An improved radiosynthesis of O-(2-[18F]fluoroethyl)-O-(p-nitrophenyl)methylphosphonate: A first-in-class cholinesterase PET tracer

Article abstract of DOI:10.1002/jlcr.3511

O-(2-Fluoroethyl)-O-(p-nitrophenyl) methylphosphonate 1 is an organophosphate cholinesterase inhibitor that creates a phosphonyl-serine covalent adduct at the enzyme active site blocking cholinesterase activity in vivo. The corresponding radiolabeled O-(2-[¹⁸F]fluoroethyl)-O-(p-nitrophenyl) methylphosphonate, [¹⁸F]1, has been previously prepared and found to be an excellent positron emission tomography imaging tracer for assessment of cholinesterases in live brain, peripheral tissues, and blood. However, the previously reported [¹⁸F]1 tracer synthesis was slow even with microwave acceleration, required high-performance liquid chromatography separation of the tracer from impurities, and gave less optimal radiochemical yields. In this paper, we report a new synthetic approach to circumvent these shortcomings that is reliant on the facile reactivity of bis-(O,O-p-nitrophenyl) methylphosphonate, 2, with 2-fluoroethanol in the presence of DBU. The cold synthesis was successfully translated to provide a more robust radiosynthesis. Using this new strategy, the desired tracer, [¹⁸F]1, was obtained in a non-decay–corrected radiochemical yield of 8?±?2% (n?=?7) in >99% radiochemical and >95% chemical purity with a specific activity of 3174?±?345 Ci/mmol (EOS). This new facile radiosynthesis routinely affords highly pure quantities of [¹⁸F]1, which will further enable tracer development of OP cholinesterase inhibitors and their evaluation in vivo.

Full text of DOI:10.1002/jlcr.3511

An Improved Radiosynthesis of O-(2-[18F]Fluoroethyl)-O-(p-
nitrophenyl)methylphosphonate: A First-in-Class Cholinesterase PET Tracer.
Abbreviated Title: Optimizing preparation of [18F]cholinesterase inhibitors.
Kiel D. Neumann,a* Charles M. Thompson,b Joseph E. Blecha,a John M. Gerdes,b
and Henry F. VanBrocklina
a Department of Radiology and Biomedical Imaging, University of California – San
Francisco, 185 Berry St. Suite 350, San Francisco, CA 94107
b Department of Biomedical and Pharmaceutical Sciences, University of Montana,
Missoula, Montana, 59812
Corresponding Author: Kiel D. Neumann, Department of Radiology and Biomedical
Imaging, University of California – San Francisco, 185 Berry St. Suite 350, San Francisco,
CA 94107.
Phone: 415-353-4569
FAX: Fax: (415) 476-0616
E-mail: kiel.neumann@ucsf.edu
Keywords: β-fluoroethoxyphosphonate, fluorine-18, acetylcholinesterase, positron emission
tomography (PET), [18F]-fluoroethanol, bis-(p-nitrophenyl) methylphosphonate.
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been through the copyediting, typesetting, pagination and proofreading process which may
lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1002/jlcr.3511
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Abstract
O-(2-Fluoroethyl)-O-(p-nitrophenyl) methylphosphonate 1 is an organophosphate (OP)
cholinesterase inhibitor, which creates a phosphonyl-serine covalent adduct at the enzyme
active site blocking cholinesterase activity in vivo. The corresponding radiolabeled O-(2-
[18F]fluoroethyl)-O-(p-nitrophenyl) methylphosphonate, [18F]1, has been previously prepared
and found to be an excellent positron emission tomography (PET) imaging tracer for
assessment of cholinesterases in live brain, peripheral tissues and blood. However, the
previously reported [18F]1 tracer synthesis was slow even with microwave acceleration,
required HPLC separation of the tracer from impurities and gave less optimal radiochemical
yields. In this paper, we report a new synthetic approach to circumvent these shortcomings
that is reliant on the facile reactivity of bis-(O,O-p-nitrophenyl) methylphosphonate, 2, with
2-fluoroethanol in the presence of DBU. The cold synthesis was successfully translated to
provide a more robust radiosynthesis. Using this new strategy, the desired tracer, [18F]1 was
obtained in a non-decay corrected radiochemical yield of 8 ± 2% (n = 7) in > 99%
radiochemical and > 95% chemical purity with a specific activity of 3,174 ± 345 Ci/mmol
(EOS). This new facile radiosynthesis routinely affords highly pure quantities of [18F]1,
which will further enable tracer development of OP cholinesterase inhibitors and their
evaluation in vivo.
