Synthesis and simple 18F-labeling of 3-fluoro-5-(2-(2- (fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile as a high affinity radioligand for imaging monkey brain metabotropic glutamate subtype-5 receptors with positron emission tomography

Article abstract of DOI:10.1021/jm0701268

2-Fluoromethyl analogs of (3-[(2-methyl-1,3-thiazol-4yl)ethynyl]pyridine) were synthesized as potential ligands for metabotropic glutamate subtype-5 receptors (mGluR5s). One of these, namely, 3-fluoro-5-(2-(2-(fluoromethyl) thiazol-4-yl)ethynyl)benzonitri

Full text of DOI:10.1021/jm0701268

3256
J. Med. Chem. 2007, 50, 3256-3266
18
Synthesis and Simple F-Labeling of
3-Fluoro-5-(2-(2-(fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile as a High Affinity Radioligand
for Imaging Monkey Brain Metabotropic Glutamate Subtype-5 Receptors with Positron
Emission Tomography
Fabrice G. Sime´on,* Amira K. Brown, Sami S. Zoghbi, Velvet M. Patterson, Robert B. Innis, and Victor W. Pike
Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Building 10, Room B3 C346A, 10 Center DriVe,
Bethesda, Maryland 20892-1003
ReceiVed February 1, 2007
2-Fluoromethyl analogs of (3-[(2-methyl-1,3-thiazol-4yl)ethynyl]pyridine) were synthesized as potential
ligands for metabotropic glutamate subtype-5 receptors (mGluR5s). One of these, namely, 3-fluoro-5-(2-
(2-(fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile (3), was found to have exceptionally high affinity (IC50
) 36 pM) and potency in a phosphoinositol hydrolysis assay (IC50 ) 0.714 pM) for mGluR5. Compound
3 was labeled with fluorine-18 (t1/2 ) 109.7 min) in high radiochemical yield (87%) by treatment of its
synthesized bromomethyl analog (17) with [18F]fluoride ion and its radioligand behavior was assessed with
positron emission tomography (PET). Following intravenous injection of [18F]3 into rhesus monkey,
radioactivity was avidly taken up into brain with high uptake in mGluR5 receptor-rich regions such as
striata. [18F]3 was stable in monkey plasma and human whole blood in vitro and in monkey and human
brain homogenates. In monkey in vivo, a single polar radiometabolite of [18F]3 appeared rapidly in plasma.
[18F]3 merits further evaluation as a PET radioligand for mGluR5 in human subjects.
Introduction
Glutamate is a ubiquitous excitatory neurotransmitter within
the central nervous system (CNS) and acts on diverse sets of
receptors, all with potential to modulate neurological function
and, consequently, mental state and behavior. Glutamate recep-
tors include NMDA, AMPA, and kainate receptors, which
belong to the glutamate ionotropic receptor group. Glutamate
also activates a family of G-protein-coupled receptors, the
metabotropic receptors (mGluRs).1 Based on sequence homol-
ogy, functional coupling, and pharmacology,2 mGluRs are
classified as belonging to groups I, II, or III. Group I receptors
are primarily localized postsynaptically and include subtype 1
(mGluR1) and subtype 5 (mGluR5), which exhibit different
patterns of expression in the CNS.
Figure 1. Structures of some mGluR5 ligands. Asterisks mark positions
in which ligands have been labeled with either carbon-11 or fluorine-
18.
Activation of mGluR5 stimulates phospholipase C, resulting
in phosphoinositide hydrolysis and increase of intracellular Ca2+
levels.2,3 Several potent noncompetitive antagonists for mGluR5
have been developed, including 6-methyl-2-(phenylethynyl)-
pyridine4 (MPEP; Figure 1) and 3-[(2-methyl-1,3-thiazol-4yl)-
ethynyl]-pyridine5 (MTEP; Figure 1).
Modulation of mGluR5s has potential for the treatment of
schizophrenia6 and Alzheimer’s disease.7 Further evidence
supports the use of MPEP or MTEP in the treatment of anxiety,8
depression,9 and pain.10 mGluR5s may also play an important
role in drug-related behaviors, particularly in drug abuse,11 drug
addiction,12 and alcohol withdrawal.13 Hence, mGluR5 antago-
nists may turn out to be useful therapeutics for a variety of CNS
disorders.
) 109.7 min; iodine-123, t1/2 ) 13.13 h) is a challenge,
especially if high affinity and selectivity for mGluR5 are to be
retained. Until recently, no simple derivatives of MPEP and
MTEP had proven to be useful for in vivo imaging.
Both 11C-labeled 3-methoxy-MPEP (Figure 1)14 and 3-meth-
oxy-MTEP15 (Figure 1) gave the desired rapid uptake of radio-
activity into monkey and rat brain. However, this uptake was
immediately followed by fast washout, suggesting little retention
by high affinity receptor binding. Moreover, these radioligands
showed a uniform distribution across most cerebral regions.
More promising results have been obtained with [11C]ABP688
(Figure 1), which was found to be a selective and effective radio-
tracer for imaging mGluR5 in rodents16 and humans17 in vivo.
