Optimization of a Nucleophilic Two-Step Radiosynthesis of 6-O-(2-[18F]fluoroethyl)-6-O-desmethyl-diprenorphine ([18F]FE-DPN) for PET Imaging of Brain Opioid Receptors

We have established a method for nucleophilic one-pot, two-step radiosynthesis of the popular opioid receptor radioligand 6-O-(2-[18F]fluoroethyl)-6-O-desmethyl-diprenorphine ([18F]FE-DPN) from the novel precursor 6-O-(2-tosyloxyethyl)-6-O-desmethyl- 3-O-trityl-diprenorphine (TE-TDDPN), which we designate as the Henriksen precursor. We undertook an optimization of the synthesis conditions, aiming to enhance the accessibility of [18F]FE-DPN for positron emission tomography (PET) studies of μ-opioid receptors. Herein, we report an optimized direct nucleophilic 18F-fluorination and the deprotection conditions for a fully automated radiosynthesis of [18F]FE-DPN on a modified GE Tracerlab FX FE synthesis panel. Starting from 1–1.5 GBq of [18F]fluoride and applying an Oasis Max 1cc cartridge for fluorine-18 trapping with a reduced amount of K2CO3 (5.06 μmol K+ ion), [18F]FE-DPN ([18F]11) was produced with 44.5 ± 10.6 RCY (decay-corrected), high radiochemical purity (>99%), and a molar activity of 32.2 ± 11.8 GBq/μmol in 60–65 min.


Introduction
Opioid receptors (ORs) are plasma membrane proteins belonging to the G-proteincoupled receptor family (GPCRs) [1], which have wide expression throughout the central nervous system, the periphery, and the immune system, where they mediate diverse effects of endogenous opioid peptides and poppy alkaloids [2].The ORs consist of three classic types (µ-OR, δ-OR and κ-OR) and an additional nonclassical type known as the opioid-like 1 receptor (nociceptin/orphanin FQ peptide receptor, ORL-1/NOP) [3].The opioid systems are involved in numerous physiological processes [4] and are implicated in pathophysiological processes such as chronic pain, movement disorders, substance abuse, epilepsy, and schizophrenia [5,6].
In the 1980s, a research group of the Hammersmith Hospital developed a new type of 11 C-labeled diprenorphine derivative, namely [N 17 -cyclopropylmethyl- 11 C] diprenorphine [38].The synthesis entailed a reaction of N 17 -nordiprenorphine with [carboxy- 11 C]cyclopropanecarbonyl chloride, followed by a reduction of the resultant amide with LiAlH4 to the labelled orvinol-type radiotracer.The synthesis of the metabolically more stable [6-O-methyl- 11 C]diprenorphine was developed by Lever [39,40]    [ 3 H]DPN ([15,16-3 H]diprenorphine) was first applied to identify and characterize brain ORs in vitro [33].In the earliest report of its pharmacological selectivity, Frost et al. [34] found that [ 3 H]DPN was µ-OR selective, according to displacement studies in vivo.Subsequently, Jones et al. [35] concluded that the initial attribution of µ-OR selectivity had been due to invalid extrapolation of binding data in vitro to the interpretation of studies ex vivo.Indeed, Raynor et al. [36] found that diprenorphine (13) was nearly equipotent in vitro for displacing selective ligands from transformed CHO cells expressing human ORs, i.e., [ 3 H]DAMGO (µ-OR, K i = 0.07 nM), [ 3 H]naltrindole (δ-OR, K i = 0.23 nM), and [ 3 H]U-69,593 (κ-OR, K i = 0.02 nM).Returning to this topic of selectivity of orvinols, recent small animal PET results indicate considerable selectivity of [ 18 F]FE-DPN ([ 18 F]11) for µ-ORs in the brain tissue of living rodents [37], which should favor its use in PET research.
However, the short physical half-life of carbon-11 (20.34 min) is an impediment to the use of diprenorphine derivatives, as new radiosynthesis is required for each successive PET examination.The longer physical half-life of 18 F-fluorine (110 min) can thus be logistically advantageous compared to 11 C-carbon, with respect to longer PET recording times and the logistics of radioligand preparation and delivery to nearby PET centers.Furthermore, favorable nuclear properties of 18 F-fluorine (97% β+ decay and 635 keV positron energy) enable higher resolution imaging.
Herein, we described a detailed optimized radiosynthetic approach for the preparation of [ 18 F]FE-DPN ([ 18 F]11) applying direct aliphatic 18 F-fluorination on a GE Tracerlab FX FE automated synthesis module.This convenient radiolabeling method should ultimately enhance the accessibility of [ 18 F]FE-DPN ([ 18 F]11)for PET investigations of µ-ORs.

