Radiosynthesis of [18F]ATPFU: A potential PET ligand for mTOR

Article abstract of DOI:10.1002/jlcr.3239

Mammalian target of rapamycin (mTOR) plays a pivotal role in many aspects of cellular proliferation, and recent evidence suggests that an altered mTOR signaling pathway plays a central role in the pathogenesis of aging, tumor progression, neuropsychiatric, and major depressive disorder. Availability of a mTOR-specific PET tracer will facilitate monitoring early response to treatment with mTOR inhibitors that are under clinical development. Towards this we have developed the radiosynthesis of [¹⁸F]1-(4-(4-(8-oxa-3-azabicyclo[3.2.1]octan-3-yl)-1-(2,2,2-trifluoroethyl)-1H-pyrazolo[3,4-d]pyrimidin-6-yl) phenyl)-3-(2-fluoroethyl)urea [¹⁸F]ATPFU ([¹⁸F]1) as an mTOR PET ligand. Synthesis of reference 1 and the precursor for radiolabeling, 4-(4-8-oxa-3-azabicyclo[3.2.1]-octan-3yl)-1-(2,2,2-trifluoroethyl)-1H-pyrazolo[3,4-d]pyrimidin-6yl)aniline (10), were achieved from beta-chloroaldehyde 3 in 4 and 5 steps, respectively, with an overall yield of 25-28%. [¹⁸F] Fluoroethylamine was prepared by heating N-[2-(toluene-4-sulfonyloxy)ethyl]phthalimide with [¹⁸F]fluoride ion in acetonitrile. [¹⁸F]1 was obtained by slow distillation under argon of [¹⁸F]FCH₂CH₂NH₂ into amine 10 that was pre-treated with triphosgene at 0-5 °C. The total time required for the two-step radiosynthesis including semi-preparative HPLC purification was 90 min, and the overall radiochemical yield of [¹⁸F]1 for the process was 15 ± 5% based on [¹⁸F]fluoride ion (decay corrected). At the end of synthesis (EOS), the specific activity was 37-74 GBq/μmol (N= 6).

Full text of DOI:10.1002/jlcr.3239

Research Article
Received 28 July 2014,
Revised 17 September 2014,
Accepted 22 September 2014
Published online 31 October 2014 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3239
18
Radiosynthesis of [ F]ATPFU: a potential PET
ligand for mTOR
Vattoly J. Majo,a Norman R. Simpson,a Jaya Prabhakaran,b
J. John Mann,a,b and J. S. Dileep Kumara,c
*
Mammalian target of rapamycin (mTOR) plays a pivotal role in many aspects of cellular proliferation, and recent evidence
suggests that an altered mTOR signaling pathway plays a central role in the pathogenesis of aging, tumor progression,
neuropsychiatric, and major depressive disorder. Availability of a mTOR-specific PET tracer will facilitate monitoring early
response to treatment with mTOR inhibitors that are under clinical development. Towards this we have developed the
radiosynthesis of [18F]1-(4-(4-(8-oxa-3-azabicyclo[3.2.1]octan-3-yl)-1-(2,2,2-trifluoroethyl)-1H-pyrazolo[3,4-d]pyrimidin-6-yl)
phenyl)-3-(2-fluoroethyl)urea [18F]ATPFU ([18F]1) as an mTOR PET ligand. Synthesis of reference 1 and the precursor for
radiolabeling, 4-(4-8-oxa-3-azabicyclo[3.2.1]-octan-3yl)-1-(2,2,2-trifluoroethyl)-1H-pyrazolo[3,4-d]pyrimidin-6yl)aniline (10),
were achieved from beta-chloroaldehyde 3 in 4 and 5 steps, respectively, with an overall yield of 25–28%. [18F]
Fluoroethylamine was prepared by heating N-[2-(toluene-4-sulfonyloxy)ethyl]phthalimide with [18F]fluoride ion in
acetonitrile. [18F]1 was obtained by slow distillation under argon of [18F]FCH2CH2NH2 into amine 10 that was pre-treated
with triphosgene at 0–5 °C. The total time required for the two-step radiosynthesis including semi-preparative HPLC
purification was 90 min, and the overall radiochemical yield of [18F]1 for the process was 15 5% based on [18F]fluoride
ion (decay corrected). At the end of synthesis (EOS), the specific activity was 37–74 GBq/μmol (N = 6).