Recently, researchers at Merck disclosed a selective and high
affinity (Ki ) 80 pM) mGluR5 ligand (F-MTEB, Figure 1) that,
when labeled with fluorine-18, produced the first successful PET
imaging of brain mGluR5s in monkey.18 However, the radio-
synthesis of this radioligand is low yielding from cyclotron-
produced [18F]fluoride ion (2-5%, decay-corrected) and limits
the amount of radioligand that may be produced for PET studies
in human subjects.
MPEP and MTEP have provided leads to some candidate
radioligands for imaging human mGluR5 receptors with PET
in vivo. Due to the small size and limited functionalization of
MPEP and MTEP, labeling of their derivatives with an imaging
radionuclide (e.g., carbon-11, t1/2 ) 20.4 min; fluorine-18, t1/2
* To whom correspondence should be addressed. Tel.: 301 451 3907.
Fax: 301 480 5112. E-mail: simeonf@mail.nih.gov.
10.1021/jm0701268 This article not subject to U.S. Copyright. Published 2007 by the American Chemical Society
Published on Web 06/16/2007

Imaging Metabotropic Glutamate Subtype-5 Receptors
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14 3257
Scheme 1a
a Reagents, conditions, and yields: (a) Lawesson’s reagent, HMPA, 80 °C, 4 h, 78%; (b) chloroacetyltrimethylsilyl-acetylene, 50 °C, 36 h, 65%; (c)
ArBr, Pd(Ph3)4, CuI, DME, TBAF, 80 °C, 10-16 h, 44-70%.
Scheme 2a
a Reagents, conditions, and yields: (a) BuLi, DMF, -78 °C, 30 min, ∼71%; (b) NaBH4, MeOH, 25 °C, 45 min, 69%; (c) imidazole, TBSCl, CH2Cl2,
25 °C, 45 min, 99%; (d) BuLi, SnBu3Cl, Et2O-hexanes, -78 °C, 25 min, 71%; (e) I2, CHCl3, rt, 25 min, 96%.
Scheme 3a
We pursued a strategy for 18F-labeling in a fluoromethyl
group at the 2-position of a thiazole ring in mGluR5 ligands
related to MTEP. By selecting this group for radiolabeling, we
expected to open up a generic route to 18F-labeled analogs of
MTEP with almost any substitution pattern in the distal aryl
ring. Based on this strategy, we now describe the synthesis of
a potent mGluR5 ligand (3; Scheme 1) and its high-yield one-
a Reagents, conditions, and yields: (a) TMSA, CuI, Pd(PPh3)4, DMF,
step labeling with [18F]fluoride ion to give a new radioligand
rt, overnight, 94%; (b) TBAF, THF, 30 min, rt, 95%.
for imaging brain mGluR5 in vivo.
tive for Sonogashira coupling with a partner alkyne. Starting
with 2,4-dibromo-thiazole, 10 was obtained using the reaction
Results and Discussion
Chemistry. 2-Fluoromethylthiazole derivatives were synthe-
sized in three-steps from commercially available materials
(Scheme 1). Thiation of 2-fluoroacetamide gave 2-fluorothio-
acetamide (1), which was then cyclized by treatment with
chloroacetyltrimethylsilylacetylene to give 2-(fluoromethyl)-4-
(2-(trimethylsilyl)ethynyl)thiazole (2). The reaction, which was
very slow at room temperature, reached a conversion of 89%
(65% yield after purification) when heated to 50 °C for 36 h.
Alkyne 2 was used as a synthon for the preparation of several
new ligands (3-7) through Sonogashira19 cross-coupling reac-
tions.
We envisaged that the radiolabeling of ligands 3-7 might
be achieved in each case through nucleophilic substitution in a
2-bromomethyl analog with cyclotron-produced [18F]fluorine ion
to give the corresponding candidate PET radioligand.
The preparation of such a bromo precursor required another
synthetic route, here exemplified by the convergent synthesis
of the 2-bromomethyl analog of 3, namely, compound 17
(Schemes 2-4). The synthesis requires a 4-halo-thiazole deriva-
steps described previously for the synthesis of 2-hydroxy-
methylthiazole from 2-bromothiazole.20,21 The 4-bromo group
was well retained throughout this process. The bromo compound
10 could be used for Sonogashira coupling, but we decided to
convert this into the more reactive iodo compound 12 through
two high-yielding steps (d and e, Scheme 2).
The coupling of 3-bromo-5-fluorobenzonitrile with trimeth-
ylsilylacetylene (TMSA) gave the silylated alkyne 13 in high
yield. This was efficiently desilylated with tetrabutylammonium
fluoride (TBAF) in tetrahydrofuran (THF) at room temperature
to give alkyne 14 (Scheme 3).
The Sonogashira coupling of 13 or 14 with 12 gave compound
15. As expected, the yield of 15 from 14 was much higher (90%)
than from the silyl compound 13 (20%). In another experiment,
we observed that the bromo derivative 10 could be coupled with
14 to give 15 in only 20% yield, so justifying the conversion
of 10 into the more reactive iodo compound 12. After treatment
of 15 with TBAF, alcohol 16 was obtained in good yield. This

3258 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14
Sime´on et al.
