Radiosynthesis of [11C]BBAC and [11C]BBPC as potential PET tracers for orexin2 receptors

Radiosynthesis of [N-methyl-(11)C](S)-N-([1,1'-biphenyl]-2-yl)-1-(2-((1-methyl-1H-benzo[d]imidazol-2-yl)thio)acetyl)pyrrolidine-2-carboxamide ([(11)C]BBAC or [(11)C]3) and [N-methyl-(11)C] (S)-N-([1,1'-biphenyl]-2-yl)-1-(3-(1-methyl-1H-benzo[d]imidazol-2-yl)propanoyl)pyrrolidine-2-carboxamide ([(11)C]BBPC or [(11)C]-4), two potential PET tracers for orexin2 receptors are described. Syntheses of non-radioactive standards 3, 4 and corresponding desmethyl precursors 1, 2 were achieved from common intermediate (S)-2-([1,1'-biphenyl]-2-yl)-1-(pyrrolidin-2-yl)ethanone. Methylation using [(11)C]CH(3)OTf in the presence of base in acetone afforded [(11)C]3 and [(11)C]4 in 30±5% yield (EOS) with >99 % radiochemical purities with a specific activity ranged from 2.5±0.5 Ci/μmol (EOB). The logP of [(11)C]3 and [(11)C]4 were determined as 3.4 and 2.8, respectively. The total synthesis time was 30 min from EOB. However, PET scans performed in a rhesus monkey did not show tracer retention or appropriate brain uptake. Hence [(11)C]3 and [(11)C]4 cannot be used as PET tracers for imaging orexin2 receptors.     

Synthesis and biodistribution of new radiolabeled high-affinity choline transporter inhibitors [11C]hemicholinium-3 and [18F]hemicholinium-3

The high-affinity choline transporter (CHT1) system is an attractive target for the development of positron emission tomography (PET) biomarkers to probe brain, cardiac, and cancer diseases. An efficient and convenient synthesis of new radiolabeled CHT1 inhibitors [(11)C]hemicholinium-3 and [(18)F]hemicholinium-3 by solid-phase extraction (SPE) technique using a cation-exchange CM Sep-Pak cartridge has been well developed. The preliminary evaluation of both tracers through biodistribution studies in 9L-glioma rats has been performed, and the uptakes in the heart and tumor were observed, while very low brain uptake was seen.     

Quantitative analysis and comparison study of [18F]AlF-NOTA-PRGD2, [18F]FPPRGD2 and [68Ga]Ga-NOTA-PRGD2 using a reference tissue model

With favorable pharmacokinetics and binding affinity for α(v)β(3) integrin, (18)F-labeled dimeric cyclic RGD peptide ([(18)F]FPPRGD2) has been intensively used as a PET imaging probe for lesion detection and therapy response monitoring. A recently introduced kit formulation method, which uses an (18)F-fluoride-aluminum complex labeled RGD tracer ([(18)F]AlF-NOTA-PRGD2), provides a strategy for simplifying the labeling procedure to facilitate clinical translation. Meanwhile, an easy-to-prepare (68)Ga-labeled NOTA-PRGD2 has also been reported to have promising properties for imaging integrin α(v)β(3). The purpose of this study is to quantitatively compare the pharmacokinetic parameters of [(18)F]FPPRGD2, [(18)F]AlF-NOTA-PRGD2, and [(68)Ga]Ga-NOTA-PRGD2. U87MG tumor-bearing mice underwent 60-min dynamic PET scans following the injection of three tracers. Kinetic parameters were calculated using Logan graphical analysis with reference tissue. Parametric maps were generated using voxel-level modeling. All three compounds showed high binding potential (Bp(ND)?=?k(3)/k(4)) in tumor voxels. [(18)F]AlF-NOTA-PRGD2 showed comparable Bp(ND) value (3.75±0.65) with those of [(18)F]FPPRGD2 (3.39±0.84) and [(68)Ga]Ga-NOTA-PRGD2 (3.09±0.21) (p>0.05). Little difference was found in volume of distribution (V(T)) among these three RGD tracers in tumor, liver and muscle. Parametric maps showed similar kinetic parameters for all three tracers. We also demonstrated that the impact of non-specific binding could be eliminated in the kinetic analysis. Consequently, kinetic parameter estimation showed more comparable results among groups than static image analysis. In conclusion, [(18)F]AlF-NOTA-PRGD2 and [(68)Ga]Ga-NOTA-PRGD2 have comparable pharmacokinetics and quantitative parameters compared to those of [(18)F]FPPRGD2. Despite the apparent difference in tumor uptake (%ID/g determined from static images) and clearance pattern, the actual specific binding component extrapolated from kinetic modeling appears to be comparable for all three dimeric RGD tracers.     

