Synthesis of [¹⁸F]-labeled (2-(2-fluoroethoxy)ethyl)triphenylphosphonium cation as a potential agent for myocardial imaging using positron emission tomography

[(18)F]-labeled molecular probe for the detection of myocardial perfusion deficit is driving particular interest due to its high clinical applicability. Thus, we synthesized (2-(2-[(18)F]fluoroethoxy)ethyl)triphenylphosphonium salt ([(18)F]3) that specifically accumulates in myocardium according to mitochondrial membrane potential. Here, we evaluated the performance of [(18)F]3 as a mitochondrial voltage sensor in vitro and in vivo. The [(18)F]3 was synthesized with 20～30% radiochemical yield and radiochemical purity was >98% by analytical HPLC. Specific activity was >6.7TBq/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.     

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.     

Synthesis, radiolabeling, and in vivo evaluation of an 18F-labeled isatin analog for imaging caspase-3 activation in apoptosis

A non-peptide-based isatin sulfonamide analog, WC-II-89, was synthesized and its inhibition toward recombinant human caspase-3 and other caspases was determined. This compound showed high potency for inhibiting caspase-3 and -7, and high selectivity against caspases-1, -6, and -8. [(18)F]WC-II-89 was synthesized via a nucleophilic substitution of the corresponding mesylate precursor in high yield and radiochemical purity. Biodistribution studies using [(18)F]WC-II-89 revealed higher uptake in liver and spleen of cycloheximide-treated rats, an animal model of apoptosis, relative to control animals. Western blot analysis confirmed the presence of activated caspase-3 in the liver and spleen of cycloheximide-treated animals. MicroPET imaging studies revealed a high uptake of the radiotracer in the liver of a cycloheximide-treated rat relative to the untreated control. These data suggest that [(18)F]WC-II-89 is a potential radiotracer for imaging caspase-3 activation in tissues undergoing apoptosis.     

Synthesis and evaluation of novel F-18 labeled 4-aminoquinazoline derivatives: potential PET imaging agents for tumor detection

Three novel (18)F-labeled 4-aminoquinazoline derivatives, N-(3-chloro-4-fluorophenyl)-6-(2-[(18)F]fluoroethoxy)-7-methoxyquinazolin-4-amine([(18)F]1), N-(3-ethynylphenyl)-6-(2-[(18)F]fluoroethoxy)-7-methoxyquinazolin-4-amine([(18)F]2), and N-(3-bromophenyl)-6-(2-[(18)F]fluoroethoxy)-7-methoxyquinazolin-4-amine([(18)F]3) were synthesized and radiolabeled by two-step reaction with overall radiochemical yield of 21-24% (without decay corrected). Then we carried out their biodistribution experiments in S180 tumor-bearing mice. Results showed that they had certain concentration accumulation in tumor and fast clearance from muscle and blood. It was encouraging that [(18)F]3 was competitive among three (18)F-labeled 4-aminoquinazoline derivatives in some aspects such as tumor/muscle uptake ratio reaching 7.70 at 60 min post-injection, tumor/blood uptake ratio reaching 6.61 at 120 min post-injection. So we compared radioactivity characteristics of [(18)F]3 with those of [(18)F]-FDG and L-[(18)F]-FET in the same animal model. The absolute radioactivity uptake of [(18)F]3 in tumor reached 3.31 at 60 min p.i., which was slightly higher than [(18)F]-FDG (2.16) and L-[(18)F]-FET (2.75) at the same time phase. For [(18)F]3, tumor/muscle uptake ratio peaked 7.70 at 60 min, which was obviously superior to those of [(18)F]-FDG and L-[(18)F]-FET at all time points. The tumor/brain uptake ratios of [(18)F]3 were 10.36, 17.42, 41.11 at 30 min, 60 min and 120 min post-injection, respectively, and are much higher than those of L-[(18)F] FET (2.54, 2.92 and 2.95) and [(18)F]-FDG (0.61, 1.02 and 1.33) at the same time points. All these results indicate that [(18)F]3 is promising to become a potential PET tumor imaging agent.     

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.     

