We demonstrate the semi-automated radiochemical synthesis of [18F]3F4AP and quality control procedures.
3-[18F]fluoro-4-aminopyridine, [18F]3F4AP, is a radiofluorinated analog of the FDA-approved drug for multiple sclerosis 4-aminopyridine (4AP). This compound is currently under investigation as a PET tracer for demyelination. We recently described a novel chemical reaction to produce metafluorinated pyridines consisting of direct fluorination of a pyridine N-oxide and the utilization of this reaction for the radiochemical synthesis of [18F]3F4AP. In this article, we demonstrate how to produce this tracer using an automated synthesizer and an in-house made flow hydrogenation reactor. We also show the standard quality control procedures performed before releasing the radiotracer for preclinical animal imaging studies. This semi-automated procedure may serve as the basis for future production of [18F]3F4AP for clinical studies.
The ability to trace a small-molecule drug non-invasively within the human body has great potential towards precision medicine. Among molecular imaging techniques, positron emission tomography (PET) has many favorable characteristics: the high sensitivity of PET detectors allows detection and quantification of very small amounts of radioactive material and the characteristics of the scanners allow accurate spatial mapping of the drug localization1,2,3. For example, PET allows detection and localization of tumors and metastasis based on the level of uptake of a radioactive glucose analog, [18F]FDG4. PET can also provide localization and quantification of specific brain receptors and their occupancy which can be valuable for diagnosing and understanding neurological and psychiatric disorders5. In order to develop a small molecule PET tracer, the compound of interest must be labeled with a positron-emitting isotope, typically 11C or 18F. Between these two radioisotopes, 18F has a longer half-life (109 min vs. 20.3 for 11C), which allows multi-dose and offsite production. Nevertheless, adding 18F to a molecule can be challenging. 18F labeling requires fast reactions compatible with automation relieving the chemist of direct handling of the activity and receiving high-absorbed radiation doses.
We recently described the use of pyridine N-oxides as precursors for the fluorination of pyridines and the use of this chemistry in the radiochemical synthesis of [18F]3F4AP6, a radiofluorinated analog of the FDA-approved drug for multiple sclerosis, 4-aminopyridine (4AP)7,8,9. This novel radiotracer is currently under investigation as a PET tracer for demyelination10,11,12. In this video article, we demonstrate the semi-automated synthesis of this compound using an IBA Synthera Synthesis Unit (henceforth referred to as "the synthesizer") and an in-house made flow hydrogenation device. The synthesis is based on the reaction shown in Figure 1. Preparation for the procedure takes approximately 1 h, radiolabeling and purification 1.5 h and quality control procedures 0.5 h.
CAUTION: All procedures involving the use of radioactive materials must be approved by the local Office of Radiation Safety. When working with radioactive materials wear a lab coat and personal radiation badges. Use two layers of gloves at all times and check hands with a Geiger counter after each step that involves handling radioactivity. If the gloves are contaminated with radioactivity discard and replace outer gloves. Use appropriate shielding, minimize time in contact with the radiation source and maximize distance.
1. One Week before Experiment: Preparation of Materials
2. Day of Experiment: Before Arrival of Fluorine-18
3. Day of Experiment: 18F Labeling
4. Day of Experiment: Hydrogenation
CAUTION: Injection of the product into the hydrogenator has to be done using proper shielding precautions. Hydrogen gas must be properly handled and vented.
NOTE: the hydrogenation reactor can be connected in place of the HPLC column on the synthesizer and controlled using the synthesizer software.
5. Day of Experiment: Purification and Preparation of the Dose
6. Day of Experiment: Quality Control (QC) Tests
7. Day of Experiment: Calculations (Table 1)
The radiochemical synthesis of [18F]3F4AP comprises two steps (Figure 1). The first step is carried out in a fully automated fashion using the synthesis unit (Figure 3). This cassette-based system uses four reagent vials and one reactor vial and has computer-controlled valves that allow transfer and mixing of reagents as well as heating, pressurizing and evacuating the reactor. In addition, it supports standard solid-phase extraction cartridges for separation of reagents. The computer interface allows users to write and modify scripts in order to run their own syntheses. In the case of [18F]3F4AP, the synthesis procedure is comprised of five basic parts. In the first part, the synthesizer performs self-check steps, preheats the reactor and waits for operator's signal that the 18F is ready. During the second part, the [18F]fluoride is transferred from the 18F vial into the anion exchange cartridge and eluted from the cartridge into the reactor using a solution tetrabutyl ammonium bicarbonate. The third part, the synthesizer azeotropically dries the [18F]fluoride under vacuum to make it reactive towards nucleophilic displacement. In the fourth part, the precursor is automatically added to the reactor where it reacts with the 18F– to generate the labeled compound. Finally, the reaction is quenched by the addition of 0.2% oxalic acid in methanol, which prevents base-promoted decomposition of the product, and the final solution is pressure-transferred to the collection vial after passing through an alumina-N cartridge that traps any unreacted fluoride.