Keywords: mTOR; radiotracer; PET; fluorine-18
Availability of an mTOR-specific PET tracer would facilitate
Introduction
monitoring early response to treatment with mTOR inhibitors
Mammalian target of rapamycin (mTOR) is an evolutionarily
that are under clinical development for human cancers and
conserved 289-kDa serine/threonine protein kinase and a
CNS disorders. Although GSK2126458, a potent mTOR inhibitor,
member of the phosphoinositide-3-kinase (PI3K) family that
was recently radiolabeled, subnanomolar affinity against PI3K
integrates signals from multiple pathways, including nutrients,
subtypes and protein kinase B (Akt) makes it unsuitable for
specific in vivo evaluation of mTOR.15
growth factors, hormones, and stresses to regulate cellular
functions such as translation, transcription, protein turnover, cell
Pyrazolopyrimidines and pyridopyrimidines substituted with
morpholine moiety that have examples of nanomolar affinity
growth, differentiation, cell survival, metabolism, energy balance,
and stress response.1–3 Thus, mTOR functions as a molecular
mTOR inhibitors with high selectivity over PI3K subtypes,
sensor, which regulates protein synthesis, enhancing mRNA
favorable lipophilicity, and easy access to radiolabeling were
translation of genes involved in the regulation of cell
selected as candidates for developing mTOR PET ligands. Among
proliferation and survival, working as part of two distinct
multimeric complexes known as mTORC1 and mTORC2. mTOR
pathway plays a central role in controlling protein homeostasis
and negatively regulates autophagy. Recent evidence suggests
these, WYE-125132 is a highly potent, ATP-competitive, and
specific inhibitor of mTOR that inhibited mTORC1 and mTORC2
in diverse cancer models in vitro and in vivo (Figure 1).18 In the
SAR studies, the presence of fluoroalkyl group connected to the
amino group of urea linkage as in compound 1-(4-(4-(8-oxa-3-
azabicyclo[3.2.1]octan-3-yl)-1-(2,2,2-trifluoroethyl)-1H-pyrazolo-
that an altered mTOR signaling pathway plays a central role in
aging4, tumors including glioblastoma5 Alzheimer’s disease
(AD)6,7, and epileptogenesis.
A 50% reduction in mTOR
8,9
[3,4-d]pyrimidin-6-yl)phenyl)-3-(2-fluoroethyl)urea (ATPFU or 1)
was found to be equally potent as WYE-125132 towards mTOR.16,17
expression is reported in the prefrontal cortex of major
depressive disorder (MDD) subjects compared to controls.10,11
Recently, Tang et al. demonstrated that mTOR accumulates in
tangle-bearing neurons at early stages in AD brains and
mediates the synthesis and aggregation of tau.12
Pharmacological reduction of mTOR signaling with rapamycin
alleviates tau pathology and the associated behavioral deficits
in mouse model overexpressing mutant human tau.13 Thus,
mTOR inhibitors are currently proposed as alternate candidates
for the treatment of AD, epilepsy and depression. Several mTOR
inhibitors are also currently in Phase II/III clinical trials for cancer
aDivision of Molecular Imaging and Neuropathology New York State Psychiatric
Institute, New York, USA
bDepartment of Psychiatry, Columbia University College of Physicians and
Surgeons, New York, USA
cDepartment of Psychiatry, Stony Brook University, Stony Brook, USA
*Correspondence to: J. S. Dileep Kumar, Division of Molecular Imaging and
Neuropathology New York State Psychiatric Institute, New York, USA.
treatment through mono-targeted or combination therapy.14 E-mail: kumardi@nyspi.columbia.edu
J. Label Compd. Radiopharm 2014, 57 705–709
Copyright © 2014 John Wiley & Sons, Ltd.

