Highlighting the versatility of the tracerlab synthesis modules. Part 1: fully automated production of [18F]labelled radiopharmaceuticals using a Tracerlab FXFN

The field of radiochemistry is moving toward exclusive use of automated synthesis modules for production of clinical radiopharmaceutical doses. Such a move comes with many advantages, but also presents radiochemists with the challenge of re‐configuring synthesis modules for production of radiopharmaceuticals that require nonconventional radiochemistry while maintaining full automation. This review showcases the versatility of the Tracerlab FX_FN synthesis module by presenting simple, fully automated methods for producing [¹⁸F]FLT, [¹⁸F]FAZA, [¹⁸F]MPPF, [¹⁸F]FEOBV, [¹⁸F]sodium fluoride, [¹⁸F]fluorocholine and [¹⁸F]SFB. Copyright © 2011 John Wiley & Sons, Ltd.     

The automated radiosynthesis and purification of the opioid receptor antagonist, [6‐O‐methyl‐11C]diprenorphine on the GE TRACERlab FXFE radiochemistry module

[6‐O‐Methyl‐¹¹C]diprenorphine ([¹¹C]diprenorphine) is a positron emission tomography ligand used to probe the endogenous opioid system in vivo. Diprenorphine acts as an antagonist at all of the opioid receptor subtypes, that is, μ (mu), κ (kappa) and δ (delta). The radiosynthesis of [¹¹C]diprenorphine using [¹¹C]methyl iodide produced via the ‘wet’ method on a home‐built automated radiosynthesis set‐up has been described previously. Here, we describe a modified synthetic method to [¹¹C]diprenorphine performed using [¹¹C]methyl iodide produced via the gas phase method on a GE TRACERlab FX_FE radiochemistry module. Also described is the use of [¹¹C]methyl triflate as the carbon‐11 methylating agent for the [¹¹C]diprenorphine syntheses. [¹¹C]Diprenorphine was produced to good manufacturing practice standards for use in a clinical setting. In comparison to previously reported [¹¹C]diprenorphine radiosyntheisis, the method described herein gives a higher specific activity product which is advantageous for receptor occupancy studies. The radiochemical purity of [¹¹C]diprenorphine is similar to what has been reported previously, although the radiochemical yield produced in the method described herein is reduced, an issue that is inherent in the gas phase radiosynthesis of [¹¹C]methyl iodide. The yields of [¹¹C]diprenorphine are nonetheless sufficient for clinical research applications. Other advantages of the method described herein are an improvement to both reproducibility and reliability of the production as well as simplification of the purification and formulation steps. We suggest that our automated radiochemistry route to [¹¹C]diprenorphine should be the method of choice for routine [¹¹C]diprenorphine productions for positron emission tomography studies, and the production process could easily be transferred to other radiochemistry modules such as the TRACERlab FX C pro. © 2014 The Authors. Journal of Labelled Compounds and Radiopharmaceuticals Published by John Wiley & Sons Ltd.     

Fully automated,high yielding production of N‐succinimidyl 4‐[18F]fluorobenzoate ([18F]SFB), and its use in microwave‐enhanced radiochemical coupling reactions

A General Electric Medical Systems (GEMS) Tracerlab FX_FN fluorine‐18 synthesis module has been reconfigured to allow rapid (45 min), fully automated production of N‐succinimidyl 4‐[¹⁸F]fluorobenzoate ([¹⁸F]SFB) using the established three‐step, one‐pot synthesis procedure. Purification is by sep‐pak only and [¹⁸F]SFB is routinely obtained in 38% non‐decay corrected yield,>1 Ci/µmol specific activity, and >95% radiochemical purity (n=20). Moreover, this report includes our preliminary research efforts into improving peptide coupling reactions with [¹⁸F]SFB using microwave‐enhanced radiochemistry. Reaction times can be reduced by>90%, when compared with traditional thermal reactions, with no significant effect on radiochemical reaction yield. Copyright © 2010 John Wiley & Sons, Ltd.     