Evaluation of [11C]hemicholinium-15 and [18F]hemicholinium-15 as new potential PET tracers for the high-affinity choline uptake system in the heart

[(11)C]Hemicholinium-15 ([(11)C]HC-15) and [(18)F]hemicholinium-15 ([(18)F]HC-15) have been synthesized as new potential PET tracers for the heart high-affinity choline uptake (HACU) system. [(11)C]HC-15 was prepared by N-[(11)C]methylation of the appropriate precursor, 4-methyl-2-phenyl-morpholin-2-ol, using [(11)C]CH(3)OTf in 55-70% radiochemical yield decay corrected to end of bombardment (EOB) and 2-3Ci/mumol specific activity at end of synthesis (EOS). [(18)F]HC-15 was prepared by N-[(18)F]fluoromethylation of the precursor using [(18)F]FCH(2)OTf in 20-30% radiochemical yield decay corrected to EOB and >1.0Ci/mumol specific activity at EOS. The biodistribution of both compounds was determined in rats at 20min post-intravenous injection, and the results show the heart region uptakes 1.32+/-0.75%ID/g in R-ventricle for [(11)C]HC-15 and 1.28+/-0.81%ID/g in L-ventricle for [(18)F]HC-15, respectively. The dynamic PET imaging studies of [(11)C]HC-15 in rats were acquired 60min post-intravenous injection of the tracer using the IndyPET-II scanner. For the blocking experiments, the rats were intravenously pretreated with 3.0mg/kg of unlabeled HC-15 prior to [(11)C]HC-15 injection. [(11)C]HC-15 rat heart PET studies show rapid heart uptake to give clear heart images. The rat heart PET blocking studies found no significant blocking effect. The dynamic PET studies in normal and ablated dogs were performed using Siemens PET scanner with [(13)N]NH(3), [(11)C]HC-15, and [(18)F]HC-15. PET studies in dogs of both [(11)C]HC-15 and [(18)F]HC-15 also show significant heart uptake and give images of the heart. However, there is no significant change in [(11)C]HC-15 L-ventricle uptake following radiofrequency ablation in the dog. These results suggest that the localization of HC-15 tracers in the heart is mediated by non-specific processes, and the visualization of HC-15 tracers on the heart is related to non-specific binding of HACU.     

Synthesis and evaluation of [18F]Fluorobutyl ethacrynic amide: a potential PET tracer for studying glutathione transferase

[(18)F]Flurobutyl ethacrynic amide ([(18)F]FBuEA) was prepared from the precursor tosylate N-Boc-N-[4-(toluenesulfonyloxy)butyl]ethacrynic amide with a radiochemical yield of 3%, a specific activity of 48 GBq/μmol and radiochemical purity of 98%. Chemical conjugation of [(18)F]FBuEA with glutathione (GSH) via a self-coupling reaction and enzymatic conjugation under catalysis of glutathiontransferase alpha (GST-α) and π provided about 41% yields of radiochemical conjugated product [(18)F]FBuEA-GSH, 85% and 5-16%, respectively. The catalytic selectivity of this tracer toward GST-alpha was addressed. Positron emission tomography (PET) imaging of [(18)F]FBuEA in normal rats showed that a homogeneous pattern of radioactivity was distributed in the liver, suggesting a catalytic role of GST. By contrast, PET images of [(18)F]FBuEA in rats with thioacetamide-induced cholangiocarcinoma displayed a heterogeneous pattern of radioactive accumulation with cold spots in tumor lesions. PET imaging with [(18)F]FBuEA could be used for early diagnosis of hepatic tumor with a low GST activity as well as liver function.     

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.     

Synthesis and radiopharmacological characterization of 2beta-carbo-2'-[18F]fluoroethoxy-3beta-(4-bromo-phenyl)tropane ([18F]MCL-322) as a PET radiotracer for imaging the dopamine transporter (DAT)

The fluoroalkyl-containing tropane derivative 2beta-carbo-2'-fluoroethoxy-3beta-(4-bromo-phenyl)tropane (MCL-322) is a highly potent and moderately selective ligand for the dopamine transporter (DAT). The compound was labeled with the short-lived positron emitter (18)F in a single step by nucleophilic displacement of the corresponding tosylate precursor MCL-323 with no-carrier-added [(18)F]fluoride. The positron emission tomography (PET) radiotracer 2beta-carbo-2'-[(18)F]fluoroethoxy-3beta-(4-bromo-phenyl)tropane [(18)F]MCL-322 was obtained in decay-corrected radiochemical yields of 30-40% at a specific radioactivity of 1.6-2.4Ci/mumol (60-90GBq/mumol) at the end-of-synthesis (EOS). Small animal PET, ex vivo and in vivo biodistribution experiments in rats demonstrated a high uptake in the striatum (3.2% ID/g) 5min after injection, which increased to 4.2% ID/g after 60min. The uptake in the cerebellum was 1.8% ID/g and 0.6% ID/g after 5min and 60min post-injection, respectively. Specific binding to DAT of [(18)F]MCL-322 was confirmed by blocking experiments using the high affinity DAT ligand GBR 12909. The radiopharmacological characterization was completed with metabolite and autoradiographic studies confirming the selective uptake of [(18)F]MCL-322 in the striatum. It is concluded that the simple single-step radiosynthesis of [(18)F]MCL-322 and the promising radiopharmacological data make [(18)F]MCL-322 an attractive candidate for the further development of a PET radiotracer potentially suitable for clinical DAT imaging in the human brain.     

