Imaging of amyloid deposition in human brain using positron emission tomography and [18F]FACT: comparison with [11C]PIB.

PURPOSE: The characteristic neuropathological changes in Alzheimer's disease (AD) are deposition of amyloid senile plaques and neurofibrillary tangles. The (18)F-labeled amyloid tracer, [(18)F]2-[(2-{(E)-2-[2-(dimethylamino)-1,3-thiazol-5-yl]vinyl}-1,3-benzoxazol-6-yl)oxy]-3-fluoropropan-1-ol (FACT), one of the benzoxazole derivatives, was recently developed. In the present study, deposition of amyloid senile plaques was measured by positron emission tomography (PET) with both [(11)C]Pittsburgh compound B (PIB) and [(18)F]FACT in the same subjects, and the regional uptakes of both radiotracers were directly compared. METHODS: Two PET scans, one of each with [(11)C]PIB and [(18)F]FACT, were performed sequentially on six normal control subjects, two mild cognitive impairment (MCI) patients, and six AD patients. The standardized uptake value ratio of brain regions to the cerebellum was calculated with partial volume correction using magnetic resonance (MR) images to remove the effects of white matter accumulation. RESULTS: No significant differences in the cerebral cortical uptake were observed between normal control subjects and AD patients in [(18)F]FACT studies without partial volume correction, while significant differences were observed in [(11)C]PIB. After partial volume correction, the cerebral cortical uptake was significantly larger in AD patients than in normal control subjects for [(18)F]FACT studies as well as [(11)C]PIB. Relatively lower uptakes of [(11)C]PIB in distribution were observed in the medial side of the temporal cortex and in the occipital cortex as compared with [(18)F]FACT. Relatively higher uptake of [(11)C]PIB in distribution was observed in the frontal and parietal cortices. CONCLUSION: Since [(18)F]FACT might bind more preferentially to dense-cored amyloid deposition, regional differences in cerebral cortical uptake between [(11)C]PIB and [(18)F]FACT might be due to differences in regional distribution between diffuse and dense-cored amyloid plaque shown in the autoradiographic and histochemical assays of postmortem AD brain sections.

Micro-positron emission tomography/contrast-enhanced computed tomography imaging of orthotopic pancreatic tumor-bearing mice using the αvß3 integrin tracer 64Cu-labeled cyclam-RAFT-c(-RGDfK-)4.

The purpose of this study was to develop a clinically relevant orthotopic xenotransplantation model of pancreatic cancer and to perform a preclinical evaluation of a new positron emission tomography (PET) imaging probe, 64Cu-labeled cyclam-RAFT-c(-RGDfK-)4 peptide (64Cu-RAFT-RGD), using this model. Varying degrees of αvß3 integrin expression in several human pancreatic cancer cell lines were examined by flow cytometry and Western blotting. The cell line BxPC-3, which is stably transfected with a red fluorescence protein (RFP), was used for surgical orthotopic implantation. Orthotopic xenograft was established in the pancreas of recipient nude mice. An in vivo probe biodistribution and receptor blocking study, preclinical PET imaging coregistered with contrast-enhanced computed tomography (CECT) comparing 64Cu-RAFT-RGD and ¹8F-fluoro-2-deoxy-d-glucose (¹8F-FDG) accumulation in tumor, postimaging autoradiography, and histologic and immunohistochemical examinations were done. Biodistribution evaluation with a blocking study confirmed that efficient binding of probe to tumor is highly αvß3 integrin specific. 64Cu-RAFT-RGD PET combined with CECT provided for precise and easy detection of cancer lesions. Autoradiography, histologic, and immunohistochemical examinations confirmed the accumulation of 64Cu-RAFT-RGD in tumor versus nontumor tissues. In comparative PET studies, 64Cu-RAFT-RGD accumulation provided better tumor contrast to background than ¹8F-FDG. Our results suggest that 64Cu-RAFT-RGD PET imaging is potentially applicable for the diagnosis of αvß3 integrin-expressing pancreatic tumors.

Carbon-11 radiolabeling of an oligopeptide containing tryptophan hydrochloride via a Pictet-Spengler reaction using carbon-11 formaldehyde.