Development of a fully automated low-pressure [11C]CO carbonylation apparatus

[¹¹C]carbon monoxide ([¹¹C]CO) is a versatile synthon for radiolabeling of drug-like molecules for imaging studies with positron emission tomography (PET). We here report the development of a novel, user-friendly, fully automated, and good manufacturing practice (GMP) compliant low-pressure synthesis module for ¹¹C-carbonylation reactions using [¹¹C]CO. In this synthesis module, [¹¹C]CO was reliably prepared from cyclotron-produced [¹¹C]carbon dioxide ([¹¹C]CO₂) by reduction over heated molybdenum and delivered to the reaction vessel within 7 min after end of bombardment, with an overall radiochemical yield (RCY) of 71%. [¹¹C]AZ13198083, a histamine type-3 receptor ligand, was used as a model compound to assess the functionality of the radiochemistry module. At full batch production conditions (55 μA, 30 min), our newly developed low-pressure ¹¹C-carbonylation apparatus enabled us to prepare [¹¹C]AZ13198083 in an isolated radioactivity of 8540 ± 1400 MBq (n = 3). The radiochemical purity of each of the final formulated batches exceeded 99%, and all other quality control tests results conformed with specifications typically set for carbon-11 labeled radiopharmaceuticals. In conclusion, this novel radiochemistry system offers a convenient GMP compliant production drugs and radioligands for imaging studies in human subjects.     

Synthesis of (±) 3‐(6‐nitro‐2‐quinolinyl)‐[9‐methyl‐11C]‐3,9‐diazabicyclo‐[4.2.1]‐nonane ([11C‐methyl]NS 4194)

(±) 3‐(6‐Nitro‐2‐quinolinyl)‐[9‐methyl‐¹¹C]‐3,9‐diazabicyclo‐[4.2.1]‐nonane ([¹¹C‐methyl]NS 4194), a selective serotonin reuptake inhibitor (SSRI), was synthesised within 35 min after end of bombardment with a radiochemical purity >98%. It had a decay‐corrected radiochemical yield of 7% after preparative HPLC, and a specific radioactivity around 37 GBq/μmol (EOS). A typical production starting with 40 GBq [¹¹C]CO₂ yielded 800 MBq of radiolabelled [¹¹C‐methyl]NS 4194 in a formulated solution. The synthesis of the precursor to [¹¹C‐methyl]NS 4194, (±) 9‐H‐3‐[6‐nitro‐(2‐quinolinyl)]‐3,9‐diazabicyclo‐[4.2.1]‐nonane, as well as the unlabelled analogue (±) 9‐methyl 3‐[6‐nitro‐(2‐quinolinyl)]‐3,9‐diazabicyclo‐[4.2.1]‐nonane (NS 4194), are also described. Copyright © 2002 John Wiley & Sons, Ltd.     

Preparation of [11C]methyl nona‐fluorobutyl‐1‐sulfonate ([11C]MeONf) and its use in the synthesis of [11C]‐6‐OH‐BTA‐1

The rapid, simple and high‐yield synthesis of the extraordinarily reactive ¹¹C‐methylating agent, [¹¹C]methyl nona‐fluorobutyl‐1‐sulfonate ([¹¹C]MeONf), and its use in the synthesis of the promising β‐amyloid imaging agent, [¹¹C]‐6‐OH‐BTA‐1, is reported. In terms of radioactive methylation yields, [¹¹C]MeONf seems to surpass [¹¹C]methyl trifluoromethansulfonate ([¹¹C]MeOTf) as a methylating agent in this particular case giving the ¹¹C‐labelled compound in high‐preparative radiochemical yields between 27 and 29% EOS with a minimum formation of radioactive by‐products. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis of [O‐methyl‐11C]‐4‐(1,3‐dimethoxy‐2‐propylamino)‐2,7‐dimethyl‐8‐(2,4‐dichlorophenyl)[1,5‐a]pyrazolo‐1,3,5‐triazine ([11C]DMP696): a potential PET ligand for CRF1 receptors

