Evaluation of potential PET imaging probes for the orexin 2 receptors

A wide range of central nervous system (CNS) disorders, particularly those related to sleep, are associated with the abnormal function of orexin (OX) receptors. Several orexin receptor antagonists have been reported in recent years, but currently there are no imaging tools to probe the density and function of orexin receptors in vivo. To date there are no published data on the pharmacokinetics (PK) and accumulation of some lead orexin receptor antagonists. Evaluation of CNS pharmacokinetics in the pursuit of positron emission tomography (PET) radiotracer development could be used to elucidate the association of orexin receptors with diseases and to facilitate the drug discovery and development. To this end, we designed and evaluated carbon-11 labeled compounds based on diazepane orexin receptor antagonists previously described. One of the synthesized compounds, [¹¹C]CW4, showed high brain uptake in rats and further evaluated in non-human primate (NHP) using PET-MR imaging. PET scans performed in a baboon showed appropriate early brain uptake for consideration as a radiotracer. However, [¹¹C]CW4 exhibited fast kinetics and high nonspecific binding, as determined after co-administration of [¹¹C]CW4 and unlabeled CW4. These properties indicate that [¹¹C]CW4 has excellent brain penetrance and could be used as a lead compound for developing new CNS-penetrant PET imaging probes of orexin receptors.     

HER-2阳性乳腺癌PET及SPECT靶向探针研究进展

HER-2阳性乳腺癌的恶性度高,易复发和转移。正电子发射体层成像（PET）和单光子发射体层成像（SPECT）可特异性提供靶组织的HER-2表达信息,并能对乳腺癌的疗效及预后实时评估,在乳腺癌诊疗过程中发挥了重要作用。随着HER-2靶向分子探针研究不断深入,降低正常器官和组织的非特异性摄取成为研究的重点。对PET及SPECT成像的HER-2分子靶向探针作一介绍,并综述HER-2分子探针的应用现状及进展,客观分析各种靶向探针的优势及不足。     

Synthesis and in vivo evaluation of a PET radioligand for imaging the endothelin-A receptor

The endothelin-A receptor ligand Atrasentan (ABT-627) was radiolabeled by (11)C-methylaton of the desmethyl precursor in phenolate form. In mice, the highest uptake of [(11)C]ABT-627 was in the liver, kidneys and lungs. No significant binding was observed in mouse brain or heart. PET studies in a baboon, however, showed accumulation in the myocardium and lungs with a tissue/blood equilibrium reached at 40 min postinjection. Between 35 and 75 min, the heart/blood and lung/blood ratios were 1.72 and 1.31, respectively. Pretreatment with a 0.39 mg/kg dose of unlabeled ABT-627 inhibited the uptake of the tracer by 53-54% in both the myocardium and lungs at 65 min.     

Synthesis and evaluation of candidate PET radioligands for corticotropin-releasing factor type-1 receptors

IntroductionA radioligand for measuring the density of corticotropin-releasing factor subtype-1 receptors (CRF₁ receptors) in living animal and human brain with positron emission tomography (PET) would be a useful tool for neuropsychiatric investigations and the development of drugs intended to interact with this target. This study was aimed at discovery of such a radioligand from a group of CRF₁ receptor ligands based on a core 3-(phenylamino)‐pyrazin-2(1H)-one scaffold.MethodsCRF₁ receptor ligands were selected for development as possible PET radioligands based on their binding potency at CRF₁ receptors (displacement of [¹²⁵I]CRF from rat cortical membranes), measured lipophilicity, autoradiographic binding profile in rat and rhesus monkey brain sections, rat biodistribution, and suitability for radiolabeling with carbon-11 or fluorine-18. Two identified candidates (BMS-721313 and BMS-732098) were labeled with fluorine-18. A third candidate (BMS-709460) was labeled with carbon-11 and all three radioligands were evaluated in PET experiments in rhesus monkey. CRF₁ receptor density (B_max) was assessed in rhesus brain cortical and cerebellum membranes with the CRF₁ receptor ligand, [³H]BMS-728300.ResultsThe three ligands selected for development showed high binding affinity (IC₅₀ values, 0.3–8 nM) at CRF₁ receptors and moderate lipophilicity (LogD, 2.8–4.4). [³H]BMS-728300 and the two ¹⁸F-labeled ligands showed region-specific binding in rat and rhesus monkey brain autoradiography, namely higher binding density in the frontal and limbic cortex, and cerebellum than in thalamus and brainstem. CRF₁ receptor B_max in rhesus brain was found to be 50–120 fmol/mg protein across cortical regions and cerebellum. PET experiments in rhesus monkey showed that the radioligands [¹⁸F]BMS-721313, [¹⁸F]BMS-732098 and [¹¹C]BMS-709460 gave acceptably high brain radioactivity uptake but no indication of the specific binding as seen in vitro.ConclusionsCandidate CRF₁ receptor PET radioligands were identified but none proved to be effective for imaging monkey brain CRF₁ receptors. Higher affinity radioligands are likely required for successful PET imaging of CRF₁ receptors.     

