LXH-254 in targeting RAF. Overall, [ 18 F]F-LXH-254 is a promising radiotracer for screening and diagnosing patients with RAF-related disease and monitoring their treatment. This is the first attempt at using an 18 F-labeled RAF-specific radiotracer.

A novel HPLC method was developed and validated to determine radiochemical identity, radiochemical purity and chemical purity for the analysis of O-(2-[ 18 F]fluoroethyl-l-tyrosine ([ 18 F]FET). In this method, an analytical Phenomenex Gemini C18 column was used with an isocratic eluent of 7 % ethanol and 93 % 50 mM potassium phosphate buffer (pH = 6.9). The flow rate was 1.0 mL/min and the injection volume was 10 μL. A photo-diode array detector set at 220 nm was used for UV mass detection and a single channel, high sensitivity radiation detector was used. The method validation assays including specificity, linearity, precision, accuracy, and robustness were evaluated. Results show that the method was suitable for qualitative and quantitative determination of radiochemical and chemical purity of [ 18 F]FET. This system has been routinely used for the analysis of more than 120 batches of [ 18 F]FET with radiochemical yield 23.7 ± 6 % (no decay corrected) and molar activity 593 ± 284 GBq/μmole in our facility to support human use., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Background: To investigate the potential utility of quantitative parameters obtained by 18 F-fibroblast activation protein inhibitor positron emission tomography/computed tomography ([ 18 F]AlF-NOTA-FAPI-04 PET/CT) in the assessment of organ involvement and disease activity in IgG4-related disease (IgG4-RD)., Methods: This study enrolled patients who underwent [ 18 F]AlF-NOTA-FAPI-04 PET/CT scans at the Department of Rheumatology, The First Affiliated Hospital, Zhejiang University School of Medicine from August 2021 to August 2022. The PET/CT images of the included patients were re-evaluated by PET center technicians, and the maximal standardized uptake value (SUV max ), metabolic lesion volume (MLV), and total lesion FAPI (TL-FAPI) were used to evaluate the involved organs and tissues that abnormally accumulated [ 18 F]AlF-NOTA-FAPI-04. The clinical and laboratory data of patients are also systematically collected and analyzed., Results: Among the patients included in this study, 12 patients met the IgG4-RD classification criteria established by the American College of Rheumatology in 2019. Among them, 8 were males and 4 were females, with an average age of 59.3 ± 11.5 years. 50% of IgG4-RD patients were found with more organ involvement on PET/CT than physical examination, ultrasonography, and computed tomography. IgG4 levels (Rho = 0.594, p = 0.042) and IgG4-RI (Rho = 0.647, p = 0.023) were significantly positively correlated with TL-FAPI. After linear regression analysis, only TL-FAPI showed a predictive value of RI (R 2  = 0.356, B = 0.008, p = 0.041)., Conclusions: [ 18 F]AlF-NOTA-FAPI-04 PET/CT is a useful tool for identifying asymptomatic organ involvement and assessing disease activity. The TL-FAPI as an indicator was positively correlated with IgG4-RD disease activity., (© 2024. The Author(s).)

PD-L1 is expressed in many tumors but rarely in normal tissues, therefore, it can be a target of PET imaging. In this work, we developed new peptide-based PET probes [ 18 F]AlF-PAI-PDL1p and [ 68 Ga]Ga-PAI-PDL1p with yields of 20-25 % and 40-55 %, respectively. [ 18 F]AlF-PAI-PDL1p and [ 68 Ga]Ga-PAI-PDL1p were synthesized within 30 min with high molar activities. [ 18 F]AlF-PAI-PDL1p and [ 68 Ga]Ga-PAI-PDL1p showed good stability in vivo and in vitro. In vitro cell studies showed [ 18 F]AlF-PAI-PDL1p and [ 68 Ga]Ga-PAI-PDL1p target PD-L1 specifically, with high uptake of 61.52 ± 4.39 and 19.29 ± 2.17 %ID/1 million cells in B16F10 cells at 60 min, respectively. Biodistribution results showed that both [ 18 F]AlF-PAI-PDL1p and [ 68 Ga]Ga-PAI-PDL1p had lower liver accumulation. In vivo PET imaging results showed that [ 18 F]AlF-PAI-PDL1p had a high tumor uptake of 4.23 ± 0.81 %ID/g at 2 h and increased uptake of 6.60 ± 1.01 %ID/g at 12 h. [ 68 Ga]Ga-PAI-PDL1p also showed high tumor uptake of 2.30 ± 0.20 %ID/g at 2 h and slightly increased uptake of 3.80 ± 0.26 %ID/g at 6 h. In conclusion, [ 18 F]AlF-PAI-PDL1p and [ 68 Ga]Ga-PAI-PDL1 seemed to be potential tracers for PET imaging of PD-L1 expression., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

C-X-C motif chemokine receptor 4 (CXCR4) is an attractive target for the diagnosis and treatment of cancers. Here, we aimed to develop a new CXCR4-targeted PET tracer, and to investigate the translational potential for noninvasive imaging of CXCR4 expression in various cancer entities through preclinical and pilot clinical studies. Methods [ 18 F]AlF-NOTA-QHY-04 was synthesized and evaluated by cellular uptake, blocking and biolayer interferometry studies in vitro . The pharmacokinetics, biodistribution, and imaging specificity were researched in tumor-bearing mice. [ 18 F]AlF-NOTA-QHY-04 PET/CT imaging was performed on 55 patients with different types of cancers. Correlations between ex vivo CXCR4 expression and PET parameters, and CXCR4 expression characteristics in different tumors were analyzed by histopathological staining in patients. Results [ 18 F]AlF-NOTA-QHY-04 was prepared with high radiolabeling yield and radiochemical purity, exhibiting good stability, high binding affinity and specificity for CXCR4 . NCI-H69 (small cell lung cancer, SCLC) tumor-bearing mice showed the highest tumor uptake (4.98 ± 0.98%ID/mL, P < 0.0001) on PET imaging except for Daudi lymphoma xenograft model, which was consistent with the results of cellular and histological analyses. Patients with diffuse large B-cell lymphoma showed the highest tumor uptake (SUV max , 11.10 ± 4.79) followed by SCLC patients (SUV max , 7.51 ± 3.01), which were both significantly higher than other solid tumors ( P < 0.05). The radiotracer uptake of high-grade gliomas is significantly higher than that of low-grade gliomas (3.13 ± 0.58 vs. 1.18 ± 0.51, P = 0.005). Significant higher tumor-to-normal brain ratio of [ 18 F]AlF-NOTA-QHY-04 than [ 18 F]FDG was found in primary brain tumors (62.55 ± 43.24 vs 1.70 ± 0.25, P = 0.027). Positive correlations between ex vivo CXCR4 expression and [ 18 F]AlF-NOTA-QHY-04 uptake (all P < 0.01) were recorded. Multicolor immunofluorescence staining indicated the high tracer uptake in certain patients was mainly due to the high expression of CXCR4 in tumor cells, followed by macrophages. Conclusion The CXCR4-targeted radiotracer [ 18 F]AlF-NOTA-QHY-04 was successfully prepared with favorable yield, high specificity and binding affinity to CXCR4. Preclinical and pilot clinical studies demonstrated its feasibility and potential application in precise diagnosis for not only lymphoma but also SCLC and glioma. [ 18 F]AlF-NOTA-QHY-04 PET/CT can also provide a complementary mapping for brain tumors to [ 18 F]FDG PET/CT., Competing Interests: Competing Interests: The authors have declared that no competing interest exists., (© The author(s).)