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Introduction.
Organophosphorus esters (OP) are a class of compounds that include insecticides
(parathion, malathion, etc.) and chemical nerve agents (sarin, soman, VX, etc.).1-2 OP nerve
agents (Figure 1) are particularly lethal due to their reactive P=O bond and leaving group Z
that combine to rapidly react with the neurotransmitter-hydrolyzing enzyme
acetylcholinesterase (AChE). OP insecticides first require conversion to form the reactive
oxon form that reacts with AChE.7 OP nerve agents and insecticide oxons are structurally
similar sharing a phosphoester group (R = alkyl) and leaving group Z, but differ in that nerve
agents possess a P-CH3 bond whereas insecticide oxons are typically diesters.
O
P
S
P
O
P
oxidation
O R
CH3
O R
O R
O R
Z
O
R
Z
Z
OP Nerve Agent
OP insecticide
OP insecticide oxon
malathion: R = Me, Z = SCH(CO2Et)CH2CO2Et
parathion: R = Et, Z = O-Ph-pNO2
VX: R = Et ; Z = SCH2CH2N(iPr)2
sarin: R = iPr; Z = F
soman: R = CH(CH3)C(CH3)3, ; Z = F
cyclosarin: R = cyclohexyl ; Z = F
chlorpyriphos: R = Et, Z = O-[2,3,5-trichloropyridinol]
diazinon: R = Et, Z = O-[6-Me,4-iPr-pyrimidinol]
Figure 1. Examples of organophosphate chemical nerve agents and insecticides
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O
S
CH3
O
O
O
O
O
P
P
O2N
(iPr)2N
paraoxon
VX
O
CH3
O
F18F
P
O
O2N
Fluoro-VX surrogate 1; OP PET tracer [18F]1
O-(2-fluoroethyl)-O-(p-nitrophenyl) methyphosphonate
Figure 2. Fluoro-VX surrogate based on VX and paraoxon structures
Despite this minor difference, nerve agents and insecticide oxons are covalent
inhibitors of AChE. Given these common structural components, we developed a hybrid OP
structure based upon the nerve agent VX and paraoxon (insecticide oxon)8 modified to
incorporate an [18F]fluorine atom to produce an OP-based PET imaging tracer (Figure 2).9
The resultant fluoro-VX surrogate, O-(2-fluoroethyl)-O-(p-nitrophenyl) methylphosphonate,
utilizes a p-nitrophenoxy leaving group and acts similarly in vitro and in vivo to VX without
handling concerns.10-11 The fluorine tracer atom is strategically positioned four bonds from
the reactive phosphorus atom to minimize steric or electronic perturbation.12 Injection of high
specific activity O-(2-[18F]fluoroethyl)-O-(p-nitrophenyl) methylphosphonate, [18F]1, in rats
shows localization of radioactivity in AChE rich regions of brain and other tissues9 where it
is likely exists as (CH3)(18FCH2CH2O)P(O)-AChE and other esterase adducts.12
Although in vivo
studies with O-(2-[18F]fluoroethyl)-O-(p-nitrophenyl)
methylphosphonate were highly successful, the synthesis was slow (~150 min), requiring
microwave acceleration and yields were low due to low-purity acquisition of the precursor
[18F]-2-fluoroethyltosylate.9 To further advance tracer [18F]1 studies, our goal was to develop
a faster, more reliable and higher yielding synthetic approach.
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Experimental
Materials. All reagents were obtained from Sigma-Aldrich (St. Louis, MO) and used without
purification, unless otherwise noted. Solvents were purchased in their anhydrous forms and
used without any additional purification. For cold chemistry, flash chromatography was
conducted using silica gel (200-300 mesh) and thin-layer chromatography (TLC) was
visualized by UV and/or staining by 2,6-dibromoquinone-4-chloroimide (DBQ) or iodine.
