Gengyang
Yuan
*,
Timothy M.
Shoup
,
Sung-Hyun
Moon
and
Anna-Liisa
Brownell
*
Gordon Center for Medical Imaging, Massachusetts General Hospital and Harvard Medical School, 3rd Avenue, Charlestown, MA 02129, USA. E-mail: gyyuan@mgh.harvard.edu; abrownell@mgh.harvard.edu
First published on 2nd July 2020
A modified alcohol-enhanced 18F-fluorodeboronation has been developed for the radiosyntheses of [18F]JNJ-46356479 and [18F]FITM. Unlike the [18F]KF/K222 approach, this method tolerates the presence of sensitive heterocycles in Bpin precursors 4 and 8 allowing a one-step 18F-fluorodeboronation on the fully automated TRACERlab™ FXFN platform.
Application of Cu-mediated 18F-fluorination in automated platforms is essential in producing large-scale radioligands to support preclinical and clinical studies. Successful automated radiosyntheses with this method are the preparation of [18F]TRACK on TRACERlab™ FXFN platform20 and the synthesis of [18F]olaparib on Ziegler Modular-Lab.21 In these reports, automated SNAr radiofluorination occurred with the [18F]KF/Kryptofix® 222 (K222) complex. Recently, Zischler et al. revealed that the milder [18F]TEAF complex was also compatible with this radiolabeling method (Fig. 1B).24 Moreover, addition of alcohols, such as methanol and n-butanol, as co-solvents, seemed to enhance the functionality tolerance of this 18F-fluorodeboronation method to include indoles, phenols, and anilines radiolabeled via unprotected precursors. However, the authors investigated this approach manually where extensive physical operations were required. For example, the aqueous 18F was loaded onto the quaternary methyl ammonium (QMA) cartridge, an anion-exchanger to enrich 18F from [18O]H2O, from the male side instead of the female side followed by an alcohol wash and air flush from the female side to remove the [18O]water and increase the recover yield of 18F. Air was also introduced into the reactor to promote reaction. These special and tedious operations hinder its application toward the fully automated synthetic modules such as GE TRACERlab™ FXFN platform. Nevertheless, this strategy has been recently utilized by Bernard-Gauthier et al. for the radiosynthesis of [18F]TRACK using semiautomated radiosynthesis module Scintomics GRP (Germany).20
To investigate the suitability of [18F]JNJ-46356479 as a PET imaging ligand for metabotropic glutamate receptor 2 (mGluR2) in the brain, we modified this alcohol-enhanced Cu-mediated 18F-fluorination to (a) utilize its potential in tolerating sensitive organoboranes that are not compatible with the [18F]KF/K222 system, and (b) avoid manual manipulations to allow a fully automated radiosynthesis on a GE TRACERlab™ FXFN platform (Fig. 1). After a thorough investigation of radiofluorination conditions for [18F]JNJ-46356479, a fully automated method was developed, which was also applicable to the automated one-step radiosynthesis of a mGluR1 negative allosteric modulator (NAM) [18F]FITM.
The synthesis of non-labeled JNJ-46356479 was achieved via the method reported by Cid et al., starting from 2,4-dichloro-3-(trifluoromethyl) pyridine using over 6 steps to allow final reductive coupling reaction between aldehyde 1 and 1-(2,4-difluorophenyl) piperazine 2 (Scheme 1).25 The para-aryl fluoride of JNJ-46356479 was selected as a radiolabeling site to avoid steric hinderance when introducing 18F or bulky leaving groups. Traditional nucleophilic SNAr substitution of nitro- or iodo-leaving groups was assumed not to be feasible due to the poor activating effect of adjacent fluoride. The sulfonium or iodonium salt or iodonium ylide radiolabeling methods, however, required oxidative reaction conditions that might not be tolerated by the heterocycle containing JNJ-46356479.
The 18F-fluorodeboronation precursor 4 was synthesized in a similar manner as that of JNJ-46356479 with the boronic ester 3. However, direct application of the same reductive amination reaction conditions, using HOAc and NaBH(OAc)3, resulted in substantial side products that were in close vicinity with the desired product 4, including the deboronation product (aryl-H) and boronic species 5. Interestingly, this decomposition did not occur to the structurally simpler fragment 3, which was prepared via the de-Boc protection of compound 6 under strong acidic condition at room temperature (Scheme 2). To improve the yield of compound 4, the basic reductive amination conditions with triethylamine (TEA) and NaBH(OAc)3 were applied, resulting in much less side products and an increased yield of compound 4.