Synthesis of [O‐methyl‐¹¹C]‐4‐(1,3‐dimethoxy‐2‐propylamino)‐2,7‐dimethyl‐8‐(2,4‐dichlorophenyl)[1,5‐a]pyrazolo‐1,3,5‐triazine ([¹¹C]DMP696), a highly selective CRF₁ antagonist has been achieved. The total time required for the synthesis of [¹¹C]DMP696 is 30 min from EOB using [¹¹C]methyl triflate in THF, with a 16% yield (EOS) and >99% chemical and radiochemical purities along with a specific activity of >2000 Ci/mmol (EOS). Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

Radiosynthesis of [N‐methyl‐11C]methylene blue

In this paper we present the radiochemical synthesis of the novel compound [N‐methyl‐¹¹C]methylene blue. The synthesis of [N‐methyl‐¹¹C]methylene blue was accomplished by means of ¹¹C‐methylation of commercially available Azure B using [¹¹C]methyl trifluoromethanesulfonate ([¹¹C]methyl triflate). Following purification [N‐methyl‐¹¹C]methylene blue was obtained with a radiochemical purity greater than 97% in a 4–6% decay corrected radiochemical yield. The synthesis was completed in an average of 35 min following the end of bombardment. Copyright © 2003 John Wiley & Sons, Ltd.     

Fine‐tuning of the automated [18F]PSMA‐1007 radiosynthesis

Radiolabeled prostate‐specific membrane antigen (PSMA) targeting PET‐tracers have become desirable radiopharmaceuticals for the imaging of prostate cancer (PC). Recently, the PET radiotracer [¹⁸F]PSMA‐1007 was introduced as an alternative to [⁶⁸Ga]Ga‐PSMA‐11, for staging and diagnosing biochemically recurrent PC. We incorporated a one‐step procedure for [¹⁸F]PSMA‐1007 radiosynthesis, using both Synthra RNplus and GE TRACERlab FxFN automated modules, in accordance with the recently described radiolabeling procedure. Although the adapted [¹⁸F]PSMA‐1007 synthesis resulted in repeatable radiochemical yields (55 ± 5%, NDC), suboptimal radiochemical purities of 87 ± 8% were obtained using both modules. As described here, modifications made to the radiolabeling and the solid‐phase extraction purification steps reduced synthesis time to 32 minutes and improved radiochemical purity to 96.10%, using both modules, without shearing the radiochemical yield.     

Automated radiosynthesis of no‐carrier‐added 4‐[18F]fluoroiodobenzene: a versatile building block in 18F radiochemistry

4‐[¹⁸F]Fluoroiodobenzene ([¹⁸F]FIB) is a versatile building block in ¹⁸F radiochemistry used in various transition metal‐mediated C–C and C–N cross‐coupling reactions and [¹⁸F]fluoroarylation reactions. Various synthesis routes have been described for the preparation of [¹⁸F]FIB. However, to date, no automated synthesis of [¹⁸F]FIB has been reported to allow access to larger amounts of [¹⁸F]FIB in high radiochemical and chemical purity. Herein, we describe an automated synthesis of no‐carrier‐added [¹⁸F]FIB on a GE TRACERlab? FX automated synthesis unit starting from commercially available (4‐iodophenyl)diphenylsulfonium triflate as the labelling precursor. [¹⁸F]FIB was prepared in high radiochemical yields of 89 ± 10% (decay‐corrected, n = 7) within 60 min, including HPLC purification. The radiochemical purity exceeded 95%, and specific activity was greater than 40 GBq/µmol. Typically, from an experiment, 6.4 GBq of [¹⁸F]FIB could be obtained starting from 10.4 GBq of [¹⁸F]fluoride. Copyright © 2013 John Wiley & Sons, Ltd.     

