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We report an efficient carbon-11 radiolabeling technique to produce clinically relevant tracers for Positron Emission Tomography (PET) using solid phase extraction cartridges. 11C-methylating agent is passed through a cartridge preloaded with precursor and successive elution with aqueous ethanol provides chemically and radiochemically pure PET tracers in high radiochemical yields.
Routine production of radiotracers used in positron emission tomography (PET) mostly relies on wet chemistry where the radioactive synthon reacts with a non-radioactive precursor in solution. This approach necessitates purification of the tracer by high performance liquid chromatography (HPLC) followed by reformulation in a biocompatible solvent for human administration. We recently developed a novel 11C-methylation approach for the highly efficient synthesis of carbon-11 labeled PET radiopharmaceuticals, taking advantage of solid phase cartridges as disposable "3-in-1" units for the synthesis, purification and reformulation of the tracers. This approach obviates the use of preparative HPLC and reduces the losses of the tracer in transfer lines and due to radioactive decay. Furthermore, the cartridge-based technique improves synthesis reliability, simplifies the automation process and facilitates compliance with the Good Manufacturing Practice (GMP) requirements. Here, we demonstrate this technique on the example of production of a PET tracer Pittsburgh compound B ([11C]PiB), a gold standard in vivo imaging agent for amyloid plaques in the human brains.
Positron emission tomography (PET) is a molecular imaging modality which relies on detecting the radioactive decay of an isotope attached to a biologically active molecule to enable the in vivo visualization of biochemical processes, signals and transformations. Carbon-11 (t1/2 = 20.3 min) is one of the most commonly used radioisotopes in PET because of its abundance in organic molecules and short half-life which allows for multiple tracer administrations on the same day to the same human or animal subject and reduces the radiation burden on the patients. Many tracers labeled with this isotope are used in clinical studies and in basic health research for in vivo PET imaging of classical and emerging biologically relevant targets - [11C]raclopride for D2/D3 receptors, [11C]PiB for amyloid plaques, [11C]PBR28 for translocator protein - to name just a few.
Carbon-11 labeled PET tracers are predominantly produced via 11C-methylation of non-radioactive precursors containing -OH (alcohol, phenol and carboxylic acid), -NH (amine and amide) or -SH (thiol) groups. Briefly, the isotope is generated in the gas target of a cyclotron via a 14N(p,α)11C nuclear reaction in the chemical form of [11C]CO2. The latter is then converted into [11C]methyl iodide ([11C]CH3I) via either wet chemistry (reduction to [11C]CH3OH with LiAlH4 followed by quenching with HI)1 or dry chemistry (catalytic reduction to [11C]CH4 followed by radical iodination with molecular I2)2. [11C]CH3I can then be further converted to the more reactive 11C-methyl triflate ([11C]CH3OTf) by passing it over a silver triflate column3. The 11C-methylation is then performed by either bubbling the radioactive gas into a solution of non-radioactive precursor in organic solvent or via the more elegant captive solvent "loop" method4,5. The 11C-tracer is then purified by means of HPLC, reformulated in a biocompatible solvent, and passed through a sterile filter before being administered to human subjects. All of these manipulations must be fast and reliable given the short half-life of carbon-11. However, the use of an HPLC system significantly increases the losses of the tracer and production time, often necessitates the use of toxic solvents, complicates automation and occasionally leads to failed syntheses. Furthermore, the required cleaning of the reactors and HPLC column prolongs delays between the syntheses of subsequent tracer batches and increases the exposure of personnel to radiation.
The radiochemistry of fluorine-18 (t1/2 = 109.7 min), the other widely used PET isotope, has been recently advanced via the development of cassette-based kits that obviate the need for HPLC purification. By employing solid phase extraction (SPE) cartridges, these fully disposable kits allow for the reliable routine production of 18F-tracers, including [18F]FDG, [18F]FMISO, [18F]FMC and others, with shorter synthesis times, reduced personnel involvement and minimal maintenance of the equipment. One of the reasons carbon-11 remains a less popular isotope in PET imaging is a lack of similar kits for the routine production of 11C-tracers. Their development would significantly improve synthetic reliability, increase radiochemical yields and simplify automation and preventive maintenance of the production modules.
Currently available production kits take advantage of inexpensive, disposable, SPE cartridges instead of HPLC columns for the separation of the radiotracer from unreacted radioactive isotope, precursor and other radioactive and non-radioactive by-products. Ideally, the radiolabeling reaction also proceeds on the same cartridge; for example, the [18F]fluoromethylation of dimethylaminoethanol with gaseous [18F]CH2BrF in the production of prostate cancer imaging PET tracer [18F]fluoromethylcholine occurs on a cation-exchange resin cartridge6. Although similar procedures for the radiolabeling of several 11C-tracers on cartridges have been reported7,8 and became especially powerful for the radiosynthesis of [11C]choline9 and [11C]methionine10, these examples remain limited to oncological PET tracers where the separation from the precursor is often not required. We recently reported the development of "[11C]kits" for the production of [11C]CH3I11 and subsequent 11C-methylation, as well as solid phase-supported synthesis12 in our endeavours to simplify the routine production of 11C-tracers. Here, we wish to demonstrate our progress using the example of the solid phase supported radiosynthesis of [11C]PiB, a radiotracer for Aβ imaging which revolutionized the field of Alzheimer's disease (AD) imaging when it was first developed in 2003 (Figure 1)13,14. In this method, volatile [11C]CH3OTf (bp 100 °C) is passed over 6-OH-BTA-0 precursor deposited on the resin of a disposable cartridge. PET tracer [11C]PiB is then separated from the precursor and radioactive impurities by elution from the cartridge with biocompatible aqueous ethanol. Further, we automated this method of [11C]PiB radiosynthesis using a remotely operated radiochemistry synthesis module and disposable cassette kits. Specifically, we implemented this radiosynthesis on a 20-valve radiochemistry module, equipped with syringe drive (dispenser) which fits standard 20 mL disposable plastic syringe, gas flow controller, vacuum pump and gauge. Due to the simplicity of this method, we are confident that it can be modified to most commercially available automated synthesizers, either cassette-based or those equipped with stationary valves. This solid phase supported technique facilitates [11C]PiB production compliant with Good Manufacturing Practice (GMP) regulations and improves synthesis reliability. The technique described here also reduces the amount of precursor required for radiosynthesis, uses only "green" biocompatible solvents and decreases the time between consecutive production batches.
