Development of a new epidermal growth factor receptor positron emission tomography imaging agent based on the 3-cyanoquinoline core: Synthesis and biological evaluation

The epidermal growth factor receptor (EGFR/c-ErbB1/HER1) is overexpressed in many cancers including breast, ovarian, endometrial, and non-small cell lung cancer. An EGFR specific imaging agent could facilitate clinical evaluation of primary tumors and/or metastases. To achieve this goal we designed and synthesized a small array of fluorine containing compounds based on a 3-cyanoquinoline core. A lead compound, 16, incorporating 2′-fluoroethyl-1,2,3-triazole was selected for evaluation as a radioligand based on its high affinity for EGFR kinase (IC₅₀ = 1.81 ± 0.18 nM), good cellular potency (IC₅₀ = 21.97 ± 9.06 nM), low lipophilicity and good metabolic stability. ‘Click’ labeling afforded [¹⁸F]16 in 37.0 ± 3.6% decay corrected radiochemical yield based on azide [¹⁸F]14 and 7% end of synthesis (EOS) yield from aqueous fluoride. Compound [¹⁸F]16 was obtained with >99% radiochemical purity in a total synthesis time of 3 h. The compound showed good stability in vivo and a fourfold higher uptake in high EGFR expressing A431 tumor xenografts compared to low EGFR expressing HCT116 tumor xenografts. Furthermore, the radiotracer could be visualized in A431 tumor bearing mice by small animal PET imaging. Compound [¹⁸F]16 therefore constitutes a promising radiotracer for further evaluation for imaging of EGFR status.     

Preparation and biodistribution of [18F]FP2OP as myocardial perfusion imaging agent for positron emission tomography

Myocardial extractions of pyridaben, a mitochondrial complex I (MC-I) inhibitor, is well correlated with blood flow. Based on the synthesis and characterization of pyridaben analogue 2-tert-butyl-5-[2-(2-[¹⁸F]fluroethoxy)ethoxy]benzyloxy]-4-chloro-2H-pyridazin-3-one ([¹⁸F]FP2OP), this study assessed its potential to be developed as myocardial perfusion imaging (MPI) agent.Methods: The tosylate labeling precursor 2-(2-(4-(tert-butyl-5-chloro-6-oxo-1,6-dihydro-pyridazin-4-yloxymethyl)benzyloxy)ethoxy)ethyl ester (OTs-P2OP) and the nonradioactive 2-tert-butyl-5-[2-(2-[¹⁹F]fluroethoxy)ethoxy]benzyloxy]-4-chloro-2H-pyridazin-3-one ([¹⁹F]FP2OP) were synthesized and characterized by IR, ¹H NMR, ¹³C NMR and MS analysis. By substituting tosyl of precursor OTs-P2OP with ¹⁸F, the radiolabeled complex [¹⁸F]FP2OP was prepared and further evaluated for its in vitro physicochemical properties, in vivo biodistribution, the metabolic stability in mice, ex vivo autoradiography and cardiac PET/CT imaging.Results: Starting with [¹⁸F]F^? Kryptofix 2.2.2./K₂CO₃ solution, the total reaction time for [¹⁸F]FP2OP was about 100 min, with final high-performance liquid chromatography purification included. Typical decay-corrected radiochemical yield stayed at 41 ± 5.3%, the radiochemical purity, 98% or more. Biodistribution in mice showed that the heart uptake of [¹⁸F]FP2OP was 41.90 ± 4.52%ID/g at 2 min post-injection time, when the ratio of heart/liver, heart/lung and heart/blood reached 6.83, 9.49 and 35.74, respectively. Lipophilic molecule was further produced by metabolized [¹⁸F]FP2OP in blood and urine at 30 min. Ex vivo autoradiography demonstrates that [¹⁸F]FP2OP may have high affinity with MC-I and that can be blocked by [¹⁹F]FP2OP or rotenone (a known MC-I inhibitor). Cardiac PET images were obtained in a Chinese mini-swine at 5, 15, 30 and 60 min post-injection time with high quality.Conclusion: [¹⁸F]FP2OP was synthesized with high radiochemical yield. The promising biological properties of [¹⁸F]FP2OP suggest high potential as MPI agent for positron emission tomography in the future.     