V. J. Majo et al.
Figure 1. The structures of highly specific pyrazolopyrimidine mTOR inhibitor WYE-132 and the proposed mTOR PET tracer ATPFU.
1H-pyrazolo[3,4-d]pyrimidin-4-yl)-8-oxa-3-azabicyclo[3.2.1]octane (100 mg,
0.288 mmol), and sodium carbonate (61.0 mg, 0.575 mmol) were combined
in acetonitrile:water (1:1, volume: 2 mL) in a 10-mL round bottom flask to
give a colorless suspension. The mixture was purged under Ar. Pd(PPh3)4
(25 mg, 0.022 mmol) was added, and the reaction mixture was heated
overnight at 90–95 °C. After cooling to rt, the reaction mixture was
quenched with water and repeatedly extracted with EtOAc, washed with
brine, dried over MgSO4, concentrated under vacuum, and column
chromatographed (gradient from 25% EtOAC to 80% EtOAc in hexane) to
yield the desired product 1a as a colorless solid (91 mg, 64%); 1H NMR
(400 MHz, CDCl3) δ 8.38–8.16 (m, 3H), 7.90 (d, J = 2.2 Hz, 1H), 7.54–7.34
(m, 2H), 6.22 (s, 1H), 4.96 (q, J = 8.4 Hz, 2H), 4.51 (dd, J = 5.4, 3.4 Hz, 3H),
4.39 (t, J = 4.8 Hz, 1H), 3.51–3.36 (m, 4H), 2.55–2.48 (m, 2H), 1.95 (dd,
J = 7.9, 4.2 Hz, 2H), 1.81 (d, J = 5.8 Hz, 2H); HRMS Calcd for C22H24F4N7O2
(MH+): 494.1928; Found: 494.1917.
ATPFU is a highly potent and specific inhibitor of mTOR that
inhibited mTORC1 and mTORC2 in vitro (IC50 = 0.23 nM). ATPFU
has no significant binding to P13KR (IC50 = 2746 nM) and a
favorable ClogP (2.03) for facile blood brain barrier (BBB)
penetration.17 Herein we describe the radiosynthesis of [18F]ATPFU
as a potential mTOR PET ligand.
Experimental section
General methods
All commercial reagents and solvents were used without further
purification unless otherwise specified. The pyrazolopyrimidine 5 was
prepared from barbituric acid (2) in 3 steps based on a literature
procedure.18 The boronate esters 8a19 and 8b19 were prepared from
commercially available 4-aminophenylboronic acid pinacol ester (6) on
treatment with triphosgene followed by the corresponding 2-fluoro (or)
2-hydroxy ethylamine. High-resolution mass spectra were acquired
under fast atom bombardment (FAB1) mode using a tandem mass
spectrometer (JKS-HX, 11UHF/HX110 HF). The 1H NMR spectra of all the
compounds were recorded on a 400-MHz spectrometer (Bruker, PPX-
400). The spectra were recorded in CDCl3 or DMSO-d6, and chemical shift
(δ) data for the proton resonances were reported in parts per million
(ppm) relative to internal standard TMS. Thin-layer chromatography
was performed using Silica Gel 60F254 plates (EM Science) and visualized
by UV light. Flash column chromatography was carried out using silica
gel 60 (Fisher Scientific, 230–400 mesh). Analytical HPLC was performed
using a reverse phase column (4.6 × 250 mm, 5 μm, Phenomenex Prodigy
ODS(3)) and eluted with mobile phase; 50:50 acetonitrile:0.1 M
ammonium formate buffer. Semipreparative HPLC was performed using
a reverse phase column (Phenomenex Prodigy ODS-Prep 10 × 250 mm,
10 μm) and eluted with 40:60:0.5 acetonitrile:0.1 M ammonium formate
solution:acetic acid buffer. For detection of radiolabeled compounds, γ-
ray detector (Bioscan Flow-Count fitted with a NaI detector) was used
in series with the UV absorbance (Waters Model 2487 set at 254 nm).
