A combined simple bubbling method with high performance liquid chromatography purification strategy, higher radiochemical yield and purity and faster preparation of carbon-11-raclopride.

OBJECTIVE: Carbon-11-raclopride (¹¹C-R) is a positron-emitting radiotracer successfully used for the study of cognitive control and widely applied in PET imaging. A simple automated preparation of ¹¹C-R by using the reaction of carbon-(11)-methyl triflate (¹¹C-MeOTF) or ¹¹C-methyl iodide (¹¹C-MeI) with demethylraclopride is described. METHODS: Specifically we used a simple setup applied an additional "U" reaction vessel for ¹¹C-MeOTf compared with ¹¹C-MeI and assessed the influence of several solvents and of the amount of the percussor for ¹¹C-methylation of demethylraclopride by the bubbling method. The reversal of retention order between product and its precursor has been achieved for ¹¹C-R, enabling collection of the purified ¹¹C-R by using the HPLC column after shorter retention time. RESULTS: By the improved radiosynthesis and purification strategy, ¹¹C-R could be prepared with higher radiochemical yield than that of the previous studies. The yield for ¹¹C-MeOTf was 76% and for ¹¹C-CH3I >26% and with better radiochemical purity (>99% based on both ¹¹C-MeOTf and ¹¹C-MeI) as compared to the previously obtained purity of ¹¹C-R using HPLC method with acetonitrile as a part of mobile phase. Furthermore, by using ethanol as the organic modifier, residual solvent analysis prior to human injection could be avoided and ¹¹C-R could be injected directly following simple dilution and sterile filtration. CONCLUSION: Improved radiosynthesis and HPLC purification in combination with ethanol containing eluent, extremely shortened the time for preparation of ¹¹C-R, gave a higher radiochemical yield and purity for ¹¹C-R and can be used for multiple and faster synthesis of ¹¹C-R and probably for other ¹¹C-labeled radiopharmaceuticals.

Application of Microreactor to the Preparation of C-11-Labeled Compounds via O-[11C]Methylation with [11C]CH3I: Rapid Synthesis of [11C]Raclopride.

A new radiolabeling method using a microreactor was developed for the rapid synthesis of [(11)C]raclopride. A chip bearing a Y-shaped mixing junction with a 200 µm (width)×20 µm (depth)×250 mm (length) flow channel was designed, and the efficiency of O-[11C]methylation was evaluated. Dimethyl sulfoxide solutions containing the O-desmethyl precursor or [11C]CH3I were introduced into separate injection ports by infusion syringes, and the radiochemical yields were measured under various conditions. The decay-corrected radiochemical yield of microreactor-derived [11C]raclopride reached 12% in 20 s at 25 °C, which was observed to increase with increasing temperature. In contrast, batch synthesis at 25 °C produced a yield of 5%: this indicates that this device could effectively achieve O-[11C]methylation in a shorter period of time. The microreactor technique may facilitate simple and efficient routine production of 11C-labeled compounds via O-[11C]methylation with [11C]CH3I.

Synthesis of ([(11)C]carbonyl)raclopride and a comparison with ([(11)C]methyl)raclopride in a monkey PET study.

INTRODUCTION: The selective dopamine D2 receptor antagonist raclopride is usually labeled with carbon-11 using [(11)C]methyl iodide or [(11)C]methyl triflate for use in the quantification of dopamine D2 receptors in human brain. The aim of this work was to label raclopride at the carbonyl position using [(11)C]carbon monoxide chemistry and to compare ([(11)C]carbonyl)raclopride with ([(11)C]methyl)raclopride in non-human primate (NHP) using PET with regard to quantitative outcome measurement, metabolism of the labeled tracers and protein binding. METHODS: Palladium-mediated carbonylation using [(11)C]carbon monoxide, 4,6-dichloro-2-iodo-3-methoxyphenol and (S)-(-)-2-aminomethyl-1-ethylpyrrolidine was applied in the synthesis of ([(11)C]carbonyl)raclopride. The reaction was performed at atmospheric pressure using xantphos as supporting phosphine ligand and palladium (π-cinnamyl) chloride dimer as the palladium source. ([(11)C]Methyl)raclopride was prepared by a previously published method. In the PET study, two female cynomolgus monkeys were used under gas anesthesia of sevoflurane. A dynamic PET measurement was performed for 63 min with an HRRT PET camera after intravenous injection of ([(11)C]carbonyl)raclopride and ([(11)C]methyl)raclopride, respectively, during the same day. The order of injection of the two PET radioligands was changed between the two monkeys. The venous blood sample for measurement of protein binding was taken 3 min prior to the PET scan. Binding potential (BPND) of the putamen and caudate was calculated with SRTM using the cerebellum as a reference region. RESULTS: The target compound ([(11)C]carbonyl)raclopride was obtained with 50 ± 5% decay corrected radiochemical yield and 95% radiochemical purity. The trapping efficiency (TE) of [(11)C]carbon monoxide was 65 ± 5% and the specific radioactivity of the final product was 34 ± 1 GBq/µmol after a 50 min of synthesis time. The radiochemical yield of ([(11)C]methyl)raclopride was in the same range as published previously i. e. 50-60% and specific radioactivity of those two batches which were used in the present PET study were 192 GBq/µmol and 638 GBq/µmol respectively after a synthesis time of 32 min. In monkey PET studies, the percentage difference of BPND in putamen was <3% and that in caudate was <9% for the two radioligands. The plasma protein binding was 86.2 ± 0.3% and 85.7 ± 0.6% for ([(11)C]carbonyl)raclopride and ([(11)C]methyl)raclopride, respectively. The radiometabolite pattern was similar for both radioligands. CONCLUSION: Raclopride was (11)C-labeled at the carbonyl position using a palladium-mediated [(11)C]carbonylation reaction. A comparison between ([(11)C]carbonyl)raclopride and ([(11)C]methyl)raclopride with regard to quantitative PET outcome measurements, metabolism of radioligands and protein binding in monkey was performed. The monkey PET study with ([(11)C]carbonyl)raclopride showed similar results as for ([(11)C]methyl)raclopride. The PET studies were performed on 2 subjects.