Synthesis and preclinical characterization of 1-(6′-deoxy-6′-[18F]fluoro-β-d-allofuranosyl)-2-nitroimidazole (β-6′-[18F]FAZAL) as a positron emission tomography radiotracer to assess tumor hypoxia

Positron emission tomography (PET) using fluorine-18 (¹⁸F)-labeled 2-nitroimidazole radiotracers has proven useful for assessment of tumor oxygenation. However, the passive diffusion-driven cellular uptake of currently available radiotracers results in slow kinetics and low tumor-to-background ratios. With the aim to develop a compound that is actively transported into cells, 1-(6′-deoxy-6′-[¹⁸F]fluoro-β-d-allofuranosyl)-2-nitroimidazole (β-[¹⁸F]1), a putative nucleoside transporter substrate, was synthetized by nucleophilic [¹⁸F]fluoride substitution of an acetyl protected labeling precursor with a tosylate leaving group (β-6) in a final radiochemical yield of 12 ± 8% (n = 10, based on [¹⁸F]fluoride starting activity) in a total synthesis time of 60 min with a specific activity at end of synthesis of 218 ± 58 GBq/μmol (n = 10). Both radiolabeling precursor β-6 and unlabeled reference compound β-1 were prepared in multistep syntheses starting from 1,2:5,6-di-O-isopropylidene-α-d-allofuranose. In vitro experiments demonstrated an interaction of β-1 with SLC29A1 and SLC28A1/2/3 nucleoside transporter as well as hypoxia specific retention of β-[¹⁸F]1 in tumor cell lines. In biodistribution studies in healthy mice β-[¹⁸F]1 showed homogenous tissue distribution and excellent metabolic stability, which was unaffected by tissue oxygenation. PET studies in tumor bearing mice showed tumor-to-muscle ratios of 2.13 ± 0.22 (n = 4) at 2 h after administration of β-[¹⁸F]1. In ex vivo autoradiography experiments β-[¹⁸F]1 distribution closely matched staining with the hypoxia marker pimonidazole. In conclusion, β-[¹⁸F]1 shows potential as PET hypoxia radiotracer which merits further investigation.     

Syntheses and pharmacological characterization of novel thiazole derivatives as potential mGluR5 PET ligands

Four novel thiazole containing ABP688 derivatives were synthesized and evaluated for their binding affinity towards the metabotropic glutamate receptor subtype 5 (mGluR5). (E)-3-((2-(Fluoromethyl)thiazol-4-yl)ethynyl)cyclohex-2-enone O-methyl oxime (FTECMO), the ligand with the highest binding affinity (K_i = 5.5 ± 1.1 nM), was labeled with fluorine-18. [¹⁸F]-FTECMO displayed optimal lipophilicity (log D_pH7.4 = 1.6 ± 0.2) and high stability in rat and human plasma as well as sufficient stability in rat liver microsomes. In vitro autoradiography with [¹⁸F]-FTECMO revealed a heterogeneous and displaceable binding in mGluR5-rich brain regions. PET imaging with [¹⁸F]-FTECMO in Wistar rats, however, showed low brain uptake. Uptake of radioactivity into the skull was observed suggesting in vivo defluorination. Thus, although [¹⁸F]-FTECMO is an excellent ligand for the detection of mGluR5 in vitro, its in vivo characteristics are not optimal for the imaging of mGluR5 in rats in vivo.     