Receptor-interacting serine/threonine protein kinase 1 (RIPK1) has emerged as an important regulator of pathologic cell death and inflammation and is implicated in the pathologies of various central nervous system diseases. In this study, we reported the development of three potent dihydropyrazole-cored RIPK1 positron emission tomography (PET) ligands [ 18 F] WL1 - 3 . Among these, [ 18 F] WL1 showed specific binding to RIPK1 in mouse brain sections in vitro through autoradiography and exhibited favorable brain kinetics in mice, characterized by a high initial uptake (brain 2 min = 4.89% ID/g) and rapid washout (brain 60 min = 0.21% ID/g). PET studies in rat brains revealed that [ 18 F] WL1 could readily penetrate the brain with specific binding confirmed by inhibition effects of unlabeled WL1 and GSK'547. Notably, [ 18 F] WL1 showed significant potential in imaging the alterations of RIPK1 in a rat brain of tumor necrosis factor α-induced systemic inflammatory response syndrome model. These findings may pave the way for the future design of potent RIPK1 PET ligands.

A 2-phenyl-3-difluoromethoxy-pyridinyl moiety features in potent phosphodiesterase 4D inhibitors that are considered to be candidate radiotracers for positron emission tomography if they are labeled with fluorine-18. Fluorine-18 could be installed as desired at the 3'-phenyl position with acridinium-mediated photoredox radiodeoxyfluorination in homologues bearing variously substituted 3'-aryloxy groups. However, a distal 3-difluoromethoxide (-OCHF 2 ) group strongly competes as a leaving group, especially when an electron-deficient aryloxy group is present at position 3'. A yield of up to 50% may occur without observable 19 F for 18 F exchange.

Single-domain antibodies, or nanobodies (Nbs), are promising biomolecules for use in molecular imaging due to their excellent affinity, specificity, and fast clearance from the blood. Given their short blood half-life, pairing Nbs with short-lived imaging radioisotopes is desirable. Because fluorine-18 ( 18 F) is routinely used for clinical imaging, it is an attractive radioisotope for Nbs. We report a novel sortase-based, site-specific 18 F-labeling method applied to three nanobodies. Labeled nanobodies were synthesized either by a two-step indirect radiolabeling method in one pot or by a one-step direct labeling method using a sortase-mediated conjugation of either the radiolabeled chelator (H-GGGK((±)-Al[ 18 F]FH 3 RESCA)-NH 2 ) or the unlabeled chelator (H-GGGK((±)-H 3 RESCA)-NH 2 ) followed by labeling with Al[ 18 F]F, respectively. The overall radiochemical yields were 15-43% ( n = 22, decay-corrected) in 70 min (indirect labeling) and 23-58% ( n = 12, decay-corrected) in 50 min (direct labeling). The radiochemical purities of the labeled nanobodies prepared by both methods were >98% with a specific activity of 400-600 Ci/mmol ( n = 22) for each of the three Nbs tested and exhibited excellent stability profiles under physiological conditions. This simple, site-specific, reproducible, and generalizable 18 F-labeling method to prepare nanobodies (Nb-Al[ 18 F]F-RESCA) or other low molecular weight biomolecules can easily be adopted in various settings for preclinical and clinical studies.

The Al 18 F-labeling approach offers a one-step access to radiofluorinated biomolecules by mimicking the labeling process for radiometals. Although these labeling conditions are considered to be mild compared to classic radiofluorinations, improvements of the chelating units have led to the discovery of (±)-H 3 RESCA , which allows Al 18 F-labeling already at ambient temperature. While the suitability of (±)-H 3 RESCA for functionalization and radiofluorination of proteins is well established, its use for small molecules or peptides is less explored. Herein, we advanced this acyclic pentadentate ligand by introducing an alkyne moiety for the late-stage functionalization of biomolecules via click chemistry. We show that in addition to Al 18 F-labeling, the cyclohexanediamine triazole (CHDT) moiety allows stable complexation of 68 Ga and 111 In. Three novel CHDT-functionalized PSMA inhibitors were synthesized and their Al 18 F-, 68 Ga-, and 111 In-labeled analogs were subjected to a detailed in vitro radiopharmacological characterization. Stability studies in vitro in human serum revealed among others a high kinetic inertness of all radiometal complexes. Furthermore, the Al 18 F-labeled PSMA ligands were characterized for their biodistribution in a LNCaP derived tumor xenograft mouse model by PET imaging. One radioligand, Al[ 18 F]F-CHDT-PSMA-1 , bearing a small azidoacetyl linker at the glutamate-urea-lysine motif, provided an in vivo performance comparable to that of [ 18 F]PSMA-1007 but with even higher tumor-to-blood and tumor-to-muscle ratios at 120 min p.i. Overall, our results highlight the suitability of the novel CHDT moiety for functionalization and radiolabeling of small molecules or peptides with Al 18 F, 68 Ga, and 111 In and the triazole ring seems to entail favorable pharmacokinetic properties for molecular imaging purposes.

In 2017, the FDA authorized 5-aminolevulinic acid (5-ALA) for intraoperative optical imaging of suspected high-grade gliomas. This was the first authorized optical imaging agent for brain tumor surgery to enhance the visualization of malignant tissue. Herein we report the synthesis of a racemic and enantiopure fluorinated analog of 5-ALA, i.e., 3-fluoro-5-aminolevulinic acid (3F-5-ALA). We anticipate that these studies will provide the foundation for the future construction of a fluorine-18-labeled 5-ALA PET tracer to be used for functional and metabolic imaging of gliomas.

