Radiosynthesis and Preclinical Evaluation of m-[18F]FET and [18F]FET-OMe as Novel [18F]FET Analogs for Brain Tumor Imaging.

O-([18F]Fluoroethyl)-l-tyrosine ([18F]FET) is actively transported into the brain and cancer cells by LAT1 and possibly other amino acid transporters, which enables brain tumor imaging by positron emission tomography (PET). However, tumor delivery of this probe in the presence of competing amino acids may be limited by a relatively low affinity for LAT1. The aim of the present work was to evaluate the meta-substituted [18F]FET analog m-[18F]FET and the methyl ester [18F]FET-OMe, which were designed to improve tumor delivery by altering the physicochemical, pharmacokinetic, and/or transport properties. Both tracers could be prepared with good radiochemical yields of 41-56% within 66-90 min. Preclinical evaluation with [18F]FET as a reference tracer demonstrated reduced in vitro uptake of [18F]FET-OMe by U87 glioblastoma cells and no advantage for in vivo tumor imaging. In contrast, m-[18F]FET showed significantly improved in vitro uptake and accelerated in vivo tumor accumulation in an orthotopic glioblastoma model. As such, our work identifies m-[18F]FET as a promising alternative to [18F]FET for brain tumor imaging that deserves further evaluation with regard to its transport properties and in vivo biodistribution.

Desmethyl SuFEx-IT: SO2F2-Free Synthesis and Evaluation as a Fluorosulfurylating Agent.

Access to SuFExable compounds was remarkably simplified by introduction of the solid FO2S-donor SuFEx-IT. However, the published process for preparation of this reagent relies on the use of sulfuryl fluoride (SO2F2), which is difficult to obtain and highly toxic. Herein, we disclose a simple protocol for SO2F2-free, hectogram-scale preparation of the analogous desmethyl SuFEx-IT from inexpensive starting materials. The reagent was prepared in a high (85%) total yield and without chromatographic purification steps. In addition, we demonstrate the utility of desmethyl SuFEx-IT by successful preparation of a series of fluorosulfates and sulfamoyl fluorides in high to excellent yields. As such, our work recognizes desmethyl SuFEx-IT as a valuable alternative to common FO2S-donors and enables cost-efficient access to substrates for SuFEx click chemistry.

Next Generation Copper Mediators for the Efficient Production of 18 F-Labeled Aromatics.

Cu-mediated radiofluorination is a versatile tool for the preparation of 18 F-labeled (hetero)aromatics. In this work, we systematically evaluated a series of complexes and identified several generally applicable mediators for highly efficient radiofluorination of aryl boronic and stannyl substrates. Utilization of these mediators in nBuOH/DMI or DMI significantly improved 18 F-labeling yields despite use of lower precursor amounts. Impressively, application of 2.5 µmol aryl boronic acids was sufficient to achieve 18 F-labeling yields of up to 75 %. The practicality of the novel mediators was demonstrated by efficient production of five PET-tracers and transfer of the method to an automated radiosynthesis module. In addition, (S)-3-[18 F]FPhe and 6-[18 F]FDOPA were prepared in activity yields of 23±1 % and 30±3 % using only 2.5 µmol of the corresponding boronic acid or trimethylstannyl precursor.

Preparation of NIn-Methyl-6-[18F]fluoro- and 5-Hydroxy-7-[18F]fluorotryptophans as Candidate PET-Tracers for Pathway-Specific Visualization of Tryptophan Metabolism.