A procedure for the synthesis of a(11)C-labeled oligopeptide containing [1-(11)C]1,2,3,4-tetrahydro-ß-carboline-3-carboxylic acid ([1-(11)C]Tpi) from the corresponding Trp•HCl-containing peptides has been developed involving a Pictet-Spengler reaction with [(11) C]formaldehyde. The synthesis of [1-(11)C]Tpi from Trp and [(11)C]formaldehyde was examined as a model reaction with the aim of developing a facile and effective method for the labeling of peptides with carbon-11. The Pictet-Spengler reaction of Trp and [(11)C]formaldehyde in acidic media (TsOH or HCl) afforded the desired [1-(11)C]Tpi in a moderate radiochemical yield. Herein, the application of a Pictet-Spengler reaction to an aqueous solution of Trp•HCl gave the desired product with a radiochemical yield of 45.2%. The RGD peptide cyclo[Arg-Gly-Asp-D-Tyr-Lys] was then selected as a substrate for the labeling reaction with [(11)C]formaldehyde. The radiolabeling of a Trp•HCl-containing RGD peptide using the Pictet-Spengler reaction was successful. Furthermore, the remote-controlled synthesis of a [1-(11)C]Tpi-containing RGD peptide was attempted by using an automatic production system to generate [(11)C]CH3 I. The radiochemical yield of the [1-(11) C]Tpi-containing RGD at the end of synthesis (EOS) was 5.9 ± 1.9% (n = 4), for a total synthesis time of about 35 min. The specific activity was 85.7 ± 9.4 GBq/µmol at the EOS.

In vivo positron emission tomographic imaging of glial responses to amyloid-beta and tau pathologies in mouse models of Alzheimer's disease and related disorders.

Core pathologies of Alzheimer's disease (AD) are aggregated amyloid-ß peptides (Aß) and tau, and the latter is also characteristic of diverse neurodegenerative tauopathies. These amyloid lesions provoke microglial activation, and recent neuroimaging technologies have enabled visualization of this response in living brains using radioligands for the peripheral benzodiazepine receptor also known as the 18 kDa translocator protein (TSPO). Here, we elucidated contributions of Aß and tau deposits to in vivo TSPO signals in pursuit of mechanistic and diagnostic significance of TSPO imaging in AD and other tauopathies. A new antibody to human TSPO revealed induction of TSPO-positive microgliosis by tau fibrils in tauopathy brains. Emergence of TSPO signals before occurrence of brain atrophy and thioflavin-S-positive tau amyloidosis was also demonstrated in living mice transgenic for mutant tau by positron emission tomography (PET) with two classes of TSPO radioligands, [(11)C]AC-5216 and [(18)F]fluoroethoxy-DAA1106. Meanwhile, only modest TSPO elevation was observed in aged mice modeling Aß plaque deposition, despite the notably enhanced in vivo binding of amyloid radiotracer, [(11)C]Pittsburgh Compound-B, to plaques. In these animals, [(11)C]AC-5216 yielded better TSPO contrasts than [(18)F]fluoroethoxy-DAA1106, supporting the possibility of capturing early neurotoxicity with high-performance TSPO probes. Furthermore, an additional line of mice modeling intraneuronal Aß accumulation displayed elevated TSPO signals following noticeable neuronal loss, unlike TSPO upregulation heralding massive neuronal death in tauopathy model mice. Our data corroborate the utility of TSPO-PET imaging as a biomarker for tau-triggered toxicity, and as a complement to amyloid scans for diagnostic assessment of tauopathies with and without Aß pathologies.

Characterization of 1-(2-[18F] fluoro-3-pyridyl)-4-(2-isopropyl-1-oxo- isoindoline-5-yl)-5-methyl-1H-1,2,3-triazole, a PET ligand for imaging the metabotropic glutamate receptor type 1 in rat and monkey brains.