Synthesis and biological evaluation of 18F-Norfallypride in the rodent brain using PET imaging

Norfallypride (N-[(2-pyrolidinyl)methyl]-2,3-dimethoxy-5-(3’-fluoropropyl)benzamide), an analog of fallypride, has been synthesized and evaluated as a potential PET imaging agent for dopamine receptors with increased subtype selectivity. In order to synthesize ¹⁸F-Norfallypride, the substituted benzamide tosylate (S)-N-[(1-BOC-2-pyrolidinyl)methyl]-2,3-dimethoxy-5-(3’-tosyloxypropyl)-benzamide) was radiolabeled with ¹⁸F using Kryptofix and K₂CO₃ in acetonitrile and deprotected with trifluoroacetic acid to yield (S)-¹⁸F-Norfallypride in approx. 10% radiochemical yields. Norfallypride exhibited an IC₅₀ of 0.63 μM for displacing ¹⁸F-fallypride in rat brain slices. In vitro rat brain autoradiographic studies revealed weak binding of ¹⁸F-norfallypride to striatal regions. PET imaging in rats showed low brain uptake of ¹⁸F-norfallypride in the rat brain. Ex vivo brain PET analysis displayed binding of ¹⁸F-norfallypride in several brain regions. With respect to the cerebellum, ex vivo PET ratios were: striatum > 3; hypothalamus > 2; hippocampus ~ 2; cerebellar nuclei > 2 while autoradiographic ratios were 14, 9, 4 and 6 respectively. ¹⁸F-Norfallypride exhibited a unique binding profile to rat brain regions known to contain significant amounts of dopamine D3 and serotonin 5HT₃ receptors. Efforts are currently under way to increase brain permeability and fully characterize the binding of ¹⁸F-norfallypride in vivo.     

胃癌PET/CT分子影像探针的研究进展

胃癌是最常见的消化系统恶性肿瘤之一,预后较差,准确地诊断及分期对胃癌临床治疗决策的制定具有重要意义。放射性核素显像如PET/CT或PET/MR可以在分子或细胞水平对肿瘤进行显像,在胃癌的诊断、分期、再分期和疗效评估方面提供重要的信息。由于传统的正电子核素示踪剂18F-FDG在胃癌的应用中存在一些局限性,多种新型分子探针逐渐被开发并应用于胃癌诊断中。笔者就PET/CT分子影像探针在胃癌诊断中的临床应用现状和研究进展进行综述。     

Synthesis and preliminary evaluation of [18F]-labeled 2-oxoquinoline derivatives for PET imaging of cannabinoid CB2 receptor

IntroductionThe cannabinoid receptor type 2 (CB₂) is an important target for development of drugs and imaging agents for diseases, such as neuroinflammation, neurodegeneration and cancer. Recently, we reported synthesis and results of in vitro receptor binding of a focused library of fluorinated 2-oxoquinoline derivatives as CB₂ receptor ligands. Some of the compounds demonstrated to be good CB₂-specific ligands with Ki values in the nanomolar to subnanomolar concentrations; therefore, we pursued the development of their ¹⁸F-labeled analogues that should be useful for positron emission tomography (PET) imaging of CB₂ receptor expression. Here, we report the radiosynthesis of two ¹⁸F-labeled 2-oxoquinoline derivatives and the preliminary in vitro and ex vivo evaluation of one compound as a CB₂-specific radioligand.Methods4-[¹⁸F]fluorobenzyl amine [¹⁸F]-3 was prepared by radiofluorination of 4-cyano-N,N,N-trimethylanilinium triflate salt followed by reduction with LiAlH₄ and then coupled with acid chlorides 11 and 12 to afford [¹⁸F]-13 and [¹⁸F]-14. In vitro CB₂ receptor binding assay was performed using U87 cells transduced with CB₂ and CB₁ receptor. Ex vivo autoradiography was performed with [¹⁸F]-14 on spleen and on CB₂- and CB₁-expressing and wild-type U87 subcutaneous tumors grown in mice.ResultsThe radiochemical yields of [¹⁸F]-13 and [¹⁸F]-14 were 10%–15.0% with an average of 12% (n=10); radiochemical purity was >99% with specific activity 1200 mCi/μmol. The dissociation constant Kd for [¹⁸F]-14 was 3.4 nM. Ex vivo autoradiography showed accumulation of [¹⁸F]-14 in the CB₂-expressing tumor.ConclusionTwo new [¹⁸F]-labeled CB₂ ligands have been synthesized. Compound [¹⁸F]-14 appears to be a potential PET imaging agent for the assessment of CB₂ receptor expression; however, poor solubility restrain its use in vivo.     