Despite the availability of various 11 C-labeled positron emission tomography (PET) tracers for assessing P-glycoprotein (P-gp) function, there are still limitations related to complex metabolism, high lipophilicity, and low baseline uptake. This study aimed to address these issues by exploring a series of customized dihydropyridines (DHPs) with enhanced stability and reduced lipophilicity as alternative PET tracers for P-gp dysfunction. Compared with verapamil and the rest DHPs, dimethyl 4-(4-fluorophenyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate (1) exhibited superior cellular uptake differences between the human gastric cancer cell line SGC7901 and its drug-resistant counterpart. [ 18 F]1 is successfully synthesized using a novel "hot-Hantzsch" approach in 22.1 ± 0.1 % radiochemical yields. MicroPET/CT imaging demonstrated that the uptake of [ 18 F]1 in the brains of P-gp blocked mice increased by > 3 times compared to the control group. Additionally, [ 18 F]1 displayed favorable lipophilicity (log D = 2.3) and excellent clearance characteristics, making it a promising tracer candidate with low background noise and high contrast., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) regulates programmed cell death and inflammation, contributing to a wide range of human pathologies, including inflammatory disorders, neurodegenerative conditions, and cancer. Despite this, no RIPK1 positron emission tomography (PET) ligand with significant in vivo specificity has been reported to date. In this work, we designed and synthesized a new family of dihydropyrazole-cored ligands suitable for 18 F-labeling at the late stage. Among these, WL8 showed a strong binding affinity to RIPK1 (EC 50  = 19.9 nM, K d  = 25 nM) and was successfully labeled with 18 F in the 6-position of pyridine ring, yielding a high radiochemistry yield of 27.9 % (decay-corrected) and a high molar activity of 18.8-31.2 GBq/μmol. In in vitro autoradiography, [ 18 F]WL8 showed some specific binding in the brain sections of rats and lipopolysaccharide (LPS) model mice. Preliminary PET studies in rat brains revealed that [ 18 F]WL8 could efficiently penetrate the blood-brain barrier and was rapidly washed out. As anticipated, [ 18 F]WL8 exhibited a high initial uptake (brain 2min  = 4.80 % ID/g) in mouse brains, followed by a rapid washout (brain 60min  = 0.14 % ID/g), although no clear specific binding to RIPK1 was observed. Moderate in vivo stability was noted for [ 18 F]WL8 in mouse brains with 35.2 % of the parent fraction remaining after 30 min post-administration. Altogether, our work broadens the landscape and offers a new chemotype for RIPK1 PET ligand development., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Masson SAS. All rights reserved.)

Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) is a promising target for the diagnosis and treatment of various diseases, especially neurodegenerative disorders. Developing PET imaging probes targeting RIPK1 is beneficial for visualizing the connections between RIPK1 and diseases, as well as for related drug development. In this study, we report the design and synthesis of a series of novel RIPK1 inhibitors. Three potent inhibitors, 7i, 7k, and 8a, with good cell anti-necroptosis potency and physicochemical properties, were identified and selected for PET imaging probe development. Subsequently, three PET imaging radioligands ([ 11 C]7k, [ 18 F]7i, and [ 18 F]8a) were successfully synthesized. In mouse PET imaging studies, all three radioligands showed good brain uptake. Among them, probe [ 18 F]8a exhibited good binding specificity in both in vitro autoradiography and in vivo PET imaging studies. Additionally, [ 18 F]8a demonstrated good in vivo metabolic stability. This work highlights the potential of probe [ 18 F]8a for imaging brain RIPK1 in live animals, laying the groundwork for the future development of RIPK1 PET radioligands., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Masson SAS. All rights reserved.)

d-Amino acids such as d-alanine are substrates for bacterial peptidoglycan biosynthesis and are selectively taken up by bacteria and not mammalian cells. Consequently, d-amino acid metabolism is an attractive target for antibiotic discovery and the development of bacteria-specific imaging agents. d-Fluoroalanine and the deuterium-labeled analogue fludalanine (MK641) were originally explored as antibiotics by Merck but failed in clinical trials due to unaccepted toxicity. Herein, we synthesized a fluorine-18 labeled d-fluoroalanine, d-3-[ 18 F]fluoroalanine (d-[ 18 F]FAla), and its deuterated analogue, d-3-[ 18 F]fluoroalanine- d 3 (d-[ 18 F]FAla- d 3 ), and evaluated their capability to image bacterial infection. Both d-[ 18 F]FAla and d-[ 18 F]FAla- d 3 can accumulate up to 0.64-0.78% ID/cc in the infectious area at 15 min postinjection. Despite the reduction of in vivo defluorination not being observed for deuterated 18 F-labeled d-fluoroalanine, these radiolabeled d-alanine analogues were able to differentiate bacterial infection from sterile inflammation in a soft-tissue model of S. aureus infection.

The radiohybrid (rh) concept to design targeted (and chemically identical) radiotracers for imaging or radionuclide therapy of tumors has gained momentum. For this strategy, a new bifunctional Silicon-based Fluoride Acceptor (SiFA) moiety (SiFA)SeFe was synthesized, endowed with improved hydrophilicity and high versatility of integration into rh-compounds. Preliminary radiolabeling and stability studies under different conditions were conducted using model bioconjugate peptides. Further, three somatostatin receptor 2 (sstR2)-targeted rh-compounds ( (SiFA)SeFe-rhTATE1-3 , TATE = (Tyr 3 )-octreotate) were developed. Compound (SiFA)SeFe-rhTATE3 , enables labeling with 18 F for PET imaging or chelation of 177 Lu for therapy. The rh-compounds possess comparable receptor binding affinity and in vitro performance as good as the clinically proven gold standards. SstR2-specificity was further shown for (SiFA)SeFe-rhTATE2 using the chicken chorioallantoic membrane (CAM) model. The biodistribution of two compounds in mice showed high accumulation in tumors and excretion via the kidneys, demonstrating the clinical applicability of the (SiFA)SeFe moiety.

Mutations of isocitrate dehydrogenase 1 (IDH1) are key biomarkers for glioma classification, but current methods for detection of mutated IDH1 (mIDH1) require invasive tissue sampling and cannot be used for longitudinal studies. Positron emission tomography (PET) imaging with mIDH1-selective radioligands is a promising alternative approach that could enable non-invasive assessment of the IDH status. In the present work, we developed efficient protocols for the preparation of four 18 F-labeled derivatives of the mIDH1-selective inhibitor olutasidenib. All four probes were characterized by cellular uptake studies with U87 glioma cells harboring a heterozygous IDH1 mutation (U87-mIDH) and the corresponding wildtype cells (U87-WT). In addition, the most promising probe was evaluated by PET imaging in healthy mice and mice bearing subcutaneous U87-mIDH and U87-WT tumors. Although all four probes inhibited mIDH1 with variable potencies, only one of them ([ 18 F]mIDH-138) showed significantly higher in vitro uptake into U87-mIDH compared to U87-WT cells. In addition, PET imaging with [ 18 F]mIDH-138 in mice demonstrated good in vivo stability and low non-specific uptake of the probe, but also revealed significantly higher uptake into U87-WT compared to U87-mIDH tumors. Finally, application of a two-tissue compartment model (2TCM) to the PET data indicated that preferential tracer uptake into U87-WT tumors results from higher specific binding rather than from differences in tracer perfusion. In conclusion, these results corroborate recent findings that mIDH1-selective inhibition may not directly correlate with mIDH1-selective target engagement and indicate that in vivo engagement of wildtype and mutated IDH1 may be governed by factors that are not faithfully reproduced by in vitro assays, both of which could complicate development of PET probes.