A combined loop‐SPE method for the automated preparation of [11C]doxepin

A simple and versatile loop‐solid phase extraction (SPE) method was developed for the automated preparation of [¹¹C]doxepin, a histamine H₁ receptor antagonist, from [¹¹C]methyl triflate ([¹¹C]MeOTf). This labeling agent was passed through a Teflon or Tefzel loop coated internally with a film of the precursor solution. The reaction products were then flushed from the loop to a short SPE column, where they were concentrated and then injected onto a semi‐preparative HPLC column simply by switching an injection valve. By applying this combined loop‐SPE technique the whole procedure turned out to be easily automated. The formation of [¹¹C]methylated doxepin ([¹¹C]methyldoxepin) was observed and the ratio of doxepin to methyldoxepin was found to be clearly correlated with the mass ratio of nordoxepin to MeOTf. This observation highlights the importance of [¹¹C]MeOTf specific activity in the [¹¹C]methylation of secondary amines. Using this method, [¹¹C]Doxepin was prepared in over 40% radiochemical yield from high specific activity [¹¹C]MeOTf. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis and biological evaluation of carbon‐11‐labeled acyclic and furo[2,3‐d]pyrimidine derivatives of bicyclic nucleoside analogues (BCNAs) for structure–brain uptake relationship study of BCNA tracers

We reported earlier on radiolabeled alkoxyphenyl bicyclic nucleoside analogues (BCNAs) as potential positron emission tomography (PET) reporter probes for imaging of varicella zoster virus thymidine kinase (VZV‐tk) gene in vivo. Despite their favorable physicochemical properties, these tracers are not taken up in the brain in mice. In order to probe the role of the deoxyribose sugar moiety in blood‐brain barrier (BBB) penetration of these molecules, we have synthesized and evaluated a carbon‐11‐labeled acyclic bicyclic nucleoside derivative ([¹¹C]‐10) where the 2′‐deoxyribose sugar is replaced with a (2‐hydroxyethoxy)methyl group and [¹¹C]‐12, which has no sugar moiety but a [¹¹C]methyl group on the N‐3 position of the pyrimidine ring. Methylation was achieved on the phenol ([¹¹C]‐10) or the N‐3 position ([¹¹C]‐12) using [¹¹C]methyl triflate (radiosynthesis). The (non‐radioactive) acyclic O‐methyl derivative 10 has rather poor affinity for the enzyme VZV‐TK in vitro (IC₅₀: 430 µM), compared with the moderate affinity of the BCNA‐base N‐methyl derivative 12 (IC₅₀: 79 µM). In normal mice, none of the two tracers ([¹¹C]‐10 or [¹¹C]‐12) showed significant uptake in the brain, suggesting that compounds containing a furo[2,3‐d]pyrimidine system do not cross the BBB. Copyright © 2008 John Wiley & Sons, Ltd.     

Gas phase production of [11C]methyl iodide‐d3. Synthesis and biological evaluation of S‐[N‐methyl‐11C]citalopram and deuterated analogues

Three ¹¹C‐labelled tracers for the serotonin reuptake site, S‐[N‐methyl‐¹¹C]citalopram ( [¹¹C]‐4 ), S‐[N‐methyl‐d₃‐¹¹C]citalopram ( [¹¹C]‐12 ), and S‐[N‐methyl‐¹¹C]citalopram‐α,α‐d₂ ( [¹¹C]‐13 ) were synthesized and the distribution of radioactivity after injection of radioligand was examined ex vivo in rats. The deuterated analogue of (S)‐desmethylcitalopram, (S)‐1‐(4‐fluorophenyl)‐1‐(3‐methylamino‐[3‐d₂]‐propyl)‐1,3‐di‐hydro‐isobenzofuran‐5‐carbonitrile ( 11 ), was synthesized in a multi‐step synthesis from escitalopram ( 4 ) and used as precursor in the synthesis of [¹¹C]‐13 . In analogy with the reported gas phase synthesis of [ ¹¹ C]methyl iodide the first gas phase synthesis of [¹¹C]Methyl iodide‐d₃ is reported. The ¹H/²H kinetic isotope effect related to the synthesized compounds were investigated in ex vivo rat studies, where the brain regions of interest to cerebellum ratios of the tracers [¹¹C]‐4 , [¹¹C]‐12 and [¹¹C]‐13 were compared. The ex vivo data indicated no significant differences in binding in any of the investigated brain regions after injection of the three tracers. Copyright © 2004 John Wiley & Sons, Ltd.     