Synthesis and evaluation of iodinated TZTP-derivatives as potential radioligands for imaging of muscarinic M2 receptors with SPET

A series of iodinated thiadiazolyltetrahydro-1-methyl-pyridine (TZTP) compounds was synthesized and evaluated in vitro and in vivo as potential radioligands for imaging of the muscarinic M2 receptor subtype with SPET. One of these compounds, 5-(E)-iodopentenylthio-TZTP, has high in vitro affinity (Ki = 4.9 nM) and moderate selectivity for the muscarinic M2 receptor subtype. Although the uptake pattern in the biodistribution studies in rats is consistent with muscarinic M2 receptor disribution, specific in vivo binding to these receptors could not be demonstrated. The usefulness of this tracer in human SPET imaging may therefore be limited.     

肿瘤短链脂肪酸代谢PET显像剂研究进展

18F-FDG PET显像在临床上发挥着越来越重要的作用,但其在某些肿瘤的应用中存在一定的弊端,易产生假阳性和假阴性。脂肪酸代谢PET显像在脂类代谢显像中占有非常重要的地位,在一定程度上可弥补糖代谢的不足,能提高对肿瘤诊断的灵敏度和准确率。笔者就目前应用于临床前和临床研究中的肿瘤短链脂肪酸代谢PET显像剂进行综述。     

PET imaging of human cardiac opioid receptors

The presence of opioid peptides and receptors and their role in the regulation of cardiovascular function has been previously demonstrated in the mammalian heart. The aim of this study was to image micro and delta opioid receptors in the human heart using positron emission tomography (PET). Five subjects (three females, two males, 65+/-8 years old) underwent PET scanning of the chest with [(11)C]carfentanil ([(11)C]CFN) and [(11)C]- N-methyl-naltrindole ([(11)C]MeNTI) and the images were analyzed for evidence of opioid receptor binding in the heart. Either [(11)C]CFN or [(11)C]MeNTI (20 mCi) was injected i.v. with subsequent dynamic acquisitions over 90 min. For the blocking studies, either 0.2 mg/kg or 1 mg/kg of naloxone was injected i.v. 5 min prior to the injection of [(11)C]CFN and [(11)C]MeNTI, respectively. Regions of interest were placed over the left ventricle, left ventricular chamber, lung and skeletal muscle. Graphical analysis demonstrated average baseline myocardial binding potentials (BP) of 4.37+/-0.91 with [(11)C]CFN and 3.86+/-0.60 with [(11)C]MeNTI. Administration of 0.2 mg/kg naloxone prior to [(11)C]CFN produced a 25% reduction in BP in one subject in comparison with baseline values, and a 19% decrease in myocardial distribution volume (DV). Administration of 1 mg/kg of naloxone before [(11)C]MeNTI in another subject produced a 14% decrease in BP and a 21% decrease in the myocardial DV. These results demonstrate the ability to image these receptors in vivo by PET. PET imaging of cardiac opioid receptors may help to better understand their role in cardiovascular pathophysiology and the effect of abuse of opioids and drugs on heart function.     

Synthesis and evaluation of a novel 68Ga-chelate-conjugated bisphosphonate as a bone-seeking agent for PET imaging

Introduction

⁶⁸Ga is a positron-emitting nuclide that has significant imaging potential given that, unlike cyclotron-produced ¹⁸F, the isotope can be produced on-site utilizing a ⁶⁸Ge/⁶⁸Ga generator. We recently synthesized a novel bone-seeking agent by coupling a bisphosphonate with the ⁶⁸Ga chelator 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). This study presents a first report on the potential of this ⁶⁸Ga bone-seeking radiopharmaceutical in the detection of bone metastases.