1
19
31
The non-radioactive molecular characterizations utilized H-, F- and P-NMR that were
recorded in CDCl3 on a Varian Avance 400-MHz spectrometer; chemical shifts (δ) are
reported in ppm (relative to 7.26 ppm) and coupling constants (J) in Hertz (Hz). For
radiochemical transformations, analytical HPLC was performed using a Waters 590 LC pump
(Milford, MA), connected in series to a Shimadzu SPD-UV-visible detector (Columbia, MD)
(λ = 254 nm) and a gamma counting in-line radiation flow detector (Model 105s, CRA;
Berkeley, CA). Semi-preparative HPLC was performed with a Waters 600 LC pump
(Milford, MA) connected in series to a Shimadzu SPD-UV-visible detector (Columbia, MD)
(λ = 254 nm) and a gamma counting in-line radiation flow detector (Model 105s, CRA;
Berkeley, CA). HPLC chromatograms were acquired using SRI PeakSimple software
(version 304 – Torrence, CA). QMA cartridges for concentrating [18F]fluoride from cyclotron
production were purchased from Myja Scientific (O’Neill, NE).
Synthesis of O-(2-fluoroethyl)-O-(4-nitrophenyl) methylphosphonate (1). Solid bis-(O,O-
p-nitrophenyl) methylphosphonate (0.068 g; 0.200 mmol)13 was added to a Wheaton vial (0.5
mL total volume) containing 0.2 mL of anhydrous dichloromethane. The flask was fitted
with a septa, a triangular stir vane added, and the atmosphere exchanged for nitrogen (balloon
needle inlet) at 25 C. To this solution was added 1,8-diazabicycloundec-7-ene (DBU; 0.016
mL; 0.100 mmol), whereupon the solution turned a light yellow (due to formation of some p-
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nitrophenoxy oxyanion). The solution was stirred at 25˚ C for 15 min (TLC shows no
decomposition of the starting material) and 2-fluoroethanol (6.6 mg; 6.5 µL; 0.1 mmol) was
added via a 10 µL Hamilton syringe. Within a few seconds, the yellow solution turns to
darker yellow or burnt orange. Thin-layer chromatography shows formation of the product
(Rf ~ 0.25-0.30, EtOAc:hex, 7:3) that was confirmed by 31P NMR (29.3 ppm) at 3 min post-
addition of 2-fluoroethanol. After an additional 2 min, the reaction indicates complete
conversion to O-(2-fluoroethyl)-O-(p-nitrophenyl)methylphosphonate 1 as determined by
31
ratio of product to starting material by P NMR along with co-identification of the starting
19
material (Rf ~ 0.40; two-fold excess in this experiment) and p-nitrophenol (Rf ~ 0.55). F
NMR indicates > 95% consumption of fluoroethanol and formation of O-(2-fluoroethyl)-O-
(p-nitrophenyl)methylphosphonate (-224.5 ppm). Flash chromatography with EtOAc:hex
(7:3) affords 24 mg (90% based on fluoroethanol) that was > 98% pure as determined by
HPLC using a Sonoma C18(2) 5m 100Å 15 cm x 4.6 mm (ES Industries; SMA-C18) column
at 275 nm with 35% CH3CN/0.01% H3PO4 in H2O at 1.50 mL/min. Spectral data:14 1H NMR
(400.18 MHz, CDCl3) δ 8.84 (d, J = 9.2 Hz, 2H), 7.41 (d, J = 9.2 Hz, 2H), 4.50-4.67 (m, 2H),
13
4.24-4.47 (m, 2H), 1.75 (d, J = 17.8 Hz, 3H). C NMR (100.63 MHz, CDCl3) δ 155.14,
31
144.75, 125.89, 121.11, 82.72, 81.35, 12.21 (d, JCP = 144.15 Hz). P NMR (162.0 MHz,
CDCl3) δ 29.3. 19F NMR (376.55 MHz, CDCl3) δ -224.47.
Radiosynthesis of O-(2-[18F]fluoroethyl)-O-(p-nitrophenyl) methylphosphonate; [18F]1.