Scheme 4a
a Reagents, conditions, and yields: (a) 13 or 14, CuI, Pd(PPh3)4, Et3N, DMF, 80 °C, 10 min, 20% from 13, 90% from 14; (b) TBAF, THF, rt, 30 min,
82%; (c) CBr4, PPh3, benzene, reflux, 1 h, 51%.
Scheme 5a
Table 1. Affinities, cLogP, cLogD, and LogD Values of Ligands 3-7
a Reagents, conditions, and RCY: (a) [18F]fluoride ion, K2CO3, Krypto-
fix2.2.2, MeCN, 45 W in two 1-min pulses, 85 °C, 87% RCY.
a All values represent the mean ( SEM of three determinations, with
each determination lying within 0.3 log unit of the mean. b cLogP was
calculated by using the Pallas 3.0 software (Compudrug, U.S.A.).
was brominated with carbon tetrabromide in the presence of
triphenylphosphine to give the desired precursor 17 (Scheme
4).
Measurement and Calculation of LogD and LogP. The
LogD of [18F]3 was found experimentally to be 2.18 ( 0.22 (n
) 6; Table 1) and cLogD and cLogP were each computed to
be 3.31. These values are within the range generally considered
desirable for a prospective brain imaging agent.22,23 cLogD
values for ligands 4-7 were in the range 1.68-3.34. For each
ligand, 3-7, cLogP values were identical to cLogD values
because these compounds are nonionized at physiological pH
(7.4; Table 1).
In Vitro Binding Assay. All the prepared 2-fluoromethyl
ligands 3-7 exhibited subnanomolar affinity for mGluR5s
(Table 1). The direct 2-methyl analogs of ligands 3-6 are known
and their affinities for binding to mGluR5 are reported.18,24 Thus,
the direct 2-methyl derivative of ligand 3 (F-MTEB) is reported
to have a Ki value of 80 pM in a radioligand binding assay,18
while the direct 2-methyl analogs of ligands 4-6 are reported
to have IC50 values of 0.94, 23, and 49 nM, respectively, for
inhibition of agonist-induced phosphoinositide hydrolysis.24
Although these data are not strictly comparable between each
other and with those of Table 1 because of differences in type
of assay employed, it appears that the introduction of fluorine
at the 2-methyl group is without detriment to binding affinity;
that is, introduction of a fluoro substituent at the 2-methyl group
allows high affinity for mGluR5 to be retained.
Figure 2. Variation with temperature in the decay-corrected radio-
chemical yields (RCYs) of [18F]3 from the radiofluoridation of 17 in
acetonitrile-trace water with KHCO3-18 crown-6 as base. Key: 10
min reaction, b; 20 min reaction, O. Points show mean of data (n )
2) and error bars show the minimum and maximum value.
reported for its direct 2-methyl analog, F-MTEB (Ki ) 80 pM).18
Also, 3 exhibited just slightly lower cLogP and cLogD (3.31)
than F-MTEB (cLogP and cLogD ) 3.32; cf. LogD ) 3.2
(reported in ref 16)). In view of the similarity of properties
between 3 and F-MTEB and the reported effectiveness of [18F]F-
MTEB as a PET radioligand,18 we selected 3 for further study,
including receptor screening, labeling with fluorine-18, and in
vitro and in vivo studies.
Phosphoinositol Hydrolysis Assay of 3 and F-MTEB.
Ligand 3 exhibited an exceptionally high potency (IC50 ) 0.714
pM, n ) 6) for inhibition of phosphoionisitol hydrolysis in this
assay,4 higher than that exhibited by F-MTEB in the same assay
(IC50 ) 9.4 pM, n ) 6).
Receptor Screening of 3. Ligand 3 was found to be highly
selective for binding to mGluR5 receptors over mGluR1,2,4,6,
and 8, 5-HT1A,1B,1D,1E,2A,2B,2C,3,5A,6, and 7, RA,1B,2A,2B, and 2C, â1 and 2
1-5, σ1, NMDA/MK801, H1-4, M1-5, DOR, MOR, KOR,
DAT, NET, SERT (all Ki > 10 µM).
,
D
Introduction of a 3-nitrile group enhanced affinity for mGluR5
(cf. 3 and 5). Reducing the lipophilicity by introducing a
pyridinyl moiety, as in 7, in place of a phenyl moiety (cf. 4)
somewhat reduced affinity.
Ligand 3 bound with exceptionally high affinity to the
mGluR5 receptor (IC50 ) 36 pM), an affinity similar to that
Radiochemistry. The conversion of 17 into [18F]3 was
initially investigated using Kryptofix 2.2.2-K2CO3 as base in
acetonitrile (Scheme 5). Reaction for 30 min at 80 and 110 °C
gave decay-corrected radiochemical yields (RCYs) of 48 and
77%, respectively. We next found that radiofluoridation condi-
tions described recently in our laboratory25 (18-crown-6-KHCO3

Imaging Metabotropic Glutamate Subtype-5 Receptors
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14 3259
Figure 3. Chromatograms from the HPLC separation of [18F]3, produced in a Synthia device. Key: a, [18F]fluoride ion; b, [18F]3; c, 3; d, 17.
in MeCN-trace H2O) gave very high RCYs from short reaction
times (10 or 20 min) at moderate temperature (80 or 85 °C).