Photoredox is a powerful synthetic tool in organic chemistry and has been widely used in various fields, including nuclear medicine and molecular imaging. In particular, acridinium-based organophotoredox radiolabeling has significantly impacted the production and discovery of positron emission tomography (PET) agents. Despite their extensive use in preclinical research, no PET agents synthesized by acridinium photoredox labeling have been tested in humans. [ 18 F]FDOPA is clinically used for tumor diagnosis and the evaluation of neuropsychiatric disorders, but its application is limited by complex synthesis methods, the need for expensive modules, and/or the high cost of consumable materials/cassettes. In this report, we integrated a photoredox labeling unit with an automated module and produced [ 18 F]FDOPA for human study. This research not only represents the first human study of a PET agent generated by acridinium-based organophotoredox reactions but also demonstrates the safety of this novel labeling method, serving as a milestone/reference for the clinical translation of other PET agents generated by this technique in the future.

α-synuclein (α-syn) pathologies are central to the development of synucleinopathies including Parkinson's disease (PD). Positron emission tomography (PET) imaging of α-syn pathologies is one strategy to facilitate the diagnosis, understanding, and treatment of synucleinopathies, but has been restricted by the lack of specific α-syn PET probes. In this work, we identified 2,6-disubstituted imidazo[2,1- b ][1,3,4]thiadiazole (ITA) as a new α-syn-binding scaffold. Through autoradiography studies, we discovered an iodinated lead compound [ 125 I] ITA-3 , with moderate binding affinity (IC 50 = 55 nM) to α-syn pathologies in human PD brain sections. Modified from [ 125 I] ITA-3 , we developed a potential PET tracer, [ 18 F] FITA-2 (radiochemical yield >25%, molar activity >110 GBq/μmol), which demonstrated clear signals in α-syn-rich regions in human PD brain tissues (IC 50 = 245 nM), good brain uptake (SUV peak = 2.80 ± 0.45), and fast clearance rate in rats. Overall, [ 18 F] FITA-2 appears to be a promising candidate for α-syn PET imaging and merits further development.

Activation of the adenosine 2A receptor (A 2A R) can lead to tumor immunosuppression, which results in poor prognosis of immunotherapy. The aim of this study was to design novel 18 F-labeled probes ([ 18 F]F-PFP 2 and [ 18 F]F-PFP 4 ) to visualize A 2A R in the tumor. The uptake of radioprobes in A 2A R-negative 4T1 breast tumor was lower than that of A 2A R-positive B16F10 melanoma at 1 h p.i. (1.22 ± 0.36% ID/g vs 2.80 ± 0.72% ID/g), 2 h p.i. (1.09 ± 0.20% ID/g vs 2.93 ± 0.76% ID/g) and 3 h p.i. (0.89 ± 0.27% ID/g vs 2.73 ± 0.58% ID/g), respectively. B16F10 lung metastasis models were employed to expand the application scenarios, observing significantly higher uptake of [ 18 F]F-PFP 2 in metastatic lesions compared to normal lung tissue (5.55 ± 2.18% ID/g vs 1.89 ± 0.65% ID/g, tumor/lung ratio ∼3). It is given that [ 18 F]F-PFP 2 might lay the foundation for establishing an A 2A R-targeted imaging evaluation system for tumors, which will provide more precise guidance for personalized treatment.

Fatty acid binding protein 3 (FABP3) is expressed both in tumor cells and in the tumor vasculature, making it a potential target for medical imaging and therapy. In this study, we aimed to radiolabel a CooP peptide with a free amino and thiol group, and evaluate the radiolabeled product [ 18 F]FNA- N -CooP for imaging FABP3 expression in breast cancer brain metastases by positron emission tomography. [ 18 F]FNA- N -CooP was prepared by highly chemoselective N -acylation and characterized using different chemical approaches. We validated its binding to the target using in vitro tissue section autoradiography and performed stability tests in vitro and in vivo. [ 18 F]FNA- N -CooP was successfully synthesized in 16.8% decay-corrected radiochemical yield with high radiochemical purity (98.5%). It exhibited heterogeneous binding on brain metastasis tissue sections from a patient with breast cancer, with foci of radioactivity binding corresponding to FABP3 positivity. Furthermore, the tracer binding was reduced by 55% in the presence of nonradioactive FNA- N -CooP a blocker, indicating specific tracer binding and that FABP3 is a viable target for [ 18 F]FNA- N -CooP. Favorably, the tracer did not bind to necrotic tumor tissue. However, [ 18 F]FNA- N -CooP displayed limited stability both in vitro in mouse plasma or human serum and in vivo in mouse, therefore further studies are needed to improve the stability [ 18 F]FNA- N -CooP to be used for in vivo applications.

Molecular imaging using positron emission tomography (PET) can serve as a promising tool for visualizing biological targets in the brain. Insights into the expression pattern and the in vivo imaging of the G protein-coupled orexin receptors OX1R and OX2R will further our understanding of the orexin system and its role in various physiological and pathophysiological processes. Guided by crystal structures of our lead compound JH112 and the approved hypnotic drug suvorexant bound to OX1R and OX2R, respectively, we herein describe the design and synthesis of two novel radioligands, [ 18 F]KD23 and [ 18 F]KD10. Key to the success of our structural modifications was a bioisosteric replacement of the triazole moiety with a fluorophenyl group. The 19 F-substituted analog KD23 showed high affinity for the OX1R and selectivity over OX2R, while the high affinity ligand KD10 displayed similar K i values for both subtypes. Radiolabeling starting from the respective pinacol ester precursors resulted in excellent radiochemical yields of 93% and 88% for [ 18 F]KD23 and [ 18 F]KD10, respectively, within 20 min. The new compounds will be useful in PET studies aimed at subtype-selective imaging of orexin receptors in brain tissue., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Purpose: There is currently no ideal radiotracer for imaging bacterial infections. Radiolabelled D-amino acids are promising candidates because they are actively incorporated into the peptidoglycan of the bacterial cell wall, a structural feature which is absent in human cells. This work describes fluorine-18 labelled analogues of D-tyrosine and D-methionine, O-(2-[ 18 F]fluoroethyl)-D-tyrosine (D-[ 18 F]FET) and S-(3-[ 18 F]fluoropropyl)-D-homocysteine (D-[ 18 F]FPHCys), and their pilot evaluation studies as potential radiotracers for imaging bacterial infection., Procedures: D-[ 18 F]FET and D-[ 18 F]FPHCys were prepared in classical fluorination-deprotection reactions, and their uptake in Staphylococcus aureus and Pseudomonas aeruginosa was evaluated over 2 h. Heat killed bacteria were used as controls. A clinically-relevant foreign body model of S. aureus infection was established in Balb/c mice, as well as a sterile foreign body to mimic inflammation. The ex vivo biodistribution of D-[ 18 F]FPHCys in the infected and inflamed mice was evaluated after 1 h, by dissection and gamma counting. The uptake was compared to that of [ 18 F]FDG., Results: In vitro uptake of both D-[ 18 F]FET and D-[ 18 F]FPHCys was specific to live bacteria. Uptake was higher in S. aureus than in P. aeruginosa for both radiotracers, and of the two, higher for D-[ 18 F]FPHCys than D-[ 18 F]FET. Blocking experiments with non-radioactive D-[ 19 F]FPHCys confirmed specificity of uptake. In vivo, D-[ 18 F]FPHCys had greater accumulation in S. aureus infection compared with sterile inflammation, which was statistically significant. As anticipated, [ 18 F]FDG showed no significant difference in uptake between infection and inflammation., Conclusions: D-[ 18 F]FPHCys uptake was higher in infected tissues than inflammation, and represents a fluorine-18 labelled D-AA with potential to detect a S. aureus reference strain (Xen29) in vivo. Additional studies are needed to evaluate uptake of this radiotracer in clinical isolates., (© 2024. The Author(s).)