N-[18F]fluoroethyl-4-piperidyl acetate ([18F]FEtP4A): A PET tracer for imaging brain acetylcholinesterase in vivo

N-[(18)F]Fluoroethyl-4-piperidyl acetate ([(18)F]FEtP4A) was synthesized and evaluated as a PET tracer for imaging brain acetylcholinesterase (AchE) in vivo. [(18)F]FEtP4A was previously prepared by reacting 4-piperidyl acetate (P4A) with 2-[(18)F]fluoroethyl bromide ([(18)F]FEtBr) at 130 degrees C for 30 min in 37% radiochemical yield using an automated synthetic system. In this work, [(18)F]FEtP4A was synthesized by reacting P4A with 2-[(18)F]fluoroethyl iodide ([(18)F]FEtI) or 2-[(18)F]fluoroethyl triflate ([(18)F]FEtOTf in improved radiochemical yields, compared with [(18)F]FEtBr under the corresponding condition. Ex vivo autoradiogram of rat brain and PET summation image of monkey brain after iv injection of [(18)F]FEtP4A displayed a high radioactivity in the striatum, a region with the highest AchE activity in the brain. Moreover, the distribution pattern of (18)F radioactivity was consistent with that of AchE in the brain: striatum>frontal cortex>cerebellum. In the rat and monkey plasma, two radioactive metabolites were detected. However, their presence might not preclude the imaging studies for AchE in the brain, because they were too hydrophilic to pass the blood-brain barrier and to enter the brain. In the rat brain, only [(18)F]fluoroethyl-4-piperidinol ([(18)F]FEtP4OH) was detected at 30 min postinjection. The hydrolytic [(18)F]FEtP4OH displayed a slow washout and a long retention in the monkey brain until the PET experiment (120 min). Although [(18)F]FEtP4A is a potential PET tracer for imaging AchE in vivo, its lower hydrolytic rate and lower specificity for AchE than those of [(11)C]MP4A may limit its usefulness for the quantitative measurement for AchE in the primate brain.     

Facile synthesis and PET imaging of a novel potential heart acetylcholinesterase tracer N-[11C]methyl-3-[[(dimethylamino)carbonyl]oxy]-2-(2',2'-diphenylpropionoxymethyl)pyridinium

A new AChE tracer N-[(11)C]methyl-3-[[(dimethylamino)carbonyl]oxy]-2-(2',2'-diphenylpropionoxymethyl)pyridinium ([(11)C]MDDP, [(11)C]1) has been synthesized in 40-65% radiochemical yield. Initial PET dynamic studies of [(11)C]MDDP in rat heart showed rapid heart uptake and blood pool clearance to give high-quality heart images. Blocking studies of [(11)C]MDDP with pretreatment drug neostigmine in rats found only minor reductions in rat heart [(11)C]MDDP retention. The results suggest that [(11)C]MDDP delineates the heart very clearly, and the uptakes of [(11)C]MDDP in rat heart might be related to non-specific binding.     