Synthesis and evaluation of 18F-labeled mitiglinide derivatives as positron emission tomography tracers for β-cell imaging

Measuring changes in β-cell mass in vivo during progression of diabetes mellitus is important for understanding the pathogenesis, facilitating early diagnosis, and developing novel therapeutics for this disease. However, a non-invasive method has not been developed. A novel series of mitiglinide derivatives (o-FMIT, m-FMIT and p-FMIT; FMITs) were synthesized and their binding affinity for the sulfonylurea receptor 1 (SUR1) of pancreatic islets were evaluated by inhibition studies. (+)-(S)-o-FMIT had the highest affinity of our synthesized FMITs (IC₅₀ = 1.8 μM). (+)-(S)-o-[¹⁸F]FMIT was obtained with radiochemical yield of 18% by radiofluorination of racemic precursor 7, hydrolysis, and optical resolution with chiral HPLC; its radiochemical purity was >99%. In biodistribution experiments using normal mice, (+)-(S)-o-[¹⁸F]FMIT showed 1.94 ± 0.42% ID/g of pancreatic uptake at 5 min p.i., and decreases in radioactivity in the liver (located close to the pancreas) was relatively rapid. Ex vivo autoradiography experiments using pancreatic sections confirmed accumulation of (+)-(S)-o-[¹⁸F]FMIT in pancreatic β-cells. These results suggest that (+)-(S)-o-[¹⁸F]FMIT meets the basic requirements for an radiotracer, and could be a candidate positron emission tomography tracer for in vivo imaging of pancreatic β-cells.     

Synthesis and biological evaluation of 18F labeled fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives for sigma-1 receptor imaging

We report the synthesis and evaluation of a series of fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives as potential σ₁ receptor ligands. In vitro competition binding assays showed that 1-(1,3-benzodioxol-5-ylmethyl)-4-(4-(2-fluoroethoxy)benzyl)piperazine (6) exhibits low nanomolar affinity for σ₁ receptors (K_i = 1.85 ± 1.59 nM) and high subtype selectivity (σ₂ receptor: K_i = 291 ± 111 nM; K_iσ₂/K_iσ₁ = 157). [¹⁸F]6 was prepared in 30–50% isolated radiochemical yield, with radiochemical purity of >99% by HPLC analysis after purification, via nucleophilic ¹⁸F^? substitution of the corresponding tosylate precursor. The log D_{pH 7.4} value of [¹⁸F]6 was found to be 2.57 ± 0.10, which is within the range expected to give high brain uptake. Biodistribution studies in mice demonstrated relatively high concentration of radiotracers in organs known to contain σ₁ receptors, including the brain, lungs, kidneys, heart, and spleen. Administration of haloperidol 5 min prior to injection of [¹⁸F]6 significantly reduced the concentration of radiotracers in the above-mentioned organs. The accumulation of radiotracers in the bone was quite low suggesting that [¹⁸F]6 is relatively stable to in vivo defluorination. The ex vivo autoradiography in rat brain showed high accumulation of radiotracers in the brain areas known to possess high expression of σ₁ receptors. These findings suggest that [¹⁸F]6 is a suitable radiotracer for imaging σ₁ receptors with PET in vivo.     

Synthesis of 18F-labelled 2-(4′-fluorophenyl)-1,3-benzothiazole and evaluation as amyloid imaging agent in comparison with [11C]PIB

2-(4′-[¹⁸F]fluorophenyl)-1,3-benzothiazole was synthesized as a fluorine-18 labelled derivative of the Pittsburg Compound-B (PIB), which has known affinity for amyloid β and promising characteristics as tracer for in vivo visualisation of amyloid deposits in patients suffering from Alzheimer’s disease (AD). Both the nitro-precursor 2-(4′-nitrophenyl)-1,3-benzothiazole and the non-radioactive reference compound were synthesized using a 1-step synthesis pathway. Labelling was achieved by direct aromatic nucleophilic substitution of the nitro-precursor using [¹⁸F]fluoride by heating for 20 min at 150 °C and with a radiochemical yield of 38%. The reference compound showed high affinity for amyloid in an in vitro competition binding study using human AD brain homogenates (K_i = 9.0 nM) and fluorescence imaging of incubated transgenic APP mouse brain slices confirmed binding to amyloid plaques. A biodistribution study in normal mice showed a high brain uptake at 2 min pi (3.20% ID/g) followed by a fast washout (60 min pi: 0.21% ID/g). A dynamic μPET study was performed in a transgenic APP and normal WT mouse, but, similar to [¹¹C]PIB, no difference was seen in tracer retention between both kind of mice. The new ¹⁸F-labelled 2-phenylbenzothiazole showed excellent preclinical characteristics comparable with those of the ¹¹C-labelled PIB.     