The chemical identity of the radiotracer was established by co-injecting
with a sample of nonradioactive standard. The specific activity was
determined based on a standard 5 point mass curve of cold ligand using
analytical HPLC. The F-18 isotope for radiosynthesis (25–50 mCi in 0.5 mL
of O-18 water) was supplied by IBA Molecular, North America. Data
acquisition for both the analytical and preparative systems was
accomplished using a USB data acquisition starter kit (DATAQ DI-158U)
equipped with high speed acquisition software (WinDaq). The
experimental partition coefficient (logD at pH 7.4) of the radiotracer
was determined by partitioning between 1-octanol and freshly prepared
PBS buffer (pH 7.4) and measuring the radioactivity in both phases with a
Gamma Counter (Packard Instruments) using the modified method of
Wilson et. al.20
1-(4-(4-(8-Oxa-3-azabicyclo[3.2.1]octan-3-yl)-1-(2,2,2-trifluoroethyl)-1H-
pyrazolo[3,4-d]pyrimidin-6-yl)phenyl)-3-(2-hydroxyethyl)urea (1b). In
a
10-mL round bottom flask, was 1-(2-hydroxyethyl)-3-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)urea (114 mg, 0.374 mmol),
(1R,5S)-3-(6-chloro-1-(2,2,2-trifluoroethyl)-1H-pyrazolo[3,4-d]pyrimidin-4-
yl)-8-oxa-3-azabicyclo[3.2.1]octane (100 mg, 0.288 mmol), and sodium
carbonate (61.0 mg, 0.575 mmol) combined in CH3CN:water (1:1 ratio,
4 mL) to give a colorless solution. The reaction mixture was purged under
Ar. Pd(PPh3)4 (25 mg, 0.022 mmol) was then added in one portion. The
reaction mixture was heated overnight at 90–95 °C. After cooling to rt,
the reaction mixture was quenched with water and repeatedly extracted
with EtOAc, washed with brine, dried over MgSO4, concentrated under
vacuum, and column chromatographed (3% MeOH in EtOAc) to yield
the desired product 1b as a colorless solid (82 mg, 0.167 mmol) in 58%
yield. RP-HPLC: Phenomenex 4.6 × 250 mm, ODS-3, 5 μm column, mobile
phase: 40:60 CH3CN: 0.1 M ammonium formate solution, flow rate:
2 mL/min, tR = 5.6 min. 1H NMR (400 MHz, DMSO-d6) δ 8.86 (s, 1H),
8.49 À 8.25 (m, 3H), 7.62 À 7.41 (m, 2H), 6.27 (t, J = 5.6 Hz, 1H), 5.29
(q, J = 9.1 Hz, 2H), 4.69 À 4.46 (m, 3H), 3.47 (t, J = 5.7 Hz, 3H), 3.43 À 3.24
(m, 2H), 3.19 (q, J = 5.6 Hz, 2H), 1.96 À 1.86 (m, 2H), 1.79 (d, J = 7.0 Hz,
2H); HRMS Calcd for C22H25F3N7O3 (MH+): 492.1971; Found: 492.1966.