18F]fluoro-deoxy-glucose folate: a novel PET radiotracer with improved in vivo properties for folate receptor targeting

The folate receptor (FR) is upregulated in various cancer types (FR-α isoform) and in activated macrophages (FR-β isoform) which are involved in inflammatory and autoimmune diseases, but its expression in healthy tissues and organs is highly restricted to only a few sites (e.g kidneys). Therefore, the FR is a promising target for imaging and therapy of cancer and inflammation using folate-based radiopharmaceuticals. Herein, we report the synthesis and evaluation of a novel folic acid conjugate with improved properties suitable for positron emission tomography (PET). [(18)F]-fluoro-deoxy-glucose folate ([(18)F]3) was synthesized based on the click chemistry approach using 2-deoxy-2-[(18)F]fluoroglucopyranosyl azide and a folate alkyne derivative. The novel radiotracer [(18)F]3 was produced in good radiochemical yields (25% d.c.) and high specific radioactivity (90 GBq/μmol). Compared to previously published (18)F-folic acid derivatives, an increase in hydrophilicity was achieved by using a glucose entity as a prosthetic group. Biodistribution and PET imaging studies in KB tumor-bearing mice showed a high and specific uptake of the radiotracer in FR-positive tumors (10.03 ± 1.12%ID/g, 60 min p.i.) and kidneys (42.94 ± 2.04%ID/g, 60 min p.i.). FR-unspecific accumulation of radioactivity was only found in the liver (9.49 ± 1.13%ID/g, 60 min p.i.) and gallbladder (17.59 ± 7.22%ID/g, 60 min p.i.). No radiometabolites were detected in blood, urine, and liver tissue up to 30 min after injection of [(18)F]3. [(18)F]-fluoro-deoxy-glucose-folate ([(18)F]3) is thus a promising PET radioligand for imaging FR-positive tumors.     

Synthesis and application of [18F]FDG-maleimidehexyloxime ([18F]FDG-MHO): a [18F]FDG-based prosthetic group for the chemoselective 18F-labeling of peptides and proteins

2-[(18)F]Fluoro-2-deoxy-D-glucose ([(18)F]FDG) as the most important PET radiotracer is available in almost every PET center. However, there are only very few examples using [(18)F]FDG as a building block for the synthesis of (18)F-labeled compounds. The present study describes the use of [(18)F]FDG as a building block for the synthesis of (18)F-labeled peptides and proteins. [(18)F]FDG was converted into [(18)F]FDG-maleimidehexyloxime ([(18)F]FDG-MHO), a novel [(18)F]FDG-based prosthetic group for the mild and thiol group-specific (18)F labeling of peptides and proteins. The reaction was performed at 100 degrees C for 15 min in a sealed vial containing [(18)F]FDG and N-(6-aminoxy-hexyl)maleimide in 80% ethanol. [(18)F]FDG-MHO was obtained in 45-69% radiochemical yield (based upon [(18)F]FDG) after HPLC purification in a total synthesis time of 45 min. Chemoselecetive conjugation of [(18)F]FDG-MHO to thiol groups was investigated by the reaction with the tripeptide glutathione (GSH) and the single cysteine containing protein annexin A5 (anxA5). Radiolabeled annexin A5 ([(18)F]FDG-MHO-anxA5) was obtained in 43-58% radiochemical yield (based upon [(18)F]FDG-MHO, n = 6), and [(18)F]FDG-MHO-anxA5 was used for a pilot small animal PET study to assess in vivo biodistribution and kinetics in a HT-29 murine xenograft model.     

Synthesis and evaluation of 18F- and 11C-labeled phenyl-galactopyranosides as potential probes for in vivo visualization of LacZ gene expression using positron emission tomography