Tryptophan (Trp) is an essential proteinogenic amino acid and metabolic precursor for several signaling molecules that has been implicated in many physiological and pathological processes. Since the two main branches of Trp metabolism-serotonin biosynthesis and kynurenine pathway-are differently affected by a variety of neurological and neoplastic diseases, selective visualization of these pathways is of high clinical relevance. However, while positron emission tomography (PET) with existing probes can be used for non-invasive assessment of total Trp metabolism, optimal imaging agents for pathway-specific PET imaging are still lacking. In this work, we describe the preparation of two 18F-labeled Trp derivatives, NIn-methyl-6-[18F]fluorotryptophan (NIn-Me-6-[18F]FTrp) and 5-hydroxy-7-[18F]fluorotryptophan (5-HO-7-[18F]FTrp). We also report feasible synthetic routes for the preparation of the hitherto unknown boronate radiolabeling precursors and non-radioactive reference compounds. Under optimized conditions, alcohol-enhanced Cu-mediated radiofluorination of the respective precursors afforded NIn-Me-6-[18F]FTrp and 5-HO-7-[18F]FTrp as application-ready solutions in radiochemical yields of 45 ± 7% and 29 ± 4%, respectively. As such, our work provides access to two promising candidate probes for pathway-specific visualization of Trp metabolism in amounts sufficient for their preclinical evaluation.

Mutated Isocitrate Dehydrogenase (mIDH) as Target for PET Imaging in Gliomas.

Gliomas are the most common primary brain tumors in adults. A diffuse infiltrative growth pattern and high resistance to therapy make them largely incurable, but there are significant differences in the prognosis of patients with different subtypes of glioma. Mutations in isocitrate dehydrogenase (IDH) have been recognized as an important biomarker for glioma classification and a potential therapeutic target. However, current clinical methods for detecting mutated IDH (mIDH) require invasive tissue sampling and cannot be used for follow-up examinations or longitudinal studies. PET imaging could be a promising approach for non-invasive assessment of the IDH status in gliomas, owing to the availability of various mIDH-selective inhibitors as potential leads for the development of PET tracers. In the present review, we summarize the rationale for the development of mIDH-selective PET probes, describe their potential applications beyond the assessment of the IDH status and highlight potential challenges that may complicate tracer development. In addition, we compile the major chemical classes of mIDH-selective inhibitors that have been described to date and briefly consider possible strategies for radiolabeling of the most promising candidates. Where available, we also summarize previous studies with radiolabeled analogs of mIDH inhibitors and assess their suitability for PET imaging in gliomas.

Imaging of cerebral tryptophan metabolism using 7-[18F]FTrp-PET in a unilateral Parkinsonian rat model.

Degradation products of the essential amino acid tryptophan (Trp) are important signaling molecules in the mammalian brain. Trp is metabolized either through the kynurenine pathway or enters serotonin and melatonin syntheses. The aim of the present work was to examine the potential of the novel PET tracer 7-[18F]fluorotryptophan ([18F]FTrp) to visualize all three pathways in a unilateral 6-OHDA rat model. [18F]FDOPA-PET scans were performed in nine 6-OHDA-injected and six sham-operated rats to assess unilateral dopamine depletion severity four weeks after lesion placement. Afterwards, 7-[18F]FTrp-PET scans were conducted at different timepoints up to seven months after 6-OHDA injection. In addition, two 6-OHDA-injected rats were examined for neuroinflammation using [18F]DAA1106-PET. 7-[18F]FTrp-PET showed significantly increased tracer uptake at the 6-OHDA injection site which was negatively correlated to time after lesion placement. Accumulation of [18F]DAA1106 at the injection site was increased as well, suggesting that 7-[18F]FTrp uptake in this region may reflect kynurenine pathway activity associated with inflammation. Bilaterally in the dorsal hippocampus, 7-[18F]FTrp uptake was significantly decreased and was inversely correlated to dopamine depletion severity, indicating that it reflects reduced serotonin synthesis. Finally, 7-[18F]FTrp uptake in the pineal gland was significantly increased in relation with dopamine depletion severity, providing evidence that melatonin synthesis is increased in the 6-OHDA rat model. We conclude that 7-[18F]FTrp is able to detect alterations in both serotonin/melatonin and kynurenine metabolic pathways, and can be applied to visualize pathologic changes related to neurodegenerative processes.