Optimisation of [11C]Raclopride production using a Synthra GPextent system.

The dopamine D2 receptor radiotracer [(11)C]Raclopride is used extensively in clinical and preclinical imaging. Currently, a wide range of methods to produce [(11)C]Raclopride have been developed using traditional vessel reactions as well as cartridge or captive solvent. This work reports the optimisation of the production of [(11)C]Raclopride using a Synthra GPextent, comparing various methods. With optimised conditions, we were able to obtain 4±2% (ndc) yield of [(11)C]Raclopride (100 GBq [(11)C]CO2, n = 42) in 25 min. The radiochemical purity was >95% with specific activities of 135±41 MBq/nmol at end of synthesis.

Improved HPLC purification strategy for [11C]raclopride and [11C]DASB leading to high radiochemical yields and more practical high quality radiopharmaceutical formulations.

A downscaled analytical HPLC purification strategy for [(11)C]raclopride and [(11)C]DASB was proposed to obtain a straightforward produced injectable solution with a substantially reduced volume. Both tracers were radiosynthesized using [(11)C]methyl triflate, and several analytical columns, in combination with ethanol containing eluents, were examined to optimize the chromatographic resolution. This resulted in a 5 mL solution of [(11)C]raclopride, obtained after 6 min, and a 7 mL solution of [(11)C]DASB, obtained after 10 min, using a C16-alkylamide and CN column respectively.

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

INTRODUCTION: To 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. METHODS: Ethanolic 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. RESULTS: Both [¹¹C]raclopride (3.7% RCY; >95% RCP; SA=20831 Ci/mmol; n=64) and [¹¹C]DASB, both with (3.0% RCY; >95% RCP; SA=15152Ci/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. CONCLUSIONS: Ethanolic 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.).

Fully automated and reproducible radiosynthesis of high specific activity [¹¹C]raclopride and [¹¹C]Pittsburgh compound-B using the combination of two commercial synthesizers.

INTRODUCTION: The use of ¹¹C-labeled radiotracers in routine positron emission tomography studies is dependent on the production capability of radiochemistry laboratories. Therefore, considerable efforts are being focused on the development of fast, efficient, and robust methods for the preparation of such radiotracers. METHODS: The fully automated syntheses of [¹¹C]raclopride and [¹¹C]Pittsburgh compound-B (PIB) starting from cyclotron-produced [¹¹C]CH4 are reported. [¹¹C]methyl iodide and [¹¹C]methyl triflate were produced in the TRACERlab FXC Pro synthesis box. Methylation reactions and the final formulation were performed using the AutoLoop (captive solvent method) and the ReFORM-plus systems, respectively. RESULTS: [¹¹C]raclopride (n=30) and [¹¹C]PIB (n=24) were synthesized by O-[¹¹C]-methylation and N-[¹¹C]-methylation of (S)-O-desmethylraclopride and 6-OH-BTA-0 using [¹¹C]methyl iodide and [¹¹C]methyl triflate, respectively. Good radiochemical yields (51.3 ± 11.2 and 32.9 ± 6.6%, referred to as [¹¹C]methyl iodide, decay corrected) and specific activities (109 ± 20 and 143 ± 26 GBq/µmol) were obtained for [¹¹C]raclopride and [¹¹C]PIB, respectively, in a fully automated process. Radiochemical purity was higher than 99% in all cases. CONCLUSION: The fast, robust and fully automated processes reported here allow [¹¹C]raclopride and [¹¹C]PIB synthesis with good radiochemical yields and high specific activities. Consecutive productions can be performed with minimal intervention on the synthesis modules and minimal exposure to radiation.

