Preclinical assessment of 68Ga‐PSMA‐617 entrapped in a microemulsion delivery system for applications in prostate cancer PET/CT imaging

It has in recent years been reported that microemulsion (ME) delivery systems provide an opportunity to improve the efficacy of a therapeutic agent whilst minimising side effects and also offer the advantage of favourable treatment regimens. The prostate‐specific membrane antigen (PSMA) targeting agents PSMA‐11 and PSMA‐617, which accumulate in prostate tumours, allow for [⁶⁸Ga]Ga³⁺‐radiolabelling and positron emission tomography/computed tomography (PET) imaging of PSMA expression in vivo. We herein report the formulation of [⁶⁸Ga]Ga‐PSMA‐617 into a ME ≤40 nm including its evaluation for improved cellular toxicity and in vivo biodistribution. The [⁶⁸Ga]Ga‐PSMA‐617‐ME was tested in vitro for its cytotoxicity to HEK293 and PC3 cells. [⁶⁸Ga]Ga‐PSMA‐617‐ME was administered intravenously in BALB/c mice followed by microPET/computed tomography (CT) imaging and ex vivo biodistribution determination. [⁶⁸Ga]Ga‐PSMA‐617‐ME indicated negligible cellular toxicity at different concentrations. A statistically higher tolerance towards the [⁶⁸Ga]Ga‐PSMA‐617‐ME occurred at 0.125 mg/mL by HEK293 cells compared with PC3 cells. The biodistribution in wild‐type BALB/C mice showed the highest amounts of radioactivity (%ID/g) presented in the kidneys (31%) followed by the small intestine (10%) and stomach (9%); the lowest uptake was seen in the brain (0.5%). The incorporation of [⁶⁸Ga]Ga‐PSMA‐617 into ME was successfully demonstrated and resulted in a stable nontoxic formulation as evaluated by in vitro and in vivo means.     

Automated production of [68Ga]Ga‐DOTANOC and [68Ga]Ga‐PSMA‐11 using a TRACERlab FXFN synthesis module

The interest in gallium‐68 labelled positron‐emission tomography probes continues to increase around the world. However, one of the barriers for routine clinical use is the cost of the automated synthesis units for relatively simple labelling procedures. Herein, we describe the adaptation of a TRACERlab FX_FN synthesis module for the automated production of gallium‐68 radiopharmaceuticals using a cation‐exchange cartridge for postprocessing of the ⁶⁸Ge/⁶⁸Ga generator eluate. The recovery of activity from the cartridge was 95.6% to 98.9% using solutions of acidified sodium chloride (5 M with pH = 1‐3). The radiosyntheses of [⁶⁸Ga]Ga‐DOTANOC and [⁶⁸Ga]Ga‐PSMA‐11 were performed using acetate sodium buffer or 4‐(2‐hydroxyethyl)piperazine‐1‐ethanesulfonic acid, with a total duration of 21 and 23 minutes, respectively, including generator elution and radiopharmaceutical dispensing. Activity yields were 77% ± 2% for [⁶⁸Ga]Ga‐PSMA‐11 and 68% ± 3% for [⁶⁸Ga]Ga‐DOTANOC (n > 100). The labelled peptides had a radiochemical purity exceeding 97%, and all quality control parameters were in conformity with the limits prescribed by the European Pharmacopoeia.     

Synthesis of 18F‐labeled cyclooxygenase‐2 (COX‐2) inhibitor as a potential PET imaging agent

A new PET tracer for COX‐2 imaging, the 6‐ethoxy‐3‐(4‐methanesulfonylphenyl)‐4‐(4‐[¹⁸F]fluorophenyl)pyran‐2‐one ([¹⁸F]EFMP), was synthesized. For F‐18 radiolabeling, a trimethylammonium precursor and a brominated precursor were synthesized from 1,1,2,3‐tetrachlorocycloprop‐2‐ene in 6 steps. The radiolabeling was achieved through nucleophilic substitution using no‐carrier‐added (n.c.a.) fluorine‐18. Solid‐phase extraction and semi‐preparative‐HPLC purification produced [¹⁸F]EFMP in 14.6±3.3% (n =4) decay corrected radiochemical yield with a specific activity of 487±85.1 (n =4) Ci/mmol and greater than 98% radiochemical purity. Copyright © 2006 John Wiley & Sons, Ltd.     