Methods

4-Amino-1-hydroxybutylidene-1,1-bisphosphonate was conjugated with 2-[4,7-di(carboxymethyl)-1,4,7-triazonan-1-yl]pentanedioic acid, yielding 2-[4,7-di(carboxymethyl)-1,4,7-triazonan-1-yl]-5-[(4-hydroxy-4,4-diphosphonobutyl)amino]-5-oxopentanoic acid (NOTA-BP). ⁶⁸Ga-labeled NOTA-BP ([⁶⁸Ga]NOTA-BP) was prepared by complexation of NOTA-BP with [⁶⁸Ga] gallium chloride and evaluated in in vitro experiments, biodistribution experiments and micro-positron emission tomography (PET) imaging experiments.

Results

The labeling of NOTA-BP with ⁶⁸Ga was completed by heating for 10 min. [⁶⁸Ga]NOTA-BP was determined to have a radiochemical purity of over 95%, a high affinity for hydroxyapatite and a high stability in plasma. In in vivo biodistribution experiments, [⁶⁸Ga]NOTA-BP demonstrated high bone uptake potential. Compared with ^99mTc-labeled methylene diphosphonate ([^99mTc]MDP) and [¹⁸F]fluoride, [⁶⁸Ga]NOTA-BP exhibited faster blood clearance and a higher bone-to-blood ratio. In addition, mouse model bone metastasis was detected by micro-PET imaging at 1 h postinjection of [⁶⁸Ga]NOTA-BP.

Conclusion

We have developed a novel ⁶⁸Ga-radiolabeled bone-seeking agent. This [⁶⁸Ga]NOTA-BP complex was found to have a high bone affinity and rapid blood clearance, and may thus prove to be useful as a bone-seeking agent for clinical PET.     

Dynamic imaging of striatal D2 receptors in mice using quad-HIDAC PET.

The novel, dedicated small animal PET tomograph, quad-HIDAC, offers submillimeter resolution in instrumental characterization experiments. The aim of this study was to establish the tomograph's utility in a biologic application and to demonstrate the feasibility of rapid dynamic neuroreceptor imaging in mice. METHODS: We used the well-established, high-affinity dopamine D(2) receptor PET ligand (18)F-fallypride for imaging striatal D(2) receptors in NMRI mice. Dynamic PET data were acquired using the quad-HIDAC tomograph and subject to 2 different kinetic modeling approaches. The cerebellum, a brain region devoid of D(2) receptors, was chosen as a reference region for kinetic modeling. RESULTS: The resolution of the quad-HIDAC camera allowed clear visualization of the left and right mouse striatum with high target-to-nontarget signal ratios. The sensitivity of the tomograph permitted the generation of time-activity curves with initial time frames of 120 s. PET experiments acquiring data for 150 min demonstrated that the binding potential of (18)F-fallypride could be fitted robustly with both reference tissue models for scan durations of >or=40 min. Voxel-wise modeling resulted in parametric maps of high quality. The values for the binding potential in the striatum reached approximately 14, consistent with striatum-to-cerebellum ratios extracted from regional time-activity curves. Comparison of in vivo PET imaging results with ex vivo postmortem tissue sampling analyses indicated discrepancies in signal intensity, possibly resulting from scatter and random background in the cerebellum region of interest and leading to an overestimation of cerebellar activity concentrations and degradation of striatum-to-cerebellum ratios in PET experiments. Intraperitoneal injection of the unlabeled D(2) receptor antagonist haloperidol 30 min before intravenous injection of (18)F-fallypride blocked tracer accumulation in the striatum by >95%. CONCLUSION: The quad-HIDAC camera represents a powerful tool for future dynamic neuroreceptor PET studies in mice and rats under numerous pharmacologic or pathophysiologic conditions.     