Production of 6-l-[18F]Fluoro-m-tyrosine in an Automated Synthesis Module for 11C-Labeling.

6-l-[18F]Fluoro-m-tyrosine (6-l-[18F]FMT) represents a valuable alternative to 6-l-[18F]FDOPA which is conventionally used for the diagnosis and staging of Parkinson's disease. However, clinical applications of 6-l-[18F]FMT have been limited by the paucity of practical production methods for its automated production. Herein we describe the practical preparation of 6-l-[18F]FMT using alcohol-enhanced Cu-mediated radiofluorination of Bpin-substituted chiral Ni(II) complex in the presence of non-basic Bu4ONTf using a volatile iPrOH/MeCN mixture as reaction solvent. A simple and fast radiolabeling procedure afforded the tracer in 20.0 ± 3.0% activity yield within 70 min. The developed method was directly implemented onto a modified TracerLab FX C Pro platform originally designed for 11C-labeling. This method enables an uncomplicated switch between 11C- and 18F-labeling. The simplicity of the developed procedure enables its easy adaptation to other commercially available remote-controlled synthesis units and paves the way for a widespread application of 6-l-[18F]FMT in the clinic.

Preparation of a First 18F-Labeled Agonist for M1 Muscarinic Acetylcholine Receptors.

M1 muscarinic acetylcholine receptors (mAChRs) are abundant in postsynaptic nerve terminals of all forebrain regions and have been implicated in the cognitive decline associated with Alzheimer's disease and other CNS pathologies. Consequently, major efforts have been spent in the development of subtype-selective positron emission tomography (PET) tracers for mAChRs resulting in the development of several 11C-labeled probes. However, protocols for the preparation of 18F-labeled mAChR-ligands have not been published so far. Here, we describe a straightforward procedure for the preparation of an 18F-labeled M1 mAChR agonist and its corresponding pinacol boronate radiolabeling precursor and the non-radioactive reference compound. The target compounds were prepared from commercially available aryl fluorides and Boc protected 4-aminopiperidine using a convergent reaction protocol. The radiolabeling precursor was prepared by a modification of the Miyaura reaction and labeled via the alcohol-enhanced Cu-mediated radiofluorination. The developed procedure afforded the radiotracer in a non-decay-corrected radiochemical yield of 17 ± 3% (n = 3) and in excellent radiochemical purity (>99%) on a preparative scale. Taken together, we developed a straightforward protocol for the preparation of an 18F-labeled M1 mAChR agonist that is amenable for automation and thus provides an important step towards the routine production of a 18F-labeled M1 selective PET tracer for experimental and diagnostic applications.

Peripheral ganglia in healthy rats as target structures for the evaluation of PSMA imaging agents.

BACKGROUND: The recent implementation of PET with prostate specific membrane antigen (PSMA)-specific radiotracers into the clinical practice has resulted in the significant improvement of accuracy in the detection of prostate carcinoma (PCa). PSMA-expression in ganglia has been regarded as an important pitfall in prostate carcinoma-PET diagnostics but has not found any practical use for diagnosis or therapy. METHODS: We explored this phenomenon and demonstrated the applicability of peripheral ganglia in healthy rats as surrogates for small PSMA positive lesions for the preclinical evaluation of diagnostic PCa PET probes. Healthy rats were measured with PET/CT using the tracers [18F]DCFPyL, [Al18F]PSMA-11 and [68Ga]PSMA-11. Sections of ganglia were stained with an anti-PSMA antibody. [18F]DCFPyL uptake in ganglia was compared to that in LNCaP tumor xenografts in mice. RESULTS: Whereas [18F]DCFPyL and [68Ga]PSMA-11 were stable in vivo and accumulated in peripheral ganglia, [Al18F]PSMA-11 suffered from fast in vivo deflourination resulting in high bone uptake. Ganglionic PSMA expression was confirmed by immunohistochemistry. [18F]DCFPyL uptake and signal-to-noise ratio in the superior cervical ganglion was not significantly different from LNCaP xenografts. CONCLUSIONS: Our results demonstrated the non-inferiority of the novel model compared to conventionally used tumor xenografts in immune compromised rodents with regard to reproducibility and stability of the PSMA signal. Furthermore, the model involves less expense and efforts while it is permanently available and avoids tumor-growth associated animal morbidity and distress. To the best of our knowledge, this is the first tumor-free model suitable for the in vivo evaluation of tumor imaging agents.