18F Labeled benzimidazole derivatives as potential radiotracer for positron emission tomography (PET) tumor imaging

This article reported the synthesis and bioevaluation of two [¹⁸F] labeled benzimidazole derivatives, 4-(5-(2-[¹⁸F] fluoro-4-nitrobenzamido)-1-methyl-1H-benzimidazol-2-yl) butanoic acid ([¹⁸F] FNBMBBA, [¹⁸F]a1) and 3-(2-fluoroethyl)-7-methyl-2-propyl-3H-benzimidazole-5-carboxylic acid ([¹⁸F] FEMPBBA, [¹⁸F]b1) for PET tumor imaging. The preparation [¹⁸F] FEMPBBA was completed in 1 h with overall radiochemical yield of 50–60% (without decay corrected). Biodistribution assay in S180 tumor bearing mice of both compounds were carried out, and the results are both meaningful. [¹⁸F] FEMPBBA which can be taken as a revision of [¹⁸F] FNBMBBA got an excellent result, and has significant advantages in some aspects compared with L-[¹⁸F] FET and [¹⁸F]-FDG in the same animal model, especially in tumor/brain uptake ratio. The tumor/brain uptake ratio of [¹⁸F] FEMPBBA gets to 4.81, 7.15, and 9.8 at 30 min, 60 min and 120 min, and is much higher than that of L-[¹⁸F] FET (2.54, 2.92 and 2.95) and [¹⁸F]-FDG (0.61, 1.02, 1.33) at the same time point. The tumor/muscle and tumor/blood uptake ratio of [¹⁸F] FEMPBBA is also higher than that of L-[¹⁸F] FET at 30 min and 60 min. This result indicates compound [¹⁸F] FEMPBBA is a promising radiotracer for PET tumor imaging.     

Microfluidic technology: an economical and versatile approach for the synthesis of O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET)

A new synthesis of O-(2-[¹⁸F]fluoroethyl)-l-tyrosine [¹⁸F]FET was developed using a NanoTek® microfluidic synthesis system (Advion BioSciences, Inc.). Optimal reaction conditions were studied through screening different reaction parameters like temperature, flow rate, reaction time, concentration of the labeling precursor, and the applied volume ratio between the labeling precursor and [¹⁸F]fluoride. [¹⁸F]FET was obtained after HPLC purification with 50% decay-corrected radiochemical yield starting from as little as 40 μg of labeling precursor. Small animal PET studies in EMT-6 tumor bearing mice showed radioactivity accumulation in the tumor (SUV_60min 1.21 ± 0.2) resulting in an slightly increasing tumor-to-muscle ratio over time.     

Synthesis and radiopharmacological investigation of 3-[4′-[18F]fluorobenzylidene]indolin-2-one as possible tyrosine kinase inhibitor

The radiosynthesis and radiopharmacological evaluation of 3-[4′-[¹⁸F]fluorobenzylidene]indolin-2-one, a derivative of tyrosine kinase inhibitor SU5416, is described. The radiosynthesis was accomplished by Knoevenagel condensation of 4-[¹⁸F]fluorobenzaldehyde with oxindole in a remotely controlled synthesis module. The reaction conditions were optimized through screening the influence of different bases on the radiochemical yield. The radiotracer was obtained after a two-step labelling procedure in 4% decay-corrected radiochemical yield at a specific activity of 48–61 GBq/μmol within 90 min. The radiochemical purity after semi-preparative HPLC purification exceeded 98%.The biodistribution was studied in Wistar rats. After distribution the radiotracer was rapidly accumulated in the adrenals, liver and kidneys, however, it was cleared from these and the most other organs. Only the adipose tissue remained the activity over 60 min. Unexpected high transient uptake was observed in the brain, pancreas, heart and lung. The fast clearance of 3-[4′-[¹⁸F]fluorobenzylidene]indolin-2-one was caused by excretion, approximately one half each was renal and biliary excreted and the other part cleared by metabolic processes. In arterial blood plasma two more polar metabolites were found by radio-HPLC. After 20 min post-injection, only 12% of intact radiotracer has been detected. Consequently, in small animal PET studies with FaDu tumour bearing mice no specific uptake in the tumours could be observed.     