We developed 1-(2-[(18) F]fluoro-3-pyridyl)-4-(2-isopropyl-1-oxo-isoindoline-5-yl)-5-methyl-1H-1,2,3-triazole ([(18) F]FPIT) as a promising positron emission tomography (PET) ligand for in vitro and in vivo imaging of metabotropic glutamate receptor type 1 (mGluR1) in rat and monkey brains. In vitro autoradiography with [(18) F]FPIT was used to determine the distribution of radioactivity in rat and monkey brains. In vivo experiments were performed using dissection and small-animal PET on rats, and PET on monkey. Metabolite analysis was performed on rat plasma and brain, and monkey plasma. Autoradiography of rat and monkey brains showed that [(18) F]FPIT binding is aligned with the reported distribution of mGluR1 with high specific binding in the cerebellum and thalamus. PET study on rat and monkey showed high brain uptake and distribution patterns consistent with those seen in the autoradiographic studies. The radioactivity in the brain was significantly decreased by pre-treatment with unlabeled FPIT, indicative of a specific signal for mGluR1 that was inhibited by mGluR1-selective ligand JNJ-16259865 in the brain. Metabolite analysis showed that the percentage of unchanged [(18) F]FPIT was 89% in the brain homogenate of rat at 90 min after injection. In the monkey plasma, the percentage of unchanged form was 50% at 90 min. [(18) F]FPIT produced in vitro and in vivo signals to visualize mGluR1 expression in rat and monkey brains, suggesting the usefulness of [(18) F]FPIT for imaging mGluR1 in human brain.

Positron emission tomography imaging of tumor angiogenesis and monitoring of antiangiogenic efficacy using the novel tetrameric peptide probe 64Cu-cyclam-RAFT-c(-RGDfK-)4.

64Cu-cyclam-RAFT-c(-RGDfK-)4 is a novel multimeric positron emission tomography (PET) probe for αVß3 integrin imaging. Its uptake and αVß3 expression in tumors showed a linear correlation. Since αVß3 integrin is strongly expressed on activated endothelial cells during angiogenesis, we aimed to determine whether 64Cu-cyclam-RAFT-c(-RGDfK-)4 PET can be used to image tumor angiogenesis and monitor the antiangiogenic effect of a novel multi-targeted tyrosine kinase inhibitor, TSU-68. Athymic nude mice bearing human hepatocellular carcinoma HuH-7 xenografts, which expressed negligible αVß3 levels on the tumor cells, received intraperitoneal injections of TSU-68 or the vehicle for 14 days. Antiangiogenic effects were determined at the end of therapy in terms of 64Cu-cyclam-RAFT-c(-RGDfK-)4 uptake evaluated using PET, biodistribution assay, and autoradiography, and they were compared with microvessel density (MVD) determined by CD31 immunostaining. 64Cu-cyclam-RAFT-c(-RGDfK-)4 PET enabled clear tumor visualization by targeting the vasculature, and the biodistribution assay indicated high tumor-to-blood and tumor-to-muscle ratios of 31.6 ± 6.3 and 6.7 ± 1.1, respectively, 3 h after probe injection. TSU-68 significantly slowed tumor growth and reduced MVD; these findings were consistent with a significant reduction in the tumor 64Cu-cyclam-RAFT-c(-RGDfK-)4 uptake. Moreover, a linear correlation was observed between tumor MVD and the corresponding standardized uptake value (SUV) (r = 0.829, P = 0.011 for SUV(mean); r = 0.776, P = 0.024 for SUV(max)) determined by quantitative PET. Autoradiography and immunostaining showed that the distribution of intratumoral radioactivity and tumor vasculature corresponded. We concluded that 64Cu-cyclam-RAFT-c(-RGDfK-)4 PET can be used for in vivo angiogenesis imaging and monitoring of tumor response to antiangiogenic therapy.

Imaging for metabotropic glutamate receptor subtype 1 in rat and monkey brains using PET with [18F]FITM.

PURPOSE: In this study, we evaluate the utility of 4-[(18)F]fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]FITM) as a positron emission tomography (PET) ligand for imaging of the metabotropic glutamate receptor subtype 1 (mGluR1) in rat and monkey brains. METHODS: In vivo distribution of [(18)F]FITM in brains was evaluated by PET scans with or without the mGluR1-selective antagonist (JNJ16259685). Kinetic parameters of monkey PET data were obtained using the two-tissue compartment model with arterial blood sampling. RESULTS: In PET studies in rat and monkey brains, the highest uptake of radioactivity was in the cerebellum, followed by moderate uptake in the thalamus, hippocampus and striatum. The lowest uptake of radioactivity was detected in the pons. These uptakes in all brain regions were dramatically decreased by pre-administration of JNJ16259685. In kinetic analysis of monkey PET, the highest volume of distribution (V(T)) was detected in the cerebellum (V(T) = 11.5). CONCLUSION: [(18)F]FITM has an excellent profile as a PET ligand for mGluR1 imaging. PET with [(18)F]FITM may prove useful for determining the regional distribution and density of mGluR1 and the mGluR1 occupancy of drugs in human brains.