Minimalist approach meets green chemistry: Synthesis of 18 F- labeled (hetero)aromatics in pure ethanol.

The application of toxic solvents and additives is inevitable for most of the described protocols for 18 F-labeling. Herein, a novel "green" procedure for nucleophilic aromatic radiofluorination of highly activated (hetero)aromatic substrates in pure EtOH is described. Using this method a series of 18 F-labeled (hetero)arenes have been synthesized in radiochemical yields (RCYs) of up to 97%.

Alcohol-Supported Cu-Mediated 18F-Fluorination of Iodonium Salts under "Minimalist" Conditions.

In the era of personalized precision medicine, positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI gain recognition as indispensable tools of clinical diagnostics. A broader implementation of these imaging modalities in clinical routine is closely dependent on the increased availability of established and emerging PET-tracers, which in turn could be accessible by the development of simple, reliable, and efficient radiolabeling procedures. A further requirement is a cGMP production of imaging probes in automated synthesis modules. Herein, a novel protocol for the efficient preparation of 18F-labeled aromatics via Cu-mediated radiofluorination of (aryl)(mesityl)iodonium salts without the need of evaporation steps is described. Labeled aromatics were prepared in high radiochemical yields simply by heating of iodonium [18F]fluorides with the Cu-mediator in methanolic DMF. The iodonium [18F]fluorides were prepared by direct elution of 18F- from an anion exchange resin with solutions of the corresponding precursors in MeOH/DMF. The practicality of the novel method was confirmed by the racemization-free production of radiolabeled fluorophenylalanines, including hitherto unknown 3-[18F]FPhe, in 22-69% isolated radiochemical yields as well as its direct implementation into a remote-controlled synthesis unit.

Alcohol-Enhanced Cu-Mediated Radiofluorination.

The potential of many 18 F-labeled (hetero)aromatics for applications in positron emission tomography remains underexplored because convenient procedures for their radiosynthesis are lacking. Consequently, simple methods to prepare radiofluorinated (hetero)arenes are highly sought after. Herein, we report the beneficial effect of primary and secondary alcohols on Cu-mediated 18 F-labeling. This observation contradicts the assumption that such alcohols are inappropriate solvents for aromatic fluorination. Therefore, we developed a protocol for rapid radiolabeling of an extraordinarily broad scope of boronic and stannyl substrates under general reaction conditions. Notably, radiofluorinated indoles, phenols, and anilines were synthesized directly from the corresponding unprotected precursors. Furthermore, the novel method enabled the preparation of radiofluorinated tryptophans, [18 F]F-DPA, [18 F]DAA1106, 6-[18 F]FDA, and 6-[18 F]FDOPA.

A Practical Method for the Preparation of 18F-Labeled Aromatic Amino Acids from Nucleophilic [18F]Fluoride and Stannyl Precursors for Electrophilic Radiohalogenation.

