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.     

Enzymatic synthesis of [1-11C]pyruvic acid, L-[1-11C]lactic acid and L-[1-11C]alanine via DL-[1-11C]alanine

L-[1-11C]Lactic acid was prepared enzymatically from [1-11C]pyruvic acid by way of DL-[1-11C]alanine, using remote, semiautomated procedures. The DL isomers of alanine were prepared by a modification of the Bucherer-Strecker reaction from no-carrier-added (NCA) hydrogen [11C]cyanide. The enantiomer mixture was transformed to [1-11C]pyruvic acid by successive elution through columns of (a) immobilized D-amino acid oxidase (D-AAO)/catalase and (b) immobilized L-alanine dehydrogenase (L-AID) or L-amino acid oxidase (L-AAO/catalase). [1-11C]-Pyruvic acid was subsequently converted to L-[1-11C]lactic acid by passage through a L-lactic dehydrogenase (L-LDH) column. L-[1-11C]Alanine and [1-11C]-pyruvic acid were separated chromatographically by way of a cation-exchange column (AG50W-X2, H+ form). Typically the synthesis time was 35-40 min after cyclotron production of hydrogen [11C]cyanide (400 mCi), with radiochemical yields of 25 mCi (25%) for L-[1-11C]lactic acid, 35 mCi (29%) for [1-11C]pyruvic acid, and 20 mCi (20%) for L-[1-11C]alanine. The use of immobilized enzymes eliminates the possibility of protein contamination and assures the production of sterile, pyrogen-free products, allowing for rapid and effective regio- and stereo-specific transformations.     

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).     

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 new synthesis of [3-C]pyruvic acid using alanine racemase

The synthesis of [3-¹¹C]pyruvic acid was attempted by two reaction systems (A: alanine racemase and -amino acid oxidase, B: alanine racemase and -alanine dehydrogenase) utilizing a new thermostable enzyme, alanine racemase. Conversion rates from , -[3-¹¹C]alanine to [3-¹¹C]pyruvic acid were almost 100% in both methods. Similar results were obtained with immobilized enzymes packed in a single column. Furthermore, the same column could be used repeatedly without a remarkable decrease of the [3-¹¹C]pyruvic acid yield. Various matrices were tested for the immobilizing enzyme, and Aminopropyl-CPG was concluded to be the most suitable since the loss of the enzyme activity was the least in the studied matrices.     

[11C]octopamine synthesis using [11C]cyanide: Chemical and enzymatic approaches for the [11C]cyanohydrin synthesis

[¹¹C]-p- and m-octopamine hydrochloride were synthesized from [¹¹C]HCN in a two-step sequence. Chemical and enzymatic approaches were used for the formation of the [¹¹C]cyanohydrin intermediates as the key step. Isolated radiochemical yields of 0.7–2.3% at the end-of-synthesis were obtained with an overall preparation time of 40–60 min. The enantiomeric purity of the [¹¹C]-p-octopamine obtained through the enzymatic process was 92% e.e. in the (S)-enantiomer, whereas that of the [¹¹C]-m-octopamine was 42% e.e. in the (R)-enantiomer, as determined by HPLC without any derivatization.     

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.     

On-line synthesis of [11C]nitroalkanes

The on-line synthesis of 1-¹¹C-labelled nitroalkanes [nitromethane (I), nitroethane (II) and nitropropane (III)], from their corresponding 1-¹¹C-labelled alkyl iodides, by use of a heated silver nitrite column is described. The radiochemical yields of I, II and III were of the order of 40–70%, based on the corresponding [1-¹¹C]alkyl iodides. The total radiochemical yields, starting from [¹¹C]carbon dioxide, were of the order of 25–55% with an overall synthesis time of 8–15 min.     

The radiosynthesis of [-methyl-11C]-sertraline

[N-methyl-¹¹C]Sertraline, a potential agent for the study of the serotonergic system in vivo with positron emission tomography, was prepared by N-methylation of the corresponding norcompound with [¹¹C]iodomethane, which was itself prepared from cyclotron-produced [¹¹C]carbon dioxide. Under the best conditions found [norsertraline free base (20 mM) in DMF (0.70 mL), 120°C for 8 min] [N-methyl-¹¹C]-sertraline can be prepared in 43% radiochemical yield from [¹¹C]iodomethane (decay-corrected), corresponding to 20% overall radiochemical yield from [¹¹C]carbon dioxide (decay-corrected), with high specific radioactivity. Preparations can be ready for i.v. injection 50 min from the end of radionuclide production.     

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.     

A simple,versatile, low-cost and remotely operated apparatus for [11C]acetate, [11C]choline, [11C]methionine and [11C]PIB synthesis

A simple, efficient and remotely operated synthesis apparatus for carrying out routine [¹¹C]carboxylation, on-column and bubbling [¹¹C]methylation was essential for reliable, day-to-day production of [¹¹C]-labelled PET radiopharmaceuticals. We developed an in-house apparatus specifically applied to the synthesis of [¹¹C]acetate, [¹¹C]choline, [¹¹C]methionine and 2-(4′-N-[¹¹C]methylaminophenyl)-6-hydroxybenzothiazole ([¹¹C]PIB), where high radiochemical purity (⩾97%) and moderate radiochemical yields (18% for [¹¹C]PIB, 41–55% for the others) could be achieved. These findings provided evidence that this was a fast, versatile and reliable apparatus suitable for a PET/CT centre with limited financial budget and hot cell space for synthesis of [¹¹C]-labelled radiopharmaceuticals.     

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.     

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.     

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.     

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.     

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%.     

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.     

Synthesis of [isopropyl-11C]nimodipine for in vivo studies of dihydropyridine binding in man using positron emission tomography

Nimodipine, an antagonist of the L-type calcium ion channel, was labelled with ¹¹C for in vivo positron emission tomography studies of dihydropyridine binding in the human brain. The synthesis was based on esterification of the corresponding acid (W2100) using [2-¹¹C]isopropyl iodide as the labelling precursor. The effects of different bases, solvent mixtures and reaction temperatures on radiochemical yields were investigated. The synthesis, including purification by semi-preparative reversed-phase HPLC, required 60–65 min. Conversion of [2-¹¹C]isopropyl iodide to [isopropyl-¹¹C]nimodipine was of the order of 60–80%. The radiochemical yield (isolated) was 20–25%, based on [¹¹C]carbon dioxide. The specific activity of the isolated product varied from 4–40 GBq/μmol (end-of-synthesis).     

Efficient and reproducible synthesis of [1-11C]acetyl chloride using the loop method

[1-¹¹C]Acetyl chloride ([¹¹C]AcCl), an important [¹¹C]acylating agent, was synthesized by reacting [¹¹C]CO₂ with methylmagnesium bromide coated on the inner surface of a polyethylene loop (loop method). By optimizing the reaction conditions and synthesis parameters, [1-¹¹C]phenylacetate and [1-¹¹C]benzylacetate were produced from [¹¹C]AcCl in high radiochemical yield and specific activity.