RCYs reached near maximal values after 10 min heating; they
were only slightly improved with longer heating (20 min; Figure
2). Longer reaction times (>30 min) gave lower RCYs and
higher temperatures (>85 °C), only slightly increased RCYs,
probably due to precursor degradation in each case. No exchange
of the aryl fluoro group with [18F]fluoride ion was observed
under these mild conditions.
For the production of [18F]3 in a formulated form ready for
intravenous injection into monkey, higher starting activities of
[18F]fluoride ion (>100 mCi) needed to be used. Productions
were initially performed within a lead-shielded automated
TRACERlab FXF-N apparatus. The use of anhydrous acetonitrile
at 88 °C with Kryptofix 2.2.2-K2CO3 as base gave reproducible
yields (overall isolated RCYs; 15-25%). These isolated RCYs
were increased considerably (to 54%) when using microwave
irradiation (45 W, 82-85 °C, 2 min) for the same radiofluori-
dation reaction in a lead-shielded Synthia device.26 The incor-
poration of [18F]fluoride ion into [18F]3 was up to 87%, decay-
corrected. The radioligand was readily purified by HPLC (Figure
3) in high chemical purity (>95%), high radiochemical purity
(>99%), and with specific activities up to 9 Ci/µmol at the end
of synthesis (about 150 min from the end of radionuclide
production).
PET Imaging in Monkey. Time-activity curves (TACs)
were obtained for striata, hippocampus, occipital cortex, cer-
ebellum, and mandible. In experiments in which [18F]3 was
administered alone (baseline experiments), brain uptake of
radioactivity was high, ranging between 450 and 700% SUV
across regions. Hamill et al.18 reported that, after administration
of [18F]F-MTEB into rhesus monkey, the uptake of radioactivity
into brain was maximally about 320% SUV in striatum. (In our
PET experiment with [18F]F-MTEB, we found the maximal
uptake of radioactivity to be similar to that of [18F]3, i.e., 600-
900% SUV across all regions.) The brain distribution of
radioactivity by 180 min after administration of [18F]3 matched
the expected distribution of mGluR5 receptors,18 with regions
of high receptor density (e.g., striatum; Bmax ∼63 nM in rhesus
monkey caudate18) having high levels of radioactivity and
receptor-poor cerebellum (Bmax ∼ 24 nM in rhesus monkey18)
having a low level of radioactivity (panel A, Figure 4).
The uptake of radioactivity in mGluR5-rich regions (e.g.,
striata and hippocampus) and moderately rich cerebellum were
reduced to about the same level when [18F]3 was administered
at 15 min after a receptor saturating dose of the mGluR5-
selective ligand MTEP (5 mg/kg, i.v.; panel B, Figure 4), thereby
showing that there was a substantial mGluR5-specific signal in
gray matter regions in the baseline experiment. Figure 5 shows
horizontal PET scans acquired between 60 and 180 min after
injection of [18F]3 in brain regions at the level of the striata in
both a baseline and a pretreatment experiment in the same
monkey. Radioactivity in the baseline experiment largely
localizes to mGluR5-rich regions (e.g., striata), as shown by
coregistration of brain structures with an MRI scan. In the
pretreatment experiment, radioactivity is greatly diminished and
uniform across the brain. One displacement study was performed
in rhesus monkey, with MTEP (3 mg/kg) given at 40 min after
the injection of [18F]3 (3.44 mCi). This showed a marginally
quicker washout of radioactivity from mGluR5 high-density
regions, including the striatum and hippocampus. A more
striking effect was achieved when 3 (2 mg/kg, i.v.) was used
as displacing agent at 40 min (see Supporting Information).
Under baseline conditions, radioactivity gradually accumu-
lated in mandible, indicative of some defluorination of the
radioligand to [18F]fluoride ion, and reached values of about
400% SUV at 180 min (panel A, Figure 4). Because of the
relatively high uptake in adjacent neocortex, we could not
accurately measure skull uptake without spillover of activity
from brain.

3260 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14
Sime´on et al.
Figure 4. Time-activity curves in brain regions after (panel A) intravenous injection of [18F]3 (4.0 mCi) alone into rhesus monkey and (panel B)
after intravenous injection of [18F]3 (3.8 mCi) into the same rhesus monkey at 15 min after intravenous injection of MPEP (5 mg/kg). Key: cerebellum,
]; occipital cortex, 3; hippocampus 4; striatum, 0; mandible, O.
Figure 5. Horizontal PET images, obtained at the level of the striata, summed from data acquired between 60 and 180 min after injection of
rhesus monkey with [18F]3 (4.0 mCi) alone (panel A) or after injection in the same monkey with [18F]3 (3.8 mCi) at 15 min later than injection
of MPEP (5 mg/kg, i.v.; panel B). Panel C shows the corresponding fused magnetic resonance-PET image used to identify structures in the PET
scans.