In a recent contribution of Scott et al., the substrate scope of Cu-mediated nucleophilic radiofluorination with [18F]KF for the preparation of 18F-labeled arenes was extended to aryl- and vinylstannanes. Based on these findings, the potential of this reaction for the production of clinically relevant positron emission tomography (PET) tracers was investigated. To this end, Cu-mediated radiofluorodestannylation using trimethyl(phenyl)tin as a model substrate was re-evaluated with respect to different reaction parameters. The resulting labeling protocol was applied for 18F-fluorination of different electron-rich, -neutral and -poor arylstannyl substrates in RCCs of 16-88%. Furthermore, this method was utilized for the synthesis of 18F-labeled aromatic amino acids from additionally N-Boc protected commercially available stannyl precursors routinely applied for electrophilic radiohalogenation. Finally, an automated synthesis of 6-[18F]fluoro-l-m-tyrosine (6-[18F]FMT), 2-[18F]fluoro-l-tyrosine (2-[18F]F-Tyr), 6-[18F]fluoro-l-3,4-dihydroxyphenylalanine (6-[18F]FDOPA) and 3-O-methyl-6-[18F]FDOPA ([18F]OMFD) was established furnishing these PET probes in isolated radiochemical yields (RCYs) of 32-54% on a preparative scale. Remarkably, the automated radiosynthesis of 6-[18F]FDOPA afforded an exceptionally high RCY of 54 ± 5% (n = 5).

Copper-mediated aromatic radiofluorination revisited: efficient production of PET tracers on a preparative scale.

Two novel methods for copper-mediated aromatic nucleophilic radiofluorination were recently reported. Evaluation of these methods reveals that, although both are efficient in small-scale experiments, they are inoperative for the production of positron emission tomography (PET) tracers. Since high base content turned out to be responsible for low radiochemical conversions, a "low base" protocol has been developed which affords (18)F-labeled arenes from diaryliodonium salts and aryl pinacol boronates in reasonable yields. Furthermore, implementation of our "minimalist" approach to the copper-mediated [(18)F]-fluorination of (mesityl)(aryl)iodonium salts allows the preparation of (18)F-labeled arenes in excellent RCCs. The novel radiofluorination method circumvents time-consuming azeotropic drying and avoids the utilization of base and other additives, such as cryptands. Furthermore, this procedure enables the production of clinically relevant PET tracers; [(18)F]FDA, 4-[(18)F]FPhe, and [(18)F]DAA1106 are obtained in good isolated radiochemical yields. Additionally, [(18)F]DAA1106 has been evaluated in a rat stroke model and demonstrates excellent potential for visualization of translocator protein 18 kDa overexpression associated with neuroinflammation after ischemic stroke.

Synthesis of ¹8F-labelled ß-lactams by using the Kinugasa reaction.

Owing to their broad spectrum of biological activities and low toxicity, ß-lactams are attractive lead structures for the design of novel molecular probes. However, the synthesis of positron emission tomography (PET)-isotope-labelled ß-lactams has not yet been reported. Herein, we describe the simple preparation of radiofluorinated ß-lactams by using the fast Kinugasa reaction between (18)F-labelled nitrone [(18)F]-1 and alkynes of different reactivity. Additionally, (18)F-labelled fused ß-lactams were obtained through the reaction of a cyclic nitrone 7 with radiofluorinated alkynes [(18)F]-6 a,b. Radiochemical yields of the Kinugasa reaction products could be significantly increased by the use of different Cu(I) ligands, which additionally allowed a reduction in the amount of precursor and/or reaction time. Model radiofluorinated ß-lactam-peptide and protein conjugates ([(18)F]-10 and (18)F-labelled BSA conjugate) were efficiently obtained in high yield under mild conditions (aq. MeCN, ambient temperature) within a short reaction time, demonstrating the suitability of the developed method for radiolabelling of sensitive molecules such as biopolymers.

Validation of analytical HPLC with post-column injection as a method for rapid and precise quantification of radiochemical yields.