Recently, the folate receptor (FR) has become an exciting target for the diagnosis of FR-positive malignancies. Nevertheless, suboptimal in vivo pharmacokinetic properties, particularly high uptake in the renal and hepatobiliary systems, are important limiting factors for the clinical translation of most FR-based radiotracers. In this study, we developed a novel 18 F-labeled FR-targeted positron emission tomography (PET) tracer [ 18 F]AlF-NOTA-Asp 2 -PEG 2 -Folate modified with a hydrophilic linker (-Asp 2 -PEG 2 ) to optimize its pharmacokinetic properties and conducted a comprehensive preclinical assessment. The [ 18 F]AlF-NOTA-Asp 2 -PEG 2 -Folate was manually synthesized within 30 min with a non-decay-corrected radiochemical yield of 16.3 ± 2.0% (n = 5). Among KB cells, [ 18 F]AlF-NOTA-Asp 2 -PEG 2 -Folate exhibited high specificity and affinity for FR. PET/CT imaging and biodistribution experiments in KB tumor-bearing mice showed decent tumor uptake (1.7 ± 0.3% ID/g) and significantly decreased uptake in kidneys and liver (22.2 ± 2.1 and 0.3 ± 0.1% ID/g at 60 min p.i., respectively) of [ 18 F]AlF-NOTA-Asp 2 -PEG 2 -Folate, compared to the known tracer [ 18 F]AlF-NOTA-Folate (78.6 ± 5.1 and 5.3 ± 0.5 % ID/g at 90 min p.i., respectively). The favorable properties of [ 18 F]AlF-NOTA-Asp 2 -PEG 2 -Folate, including its efficient synthesis, decent tumor uptake, relatively low renal uptake, and rapid clearance from most normal organs, portray it as a promising PET tracer for FR-positive tumors., (© 2024 John Wiley & Sons Ltd.)

Purpose: To describe the pharmacokinetic properties of the [ 18 F]fluoro-polyethylene glycol(PEG)-folate radiotracer in PET/CT imaging of patients with advanced stage epithelial ovarian cancer (EOC)., Procedures: In five patients with advanced EOC (FIGO stage IIIB/IIIC, Fédération Internationale de Gynécologie et d'Obstétrique), a 90-min dynamic PET acquisition of the pelvis was performed directly after i.v. administration of 185 MBq [ 18 F]fluoro-PEG 6 -folate. Arterial blood samples collected at nineteen timepoints were used to determine the plasma input function. A static volume of interest (VOI) for included tumor lesions was drawn manually on the PET images. Modelling was performed using PMOD software. Three different models (a 1-tissue compartment model (1T2k) and two 2-tissue compartment models, irreversible (2T3k) and reversible (2T4k)) were compared in goodness of fit with the time activity curves by means of the Akaike information criterion., Results: The pharmacokinetic analysis in the pelvic area has proven to be much more challenging than expected. Only four out of 22 tumor lesions in five patients were considered suitable to perform modelling on. The remaining tumor lesions were inapt due to either low tracer uptake, small size, proximity to other [ 18 F]fluoro-PEG 6 -folate -avid structures and/or displacement by abdominal organ motion in the dynamic scan. Data from the four analyzed tumor lesions suggest that the irreversible 2T3k may best describe the pharmacokinetics. All 22 lesions were immunohistochemically stained positive for the folate receptor alpha (FRα) after resection., Conclusion: Performing pharmacokinetic analysis in the abdominal pelvic region is very challenging. This brief article describes the challenges and pitfalls in pharmacokinetic analysis of a tracer with high physiological accumulation in the intestines, in case of lesions of limited size in the abdominal pelvic area., (© 2024. The Author(s).)

The postsynaptic density (PSD) comprises numerous scaffolding proteins, receptors, and signaling molecules that coordinate synaptic transmission in the brain. Postsynaptic density protein 95 (PSD-95) is a master scaffold protein within the PSD and one of its most abundant proteins and therefore constitutes a very attractive biomarker of PSD function and its pathological changes. Here, we exploit a high-affinity inhibitor of PSD-95, AVLX-144, as a template for developing probes for molecular imaging of the PSD. AVLX-144-based probes were labeled with the radioisotopes fluorine-18 and tritium, as well as a fluorescent tag. Tracer binding showed saturable, displaceable, and uneven distribution in rat brain slices, proving effective in quantitative autoradiography and cell imaging studies. Notably, we observed diminished tracer binding in human post-mortem Parkinson's disease (PD) brain slices, suggesting postsynaptic impairment in PD. We thus offer a suite of translational probes for visualizing and understanding PSD-related pathologies.

Granzyme B is an immune-related biomarker that closely correlates with cytotoxic T lymphocytes (CTLs), and hence detecting the expression level of granzyme B can provide a dependable scheme for clinical immune response assessment. In this study, two positron emission tomography (PET) probes [ 18 F]SF-M-14 and [ 18 F]SF-H-14 targeting granzyme B are designed based on the intramolecular cyclization scaffold SF. [ 18 F]SF-M-14 and [ 18 F]SF-H-14 can respond to granzyme B and glutathione (GSH) to conduct intramolecular cyclization and self-assemble into nanoaggregates to enhance the retention of probe at the target site. Both probes are prepared with high radiochemical purity (>98%) and high stability in PBS and mouse serum. In 4T1 cells cocultured with T lymphocytes, [ 18 F]SF-M-14 and [ 18 F]SF-H-14 reach the maximum uptake of 6.71 ± 0.29 and 3.47 ± 0.09% ID/mg at 0.5 h, respectively, but they remain below 1.95 ± 0.22 and 1.47 ± 0.21% ID/mg in 4T1 cells without coculture of T lymphocytes. In vivo PET imaging shows that the tumor uptake in 4T1-tumor-bearing mice after immunotherapy is significantly higher (3.5 times) than that in the untreated group. The maximum tumor uptake of [ 18 F]SF-M-14 and [ 18 F]SF-H-14 in the mice treated with BEC was 4.08 ± 0.16 and 3.43 ± 0.12% ID/g, respectively, while that in the untreated mice was 1.04 ± 0.79 and 1.41 ± 0.11% ID/g, respectively. These results indicate that both probes have great potential in the early evaluation of clinical immunotherapy efficacy.