In this study, the percentage of specific (i.e., receptor-bound)
radioligand in brain was not quantified with arterial sampling
and compartmental modeling. As described below, [18F]3 was
unstable in the whole blood of monkey and was difficult to
measure with rapid sampling. Nevertheless, the brain-time
activity curves allow an estimate of specific and nondisplaceable
uptake, with the latter including nonspecific binding plus free
radioligand in tissue water. Early time points are highly affected
by blood flow and passage of the radioligand through the blood-
brain barrier. During the period 60 to 120 min, when uptake is
more dependent on receptor binding, uptake in hippocampus
and striatum was about 500% SUV at baseline and about 200%
Figure 6. Clearance of total radioactivity (O) and parent radioligand
radioactivity (b) from rhesus monkey plasma after administration of
[18F]3.
SUV after blockade (Figure 4). Thus, about 40% of uptake in
these regions appeared to be nondisplaceable and 60% specific.
These estimates are prone to error because they were not
corrected for any changes in the concentration of parent
radioligand in plasma that may be associated with preblockade.
In other words, visual inspection of the baseline and preblocked
time-activity curves confirm a substantial amount of receptor-
specific binding, but accurate quantitation is impossible without
knowledge of arterial plasma concentrations of the parent
radioligand over time.
Emergence of Radio-Metabolite in Monkey Blood In Vivo.
For whole monkey blood collected from 1 to 180 min after the
administration of [18F]3 into rhesus monkey, radioactivity
distributed 33.9 ( 5.5% (n ) 24) into cellular components.
MTEP had no influence on this distribution (data not shown).
After administration of [18F]3 to monkey, radioactivity cleared
quite rapidly from plasma (Figure 6). Extraction of radioactivity
from plasma into HPLC analyte was generally quite efficient
(79-86%), although plasma taken at certain time points (3, 5,
and 10 min) gave lower extractions (65, 52, and 48%,
respectively). Reverse phase radio-HPLC analysis of plasma
showed that a single radio-metabolite appeared rapidly and
Figure 7. Time course of percentage of radioactivity in plasma
represented by parent radioligand after intravenous injection of rhesus
monkey with [18F]3.
represented nearly all the radioactivity by 25 min (Figure 7).
This radio-metabolite eluted near the void volume (tR ) 2.0 (
0.04 min, n ) 14) and was much more polar than parent

Imaging Metabotropic Glutamate Subtype-5 Receptors
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14 3261
during its incubation in vitro with monkey brain homogenate
at 37 °C for 90 min. The extraction of radioactivity into HPLC
analytes was high (95-98%). Similarly, [18F]3 degraded only
1.4% during its incubation in vitro with human brain homoge-
nate at 37 °C for 3 h. Extractions of radioactivity into analytes
were again high (96-98%).
Conclusions
Several 2-(fluoromethyl)thiazoles were prepared as high
affinity ligands for mGluR5 and as prospective PET radioli-
gands. Introduction of a fluorine atom into the methyl group at
the 2-position of the 1,3-thiazole ring was beneficial to the
affinity of ligands for mGluR5 and also allowed easy labeling
with [18F]fluoride ion. Labeling at this position allows multiple
variations in the substitution pattern of the distal aryl (phenyl
or pyridinyl) ring in a search for effective PET radioligands.
The new synthon 2 is exceptionally useful for preparing such
ligands through Sonogashira cross-coupling reactions with
substituted bromobenzenes. The new radioligand [18F]3 shows
high penetration into rhesus monkey brain and a substantial
percentage of brain uptake is displaceable (i.e., specifically
bound to mGluR5). Quantitation of monkey brain uptake was
limited because the radioligand was metabolized in whole blood
of this species. We found that [18F]3 was, however, stable in
human whole blood, suggesting that compartmental quantitation
will be feasible in human species. In summary, [18F]3 shows
promise as a new PET radioligand for quantifying mGluR5
receptors and merits evaluation in human subjects.
Figure 8. Time course of composition of radioactivity in whole
monkey blood after addition of [18F]3. Key: b, [18F]3; 9, polar
radiometabolite; 2, radioactivity not extracted into analyte.
Figure 9. Time course of composition of radioactivity in whole human
blood after addition of [18F]3. Key: b, [18F]3; 9, polar radiometabolite;
2, radioactivity not extracted into analyte.