One pot radiofluorination of a new potential PSMA ligand [Al18F]NOTA‐DUPA‐Pep

Up to now, many radiolabeled ligands targeting prostate‐specific membrane antigen (PSMA) have been synthesized. To carry out radiofluorinations, there have been several approaches mainly involving two steps of radiosynthesis. However, in case of labelings with high activities, one radiosynthetic step (‘one pot synthesis’) is highly desirable. As an alternative, radiofluorinations of peptides coupled to the NOTA complex via Al¹⁸F appear to be very promising, both in terms of feasible labelling procedure and stability in vivo. Therefore, in the present study, we synthesized a new PSMA‐ligand, that is, NOTA‐DUPA‐Pep, labeled with Al¹⁸F and examined the reaction kinetics in dependence on temperature, time, concentration of precursor, and AlCl₃ solution. Highest radiochemical yields of 83 ± 1.1% were obtained at 105 °C after 15 min of reaction time. At the end of synthesis, [Al¹⁸F]NOTA‐DUPA‐Pep was prepared with a radiochemical purity of ≥98% with an overall yield of 79 ± 0.7% (n = 3) (decay uncorrected) within 55 min. Copyright © 2012 John Wiley & Sons, Ltd.     

Simplified and robust one‐step radiosynthesis of [18F]DCFPyL via direct radiofluorination and cartridge‐based purification

[¹⁸F]DCFPyL is a clinical‐stage PET radiotracer used to image prostate cancer. This report details the efficient production of [¹⁸F]DCFPyL using single‐step direct radiofluorination, without the use of carboxylic acid‐protecting groups. Radiolabeling reaction optimization studies revealed an inverse correlation between the amount of precursor used and the radiochemical yield. This simplified approach enabled automated preparation of [¹⁸F]DCFPyL within 28 minutes using HPLC purification (26% ± 6%, at EOS, n = 4), which was then scaled up for large‐batch production to generate 1.46 ± 0.23 Ci of [¹⁸F]DCFPyL at EOS (n = 7) in high molar activity (37 933 ± 4158 mCi/μmol, 1403 ± 153 GBq/μmol, at EOS, n = 7). Further, this work enabled the development of [¹⁸F]DCFPyL production in 21 minutes using an easy cartridge‐based purification (25% ± 9% radiochemical yield, at EOS, n = 3).     

Prospects of medium specific activity 177Lu in targeted therapy of prostate cancer using 177Lu‐labeled PSMA inhibitor

Targeted radionuclide therapy using ¹⁷⁷Lu‐labeled peptidomimetic inhibitor of prostate specific membrane antigen (PSMA) viz. PSMA‐617 is emerging as one the most effective strategies for management of metastatic prostate cancer, which is one of the leading causes of cancer related death. The aim of the present study is to develop a robust and easily adaptable protocol for formulation of therapeutic dose of ¹⁷⁷Lu‐PSMA‐617 at hospital radiopharmacy using moderate specific activity ¹⁷⁷Lu available at an affordable cost. Extensive radiochemical studies were performed to optimize the required [PSMA‐617] / [Lu] ratio and other parameters to formulate 7.4 GBq dose of ¹⁷⁷Lu‐PSMA‐617. Based on these, 7.4 GBq therapeutic dose of ¹⁷⁷Lu‐PSMA‐617 was formulated by incubating 160 µg of PSMA‐617 with indigenously produced ¹⁷⁷LuCl₃ (555 GBq/µg specific activity of ¹⁷⁷Lu) at 90 °C for 30 min. The radiochemical purity of the formulation was 98.3 ± 0.6% (n = 7) which was retained to the extent of >95% after 7 d in normal saline at room temperature and >96% after 2 d in human serum at 37 °C. Preliminary clinical studies showed specific targeting of the agent in the lesion sites and similar physiological distribution as in diagnostic ⁶⁸Ga‐PSMA‐11 PET scans performed earlier. The developed optimized protocol for formulating therapeutic dose of ¹⁷⁷Lu‐PSMA‐617 could be useful for large number of nuclear medicine therapy clinics across the world having access to moderate specific activity ¹⁷⁷Lu at an affordable cost.     