After the labeling step is completed a small sample can be taken for quality control. Running a sample on the HPLC provides confirmation that the labeling step worked and an estimation of the radiochemical purity (Figure 4). Also, from the UV trace on the HPLC the mass amount of product can be calculated using a pre-established calibration curve.
While the in-process quality control HPLC is running, the second reaction step, reduction of the N-oxide and nitro groups, is performed. In order to do this, the labeled product is automatedly injected into an in-house hydrogenation device based on the method published by Yoswathananont et al.13 (Figure 2). This device consists of an HPLC pump and a compressed hydrogen tank connected to the flow hydrogenation device through lines equipped with check valves to prevent back-streaming. The product is pushed by the HPLC pump and mixed with hydrogen in a T-shaped mixer. This mixture is then passed through a small cartridge containing 10% Pd/C catalyst on a solid support. After passing through the catalyst the reduced product is then collected in small fractions.
Following hydrogenation, the crude product is transported and manually injected into the HPLC for purification of the final product (Figure 5). The mobile phase of the HPLC has been selected to be compatible with animal injection. The peaks corresponding to the product are then collected and filtered-sterilized to obtain the final dose.
Prior to releasing the dose for PET imaging studies, quality control tests are performed. These tests are performed to ensure that the tracer is the chemical entity that it is supposed to be and that it is safe for injection. Some of these tests may not be required for injection into animals but it is generally recommended to follow the human use guidelines. Doing so ensures quality of the product, which increases confidence in the results and greatly facilitates future transition to manufacturing the product for human injection.
Table 1 contains the typical synthesis parameters including initial amount radioactivity, initial amount of precursor, yield for each step, specific activity, filtering loses, etc. These parameters are useful troubleshooting occasional failures and future optimization of the procedure.
Figure 1. Reaction scheme. Radiochemical synthesis consists of labeling by 19F/18F exchange followed by palladium-catalyzed hydrogenation. Please click here to view a larger version of this figure.
Figure 2. Hydrogenation system. Schematic of the device. This device is based on the publication by Yoswathananont et al. (ref 13).
Figure 3. Scheme of synthesizer integrated fluidic processor (IFP) and reagents. IFP contains four reagent vials, a QMA cartridge and one reactor vial. Please click here to view a larger version of this figure.
Figure 4. UV and radioHPLC tracers for intermediate product. 3-fluoro-4-nitropyridine N-oxide has a characteristic absorption at 313 nm. Please click here to view a larger version of this figure.
Figure 5. UV and radioHPLC tracers for final product. 3-fluoro-4-aminoopyridine absorbs at a 254 nm. Please click here to view a larger version of this figure.
Concept | Mean (n = 4) | S.D. | Comments |
Initial 18F activity (mCi) | 148.0 | 44.9 | Start of synthesis |
Precursor amount (μg) | 50 | Use 50 μL of 1.0 mg/mL stock | |
Activity left in QMA (mCi) | 3.0 | 1.7 | Measured at the end of labeling step |
Radiolabeling yield | 29.7% | 6.3% | Act_collection_vial ÷ (Act_collection_vial + Act_AluN) |
Radiochemical purity (HPLC-1) | > 98% | From HPLC-1 QC | |
Spec. act. intermediate (mCi/μmol) | 122.9 | 29.7 | From HPLC-1 using calibration curve |
Hydrogenation recovery (d.c.) | 74% | 9.0% | Corrected for decay |
HPLC radiochemical purity (HPLC-2) | 90.7% | 2.9% | Calculated from HPLC-2 |
Drying efficiency | > 98% | Corrected for decay | |
Filtering recovery | 93.5% | 1.7% | Corrected for decay |
Dose volume (mL) | 3.3 | Collect fractions with highest radioactivity | |
Spec. act. final product (mCi/µmol) | 75.5 | 30.0 | From HPLC-3 using calibration curve |
Synthesis efficiency | 8.5% | 3.6% | Non-decay corrected |
Synthesis time (min) | 104 | 11.2 |
Table 1. Radiochemical synthesis parameters.
