An alternative synthesis of DL-[1-11C]Alanine from [11C]HCN

The synthesis of dl-[1-¹¹C]alanine from [1-¹¹C]HCN via dl-[1-¹¹C]2-aminopropanenitrile and its use in an enzymatic synthesis of [1-¹¹C]pyruvic acid is reported. The purified dl-[1-¹¹C]alanine was obtained in a radiochemical yield of 75% with a radiochemical purity higher than 98% within 40 min after end of bombardment (EOB). [1-¹¹C]Pyruvic acid was prepared by enzymatic synthesis from crude dl-[1-¹¹C]alanine using a previously reported procedure for the synthesis of [3-¹¹C]pyruvic acid. dl-[1-¹¹C]Alanine and [1-¹¹C]pyruvic acid prepared by these methods have been found to be sterile and free of pyrogens, and can thus be used in studies of the distribution of these radiopharmaceuticals in man using positron emission tomography (PET).     

A simple modification of GE tracerlab FX C Pro for rapid sequential preparation of [11C]carfentanil and [11C]raclopride

A simple modification of the GE Tracerlab FX C Pro system which enabled performance of both solvent capture (loop method) and conventional solution phase [¹¹C]methylation in the same module is described. By the quick setup and automated method, [¹¹C]carfentanil and [¹¹C]raclopride could be prepared in rapid succession without opening the hot cell. The radiochemical yields were over 40% and 30% (decay-corrected and based on [¹¹C]methyl iodide) for [¹¹C]carfentanil and [¹¹C]raclopride, respectively. The radiochemical purities were greater than 95% and specific activities over 5 Ci/mmol for both tracers. The modification is extremely easy and can be utilized for multiple syntheses of other ¹¹C-labeled radiopharmaceuticals in a fast and reliable manner.     

PET imaging of focal demyelination and remyelination in a rat model of multiple sclerosis: comparison of [11C]MeDAS, [11C]CIC and [11C]PIB

Purpose

In this study, we compared the ability of [¹¹C]CIC, [¹¹C]MeDAS and [¹¹C]PIB to reveal temporal changes in myelin content in focal lesions in the lysolecithin rat model of multiple sclerosis. Pharmacokinetic modelling was performed to determine the best method to quantify tracer uptake.

Methods

Sprague-Dawley rats were stereotactically injected with either 1 % lysolecithin or saline into the corpus callosum and striatum of the right brain hemisphere. Dynamic PET imaging with simultaneous arterial blood sampling was performed 7 days after saline injection (control group), 7 days after lysolecithin injection (demyelination group) and 4 weeks after lysolecithin injection (remyelination group).

Results

The kinetics of [¹¹C]CIC, [¹¹C]MeDAS and [¹¹C]PIB was best fitted by Logan graphical analysis, suggesting that tracer binding is reversible. Compartment modelling revealed that all tracers were fitted best with the reversible two-tissue compartment model. Tracer uptake and distribution volume in lesions were in agreement with myelin status. However, the slow kinetics and homogeneous brain uptake of [¹¹C]CIC make this tracer less suitable for in vivo PET imaging. [¹¹C]PIB showed good uptake in the white matter in the cerebrum, but [¹¹C]PIB uptake in the cerebellum was low, despite high myelin density in this region. [¹¹C]MeDAS distribution correlated well with myelin density in different brain regions.

Conclusion

This study showed that PET imaging of demyelination and remyelination processes in focal lesions is feasible. Our comparison of three myelin tracers showed that [¹¹C]MeDAS has more favourable properties for quantitative PET imaging of demyelinated and remyelinated lesions throughout the CNS than [¹¹C]CIC and [¹¹C]PIB.     

Synthesis of DL-[3-11C]valine using [2-11C]isopropyl iodide,and preparation of L-[3-11C]valine by treatment with D-amino acid oxidase

DL-[3-¹¹C]Valine, synthesized by phase-transfer alkylation of N-(diphenylmethylene)glycine t-butyl ester with [2-¹¹C]isopropyl iodide, followed by acidic hydrolysis, was obtained in 20–30% radiochemical yield (decay corrected and calculated on the amount of [¹¹C]carbon dioxide used) and with 93–99% radiochemical purity with a total synthesis time of 50 min. Following treatment with immobilized D-amino acid oxidase, L-[3-¹¹C]valine was obtained in 90–99% enantiomeric excess with a total synthesis time of 85 min. [2-¹¹C]Isopropyl iodide was obtained in 40 and 90% radiochemical yield and purity respectively, within 12 min calculated from [¹¹C]carbon dioxide. In a typical experiment starting with 150 mCi of [¹¹C]carbon dioxide, 7 mCi of DL-[3-¹¹C]valine and 0.8 mCi of L-[3-¹¹C]valine were obtained.     