Synthesis and in vitro evaluation of [18F]BMS-754807: A potential PET ligand for IGF-1R

Radiosynthesis and in vitro evaluation of [¹⁸F](S)-1-(4-((5-cyclopropyl-1H-pyrazol-3-yl)amino)pyrrolo[2,1-f][1,2,4]triazin-2-yl)-N-(6-fluoropyridin-3-yl)-2-methylpyrrolidine-2-carboxamide ([¹⁸F]BMS-754807 or [¹⁸F]1) a specific IGF-1R inhibitor was performed. [¹⁸F]1 demonstrated specific binding in vitro to human cancer tissues. Synthesis of reference standard 1 and corresponding bromo derivative (1a), the precursor for radiolabeling were achieved from 2,4-dichloropyrrolo[2,1-f][1,2,4]triazine (4) in three steps with 50% overall yield. The radioproduct was obtained in 8% yield by reacting 1a with [¹⁸F]TBAF in DMSO at 170 °C at high radiochemical purity and specific activity (1–2 Ci/μmol, N = 10). The proof of concept of IGF-IR imaging with [¹⁸F]1 was demonstrated by in vitro autoradiography studies using pathologically identified surgically removed grade IV glioblastoma, breast cancer and pancreatic tumor tissues. These studies indicate that [¹⁸F]1 can be a potential PET tracer for monitoring IGF-1R.     

Synthesis and bioevaluation of [18F]4-fluoro-m-hydroxyphenethylguanidine ([18F]4F-MHPG): A novel radiotracer for quantitative PET studies of cardiac sympathetic innervation

A new cardiac sympathetic nerve imaging agent, [¹⁸F]4-fluoro-m-hydroxyphenethylguanidine ([¹⁸F]4F-MHPG), was synthesized and evaluated. The radiosynthetic intermediate [¹⁸F]4-fluoro-m-tyramine ([¹⁸F]4F-MTA) was prepared and then sequentially reacted with cyanogen bromide and NH₄Br/NH₄OH to afford [¹⁸F]4F-MHPG. Initial bioevaluations of [¹⁸F]4F-MHPG (biodistribution studies in rats and kinetic studies in the isolated rat heart) were similar to results previously reported for the carbon-11 labeled analog [¹¹C]4F-MHPG. The neuronal uptake rate of [¹⁸F]4F-MHPG into the isolated rat heart was 0.68 ml/min/g wet and its retention time in sympathetic neurons was very long (T_1/2 >13 h). A PET imaging study in a nonhuman primate with [¹⁸F]4F-MHPG provided high quality images of the heart, with heart-to-blood ratios at 80–90 min after injection of 5-to-1. These initial kinetic and imaging studies of [¹⁸F]4F-MHPG suggest that this radiotracer may allow for more accurate quantification of regional cardiac sympathetic nerve density than is currently possible with existing neuronal imaging agents.     

Synthesis and biological evaluation of 2-(3,4-dimethoxyphenyl)-6-(2-[18F]fluoroethoxy)benzothiazole ([18F]FEDBT) for PET imaging of breast cancer

