Synthesis and biodistribution of a new radiotracer for in vivo labeling of serotonin uptake sites by PET,cis-N,N-[11C]dimethyl-3-(2′,4′-dichlorophenyl)-indanamine (cis-[11C]DDPI)

A new PET radiotracer for in vivo labeling of serotonin (5-HT) uptake sites, cis-N, N-[¹¹C]dimethyl-3-(2′,4′-dichlorophenyl)-indanamine, cis-[¹¹C]DDPI, was synthesized and its biological behavior was studied. The radiosynthesis of cis-[¹¹C]DDPI was performed by N-methylation of cis-N-methyl-3-(2′,4′-dichlorophenyl)-indanamine with [¹¹C]iodomethane. The average radiochemical yield was approx. 8%, with an average specific activity of 600mCi/μmol. Following intravenous administration, cis-[¹¹C]DDPI accumulated in mouse brain regions rich in 5-HT uptake sites, such as olfactory tubercles, hypothalamus and frontal cortex. Following pre-injection of 1 mg/kg of paroxetine, a high affinity 5-HT uptake blocker, the binding of cis-[¹¹C]DDPI in the olfactory tubercles, hypothalamus and frontal cortex was decreased by 23, 25 and 16%; this corresponds to 73, 82 and 59% of the specific binding in these regions. These results suggest that the accumulation of cis-[¹¹C]DDPI in the tissues rich in 5-HT sites is a result of specific binding of cis-[¹¹C]DDPI to 5-HT uptake sites. Due to the relatively high non-specific uptake and slow clearance of this compound from non-specific binding sites, the ratio between specific and non-specific binding increased slowly with time, reaching 1.5:1 at 60 min after injection.     

Synthesis and preliminary biological evaluation of a novel P2X7R radioligand [18F]IUR-1601

The reference standard IUR-1601 ((S)-N-(2-chloro-3-(trifluoromethyl)benzyl)-1-(2-fluoroethyl)-5-oxopyrrolidine-2-carboxamide) was synthesized from tert-butyl (S)-5-oxopyrrolidine-2-carboxylate, fluoroethylbromide, and 2-chloro-3-(trifluoromethyl)benzylamine with overall chemical yield 12% in three steps. The target tracer [¹⁸F]IUR-1601 ((S)-N-(2-chloro-3-(trifluoromethyl)benzyl)-1-(2-[¹⁸F]fluoroethyl)-5-oxopyrrolidine-2-carboxamide) was synthesized from desmethyl-GSK1482160 with 2-[¹⁸F]fluoroethyl tosylate, prepared from 1,2-ethylene glycol-bis-tosylate and K[¹⁸F]F/Kryptofix2.2.2, in two steps and isolated by HPLC combined with SPE in 1–3% decay corrected radiochemical yield. The radiochemical purity was >99%, and the molar activity at end of bombardment (EOB) was 74–370?GBq/μmol. The potency of IUR-1601 in comparison with GSK1482160 was determined by a radioligand competitive binding assay using [¹¹C]GSK1482160, and the binding affinity K_i values for IUR-1601 and GSK1482160 are 4.31 and 5.14?nM, respectively.     

Synthesis of a fluorinated fatty acid,dl-erythro-9,10-[18F]difluoropalmitic acid,and biodistribution studies in rats

9,10-Difluoropalmitic acid (DFPA) labeled with the cyclotron produced, positron emitting radionuclide ¹⁸F has been synthesized as a potential analogue of 9,10-[³H]palmitic acid, a fatty acid which has been used to study lipid metabolism in rat brain and pituitary. [¹⁸F]DFPA was prepared by the direct and stereoselective addition of [¹⁸F]F₂ to the double bond of cis-9,10-palmitoleic acid. The fluorination was carried out in FCCl₃ at −70 °C using a low concentration of F₂ (0.5%) in neon. [¹⁸F]DFPA has been obtained in radiochemical yields of 12–16% from end-of-bombardment (EOB) in approx. 2.5 h. Chemical and radiochemical purity exceeded 95%, and specific activities calculated to EOB ranged from 500 to 1000 mCi/mmol. [¹⁸F]DFPA crosses the blood-brain barrier and is incorporated into rat brain at about twice the level of that of 9,10-[³H]palmitic acid. The synthesis of [¹⁸F]DFPA permits us to study the biological disposition and metabolism of a vicinal-difluoro fatty acid.     