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, radiofluorination and pharmacological evaluation of a fluoromethyl spirocyclic PET tracer for central σ1 receptors and comparison with fluoroalkyl homologs

The spirocyclic σ(1) receptor ligand 1 (1'-benzyl-3-(fluoromethyl)-3H-spiro[[2]benzofuran-1,4'-piperidine]) was prepared in four steps starting from methoxy derivative 5. Due to its high σ(1) affinity (K(i)=0.74nM) and selectivity against several other relevant targets, 1 was investigated as (18)F-labeled PET tracer and its biological properties were compared with those of homologous fluoroalkyl derivatives 2-4. The fluoromethyl derivative 1 was faster metabolized in vitro than homologs 2-4. In contrast to the radiosynthesis of [(18)F]2-4, the nucleophilic substitution of the tosylate 15 using the K[(18)F]F-K(222)-carbonate complex required heating to 150°C in DMSO to achieve high labeling efficiencies. Whereas radiometabolites of [(18)F]2-4 were not detected in vivo in the brain of mice, two radiometabolites of [(18)F]1 were found. Analysis of ex vivo autoradiography images provided rather low target-to-nontarget ratio for [(18)F]1 compared with [(18)F]2-4. [(18)F]1 showed a fast uptake in the brain, which decreased continuously over time. The brain-to-plasma ratio of the radiotracer [(18)F]1 was only exceeded by the fluoroethyl tracer [(18)F]2.     

A Tc-99m-labeled long chain fatty acid derivative for myocardial imaging

C-11- and I-123-labeled long chain fatty acid derivatives have been reported as useful radiopharmaceuticals for the estimation of myocardial fatty acid metabolism. We have reported that Tc-99m-labeled N-[[[(2-mercaptoethyl)amino]carbonyl]methyl]-N-(2-mercaptoethyl)-6-aminohexanoic acid ([(99m)Tc]MAMA-HA), a medium chain fatty acid derivative, is metabolized by beta-oxidation in the liver and that the MAMA ligand is useful for attaching to the omega-position of fatty acid derivatives as a chelating group for Tc-99m. On the basis of these findings, we focused on developing a Tc-99m-labeled long chain fatty acid derivative that reflected fatty acid metabolism in the myocardium. In this study, we synthesized a dodecanoic acid derivative, MAMA-DA, and a hexadecanoic acid derivative, MAMA-HDA, and performed radiolabeling and biodistribution studies. [(99m)Tc]MAMA-DA and [(99m)Tc]MAMA-HDA were prepared using a ligand-exchange reaction. Biodistribution studies were carried out in normal mice and rats. Then, a high initial uptake of Tc-99m was observed, followed by a rapid clearance from the heart. The maximum heart/blood ratio was 3.6 at 2 min postinjection of [(99m)Tc]MAMA-HDA. These kinetics were similar to those with postinjection of p-[(125)I]iodophenylpentadecanoic acid. Metabolite analysis showed [(99m)Tc]MAMA-HDA was metabolized by beta-oxidation in the body. In conclusion, [(99m)Tc]MAMA-HDA is a promising compound as a long chain fatty acid analogue for estimating beta-oxidation of fatty acid in the heart.     

18F]FHPG positron emission tomography for detection of herpes simplex virus (HSV) in experimental HSV encephalitis

Herpes simplex virus type 1 (HSV-1) is one of the most common causes of sporadic encephalitis. The initial clinical course of HSV encephalitis (HSE) is highly variable, and the infection may be rapidly fatal. For effective treatment with antiviral medication, an early diagnosis of HSE is crucial. Subtle brain infections with HSV may be causally related to neuropsychiatric disorders such as Alzheimer's dementia. We investigated the feasibility of a noninvasive positron emission tomography (PET) imaging technique using [(18)F]FHPG as a tracer for the detection of HSE. For this purpose, rats received HSV-1 (infected group) or phosphate-buffered saline (control group) by intranasal application, and dynamic PET scans were acquired. In addition, the distribution of tracer accumulation in specific brain areas was studied with phosphor storage imaging. The PET images revealed that the overall brain uptake of [(18)F]FHPG was significantly higher for the infected group than for control animals. Phosphor storage images showed an enhanced accumulation of [(18)F]FHPG in regions known to be affected after intranasal infection with HSV. High-performance liquid chromatography metabolite analysis showed phosphorylated metabolites of [(18)F]FHPG in infected brains, proving that the increased [(18)F]FHPG uptake in infected brains was due to HSV thymidine kinase-mediated trapping. Freeze lesion experiments showed that damage to the blood-brain barrier could in principle induce elevated [(18)F]FHPG uptake, but this nonspecific tracer uptake could easily be discriminated from HSE-derived uptake by differences in the tracer kinetics. Our results show that [(18)F]FHPG PET is a promising tool for the detection of HSV encephalitis.     