2-(3-(4-(4-(8-Oxa-3-azabicyclo[3.2.1]octan-3-yl)-1-(2,2,2-trifluoroethyl)-1H-
pyrazolo[3,4-d]pyrimidin-6-yl)phenyl)ureido)ethyl 4-methylbenzenesulfonate
(1c). In a 10-mL round bottom flask was 1-(4-(4-((1R,5S)-8-oxa-3-azabicyclo
[3.2.1]octan-3-yl)-1-(2,2,2-trifluoroethyl)-1H-pyrazolo[3,4-d]pyrimidin-6-yl)
phenyl)-3-(2-hydroxyethyl)urea (10 mg, 0.02 mmol) in CH2Cl2 (2 mL) to
give a colorless suspension. Triethylamine (9 μL, 0.061 mmol) was added
followed by p-toluenesulfonic anhydride (9.96 mg, 0.031 mmol). A clear
colorless solution was formed. The reaction mixture was stirred at rt for
30 min. The reaction mixture was directly column chromatographed
(CH2Cl2:methanol 92:8 to 96:4) on a neutral alumina column under Ar to
give the desired product 1c as a light yellow solid (10 mg, 78%). RP-HPLC:
Phenomenex 4.6 × 250 mm, ODS-3, 5-μm column, mobile phase: 45:55
CH3CN:0.1 M ammonium formate solution, flow rate: 2 mL/min,
tR = 16 min. 1H NMR (400 MHz, CDCl3) δ 8.43 (d, J = 8.2 Hz, 2H), 8.01
(d, J = 6.0 Hz, 1H), 7.81 (d, J = 7.9 Hz, 2H), 7.43 (d, J = 8.3 Hz, 2H), 7.33
(d, J = 8.3 Hz, 2H), 7.01 (s, 1H), 5.53 (t, J = 6.0 Hz, 1H), 5.06 (q, J = 8.2 Hz,
2H), 4.62 (d, J = 5.4 Hz, 3H), 4.16 (dd, J = 9.4, 5.5 Hz, 3H), 3.56 (dd, J = 10.3,
5.2 Hz, 4H), 2.39 (s, 3H), 2.06–2.09 (m, 2H), 1.89 (d, J = 7.1 Hz, 2H).
Chemistry and radiochemistry
1-(2-Fluoroethyl)-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)
urea (1a). (106 mg, 0.345 mmol), (1R,5S)-3-(6-chloro-1-(2,2,2-trifluoroethyl)-
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V. J. Majo et al.
4-(4-(8-Oxa-3-azabicyclo[3.2.1]octan-3-yl)-1-(2,2,2-trifluoroethyl)-1H-pyrazolo
CDCl3) δ 8.41–8.28 (m, 2H), 7.98 (d, J = 2.6Hz, 1H), 6.84–6.71 (m, 2H), 5.05
[3,4-d]pyrimidin-6-yl)aniline (10). 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) (q, J = 8.5 Hz, 2H), 4.61 (d, J = 4.5 Hz, 2H), 3.93 (brs, 2H), 3.58 (brs, 2H), 2.06
aniline (82 mg, 0.374 mmol), (1R,5S)-3-(6-chloro-1-(2,2,2-trifluoroethyl)-
1H-pyrazolo[3,4-d]pyrimidin-4-yl)-8-oxa-3-azabicyclo[3.2.1]-octane (100 mg,
(dd, J = 8.5, 4.3 Hz, 2H), 1.90 (d, J = 7.3 Hz, 2H).
[
18F]1-(2-Fluoroethyl)-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
0.288 mmol), and sodium carbonate (61.0 mg, 0.575 mmol) were combined phenyl)urea ([18F]1a). An aqueous solution of [18F]fluoride ion (25–50 mCi
in CH3CN:water (1:1 ratio, 4 mL) in a 10-mL round bottom flask to give a
colorless suspension. The mixture was purged under Ar. Pd(PPh3)4 (25 mg,
0.022 mmol) was then added in one portion. The reaction mixture was
heated overnight at 90–95 °C. After cooling to rt, the reaction mixture was
quenched with water and repeatedly extracted with EtOAc, washed with
brine, dried over MgSO4, passed through a celite pad and concentrated
under vacuum and column chromatographed (hexane:EtOAc 20:80) to give
in 0.5 mL of O-18 water) in a 5-mL V-vial was treated with 200 μL of 15:1
acetonitrile:water containing kryptofix K222 (36 mg) and potassium
carbonate (2 mg). The mixture was azeotropically heated and dried at
95 °C under a stream of argon by the repeated addition of acetonitrile
(3 × 0.5 mL).22 A solution of 30 mg of N-[2-(toluene-4-sulfonyloxy)ethyl]
phthalimide in 1 mL of anhydrous acetonitrile was then added to the
reaction vial, sealed, and heated for 10 min at 100 °C.23 The reaction
mixture was then evaporated under vacuum and hydrazine monohydrate
1
the desired product 10 as a brown solid (75 mg, 65%). H NMR (400 MHz,
Scheme 1. Synthesis of ATPFU, hydroxyl, and tosyl analogues. i) DMF/POCl3, rt, 72%; ii) CF3CH2NHNH2, EtOH, Et3N, À78 °C, 64%; iii) (1R,5S)8-oxa-3-azabicyclo[3.2.1]octane
hydrochloride, Et3N, rt, 60%; iv) triphosgene, CH2Cl2, rt; 84%; v) Na2CO3, acetonitrile, Pd(PPh3)4, 90 °C, 64%; vi) TsCl, CH2Cl2, Et3N, rt, 78%.