[18F]DAA1106: Automated radiosynthesis using spirocyclic iodonium ylide and preclinical evaluation for positron emission tomography imaging of translocator protein (18 kDa)

DAA1106 (N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide), is a potent and selective ligand for the translocator protein (18?kDa, TSPO) in brain mitochondrial fractions of rats and monkey (K_i?=?0.043 and 0.188?nM, respectively). In this study, to translate [¹⁸F]DAA1106 for clinical studies, we performed automated syntheses of [¹⁸F]DAA1106 using the spirocyclic iodonium ylide (1) as a radiolabelling precursor and conducted preclinical studies including positron emission tomography (PET) imaging of TSPO in ischemic rat brains. Radiofluorination of the ylide precursor 1 with [¹⁸F]F^?, followed by HPLC separation and formulation, produced the [¹⁸F]DAA1106 solution for injection in 6% average (n?=?10) radiochemical yield (based on [¹⁸F]F^?) with >98% radiochemical purity and molar activity of 60–100?GBq/μmol at the end of synthesis. The synthesis time was 87?min from the end of bombardment. The automated synthesis achieved [¹⁸F]DAA1106 with sufficient radioactivity available for preclinical and clinical use. Biodistribution study of [¹⁸F]DAA1106 showed a low uptake of radioactivity in the mouse bones. Metabolite analysis showed that >96% of total radioactivity in the mouse brain at 60?min after the radiotracer injection was unmetabolized [¹⁸F]DAA1106. PET study of ischemic rat brains visualized ischemic areas with a high uptake ratio (1.9?±?0.3) compared with the contralateral side. We have provided evidence that [¹⁸F]DAA1106 could be routinely produced for clinical studies.     

Labeling proteins with fluorine-18 using N-succinimidyl 4-[18F]fluorobenzoate

Two methods were investigated for the no-carrier-added synthesis of N-succinimidyl 4-[¹⁸F]fluorobenzoate (S[¹⁸F]FB). The first, an attempted nucleophilic aromatic substitution by [¹⁸F]fluoride on N-succinimidyl 4-nitrobenzoate was unsuccessful. The second method involved three steps; [¹⁸F]fluoride for trimethylammonium substitution on 4-formyl-N,N,N-trimethylanilinium triflate, oxidation to 4-[¹⁸F]fluorobenzoic acid, followed by reaction with N-hydroxysuccinimide and dicyclohexylcarbodiimide to form S[¹⁸F]FB. Total synthesis and purification time was 100 min and the overall radiochemical yield was 25% (decay corrected). A monoclonal antibody F(ab′)₂ fragment could be labeled in 40–60% yield by reaction with S[¹⁸F]FB for 15–20 min. The tissue distribution in normal mice and in vitro tumor binding of the antibody F(ab′)₂ labeled by reaction with S[¹⁸F]FB were comparable to those observed for the fragment after radioiodination using N-succinimidyl 4-[¹²⁵I]iodobenzoate.     

Imaging prostate derived tumors with PET and N-(3-[18F]fluoropropyl)putrescine

Because of the high uptake of polyamines by the prostate and by prostate derived tumors, polyamines have been considered as potential imaging agents for metastatic prostate cancer. We now report the successful PET imaging of the Dunning R3327H prostatic carcinoma with N-(3-[¹⁸F]fluoropropyl)-putrescine (FPP), a positron-labeled putrescine analog. Additionally, the biodistribution of FPP in tumor bearing Copenhagen male rats is analyzed. The tumor uptake of FPP was high, and the tumor-to-muscle ratios at 1,2,3 and 4.5 h post-injection were 7.2 ± 1.0, 5.61 ± 1.65, 4.62 ± 0.21 and 3.51 ± 0.91 respectively. The estimated radiation dose for FPP was calculated from rat biodistribution data. The radiation dose estimates suggest that the critical organ, following the administration of FPP, is the upper large intenstine which receives 0.3 rad/mCi administered.     

No-carrier-added (NCA) (±)-N-(3-[18F]fluoropropyl)-N-normetazocine-Synthesis and PET studies in a baboon

No-carrier-added (NCA) (±)-N-(3-[¹⁸F]Fluoropropyl)-N-normetazocine (2) was synthesized by N-alkylation of (±)-N-normetazocine (1) with NCA 1-[¹⁸F]fluoro-3-iodopropane in an overall radiochemical yield of 10% (EOB) with a mass of 3.5 nmol in a synthesis time of 90 min from end of bombardment (EOB). PET studies of 2 in a baboon did not indicate specificity for opiate receptor sites alone: The activity declined rapidly in the striatum the frontal cortex and the cerebellum. The baboon total arterial plasma radioactivity clearance was very rapid and the metabolism of compound 2 in plasma was also very rapid. These results suggest that compound 2 is not a suitable radioligand for imaging opiate receptors in the human brain by positron tomography.     