Experimental Section
Materials and general methods. [3H]3-Methoxy-5-(pyridine-
2-yl-ethynyl)-pyridine ([3H]MPEPy;27 84 Ci/mmol) was purchased
from Amersham Biosciences (Buckinghamshire, England). MPEPy
was purchased from Tocris Bioscience (Ellisville, MO). 2,4-
Dibromothiazole was obtained from Frontier Scientific Inc. (Logan,
UT). 3-Bromo-5-fluoro-benzonitrile was obtained from Oakwood
Products Inc. (West Columbia, SC). THF was freshly distilled over
sodium with benzophenone before use. Methanol was dried by
passage through a short column of neutral alumina. Other reagents
and solvents were purchased from Aldrich Chemical Co. or
Lancaster Chemical and used without further purification unless
otherwise indicated. All reactions were performed under argon
unless otherwise indicated.
radioligand (tR ) 4.8 ( 0.2 min, n ) 14). In view of the
observed uptake of radioactivity into mandible and the polarity
of the radio-metabolite, its identity is likely to be [18F]fluoride
ion. The uptake of [18F]fluoride ion into skull has potential to
compromise accurate quantitation of receptor-radioligand in-
teractions in closely proximal brain regions. However, the
peripheral metabolism of [18F]3 in monkey does not appear to
generate brain-penetrant radio-metabolites that would be trouble-
some for quantitation throughout the brain. It may be noted that
in rhesus monkey [18F]F-MTEB is not defluorinated but gives
two less polar radio-metabolites in plasma (Zoghbi, S. S. et al.;
unpublished results). We do not know whether these radio-
metabolites may enter the brain.
The 1H (400 MHz), 19F (376.5 MHz), and 13C NMR (100 MHz)
spectra of all compounds were acquired on an Advance (Bruker)
spectrometer using the chemical shifts of residual deuterated solvent
as internal standard; chemical shifts (δ) for the proton and carbon
resonance are reported in parts per million (ppm) downfield from
TMS (δ ) 0). Thin layer chromatography (TLC) was performed
with silica gel layers (type 60 F254; EM Science), and compounds
were visualized under UV light. Flash chromatography was carried
out using a Horizon HPFC system (Biotage, Charlottesville, VA;
column sizes: 12 × 150 mm, 25 × 150 mm, and 40 × 150 mm)
with hexanes and ethyl acetate (EtOAc) as eluents. Constituent
proportions in chromatographic mobile phases are expressed by
volume. HPLC for determination of compound purity was per-
formed on a Prodigy column (5 µm, 250 × 4.6 mm; Phenomenex,
Torrance, CA) eluted with 10 mM ammonium formate-acetonitrile
(40: 60 v/v) at 1.25 mL/min. HPLC eluates were monitored both
for absorbance at 254 nm and, where appropriate, for radioactivity.
Mass spectra were acquired with a PolarisQ GC-MS instrument
(Thermo Finnigan) equipped with a capillary RTX-5ms column
(30 m × 0.25 mm; flow rate, 1 mL/min; carrier gas, He). High-
resolution mass spectra were acquired under electron ionization
conditions using a double-focusing high-resolution mass spectrom-
eter (AUTOSPEC, Micromass Inc.) with samples introduced
through a direct insertion probe. Yields are recorded for chromato-
graphically and spectroscopically (1H NMR) pure materials.
Stability of [18F]3 in Monkey and Human Whole Blood
In Vitro. [18F]3 was found to be stable when incubated in
aqueous solution for 159 min, in phosphate-buffered saline (pH
) 7.4) for 71 min or in monkey plasma for 168 min. Incubation
of [18F]3 with monkey whole blood over 90 min at 37 °C
resulted in progressive metabolism of the radioligand predomi-
nantly to a radioactive component that was not extracted into
acetonitrile and also to an acetonitrile-soluble component eluting
at the void volume on reverse phase HPLC (Figure 8), consistent
with possible identity (in each case) as [18F]fluoride ion. This
metabolism was unaffected by azide ion, which, because of its
negative charge, is assumed to be unable to enter blood cells.
These findings suggest that the metabolism occurs in cellular
elements of monkey blood and not in plasma.
Incubation of [18F]3 with human whole blood gave strikingly
different results to those of monkey whole blood. Over the time
span of the experiment (90 min), the extraction of radioactivity
into acetonitrile was high and this radioactivity was found to
be predominantly (>98%) parent radioligand (Figure 9).
Stability of [18F]3 in Monkey and Human Brain Homo-
genate In Vitro. [18F]3 degraded only to very low extent (1.8%)

3262 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14
Sime´on et al.
2-Fluorothioacetamide (1). Hexamethylphosphoramide (HMPA;
12 mL) was added to Lawesson’s reagent28 (4.05 g, 10 mmol) and
2-fluoroacetamide (1.54 g, 20 mmol). The reaction mixture was
heated to 80 °C and stirred for 4 h, then cooled, poured into water,
and extracted three times with AcOEt (40 mL). The organic phases
were combined, dried over MgSO4, and evaporated to dryness under
vacuum. The residue was purified on a silica gel column (hexane-
diethyl ether, 80:20 to 70:30) to give 1 (1.45 g, 78%) as an oil that
rapidly turned into yellow crystals. 1H NMR (CDCl3) δ 5.12 (d, J
) 48.0 Hz, 2H); 13C NMR (CDCl3) δ 200.7 (d, J ) 12.0 Hz), 85.0
(d, J ) 194 Hz); GC-MS m/z 93.01 (M+) and 95.01 (M + 2H+).
2-(Fluoromethyl)-4-(2-(trimethylsilyl)ethynyl)thiazole (2).