Given the ever-present demand for improved PET radiotracer in oncology imaging, we have synthesized 2-(3,4-dimethoxyphenyl)-6-(2-[¹⁸F]fluoroethoxy)benzothiazole ([¹⁸F]FEDBT), a fluorine-18-containing fluoroethylated benzothiazole to explore its utility as a PET imaging tracer. [¹⁸F]FEDBT was prepared via kryptofix-mediated nucleophilic substitution of the tosyl group precursor. Fractionated ethanol-based solid-phase (SPE cartridge-based) purification afforded [¹⁸F]FEDBT in 60% radiochemical yield (EOB), with radiochemical purity in excess of 98% and the specific activity was 35 GBq/μmol. The radiotracer displayed clearly higher cellular uptake ratio in various breast cancer cell lines MCF7, MDA-MB-468 and MDA-MB-231. However, both biodistribution and microPET studies have showed an higher abdominal accumulation of [¹⁸F]FEDMBT and the tumor/muscle ratio of 1.8 was observed in the MDA-MB-231 xenograft tumors mice model. Further the lipophilic improvement is needed for the reducement of hepatobilliary accumulation and to promote the tumor uptake for PET imaging of breast cancer.     

Synthesis and biological evaluation of 18F-labled 2-phenylindole derivatives as PET imaging probes for β-amyloid plaques

A novel series of fluorinated 2-phenylindole derivatives were synthesized and evaluated as β-amyloid imaging probes for PET. The in vitro inhibition assay demonstrated that their binding affinities for Aβ_1–42 aggregates ranged from 28.4 to 1097.8 nM. One ligand was labeled with ¹⁸F ([¹⁸F]1a) for its high affinity (K_i = 28.4 nM), which was also confirmed by in vitro autoradiography experiments on brain sections of transgenic mouse (C57BL6, APPswe/PSEN1, 11 months old, male). In vivo biodistribution experiments in normal mice showed that this radiotracer displayed high initial uptake (5.82 ± 0.51% ID/g at 2 min) into and moderate washout (2.77 ± 0.31% ID/g at 60 min) from the brain. [¹⁸F]1a could be developed as a promising new PET imaging probe for Aβ plaques although necessary modifications are still needed.     

Synthesis and evaluation of a 18F-labeled spirocyclic piperidine derivative as promising σ1 receptor imaging agent

Several spirocyclic piperidine derivatives were designed and synthesized as σ₁ receptor ligands. In vitro competition binding assays showed that the fluoroalkoxy analogues with small substituents possessed high affinity towards σ₁ receptors and subtype selectivity. Particularly for ligand 1′-((6-(2-fluoroethoxy)pyridin-3-yl)methyl)-3H-spiro[2-benzofuran-1,4′-piperidine] (2), high σ₁ receptor affinity (K_i = 2.30 nM) and high σ₁/σ₂ subtype selectivity (142-fold) as well as high σ₁/VAChT selectivity (234-fold) were observed. [¹⁸F]2 was synthesized using an efficient one-pot, two-step reaction method in a home-made automated synthesis module, with an overall isolated radiochemical yield of 8–10%, a radiochemical purity of higher than 99%, and specific activity of 56–78 GBq/μmol. Biodistribution studies of [¹⁸F]2 in ICR mice indicated high initial brain uptake and a relatively fast washout. Administration of haloperidol, compound 1 and different concentrations of SA4503 (3, 5, or 10 μmol/kg) 5 min prior to injection of [¹⁸F]2 significantly decreased the accumulation of radiotracer in organs known to contain σ₁ receptors. Ex vivo autoradiography in Sprague–Dawley rats demonstrated high accumulation of radiotracer in brain areas with high expression of σ₁ receptors. These encouraging results prove that [¹⁸F]2 is a suitable candidate for σ₁ receptor imaging with PET in humans.     

Radiosynthesis of N-(4-chloro-3-[11C]methoxyphenyl)-2-picolinamide ([11C]ML128) as a PET radiotracer for metabotropic glutamate receptor subtype 4 (mGlu4)

