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, [18F]KD23 and [18F]KD10. Key to the success of our structural modifications was a bioisosteric replacement of the triazole moiety with a fluorophenyl group. The 19F-substituted analog KD23 showed high affinity for the OX1R and selectivity over OX2R, while the high affinity ligand KD10 displayed similar Ki values for both subtypes. Radiolabeling starting from the respective pinacol ester precursors resulted in excellent radiochemical yields of 93% and 88% for [18F]KD23 and [18F]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.

Malignant melanoma has an aggressive nature and a high metastatic propensity resulting in the highest mortality rate of any skin cancer. In this study, we synthesized 18F-labeled procainamide (PCA) for detection of melanoma using positron emission tomography (PET), and evaluated its biological characteristics. The non-decay-corrected radiochemical yield of 18F-PCA was 10-15% and its in vitro stability was over 98% for 2 h. At 1 h, cellular uptake of 18F-PCA was 3.8-fold higher in a group with the presence of l-tyrosine than in a non-l-tyrosine-treated group. Furthermore, 18F-PCA permitted visualization of B16F10 (mouse melanoma) xenografts on microPET after intravenous injection, and was retained in the tumor for 60 min, with a high tumor-to-liver uptake ratio. 18F-PCA showed specific melanoma uptake in primary lesions with a high melanin targeting ability in small animal models. 18F-PCA may have potential as a PET imaging agent for direct melanoma detection.

The dopamine D4 receptor (D4R) has attracted considerable attention as potential target for the treatment of a broad range of central nervous system disorders. Although many efforts have been made to improve the performance of putative radioligand candidates, there is still a lack of D4R selective tracers suitable for in vivo PET imaging. Thus, the objective of this work was to develop a D4-selective PET ligand for clinical applications.
Four compounds based on previous and new lead structures were prepared and characterized with regard to their D4R subtype selectivity and predicted lipophilicity. From these, 3-((4-(2-fluorophenyl)piperazin-1-yl)methyl)-1H-pyrrolo[2,3-b]pyridine I and (S)-4-(3-fluoro-4-methoxybenzyl)-2-(phenoxymethyl)morpholine II were selected for labeling with fluorine-18 and subsequent evaluation by in vitro autoradiography to assess their suitability as D4 radioligand candidates for in vivo imaging.
The radiosynthesis of [18F]I and [18F]II was successfully achieved by copper-mediated radiofluorination with radiochemical yields of 7% and 66%, respectively. The radioligand [18F]II showed specific binding in areas where D4 expression is expected, whereas [18F]I did not show any uptake in distinct brain regions and exhibited an unacceptable degree of non-specific binding.
The compounds studied exhibited high D4R subtype selectivity and logP values compatible with high brain uptake, but only ligand [18F]II showed low non-specific binding and is therefore a good candidate for further evaluation.
The discovery of new lead structures for high-affinity D4 ligands opens up new possibilities for the development of suitable PET-radioligands.
PET-imaging of dopamine D4-receptors could facilitate understanding, diagnosis and treatment of neuropsychiatric and neurodegenerative diseases.

Hyperphosphorylated tau proteins are one of the neuropathological hallmarks in the Alzheimer\'s disease (AD) brain. The in vivo imaging of tau aggregates with nuclear medical imaging probes is helpful for the further comprehension of and medical intervention in the AD pathology. For tau-selective PET imaging, we newly designed and synthesized 18F-labeled benzimidazopyridine (BIP) derivatives with fluoroalkylamino groups, [18F]IBIPF1 and [18F]IBIPF2, and evaluated their utilities as tau imaging probes. They both bound selectively to tau against amyloid β (Aβ) aggregates in AD brain sections in vitro, and showed good pharmacokinetics in mouse brains in vivo. Notably, [18F]IBIPF1 exhibited high tau-selectivity (Tau/Aβ ratio = 34.8), high brain uptake (6.22% ID/g at 2 min postinjection), and subsequent washout (2.77% ID/g at 30 min postinjection). In vivo analysis of radiometabolites indicated that [18F]IBIPF1 was stable against metabolism in the mouse brain. These encouraging preclinical results suggest that further structural optimization based on the BIP scaffold may lead to the development of more useful tau imaging probes.

Carbonic anhydrase IX (CA IX) is the first carbonic anhydrase found to be associated with cancer that is over-expressed in a variety of human solid tumors. As a surrogate marker for hypoxia, the expression of CA IX is strongly upregulated in hypoxic tumors by hypoxia and hypoxia-inducible factor 1a (HIF-1a). In our pursuit of a CA IX-specific PET probe, we designed and synthesized a peptide-based CA IX imaging probe by the efficient click reaction of 1,3-dipolar cycloaddition of terminal alkynes and organic azides. The probe 18F-CA IX-P1-4-10 was obtained with a radiochemical yield of 35-45% (n = 5) and radiochemical purity of >99% in 70-80 min (HPLC purification time included). 18F-CA IX-P1-4-10 had good stability in phosphate buffered saline (PBS), but about 51% peptide degradation was detected in new-born calf serum (NBCS) after incubation. Preliminary microPET/CT experiments demonstrated a specific uptake of 18F-CA IX-P1-4-10 in HT29 tumor and the uptake of 18F-CA IX-P1-4-10 was blocked by peptide CA IX-P1-4-10-Yne pretreatment. Immunohistochemical staining and western blotting studies confirmed the HT29 tumor was CA IX-positive which further proved tumor accumulation of 18F-CA IX-P1-4-10 was correlated with CA IX expression. The results suggest that 18F-CA IX-P1-4-10 is a promising PET tracer for the specific imaging of CA IX-expressing tumors at the molecular level.