1-Chloro-4-(trimethylsilyl)-3-butyne-2-one (2.46 g, 14.1 mmol) was
placed in N,N-dimethylformamide (DMF; 20 mL). Compound 1
(1.01 g, 10.85 mmol) was added and the mixture was heated to
50 °C for 36 h. The reaction mixture was then cooled, poured into
water (50 mL), and extracted twice with AcOEt (20 mL). The
organic layers were combined and dried. Solvent was then
evaporated off under vacuum. The residue was purified on silica
gel to give 2 (1.3 g, 65%) as a heavy oil that slowly crystallized.
1H NMR (CDCl3) δ 7.55 (d, J ) 0.92 Hz, 1H), 5.60 (dd, J1 ) 48.0
Hz, J2 ) 0.66 Hz, 2H, CH2), 0.26 (s, 9H); 13C NMR (CDCl3) δ
164.8 (d, J ) 24.93 Hz), 137.9, 124.6 (d, J ) 2.13 Hz), 97.8, 95.7,
81.9 (d, J ) 170.0 Hz), -0.2; mp 42-44 °C; HRMS calcd for
C9H13FNSiS (M+ + H), 214.0522; found, 214.0503.
General Method for the Sonogashira Cross-Coupling of 2
with Aryl Bromides. Compound 2 (250 mg, 1.17 mmol), the
appropriate aryl bromide (1.4 mmol), CuI (15 mg, 79 µmol), Pd-
(PPh3)4 (45 mg, 39 µmol), and triethylamine (0.8 mL) were added
to dimethylethylene glycol (DME; 8 mL). Argon was bubbled into
the resulting dark solution while it was heated to 80 °C. TBAF (2
mmol; 1.0 M solution in THF, 2 mL) was added via syringe over
45 min. The resulting black reaction mixture was heated at 80 °C
until TLC analysis showed no starting material present (10-16 h).
The reaction mixture was cooled to room temperature, filtered
through celite, and then evaporated to dryness. The resulting dark
residue was dissolved in dichloromethane (25 mL) and washed with
water (2 × 20 mL) and then brine (2 × 20 mL). The organic phase
was dried over MgSO4, evaporated under vacuum, and purified by
chromatography on silica gel (hexane-AcOEt, 80:20 to 60:40).
3-Fluoro-5-(2-(2-(fluoromethyl)thiazol-4-yl)ethynyl)benzoni-
trile (3). Sonogashira cross-coupling was applied to 3-bromo-5-
fluorobenzonitrile (280 mg; 1.4 mmol) to afford 3 (134 mg, 44%)
as a white powder. 1H NMR (CDCl3) δ 7.69 (s, 1H), 7.64 (s, 1H),
7.48 (m, 1H), 7.36 (dd, J ) 7.74 Hz, J ) 1.30 Hz, 1H), 5.65 (d,
2H, J ) 46.5 Hz, CH2); 13C NMR (CDCl3) δ 165.3 (d, J ) 25.08
Hz), 161.9 (d, J ) 251.1 Hz), 136.5, 131.2 (d, J ) 3.53 Hz), 125.5
(d, J ) 1.94 Hz), 123.5 (d, J ) 9.82 Hz), 123.2 (d, J ) 23.03 Hz),
119.4 (d, J ) 24.71 Hz), 116.8 (d, J ) 3.11 Hz), 114.4 (d, J )
10.20 Hz), 86.1, 85.7 (d, J ) 3.28 Hz), 80.6 (d, J ) 170.71 Hz);
mp 110-112 °C; HRMS calcd for C12H8F2NS (M+ + H), 261.0298;
found, 261.0306; HPLC (tR ) 7.45 min; >99.8% purity).
Hz), 124.2 (d, J ) 22.11 Hz), 123.9 (d, J ) 9.77 Hz), 118.5 (d, J
) 21.10 Hz), 114.9 (d, J ) 11.5 Hz), 83.9, 81.5 (d, J ) 168.00
Hz); mp 110-112 °C; HRMS calcd for C12H8F2NS (M+ + H),
236.0346; found, 236.0336; HPLC (tR ) 9.15 min; >99.8% purity).
2-(Fluoromethyl)-4-(2-(3-methoxypheny)ethynyl)thiazole (6).
Sonogashira cross-coupling was applied to 3-bromoanisole (262
1
mg; 1.4 mmol) to afford 7 (185 mg; 64%) as a beige powder. H
NMR (CDCl3) δ 7.60 (s, 1H), 7.27 (m, 1H), 7.16 (ddd, d, J1 )
1.16 Hz, d, J2 ) 1.24 Hz, d, J3 ) 7.56 Hz, 1H), 7.10 (dd, d, J1 )
1.40 Hz, d, J2 ) 1.16 Hz, 1H), 6.92 (ddd, d, J1 ) 1.00 Hz, d, J2 )
1.64 Hz, d, J3 ) 8.32 Hz, 1H), 5.64 (d, J ) 46.77 Hz, 2H, CH2),
3.82 (s, 3H); 13C NMR (CDCl3) δ 164.69 (d, J ) 24.85 Hz), 159.31,
137.74, 129.50, 124.32, 123.67 (d, J ) 2.16 Hz), 123.08, 116.42,
115.69, 89.44, 82.47, 80.67 (d, J ) 170.3 Hz), 55.32; mp 61-
62 °C; HRMS calcd for C13H11FNOS (M+ + H), 248.0545; found,
248.0555; HPLC (tR ) 8.48 min; >99.8% purity).