The original manual radiolabeling protocol was not directly applied for [18F]JNJ-46356479 due to the reproducibility issue witnessed during the radiofluorination of compound 6 and incomplete removal of [18O]H2O from QMA cartridge by air would inhibit the reaction. Instead, the labeling protocol was comprehensively investigated to reveal critical influential factors that would affect the reaction outcomes. More importantly, efforts were focused on simplifying the manual handlings of this method to apply it to the fully automated synthetic modules.
As shown in Table 1, based on the original reaction conditions, the reaction temperature, time as well as the amounts of TEAB, precursor 4 and catalyst [Cu(OTf)2py4] were optimized. The anhydrous dimethylacetamide (DMA) was used as a solvent. The optimal reaction temperature was 130 °C and further elevation of reaction temperature did not significantly increase the RCCs (entries 1–3). Increasing the amount of base was detrimental to the RCCs (entry 4), though it could improve the elution efficiency of 18F from QMA cartridge (∼95–99%). On the other hand, increased precursor amount was beneficial to enhance the RCCs (entries 5–8). These results indicated that boronic pinacol ester (Bpin) 4 was vulnerable to the excess amount of base. This sensitivity of organoboranes was also noted by Zhang et al., where use of Py-OTf or DMAP-OTf as 18F eluting agent and base alleviated this issue.27 To maintain a balanced recovery of 18F (85–90%) and to limit precursor use, 3.5 mg (6.25 μmol) of 4 and 2.7 mg (14.1 μmol) of TEAB were considered as optimal amounts (entry 8). The reaction time showed little effect on the resulting RCCs (entries 9–11), thus a shorter reaction time was preferred. The amount of [Cu(OTf)2(py)4] was also optimized, where reduction of the catalyst by half (i.e., 9 mg, 13.3 μmol) led to similar RCCs (entries 12–14). In addition, the boronic acid precursor 5 was also examined for the synthesis of [18F]JNJ-46356479 with the same method. Although MeOH was claimed as an accompanying alcohol for boronic acid precursors by Zischler and coworkers,24 it failed to give the desired product in this radiofluorination reaction (Table 1, entry 15; Table S2,† entries 1 and 2). However, n-BuOH led to the formation of [18F]JNJ-46356479 (Table 1, entry 16; Table S2,† entries 3–6). The RCCs ranged from 5% to 13% with elevated temperature and increased amount of precursor favored the radiochemical conversion (Table S2,† entries 3–6). Therefore, it was reasonable to conclude that the boronic pinacol ester 4 was more reactive than the boronic acid 5 for this radiofluorination method.
Entry | Precursor (mg per equiv.) | TEAB (mg per equiv.) | [Cu(OTf)2(py)4] (mg per equiv.) | Co-solvent (mL) | T (°C) | t (min) | RCC by rTLC (%) |
---|---|---|---|---|---|---|---|
1 | 4 (3.0/1.0) | 2.7/2.6 | 18.0/4.9 | n-BuOH (0.4) | 110 | 20 | 22 ± 2 (n = 2) |
2 | 4 (3.0/1.0) | 2.7/2.6 | 18.0/4.9 | n-BuOH (0.4) | 130 | 20 | 25 ± 2 (n = 5) |
3 | 4 (3.0/1.0) | 2.7/2.6 | 18.0/4.9 | n-BuOH (0.4) | 150 | 20 | 20 ± 2 (n = 2) |
4 | 4 (3.0/1.0) | 8.2/7.8 | 18.0/4.9 | n-BuOH (0.4) | 130 | 20 | 7 ± 2 (n = 2) |
5 | 4 (0.5/1.0) | 2.7/15.6 | 18.0/29.4 | n-BuOH (0.4) | 130 | 20 | 2 ± 1 (n = 2) |
6 | 4 (1.0/1.0) | 2.7/7.8 | 18.0/14.7 | n-BuOH (0.4) | 130 | 20 | 5 ± 1 (n = 2) |
7 | 4 (1.5/1.0) | 2.7/5.2 | 18.0/9.8 | n-BuOH (0.4) | 130 | 20 | 12 ± 1 (n = 2) |
8 | 4 (3.5/1.0) | 2.7/2.3 | 18.0/4.3 | n-BuOH (0.4) | 130 | 5 | 28 ± 3 (n = 2) |
9 | 4 (2.0/1.0) | 2.7/3.9 | 18.0/7.4 | n-BuOH (0.4) | 130 | 5 | 15 ± 1 (n = 2) |
10 | 4 (2.0/1.0) | 2.7/3.9 | 18.0/7.4 | n-BuOH (0.4) | 130 | 10 | 14 ± 2 (n = 2) |
11 | 4 (2.0/1.0) | 2.7/3.9 | 18.0/7.4 | n-BuOH (0.4) | 130 | 20 | 15 ± 1 (n = 2) |
12 | 4 (2.0/1.0) | 2.7/3.9 | 4.5/1.8 | n-BuOH (0.4) | 130 | 20 | 6 ± 1 (n = 2) |
13 | 4 (2.0/1.0) | 2.7/3.9 | 9.0/3.6 | n-BuOH (0.4) | 130 | 20 | 21 ± 1 (n = 2) |
14 | 4 (3.5/1.0) | 2.7/2.3 | 9.0/2.1 | n-BuOH (0.4) | 130 | 10 | 28 ± 2 (n = 4) |
15 | 5 (2.0/1.0) | 2.7/3.4 | 18.0/6.3 | MeOH (0.4) | 110 | 10 | 0 (n = 2) |
16 | 5 (5.0/1.0) | 2.7/1.3 | 18.0/2.5 | n-BuOH (0.4) | 150 | 20 | 13 ± 2 (n = 3) |
Noteworthy, the 18F in [18O]water was loaded onto the QMA cartridge from the female side and eluted out from the male side in the normal manner. The [18F]TEAF complex was obtained after conventional azeotropic dryings and no air was flushed into the reactor during reaction. These modifications facilitated the subsequent utilization of fully automated synthetic modules.