3-Hydroxy-2-nitrophenyl 2,3,4,6-tetra-O-acetyl-beta-D-galactopyranoside, a derivative of the chromogenic beta-galactosidase (beta-gal) substrate o-nitrophenyl beta-D-galactopyranoside (ONPG) was synthesized using a Koenigs-Knorr glycosylation reaction. It was alkylated with 2-[(18)F]fluoroethyl triflate or [(11)C]methyl triflate, followed by deacetylation of the sugar hydroxyl groups to obtain radiolabeled 3-(2'-[(18)F]fluoroethoxy)-2-nitrophenyl beta-D-galactopyranoside ([(18)F]-2c) and 3-[(11)C]methoxy-2-nitrophenyl beta- d-galactopyranoside ([(11)C]-3c), which were evaluated as potential reporter probes for in vivo visualization of LacZ gene expression with positron emission tomography (PET). In vitro, [(18)F]- 2c and [(11)C]-3c were good substrates of beta-gal and showed, respectively, a 7.5- and 2.5-fold higher uptake into beta-gal expressing cells (LacZ cells) compared to control cells. However, reversed-phase HPLC analysis of the LacZ cell lysate and supernatant showed that labeled 3-(2'-[(18)F]fluoroethoxy)-2-nitrophenol, the hydrolysis product formed by beta-gal-mediated cleavage of [(18)F]-2c, substantially leaked out of the cells, which would lead to loss of PET signal. In a microPET study of [(18)F]-2c in a mouse with a beta-gal expressing tumor, high retention was observed in liver and kidneys, but only negligible accumulation was seen in the tumor. As a general conclusion, it can be stated that the synthesized PET tracers [ (18)F]-2c and [(11)C]-3c are not suitable for use as LacZ reporter probes. Further structural modifications to improve the diffusion over the tumor cell membrane and to increase retention in beta-gal expressing cells may lead to more favorable in vivo imaging probes.     

Novel radiosynthesis of PET HSV-tk gene reporter probes [18F]FHPG and [18F]FHBG employing dual Sep-Pak SPE techniques

Positron emission tomography (PET) herpes simplex virus thymidine kinase (HSV-tk) gene reporter probes 9-[(3-[(18)F]fluoro-1-hydroxy-2-propoxy)methyl]guanine ([(18)F]FHPG) and 9-(4-[(18)F]fluoro-3-hydroxymethylbutyl)guanine ([(18)F]FHBG) were prepared by nucleophilic substitution of the appropriate tosylated precursors with [(18)F]KF/Kryptofix 2.2.2 followed by a quick deprotection reaction and purification with a simplified dual Silica Sep-Pak solid-phase extraction (SPE) method in 15-30% radiochemical yield.     

N-Succinimidyl 4-[(18)F]-fluoromethylbenzoate-labeled dimeric RGD peptide for imaging tumor integrin expression

RGD peptides, radiolabeled with (18)F, have been used in the clinic for PET imaging of tumor angiogenesis in cancer patients. RGD peptides are typically labeled using a prosthetic group such as N-succinimidyl 4-[(18)F]-fluorobenzoate ([(18)F]SFB) or 4-nitrophenyl 2-[(18)F]-fluoropropionate ([(18)F]NPFP). However, the complex radiosynthetic procedures have impeded their broad application in clinical studies. We previously radiolabeled proteins and peptides with the prosthetic group, N-succinimidyl 4-[(18)F]-fluoromethylbenzoate ([(18)F]SFMB), which was prepared in a simple one-step procedure. In this study, we labeled a PEGylated cyclic RGD peptide dimer, PEG(3)-E[c(RGDyK)](2) (PRGD2), using [(18)F]SFMB and evaluated for imaging tumor αvβ3 integrin expression with positron emission tomography (PET). [(18)F]SFMB was prepared in one step using [(18)F]fluoride displacement of a nitrobenzenesulfonate leaving group under mild reaction conditions followed by HPLC purification. The (18)F-labeled peptide, [(18)F]FMBPRGD2 was prepared by coupling PRGD2 with [(18)F]SFMB in pH 8.6 borate buffer and purified with HPLC. The direct labeling on BMBPRGD2 was also attempted. A Siemens Inveon PET was used to image the uptake of the [(18)F]FMBPRGD2 into a U87MG xenograft mouse model. [(18)F]FMBPRGD2, was prepared with a 15% overall radiochemical yield (uncorrected) in a total synthesis time of 90?min, which was considerably shorter than the preparation of [(18)F]SFB- and [(18)F]NPFP-labeled RGD peptides. The direct labeling, however, was not successful. High quality microPET images using [(18)F]FMBPRGD2 clearly visualized tumors by 15?min with good target to background ratio. Early tracer accumulation in the bladder suggests fast renal clearance. No obvious bone uptake can be detected even at 4-h time point indicating that fluorine attachment is stable in mice. In conclusion, N-succinimidyl 4-[(18)F]-fluoromethylbenzoate ([(18)F]SFMB) prosthetic group can be a good alternative for labeling RGD peptides to image αvβ3 integrin expression and for labeling other peptides.     