Utilization of [11C]phosgene for radiosynthesis of N-(2-{3-[3,5-bis(trifluoromethyl)]phenyl[11C]ureido}ethyl)glycyrrhetinamide, an inhibitory agent for proteasome and kinase in tumors.

N-(2-{3-[3,5-Bis(trifluoromethyl)]phenylureido}ethyl)glycyrrhetinamide (2), an ureido-substituted derivative of glycyrrhetinic acid (1), has been reported to display potent inhibitory activity for proteasome and kinase, which are overexpressed in tumors. In this study, we labeled this unsymmetrical urea 2 using [(11)C]phosgene ([(11)C]COCl(2)) as a labeling agent with the expectation that [(11)C]2 could become a positron emission tomography ligand for the imaging of proteasome and kinase in tumors. The strategy for the radiosynthesis of [(11)C]2 was to react hydrochloride of 3,5-bis(trifluoromethyl)aniline (4·HCl) with [(11)C]COCl(2) to possibly give isocyanate [(11)C]6, followed by the reaction of [(11)C]6 with N-(2-aminoethyl)glycyrrhetinamide (3).

Radiosynthesis of [13N]dantrolene, a positron emission tomography probe for breast cancer resistant protein, using no-carrier-added [13N]ammonia.

Dantrolene (1) is a substrate for breast cancer resistant protein, which is widely distributed in the blood-brain-barrier, intestine, gall bladder, and liver. PET study with 1 labeled with a positron emitter can be used to visualize BCRP and to elucidate the effect of BCRP on the pharmacokinetics of drugs. The objective of this study was to label 1 using nitrogen-13 ((13)N, a positron emitter; half-life: 9.9min). Using no-carrier-added [(13)N]NH(3) as the labeling agent, we synthesized [(13)N]dantrolene ([(13)N]1) for the first time. The reaction of carbomyl chloride 2b with [(13)N]NH(3) gave an unsymmetrical urea [(13)N]3, followed by cyclization of [(13)N]3 to afford [(13)N]1. Due to its instability, 2b was prepared in situ by treating amine 5 with triphosgene in a ratio of 4 to 1 and used for subsequent [(13)N]ammonolysis without purification.

Effect of radiolabeled metabolite elimination from the brain on the accuracy of cerebral enzyme activity estimation using positron emission tomography with substrate tracers.

Cerebral enzyme activity can be quantified using positron emission tomography (PET) in conjunction with a radiolabeled enzyme substrate. We investigated the relationship between the elimination rate (k(el)) of tracer metabolites from the brain and the precision of target enzyme activity estimation (k(3)). An initial simulation study indicated that the precision of k(3) estimates was highly dependent on k(el), and was characterized by several kinetic parameters including the ratio of k(el) and the efflux rate (k(2)) of authentic tracer (ß≡k(el)/k(2)). The optimal tracer condition for high sensitivity was found to be ß<0.1. To verify the simulation results, we performed a PET study with a single monkey using two PET tracers, N-[(18)F]fluoroethylpiperidin-4-ylmethyl acetate ([(18)F]FEP-4MA) and N-[(11)C]methylpiperidin-4-yl acetate ([(11)C]MP4A). Both of these substrate type tracers were developed for measuring cerebral acetylcholinesterase activity. There was good retention of the radioactive metabolite of [(11)C]MP4A in the brain (k(el)=0.0036±0.0013 min(-1), ß=0.028), whereas that of [(18)F]FEP-4MA was eliminated from the brain (k(el)=0.012±0.0010 min(-1), ß=0.085). A non-linear least square analysis for simultaneous estimation of all parameters showed that the precision of the k(3) estimate for [(18)F]FEP-4MA was as high (7.4%) as that for [(11)C]MP4A (10%). These results indicate that tracers with metabolites that are eliminated from the brain at a slow rate (ß<0.1) may be useful for the quantitative measurement of target enzyme activity.

Visualization of early infarction in rat brain after ischemia using a translocator protein (18 kDa) PET ligand [11C]DAC with ultra-high specific activity.