Synthesis of [18F] 4‐amino‐N‐(3‐chloro‐4‐fluorophenyl)‐N′‐hydroxy‐1,2,5‐oxadiazole‐3‐carboximidamide (IDO5L): a novel potential PET probe for imaging of IDO1 expression

To synthesize ¹⁸F‐labeled positron emission tomography (PET) ligands, reliable labeling techniques inserting ¹⁸F into a target molecule are necessary. The ¹⁸F‐fluorobenzene moiety has been widely utilized in the synthesis of ¹⁸F‐labeled compounds. The present study utilized [¹⁸F]‐labeled aniline as intermediate in [¹⁸F]‐radiolabeling chemistry for the facile radiosynthesis of 4‐amino‐N‐(3‐chloro‐4‐fluorophenyl)‐N′‐hydroxy‐1,2,5‐oxadiazole‐3‐carboximidamide ([¹⁸F]IDO5L) as indoleamine 2,3‐dioxygenase 1 (IDO1) targeted tracer. IDO5L is a highly potent inhibitor of IDO1 with low nanomolar IC₅₀. [¹⁸F]IDO5L was synthesized via coupling [¹⁸F]3‐chloro‐4‐fluoroaniline with carboximidamidoyl chloride as a potential PET probe for imaging IDO1 expression. Under the optimized labeling conditions, chemically and radiochemically pure (>98%) [¹⁸F]IDO5L was obtained with specific radioactivity ranging from 11 to 15 GBq/µmol at the end of synthesis within ~90 min, and the decay‐corrected radiochemical yield was 18.2 ± 2.1% (n = 4).     

cGMP production of the radiopharmaceutical [18F]MK‐6240 for PET imaging of human neurofibrillary tangles

Fluorine‐18–labelled 6‐(fluoro)‐3‐(1H‐pyrrolo[2,3‐c]pyridin‐1‐yl)isoquinolin‐5‐amine ([¹⁸F]MK‐6240) is a novel potent and selective positron emission tomography (PET) radiopharmaceutical for detecting human neurofibrillary tangles, which are made up of aggregated tau protein. Herein, we report the fully automated 2‐step radiosynthesis of [¹⁸F]MK‐6240 using a commercially available radiosynthesis module, GE Healthcare TRACERlab FX_FN. Nucleophilic fluorination of the 5‐diBoc‐6‐nitro precursor with potassium cryptand [¹⁸F]fluoride (K[¹⁸F]/K₂₂₂) was performed by conventional heating, followed by acid deprotection and semipreparative high‐performance liquid chromatography under isocratic conditions. The isolated product was diluted with formulation solution and sterile filtered under Current Good Manufacturing Practices, and quality control procedures were established to validate this radiopharmaceutical for human use. At the end of synthesis, 6.3 to 9.3 GBq (170‐250 mCi) of [¹⁸F]MK‐6240 was formulated and ready for injection, in an uncorrected radiochemical yield of 7.5% ± 1.9% (relative to starting [¹⁸F]fluoride) with a specific activity of 222 ± 67 GBq/μmol (6.0 ± 1.8 Ci/μmol) at the end of synthesis (90 minutes; n = 3). [¹⁸F]MK‐6240 was successfully validated for human PET studies meeting all Food and Drug Administration and United States Pharmacopeia requirements for a PET radiopharmaceutical. The present method can be easily adopted for use with other radiofluorination modules for widespread clinical research use.     

Simple and efficient automated radiosynthesis of 2‐18F‐fluoropropionic acid using solid‐phase extraction cartridges purification

2‐¹⁸F‐Fluoropropionic acid (¹⁸F‐FPA), a fluorinated analog of ¹¹C‐acetate, shows great potential for positron emission tomography (PET) imaging of prostate cancer. The present study reports a simple automated synthesis using cartridges purification, which affords ¹⁸F‐FPA suitable for human use. Automated synthesis of ¹⁸F‐FPA was performed using two‐step one‐pot synthesis procedure, consisting of [¹⁸F]fluorination of methyl‐2‐bromopropionate, hydrolysis with sodium hydroxide, and purification with a series of commercial solid‐phase extraction cartridges instead of a high‐performance liquid chromatography system. Automated synthesis of ¹⁸F‐FPA was also carried out via the on‐column hydrolysis on the HLB cartridges, using a similar procedure to the automated synthesis of ¹⁸F‐fluoro‐2‐deoxy‐ d ‐glucose (¹⁸F‐FDG). Using one‐pot procedures on the commercial modified PET‐MF‐2V‐IT‐I synthesizer, the final ¹⁸F‐FPA solution could be obtained with high uncorrected radiochemical yields (>35%) and radiochemical purity (>95%). For the on‐column hydrolysis procedure, the uncorrected radiochemical yield of ¹⁸F‐FPA was 46 ± 7% (n = 5, based on [¹⁸F]fluoride) within 36 min and the radiochemical purity was above 95% with the specific activity of 40 GBq/µmol. Automated synthesis of ¹⁸F‐FPA by on‐column hydrolysis procedure is similar to the ¹⁸F‐FDG synthesis, and it can easily be applied to the automated synthesis of ¹⁸F‐FPA on the commercial ¹⁸F‐FDG synthesizer. Copyright © 2012 John Wiley & Sons, Ltd.     