Scheme 2. Reactions of 1c with [18F]fluoride ion. i) [18F]TBAF (or) [18F]KF/K222, heat.
J. Label Compd. Radiopharm 2014, 57 705–709
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V. J. Majo et al.
Scheme 3. Radiosynthesis of [18F]ATPFU. i) Na2CO3, acetonitrile, Pd(PPh3)4, 90 °C, 65%.
(100 μL, 99 + %, 64% NH2NH2 in water) was added for deprotection of the
phthalimide. The resultant free [18F]fluoroethylamine was distilled under
Ar at 75 °C into a second V-vial containing a solution of arylamine 10
(2 mg) in CH2Cl2 pretreated with triphosgene (1.4 mg) and triethylamine
(20 μL) at 0 °C. A slow argon flow (<10 psi) is maintained for a steady
transfer of activity into the second V-vial. After 15–20 min, the CH2Cl2
was removed under Ar at rt, and the precipitate was dissolved in
acetonitrile (0.5 mL), and then injected onto a semipreparative HPLC
column (Phenomenex, Prodigy ODS-Prep 10 × 250 mm, 10 μm; and
eluted with 40:60:0.5 acetonitrile:0.1 M ammonium formate solution:
acetic acid; pH = 5.5–6.0; flow rate: 10 mL/min). The product fraction of
beta-chlorovinylaldehyde 3 obtained by the Vilsmeier formylation
of pyrimidine-2,4,6(1H,3H,5H)trione (2) in 72% yield.21 The
aldehyde 3 was reacted with trifluoroethyl hydrazine at À78°C
followed by treatment with bridged morpholine (1R,5S)8-oxa-3-
azabicyclo[3.2.1]octane to provide 5 in 60% yield. Compound 5
was then coupled with phenylboronate 8a to give reference
standard 1a in 64% yield (Scheme 1).
Our initial strategy was to perform a tosyl to [18F]fluoro
nucleophilic displacement reaction to achieve [18F]ATPFU.
Towards this, the hydroxyl derivative 1b was synthesized by
coupling of compound 5 with boronic ester 8b in 58% yield.
The compound 1b upon reaction with TsCl yielded tosyl-
precursor 1c in 78% yield (Scheme 1). The radiosynthesis of
[18F]ATPFU was attempted with tosyl-ATPFU (1c) with [18F]KF
and [18F]TBAF under a variety of conditions. However, the tosyl
precursor 1c did not undergo radiofluorination, instead,
imidazolidin-2-one 9a and 4,5-dihydrooxazol-2-amine 9b were
[
18F]1a eluted at 11–12 min was collected based on the γ-detector,
diluted with 100 mL of deionized water, and passed through a classic C-
18 Sep-Pak® cartridge, washed with 10 mL of deionized water and eluted
with 1 mL of absolute ethanol. Reconstitution of the product in 1 mL of
ethanol and 9 mL of 0.9% sodium chloride solution for injection, USP
afforded [18F]ATPFU. A portion of the ethanol solution was injected into
an analytical HPLC (Phenomenex, Prodigy ODS(3) 4.6 × 250 mm, 5 μm;
mobile phase; 50:50 acetonitrile:0.1 M ammonium formate solution, flow
rate 2 mL/min, tR = 7–8 min) to determine the specific activity, and obtained as the only products under experimental conditions
chemical and radiochemical purities.