Automated radiosynthesis of [18F]JNJ-46356479 was performed with the optimal conditions depicted in Table 1 entry 14, using a computer controlled GE TRACERlab™ FXFN module (see ESI, Fig. S1†). The final [18F]JNJ-46356479 was obtained with a radiochemical yield (RCY) of 5 ± 3% (n > 10, non-decay-corrected), a molar activity of 180 ± 102 GBq μmol−1 at end of synthesis (EOS, 45 min, n > 10) and excellent chemical and radiochemical purities (>95%, see ESI, Fig. S3†). Noteworthy, it was crucial to add n-BuOH first to dissolve the azeotropically dried [18F]TEAF complex before applying the precursor solution, otherwise only negligible amount of [18F]JNJ-46356479 (RCY < 0.1%, n = 2) would be obtained (see ESI, Fig. S4†).
In fact, the presence of sensitive heterocycles also made compounds 4 and 8 chemically unstable under certain conditions. They slowly decomposed to their aryl boronic acid analogues during flash column purification with either silica gel or alumina. Initial purifications of 4via a preparative C-18 column with a gradient elution (mobile phase: CH3CN/0.1% formic acid in water, flow rate: 15 min mL−1) resulted in a 75% degradation of 4 to its boronic acid analog 5 after the collected product fractions were dried in lyophilizer overnight. Although the boronic pinacol esters are generally considered more stable than boronic acids under protodeboronation conditions,29 the protodeboronation side product was minor (<5%). In addition, slight decompositions were also noticed for 4 and 8 during NMR characterizations in deuterated solvents of CDCl3, CD3OD and dimethyl sulfoxide-d6. In practice, the Bpin precursors were purified via flash column chromatography immediately after workup and characterized by LC-MS and NMR for their purities and identities. Fortunately, the boronic acid analogues were also suitable for the radiofluorination method as tested here with compound 5. Moreover, once isolated, both precursors are quite stable at room temperature. During the reaction, compounds 4 and 8 maintained a high purity of >95%, with marginal amount of their boronic acid analogues. Although pure compound 5 was not directly tested under the [18F]KF/K222 conditions, a mixture containing 62% of 4, 35% of 5 and 3% of their protodeboronation side product did not lead to the desired [18F]JNJ-46356479, whereas under the optimized reaction conditions shown in Table 1 entry 14, [18F]JNJ-46356479 can be obtained with the same mixture.
Switching the base from K2CO3 to TEAB together with the addition of n-BuOH had alleviated the harshness of the reaction conditions and led to the desired products from their Bpin precursors. Even under this modified protocol, the amount of TEAB had to be limited to ensure an optimal RCC (Table 1, entry 4). The n-BuOH was positioned separately in vial 2 and added to reaction vessel before the precursor solution in the FXFN platform, which proved to be crucial, otherwise only negligible [18F]JNJ-46356479 would be obtained. The successful automated radiosynthesis of [18F]JNJ-46356479 and [18F]FTIM demonstrated that employment of traditional 18F trap & release on QMA cartridge, conventional azeotropic drying, as well as devoid of air flush to the reaction mixture, was applicable to the alcohol-enhanced 18F-fluorodeboronation.
Footnote |
† Electronic supplementary information (ESI) available: Manual & automated radiosynthesis, radioTLC, radioHPLC, NMR spectra and detailed reaction condition optimizations. See DOI: 10.1039/d0ra04943c |
This journal is © The Royal Society of Chemistry 2020 |