Microfluidic radiosynthesis and biodistribution of [18F] 2‐(5‐fluoro‐pentyl)‐2‐methyl malonic acid

Microfluidics technology has emerged as a powerful tool for the radiosynthesis of positron emission tomography (PET) and single‐photon emission computed tomography radiolabeled compounds. In this work, we have exploited a continuous flow microfluidic system (Advion, Inc., USA) for the [¹⁸F]‐fluorine radiolabeling of the malonic acid derivative, [¹⁸F] 2‐(5‐fluoro‐pentyl)‐2‐methyl malonic acid ([¹⁸F]‐FPMA), also known as [¹⁸F]‐ML‐10, a radiotracer proposed as a potential apoptosis PET imaging agent. The radiosynthesis was developed using a new tosylated precursor. Radiofluorination was initially optimized by manual synthesis and served as a basis to optimize reaction parameters for the microfluidic radiosynthesis. Under optimized conditions, radio‐thin‐layer chromatography analysis showed 79% [¹⁸F]‐fluorine incorporation prior to hydrolysis and purification. Following hydrolysis, the [¹⁸F]‐FPMA was purified by C18 Sep‐Pak, and the final product was analyzed by radio‐HPLC (high‐performance liquid chromatography). This resulted in a decay‐corrected 60% radiochemical yield and ≥98% radiochemical purity. Biodistribution data demonstrated rapid blood clearance with less than 2% of intact [¹⁸F]‐FPMA radioactivity remaining in the circulation 60 min post‐injection. Most organs showed low accumulation of the radiotracer, and radioactivity was predominately cleared through kidneys (95% in 1 h). Radio‐HPLC analysis of plasma and urine samples showed a stable radiotracer at least up to 60 min post‐injection.     

Automated production at the curie level of no‐carrier‐added 6‐[18F]fluoro‐ l‐dopa and 2‐[18F]fluoro‐ l‐tyrosine on a FASTlab synthesizer

An efficient, fully automated, enantioselective multi‐step synthesis of no‐carrier‐added (nca) 6‐[¹⁸F]fluoro‐L‐dopa ([¹⁸F]FDOPA) and 2‐[¹⁸F]fluoro‐L‐tyrosine ([¹⁸F]FTYR) on a GE FASTlab synthesizer in conjunction with an additional high‐ performance liquid chromatography (HPLC) purification has been developed. A PTC (phase‐transfer catalyst) strategy was used to synthesize these two important radiopharmaceuticals. According to recent chemistry improvements, automation of the whole process was implemented in a commercially available GE FASTlab module, with slight hardware modification using single use cassettes and stand‐alone HPLC. [¹⁸F]FDOPA and [¹⁸F]FTYR were produced in 36.3 ± 3.0 % (n = 8) and 50.5 ± 2.7 % (n = 10) FASTlab radiochemical yield (decay corrected). The automated radiosynthesis on the FASTlab module requires about 52 min. Total synthesis time including HPLC purification and formulation was about 62 min. Enantiomeric excesses for these two aromatic amino acids were always >95 %, and the specific activity of was >740 GBq/µmol. This automated synthesis provides high amount of [¹⁸F]FDOPA and [¹⁸F]FTYR (>37 GBq end of synthesis (EOS)). The process, fully adaptable for reliable production across multiple PET sites, could be readily implemented into a clinical good manufacturing process (GMP) environment.     

