Synthesis of 1-11C-labeled fatty acid from [11C]HCN

A new chemical approach to the preparation of [1-¹¹C]fatty acids from [¹¹C]HCN has been developed. Four straight chain [1-¹¹C]fatty acids (C₁₅C₁₈) and one branched chain [1-¹¹C]fatty acid (3-methylheptadecanoic acid) were produced by the reaction of alkyl bromides with ¹¹CN⁻ and subsequent hydrolysis. The synthesis time was 47–67 min after the end of bombardment, with radiochemical yields of 16–20 mCi (70–83%) for a series of straight chain [1-¹¹C]fatty acids and 4–8 mCi (33–42%) for 3-methyl [1-¹¹C]heptadecanoic acid (from 120 mCi of trapped [¹¹C]HCN). This method has some advantages, such as no requirement of absolutely anhydrous solvent, easy ¹¹C-labeling using an extremely small amount of substrate, and good reproducibility.     

Metabolism of methyl-branched iodo palmitic acids in cultured hepatocytes

The metabolic fate of methyl-branched iodo fatty acids was studied in primary culture of rat hepatocytes. We compared 16-iodo-2-R,S-methyl palmitic acid (2-Me), which can be oxidized, with 16-iodo-3-R,S-methyl palmitic acid (3-Me) which can be oxidized only after an initial oxydation and with 16-iodo-2,2-dimethyl palmitic acid (2,2-Me₂) and 16-iodo-3,3-dimethyl palmitic acid (3,3-Me₂) which cannot be oxidized at all. The normal fate of natural fatty acids was given by comparative experiments with [1-¹⁴C] palmitic acid. Monomethyl-branched iodo fatty acids were taken up in the same range as palmitic acid but more than dimethyl-branched iodo fatty acids. After a 15-h incubation, acido-soluble products (ASP) accounted for 75% of the radioactivity taken up as 16-iodo-2-methyl palmitic acid, 50% as other methyl-branched iodo fatty acids and only 30% as palmitic acid, which indicated that all the methyl-branched iodo fatty acids underwent a strong deiodination process. Fatty acids were esterified in the following order: palmitic acid >16-iodo-3-R,S-methyl palmitic acid>16-iodo-2-R,S-methyl palmitic acid>16-iodo-2,2-dimethyl palmitic acid>16-iodo-3,3-dimethyl palmitic acid. Cultured hepatocytes, labelled for 3 h with the various fatty acids and reincubated for 12 h without fatty acid, secreted large amounts of free dimethylbranched iodo fatty acids as compared to the monomethyl ones and palmitic acid. Only hepatocytes prelabelled with 16-[¹²⁵I]iodo-2,2-dimethyl palmitic acid exhibited an appreciable secretion of labeled triglycerides, but at a lower rate than with [1-¹⁴C] palmitic acid. Conversely, the 16-iodo-monomethyl palmitic acids remained chiefly in hepatocyte triglycerides. Minute amounts of 16-iodo-methyl-branched-palmitic acids were found in hepatocyte or secred phospholipids as compared with palmitic acid. This metabolic fate of methyl-branched iodo palmitic acids argues against their utilization as imaging probes to monitor in vivo the synthesis and the secretion of triglycerides by the liver.     

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.     

Syntheses of racemic [1-11C]-alanine and partially resolved [3-11C]-alanine

Irradiation of N₂(g) with 10 MeV protons gave [¹¹C]-carbon dioxide which was used as such in the synthesis of racemic [1-¹¹C]-alanine. Alternatively it was transformed to [¹¹C]-methyl iodide for the asymmetric synthesis of [3-¹¹C]-alanine, giving the L(+) form in excess, the ratio L(+)/D(−) being 2.9. The syntheses were completed in 20 and 60 min, respectively.     

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).     

Radiosynthesis of NCA [carbonyl-11C]6-fluoromelatonin

A method is described for the preparation of [carbonyl]-¹¹C]6-fluoromelatonin for intravenous injection and potential study of the melatonin system in vivo by positron emission tomography. The preparation is based on the acetylation of 6-fluoro-5-methoxytryptamine with NCA [1-¹¹C]acetyl chloride (itself prepared from cyclotron-produced [¹¹C]carbon dioxide) and purification by HPLC. It gives chemically and radiochemically pure [carbonyl-¹¹C]6-fluoromelatonin in 35% radiochemical yield (from [¹¹C]CO₂, decay-corrected) within 35 min from the end of radionuclide production and with high specific activity e.g. 1.6 GBq/μmol (43 mCi/μmol) at the end of synthesis from 1.1 GBq (30 mCi) of [¹¹C]carbon dioxide.     