A Kryptofix K2.2.2/KHCO3 solution was prepared from K2.2.2 (12 mg) in 0.8 mL CH3CN
followed by the addition of 0.1 mL of a 10 mg/mL KHCO3 solution in water. [18F]Fluoride,
18
obtained by the nuclear reaction of O(p,n)18F from a GE PETtrace cyclotron, was eluted
from a miniature QMA cartridge (Myja Scientific) using the Kryptofix/KHCO3 solution and
the solvent was azeotropically distilled at 115 °C under a stream of nitrogen and reduced
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pressure. Once the vial was dry, an additional 0.8 mL of anhydrous CH3CN was added to the
reaction vial and the solvent was azeotropically distilled again affording apparent dry reaction
vial contents. The reaction vial was cooled to 25 °C and a solution of ethylene sulfite (20 μL
in 300 μL of anhydrous CH3CN) was added. The vial was sealed, then heated to 85 ˚C for 20
min on an aluminum block heater. The reaction vial was allowed to cool for 3 min
whereupon bis-(O,O-p-nitrophenyl) methylphosphonate (5.0 mg dissolved in 400 μL of
anhydrous CH3CN and 2 μL DBU) was added. The vial contents were manually mixed for 5
sec and allowed to stand at 25 °C for 10 min. The resultant reaction mixture was passed
through a C18 light cartridge (Waters) into a clean reaction vial and the solution was
evaporated at 40 °C under a stream of nitrogen gas and reduced pressure. The resultant
residue was dissolved in 300 μL of acetonitrile and injected onto a semi-preparative HPLC
column (Luna C18(2) column, 250 × 10 mm, 5 μm) using a gradient mobile phase (1:9
CH3CN:H2O containing 0.1% ascorbic acid for 4 min then linear increase to 8:2 over 30 min,
Rt = 16 min) at a flow rate of 3 mL/min and detection at 254 nm. The isolated product was
diluted with 20 mL of water, supplemented with 0.1% w/v L-ascorbic acid. The solution was
passed through a C18 light cartridge at a rate of 4 mL/min. The cartridge is then washed with
5 mL of deionized water and the final product is eluted from the cartridge with 200 µL of
ethanol and collected in a glass vial. The ethanol is then removed under vacuum for 20 sec.
The desired tracer product identity, [18F]1, was confirmed using the identical HPLC
conditions and co-elution with cold standard 1. Specific activity was determined by
integrating area against a standard curve. Non-decay-corrected radiochemical yields (EOB)
of [18F]1 were 8 ± 2% (n = 7). Chemical and radiochemical purities were >95% and >99%,
respectively. Specific activity was calculated to 3,174 ± 345 Ci/mmol at the end of synthesis
(n = 7). Overall synthesis time, including HPLC and product reformulation, from EOB was
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65 min. Starting from 150-300 mCi of [18F]fluoride, the range of final isolated yields of
[18F]1 were 10-50 mCi.
Results and Discussion
Phosphorylation of AChE by 1 requires nucleophilic attack by the primary alcohol of
an active serine residue, which liberates the p-nitrophenoxy (Z) moiety. Therefore, the [18F]-
label in the VX surrogate structure (Figure 2) must be introduced in either the methyl or
ethoxy groups to remain attached to AChE after inhibition. Incorporation of fluorine at the
methyl or alpha carbon of the ethoxy substituent would increase hydrolytic reactivity at
phosphorus atom and ester group, respectively, and were dismissed as possible fluorine atom
introduction sites leaving the beta ethyl position as the sole option for positron atom labeling.
The suitable viability of this beta ethyl position strategy has recently been further confirmed
by cold ligand in vitro cholinesterase studies.12
In our prior tracer preparation report, we demonstrated the microwave-assisted SN2
reaction of 2-fluoroethyl tosylate ([18F]FCH2CH2OTs) with the cesium salt of methyl (p-
nitrophenoxy) phosphonic acid, leading to [18F]1.9 Although successful, a consistent
radiosynthesis (ndc yields ~1-2.5% for 150 min preparation) was found routinely limited as a
result of the sensitive coupling between the phosphonic acid salt and 2-fluoroethyl tosylate,
and the inconsistent technical limitations using a microwave apparatus. Thus, a higher
yielding and technically streamlined synthetic methodology was sought. Herein, we report a
higher yielding and effectively reactive approach to synthesize [18F]1 quickly and in efficient
conversion directly from bis-(O,O-p-nitrophenyl) methylphosphonate (2) (prepared from
methyl phosphonic dichloride).13 This approach may be generally amenable to prepare other
radiolabeled OP agents shown in Figure 1.