Remote-controlled production of [1-11C]-d-glucose and evaluation of the effect of labelling position on loss of [11C]CO2

A remote-controlled apparatus is described for the routine production of [1-¹¹C]-d-glucose by the reaction of [¹¹C]cyanide with d(−)arabinose and reduction of the intermediate [1-¹¹C]aldonitriles with Raney nickel in formic acid. The total time of synthesis, including HPLC separation, is 55–60 min from the end-of-trapping. The decay-corrected isolated radiochemical yield of [1-¹¹C]-d-glucose is 8–11% (based on trapped [¹¹C]cyanide) and the radiochemical purity is >99%. Studies of regional cerebral metabolic rate of glucose using [1-¹¹C]-d-glucose and positron emission tomography indicate that better contrast between areas of high and low metabolic rate is obtained when glucose is selectively rather than uniformly labelled. Arteriovenous determinations showed that the egress of [¹¹C]carbon dioxide from the brain was reduced and delayed compared to that previously observed for [U-¹¹C]-d-glucose.     

A simple Tracerlab module modification for automated on-column [11C]methylation and [11C]carboxylation.

A modification of commercial [11C]methylation module which can be implemented for both on-column [11C]methylation and [11C]carboxylation in the same automated system is described. This module configuration was applied to the solid-phase synthesis of N-[11C]methyl-choline ([11C]choline) and L-(S-methyl-[11C])methionine ([11C]methionine), using [11C]CH(3)I as methylating agent, as well as to the synthesis of [11C]acetate by [11C]carboxylation with [11C]CO2 of methylmagnesium chloride with high and reproducible radiochemical yields in short reaction time, demonstrating to be a fast and reliable tool for the production of these [11C]radiopharmaceuticals for clinical use.     

A semi-automated synthesis system for routine preparation of [11C]YM-09151-2 and [11C]pyrilamine from [11C]methyl iodide

A synthesis system has been developed for routine preparation of the ¹¹C-labeled receptor ligands, [¹¹C]YM-09151-2 and [¹¹C]pyrilamine, from [¹¹C]methyl iodide produced automatically. The system features semi-automated operation, from the reaction of the desmethyl derivative with [¹¹C]methyl iodide to filtration with a specifically developed syringe pump of the final product in saline into a sterile vial. The preparations were completed within 45 min after irradiation and approx. 1 GBq (27 mCi) of [¹¹C]YM-09151-2 or [¹¹C]pyrilamine was obtained with a radiochemical and chemical purity of >99% and an average specific activity of 44 GBq/μmol (1.2 Ci/μmol) at the end of synthesis. Sterile and pyrogen-free ¹¹C-labeled receptor ligands suitable for human injection are routinely prepared using the present synthesis system.     

[11C]PIB, [18F]FDG and MR imaging in patients with mild cognitive impairment

Purpose

Cortical glucose metabolism, brain amyloid β accumulation and hippocampal atrophy imaging have all been suggested as potential biomarkers in predicting which patients with mild cognitive impairment (MCI) will convert to Alzheimer’s disease (AD). The aim of this study was to compare the prognostic ability of [¹¹C]PIB PET, [¹⁸F]FDG PET and quantitative hippocampal volumes measured with MR imaging in predicting conversion to AD in patients with MCI.

Methods

The study group comprised 29 patients with MCI who underwent [¹¹C]PIB PET and MR imaging. Of these, 22 also underwent [¹⁸F]FDG PET. All subjects were invited back for clinical evaluation after 2 years.