Results and Discussion
Tracer molecules can be radiofluorinated with direct or indirect methods [42].In the former approach, [ 18 F]fluoride directly reacts with the precursor designed for radiolabeling though the incorporation of a favorable leaving group.In such applications, the radiolabeling step is followed by the removal of the protective group and the subsequent purification of the desired labeled product [43].Indirect methods require the previous radiolabeling of a prosthetic group, which is then conjugated to the precursor biomolecule.For this purpose, 18 F-labeled prosthetic groups are prepared in one or two steps, usually followed by purification, which further increases the time required for complete radiosynthesis [44,45].Therefore, direct radiofluorination is generally preferable to the indirect method.
The availability of this novel TE-TDDPN ( 16) precursor [59] encouraged us to investigate the synthesis of [ 18 F]FE-DPN ([ 18 F]11) through direct radiofluorination (Figure 4).However, in parallel with our research, Levinstein et al. achieved the one-pot, two-step nucleophilic radiosynthesis of [ 18 F]FE-DPN ([ 18 F]11) using an in-house, custom-made radiofluorination module [37].In their procedure, TE-TDDPN ( 16) was treated with dried [ 18 F]fluoride (100 °C, 12 min) followed by a removal of the the trityl protecting group of [ 18 F]FE-TDDPN ([ 18 F]18) with 1 M hydrochloric acid (80 °C, 12 min).The average radiochemical yield was 10% (non-decay corrected), the radiochemical purity was 98%, and the molar activity 2027 GBq/µmol.For the automated radiosyntheses, we used the GE TRA-CERlab FX FE synthesis panel with significant modifications, as presented in Figure 5.While originally designed for electrophile radiosynthesis, we modified it to be suitable for nucleophilic reactions.In particular, we replaced the fluorine-neon inlet with an irradiated water inlet and added a second reactor with several extra valves, which we did not, however, use in these experiments.Moreover, we removed the formulation unit of the panel, in order to have enough reagent vessel for the reaction and the transfer of the reaction mixture for HPLC purification.For the automated radiosyntheses, we used the GE TRA-CERlab FX FE synthesis panel with significant modifications, as presented in Figure 5.While originally designed for electrophile radiosynthesis, we modified it to be suitable for nucleophilic reactions.In particular, we replaced the fluorine-neon inlet with an irradiated water inlet and added a second reactor with several extra valves, which we did not, however, use in these experiments.Moreover, we removed the formulation unit of the panel, in order to have enough reagent vessel for the reaction and the transfer of the reaction mixture for HPLC purification.We first used standard elution conditions for [ 18 F]F -ion separation.The anion exchanger column (V8 position) was a Sep-Pak QMA Plus Light column (preconditioned with 2 mL of 14 mg/mL K2CO3 solution and 10 mL of water), and the elution mixture (V1 position) contained K2CO3 and Kriptofix 222 (K222) as the phase transfer catalyst.The elution solution was a mixture of 120 µL 14 mg/mL aqueous solution of K2CO3 and 10 mg of We first used standard elution conditions for [ 18 F]F -ion separation.The anion exchanger column (V8 position) was a Sep-Pak QMA Plus Light column (preconditioned with 2 mL of 14 mg/mL K 2 CO 3 solution and 10 mL of water), and the elution mixture (V1 position) contained K 2 CO 3 and Kriptofix 222 (K 222 ) as the phase transfer catalyst.The elution solution was a mixture of 120 µL 14 mg/mL aqueous solution of K 2 CO 3 and 10 mg of K 222 in acetonitrile (2 mL).However, this resulted in low radiochemical yield and an unidentified radioactive by-product, probably due to the base sensitivity of the precursor and the labeled compound.Previous work indicated 6-O-(2-hydroxyethyl)-6-O-desmethyldiprenorphine (17, HE-DPN) to be a likely biproduct, which we confirmed by synthesis and structural identification (see in Supplementary Material: Table S1, Figures S1 and S2).The retention time of HE-DPN was 7-8 min in the semi-preparative HPLC chromatogram, while the retention time of [ 18 F]FE-DPN was 15 min, thus affording complete separation.During the initial radiosyntheses with relatively large amounts of base, we obtained significant amounts of a radioactive by-product compared to [ 18 F]FE-DPN, in a ratio of 3:7 (Supplementary Material Figure S3).We hypothesize that cleavage of the E-ring after nucleophilic 18 F-fluorination of TE-TDDPN could possibly result in the formation of two radioactive by-products (Supplementary Material Scheme S1).
Thus, it was necessary to reduce the basicity of the reaction mixture to avoid degradation during radiosynthesis.In the subsequent experiments, we applied KHCO 3 in reduced amounts to the reaction mixture, such that the elution mixture consisted of 120 µL of a 10 mg/mL aqueous KHCO 3 solution and 5.5 mg of K 222 in acetonitrile (2 mL).However, the low K + ion concentration resulted in low [ 18 F]F -elution efficiency, so we increased the amount of KHCO 3 to 1.8 mg and the amount of K 222 to 9 mg, which gave better [ 18 F]F -recovery.
In order to further reduce the amount of base and achieve adequate [ 18 F]F -elution, we studied other anion exchange columns containing less anion exchange sorbent.We had previously tested the SOLA-AX SPE anion exchange column with 10 mg sorbent for the separation of [ 18 F]F -with 80% recovery using a manual loading and elution procedure [61].Based upon these results, we tested the applicability of the SOLA-AX SPE column (preconditioned with 2 mL of 10% K 2 CO 3 solution and 10 mL of water) in the automated synthesis apparatus.However, the flow resistance of this column was very low, thus calling for a low-flow loading process for suitable fluorine-18 absorption.This approach proved difficult to implement and was insufficiently robust.Therefore, we decided to abandon using this anion exchange column for the radiosynthesis of [ 18 F]FE-DPN ([ 18 F]11).