Radiosynthesis and biodistribution of cyclic RGD peptides conjugated with novel [18F]fluorinated aldehyde-containing prosthetic groups

Achieving high-yielding, robust, and reproducible chemistry is a prerequisite for the (18)F-labeling of peptides for quantitative receptor imaging using positron emission tomography (PET). In this study, we extend the toolbox of oxime chemistry to include the novel prosthetic groups [(18)F]-(2-{2-[2-(2-fluoroethoxy)ethoxy]ethoxy}ethoxy)acetaldehyde, [(18)F]5, and [(18)F]-4-(3-fluoropropoxy)benzaldehyde, [(18)F]9, in addition to the widely used 4-[(18)F]fluorobenzaldehyde, [(18)F]12. The three (18)F-aldehydes were conjugated to the same aminooxy-bearing RGD peptide and the effect of the prosthetic group on biodistribution and tumor uptake studied in mice. The peptide conjugate [(18)F]7 was found to possess superior in vivo pharmacokinetics with higher tumor to blood, tumor to liver, tumor to muscle, and tumor to lung ratios than either [(18)F]10 or [(18)F]13. The radioactivity from the [(18)F]7 conjugate excreted more extensively through the kidney route with 79%id passing through the urine and bladder at the 2 h time point compared to around 55%id for the more hydrophobic conjugates [(18)F]10 and [(18)F]13. The chemical nature of a prosthetic group can be employed to tailor the overall biodistribution profile of the radiotracer. In this example, the hydrophilic nature of the ethylene glycol containing prosthetic group [(18)F]5 clearly influences the overall excretion pattern for the RGD peptide conjugate.     

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.     

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.     

Radiosynthesis and in vivo tumor uptake of 2-deoxy-2-[(18)F]fluoro-myo-inositol

Inositols play an important role in membrane lipid metabolism and mitogenic signaling of most cancer cells. There is paucity of data on the distribution of radiolabelled inositols. Based on work previously carried out on 1-deoxy-1-[(18)F]fluoro-scyllo-inositol ([(18)F]2), we began a program of work to label myo-inositol (2-deoxy-2-[(18)F]fluoro-myo-inositol, [(18)F]1), the most abundant inositol in cells. Fluorination of a triflate precursor 4 afforded the desired [(18)F]1 following deprotection with a radiochemical yield of 8% n.d.c. [(18)F]1 showed higher uptake in vivo in a human breast cancer xenograft model, MDA-MB-231, compared to [(18)F]2. Thus, we have developed a new inositol radiotracer that could have utility for studying inositol uptake in tumors.     

Syntheses of 18F-labeled pyrimidines and their usefulness for tumor imaging

18F-Labeled 5-fluorouracil(FUra), 5-fluoro-2'-deoxyuridine(FdUrd) and 5-fluorouridine(FUrd) were synthesized with high radiochemical purities. Biodistribution of the 18F-pyrimidines in tumor-bearing rats, mice or a rabbit was examined. Blood clearance of the 18F-pyrimidines was very rapid. The kidney and liver showed the high uptake and rapid clearance which was due to the metabolism and excretion. The 18F-concentration in the tumor was due to the metabolism and excretion. The 18F-concentration in the tumor was also high and clearance was very slow compared with those in other organs. Tumor uptakes of 18F-FdUrd were also shown by positron emission tomography and autoradiography. Biodistributions of the 18F-FdUrd and radio-deoxythymidine(dThd) were different between several organs, but similar distribution patterns in the tumor were observed by autoradiography.     

2-Deoxy-2-[18F]fluoro-d-galactose as an In Vivo tracer for imaging galactose metabolism in tumors with positron emission tomography

The feasibility of 2-deoxy-2-[¹⁸F]fluoro-D-galactose ([¹⁸F]FdGal) for imaging galactose metabolism in tumors with positron emission tomography (PET), was investigated using two hepatomas, Yoshida sarcoma, or glioma in rats, and mouse mammary carcinoma. In hepatoma-bearing rats the highest uptake of [¹⁸F]FdGal was observed in the liver followed by the kidney and tumor. The tumor uptake increased with time, and the high uptake ratios of tumor to organ were observed except for the liver and kidney. Tumor uptake was also measured in all tumors. As main metabolites in all tumors, [¹⁸F]FdGal 1-phosphate and UDP-[¹⁸F]FdGal were found by HPLC. Two hepatomas showed a slightly higher uptake and a larger percentage of UDP derivative than the other three tumors. By autoradiography the brain tumor was visualized clearly. These results indicate that [¹⁸F]FdGal has potential as a tracer for imaging galactose metabolism in tumors with PET.     

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.