Results

During the follow-up time 17 patients had converted to AD while 12 continued to meet the criteria for MCI. The two groups did not differ in age, gender or education level, but the converter group tended to have lower MMSE and Word List learning than the nonconverter group. High [¹¹C]PIB retention in the frontotemporal regions and anterior and posterior cingulate (p?<?0.05) predicted conversion to AD. Also reduced [¹⁸F]FDG uptake in the left lateral temporal cortex (LTC) predicted conversion (p?<?0.05), but quantitative hippocampal volumes did not (p?>?0.1). In receiver operating characteristic (ROC) analysis the measurements that best predicted the conversion were [¹¹C]PIB retention in the lateral frontal cortex and [¹⁸F]FDG uptake in the left LTC. Both PET methods resulted in good sensitivity and specificity and neither was significantly superior to the other.

Conclusion

The findings indicate that [¹¹C]PIB and [¹⁸F]FDG are superior to hippocampal volumes in predicting conversion to AD in patients with MCI.     

Preparation of some NCA [1-11C]acid chlorides as labelling agents

The [¹¹C]carbonation of simple Grignard reagents (RMgX, R = Et, Pr, cyclo-Bu; X = Br) with cyclotron-produced [¹¹C]carbon dioxide followed by direct treatment of the ¹¹C adducts with phthaloyl dichloride and 2,6-di-t-butylpyridine affords [1-¹¹C]acid chlorides in useful radiochemical yields (> 10–30%, decay-corrected from ¹¹CO₂), as assessed by HPLC and TLC of the labelled amides formed by reaction with 1,2,3,4-tetrahydroisoquinoline. With suitable precuations these [1-¹¹C]acid chlorides can be isolated at high specific activity within 15 min from radionuclide production as labelling agents for potential radiopharmaceuticals for PET. The preparation of [1-¹¹C]benzoyl chloride without isolation has also been demonstrated.     

Development of a modular system for the synthesis of PET [C]labelled radiopharmaceuticals

[¹¹C]labelled radiopharmaceuticals as N-[¹¹C]methyl-choline ([¹¹C]choline), l-(S-methyl-[¹¹C])methionine ([¹¹C]methionine) and [¹¹C]acetate have gained increasing importance in clinical PET and for the routine production of these radiopharmaceuticals, simple and reliable modules are needed to produce clinically relevant radioactivity. On the other hand, flexible devices are needed not only for the routine synthesis but also for more complex applications as the development of new tracers. The aim of this work was the adaptation of an Eckert Ziegler modular system for easy routine synthesis of [¹¹C]choline, [¹¹C]methionine and [¹¹C]acetate using components that account for straightforward scaling up and upgrades.     

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

Purpose

The characteristic neuropathological changes in Alzheimer’s disease (AD) are deposition of amyloid senile plaques and neurofibrillary tangles. The ¹⁸F-labeled amyloid tracer, [¹⁸F]2-[(2-{(E)-2-[2-(dimethylamino)-1,3-thiazol-5-yl]vinyl}-1,3-benzoxazol-6-yl)oxy]-3-fluoropropan-1-ol (FACT), one of the benzoxazole derivatives, was recently developed. In the present study, deposition of amyloid senile plaques was measured by positron emission tomography (PET) with both [¹¹C]Pittsburgh compound B (PIB) and [¹⁸F]FACT in the same subjects, and the regional uptakes of both radiotracers were directly compared.

Methods

Two PET scans, one of each with [¹¹C]PIB and [¹⁸F]FACT, were performed sequentially on six normal control subjects, two mild cognitive impairment (MCI) patients, and six AD patients. The standardized uptake value ratio of brain regions to the cerebellum was calculated with partial volume correction using magnetic resonance (MR) images to remove the effects of white matter accumulation.

Results

No significant differences in the cerebral cortical uptake were observed between normal control subjects and AD patients in [¹⁸F]FACT studies without partial volume correction, while significant differences were observed in [¹¹C]PIB. After partial volume correction, the cerebral cortical uptake was significantly larger in AD patients than in normal control subjects for [¹⁸F]FACT studies as well as [¹¹C]PIB. Relatively lower uptakes of [¹¹C]PIB in distribution were observed in the medial side of the temporal cortex and in the occipital cortex as compared with [¹⁸F]FACT. Relatively higher uptake of [¹¹C]PIB in distribution was observed in the frontal and parietal cortices.