Common problems | Potential reasons and solutions |
[18F]fluoride is not efficiently eluted from the QMA | · TBA-HCO3 was not prepared correctly. Ensure the concentration is adequate. |
· There are leaks on the TBA-HCO3 vial. Make sure the crimp seal is tight and the septum is not pierced prior to installing it on the IFP. | |
· TBA-HCO3 is not in good condition. Order a fresh batch. | |
Labeling yield is low | · There is moisture in the precursor solution. Dry precursor and solvents. |
· Temperature is too low. | |
Reaction solution is yellow | · The product is decomposing due to base. Use less TBA-HCO3. |
· There is too much precursor. Use less precursor. | |
· There is too little solvent for the amount of 18F–. Use more solvent. | |
Additional peaks on radioHPLC | · Nitro group is being substituted: reduce the reaction temperature or shorten reaction time. |
Hydrogenation reaction does not work | · Catalyst is not good. Use a new cartridge. |
· Flow is too fast and does not allow sufficient contact between catalyst and substrate. Decrease flow. | |
· Hydrogen pressure is too low. Increase H2 pressure. | |
Hydrogen pressure increases dramatically during procedure | · Cartridge integrity is compromised and solid support is clogging the lines. Stop the flow and shut off the gas. Let radioactivity decay. Remove catalyst cartridge and flush the system. Put a new cartridge. |
Hydrogenation yield is low | · Too many impurities competing for the catalyst (MeCN, oxalic acid). Decrease amount of impurities or increase mass of precursor (Warning: increasing precursor amount will reduce specific activity). |
Recovery of radioactivity from hydrogenation step is low | · There is a leak in the system. Check for leaks and backflush into the hydrogen line. |
· Compound is defluorinating in the reactor. Evaluate different reaction conditions (pressure, temperature, flow, etc.). | |
Too much radioactivity is lost during filtration | · Wet the filter prior to use. |
· Use filter with a lower dead volume. | |
The final product peak on the HPLC looks broad | · Too much volume injected. Inject lower amount. Use column with larger diameter. |
· The column is not well conditioned. Condition the column for at least 30 column volumes. | |
· pH of the mobile phase is low. Make sure that the pH ≥ 8. | |
· Column is not in good condition. Replace column. Use column compatible with basic pH. |
Table 2. Troubleshooting guide.
The preparation of PET tracers requires efficient labeling with minimal user intervention to minimize radiation exposure14. Here, we described the first semi-automated procedure for the radiochemical synthesis of [18F]3F4AP, a PET tracer currently under investigation for imaging demyelination. This semi-automated method produces the radiotracer with high purity and sufficient specific activity for animal studies. Prior methods for the synthesis of this compound relied on manual synthesis6, which significantly limits the amount of radioactive tracer that can be produced. Having an automated method for the synthesis also provides more reproducible yields and makes it easier to transfer the procedure to other laboratories with similar equipment. Future efforts to fully automate the procedure will be instrumental to the production of the tracer in high amounts for studies in large animals or humans.
This procedure uses nucleophilic exchange of 19F for 18F to incorporate the radioisotope into the molecule of interest. The advantages of this reaction are that it is fast and produces almost exclusively the desired product without the need to perform a potentially lengthy purification step to remove excess of precursor. One limitation of using fluoride-exchange labeling reactions such as the one used here is that due to initial mass of cold compound the final specific activity defined as amount of radioactivity in mCi over amount of compound in µmol may be limited. Under our standard conditions, starting with 100-200 mCi of 18F– and 50 µg of precursor, the typical specific activity at the end of synthesis is up to 100-200 mCi/µmol, which appears to be sufficient for preclinical PET imaging studies. Nevertheless, the specific activity may improve by increasing the starting amount for 18F– while keeping the mass amount low. There have been several reports of producing radioligands by fluoride-exchange with high specific activity (1-3 Ci/µmol) by starting with high activity and low precursor amounts15,16.
As with all radiochemical syntheses of PET tracers, it is critical to work quickly in order to minimize radioactive decay. It is also important to minimize the time handling the radioactive materials, use proper shielding and maximize the distance between the radioactive material and the user to minimize radiation exposure. These aspects are particularly important during the second half of the protocol (purification and quality control) in which the user has to manually inject the solution into the HPLC, collect the fractions and filter the final product.
As with all radiochemical syntheses of PET tracers, it is critical to work quickly in order to minimize radioactive decay. It is also important to minimize the time handling the radioactive materials, use proper shielding and maximize the distance between the radioactive material and the user to minimize radiation exposure. These aspects are particularly important during the second half of the protocol (hydrogenation and purification) in which the user has to manually inject the solution into the hydrogenator, collect the fractions, set up the drying procedure, redissolve the product in buffer and filter it. During the filtering step it is easy to lose a large amount of radioactive material in the walls of the vials. Thus, it is important to try to collect all the liquid prior to filtering. Using a greater amount of buffer to dissolve may improve the yield of recovery but its use is discouraged because it will require injecting a larger volume on the HPLC, causing the peak to broaden and increase the volume of the final dose.