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Previous investigators showed that 2 (31P-NMR  25.2) reacts effectively with a
variety of alcohols in the presence of 1,8-diazabicycloundec-7-ene (DBU) to afford methyl p-
nitrophenyl phosphonates.13 We successfully extended this reaction using 2-fluoroethanol as
the coupling alcohol to prepare compound 1 in near quantitative yield. Reaction monitoring
by chromatographic and spectroscopic (1H- and 31P-NMR) methods indicated immediate
formation of compound 1 with minimal side product formation, namely the undesired
formation of p-nitrophenol. Aqueous reaction workup using 1% HCl followed by 0.1 N
NaOH effectively removed DBU and p-nitrophenol, respectively. Evaporation of the solvent
31
19
after workup indicated a sole peak in the P-NMR at  29.3 ppm and in the F-NMR at  -
224.5 ppm. The relatively rapid reaction along with the high conversion was somewhat
unexpected as the fluorine group was anticipated to reduce the nucleophilicity of the alcohol
group. It should be noted that substituting DBU for other tertiary amine bases (TEA, Hunig’s
base, lutidine, etc.) led to dramatically lower yields and increased number of side products.
The previous report using 2 also indicated using a 5-10-fold excess of the alcohol.13
This stoichiometry would not be possible with standard radiolabeling methods in which [18F]-
containing alcohols (e.g., 2-[18F]fluoroethanol) would require a high surplus of 2 for the
transformation to tracer [18F]1. We were surprised to find that a 10-fold or greater excess of 2
could be used with 2-fluoroethanol as the limiting reagent to form 1 quantitatively, based
upon the conversion of 2-fluoroethanol to product 1 followed by 19F NMR. Thus, reversing
the stoichiometric ratio of reactants did not appear to adversely influence the rate of the
transesterification reaction to produce 1. Hence, the synthetic strategy showed promise for
radiochemical translation.
Prior reports have shown that [18F]-2-fluoroethanol could be prepared by reaction of
n-Bu4N+[18F]F- with ethylene carbonate,15 tosylate displacement with Cs18F, reduction of the
fluoroacetic acid with an aluminum hydride16 or Cs18F ring opening of ethylene sulfite.16-17
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The latter process was chosen for tracer synthesis owing to minimal steps and ease of
workup. With 2-[18F]fluoroethanol in hand, we envisioned a mild and facile
transesterification with 2, as depicted in Scheme 1.
A previous report16 described nucleophilic attack of [18F]fluoride ion on ethylene
sulfite, which in the presence of trace amounts of water, yielded 2-[18F]fluoroethanol. The
SN2-like reaction was conducted at 85 °C for 20 min, which routinely afforded 2-
[18F]fluoroethanol in yields of 50-60%, as determined by radioTLC. While the previous
report16 also described intermediate purification of 2-[18F]fluoroethanol by distillation, we did
Scheme 1. Radiochemical synthetic scheme of [18F]1.
not anticipate the subsequent transesterification reaction would be affected by the presence of
non-competing reactants from the labeling reaction. Indeed, upon cooling the reaction vial to
room temperature after completion of the 2-[18F]fluoroethanol reaction, the direct addition of
2 (dissolved in acetonitrile), in the presence of DBU, resulted in formation of [18F]1 within
10 min at room temperature in 30% yield, as determined by analytical HPLC.
As the transesterification of 2 to produce [18F]1 is ultimately limited by the amount of
2-[18F]fluoroethanol present, we sought to increase the radiochemical yields of 2-
[18F]fluoroethanol. Neither an increase in temperature nor reaction time increased the yields
in any appreciable amount. Owing to the apparent lack of sensitivity of this reaction to polar
protic solvent, we postulated the presence of bulky, aliphatic alcohols would increase the
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yields of 2-[18F]fluoroethanol as has been previously described18 for [18F]fluorination of
aliphatic electrophiles. Indeed, as evidenced by radioTLC, yields of 2-[18F]fluoroethanol
increased to 75-85%, however, this yield increase, that was the direct result of t-amyl alcohol
in the reaction mixture, did not result in a subsequent increase in yields of [18F]1. This
outcome is thought presumably due to competitive transesterification reaction of the excess t-
amyl alcohol at the phosphorus atom.