Conclusion

Since [¹⁸F]FACT might bind more preferentially to dense-cored amyloid deposition, regional differences in cerebral cortical uptake between [¹¹C]PIB and [¹⁸F]FACT might be due to differences in regional distribution between diffuse and dense-cored amyloid plaque shown in the autoradiographic and histochemical assays of postmortem AD brain sections.     

Radiosynthesis of [2-11C-carbonyl]dantrolene using [11C]phosgene for PET

Automated radiosynthesis of [2-¹¹C-carbonyl]dantrolene, the substrate of breast cancer resistance protein (BCRP/ABCG2), was performed for the first time through a multi-step/one-pot labeling sequence that started with ethyl 2-{2-[5-(4-nitrophenyl)furfurylidene]hydrazino}acetate and used [¹¹C]phosgene as a labeling agent. After optimization of the automated synthesis conditions and parameters, [2-¹¹C-carbonyl]dantrolene was obtained at a radiochemical yield of 34.0±8.4% (decay-corrected). The radiochemical purity was greater than 98% and the specific activity was 46.8±15.2 GBq/μmol at the end of the synthesis.     

Synthesis and preclinical evaluation of [11C]D617, a metabolite of (R)-[11C]verapamil

Objectives(R)-[¹¹C]verapamil is widely used as a positron emission tomography (PET) tracer to evaluate P-glycoprotein (P-gp) functionality at the blood–brain barrier in man. A disadvantage of (R)-[¹¹C]verapamil is the fact that its main metabolite, [¹¹C]D617, also enters the brain. For quantitative analysis of (R)-[¹¹C]verapamil data, it has been assumed that the cerebral kinetics of (R)-[¹¹C]verapamil and [¹¹C]D617 are the same. The aim of the present study was to investigate whether the cerebral kinetics of (R)-[¹¹C]verapamil and [¹¹C]D617 are indeed similar and, if so, whether [¹¹C]D617 itself could serve as an alternative PET tracer for P-gp.Methods[¹¹C]D617 was synthesized and its ex vivo biodistribution was investigated in male rats at four time points following intravenous administration of [¹¹C]D617 (50 MBq) without (n=4) or with (n=4) pretreatment with the P-gp inhibitor tariquidar (15 mg·kg^?1, intraperitoneally). Brain distribution was further assessed using consecutive PET scans (n=8) before and after pretreatment with tariquidar (15 mg·kg^?1, intravenously), as well as metabolite analysis (n=4).ResultsThe precursor for the radiosynthesis of [¹¹C]D617, 5-amino-2-(3,4-dimethoxy-phenyl)-2-isopropyl-pentanitrile (desmethyl D617), was synthesized in 41% overall yield. [¹¹C]D617 was synthesized in 58%–77% decay-corrected yield with a radiochemical purity of ≥99%. The homogeneously distributed cerebral volume of distribution (V_T) of [¹¹C]D617 was 1.1, and this increased 2.4-fold after tariquidar pretreatment.ConclusionV_T of [¹¹C]D617 was comparable to that of (R)-[¹¹C]verapamil, but its increase after tariquidar pretreatment was substantially lower. Hence, (R)-[¹¹C]verapamil and [¹¹C]D617 do not show similar brain kinetics after inhibition of P-gp with tariquidar.     

Enhanced radiosyntheses of [11C]raclopride and [11C]DASB using ethanolic loop chemistry

IntroductionTo improve the synthesis and quality control of carbon-11 labeled radiopharmaceuticals, we report the fully automated loop syntheses of [¹¹C]raclopride and [¹¹C]DASB using ethanol as the only organic solvent for synthesis module cleaning, carbon-11 methylation, HPLC purification, and reformulation.MethodsEthanolic loop chemistry is fully automated using a GE TRACERLab FX_C-Pro synthesis module, and is readily adaptable to any other carbon-11 synthesis apparatus. Precursors (1 mg) were dissolved in ethanol (100 μL) and loaded into the HPLC loop. [¹¹C]MeOTf was passed through the HPLC loop and then the labeled products were purified by semi-preparative HPLC and reformulated into ethanolic saline.ResultsBoth [¹¹C]raclopride (3.7% RCY; > 95% RCP; SA = 20831 Ci/mmol; n = 64) and [¹¹C]DASB, both with (3.0% RCY; > 95% RCP; SA = 15152 Ci/mmol; n = 9) and without (3.0% RCY; > 95% RCP; SA = 10931 Ci/mmol; n = 3) sodium ascorbate, have been successfully prepared using the described methodology. Doses are suitable for human use and the described methods are now employed for routine clinical production of both radiopharmaceuticals at the University of Michigan.ConclusionsEthanolic loop chemistry is a powerful technique for preparing [¹¹C]raclopride and [¹¹C]DASB, and we are in the process of adapting it for other carbon-11 radiopharmaceuticals prepared in our laboratories ([¹¹C]PMP, [¹¹C]PBR28 etc.).     