Finally, based on the results of Song et al. [62], we began testing the Oasis Max 1cc cartridge (preconditioned with 2 mL of 0.15 M KHCO 3 solution and 6 mL water) for [ 18 F]F - ion separation, which proved to give the best results among the various tested anion exchange columns.For this cartridge, we applied an elution mixture of the following composition: 7 µL of the 50 mg/mL aqueous solution of K 2 CO 3 , 100 µL of K 222 solution (10 mg/mL in acetonitrile), 100 µL water, and 400 µL acetonitrile.By this method, we succeeded in reducing the ratio of radioactive by-product to product to 6:94 (Supplementary Material Figure S4).Furthermore, we tested Bu 4 NHCO 3 as an alternative base for this column, where the elution solution consisted of 50 µL aqueous Bu 4 NHCO 3 (0.075 M) and 500 µL acetonitrile.This approach gave similar results to using K 2 CO 3 as a base but with two radioactive by-products in the analytical HPLC chromatogram, in an 8:92 ratio relative to [ 18 F]FE-DPN (Supplementary Material Figure S5).After [ 18 F]F -elution, the eluate was azeotropically evaporated by heating it at a reduced pressure.We present the [ 18 F]fluoride recoveries from different anion exchange columns and conditions in Table 1.
In the first radiosynthesis, 1 mg of TE-TDDPN precursor (16) (V2 position) was reacted in 1 mL of dry acetonitrile at 90 • C for 5 min, which failed to produce an adequate incorporation of [ 18 F]F -.Subsequently, we added, 2 mg TE-DPN precursor in 500 µL dry acetonitrile to the dried [ 18 F]fluoride in the reactor and heated the reaction mixture at 100 • C for 10 min.For deprotection, the reactor was cooled to 40 • C. In all subsequent experiments, we used these reaction conditions for the radiosynthesis.First, we preformed deprotection of the labeled compound with 1 mL of 1M HCl in 96% ethanol (V4 position) at 40 • C for 10 min.However, the reaction time of 10 min proved insufficient for the complete hydrolysis of the trityl group.Consequently, we increased the reaction time to 15 min, while holding the temperature at 40 • C.After deprotection, the reaction mixture was neutralized with 1100 µL of 1M NH 4 OH solution (V7 position).
For the purification of [ 18 F]FE-DPN ([ 18 F]11) with semipreparative RP-HPLC, the neutralized reaction mixture was diluted with 15 mL water (V6 position).Here, the HPLC retention time of the labeled compound depended on the pH of the eluent and of the sample solution; any substantial deviation from pH 7 shifted the retention time by several minutes.To eliminate this retention time drift, we replaced the semipreparative HPLC injection loop with a home-made reversed-phase pre-column packed with the C18 sorbent from a Sep-Pak C18 Plus Light cartridge.To prevent gas bubbles on the column, we installed a vented filter on the precolumn.Initially, we rinsed the reactor with 10 mL water and transferred this solution to the C18 precolumn.Later, we applied a 5% acetonitrile aqueous solution for reactor washing because the filter otherwise became clogged, probably due to the presence of water-insoluble substances.The final product was purified on a semipreparative HPLC (retention time ~15.5 min) and then formulated.Figure 6 shows the semi-preparative HPLC chromatograms and Table 2 presents the results of the optimization experiments.For the quality control of [ 18 F]FE-DPN ([ 18 F]11), we developed two analytical HPLC methods.First, we measured the radiochemical purity of the purified product by gradient elution according to the following parameters: Nucleodur C18 Pyramid 5 µm (250 × 4.6 mm) RP column with eluent A: acetonitrile and eluent B: 0.1 M ammonium formate (pH 6.67).The linear gradient was 0 min: 40% A and 60% B; 8 min: 100% A; 11 min: 100% A; 11.1 min: 40% A and 60% B, at a flow rate of 1 mL/min and a UV recording at 284.5 nm. Figure 7 shows the analytical HPLC chromatograms of the product using this gradient elution.stalled a vented filter on the precolumn.Initially, we rinsed the reactor with 10 mL water and transferred this solution to the C18 precolumn.Later, we applied a 5% acetonitrile aqueous solution for reactor washing because the filter otherwise became clogged, probably due to the presence of water-insoluble substances.The final product was purified on a semipreparative HPLC (retention time ~15.5 min) and then formulated.Figure 6 shows the semi-preparative HPLC chromatograms and Table 2 presents the results of the optimization experiments.For the quality control of [ 18 F]FE-DPN ([ 18 F]11), we developed two analytical HP methods.First, we measured the radiochemical purity of the purified product by grad elution according to the following parameters: Nucleodur C18 Pyramid 5 µm (250 × mm) RP column with eluent A: acetonitrile and eluent B: 0.1 M ammonium formate 6.67).The linear gradient was 0 min: 40% A and 60% B; 8 min: 100% A; 11 min: 100% 11.1 min: 40% A and 60% B, at a flow rate of 1 mL/min and a UV recording at 284.5 Figure 7 shows the analytical HPLC chromatograms of the product using this grad elution.Second, we developed an isocratic radio-HPLC method using the same column a the above gradient method.Chromatographic separation was achieved in 15 min u the following solvent formulation: acetonitrile (50%) and 0.1 M ammonium formate, 6.67 (50%), with the flow rate at 1 mL/min and the UV detection at 284.5 nm.Figu shows the analytical HPLC chromatograms of the [ 18 F]FE-DPN ([ 18 F]11) using this cratic HPLC method.Second, we developed an isocratic radio-HPLC method using the same column as in the above gradient method.Chromatographic separation was achieved in 15 min using the following solvent formulation: acetonitrile (50%) and 0.1 M ammonium formate, pH 6.67 (50%), with the flow rate at 1 mL/min and the UV detection at 284.5 nm. Figure 8 shows the analytical HPLC chromatograms of the [ 18 F]FE-DPN ([ 18 F]11) using this isocratic HPLC method.