The previous radiochemical synthesis reported9 for [18F]1 was also complicated by
the need of a normal phase chromatographic separation using ethyl acetate and hexanes,
which required substantial reformulation for mammalian use; namely, the final product
required a 10% (v/v) supplement of acetonitrile in PBS to ensure tracer stability prior to
injection. We attempted to streamline the isolation of pure [18F]1 by simple dilution of the
acetonitrile reaction mixture with water for HPLC purification. While the radiochemical
separation was simple, a significant amount of chemical impurity (as detected at 254 nm) co-
eluted with [18F]1. Initially, we attributed the production of this impurity to a lack of
appropriate buffer under acidic (pH 4-5) conditions. However, supplementation of the HPLC
mobile phase with various acidic buffers, including acetic acid, phosphoric acid, and citric
acid, did not reverse the formation of the chemical impurity. Supplementing the mobile phase
with 0.1% L-ascorbic acid, with direct injection of the crude reaction mixture in 300 µL of
acetonitrile, completely attenuated the formation of the co-eluting impurity. As a result,
[18F]1 could be purified by reverse-phase HPLC and reformulated in 1% ethanol/saline and
0.1% L-ascorbic acid to provide [18F]1 in 8 ± 2% (n = 7) non-decay corrected radiochemical
yield with > 99% radiochemical purity and > 95% chemical purity in a total preparation time
of 65 min. In the presence of aqueous L-ascorbic acid, [18F]1 showed no evidence of
degradation over 4 h in solution (as evidenced by HPLC).
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The method for the radiochemical preparation of [18F]1 reported herein is superior to
our prior effort.9 The improved methodology gives [18F]1 in 8 ± 2% non-decay corrected
radiochemical yield with respect to starting [18F]fluoride, while the prior method gave a 1-
2% non-decay corrected yield with respect to starting [18F]fluoride. The optimized method
reported herein is a single-pot, two-step, 65-min synthesis and offers a significant technical
and practical advantage for the radiochemical syntheses. The lower yields for the initial
preparation may reflect the excess handling required including the need for microwave
irradiation to perform the phosphonate alkylation with 2-[18F]fluoroethyl tosylate leading to a
150-min total radiochemical synthesis time. The increase in radiochemical yields of [18F]1
herein may also be attributed to a modification of synthetic strategy with the phosphorus
behaving as an electrophile on incident nucleophilic [18F]fluoroethanol as opposed to
phosphorus oxyanion behaving as a nucleophile on incident [18F]fluorethyltosylate9.
Additionally, the new methodology also provides nearly a 50% increase in specific activity as
compared to the previous method, which among other advantages, will minimize the potential
side effects of mass OPs in in vivo studies.
Conclusions
Although
a
number of radiotracers and imaging approaches to investigate
acetylcholinesterase have been reported,19-24 there are very few reports of OP PET imaging
radiotracers that are based on the covalent modification of this target. Tracer [18F]1 is the
first-in-class example of a OP agent that simulates the mechanism of inhibition of OP
insecticides and chemical warfare agents, specifically, phosphonylation of the active site
serine.12 For this reason, radioligand [18F]1 is of strategic importance to evaluate in vivo
tissue exposures to OP compounds and to interrogate new and existing countermeasures to
OP toxicity. In this paper we reported a new non-radioactive (cold) synthesis to circumvent
these shortcomings that is reliant on the facile reactivity of bis-(O,O-p-nitrophenyl)
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methylphosphonate, 2, with 2-fluoroethanol in the presence of DBU. This approach was
successfully translated to provide a routine and more optimal radiosynthesis that is mild,
rapid, and high yielding, which is considered generally applicable for the radiolabeling of
many other important OP analogs. This new facile radiosynthesis routinely affords highly
pure [18F]1 in sufficient quantities that will further enable the tracer development as a novel
radiolabeled OP cholinesterase inhibitor for assessing OP modes of action with PET imaging
in vivo.
Conflict of interest
The authors declare no competing financial interest.
Acknowledgments
This work was supported by the CounterACT Program, National Institutes of Health Office
of the Director (NIH OD), and the National Institute of Neurological Disorders and Stroke
(NINDS), Grant Award Number U01NS092495. The content is solely the responsibility of
the authors and does not necessarily represent the official views of the National Institutes of
Health.
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An Improved Radiosynthesis of O-(2-[18F]Fluoroethyl)-O-(p-
nitrophenyl)methylphosphonate: A First-in-Class Cholinesterase PET Tracer.
Abbreviated Title: Optimizing preparation of [18F]cholinesterase inhibitors.
Kiel D. Neumann,a* Charles M. Thompson,b Joseph E. Blecha,a John M. Gerdes,b
and Henry F. VanBrocklina
This article is protected by copyright. All rights reserved.