Synthesis and evaluation of 15-(4-(2-[¹⁸F]Fluoroethoxy)phenyl)pentadecanoic acid: a potential PET tracer for studying myocardial fatty acid metabolism

15-(4-(2-[1?F]fluoroethoxy)phenyl)pentadecanoic acid ([1?F]7) was synthesized as a PET probe for assessing myocardial fatty acid metabolism. The radiosynthesis of [1?F]7 was accomplished using a two-step reaction, starting with the corresponding tosylate ester, methyl 15-(4-(2-(tosyloxy)ethoxy)phenyl)pentadecanoate (5), and gave the radiolabeled fatty acid, [1?F]7 in a radiolabeling yield of 55-60% and a specific activity of >2000 Ci/mmol (decay corrected to EOB). The biological evaluation of [1?F]7 in rats displayed high uptake in heart (1.94%ID/g at 5 min), which was higher than the uptake (%ID/g) in blood, lung, muscle, pancreas, and brain. MicroPET studies of [1?F]7 in Sprague-Dawley rats demonstrated excellent images of the myocardium when compared with [11C]palmitate images in the same animal. Moreover, the tracer kinetics of [1?F]7 paralleled those seen with [11C]palmitate, with an early peak followed by biphasic washout. When compared to [11C]palmitate, [1?F]7 exhibited a slower early clearance (0.17 ± 0.01 vs 0.30 ± 0.02, P < 0.0001) and a significantly higher late clearance (0.0030 ± 0.0005 vs 0.0006 ± 0.00013, P < 0.01). These initial studies suggest that [1?F]7 could be a potentially useful clinical PET tracer to assess abnormal myocardial fatty acid metabolism.     

PET imaging and optical imaging with D-luciferin [11C]methyl ester and D-luciferin [11C]methyl ether of luciferase gene expression in tumor xenografts of living mice

New carbon-11 labeled D-luciferin analogs D-luciferin [(11)C]methyl ester ([(11)C]LMEster, [(11)C]1) and D-luciferin [(11)C]methyl ether ([(11)C]LMEther, [(11)C]2) were synthesized in 25-55% radiochemical yield. PET studies with [(11)C]LMEster and [(11)C]LMEther demonstrate a lower retention of the C-11 label at 45 min post-injection in luciferase expression tumor. Optical imaging with unlabeled substrate D-luciferin and radiotracers [(11)C]LMEster and [(11)C]LMEther gave tumor luciferase images within a few minutes of photon counting.     

Syntheses of F-18 labeled fluoroalkyltyrosine derivatives and their biological evaluation in rat bearing 9L tumor

We hereby report the synthesis of four fluorine-18 labeled tyrosine derivatives, 3-(2-[(18)F]fluoroethyl)tyrosine ([(18)F]1, [(18)F]ortho-FET), 3-(3-[(18)F]fluoropropyl)tyrosine ([(18)F]2, [(18)F]ortho-FPT) O-methyl-[3-(2-[(18)F]fluoroethyl)]tyrosine ([(18)F]3, [(18)F]MFET), and O-methyl-[3-(3-[(18)F]fluoropropyl)]tyrosine ([(18)F]4, [(18)F]MFPT). The fluorine-18 labeled tyrosine derivatives were prepared by the displacement reaction of the ethyl and propyl tosylates with K[(18)F]/K2.2.2 in acetonitrile under no-carrier-added (NCA) conditions, followed by hydrolysis with 4N HCl. The biological properties of labeled compounds were evaluated in rats bearing 9L tumor after an intravenous injection and PET image was obtained. The tumor/blood and tumor/brain ratios were 2.06, 2.92 for [(18)F]1, 2.25, 4.05 for [(18)F]2, 2.88, 1.90 for [(18)F]3, and 2.00, 2.60 for [(18)F]4 at 60 min post injection, respectively. The PET image showed localized accumulation of PET tracers in 9L glioma of the rat.     