N-(Chloro-3-methoxyphenyl)-2-picolinamide (3, ML128, VU0361737) is an mGlu₄ positive allosteric modulator (PAM), which is potent and centrally penetrating. 3 is also the first mGlu₄ PAM to show efficacy in a preclinical Parkinson disease model upon systemic dosing. As a noninvasive medical imaging technique and a powerful tool in neurological research, positron emission tomography (PET) offers a possibility to investigate mGlu₄ expression in vivo under physiologic and pathological conditions. We synthesized a carbon-11 labeled ML128 ([¹¹C]3) as a PET radiotracer for mGlu₄, and characterized its biological properties in Sprague Dawley rats. [¹¹C]3 was synthesized from N-(4-chloro-3-hydroxyphenyl)-2-picolinamide (2) using [¹¹C]CH₃I. Total synthesis time was 38 ± 2.2 min (n = 7) from the end of bombardment to the formulation. The radioligand [¹¹C]3 was obtained in 27.7 ± 5.3% (n = 5) decay corrected radiochemical yield based on the radioactivity of [¹¹C]CO₂. The radiochemical purity of [¹¹C]3 was >99%. Specific activity was 188.7 ± 88.8 GBq/mol (n = 4) at the end of synthesis (EOS).PET images were conducted in 20 normal male Sprague Dawley rats including 11 control studies, 6 studies blocking with an mGlu₄ modulator (4) to investigate specificity and 3 studies blocking with an mGlu₅ modulator (MTEP) to investigate selectivity. These studies showed fast accumulation of [¹¹C]3 (peak activity between 1–3 min) in several brain areas including striatum, thalamus, hippocampus, cerebellum, and olfactory bulb following with fast washout. Blocking studies with the mGlu₄ modulator 4 showed 22–28% decrease of [¹¹C]3 accumulation while studies of selectivity showed only minor decrease supporting good selectivity over mGlu₅. Biodistribution studies and blood analyses support fast metabolism. Altogether this is the first PET imaging ligand for mGlu₄, in which the labeled ML128 was used for imaging its in vivo distribution and pharmacokinetics in brain.     

Synthesis and evaluation of a 18F-curcumin derivate for β-amyloid plaque imaging

IntroductionCurcumin is a neuroprotective compound that inhibits the formation of amyloid oligomers and fibrils and binds to β-amyloid plaques in Alzheimer’s disease (AD). We aimed to synthesize an ¹⁸F-labeled curcumin derivate ([¹⁸F]4) and to characterize its positron emission tomography (PET) tracer-binding properties to β-amyloid plaques in a transgenic APP23 mouse model of AD.MethodsWe utilized facile one-pot synthesis of [¹⁸F]4 using nucleophilic ¹⁸F-fluorination and click chemistry. Binding of [¹⁸F]4 to β-amyloid plaques in the transgenic APP23 mouse brain cryosections was studied in vitro using heterologous competitive binding against PIB. [¹⁸F]4 uptake was studied ex vivo in rodents and in vivo using PET/computed tomography of transgenic APP23 and wild-type control mice.ResultsThe radiochemical yield of [¹⁸F]4 was 21 ± 11%, the specific activity exceeded 1 TBq/μmol, and the radiochemical purity exceeded 99.3% at the end of synthesis. In vitro studies of [¹⁸F]4 with the transgenic APP23 mouse revealed high β-amyloid plaque binding. In vivo and ex vivo studies demonstrated that [¹⁸F]4 has fast clearance from the blood, moderate metabolism but low blood–brain barrier (BBB) penetration.Conclusions[¹⁸F]4 was synthesized in high yield and excellent quality. In vitro studies, metabolite profile, and fast clearance from the blood indicated a promising tracer for Aβ imaging. However, [¹⁸F]4 has low in vivo BBB penetration and thus further studies are needed to reveal the reason for this and to possibly overcome this issue.     

Radiosynthesis and biological evaluation of an fluorine-18 labeled galactose derivative [18F]FPGal for imaging the hepatic asialoglycoprotein receptor