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 [(11)C]methylation in the same module is described. By the quick setup and automated method, [(11)C]carfentanil and [(11)C]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 [(11)C]methyl iodide) for [(11)C]carfentanil and [(11)C]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 (11)C-labeled radiopharmaceuticals in a fast and reliable manner.

In vivo vulnerability to competition by endogenous dopamine: comparison of the D2 receptor agonist radiotracer (-)-N-[11C]propyl-norapomorphine ([11C]NPA) with the D2 receptor antagonist radiotracer [11C]-raclopride.

(-)-N-Propyl-norapomorphine (NPA) is a full dopamine (DA) D2 receptor agonist and [11C]NPA is a suitable radiotracer to image D2 receptors configured in a state of high affinity for agonists with positron emission tomography (PET). In this study the vulnerability of the in vivo binding of [11C]NPA to acute fluctuation in synaptic DA was assessed with PET in baboons and compared to that of the reference D2 receptor antagonist radiotracer [11C]raclopride. Three male baboons were studied with [11C]raclopride and [11C]NPA under baseline conditions and following administration of the potent DA releaser amphetamine (0.3, 0.5, and 1.0 mg kg(-1) i.v.). Kinetic modeling with an arterial input function was used to derive the striatal specific-to-nonspecific equilibrium partition coefficient (V3"). [11C]Raclopride V3" was reduced by 24 +/- 10%, 32 +/- 6%, and 44 +/- 9% following amphetamine doses of 0.3, 0.5, and 1.0 mg kg(-1), respectively. [11C]NPA V3" was reduced by 32 +/- 2%, 45 +/- 3%, and 53 +/- 9% following amphetamine doses of 0.3, 0.5, and 1.0 mg kg(-1), respectively. Thus, endogenous DA was more effective at competing with [11C]NPA binding compared to [11C]raclopride binding, a finding consistent with the pharmacology of these tracers (agonist vs. antagonist). These results also suggest that 71% of D2 receptors are configured in a state of high affinity for agonists in vivo. In conclusion, [11C]NPA might provide a superior radiotracer to probe presynaptic DA function with PET in health and disease.

A simple loop method for the automated preparation of (11C)raclopride from (11C)methyl triflate.

A simple automated preparation of [11C]raclopride by reaction of [11C]methyl triflate with demethylraclopride triflate is described. The conventional bubbling of [11C]methyl triflate into the precursor solution was compared with two alternative methods which used a commercially available C18 cartridge (on-column method) or an empty PTFE tube (loop method) as support for the precursor solution. The influence of several solvents was assessed for all three methods. The on-column method showed excellent trapping efficiencies of [11C]methyl triflate but gave the lowest radiochemical yields. The loop method proved to be a simplified alternative to the bubbling method, giving comparable radiochemical yields with less precursor and offering an easy way to transfer the reaction mixture into an HPLC column. By the simple-loop method [11C]raclopride could be prepared in over 40% radiochemical yields (decay-corrected and based on [11C]methyl triflate).

A Peltier thermal cycling unit for radiopharmaceutical synthesis.

We have investigated the use of Peltier devices to rapidly cycle the temperature of reaction vessels in a radiopharmaceutical synthesis system. Peltier devices have the advantage that they can be actively cooled as well as heated, allowing precise and rapid control of vessel temperatures. Reaction vessel temperatures of between -6 degrees C and 110 degrees C have been obtained with commercially available devices with reasonable cycle times. Two devices have been used as the basis for a general purpose, two-pot synthesis system for production of [11C] compounds such as raclopride.

Radiotracer synthesis from [(11)C]-iodomethane: a remarkably simple captive solvent method.

A new method of [(11)C]-methylation is described, which attains the goals of simplicity, high radiochemical yields, speed, versatility, and automation. A standard high performance liquid chromatography (HPLC) injection loop on a standard HPLC injection valve is loaded with a solution (80 microL) of precursor (0.3-1.0 mg) in dimethyl formamide (DMF) or dimethyl sulfoxide (DMSO) (+ base if required). At ambient temperature [(11)C]-iodomethane is passed through the loop for 3-4 min with >90% trapping of activity. After a further 1-5 min, the contents of the loop are quantitatively injected onto the HPLC column for purification. Radiochemical yields are equal to or superior to conventional solution methods in all cases, even though no heat is applied. [(11)C]-labeled radiotracers that have been prepared by this method for human or animal studies include Raclopride, N-methylspiperone, Ro 15-1788, FLB 457, RTI-32, Rolipram, SCH 23390, and SKF 82957. Since no vials, transfer lines, cooling, heating, or sealing valves are required, no transfer losses occur, yields are high, and cleanup is minimal, this "loop method" is ideal for most radiopharmaceuticals prepared from [(11)C]-iodomethane.