Synthesis of [11C]iodomethylcyclopropane,an interesting alkylating reagent

The synthesis of [¹¹C]iodomethylcyclopropane by a four-step procedure, starting from [¹¹C]carbon dioxide, is presented. The [¹¹ carbon dioxide was trapped in a solution of cyclopropylmagnesium bromide and the acid was reduced to give the alcoholate. When treated with p-toluenesulphonyl bromide, [¹¹C]bromomethylcyclopropane was formed directly and then converted to the corresponding iodide by the Finkelstein reaction. [¹¹C]Iodomethylcyclopropane was prepared in about 13% radiochemical yield (decay-corrected) and 80% radiochemical purity, with a reaction time of 20 min and a specific radioactivity in the order of 10–100 mCi/μmol.     

Routine synthesis of N-[11C-methyl]scopolamine by phosphite mediated reductive methylation with [11C]formaldehyde

A synthesis of [¹¹C]scopolamine capable of clinical delivery of this agent in high specific activity is described. The precursor [¹¹C]formaldehyde was produced by catalytic oxidation of [¹¹C]CH₃OH over metallic silver and was used to N-¹¹C-methylate norscopolamine using aqueous neutral potassium phosphite as the reducing agent. The labeling reaction was complete after 5 min at 75–80°C and the [¹¹C]scopolamine (99% radiochemical purity) was isolated by preparative HPLC. Total synthesis time is less than 45 min. Decay corrected radiochemical yields from [¹¹C]CO₂ are presently 20–43%.     

Remote synthesis of [11C]acetate

This remote synthesis greatly simplifies previously reported liquid—liquid extraction techniques for remote production of [¹¹C]acetate. The use of solid phase extraction has reduced the total volume of solvents employed while providing [¹¹C]acetate free of labeled by-products. Remotely actuated loop flow valves have been utilized to efficiently and quickly introduce reagents. [¹¹C]Acetate is obtained within 15 min from the end of bombardment in 43% (EOB) radiochemical yield and with radiochemical purity >98%.     

The synthesis of [3-11C]pyruvic acid,a useful synthon,via an enzymatic route

The enzymatic synthesis of [3-¹¹C]pyruvic acid from racemic [3-¹¹C]alanine, using d-amino acid oxidase (D-AAO)/catalase and glutamic-pyruvic transaminase (GPT) in a coupled, one-pot reaction, is reported. The chemical synthesis of [3-¹¹C]alanine was carried out by the alkylation of N-(diphenylmethylene)glycine t-butyl ester with [¹¹C]methyl iodide in DMF/DMSO using potassium hydroxide as base. The total synthesis time was 35 min counted from release of [¹¹C]carbon dioxide giving [3-¹¹C]pyruvic acid in 73% radiochemical yield, decay corrected, and >99% radiochemical purity.     

Synthesis of L- and D-[methyl-11C]methionine

This report describes the synthesis of L- and D-[methyl-11C]methionine in pure enantiomeric forms. The compounds were prepared routinely approximately 1,000 times with less than 20 failures. Starting with carbon-11 (11C) methyl iodide, a simple one-carbon precursor produced from a one-pot or a two-pot apparatus, L- and D-[methyl-11C]methionine were prepared, respectively, with an optical purity higher than 99% in 40%-90% radiochemical yields. The total time for synthesis, starting from [11C]carbon dioxide, was 12-15 min. The crude product usually had a radiochemical purity greater than 95%. The total time for synthesis, including LC purification, was 20-30 min. The radiochemical purity of the product in each case was greater than 98%.