Rapid synthesis of maleimide functionalized fluorine‐18 labeled prosthetic group using “radio‐fluorination on the Sep‐Pak” method

Following our recently published fluorine‐18 labeling method, “Radio‐fluorination on the Sep‐Pak”, we have successfully synthesized 6‐[¹⁸F]fluoronicotinaldehyde by passing a solution (1:4 acetonitrile: t‐butanol) of its quaternary ammonium salt precursor, 6‐(N,N,N‐trimethylamino)nicotinaldehyde trifluoromethanesulfonate ( 2 ), through a fluorine‐18 containing anion exchange cartridge (PS‐HCO₃). Over 80% radiochemical conversion was observed using 10 mg of precursor within 1 minute. The [¹⁸F]fluoronicotinaldehyde ([¹⁸F] 5 ) was then conjugated with 1‐(6‐(aminooxy)hexyl)‐1H‐pyrrole‐2,5‐dione to prepare the fluorine‐18 labeled maleimide functionalized prosthetic group, 6‐[¹⁸F]fluoronicotinaldehyde O‐(6‐(2,5‐dioxo‐2,5‐dihydro‐1H‐pyrrol‐1‐yl)hexyl) oxime, 6‐[¹⁸F]FPyMHO ([¹⁸F] 6 ). The current Sep‐Pak method not only improves the overall radiochemical yield (50 ± 9%, decay‐corrected, n = 9) but also significantly reduces the synthesis time (from 60‐90 minutes to 30 minutes) when compared with literature methods for the synthesis of similar prosthetic groups.     

A report of the automated radiosynthesis of the tau positron emission tomography radiopharmaceutical, [18F]‐THK‐5351

The radiotracer, [¹⁸F]‐THK‐5351, is a highly selective and high‐binding affinity PET imaging agent for aggregates of hyper‐phosphorylated tau protein. Our report is a simplified 1‐pot, 2‐step radiosynthesis of [¹⁸F]‐THK‐5351. This report is broadly applicable for routine clinical production and multi‐center trials on account of favorable half‐life of flourine‐18 and the use of a commercially available radiosynthesis module, the GE TRACERlab™ FX_FN. First, the O‐THP protected tosyl precursor underwent nucleophilic fluorinating reaction with potassium cryptand fluoride ([¹⁸F] fluoride (K[¹⁸F]/K₂₂₂)) in Dimethyl sulfoxide at 110°C for 10 minutes followed by O‐THP removal by using diluted hydrochloric acid (HCl) at same temperature. [¹⁸F]‐THK‐5351 was purified via semi‐preparative high‐performance liquid chromatography and formulated by using 10% EtOH, United States Pharmacopeia (USP) in 0.9% sodium chloride for injection, USP and an uncorrected radiochemical yield of 21 ± 3.5%, with a specific activity of 153.11 ± 25.9 GBq/μmol (4138 ± 700 mCi/μmol) at the end of synthesis (63 minutes; n  = 3).     

An improved radiosynthesis of O‐(2‐[18F]fluoroethyl)‐O‐(p‐nitrophenyl)methylphosphonate: A first‐in‐class cholinesterase PET tracer

O‐(2‐Fluoroethyl)‐O‐(p‐nitrophenyl) methylphosphonate 1 is an organophosphate cholinesterase inhibitor that creates a phosphonyl‐serine covalent adduct at the enzyme active site blocking cholinesterase activity in vivo . The corresponding radiolabeled O‐(2‐[¹⁸F]fluoroethyl)‐O‐(p‐nitrophenyl) methylphosphonate, [ ¹⁸ F]1 , has been previously prepared and found to be an excellent positron emission tomography imaging tracer for assessment of cholinesterases in live brain, peripheral tissues, and blood. However, the previously reported [ ¹⁸ F]1 tracer synthesis was slow even with microwave acceleration, required high‐performance liquid chromatography separation of the tracer from impurities, and gave less optimal radiochemical yields. In this paper, we report a new synthetic approach to circumvent these shortcomings that is reliant on the facile reactivity of bis‐(O,O‐p‐nitrophenyl) methylphosphonate, 2 , with 2‐fluoroethanol in the presence of DBU. The cold synthesis was successfully translated to provide a more robust radiosynthesis. Using this new strategy, the desired tracer, [ ¹⁸ F]1 , was obtained in a non‐decay–corrected radiochemical yield of 8 ± 2% (n = 7) in >99% radiochemical and >95% chemical purity with a specific activity of 3174 ± 345 Ci/mmol (EOS). This new facile radiosynthesis routinely affords highly pure quantities of [ ¹⁸ F]1 , which will further enable tracer development of OP cholinesterase inhibitors and their evaluation in vivo .     