The asialoglycoprotein receptor (ASGPR) is abundantly expressed on the surface of hepatocytes where it recognizes and endocytoses glycoproteins with galactosyl and N-acetylgalactosamine groups. Given its hepatic distribution, the asialoglycoprotein receptor can be targeted by positron imaging agents to study liver function using PET imaging. In this study, the positron imaging agent [¹⁸F]FPGal was designed to specifically target hepatic asialoglycoprotein receptor and its effectiveness was assessed in in vitro and in vivo models. The radiosynthesis of [¹⁸F]FPGal required 50 min with total radiochemical yields of [¹⁸F]FPGal from [¹⁸F]fluoride as 10% (corrected radiochemical yield). The K_d of [¹⁸F]FPGal to ASGPR in HepG2 cells was 1.99 ± 0.05 mM. Uptake values of 0.55% were observed within 30 min of incubation with HepG2 cells, which could be blocked by 200 mM d(+)-galactose (<0.1%). In vivo biodistribution analysis showed that the liver accumulation of [¹⁸F]FPGal at 30 min was 4.47 ± 0.96% ID/g in normal mice compared to 1.33 ± 0.07% ID/g in hepatic fibrotic mice (P < 0.01). Reduced uptake in the hepatic fibrosis mouse models was confirmed through PET/CT images at 30 min. Compared to normal mice, the standard uptake value (SUV) in the hepatic fibrosis mice was significantly lower when assessed through dynamic data collection for 1 h. Therefore, [¹⁸F]FPGal is a feasible PET probe that provide insight into ASGPR related liver disease.     

[18F]FE@SNAP—A new PET tracer for the melanin concentrating hormone receptor 1 (MCHR1): Microfluidic and vessel-based approaches

Changes in the expression of the melanin concentrating hormone receptor 1 (MCHR1) are involved in a variety of pathologies, especially obesity and anxiety disorders. To monitor these pathologies in-vivo positron emission tomography (PET) is a suitable method. After the successful radiosynthesis of [¹¹C]SNAP-7941—the first PET-Tracer for the MCHR1, we aimed to synthesize its [¹⁸F]fluoroethylated analogue: [¹⁸F]FE@SNAP. Therefore, microfluidic and vessel-based approaches were tested. [¹⁸F]fluoroethylation was conducted via various [¹⁸F]fluoroalkylated synthons and direct [¹⁸F]fluorination. Only the direct [¹⁸F]fluorination of a tosylated precursor using a flow-through microreactor was successful, affording [¹⁸F]FE@SNAP in 44.3 ± 2.6%.     

Synthesis and biological evaluation of a novel 99mTc cyclopentadienyl tricarbonyl complex ([(Cp-R)99mTc(CO)3]) for sigma-2 receptor tumor imaging

We report the design, synthesis and biological evaluation of a novel ^99mTc 4-(4-cyclohexylpiperazine-1-yl)-butan-1-one-1-cyclopentadienyltricarbonyl technetium ([^99mTc]5) as a potential SPECT tracer for imaging of σ₂ receptors in tumors. [^99mTc]5 was prepared in 25 ± 5% isolated radiochemical yield with radiochemical purity of >99% via double-ligand transfer (DLT) reaction from the ferrocene precursor 2b (4-(4-cyclohexylpiperazine-1-yl)-1-ferrocenylbutan-1-one). The corresponding Re-complex 4 and the ferrocenyl complex 2b showed relatively high affinity towards σ₂ receptors in in vitro competition binding assay (K_i values of 4 and 2b were 64.4 ± 18.5 nM and 43.6 ± 21.3 nM, respectively) and moderate to high selectivity versus σ₁ receptors (K_iσ₁/K_iσ₂ ratios were 12.5 and 95.5, respectively). The log D value of [^99mTc]5 was determined to be 2.52 ± 0.33. Biodistribution studies in mice revealed comparably high initial brain uptake of [^99mTc]5 and slow washout. Administration of haloperidol 5 min prior to injection of [^99mTc]5 significantly reduced the radiotracer uptake in brain, heart, lung, and spleen by 40–50% at 2 h p.i.. Moreover, [^99mTc]5 showed high uptake in C6 glioma cell lines (8.6%) after incubation for 1 h. Blocking with haloperidol to compete with [^99mTc]5 significantly reduced the cell uptake. Preliminary blocking study in C6-brain-tumor bearing rats showed that [^99mTc]5 binds to σ receptors in the brain-tumor specifically. These results are encouraging for further exploration of ^99mTc-labeled probes for σ₂ receptor tumor imaging in vivo.