Aims: PET myocardial perfusion imaging (MPI) is the gold standard for the noninvasive diagnosis of ischemic myocardial. Construction of 18 F-labeled PET MPI probe showed benefits to reduce the imaging cost, and enhance the image quality and patient-friendliness., Methods: Two 18 F-labeled MPI probes ( 18 F-BoMPI) were developed. Detailed in vitro/vivo evaluation including photophysical properties, in vitro stability, myocardial cell uptake kinetics and mechanisms, cytotoxicity and IC50, biodistribution and plasma clearance curve were investigated. Resting and stressing myocardial perfusion PET imaging were performed in healthy and myocardial ischemic mice., Results: 18 F-BoMPI could be quickly labeled and easily postprocessed, and demonstrated excellent in vitro stability. Cell assays indicated that 18 F-BoMPI exhibited mitochondria-targeting but potential-independent myocardial uptake. In vivo evaluation revealed the effective myocardial uptake and rapid background clearance. PET MPI confirmed effective probe accumulation in the healthy heart, but rapidly clearance in the background, making heart clearly delineated in the images. Ischemic myocardial could be clearly distinguished as the region of radioactivity sparsity in PET MPI., Conclusion: The 18 F-labeled probes showed great potentials to reduce the practicability threshold of PET MPI.

Non-invasive imaging of GSK-3 expression in the brain will help to understand the role of GSK-3 in disease pathology and progression. Herein, we report the radiosynthesis and evaluation of two novel isonicotinamide based 18 F labeled PET probes, [ 18 F]2 and [ 18 F]6 for noninvasive imaging of GSK3. Among the developed PET probes, the in vitro blood-brain permeability coefficient of 2 (38 ± 20 × 10 -6  cm/s, n = 3) was found to be better than 6 (8.75 ± 3.90 × 10 -6  cm/s, n = 5). The reference compounds 2 and 6 showed nanomolar affinity towards GSK-3α and GSK-3β. PET probe [ 18 F]2 showed higher stability (100%) in mouse and human serums compared to [ 18 F]6 (67.01 ± 4.93%, n = 3) in mouse serum and 66.20 ± 6.38%, n = 3) in human serum at 120 min post incubation. The in vivo imaging and blocking studies were performed in wild-type mice only with [ 18 F]2 due to its observed stability. [ 18 F]2 showed a SUV of 0.92 ± 0.28 (n = 6) in mice brain as early as 5 min post-injection followed by gradual clearance over time., (© 2024. The Author(s).)

The 2,2,2-trifluoroethoxy group increasingly features in drugs and potential tracers for biomedical imaging with positron emission tomography (PET). Herein, we describe a rapid and transition metal-free conversion of fluoroform with paraformaldehyde into highly reactive potassium 2,2,2-trifluoroethoxide (CF 3 CH 2 OK) and demonstrate robust applications of this synthon in one-pot, two-stage 2,2,2-trifluoroethoxylations of both aromatic and aliphatic precursors. Moreover, we show that these transformations translate easily to fluoroform that has been labeled with either carbon-11 (t 1/2  = 20.4 min) or fluorine-18 (t 1/2  = 109.8 min), so allowing the appendage of complex molecules with a no-carrier-added 11 C- or 18 F- 2,2,2-trifluoroethoxy group. This provides scope to create candidate PET tracers with radioactive and metabolically stable 2,2,2-trifluoroethoxy moieties. We also exemplify syntheses of isotopologues of potassium 2,2,2-trifluoroethoxide and show their utility for stable isotopic labeling which can be of further benefit for drug discovery and development., (© 2024. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)

Radiolabeled peptides are valuable tools for diagnosis or therapies; they are often radiofluorinated using an indirect approach based on an F-18 prosthetic group. Herein, we are reporting our results on the F-18 radiolabeling of three peptides using two different methods based on click reactions. The first one used the well-known CuAAC reaction, and the second one is based on our recently reported hetero-Diels-Alder (HDA) using a dithioesters (thia-Diels-Alder) reaction. Both methods have been automated, and the 18 F-peptides were obtained in similar yields and synthesis time (37-39% decay corrected yields by both methods in 120-140 min). However, to obtain similar yields, the CuAAC needs a large amount of copper along with many additives, while the HDA is a catalyst and metal-free reaction necessitating only an appropriate ratio of water/ethanol. The HDA can therefore be considered as a minimalist method offering easy access to fluorine-18 labeled peptides and making it a valuable additional tool for the indirect and site-specific labeling of peptides or biomolecules.

Here we report our design and synthesis of 28 new fluorine-containing compounds as potential F-18 radiotracers for CNS imaging of sphingosine-1-phosphate receptor 1 (S1PR1), and determination of their in vitro binding potency and selectivity toward S1PR1 over other S1PR subtypes. Nine potent and selective compounds, 7c&d, 9a&c, 12b, 15b, and 18a-c with IC 50 values ranging from 0.6-12.3 nM for S1PR1 and weak binding toward S1PR2, 3, 4, and 5, were further 18 F-radiolabeled to produce [ 18 F]7c&d, [ 18 F]9a&c, [ 18 F]12b, [ 18 F]15b, and [ 18 F]18a-c. Multi-step F-18 radiochemistry procedures were investigated for radiosynthesis of [ 18 F]7c&d and [ 18 F]9a&c, and the presumed intermediates were synthesized and authenticated by analytic HPLC. We then performed nonhuman primate (NHP) PET brain imaging studies for eight radiotracers: [ 18 F]7c&d, [ 18 F]9a, [ 18 F]12b, [ 18 F]15b, and [ 18 F]18a-c. Three radiotracers, [ 18 F]7c, [ 18 F]7d, and [ 18 F]15b, had high NHP brain uptake with standardized uptake values (SUVs) at 2 h post-injection of 2.42, 2.84, and 2.00, respectively, and good brain retention. Our ex vivo biodistribution study in rats confirmed [ 18 F]7d had a high brain uptake with no in vivo defluorination. Radiometabolic analysis of [ 18 F]7c and [ 18 F]7d in rat plasma and brain samples found that [ 18 F]7c has a more favorable metabolic profile than [ 18 F]7d. However, the trend of increased brain uptake precludes [ 18 F]7c as a suitable PET radiotracer for imaging S1PR1 in the brain. Further structural optmization is warranted to identify a highly S1PR1-specific radiotracer with rapid brain uptake kinetics.