Synthesis of radiolabeled O6‐benzylguanine derivatives as new potential PET tumor imaging agents for the DNA repair protein O6‐alkylguanine‐DNA alkyltransferase

Novel radiolabeled O⁶‐benzylguanine derivatives, 2‐amino‐6‐O‐[¹¹C]‐[(methoxymethyl)benzyloxy]‐9‐benzyl purines ([¹¹C]p‐O⁶‐AMBP, 1a ; [¹¹C]m‐O⁶‐AMBP, 1b ; [¹¹C]o‐O⁶‐AMBP, 1c ), have been synthesized for evaluation as new potential positron emission tomography (PET) tumor imaging agents for the DNA repair protein, O⁶‐alkylguanine‐DNA alkyltransferase (AGT). The appropriate precursors for radiolabeling were obtained in three steps from starting material 2‐amino‐6‐chloropurine with moderate to excellent chemical yields. Tracers were prepared by O‐[¹¹C]methylation of hydroxymethyl precursors using [¹¹C]methyl triflate. Pure target compounds were isolated by solid‐phase extraction (SPE) purification procedure in 45–60% radiochemical yields (decay corrected to the end of bombardment), and a synthesis time of 20–25min. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis of 1‐(2′‐deoxy‐2′‐fluoro‐β‐D‐arabinofuranosyl)‐[Methyl‐11C]thymine ([11C]FMAU) via a Stille cross‐coupling reaction with [11C]methyl iodide

1‐(2′‐deoxy‐2′‐fluoro‐β‐D‐arabinofuranosyl)‐[methyl‐¹¹C]thymine ([¹¹C]FMAU) [¹¹C]‐ 1 was synthesised via a palladium‐mediated Stille coupling reaction of 1‐(2′‐deoxy‐2′‐fluoro‐β‐D‐arabinofuranosyl)‐5‐(trimethylstannyl)uracil 2 with [¹¹C]methyl iodide in a one‐pot procedure. The reaction conditions were optimized by screening various catalysts and solvents, and by altering concentrations and reaction temperatures. The highest yield was obtained using Pd₂(dba)₃ and P(o‐tolyl)₃ in DMF at 130°C for 5 min. Under these conditions the title compound [¹¹C]‐ 1 was obtained in 28±5% decay‐corrected radiochemical yield calculated from [¹¹C]methyl iodide (number of experiments=7). The radiochemical purity was >99% and the specific radioactivity was 0.1 GBq/μmol at 25 min after end of bombardment. In a typical experiment 700–800 MBq of [¹¹C]FMAU [¹¹C]‐ 1 was obtained starting from 6–7 GBq of [¹¹C]methyl iodide. A mixed ¹¹C/¹³C synthesis to yield [¹¹C]‐ 1 /(¹³C)‐ 1 followed by ¹³C‐NMR analysis was used to confirm the labelling position. The labelling procedure was found to be suitable for automation. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis of 11C‐labelled metomidate analogues as adrenocortical imaging agents

Clinical findings using [¹¹C]methyl 1‐[(1R)‐1‐phenylethyl]‐1H‐imidazole‐5‐carboxylate ([¹¹C]MTO, 1) show high uptake in lesions of adrenocortical origin, including adenomas, but low uptake in lesions of non‐adrenocortical origin. In this paper the synthesis and preclinical evaluation of two new ¹¹C‐labelled analogues of MTO, [¹¹C]methyl 1‐[(1R)‐1‐(4‐chlorophenyl)ethyl]‐1H‐imidazole‐5‐carboxylate ([¹¹C]CLM, 2) and [¹¹C]methyl 1‐[(1R)‐1‐(4‐bromophenyl)ethyl]‐1H‐imidazole‐5‐carboxylate ([¹¹C]BRM, 3), using frozen‐section autoradiography, organ distribution and a metabolic study are presented. Copyright © 2008 John Wiley & Sons, Ltd.     