The aim of this study was to visualize early infarction in the rat brain after ischemia using a translocator protein (TSPO) (18 kDa) PET ligand [(11)C]DAC with ultra-high specific activity (SA) of 3670-4450 GBq/µmol. An infarction model of rat brain was prepared by ischemic surgery and evaluated 2 days after ischemia using small-animal PET and in vitro autoradiography. Early infarction with a small increase of TSPO expression in the brain was visualized using PET with high SA [(11)C]DAC (average 4060 GBq/µmol), but was not distinguished clearly with usually reported SA [(11)C]DAC (37 GBq/µmol). Infarction in the rat brain 4 days after ischemia was visualized using high and usually reported SAs [(11)C]DAC. Displacement experiments with unlabeled TSPO-selective AC-5216 or PK11195 diminished the difference in radioactivity between ipsilateral and contralateral sides, confirming that the increased uptake on the infracted brain was specific to TSPO. In vitro autoradiography with high SA [(11)C]DAC showed that the TSPO expression increased on early infarction in the rat brain. High SA [(11)C]DAC is a useful and sensitive biomarker for the visualization of early infarction and the characterization of TSPO expression which was slightly elevated in the infarcted brain using PET.

Toward in vivo imaging of heart disease using a radiolabeled single-chain Fv fragment targeting tenascin-C.

Antibodies specific to a particular target molecule can be used as analytical reagents, not only for in vitro immunoassays but also for noninvasive in vivo imaging, e.g., immunoscintigraphies. In the latter case, it is important to reduce the size of antibody molecules in order to achieve suitable in vivo "diagnostic kinetics" and generate higher-resolution images. For these purposes, single-chain Fv fragments (scFvs; M(r) < 30 kDa) have greater potential than intact immunoglobulins (~150 kDa) or Fab (or Fab') fragments (~50 kDa). Our recent observation of enhanced tenascin-C (Tnc) expression at sites of cardiac repair after myocardial infarction prompted us to develop a radiolabeled scFv against Tnc for in vivo imaging of heart disease. We cloned the genes encoding the heavy and light chain variable domains of the mouse anti-Tnc monoclonal antibody 4F10, and combined them to create a single gene. The resulting scFv-4F10 gene was expressed in E. coli cells to produce soluble scFv proteins. scFv-4F10 has an affinity for Tnc (K(a) = 3.5 × 10(7) M(-1)), similar to the Fab fragment of antibody 4F10 (K(a) = 1.3 × 10(7) M(-1)) and high enough to be of practical use. A cysteine residue was then added to the C-terminus to achieve site-specific (111)In labeling via a chelating group. The resulting (111)In-labeled scFv was administered to a rat model of acute myocardial infarction. Biodistribution and quantitative autoradiographic studies indicated higher uptake of the radioactivity at the infarcted myocardium than the noninfarcted one. Single photon emission computed tomography (SPECT) provided in vivo cardiac images that coincided with the ex vivo observations. Our results will promote advances in diagnostic strategies for heart disease.

In vivo and in vitro imaging of I2 imidazoline receptors in the monkey brain.

I2 imidazoline receptors (I2Rs) are associated with depression, Alzheimer's disease, and Huntington's disease. However, in vivo imaging of I2Rs in the monkey brain has not been reported until now. We performed in vitro and in vivo imaging of (I2Rs) in the monkey brain using ¹¹C-labeled 2-(3-fluoro-4-tolyl)-4,5-dihydro-1H-imidazole ([¹¹C]FTIMD) which has high and selective affinity of I2Rs. In an auto-radiography (ARG) study, the distribution pattern of [¹¹C]FTIMD in the monkey brain was similar to that of [³H]idazoxan binging to I2Rs in the human brain, which was previously described. The specific binding of [¹¹C]FTIMD accounted for >97% of total binding in brain regions existing I2 Rs. In positron emission tomography (PET) studies, the radioactivity was accumulated in brain regions existing I2Rs ligand BU224, the accumulated radioactivity was decreased to approximately 66%-75% of the baseline measurement at 15-45 min after injection of [¹¹C]FTIMD. These results suggest that [¹¹C]FTIMD shows the specific-binging to I2Rs in the monkey brain as depicted by PET and ARG. We performed the first in vivo imaging of I2Rs using [¹¹C]FTIMD in the monkey brain.