The design and radiosynthesis of [ 18 F]NT376, a high potency inhibitor of class-IIa histone deacetylases (HDAC) is reported. We utilized a three-step radiochemical approach that led to the radiosynthesis of [ 18 F]NT376 in a good radiochemical yield, (17.0 ± 3%, decay corrected), high radiochemical purity (> 97%) and relatively high molar activity of 185.0 GBq/µmol (> 5.0 Ci/µmol). The repositioning of the 18 F-radiolabel into a phenyl ring ( 18 F-Fluoro-aryl) of the class-IIa HDAC inhibitor avoided the shortcomings of the direct radiolabeling of the 5-trifluoromethyl-1,2,4-oxadiazole moiety that was reported by us previously and was associated with low molar activity (0.74-1.51 GBq/µmol, 20-41 mCi/µmol). This radiochemical approach could find a wider application for radiolabeling similar molecules with good radiochemical yield and high molar activity., (© 2024. The Author(s).)

Introduction: Bacterial infections are a major problem in medicine, and the rapid and accurate detection of such infections is essential for optimal patient outcome. Bacterial infections can be diagnosed by nuclear imaging, but most currently available modalities are unable to discriminate infection from sterile inflammation. Bacteria-targeted positron emission tomography (PET) tracers have the potential to overcome this hurdle. In the present study, we compared three 18 F-labelled PET tracers based on the clinically applied antibiotic vancomycin for targeted imaging of Gram-positive bacteria., Methods: [ 18 F]FB-NHS and [ 18 F]BODIPY-FL-NHS were conjugated to vancomycin. The resulting conjugates, together with our previously developed [ 18 F]PQ-VE1-vancomycin, were tested for stability, lipophilicity, selective binding to Gram-positive bacteria, antimicrobial activity and biodistribution. For the first time, the pharmacokinetic properties of all three tracers were compared in healthy animals to identify potential binding sites., Results: [ 18 F]FB-vancomycin, [ 18 F]BODIPY-FL-vancomycin, and [ 18 F]PQ-VE1-vancomycin were successfully synthesized with radiochemical yields of 11.7%, 2.6%, and 0.8%, respectively. [ 18 F]FB-vancomycin exhibited poor in vitro and in vivo stability and, accordingly, no bacterial binding. In contrast, [ 18 F]BODIPY-FL-vancomycin and [ 18 F]PQ-VE1-vancomycin showed strong and specific binding to Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), which was outcompeted by unlabeled vancomycin only at concentrations exceeding clinically relevant vancomycin blood levels. Biodistribution showed renal clearance of [ 18 F]PQ-VE1-vancomycin and [ 18 F]BODIPY-FL-vancomycin with low non-specific accumulation in muscles, fat and bones., Conclusion: Here we present the synthesis and first evaluation of the vancomycin-based PET tracers [ 18 F]BODIPY-FL-vancomycin and [ 18 F]PQ-VE1-vancomycin for image-guided detection of Gram-positive bacteria. Our study paves the way towards real-time bacteria-targeted diagnosis of soft tissue and implant-associated infections that are oftentimes caused by Gram-positive bacteria, even after prophylactic treatment with vancomycin., (© 2024. The Author(s).)

Purpose: The cluster of differentiation (CD70) is a potential biomarker of clear cell renal cell carcinoma (ccRCC). This study aims to develop CD70-targeted immuno-positron emission tomography/computed tomography (immunoPET/CT) imaging tracers and explore the diagnostic value in preclinical studies and the potential value in detecting metastases in ccRCC patients., Methods: Four novel CD70-specific single-domain antibodies (sdAbs) were produced and labelled with 18 F by the aluminium fluoride restrained complexing agent (AlF-RESCA) method to develop radiotracers. The visualisation properties of the tracers were evaluated in a subcutaneous ccRCC patient-derived xenograft (PDX) model. In a registered prospective clinical trial (NCT06148220), six patients with pathologically confirmed RCC were included and underwent immunoPET/CT examination exploiting one of the developed tracers (i.e., [ 18 F]RCCB6)., Results: We engineered four sdAbs (His-tagged RCCB3 and RCCB6, His-tag-free RB3 and RB6) specifically targeting recombinant human CD70 without cross-reactivity to murine CD70. ImmunoPET/CT imaging with [ 18 F]RCCB3 and [ 18 F]RCCB6 demonstrated a high tumour-to-background ratio in a subcutaneous ccRCC PDX model, with the latter showing better diagnostic potential supported by higher tumour uptake and lower bone accumulation. In comparison, [ 18 F]RB6, developed by sequence optimisation, has significantly lower kidney accumulation than that of [ 18 F]RCCB6. In a pilot translational study, [ 18 F]RCCB6 immunoPET/CT displayed ccRCC metastases in multiple patients and demonstrated improved imaging contrast and diagnostic value than 18 F-FDG PET/CT in a patient with ccRCC., Conclusion: The work successfully developed a series of CD70-targeted immunoPET/CT imaging tracers. Of them, [ 18 F]RCCB6 clearly and specifically identified inoculated ccRCCs in preclinical studies. Clinical translation of [ 18 F]RCCB6 suggests potential for identifying recurrence and/or metastasis in ccRCC patients., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Nitric oxide (NO) plays a pivotal role as a biological signaling molecule, presenting challenges in its specific detection and differentiation from other reactive nitrogen and oxygen species within living organisms. Herein, a 18 F-labeled (fluorine-18, t 1/2 = 109.7 min) small-molecule tracer dimethyl 4-(4-(4-[ 18 F]fluorobutoxy)benzyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate ([ 18 F]BDHP) is developed based on the dihydropyridine scaffold for positron emission tomography (PET) imaging of NO in vivo . [ 18 F]BDHP exhibits a highly sensitive and efficient C-C cleavage reaction specifically triggered by NO under physiological conditions, leading to the production of a 18 F-labeled radical that is readily retained within the cells. High uptakes of [ 18 F]BDHP are found within and around NO-generating cells, such as macrophages treated with lipopolysaccharide or benzo(a)pyrene. MicroPET/CT imaging of arthritic animal model mice reveals distinct tracer accumulation in the arthritic legs, showcasing a higher distribution of NO compared with the control legs. In summary, a specific radical-generating dihydropyridine tracer with a unique radical retention strategy has been established for the marking of NO in real-time in vivo .