Conclusions
We successfully optimized the radiosynthesis the µOR PET ligand [ 18 F]FE-DPN ([ 18 F]11) through direct aliphatic nucleophilic substitution (SN2) with [ 18 F]fluoride under mild basic conditions.Since [ 18 F]FE-DPN was found to be base-sensitive, we investigated the effect of reducing the K + ion concentration on the radiochemical yield.First, we tested the recovery of [ 18 F]fluoride at reduced K + ion concentrations from various anion exchange columns, namely Sep-Pak QMA Plus Light, SOLA-AX SPE, and Oasis Max 1cc columns.For the Oasis Max 1cc column, the recovery of [ 18 F]fluoride was 92.0 ± 3.7% at 5.06 µmol K + ions, which was the lowest tested effective amount.Furthermore, starting from 1 to 1.5  The chemical identity of the radioactive product was confirmed via a co-injection of the [ 19 F]FE-DPN ([ 19 F]11) standard (Figure 9).

Conclusions
We successfully optimized the radiosynthesis of the µOR PET ligand [ 18 F]FE-DPN ([ 18 F]11) through direct aliphatic nucleophilic substitution (SN2) with [ 18 F]fluoride under mild basic conditions.Since [ 18 F]FE-DPN was found to be base-sensitive, we investigated the effect of reducing the K + ion concentration on the radiochemical yield.First, we tested the recovery of [ 18 F]fluoride at reduced K + ion concentrations from various anion exchange columns, namely Sep-Pak QMA Plus Light, SOLA-AX SPE, and Oasis Max 1cc columns.For the Oasis Max 1cc column, the recovery of [ 18 F]fluoride was 92.0 ± 3.7% at 5.06 µmol K + ions, which was the lowest tested effective amount.Furthermore, starting from 1 to 1.5