Synthesis and in vivo evaluation of [O-methyl-11C] N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)benzenesulfonamide as an imaging probe for 5-HT6 receptors

The serotonin receptor 6 (5-HT(6)) is implicated in the pathophysiology of cognitive diseases, schizophrenia, anxiety and obesity and in vivo studies of this receptor would be of value for studying the pathophysiology of these disorders. Therefore, N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)benzenesulfonamide (SB399885), a selective and high affinity (pK(i)=9.11) 5-HT(6) antagonist, has been radiolabeled with carbon-11 by O-methylation of the corresponding desmethyl analogue with [(11)C]MeOTf in order to determine the suitability of [(11)C]SB399885 to quantify 5-HT(6)R in living brain using PET. Desmethyl-SB399885 was prepared, starting from 1-(2-methoxyphenyl) piperazine hydrochloride, in excellent yield. The yield obtained for radiolabeling of [(11)C]SB399885 was 30±5% (EOS) and the total synthesis time was 30min at EOB. PET studies with [(11)C]SB399885 in baboon showed fast uptake followed by rapid clearance in the brain. Highest uptake of radioactivity of [(11)C]SB399885 in baboon brain were found in temporal cortex, parahippocampal gyrus, pareital cortex, amygdala, and hippocampus. Poor brain entry and inconsistent brain uptake of [(11)C]SB399885 compared to known 5-HT(6)R distribution limits its usefulness for the in vivo quantification of 5-HT(6)R with PET.     

Comparison study of [18F]FAl-NOTA-PRGD2, [18F]FPPRGD2, and [68Ga]Ga-NOTA-PRGD2 for PET imaging of U87MG tumors in mice

[(18)F]FPPRGD2, an F-18 labeled dimeric cyclic RGDyK peptide, has favorable properties for PET imaging of angiogenesis by targeting the α(v)β(3) integrin receptor. This radiotracer has been approved by the FDA for use in clinical trials. However, the time-consuming multiple-step synthetic procedure required for its preparation may hinder the widespread usage of this tracer. The recent development of a method using an F-18 fluoride-aluminum complex to radiolabel peptides provides a strategy for simplifying the labeling procedure. On the other hand, the easy-to-prepare [(68)Ga]-labeled NOTA-RGD derivatives have also been reported to have promising properties for imaging α(v)β(3) integrin receptors. The purpose of this study was to prepare [(18)F]FPPRGD2 [corrected] , [(18)F]FAl-NOTA-PRGD2, and [(68)Ga]Ga-NOTA-PRGD2 and to compare their pharmacokinetics and tumor imaging properties using small animal PET. All three compounds showed rapid and high tracer uptake in U87MG tumors with high target-to-background ratios. The uptake in the liver, kidneys, and muscle were similar for all three tracers, and they all showed predominant renal clearance. In conclusion, [(18)F]FAl-NOTA-PRGD2 and [(68)Ga]Ga-NOTA-PRGD2 have imaging properties and pharmacokinetics comparable to those of [(18)F]FPPRGD2. Considering their ease of preparation and good imaging qualities, [(18)F]FAl-NOTA-PRGD2 and [(68)Ga]NOTA-PRGD2 are promising alternatives to [(18)F]FPPRGD2 for PET imaging of tumor α(v)β(3) integrin expression.     

18F]azadibenzocyclooctyne ([18F]ADIBO): a biocompatible radioactive labeling synthon for peptides using catalyst free [3+2] cycloaddition

N-Terminally azido-modified peptides were labeled with the novel prosthetic labeling synthon [(18)F]azadibenzocyclooctyne ([(18)F]ADIBO) using copper-free azide-alkyne [3+2]-dipolar cycloaddition in high radiochemical yields (RCYs). (18)F-Labeled [(18)F]ADIBO was prepared by nucleophilic substitution of the corresponding tosylate in 21% overall RCY (EOB) in a fully automated synthesis unit within 55 min. [(18)F]ADIBO was incubated with azide-containing peptides at room temperature in the absence of toxic metal catalysts and the formation of the triazole conjugate was confirmed. Finally, the azide-alkyne [3+2]-dipolar cycloaddition was shown to proceed with 95% radiochemical yield in ethanol within 30 min, allowing for a development of a kit-like peptide labeling approach with [(18)F]ADIBO.     