Improved synthesis of 4‐[18F]fluoro‐m‐hydroxyphenethylguanidine using an iodonium ylide precursor

Fluorine‐18 labeled hydroxyphenethylguanidines were recently developed in our laboratory as a new class of PET radiopharmaceuticals for quantifying regional cardiac sympathetic nerve density in heart disease patients. Studies of 4‐[¹⁸F]fluoro‐m‐hydroxyphenethylguanidine ([¹⁸F]4F‐MHPG) and 3‐[¹⁸F]fluoro‐p‐hydroxyphenethylguanidine ([¹⁸F]3F‐PHPG) in human subjects have shown that these radiotracers can be used to generate high‐resolution maps of regional sympathetic nerve density using the Patlak graphical method. Previously, these compounds were synthesized using iodonium salt precursors, which provided sufficient radiochemical yields for on‐site clinical PET studies. However, we were interested in exploring new methods that could offer significantly higher radiochemical yields. Spirocyclic iodonium ylide precursors have recently been established as an attractive new approach to radiofluorination of electron‐rich aromatic compounds, offering several advantages over iodonium salt precursors. The goal of this study was to prepare a spirocyclic iodonium ylide precursor for synthesizing [¹⁸F]4F‐MHPG and evaluate its efficacy in production of this radiopharmaceutical. Under optimized automated reaction conditions, the iodonium ylide precursor provided radiochemical yields averaging 7.8% ± 1.4% (n = 8, EOS, not decay corrected), around threefold higher than those achieved previously using an iodonium salt precursor. With further optimization and scale‐up, this approach could potentially support commercial distribution of [¹⁸F]4F‐MHPG to PET centers without on‐site radiochemistry facilities.     

HPLC methods for the purification of [11C]‐labelled radiopharmaceuticals: reversal of the retention order of products and precursors

Preparative HPLC methods have been developed for a number of [¹¹C]‐methylated PET tracers, which enable elution of the labelled compounds prior to their precursors, thus reducing the overall synthesis time and avoiding contamination of the final product with precusor. This reversal of retention order has been achieved for [¹¹C]DASB, [¹¹C]raclopride, [¹¹C]FLB 457, [¹¹C]carfentanil, and 2‐fluoro‐[N‐methyl‐¹¹C]apomorphine, enabling collection of the purified radiopharmaceuticals from the HPLC system after 5–7 min. Furthermore, by using ethanol as the organic modifier, residual solvent analysis prior to human injection could be avoided and three of the radiopharmaceuticals could be injected directly following simple dilution and sterile filtration. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis of [11C]gefitinib for imaging epidermal growth factor receptor tyrosine kinase with positron emission tomography

We have synthesized N‐(3‐chloro‐4‐fluorophenyl)‐7‐[¹¹C]methoxy‐6‐[3‐(morpholin‐4‐yl)propoxy]quinazolin‐4‐amine, [¹¹C]gefitinib ([¹¹C]Iressa), a high affinity (IC₅₀ = 2 nM) inhibitor of the epidermal growth factor receptor tyrosine kinase (EGFR‐TK), in solution and in a semi‐automated stainless loop methylation system using [¹¹C]methyl triflate. The trapping efficiency for [¹¹C]methyl triflate in solution was higher than in the solvent film generated in the loop system, thus the overall radiochemical yield was considerably higher for the synthesis in solution. The average radiochemical yield for the solution chemistry was 15% with an average specific radioactivity of approximately 9000 mCi/µmole at EOS in one step from its corresponding desmethyl phenol precursor. Copyright © 2006 John Wiley & Sons, Ltd.