Conclusions
We successfully optimized the radiosynthesis of the µOR PET ligand [ 18 F]FE-DPN ([ 18 F]11) through direct aliphatic nucleophilic substitution (S N 2) with [ 18 F]fluoride under mild basic conditions.Since [ 18 F]FE-DPN was found to be base-sensitive, we investigated the effect of reducing the K + ion concentration on the radiochemical yield.First, we tested the recovery of [ 18 F]fluoride at reduced K + ion concentrations from various anion exchange columns, namely Sep-Pak QMA Plus Light, SOLA-AX SPE, and Oasis Max 1cc columns.For the Oasis Max 1cc column, the recovery of [ 18 F]fluoride was 92.0 ± 3.7% at 5.06 µmol K + ions, which was the lowest tested effective amount.Furthermore, starting from 1 to 1.5 GBq of [ 18 F]fluoride and applying the Oasis Max 1cc anion-exchange cartridge for fluorine-18 trapping with 5.06 µmol K + ions, [ 18 F]FE-DPN was prepared with 44.5 ± 10.6 RCY (decay-corrected), high radiochemical purity (>99%), and a relatively high molar activity of 32.2 ± 11.8 GBq/µmol for 60-65 min with a modified GE TRACERlab FX FE synthesis panel.With this procedure, the radiochemical yield of [ 18 F]FE-DPN radiosynthesis increased significantly while the reaction time decreased compared to previously published methods [26,37,47].
In addition, with this low K + ion concentration method, we successfully reduced the amount of radioactive by-products in the reaction mixture.During quality control, the by-product HE-DPN (17) was not detectable on the analytical UV HPLC chromatogram of the [ 18 F]FE-DPN ([ 18 F]11) product, nor were any other UV-active impurities present.Tetrabutylammonium bicarbonate was also tested as a phase transfer catalyst for the Oasis Max 1cc column gave good radiochemical yields but with the formation of new, unidentified impurities.
The results of our optimization experiments enable radiochemists to efficiently and routinely produce the [ 18 F]FE-DPN ([ 18 F]11) radiotracer for µOR PET imaging in research and clinical trials.In addition, based on our results, the application of Oasis Max 1cc anion-exchange cartridge for [ 18 F]F -ion separation can make the radiosynthesis of other base-sensitive radioactive tracers more efficient by using a reduced amount of potassium ions.