Automated synthesis of N‐(2‐[18F]Fluoropropionyl)‐l‐glutamic acid as an amino acid tracer for tumor imaging on a modified [18F]FDG synthesis module

N‐(2‐[¹⁸F]Fluoropropionyl)‐l ‐glutamic acid ([¹⁸F]FPGLU) is a potential amino acid tracer for tumor imaging with positron emission tomography. However, due to the complicated multistep synthesis, the routine production of [¹⁸F]FPGLU presents many challenging laboratory requirements. To simplify the synthesis process of this interesting radiopharmaceutical, an efficient automated synthesis of [¹⁸F]FPGLU was performed on a modified commercial fluorodeoxyglucose synthesizer via a 2‐step on‐column hydrolysis procedure, including ¹⁸F‐fluorination and on‐column hydrolysis reaction. [¹⁸F]FPGLU was synthesized in 12 ± 2% (n = 10, uncorrected) radiochemical yield based on [¹⁸F]fluoride using the tosylated precursor 2 . The radiochemical purity was ≥98%, and the overall synthesis time was 35 minutes. To further optimize the radiosynthesis conditions of [¹⁸F]FPGLU, a brominated precursor 3 was also used for the preparation of [¹⁸F]FPGLU, and the improved radiochemical yield was up to 20 ± 3% (n  = 10, uncorrected) in 35 minutes. Moreover, all these results were achieved using the similar on‐column hydrolysis procedure on the modified fluorodeoxyglucose synthesis module.     

Radiosynthesis of novel pitavastatin derivative ([18F]PTV‐F1) as a tracer for hepatic OATP using a one‐pot synthetic procedure

Pitavastatin is an antihyperlipidemic agent, a potent inhibitor of 3‐hydroxymethyl‐glutaryl‐CoA reductase, which is selectively taken up into the liver mainly via hepatic organic anion transporting polypeptide 1B1 (OATP1B1). OATP1B1 can accept a variety of organic anions, and previous reports indicated that it is responsible for the hepatic clearance of several clinically used anionic drugs. Therefore, the pharmacokinetics and the hepatic distribution of pitavastatin provide an insight into the function of OATP1B1 in humans. For the development of the in vivo evaluation of OATP1B1 function by positron emission tomography imaging, we designed a novel [¹⁸F]pitavastatin derivative ([¹⁸F]PTV‐F1), in which a [¹⁸F]fluoroethoxy group is substituted for the [¹⁸F]fluoro group of [¹⁸F]pitavastatin, with the aim of convenient radiolabeling protocol and high radiochemical yield. In vitro studies suggested that transport activities of PTV‐F1 mediated by OATP1B1 and OATP1B3 were very similar to those of pitavastatin and PTV‐F1 was metabolically stable in human liver microsomes. In the radiosynthesis of [¹⁸F]PTV‐F1 from the tosylate precursor, nucleophilic fluorination and subsequent deprotection were performed using a one‐pot procedure. [¹⁸F]PTV‐F1 was obtained with a radiochemical yield of 45% ± 3% (n = 3), and the operating time for the radiosynthesis of [¹⁸F]PTV‐F1 is very short (30 minutes) compared with [¹⁸F]pitavastatin.     

Radiosynthesis of the norepinephrine transporter tracer [18F]NS12137 via copper‐mediated 18F‐labelling

[¹⁸F]NS12137 (exo‐3‐[(6‐[¹⁸F]fluoro‐2‐pyridyl)oxy]8‐azabicyclo[3.2.1]octane) is a highly selective norepinephrine transporter (NET) tracer. NETs are responsible for the reuptake of norepinephrine and dopamine and are linked to several neurodegenerative and neuropsychiatric disorders. The aim of this study was to develop a copper‐mediated ¹⁸F‐fluorination method for the production of [¹⁸F]NS12137 with straightforward synthesis conditions and high radiochemical yield and molar activity. [¹⁸F]NS12137 was produced in two steps. Radiofluorination of [¹⁸F]NS12137 was performed via a copper‐mediated pathway starting with a stannane precursor and using [¹⁸F]F^? as the source of the fluorine‐18 isotope. Deprotection was performed via acid hydrolysis. The radiofluorination reaction was nearly quantitative as was the deprotection based on HPLC analysis. The radiochemical yield of the synthesis was 15.1 ± 0.5%. Molar activity of [¹⁸F]NS12137 was up to 300 GBq/μmol. The synthesis procedure is straightforward and can easily be automated and adapted for clinical production.     