(Scheme 2). The formation of 9a and 9b can be attributed to
the competing intramolecular nucleophilic reaction of amide or
carbonyl group with carbanion derived from 1c.
Result and discussion
Subsequently we decided to construct [18F]ATPFU from 4-(4-
Unlabeled ATPFU was synthesized based on a previous report. In (8-oxa-3-azabicyclo[3.2.1]octan-3-yl)-1-(2,2,2-trifluoroethyl)-1H-
pyrazolo[3,4-d]pyrimidin-6-yl)aniline (10) by sequential
a
brief, the pyrazolopyrimidine skeleton was constructed from
Figure 2. HPLC chromatograms of [18F]ATPFU; A. semipreparative HPLC; B. analytical QC; C. co-injection.
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J. Label Compd. Radiopharm 2014, 57 705–709

V. J. Majo et al.
carbonylation followed by tethering with fluoroethyl group
(Scheme 3). Compound 10, the precursor for radiosynthesis,
was prepared in 64% yield by the Suzuki coupling of
pyrazolopyrimidine 5 with 4-aminophenylboronic acid pinacol
ester (6). [18F]Fluoroethylamine was prepared from N-[2-
(toluene-4-sulfonyloxy)ethyl]phthalimide and [18F]fluoride ion
and purified by distillation.22,23 The radiosynthesis of [18F]1a
was achieved by freshly distilling [18F]fluoroethylamine to a
solution of 10 containing triphosgene at 0–5 °C (Scheme 3,
Figure 2). The crude product was purified via semipreparative
HPLC, followed by solid phase extraction with C18 Sep-Pak®.
[18F]1a was obtained in 90 min in 15 5% yield at end of
synthesis (EOS) in high radiochemical purity (>95%) and
specific activity (37–74 GBq/μmol). The stability of radioproduct
in 10% ethanol-saline solution was determined by HPLC
injections over a course of 6 h. [18F]1a was determined to be
stable under these conditions based on the absence of any
additional radiopeaks in HPLC. The logD at pH 7.4 of [18F]1
was measured as 1.9, indicating that the radioligand has
adequate lipophilicity for passive brain entry.20
Conflict of interest
The authors did not report any conflict of interest.
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Conclusions
In conclusion, we have successfully developed a method for
the radiosynthesis of [18F]ATPFU, a highly selective mTOR
ligand. The synthesis of standard and radiolabeling precursor
was achieved from commercially available barbituric acid in
5 and 4 steps, respectively. Conventional 18F-labeling with
tosyl precursor did not yield radioproduct under thermal and
radiochemical conditions as a result of the intramolecular
cyclization to yield imidazolidone 9a and (or) dihydrooxazlo-
2-amine 9b. The radiosynthesis of [18F]ATPFU was achieved
by reacting freshly distilled [18F]fluoroethylamine to a solution
of amine derivative 10 and triphosgene in dichloromethane
at 0–5 °C. The total time required for the radiosynthesis
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was 90 min.
from
18F] fluoride ion (decay corrected) with excellent
radiochemical purity and specific activity. Further studied will
[
18F]ATPFU was obtained in 15 5% yield
[
be aimed at determined by the in vitro and in vivo specificity [20] A. A. Wilson, L. Jin, A. Garcia, J. N. DaSilva, S. Houle, Appl. Radiat. Isot.
of [18F]ATPFU.
2001, 54, 203.
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Acknowledgements
This work was funded by NIMH grant MH062185.
Bioconjug. Chem. 2010, 21, 911.
J. Label Compd. Radiopharm 2014, 57 705–709
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