Synthesis of [1-11C]iodoethane for the preparation of [11C]ethyl labelled radiopharmaceuticals

A method for the synthesis of [¹¹C]iodoethane, a useful precursor for labelling of molecules of biomedical interest, has been developed. ¹¹CO₂ prepared by irradiation of nitrogen gas with 18 MeV protons, is converted into [¹¹C]ethanol by a Grignard reaction with methylmagnesium bromide, followed by reduction with lithium aluminum hydride and hydrolysis with hydrochloric acid. [¹¹C]Iodoethane is produced by reaction of [¹¹C]ethanol with hydriodic acid. The purification is performed by gaschromatography on Porapak Q. For a 20 min irradiation at, 15 μA intensity the method yields about 370 mCi2 of radiochemically pure [¹¹C]iodoethane with a specific activity of 180–300 mCi/μmol at the end of synthesis, 23 min after irradiation.     

Evaluation of the brain uptake properties of [1-11C]labeled hexanoate in anesthetized cats by means of positron emission tomography

Positron emission tomography (PET) was performed on the cat brain to characterize [1-¹¹C]hexanoate and other [1-¹¹C]labeled short and medium-chain fatty acids as a tracer of fatty acid oxidative metabolism. After an intravenous injection the brain uptake of [1-¹¹C]hexanoate reached a peak followed by rapid washout until 2 min (first phase). Subsequently the total brain uptake was again increased and reached to a peak 7–10 min after tracer injection (second phase). The blood radioactivity of unmetabolized [1-¹¹C]hexanoate was rapidly decreased and almost eliminated within the first 2 min, whereas the blood radioactivity of [¹¹C]CO₂/HCO₃ ^? was gradually increased and reached a peak approximately 5 min after tracer injection. As the effect of circulating [¹¹C]CO₂/ HCO₃ ^? was examined by a bolus intravenous injection of [¹¹C]CO₂/HCO₃ ^?, the brain uptake of [¹¹C]CO₂/HCO₃ ^? was rapidly increased right after the injection and changed parallel to the blood level of [¹¹C]CO₂/HCO₃ ^?. These results suggest that, in contrast to the previous mouse data, the time-activity curve in the cat brain following intravenous injection of [l-¹¹C]hexanoate has a biphasic pattern, the second phase being determined by peripherally originating [¹¹C]CO₂/HCO₃ ^?, and therefore does not reflect the metabolism of¹¹C-labeled fatty acid in the brain.     

Synthesis of racemic [3-11C]phenylalanine and [3-11C]DOPA

The synthesis of racemic [3-¹¹C]phenylalanine and [3-¹¹C]DOPA is reported. The [¹¹C]benzaldehyde and [¹¹C]veratraldehyde prepared in a two-step reaction from the corresponding [¹¹C]acid salt and [¹¹C]alcohol, by means of selective oxidation with tetrabutylammonium hydrogen chromate, were reacted with 2-phenyl-5-oxazolone or 2-(4-chloro)phenyl-5-oxazolone in the presence of a tertiary amine to give the corresponding [α-¹¹C]-4-arylene-2-aryl-5-oxazolones. Ring opening of these olefins, hydrogenation, and removal of protecting groups was carried out in one step using hydroiodic acid/phosphorus, with the production of the racemic [3-¹¹C]amino acids in 8–30% radiochemical yield (starting with ¹¹CO₂) within 52–60 min (including LC separation).     

The enzymatic synthesis of l-[3-11C]serine

l-[3-¹¹C]Serine has been prepared from [¹¹C]methanol by a series of coupled enzymatic reactions using alcoholoxidase (EC 1.1.3.13) and catalase (EC 1.11.1.6) to give [¹¹C]formaldehyde, which was condensed with tetrahydrofolate. The N⁵, N¹⁰[¹¹C]methylenetetrahydrofolate formed, was used directly with serine hydroxymethyltransferase (EC 2.1.2.1) immobilized on glutaraldehyde-activated controlled porous glass beads. l-[3-¹¹C]Serine (specific radioactivity 30–50 mCi/μmol) was obtained in a 1–2% yield after a synthesis time of 50–65 min counted from release of [¹¹C]carbon dioxide.     

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.     

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.     

Production of [2-11C]thymidine for quantification of cellular proliferation with PET

A three-step synthesis of [2-¹¹C]thymidine, using [¹¹C]urea as precursor, is described. [2-¹¹C]Thymine was obtained by cyclization of [¹¹C]urea and diethyl β-methylmalate in fuming sulfuric acid. After purification of the reaction mixture, [2-¹¹C]thymine and 2′-deoxyribose-1-phosphate were incubated in the presence of thymidine phosphorylase to form [2-¹¹C]thymidine. The whole synthesis procedure, including purification and pharmaceutical package, was achieved within 60 min with a decay corrected yield of 30–35%.     

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.     

Solid state support for the synthesis of [1−11C]-putrescine

A novel solid state support was demonstrated using [¹¹C]-HCN for the radiosynthesis of [1−¹¹C]-putrescine by the Michael addition reaction to acrylonitrile. The silica gel support allowed near quantitative trapping of the [¹¹C]-HCN and its efficient use under anhydrous conditions. The absence of water during the addition reaction eliminated by-product formation and reduced the overall synthesis and purification time required for the preparation of the radiopharmaceutical. A radiochemical yield of 53 ± 4 was achieved for purified product within 40 min of EOB. The process can be automated for the routine synthesis of [1-¹¹C]-putrescine radiopharmaceutical.     