To fully explore the potential of 18 F-labeled l-fluoroalanine for imaging cancer and other chronic diseases, a simple and mild radiosynthesis method has been established to produce optically pure l-3-[ 18 F]fluoroalanine (l-[ 18 F]FAla), using a serine-derivatized, five-membered-ring sulfamidate as the radiofluorination precursor. A deuterated analogue, l-3-[ 18 F]fluoroalanine-d 3 (l-[ 18 F]FAla-d 3 ), was also prepared to improve metabolic stability. Both l-[ 18 F]FAla and l-[ 18 F]FAla-d 3 were rapidly taken up by 9L/lacZ, MIA PaCa-2, and U87MG cells and were shown to be substrates for the alanine-serine-cysteine (ASC) amino acid transporter. The ability of l-[ 18 F]FAla, l-[ 18 F]FAla-d 3 , and the d-enantiomer, d-[ 18 F]FAla-d 3 , to image tumors was evaluated in U87MG tumor-bearing mice. Despite the significant bone uptake was observed for both l-[ 18 F]FAla and l-[ 18 F]FAla-d 3 , the latter had enhanced tumor uptake compared to l-[ 18 F]FAla, and d-[ 18 F]FAla-d 3 was not specifically taken up by the tumors. The enhanced tumor uptake of l-[ 18 F]FAla-d 3 compared with its nondeuterated counterpart, l-[ 18 F]FAla, warranted the further biological investigation of this radiotracer as a potential cancer imaging agent.

Despite the widespread use of hydrophilic building blocks to incorporate 18 F and improve tracer pharmacokinetics, achieving effective leaving group-mediated nucleophilic 18 F-fluorination in water (excluding 18 F/ 19 F-exchange) remains a formidable challenge. Here, we present a water-compatible S N 2 leaving group-mediated 18 F-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 18 F-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 18 F-fluorination in water with a 10 -2 M -1 s -1 level reaction rate constant (surpassing the 18 F/ 19 F-exchange) at room temperature. With the exergonic concerted S N 2 18 F-fluorination mechanism confirmed, this 18 F-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 18 F-tracers. [ 18 F]AquaF-Flurpiridaz demonstrates significantly improved radiochemical yield and molar activity compared to 18 F-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 S N 2 18 F-fluorination of small-molecule radiotracers.

The synthesis, biochemical evaluation and radiosynthesis of a cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitor and radioligand was performed. NT431, a newly synthesized 4-fluorobenzyl-abemaciclib, exhibited high potency to CDK4/6 and against four cancer cell lines with IC 50 similar to that of the parent abemaciclib. We performed a two-step one-pot radiosynthesis to produce [ 18 F]NT431 with good radiochemical yield (9.6 ± 3%, n = 3, decay uncorrected), high radiochemical purity (>95%), and high molar activity (>370 GBq/µmol (>10.0 Ci/µmol). In vitro autoradiography confirmed the specific binding of [ 18 F]NT431 to CDK4/6 in brain tissues. Dynamic PET imaging supports that both [ 18 F]NT431 and the parent abemaciclib crossed the BBB albeit with modest brain uptake. Therefore, we conclude that it is unlikely that NT431 or abemaciclib (FDA approved drug) can accumulate in the brain in sufficient concentrations to be potentially effective against breast cancer brain metastases or brain cancers. However, despite the modest BBB penetration, [ 18 F]NT431 represents an important step towards the development and evaluation of a new generation of CDK4/6 inhibitors with superior BBB penetration for the treatment and visualization of CDK4/6 positive tumors in the CNS. Also, [ 18 F]NT431 may have potential application in peripheral tumors such as breast cancer and other CDK4/6 positive tumors.

Herein, we disclose that pyridinium salts derived from abundant (hetero)anilines represent a novel precursor class for nucleophilic aromatic substitution reactions with [ 18 F]fluoride. The value of this new 18 F-fluorodeamination is demonstrated with the synthesis of over 30 structurally diverse and complex heteroaryl 18 F-fluorides, several derived from scaffolds that were yet to be labelled with fluorine-18. The protocol tolerates heteroarenes and functionalities commonly found in drug discovery libraries, and is amenable to scale-up and automation on a commercial radiosynthesiser., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Positron emission tomography (PET) imaging of tau aggregation in Alzheimer's disease (AD) is helping to map and quantify the in vivo progression of AD pathology. To date, no high-affinity tau-PET radiopharmaceutical has been optimized for imaging non-AD tauopathies. Here we show the properties of analogues of a first-in-class 4R-tau lead, [ 18 F]OXD-2115, using ligand-based design. Over 150 analogues of OXD-2115 were synthesized and screened in post-mortem brain tissue for tau affinity against [ 3 H]OXD-2115, and in silico models were used to predict brain uptake. [ 18 F]OXD-2314 was identified as a selective, high-affinity non-AD tau PET radiotracer with favorable brain uptake, dosimetry, and radiometabolite profiles in rats and non-human primate and is being translated for first-in-human PET studies., (© 2024. The Author(s).)

The compound 2-{[(trifluoromethyl)sulfonyl]oxy}propane-1,3-diyl bis(4-methylbenzenesulfonate) (TPB) is a crucial intermediate in the synthesis of 18 F-radiolabeled cromolyn derivatives. In this work, we combine 1 H NMR spectroscopy, X-ray crystallography, ab initio molecular dynamics, and NMR calculations to examine the structure, interactions, and solvation dynamics of the TPB molecule. In CDCl 3 , the CH 2 groups within its glyceryl-derived linker exhibit a single set of proton signals in the 1 H NMR measurements. However, when TPB is dissolved in DMSO- d 6 , distinct splitting patterns emerge despite its seemingly symmetric chemical structure. Crystallographic analysis further unveils the absence of overall symmetry in its three-dimensional arrangement. To elucidate these unique NMR features, we carry out ab initio molecular dynamics simulations and characterize the solvation structures and dynamics of TPB in CHCl 3 and DMSO solutions. In contrast to the predominantly nonpolar nature of the CHCl 3 solvents, DMSO directly participates in C-H···O hydrogen-bonding interactions with the solute molecule, leading to the splitting of its -CH 2 chemical shifts into two distinct distributions. The comprehensive understanding of the structure and solvation interactions of TPB provides essential insights into its application in the radiofluorination reactions of cromolyn derivatives and holds promise for the future development of radiolabeled dimeric drugs.

Nuclear imaging of aggregated α-synuclein pathology is an urgent clinical need for Parkinson's disease, yet promising tracers for brain α-synuclein aggregates are still rare. In this work, a class of compact benzothiazole derivatives was synthesized and evaluated for α-synuclein aggregates. Among them, azobenzothiazoles exhibited specific and selective detection of α-synuclein aggregates under physiological conditions. Fluoro-pegylated azobenzothiazole NN-F further demonstrated high-affinity binding to α-synuclein aggregates and efficient 18 F-radiolabeling via nucleophilic displacement of a tosyl precursor. [ 18 F]NN-F was stable in plasma in vitro and showed efficient brain uptake with little defluorination in vivo .