In order to troubleshoot and optimize the procedure is important to keep track of the yields of each step. For most steps this is done simply by measuring the amount of radioactivity before and after any step. In the case of the reaction the yields can be calculated through quantification of the HPLC peaks. Table 1 in the Results Section shows the typical yields for each step. Table 2 below lists many of the commonly encountered failures with potential reasons for the failure and how to correct them.
Finally, even though the procedure demonstrated here is specific for the synthesis of [18F]3F4AP, the general workflow and many of the individual steps are common to the synthesis of other compounds17. In this article we also demonstrated the typical QC tests performed on any PET tracer.
The authors have nothing to disclose.
This project was supported by grants NIH/NIBIB 1K99EB020075 to Pedro Brugarolas and an Innovation Fund Award from the Chicago Innovation Exchange to Brian Popko and Pedro Brugarolas. Prof. Brian Popko is gratefully acknowledged for his mentorship and financial support to the project. Prof. Chin-Tu Chen and the Integrated Small Animal Imaging Research Resource at the University of Chicago are acknowledged for generously sharing laboratory space and equipment. IBA is acknowledged for sponsoring open-access of this article.
Cyclotron produced [18F]fluoride | House supplied/Zevacor | IBA Cyclone 18 | 100-200 mCi |
Integrated fluid processor for production FLT/FDG | ABX | K-2715SYN | Cassette used for nucleophilic substitution |
Anhydrous acetonitrile | Janssen | 36431-0010 | Transfer under nitrogen |
Methanol | Janssen | 67-56-1 | |
ultrapure water | house supplied | Millipore MilliQ system | |
TBA-HCO3 | ABX | 808.0000.6 | abx.de |
QMA | Waters | WAT023525 | Quaternary methyl ammonium: Anion exchange solid phase extraction cartridge for trap and release of 18F- from the target water |
Sodium bicarbonate | ABX | K-28XX.03 | Prefilled 5 mL syringes |
Alumina-N | Waters | WAT020510 | Alumina-N solid phase extraction cartridge (for trapping unreacted 18F-) |
3-fluoro-4-nitropyridine N-oxide | Synthonix | 76954-0 | Store in desicator. Precursor |
3-fluoro-4-aminopyridine | Sigma Aldrich | 704490-1G | Reference standard |
Oxalic acid | Sigma Aldrich | 75688-50G | |
Sodium phosphate monobasic | Fisher Scientific | S80191-1 | |
Triethyl amine | Fisher Scientific | 04885-1 | |
Ethanol | Decon Labs | DSP-MD.43 | USP |
Final product vial | ABX | K28XX.04 | |
Millex Filter Syringe | Millex | SLGVR04NL | |
10% Pd/C cartridge | Sigma Aldrich | THS-01111-12EA | |
11 mm vials + crimp seals | Fisher Scientific | 03-250-618, 06-451-117, or equivalent | |
13 mm vials + crimp seals | Fisher Scientific | 06-718-992, 06-718-643, or equivalent | |
HPLC vials | Fisher Scientific | 03-391-16, 03-391-17, or equivalent | |
SEMIPREP C18 column | Agilent | 990967-202 | |
V-vials | Alltech | ||
Syringes: 1, 3, 10 mL | Fisher Scientific | 14-829-10D, 14-829-13Q, 14-829-18G, or equivalent | |
Compressed gases: N2, He, H2 | Airgas | UHP N300, UHP HE300, UHP H300, or equivalent | |
TLC plates | Sigma Aldrich | Z193275, or equivalent | |
Name | Company | Catalog Number | Comments |
Equipment | |||
Synthera automated synthesizer | IBA SA, Belgium, iba-worldwide.com | Synthera, 250.001 | Automatic synthesis unit |
In-house hydrogenator | See picture | See text description | |
Hot cells | Comecer | For manipulating radioactive materials | |
RadioTLC scanner | Eckert and Ziegler | For handling sterile materials | |
HPLC | Dionex | Ultimate 3000 | |
Dose calibrator | Capintec | CRC15 | Or equivalent |
Gamma counter | Capintec, 7 Vreeland Road, Florham Park, NJ 07932 | CRC 15, PET-CRC25, or equivalent | For measuring radioactivity |
Personal dosimeters | Packard | Cobra II | For measuring gamma spectrum |
Personal radiation badges and rings | Atlantic Nuclear | Rados Rad-60 Electronic Dosimeter, or equivalent | |
Rotavap + vacuum pump | Landauer | ||
Lead pigs + syringe shields | Heidolph | Or equivalent | |
Geiger counters | Pinestar | ||
Ludlum | Model 3 + Pancake GM detector, 4801605, 47-1539, or equivalent |