Brain uptake and metabolism of [1-11C]octanoate in rats: Pharmacokinetic basis for its application as a radiopharmaceutical for studying brain fatty acid metabolism

The uptake of octanoate in rat brain and its metabolism were investigated by means of intravenously injecting [1-¹¹C] or [1-¹⁴C]octanoate as a tracer. The radioactivity in the cerebrum was increased by an injection of [1-¹¹C]octanoate, and reached its peak level (0.33% ID/g) in about 2 to 5 min, and then decreased slowly. The cerebrum-to-blood ratio of the radioactivity increased with time over a period of 30 min. At 30 sec, [ 1-¹¹C]octanoate that remained unchanged in the cerebrum accounted for only 8% of the total radioactivity, in spite of there being about 90% in the blood. By means of an injection of [ 1-¹⁴C]octanoate, more than 70% of the total radioactivity in the cerebrum was found to be attributable to radiolabeled glutamate and glutamine at each time point measured between 30 sec and 30 min. The results show that [1-¹¹C]octanoate enters rat brain easily and is trapped in the cerebrum, probably in the form of glutamate and glutamine, and the usefulness of [ 1-¹¹C] octanoate as a radiopharmaceutical for studying brain fatty acid metabolism by positron emission tomography is therefore suggested.     

Preparation of 11C-methyl iodide and l-[S-methyl-11C]methionine by an automated continuous flow process

A continuous flow procedure has been developed for the synthesis of ¹¹C-methyl iodide and l-[S-methyl-¹¹C] methionine from ¹¹CO₂. The preparation can be completed in 20 min after the end of bombardment. This procedure results in the production of approximately 44 mCi ¹¹C-methyl methionine with a specific activity of 3.3 Ci/mmol. Radiochemical purity is greater than 96%.The procedure has been automated so that the preparation can be carried out remotely, thus minimizing exposure of personnel to radiation.     

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.     

Metabolism of methyl-branched iodo palmitic acids in cultured hepatocytes

The metabolic fate of methyl-branched iodo fatty acids was studied in primary culture of rat hepatocytes. We compared 16-iodo-2-R,S-methyl palmitic acid (2-Me), which can be beta oxidized, with 16-iodo-3-R,S-methyl palmitic acid (3-Me) which can be beta oxidized only after an initial alpha oxydation and with 16-iodo-2,2-dimethyl palmitic acid (2,2-Me2) and 16-iodo-3,3-dimethyl palmitic acid (3,3-Me2) which cannot be beta oxidized at all. The normal fate of natural fatty acids was given by comparative experiments with [1-14C] palmitic acid. Monomethyl-branched iodo fatty acids were taken up in the same range as palmitic acid but more than dimethyl-branched iodo fatty acids. After a 15-h incubation, acido-soluble products (ASP) accounted for 75% of the radioactivity taken up as 16-iodo-2-methyl palmitic acid, 50% as other methyl-branched iodo fatty acids and only 30% as palmitic acid, which indicated that all the methyl-branched iodo fatty acids underwent a strong deiodination process. Fatty acids were esterified in the following order: palmitic acid greater than 16-iodo-3-R,S-methyl palmitic acid greater than 16-iodo-2-R,S-methyl palmitic acid greater than 16-iodo-2,2-dimethyl palmitic acid greater than 16-iodo-3,3-dimethyl palmitic acid. Cultured hepatocytes, labelled for 3 h with the various fatty acids and reincubated for 12 h without fatty acid, secreted large amounts of free dimethyl-branched iodo fatty acids as compared to the monomethyl ones and palmitic acid.(ABSTRACT TRUNCATED AT 250 WORDS)     

Measurement of myocardial fatty acid esterification using [1-11C]palmitate and PET: comparison with direct measurements of myocardial triglyceride synthesis

Background  The purpose of the present study was to assess the accuracy of rates of myocardial fatty acid esterification (MFAE) obtained using positron emission tomography (PET). Methods and results  Sixteen dogs were studied after an overnight fast (FAST), during a euglycemic hyperinsulinemic clamp (CLAMP), or during infusion of intralipid (IL) or IL plus dobutamine (IL/DOB). MFAE was quantified using [1-¹¹C]palmitate and PET and compared to the rate of triglyceride (TG) synthesis measured using [1-¹³C]palmitate and tissue sampling. Plasma free fatty acid (FFA) concentration varied ~20-fold across groups, with this variation in FFA availability accompanied by a ~20-fold range in TG synthesis. MFAE varied ~12-fold across groups, and was significantly correlated with TG synthesis (R = 0.80, P < .001). MFAE, however, was 3- to 4-fold higher than TG synthesis in FAST, CLAMP, and IL, but only ~50% higher when cardiac work was increased in IL/DOB, suggesting that MFAE reflects, in part, the incorporation of label into amino acids via TCA cycle exchange reactions. Conclusions  Changes in MFAE parallel changes in TG synthesis, at least in the basal state. Although the data need to be interpreted cautiously, such measurements should be useful for quantifying acute changes in FFA storage by the heart in various pathophysiological states.