A new high-yield synthetic route to PET CB1 radioligands [11C]OMAR and its analogs

OMAR analogs reference standards and their corresponding desmethylated precursors were synthesized from substituted anilines either in 4 and 5 steps with 27-32% and 24-31% yield, or in 3 and 4 steps with 21-30% and 19-28% yield, respectively. [(11)C]OMAR and its analog radioligands were prepared from their desmethylated precursors with [(11)C]CH(3)OTf through O-[(11)C]methylation and isolated by HPLC combined with solid-phase extraction (SPE) in 50-65% radiochemical yields based on [(11)C]CO(2) and decay corrected to end of bombardment (EOB), with 370-740 GBq/μmol specific activity at EOB.     

A new facile synthetic route to [11C]GSK189254, a selective PET radioligand for imaging of CNS histamine H3 receptor

GSK189254 and its corresponding precursor GSK185071B were synthesized from 3-methoxyphenylacetic acid with 6-chloropyridine-3-carbolic acid or 6-chloronicotinamide in 8 and 7 steps with either 6% or 7% and either 14% or 16% yield, respectively. [(11)C]GSK189254 was prepared from GSK185071B with [(11)C]CH(3)OTf through N-[(11)C]methylation and isolated by HPLC combined with solid-phase extraction (SPE) in 50-60% radiochemical yield based on [(11)C]CO(2) and decay corrected to end of bombardment (EOB), with 370-740 GBq/μmol specific activity at EOB.     

11C]GSK2126458 and [18F]GSK2126458, the first radiosynthesis of new potential PET agents for imaging of PI3K and mTOR in cancers

GSK2126458 is a highly potent inhibitor of phosphoinositide 3-kinase (PI3K) and mammalian target of rapamycin (mTOR) with low picomolar to subnanomolar activity. [(11)C]GSK2126458 and [(18)F]GSK2126458, new potential PET agents for imaging of PI3K and mTOR in cancer, were first designed and synthesized in 40-50% and 20-30% decay corrected radiochemical yield, and 370-740 and 37-222GBq/μmol specific activity at end of bombardment (EOB), respectively.     

Synthesis of [18F]-labeled (6-fluorohexyl)triphenylphosphonium cation as a potential agent for myocardial imaging using positron emission tomography

Lipophilic cations such as phosphonium salts penetrate the hydrophobic barriers of the plasma and mitochondrial membranes and accumulate in mitochondria in response to the negative inner-transmembrane potentials. Thus, as newly developed noninvasive imaging agents, [(18)F]-labeled phosphonium salts may serve as molecular "voltage sensor" probes to investigate the role of mitochondria, particularly in myocardial disease. The present study reports the radiosynthesis of (6-fluorohexyl)triphenylphosphonium salt (3) as a potential agent for myocardial imaging by using positron emission tomography (PET). The reference compound of (6-[(18)F]fluorohexyl)triphenylphosphonium salt ([(18)F]3) was synthesized with 74% yield via three-step nucleophilic substitution reactions. The reference compound was radiolabeled via two-step nucleophilic substitution reactions of no-carrier-added [(18)F]fluoride with the precursor hexane-1,6-diyl bis(4-methylbenzenesulfonate) in the presence of Kryptofix 2.2.2 and K(2)CO(3). The radiolabeled compound was synthesized with 15-20% yield. The radiochemical purity was >98% by analytical HPLC, and the specific activity was >6.10-6.47 TBq/μmol. The cellular uptake assay showed preferential uptake of [(18)F]3 in cardiomyocytes. The results of biodistribution and micro-PET imaging studies of [(18)F]3 in mice and rats showed preferential accumulation in the myocardium. The results suggest that this compound would be a promising candidate for myocardial imaging.