1. Preparation of buffers and eluents
2. Application of the precursor to the cartridge
3. Setting up the manifold for automated synthesis
4. Radiosynthesis of [11C]PiB
CAUTION: All manipulations involving radioactive isotopes must be performed in a lead-shielded hot cell by personnel with adequate training to work with radioactive materials.
NOTE: This protocol does not cover the details of production of [11C]CO2 in the cyclotron and its conversion into [11C]CH3OTf using the radiochemistry module. These procedures will depend on the individual equipment of the radiochemistry lab and are outside the scope of this protocol. Our PET centre is equipped with an IBA cyclotron, which produces carbon-11 in the chemical form of [11C]CO2 via the 14N(p,α)11C nuclear reaction with a N2/O2 gas mixture (99.5:0.5) in the gas target, and a commercially available module for production of [11C]CH3I via the "dry method" (catalytic reduction to [11C]CH4 followed by radical iodination). [11C]CH3OTf is produced by passing [11C]CH3I over a silver triflate column heated to 175 °C at 20 mL/min.
5. Quality control procedures
CAUTION: Each batch of the radiotracer must be subjected to the appropriate quality control procedures (QC) prior to release to the PET imaging site for administration into human or animal subjects. The authors of this manuscript are not responsible for the compliance of the radiotracer produced at other centers with local health authority regulations.
To summarize a typical radiosynthesis of [11C]PiB, gaseous [11C]CH3OTf is first passed through a tC18 cartridge preloaded with a solution of precursor (Figure 1). Separation of the reaction mixture is then achieved by successive elution with aqueous ethanol solutions as follows. First, 12.5% EtOH elutes the majority of unreacted [11C]CH3OTf and 6-OH-BTA-0, then 15% EtOH washes out the residual impurities,...
Despite the recent emergence and FDA approval of several 18F-labeled PET tracers, such as florbetapir, florbetaben and flutemetamol, [11C]PiB remains a gold standard tracer for amyloid imaging due to the fast brain uptake and low non-specific binding. Currently this tracer is synthesized via either wet chemistry16 or using a "dry loop" approach4,17. Both methods require HPLC purification followed by r...
The authors declare that they have no competing financial interests.
This study was partially supported by a grant 18-05 from the Alzheimer’s Society of Canada (for A. K.) and Brain Canada Foundation with support from Health Canada. The authors would like to acknowledge the McGill University Faculty of Medicine, Montreal Neurological Institute and McConnell Brain Imaging Centre for support of this work. We also thank Mrs. Monica Lacatus-Samoila for help with quality control procedures and Drs. Jean-Paul Soucy and Gassan Massarweh for access to radioisotopes and the radiochemistry facility.
Name | Company | Catalog Number | Comments |
6-OH-BTA-0 | ABX advanced biochemical compounds | 5101 | Non-radioactive precursor of [11C]PiB |
6-OH-BTA-1 | ABX advanced biochemical compounds | 5140 | Non-radioactive standard of [11C]PiB |
Agilent 1200 HPLC system | Agilent | Agilent 1200 | Analytical HPLC system |
Ethanol absolute | Commercial alcohols | 432526 | |
Hamilton syringe (luer-tip, 250 µL) | Hamilton | HAM80701 | |
MZ Analytical PerfectSil 120 | MZ-Analysentechik GmbH | MZ1440-100040 | Analytical HPLC column |
Perkin Elmer Clarus 480 GC system | Perkin Elmer | Clarus 480 | Gas chromotograph |
polycarbonate manifold | Scintomics | ACC-101 | Synthesis manifold |
Restek MTX-Wax column (30 m, 0.53 mm) | Restek | 70625-273 | Analytical GC column |
Scintomics GRP module | Scintomics | Scintomics GRP | Automated synthesis unit |
Sep-Pak tC18 Plus | Waters | WAT020515 | Solid phase extraction cartridge |
solvent-resistant manifold | Scintomics | ACC-201 | Synthesis manifold |
Spinal needle | BD | 405181 | |
Sterile extension line | B. Braun | 8255059 | |
Sterile filter | Millipore | SLLG013SL | |
Sterile vial (20mL) | Huayi | SVV-20A | |
Sterile water | Baxter | JF7623 | |
Synthra MeIplus Research | Synthra | MeIplus Research | [11C]CH3I/[11C]CH3OTf module |
Syringe (10 mL) | BD | 309604 | |
Syringe (1mL) | BD | 309659 | |
Syringe (20 mL) | B. Braun | 4617207V | Dispenser syringe |
Vent filter | Millipore | TEFG02525 |
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