Radiosynthesis and in vivo evaluation of N-[11C]methylated imidazopyridineacetamides as PET tracers for peripheral benzodiazepine receptors

Imidazopyridineacetoamide 5–8, a series of novel and potentially selective peripheral benzodiazepine receptor (PBR) ligands with affinities comparable to those of known PBR ligands, was investigated. Radiosyntheses of [¹¹C]5, 6, 7 or 8 was accomplished by N-methylation of the corresponding desmethyl precursors with [¹¹C]methyl iodide in the presence of NaH in dimethylformamide (DMF), resulting in 25% to 77% radiochemical yield and specific activitiy of 20 to 150 MBq/nmol. Each of the labeled compounds was injected in ddY mice, and the radioactivity and weight of dissected peripheral organs and brain regions were measured. Organ distribution of [¹¹C]7 was consistent with the known PBR distribution. Moreover, [¹¹C]7 showed the best combination of brain uptake and PBR binding, leading to its high retention in the olfactory bulb and cerebellum, areas where PBR density is high in mouse brain. Coinjection of PK11195 or unlabeled 7 significantly reduced the brain uptake of [¹¹C]7. These results suggest that [¹¹C]7 could be a useful radioligand for positron emission tomography imaging of PBRs.     

Search for PET probes for imaging the globus pallidus studied with rat brainex vivo autoradiography

We have evaluated the feasibility of using four positron emission tomography (PET) tracers for imaging the globus pallidus by ex vivo autoradiography in rats. The tracers investigated were [11C]KF18446, [11C]SCH 23390 and [11C]raclopride for mapping adenosine A2A, dopamine D1 and dopamine D2 receptors, respectively, and [18F]FDG. The highest uptake by the globus pallidus was found for [11C]SCH 23390, followed by [18F]FDG, [11C]KF18446 and [11C]raclopride. The receptor-specific uptake by the globus pallidus was observed in [11C]KF18446 and [11C]SCH 23390, but not in [11C]raclopride. Uptake ratios of globus pallidus to the striatum for [18F]FDG and [11C]KF18446 were approximately 0.6, which was twice as large as that for [11C]SCH 23390. In a rat model of degeneration of striatopallidal gamma-aminobutyric acid-ergic-enkephalin neurons induced by intrastriatal injection of quinolinic acid, the uptake of [11C]KF18446 by the striatum and globus pallidus was remarkably reduced. To prove the visualization of the globus pallidus by PET with [18F]FDG and [11C]KF18446, PET-MRI registration technique and advances in PET technologies providing high-resolution PET scanner will be required. The metabolic activity of the globus pallidus could then be measured by PET with [18F]FDG, and [11C]KF18446 may be a candidate tracer for imaging the pallidal terminals projecting from the striatum.     

Comparison of MR and PET imaging for the evaluation of liver metastases

PURPOSE: To compare the accuracy of fluoro-18-deoxyglucose positron emission tomography (FDG-PET) and dynamic-enhanced magnetic resonance imaging (MRI) scans in the diagnosis of liver metastatic lesions from colon and other sources. MATERIALS AND METHODS: Thirty consecutive patients with known or suspected metastatic lesions were scanned by both MRI and PET. Histopathology and/or clinical outcome, including cross-sectional imaging follow up, were used as a gold standard. RESULTS: Of 30 patients, 16 were positive by pathology and/or clinical outcome and 14 were negative for liver metastases. The sensitivity, specificity, and positive and negative predictive values on MRI were 85.7%, 100%, 100%, and 89%, respectively, compared to 71%, 93.7%, 90.9%, and 79% on FDG-PET. The difference between the two methods was not significant (X(2) = 0.2, P > 0.05). CONCLUSION: Our study showed no significant difference in detection of liver metastases using MRI or FDG-PET. However, MRI has advantages in spatial resolution and lesion characterization.     

Synthesis of new carbon-11 labeled cyclofenil derivatives for PET imaging of breast cancer estrogen receptors.

Carbon-11 labeled cyclofenil derivatives, [(11)C]methyl-2-{4-[bis(4-hydroxyphenyl)methylene]cyclohexyl}acetate ([(11)C]16a), [(11)C]methyl-4-[bis(4-hydroxyphenyl)methylene]cyclohexanecarboxylate ([(11)C]16b), [(11)C]methyl-2-{3-[bis(4-hydroxyphenyl)methylene]cyclohexyl}acetate ([(11)C]18a), and [(11)C]methyl-3-[bis(4-hydroxyphenyl)methylene]cyclohexanecarboxylate ([(11)C]18b), have been synthesized as new potential PET agents for imaging breast cancer estrogen receptors. The target tracers were prepared by O-[(11)C]methylation of their corresponding precursors using [(11)C]CH(3)OTf and isolated by a simplified SPE purification procedure in 35-50% radiochemical yields decay corrected to EOB, 15-20 min overall synthesis time, and 74-111 GBq/micromol specific activity at EOS.