[ 18 F]fluoride Production and Separation
No-carrier added (NCA) [ 18 F]fluoride was produced through the 18 O(p,n) 18 F nuclear reaction, and 3.5 mL of 18 O-enriched water (>95% [ 18 O]H 2 O, Rotem Industries Ltd., Mishor Yamin D.N, Arava, Israel) was irradiated with a beam of protons (16.5 MeV, 70 µA) from a GE PETtrace cyclotron.For these experiments the target was rinsed with deionized water, yielding 1-3 GBq [ 18 F]fluoride, which was separated from the aqueous solution using an anion exchange column.When using the Sep-Pak QMA Plus Light cartridge, the following elution mixtures were used: A eluent: 120 µL of the 14 mg/mL aqueous solution of K 2 CO 3 , 10 mg of K 222 solution in acetonitrile (2 mL); B eluent: 120 µL of the 10 mg/mL aqueous solution of KHCO 3 and 5.5 mg of K 222 solution in acetonitrile (2 mL); C eluent: 180 µL of the 10 mg/mL aqueous solution of KHCO 3 and 9 mg of K 222 solution in acetonitrile (2 mL).When using the Oasis Max 1cc cartridge, the following elution mixtures were used: D eluent: 7 µL of the 50 mg/mL aqueous solution of K 2 CO 3 , 100 µL of K 222 solution (10 mg/mL in acetonitrile), 100 µL water and 400 µL acetonitrile; E eluent: 50 µL aqueous Bu 4 NHCO 3 (0.075 M) and 500 µL acetonitrile.In all cases, the eluate was evaporated in several steps with a flow of helium under reduced pressure (20 kPa), first at 70 • C for 70 s, followed by 105 • C for 75 s, and finally at 120 • C for 290 s.In the last step, the pressure was reduced to 5 kPa, and the reactor was cooled to 60 • C. For semipreparative RP-HPLC purification, the neutralized reaction mixture was diluted with 15 mL water.A home-made stainless-steel reverse-phase pre-column (8 × 22 mm, ID × length) was used as an HPLC injection loop, which was filled with the sorbent taken from a Sep-Pak C18 Plus Light column (Waters) and preconditioned via passage of 10 mL H 2 O, 10 mL 96% EtOH, and 10 mL H 2 O.A vented filter was applied to prevent gas bubbles in the column.First, the diluted reaction mixture was pushed through the C18 precolumn.Then, the reactor was rinsed with 5% acetonitrile aqueous solution, and the eluant was passed through the C18 precolumn.Semipreparative HPLC was performed using a Nucleodur C18 Pyramid 5 µm (250 × 10 mm) RP column.The eluent was a mixture of 65% 0.1 M ammonium formate and 35% acetonitrile (pH 7).First, the adsorbed compounds from the C18 precolumn were delivered at a flow rate of 2 mL/min to the HPLC separation column, and then the purification was carried out at a flow rate of 5 mL/min.For formulation, the fraction containing the [ 18 F]FE-DPN ([ 18 F]11) was diluted with water and then passed through a Sep-Pak C18 Plus Light column (preconditioned with 10 mL H 2 O, 10 mL 96% EtOH, and 10 mL H 2 O), which was washed with 5 mL water and then eluted with 96% ethanol in 100 µL fractions.Usually, the third fraction contained the highest radioactivity (60-80% of the total activity), which was diluted to a final volume of 1 mL with isotonic saline solution.
Int. J. Mol.Sci.2023, 24, x FOR PEER REVIEW 6 of 15 procedure for the radiosynthesis and purification of [ 18 F]FE-DPN ([ 18 F]11) using a modified GE TRACERlab FX FE synthesis panel.The preparation of [ 18 F]FE-DPN ([ 18 F]11) consists of the following steps: fluorine-18 production, [ 18 F]fluoride separation, radiolabeling, deprotection, semipreparative HPLC purification, formulation, and quality control.Due to inadequate performance of our initial procedures, we sought to improve the radiochemical yield of the [ 18 F]FE-DPN ([ 18 F]11) synthesis by investigating different labeling conditions, i.e., varying factors such as the amount and type of base used to elute [ 18 F]fluoride, precursor concentration, reaction temperature, and reaction time.

Figure 5 .
Figure 5. Schematic diagram of the modified TRACERlab FX FE synthesis panel.

Figure 5 .
Figure 5. Schematic diagram of the modified TRACERlab FX FE synthesis panel.The fluorine-18 nuclide (1-1.5 GBq) was produced with a GE PETtrace cyclotron using the 18 O(p,n) 18 F nuclear reaction.One of the critical steps in the synthesis was the separation of [ 18 F]fluoride anion from 18 O-enriched water.For separating the [ 18 F]fluoride ion from 18 O-enriched water, we tested the following anion exchanger columns: Sep-Pak QMA Plus Light column (130 mg, Waters), SOLA-AX SPE column (10 mg, Thermo Scientific), and Oasis Max 1cc (10 mg, Waters), each with elution mixtures of various compositions.We first used standard elution conditions for [ 18 F]F -ion separation.The anion exchanger column (V8 position) was a Sep-Pak QMA Plus Light column (preconditioned with 2 mL of 14 mg/mL K 2 CO 3 solution and 10 mL of water), and the elution mixture (V1 position) contained K 2 CO 3 and Kriptofix 222 (K 222 ) as the phase transfer catalyst.The elution solution was a mixture of 120 µL 14 mg/mL aqueous solution of K 2 CO 3 and 10 mg

Figure 9 .
Figure 9.The analytical UV and radio-HPLC chromatograms of the product co-injected with [ 19 F]FE-DPN ([ 19 F]11) standard using the gradient HPLC method.

Figure 9 .
Figure 9.The analytical UV and radio-HPLC chromatograms of the product co-injected with [ 19 F]FE-DPN ([ 19 F]11) standard using the gradient HPLC method.

Figure 9 .
Figure 9.The analytical UV and radio-HPLC chromatograms of the product co-injected with [ 19 F]FE-DPN ([ 19 F]11) standard using the gradient HPLC method.

Table 1 .
The [ 18 F]fluoride recovery from different anion exchange columns.

Table 2 .
The resulting radiochemical yields and molar activities during different conditions.

Table 2 .
The resulting radiochemical yields and molar activities during different conditions.