A simplified radiosynthesis of [18F]MK‐6240 for tau PET imaging

[¹⁸F]MK‐6240 (6‐(fluoro)‐3‐(1H‐pyrrolo[2,3‐c]pyridin‐1‐yl)isoquinolin‐5‐amine) is a highly selective PET radiotracer for the in vivo imaging of neurofibrillary tangles (NFTs). [¹⁸F]MK‐6240 was synthesized in one step from its bis‐Boc protected precursor N‐[(tert‐butoxy)carbonyl]‐N‐(6‐nitro‐3‐[1H‐pyrrolo[2,3‐c]pyridin‐1‐yl]isoquinolin‐5‐yl) carbamate in DMSO using [¹⁸F] fluoride with TEA HCO₃ with step‐wise heating up to 150°C, resulting in an isolated radiochemical yield of 9.8% ± 1.8% (n = 3) calculated from the end of bombardment (5.2% ± 1.0% calculated from the end of synthesis). This new synthetic approach eliminates the acidic deprotection of the bis‐Boc ¹⁸F‐labeled intermediate, which reduces the number of operations necessary for the synthesis as well as losses, which occur during deprotection and neutralization of the crude product mixture prior to the HPLC purification. The synthesis was performed automatically with a single‐use cassette on an IBA Synthera+ synthesis module. This synthesis method affords the radioligand with a reliable radiochemical yield, high radiochemical purity, and a high molar activity. [¹⁸F]MK‐6240 synthesized with this method has been regularly (n > 60) used in our ongoing human and animal PET imaging studies.     

Radiosynthesis and in vivo study of [18F]1‐(2‐fluoroethyl)‐4‐[(4‐cyanophenoxy)methyl]piperidine: a promising new sigma‐1 receptor ligand

The novel sigma‐1 receptor PET radiotracer [¹⁸F]1‐(2‐fluoroethyl)‐4‐[(4‐cyanophenoxy)methyl]piperidine ([¹⁸F]WLS1.002, [¹⁸F]‐2) was synthesized (n=6) by heating the corresponding N‐ethylmesylate precursor in an anhydrous acetonitrile solution containing [¹⁸F]fluoride, Kryptofix K₂₂₂ and potassium carbonate for 15 min. Purification was accomplished by reverse‐phase HPLC methods, providing [¹⁸F]‐2 in 59±8% radiochemical yield (EOB), with specific activity of 2.89±0.80 Ci/µmol (EOS) and radiochemical purity of 98.3±2.1%. Rat biodistribution studies revealed relatively high uptake in many organs known to contain sigma‐1 receptors, including the lungs, kidney, heart, spleen, and brain. Good clearance from normal tissues was observed over time. Blocking studies (60 min) demonstrated high (>80%) specific binding of [¹⁸F]‐2 in the brain, with reduction also noted in other organs known to express these sites. Copyright © 2005 John Wiley & Sons, Ltd.     

An efficient new method for the synthesis of 3‐[18F]fluoro‐4‐aminopyridine via Yamada‐Curtius rearrangement

4‐Aminopyridine is a clinically approved drug to improve motor symptoms in multiple sclerosis . A fluorine‐18‐labeled derivative of this drug, 3‐[¹⁸F]fluoro‐4‐aminopyridine, is currently under investigation for positron emission tomography (PET) imaging of demyelination. Herein, the Yamada‐Curtius reaction has been successfully applied for the preparation of this PET radioligand with a better radiochemical yield and improved specific activity. The overall radiochemical yield was 5 to 15% (n = 12, uncorrected) with a specific activity of 37 to 148 GBq/μmol (end of synthesis) in a 90 minute synthesis time. It is expected that this 1 pot Yamada‐Curtius reaction can be used to prepare similar fluorine‐18‐labeled amino substituted heterocycles.