Synthesis and biological evaluation of anti-1-amino-2-[18F]fluoro-cyclobutyl-1-carboxylic acid (anti-2-[18F]FACBC) in rat 9L gliosarcoma

A new [¹⁸F] labeled amino acid anti-1-amino-2-[¹⁸F]fluoro-cyclobutyl-1-carboxylic acid 9 (anti-2-[¹⁸F]FACBC) was synthesized in 30% decay-corrected yield with high radiochemical purity over 99%. The cyclic sulfamidate precursor was very stable and highly reactive towards nucleophilic radiofluorination. Cell uptake assays with rat 9L gliosarcoma cells showed that [¹⁸F]9 was transported into tumor cells via multiple amino acid transport systems, including L and A systems. Biodistribution study in rats with intracranial 9L gliosarcoma tumors demonstrated that [¹⁸F]9 had a rapid and prolonged accumulation in tumors with 26:1 tumor to brain ratio at 120 min post-injection. In this model, [¹⁸F]9 is a potential PET tracer for brain tumor imaging.     

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-[¹⁸F]fluoro-3-pyridyl)-5-methyl-1H-1,2,3-triazol-4-yl]quinoline ([¹⁸F]FPTQ, [¹⁸F]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 [¹⁸F]7a was synthesized by [¹⁸F]fluorination of 6-[1-(2-bromo-3-pyridyl)-5-methyl-1H-1,2,3-triazol-4-yl]quinoline (7b) with potassium [¹⁸F]fluoride. At the end of synthesis, 1280-1830 MBq (n = 8) of [¹⁸F]7a was obtained with >98% radiochemical purity and 118-237 GBq/??mol specific activity using 3300-4000 MBq of [¹⁸F]F⁻. In vitro autoradiography showed that [¹⁸F]7a had high specific binding with mGluR1 in the rat brain. Biodistribution study using a dissection method and small-animal PET showed that [¹⁸F]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 [¹⁸F]7a had in vivo specific binding with mGluR1. Because of a low amount of radiolabeled metabolite present in the brain, [¹⁸F]7a may have a limiting potential for the in vivo imaging of mGluR1 by PET.     

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

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

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

Synthesis and biological evaluation of novel F-18 labeled pyrazolo[1,5-a]pyrimidine derivatives: potential PET imaging agents for tumor detection

Two novel pyrazolo[1,5-a]pyrimidine derivatives, 7-(2-[¹⁸F]fluoroethylamino)-5-methylpyrazolo[1,5-a]pyrimidine-3-carbonitrile ([¹⁸F]FEMPPC, [¹⁸F]1) and N-(2-(3-cyano-5-methylpyrazolo[1,5-a]pyrimidin-7-ylamino)ethyl)-2-[¹⁸F]fluoro-4-nitrobenzamide ([¹⁸F]FCMPPN, [¹⁸F]2), have been designed and successively labeled with ¹⁸F by the nucleophilic substitution employing tosylate and nitryl as leaving groups, respectively. The radiochemical synthesis of both compounds was completed within 60 min with final high-performance liquid chromatography purification included. The corresponding radiochemical yields (without decay correction) were approximately 35% and 30%, respectively. Meanwhile, we compared the uptake characteristics of [¹⁸F]1 and [¹⁸F]2 with those of [¹⁸F]FDG and L-[¹⁸F]FET in S180 tumor cells. Furthermore, the tumor uptake of [¹⁸F]1 and [¹⁸F]2 was assessed in mice bearing S180 tumor and compared with [¹⁸F]FDG and L-[¹⁸F]FET in the same animal model. In vitro cell uptake studies showed [¹⁸F]1 had higher uptake than [¹⁸F]FDG, [¹⁸F]2 and L-[¹⁸F]FET over the 2 h period. In ex vivo biodistribution showed tumor/brain uptake ratios of [¹⁸F]2 were 12.35, 10.44, 8.69 and 5.13 at 15 min, 30 min, 60 min and 120 min post-injection, much higher than those of L-[¹⁸F]FET (2.43, 2.54, 2.93 and 2.95) and [¹⁸F]FDG (0.59, 0.61, 1.02 and 1.33) at the same time point. What’s more, the uptake of [¹⁸F]1 in tumor was 1.88, 4.37, 5.51, 2.95 and 2.88 at 5 min, 15 min, 30 min, 60 min and 120 min post-injection, respectively. There was a remarkable increasing trend before 30 min. The same trend was present for L-[¹⁸F]FET before 30 min and [¹⁸F]FDG before 60 min. Additionally, the tumor/brain uptake ratios of [¹⁸F]1 were superior to those of [¹⁸F]FDG at all the selected time points, the tumor/muscle and tumor/blood uptake ratios of [¹⁸F]1 at 30 min were higher than those of L-[¹⁸F]FET at the same time point. MicroPET image of [¹⁸F]1 administered into S180 tumor-bearing mouse acquired at 30 min post-injection illustrated that the uptake in S180 tumor was obvious. These results suggest that compound [¹⁸F]1 could be a new probe for PET tumor imaging.     