Accurate assessment of isolated radiochemical yields (RCYs) is a prerequisite for efficient and reliable optimization of labeling reactions. In practice, radiochemical conversions (RCCs) determined by HPLC analysis of crude reaction mixtures are often used to estimate RCYs. However, incomplete recovery of radioactivity from the stationary phase can lead to significant inaccuracies if RCCs are calculated based on the activity eluted from the column (i.e. the summed integrals of all peaks). Here, we validate a simple and practical method that overcomes problems associated with retention of activity on the column by determination of the total activity in the sample using post-column injection. Post-column injections were carried out using an additional injection valve, which was placed between the outlet of the HPLC column and the inlet of the detectors. 2-[18F]Fluoropyridine ([18F]FPy) and 8-cyclopentyl-3-(3-[18F]fluoropropyl)-1-propylxanthine ([18F]CPFPX) were prepared with radiochemical purities of > 99.8% and mixed with [18F]fluoride at a ratio of 1:1 to simulate reaction mixtures obtained by radiolabeling reactions with an RCC of 50%. The samples were analyzed on three different C18 HPLC columns using neutral and acidic mobile phases. RCCs determined using the summed area of all peaks in the chromatograms were compared with those determined using post-column injection. Additionally, RCCs determined by post-column injection were corrected for activity losses before, during and after radiosyntheses to afford analytical RCYs, which were compared with isolated RCYs. Determination of RCCs based on the summed area of all peaks gave correct results under certain chromatographic conditions, but led to overestimation of the actual RCCs by up to 50% in other cases. In contrast, determination of RCCs using post-column injection provided precise results in all cases, and often significantly reduced analysis time. Moreover, analytical RCYs calculated from RCCs determined by post-column injection showed excellent agreement with isolated RCYs (<3% deviation). In conclusion, HPLC analysis using post-column injection enables reliable determination of RCCs independent of the chromatographic conditions and, together with a simple activity balance, rapid and accurate prediction of isolated RCYs.

7-[18F]Fluoro-8-azaisatoic Anhydrides: Versatile Prosthetic Groups for the Preparation of PET Tracers.

18F-Fluorination of sensitive molecules is often challenging, but can be accomplished under suitably mild conditions using radiofluorinated prosthetic groups (PGs). Herein, 1-alkylamino-7-[18F]fluoro-8-azaisatoic anhydrides ([18F]AFAs) are introduced as versatile 18F-labeled building blocks that can be used as amine-reactive or "click chemistry" PGs. [18F]AFAs were efficiently prepared within 15 min by "on cartridge" radiolabeling of readily accessible trimethylammonium precursors. Conjugation with a range of amines afforded the corresponding 2-alkylamino-6-[18F]fluoronicotinamides in radiochemical conversions (RCCs) of 15-98%. In addition, radiolabeling of alkyne- or azide-functionalized precursors with azidopropyl- or propargyl-substituted [18F]AFAs using Cu-catalyzed click cycloaddition afforded the corresponding conjugates in RCCs of 44-88%. The practical utility of the PGs was confirmed by the preparation of three 18F-labeled PSMA ligands in radiochemical yields of 28-42%. Biological evaluation in rats demonstrated excellent in vivo stability of all three conjugates. In addition, one conjugate ([18F]JK-PSMA-15) showed favorable imaging properties for high-contrast visualization of small PSMA-positive lesions.

Assessment of the In Vivo Relationship Between Cerebral Hypometabolism, Tau Deposition, TSPO Expression, and Synaptic Density in a Tauopathy Mouse Model: a Multi-tracer PET Study.

Cerebral glucose hypometabolism is a typical hallmark of Alzheimer's disease (AD), usually associated with ongoing neurodegeneration and neuronal dysfunction. However, underlying pathological processes are not fully understood and reproducibility in animal models is not well established. The aim of the present study was to investigate the regional interrelation of glucose hypometabolism measured by [18F]FDG positron emission tomography (PET) with various molecular targets of AD pathophysiology using the PET tracers [18F]PI-2620 for tau deposition, [18F]DPA-714 for TSPO expression associated with neuroinflammation, and [18F]UCB-H for synaptic density in a transgenic tauopathy mouse model. Seven-month-old rTg4510 mice (n = 8) and non-transgenic littermates (n = 8) were examined in a small animal PET scanner with the tracers listed above. Hypometabolism was observed throughout the forebrain of rTg4510 mice. Tau pathology, increased TSPO expression, and synaptic loss were co-localized in the cortex and hippocampus and correlated with hypometabolism. In the thalamus, however, hypometabolism occurred in the absence of tau-related pathology. Thus, cerebral hypometabolism was associated with two regionally distinct forms of molecular pathology: (1) characteristic neuropathology of the Alzheimer-type including synaptic degeneration and neuroinflammation co-localized with tau deposition in the cerebral cortex, and (2) pathological changes in the thalamus in the absence of other markers of AD pathophysiology, possibly reflecting downstream or remote adaptive processes which may affect functional connectivity. Our study demonstrates the feasibility of a multitracer approach to explore complex interactions of distinct AD-pathomechanisms in vivo in a small animal model. The observations demonstrate that multiple, spatially heterogeneous pathomechanisms can contribute to hypometabolism observed in AD mouse models and they motivate future longitudinal studies as well as the investigation of possibly comparable pathomechanisms in human patients.