Radiosynthesis of three [11C]ureido-substituted benzenesulfonamides as PET probes for carbonic anhydrase IX in tumors.

Three ureido-substituted benzenesulfonamides 1a-c have been developed as potent inhibitors for carbonic anhydrase IX, which is overexpressed in hypoxic tumors. In this study, we labeled these unsymmetrical ureas 1a-c using [(11)C]phosgene ([(11)C]COCl(2)) as a labeling agent with the expectation that [(11)C]1a-c could become promising positron tomography probes for imaging carbonic anhydrase IX in tumors. The strategy for radiosynthesis of [(11)C]1a-c was to react hydrochloride of anilines 2a-c with [(11)C]COCl(2) to give isocyanate [(11)C]4a-c, followed by a reaction with 4-aminobenzenesulfonamide (3).

Radiosynthesis and preliminary evaluation of 4-[18F]fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide as a new positron emission tomography ligand for metabotropic glutamate receptor subtype 1.

The purpose of this study was to develop 4-[(18)F]fluoro-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]FITM, [(18)F]4) as a new PET ligand for imaging metabotropic glutamate receptor subtype 1 (mGluR1). [(18)F]4 was synthesized by [(18)F]fluorination of a novel nitro precursor 3 with [(18)F]KF in the presence of Kryptofix 222. At the end of synthesis, 429-936 MBq (n=8) of [(18)F]4 was obtained with >99% radiochemical purity and 204-559GBq/µmol specific activity starting from 6.7 to 13.0 GBq of [(18)F]F(-). The brain distribution of [(18)F]4 was determined by the in vitro and ex vivo autoradiography using rat brain sections. The in vitro and in vivo specific binding of [(18)F]4 to mGluR1 was detected in the cerebellum, thalamus, hippocampus, and striatum. These results suggest that [(18)F]4 is a promising PET ligand for the in vivo evaluation of mGluR1.

[11C]sorafenib: radiosynthesis and preliminary PET study of brain uptake in P-gp/Bcrp knockout mice.

Sorafenib (Nexavar, BAY43-9006, 1) is a second-generation, orally active multikinase inhibitor that is approved for the treatment of some cancers in patients. In this Letter, we developed [(11)C]1 as a novel positron emission tomography (PET) probe, and evaluated the influence of ABC transporters-mediated efflux on brain uptake using PET with [(11)C]1 in P-glycoprotein (P-gp)/breast cancer resistance protein (Bcrp) knockout mice versus wild-type mice. [(11)C]1 was synthesized by the reaction of hydrochloride of aniline 2 with [(11)C]phosgene ([(11)C]COCl(2)) to give isocyanate [(11)C]6, followed by reaction with another aniline 3. Small-animal PET study with [(11)C]1 indicated that the radioactivity level (AUC(0-60 min), SUV×min) in the brains of P-gp/Bcrp knockout mice was about three times higher than in wild-type mice.

Synthesis and in vivo evaluation of ¹8F-fluoroethyl GF120918 and XR9576 as positron emission tomography probes for assessing the function of drug efflux transporters.

The purpose of this study was to synthesize two new positron emission tomography (PET) probes, N-(4-(2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl)phenyl)-9,10-dihydro-5-[¹8F]fluoroethoxy-9-oxo-4-acridine carboxamide ([¹8F]3) and quinoline-3-carboxylic acid [2-(4-{2-[7-(2-[¹8F]fluoroethoxy)-6-methoxy-3,4-dihydro-1H-isoquinolin-2-yl]ethyl}phenylcarbamoyl)-4,5-dimethoxyphenyl]amide ([¹8F]4), and to evaluate the potential of these PET probes for assessing the function of two major drug efflux transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP). [¹8F]3 and [¹8F]4 were synthesized by ¹8F-alkylation of each O-desmethyl precursor with [¹8F]2-fluoroethyl bromide for injection as PET probes. In vitro accumulation assay showed that treatment with P-gp/BCRP inhibitors (1 and 2) enhanced the intracellular accumulation capacity of P-gp- and BCRP-overexpressing MES-SA/Dx5 cells. In PET studies, the uptake (AUC(brain[0-)60 (min])) of [¹8F]3 and [¹8F]4 in wild-type mice co-injected with 1 were approximately sevenfold higher than that in wild-type mice, and the uptake of [¹8F]3 and [¹8F]4 in P-gp/Bcrp knockout mice were eight- to ninefold higher than that in wild-type mice. The increased uptake of [¹8F]3 and [¹8F]4 was similar to that of parent compounds ([¹¹C]1 and [¹¹C]2) previously described, indicating that radioactivity levels in the brain after injection of [¹8F]3 and [¹8F]4 are related to the function of drug efflux transporters. Also, these results suggest that the structural difference between parent compounds ([¹¹C]1 and [¹¹C]2) and fluoroethyl analogs ([¹8F]3 and [¹8F]4) do not obviously affect the potency against drug efflux transporters. In metabolite analysis of mice, the unchanged form in the brain and plasma at 60 min after co-injection of [¹8F]4 plus 1 were higher (95% for brain; 81% for plasma) than that after co-injection of [¹8F]3 plus 1. [¹8F]4 is a promising PET probe to assess the function of drug efflux transporters.