On the basis of a pyrazine core structure, three new adenosine A2B receptor ligands (7a-c) were synthesized containing a 2-fluoropyridine moiety suitable for 18F-labeling. Compound 7a was docked into a homology model of the A2B receptor based on X-ray structures of the related A2A receptor, and its interactions with the adenosine binding site were rationalized. Binding affinity data were determined at the four human adenosine receptor subtypes. Despite a rather low selectivity regarding the A1 receptor, 7a was radiolabeled as the most suitable candidate (Ki(A2B) = 4.24 nM) in order to perform in vivo studies in mice with the aim to estimate fundamental pharmacokinetic characteristics of the compound class. Organ distribution studies and a single PET study demonstrated brain uptake of [18F]7a with a standardized uptake value (SUV) of ≈1 at 5 min post injection followed by a fast wash out. Metabolism studies of [18F]7a in mice revealed the formation of a blood-brain barrier penetrable radiometabolite, which could be structurally identified. The results of this study provide an important basis for the design of new derivatives with improved binding properties and metabolic stability in vivo.

In the amyloid cascade hypothesis, β-amyloid (Aβ) plaques is one of the major pathological biomarkers in the Alzheimer\'s disease (AD) brain. We report the synthesis and evaluation of novel radiofluorinated chalcones, [18F]4-dimethylamino-4\'-fluoro-chalcone ([18F]DMFC) and [18F]4\'-fluoro-4-methylamino-chalcone ([18F]FMC), as Aβ imaging probes. The conversion of iodine directly introduced to the chalcone backbone into fluorine was successfully carried out by 18F-labeling via the corresponding boronate precursors, achieving the direct introduction of fluorine-18 into the chalcone backbone to prepare [18F]DMFC and [18F]FMC. In a biodistribution study using normal mice, [18F]DMFC and [18F]FMC showed a higher initial uptake (4.43 and 5.47% ID/g at 2 min postinjection, respectively) into and more rapid clearance (0.52 and 0.66% ID/g at 30 min postinjection, respectively) from the brain than a Food and Drug Administration (FDA)-approved Aβ imaging agent ([18F]Florbetapir), meaning the improvement of the probability of detecting Aβ plaques and the reduction of non-specific binding in the brain. In the in vitro binding studies using aggregates of recombinant Aβ peptides, [18F]DMFC and [18F]FMC showed high binding affinity to recombinant Aβ aggregates at the Kd values of 4.47 and 6.50 nM, respectively. In the in vitro autoradiography (ARG) experiment with AD brain sections, [18F]DMFC and [18F]FMC markedly accumulated only in a region with abundant Aβ plaques, indicating that they clearly recognized human Aβ plaques in vitro. These encouraging results suggest that [18F]DMFC and [18F]FMC may be promising PET probes for the detection of an amyloid pathology and the early diagnosis of AD with marked accuracy.

BODIPYs (boron dipyrromethenes) are fluorescent dyes which show high stability and quantum yields. They feature the possibility of selective 18F-fluorination at the boron-core. Attached to a bioactive molecule and labeled with [18F]fluorine, the resulting compounds are promising tracers for multimodal imaging in vivo and can be used for PET and fluorescence imaging. A BODIPY containing a phenyl and a hydroxy substituent on boron was synthesized and characterized. Fluorinated and hydroxy substituted dyes were coupled to an isatin-based caspase inhibitor via cycloaddition and the resulting compounds were evaluated in vitro in caspase inhibition assays. The metabolic stability and the formed metabolites were investigated by incubation with mouse liver microsomes and LC-MS analysis. Subsequently the fluorophores were labeled with [18F]fluorine and an in vivo biodistribution study using dynamic PET was performed.

Positron emission tomography (PET) investigations of the 5-HT2A receptor (5-HT2AR) system can be used as a research tool in diseases such as depression, Alzheimer\'s disease and schizophrenia. We have previously developed a 11C-labeled agonist PET ligand ([11C]Cimbi-36), and the aim of this study was to identify a 18F-labeled analogue of this PET-ligand. Thus, we developed a convergent radiochemical approach giving easy access to 5 different 18F-labeled ligands structurally related to Cimbi-36 from a common 18F-labeled intermediate. After intravenous injection, all ligands entered the pig brain. However, since within-scan intervention with ketanserin, a known orthosteric 5-HT2A receptor antagonist, did not result in significant blocking, the radioligands seem unsuitable for neuroimaging of the 5-HT2AR in vivo.

A novel radiofluorinated derivative of bombesin, (18)F-AmBF3-MJ9, was synthesized and evaluated for its potential to image prostate cancer by targeting the gastrin releasing peptide receptor (GRPR). AmBF3-MJ9 was prepared from an ammoniomethyl-trifluoroborate (AmBF3) conjugated alkyne 2 and azidoacetyl-MJ9 via a copper-catalyzed click reaction, and had good binding affinity for GRPR (Ki=0.5±0.1nM). The (18)F-labeling was performed via a facile one-step (18)F-(19)F isotope exchange reaction, and (18)F-AmBF3-MJ9 was obtained in 23±5% (n=3) radiochemical yield in 25min with >99% radiochemical purity and 100±32GBq/μmol specific activity. (18)F-AmBF3-MJ9 was stable in mouse plasma, and was partially (22-30%) internalized after binding to GRPR. Positron emission tomography (PET) imaging and biodistribution studies in mice showed fast renal excretion and good uptake of (18)F-AmBF3-MJ9 by GRPR-expressing pancreas and PC-3 prostate cancer xenografts. Tumor uptake was 1.37±0.25%ID/g at 1h, and 2.20±0.13%ID/g at 2h post-injection (p.i.) with low background uptake and excellent tumor visualization (tumor-to-muscle ratios of 75.4±5.5). These data suggest that (18)F-AmBF3-MJ9 is a promising PET tracer for imaging GRPR-expressing prostate cancers.