Imatinib analogs as potential agents for PET imaging of Bcr-Abl and c-KIT expression at a kinase level

We synthesized two series of imatinib mesylate (STI-571) analogs to develop a Bcr-Abl and c-KIT receptor-specific labeling agent for positron emission tomography (PET) imaging to measure Bcr-Abl and c-KIT expression levels in a mouse model. The methods of molecular modeling, synthesis of STI-571 and its analogs, in vitro kinase assays, and radiolabeling are described. Molecular modeling revealed that these analogs bind the same Bcr-Abl and c-KIT binding sites as those bound by STI-571. The analogs potently inhibit the tyrosine kinase activity of Bcr-Abl and c-KIT, similarly to STI-571. [¹⁸F]-labeled STI-571 was prepared with high specific activity (75 GBq/μmol) by nucleophilic displacement and an average radiochemical yield of 12%. [¹³¹I]-labeled STI-571 was prepared with high purity (>95%) and an average radiochemical yield of 23%. The uptake rates of [¹⁸F]-STI-571 in K562 cells expressing Abl and in U87WT cells overexpressing c-KIT were significantly higher than those in the U87 cell and could be inhibited by STI-71 (confirming the specificity of uptake). PET scans of K562 and U87WT tumor-bearing mice with [¹⁸F]-STI-571 as a contrast agent showed visible tumor uptake and tumor-to-non-target contrast.     

Synthesis,tissue distribution in rats and PET studies in baboon brain of no-carrier-added [18F]RU 52461: in vivo evaluation as a brain glucocorticoid receptor radioligand

11,17β-Dihydroxy-6-methyl-17α -(3-[¹⁸F]fluoro-prop-1 -ynyl)androsta-1,4,6-trien-3-one ([¹⁸F]RU 52461), an ¹⁸F-analog of RU 28362, was synthesized by bromide displacement with [¹⁸F]fluoride in 12–30% overall radiochemical yield (decay-corrected) within 140 min from end of bombardment (EOB). The specific activity was 900–1500 mCi/μmol (33.3–55.5 GBq/μmol) at the end of synthesis (EOS). Biodistribution studies indicated high adrenal and pituitary retention, and uniformly low uptake of [¹⁸F]RU 52461 in all other brain regions of the rat. Except for the pituitary, no specific receptor-mediated uptake of [¹⁸F]RU 52461 could be demonstrated using saturating doses of unlabeled RU 52461 in rat brain. While no change was observed throughout the brain areas in adrenalectomized rats and in animals coinjected with dexamethasone, when compared to controls. PET studies revealed extremely low levels of radioactivity in baboon brain. Therefore, [¹⁸F]RU 52461 does not appear to cross the blood-brain barrier, suggesting that this radiopharmaceutical is not suitable to visualize the brain glucocorticoid binding sites by PET.     

Synthesis of [¹¹C]PBR06 and [¹⁸F]PBR06 as agents for positron emission tomographic (PET) imaging of the translocator protein (TSPO)