Synthesis and evaluation of 6-[1-(2-[(18)F]fluoro-3-pyridyl)-5-methyl-1H-1,2,3-triazol-4-yl]quinoline for positron emission tomography imaging of the metabotropic glutamate receptor type 1 in brain.

The purpose of this study was to synthesize 6-[1-(2-[(18)F]fluoro-3-pyridyl)-5-methyl-1H-1,2,3-triazol-4-yl]quinoline ([(18)F]FPTQ, [(18)F]7a) and to evaluate its potential as a positron emission tomography ligand for imaging metabotropic glutamate receptor type 1 (mGluR1) in the rat brain. Compound [(18)F]7a was synthesized by [(18)F]fluorination of 6-[1-(2-bromo-3-pyridyl)-5-methyl-1H-1,2,3-triazol-4-yl]quinoline (7b) with potassium [(18)F]fluoride. At the end of synthesis, 1280-1830MBq (n=8) of [(18)F]7a was obtained with >98% radiochemical purity and 118-237GBq/µmol specific activity using 3300-4000MBq of [(18)F]F(-). In vitro autoradiography showed that [(18)F]7a had high specific binding with mGluR1 in the rat brain. Biodistribution study using a dissection method and small-animal PET showed that [(18)F]7a had high uptake in the rat brain. The uptake of radioactivity in the cerebellum was reduced by unlabeled 7a and mGluR1-selective ligand JNJ-16259685 (2), indicating that [(18)F]7a had in vivo specific binding with mGluR1. Because of a low amount of radiolabeled metabolite present in the brain, [(18)F]7a may have a limiting potential for the in vivo imaging of mGluR1 by PET.

[11C]phosgene: Synthesis and application for development of PET radiotracers.

Carbon-11-labeled phosgene ([11C]phosgene, [11C]COCl2) is a useful labeling agent that connects two heteroatoms by inserting [11C]carbonyl (11C=O) function in carbamates, ureas, and carbonates, which are components of biologically important heterocyclic compounds and functional groups in drugs as a linker of fragments with in vivo stability. Development of 11C-labeled PET tracers has been performed using [11C]phosgene as a labeling agent. However, [11C]phosgene has not been frequently used for 11C-labeling because preparation of [11C]phosgene required dedicated synthesis apparatus (not commercially available) and had problems in reproducibility and reliability. In our laboratory, an improved method for synthesizing [11C]phosgene using a carbon tetrachloride detection tube kit in environmental air analysis and the automated synthesis system for preparing [11C]phosgene have been developed in 2009. This apparatus has been used for routine synthesis of 11C-labeled tracers 1-4 times/week. Using [11C]phosgene we have developed and produced many PET radiotracers containing [11C]urea and [11C]carbamate moieties. In this review, we report the performance of our method for preparing [11C]phosgene, including automated synthesis apparatus developed in house, and the application of [11C]phosgene for development and production of 11C-labeled PET tracers.

RGD-cyclam conjugate: synthesis and potential application for positron emission tomography.

Cyclam and DOTA-containing positron emission tomography radiotracers were prepared by using a modular chemical strategy based on peptide synthesis and chemoselective ligations. These molecules encompass two functional domains, one a tumour 'homing' domain and the other a chelating ligand for copper allowing nuclear imaging of tumours.