Convenient PET-tracer production via SuFEx 18F-fluorination of nanomolar precursor amounts.

Recently, a protocol for radiolabeling of aryl fluorosulfates ("SuFEx click radiolabeling") using ultrafast 18F/19F isotopic exchange has been reported. Although promising, the original procedure turned out to be rather inefficient. However, systematic optimization of the reaction parameters allowed for development of a robust method for SuFEx radiolabeling which obviates the need for azeotropic drying, base addition and HPLC purification. The developed protocol enabled efficient 18F-fluorination of low nanomolar amounts of aryl fluorosulfates in highly diluted solution (micromolar concentrations). It was successfully used to prepare a series of 29 18F-fluorosulfurylated phenols - including modified ezetimibe, α-tocopherol and etoposide, the two tyrosine derivatives Boc-Tyr([18F]FS)-OMe and H-Tyr([18F]FS)-OMe, the FAP-specific ligand [18F]FS-UAMC1110, and the DPA-714 analog [18F]FS-DPA - in fair to excellent yields. Preliminary evaluation demonstrated sufficient in vivo stability of radiofluorinated electron rich or neutral {Boc-Tyr([18F]FS)-OMe), H-Tyr([18F]FS)-OMe and [18F]FS-DPA} aryl fluorosulfates. Furthermore, [18F]FS-DPA was identified as a promising tracer for visualization of TSPO expression.

New molecular markers for prostate tumor imaging: a study on 2-methylene substituted fatty acids as new AMACR inhibitors.

The development of prostate carcinoma is associated with alterations in fatty acid metabolism. α-Methylacyl-CoA racemase (AMACR) is a peroxisomal and mitochondrial enzyme that catalyses interconversion between the (S)/(R)-isomers of a range of α-methylacyl-CoA thioesters. AMACR is involved in the ß-oxidation of the dietary branched-chain fatty acids and bile acid intermediates. It is highly expressed in prostate (more than 95 %), colon (92 %), and breast cancers (44 %) but not in the respective normal or hyperplastic tissues. Thus, targeting of AMACR could be a new strategy for molecular imaging and therapy of prostate and some other cancers. Unlabeled 2-methylenacyl-CoA thioesters (12 a-c) were designed as AMACR binding ligands. The thioesters were tested for their ability to inhibit the AMACR-mediated epimerization of (25R)-THC-CoA and were found to be strong AMACR inhibitors. Radioiodinated (E)-(131) I-13-iodo-2-methylentridec-12-enoic acid ((131) I-7 c) demonstrated preferential retention in AMACR-positive prostate tumor cells (LNCaP, LNCaP C4-2wt and DU145) compared with both AMACR-knockout LNCaP C4-2 AMACR-siRNA and benign BPH1 prostate cell lines. A significant protein-bound radioactive fraction with main bands at 47 (sum of molecular weights of AMACR plus 12 c), 70, and 75 kDa was detected in LNCaP C4-2 wt cells. In contrast, only negligible amounts of protein-bound radioactivity were found in LNCaP C4-2 AMACR-siRNA cells.