The translocator protein 18 kDa (TSPO) is an attractive target for molecular imaging of neuroinflammation and tumor progression. [¹⁸F]PBR06, a fluorine-18 labeled form of PBR06, is a promising PET TSPO radioligand originally developed at NIMH. [¹¹C]PBR06, a carbon-11 labeled form of PBR06, was designed and synthesized for the first time. The standard PBR06 was synthesized from 2,5-dimethoxybenzaldehyde in three steps with 71% overall chemical yield. The radiolabeling precursor desmethyl-PBR06 was synthesized from 2-hydroxy-5-methoxybenzaldehyde in five steps with 12% overall chemical yield. The target tracer [¹¹C]PBR06 was prepared by O-[¹¹C]methylation of desmethyl-PBR06 with [¹¹C]CH₃OTf in CH₃CN at 80 °C under basic condition and isolated by HPLC combined with SPE purification with 40–60% decay corrected radiochemical yield and 222–740 GBq/μmol specific activity at EOB. On the similar grounds, [¹⁸F]PBR06 was also designed and synthesized. The previously described Br-PBR06 precursor was synthesized from 2,5-dimethoxybenzaldehyde in two steps with 78% overall chemical yield. A new radiolabeling precursor tosyloxy-PBR06, previously undescribed tosylate congener of PBR06, was designed and synthesized from ethyl 2-hydroxyacetate, 4-methylbenzene-1-sulfonyl chloride, and N-(2,5-dimethoxybenzyl)-2-phenoxyaniline in four steps with 50% overall chemical yield. [¹⁸F]PBR06 was prepared by the nucleophilic substitution of either new tosyloxy-PBR06 precursor or known Br-PBR06 precursor in DMSO at 140 °C with K[¹⁸F]F/Kryptofix 2.2.2 for 15 min and HPLC combined with SPE purification in 20–60% decay corrected radiochemical yield, >99% radiochemical purity, 87–95% chemical purity, and 37–222 GBq/μmol specific activity at EOB. Radiosynthesis of [¹⁸F]PBR06 using new tosylated precursor gave similar radiochemical purity, and higher specific activity, radiochemical yield and chemical purity in comparison with radiosynthesis using bromine precursor.     

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

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

Facile synthesis of SSR180575 and discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide,a potent pyridazinoindole ligand for PET imaging of TSPO in cancer

A novel synthesis of the translocator protein (TSPO) ligand 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR180575, 3) was achieved in four steps from commercially available starting materials. Focused structure–activity relationship development about the pyridazinoindole ring at the N3 position led to the discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (14), a novel ligand of comparable affinity. Radiolabeling with fluorine-18 (¹⁸F) yielded 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[¹⁸F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide ([¹⁸F]-14) in high radiochemical yield and specific activity. In vivo studies of [¹⁸F]-14 revealed this agent as a promising probe for molecular imaging of glioma.     

Uptake of [18F] EF5 as a Tracer for Hypoxic and Aggressive Phenotype in Experimental Head and Neck Squamous Cell Carcinoma

PURPOSE: This study aims to investigate whether the uptake of 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide ([¹⁸F]EF5) and 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) is associated with a hypoxia-driven adverse phenotype in head and neck squamous cell carcinoma cell lines and tumor xenografts. METHODS: Xenografts were imaged in vivo, and tumor sections were stained for hypoxia-inducible factor 1α (Hif-1α), carbonic anhydrase IX (CA IX), and glucose transporter 1 (Glut-1). Tracer uptakes and the expression of Hif-1α were determined in cell lines under 1% hypoxia. RESULTS: High [¹⁸F]EF5 uptake was seen in xenografts expressing high levels of CA IX, Glut-1, and Hif-1α, whereas low [¹⁸F]EF5 uptake was detected in xenografts expressing low amounts of CA IX and Hif-1α. The uptake of [¹⁸F]EF5 between cell lines varied extensively under normoxic conditions. A clear correlation was found between the expression of Hif-1α and the uptake of [¹⁸F]FDG during hypoxia. CONCLUSIONS: The UT-SCC cell lines studied differed with respect to their hypoxic phenotypes, and these variations were detectable with [¹⁸F]EF5. Acute hypoxia increases [¹⁸F]FDG uptake in vitro, whereas a high [¹⁸F]EF5 uptake reflects a more complex phenotype associated with hypoxia and an aggressive growth pattern.     

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

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

Tumor uptake study of 18F-labeled N-acetylneuraminic acids

In order to develop positron-emitting tracers for imaging metabolic functions of tumors with positron emission tomography, tumor uptake of N-acetyl-3-[¹⁸F]fluoroneuraminic acid and N-acetyl-2-deoxy-2,3-di-[¹⁸F fluoroneuraminic acid was investigated in mice or rats. The two tracers showed similar tissue distribution patterns. After i.v. injection of each tracer into mice with an FM3A tumor, the radioactivity was very rapidly cleared from normal and tumor tissues. Only tumor-to-brain and tumor-to-muscle uptake ratios were greater than 1.0 for 2 h. In 7 types of tumor models, no selective tumor uptake of tracers was observed 30 min after injection. The metabolic alteration rate of N-acetyl-3-[¹⁸F]fluoroneuraminic acid in FM3A, liver and kidney was very slow. Neither tracer may be suitable for tumor imaging in vivo.