5-(2-(2-(Fluoromethyl)thiazol-4-yl)ethynyl)pyridine-3-carbo-
nitrile (7). Sonogashira cross-coupling was applied to 3-bromo-5-
cyanopyridine (256 mg, 1.4 mmol) to afford 5 (128 mg, 45%) as
1
a white powder. H NMR (CDCl3) δ 8.96 (s, 1H), 8.84 (s, 1H),
8.09 (t, J ) 1.90 Hz, 1H), 7.37 (s, 1H), 5.66 (d, 2H, J ) 46.65 Hz,
CH2); 13C NMR (CDCl3) δ 165.50 (d, J ) 25.05 Hz), 155.16,
151.13, 141.27, 136.25, 125.81 (d, J ) 2.05 Hz), 120.23, 115.73,
110.02, 88.48, 83.64, 80.59 (d, J ) 170.74 Hz); mp 142-144 °C;
HRMS calcd for C12H7FN3S (M+ + H), 244.0345; found, 244.0340;
HPLC (tR ) 5.10 min; >99.8% purity).
4-Bromothiazole-2-carbaldehyde (8). To a solution of 2,4-
dibromothiazole (1 g, 4.12 mmol) in anhydrous ether (40 mL, 0.1
M) at -78 °C was added n-BuLi (1.6 M in hexanes, 4.94 mmol,
3.08 mL). The resulting solution was stirred at the same temperature
for 45 min. DMF (0.64 mL, 8.24 mmol) was then added at
-78 °C. The reaction mixture was stirred at -78 °C for 30 min
and then slowly warmed to 25 °C over 2 h. Hexane (20 mL) was
added, and the resulting mixture was passed through a short silica
gel cake eluted with 30% EtOAc in hexanes. The solvents were
evaporated off to give the crude aldehyde 8 (558 mg, 71%), which
was used directly in the next step.
(4-Bromothiazol-2-yl)methanol (9). To a solution of crude 8
(558 mg, 2.92 mmol) in methanol (2.0 mL) at 25 °C was added
sodium borohydride (210 mg, 5.55 mmol). The resulting mixture
was stirred at the same temperature for 45 min. EtOAc-hexane
(1:1 v/v; 50 mL) was added, and the mixture was passed through
a short silica gel cake eluted with EtOAc. The solvents were then
evaporated off and the crude product was purified by flash column
chromatography (silica gel, 20 f 50% EtOAc in hexanes) to furnish
1
9 (385 mg, 69%). H NMR (CDCl3) δ 7.20 (s, 1H), 4.93 (s, 2H);
13C NMR (CDCl3) δ 173.0, 124.4, 117.0, 61.8.
2-(tert-Butyldimethylsilyloxymethyl)-4-bromothiazole (10). To
a solution of 9 (200 mg, 1.03 mmol) in CH2Cl2 (3.5 mL, 0.3 M)
was added imidazole (210 mg, 2.06 mmol) followed by tert-
butyldimethylchlorosilane (202 mg, 1.34 mmol) at 25 °C. The
reaction mixture was kept at 25 °C for 45 min, quenched with
MeOH (0.5 mL), and then passed through silica gel eluted with
CH2Cl2. Evaporation of solvents gave the desired silyl ether 10
3-(2-(2-(Fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile (4).
Sonogashira cross-coupling was applied to 3-bromobenzonitrile
(255 mg, 1.4 mmol) to afford 4 (162 mg; 56%) as a slightly yellow
1
1
(315 mg, 99%). H NMR (CDCl3) δ 7.16 (s, 1H), 4.93 (s, 2H),
powder. H NMR (CDCl3) δ 7.77 (dd, J1 ) 7.88 Hz, J2 ) 1.16
0.94 (s, 9H), 0.12 (s, 6H); 13C NMR (CDCl3) δ 174.5, 124.2, 116.4,
62.9, 25.7, 18.2, -5.5.
Hz, 1H), 7.66 (s, 1H), 7.64 (dd, J1 ) 8.0 Hz, J2 ) 1.30 Hz, 1H),
7.49 (d, J ) 7.84 Hz, 1H), 7.36 (dd, 1H, J ) 7.0 Hz, J ) 6.4 Hz),
5.65 (d, 2H, J ) 46.73 Hz, CH2); 13C NMR (CDCl3) δ 165.1 (d,
J ) 24.96 Hz), 136.9, 135.8, 135.0, 132.0, 129.4, 124.8 (d, J )
2.07 Hz), 123.9, 117.9, 113.1, 86.9, 85.0, 80.6 (d, J ) 170.54 Hz);
mp 82-83 °C; HRMS calcd for C13H8FN2S (M+ + H), 243.0392;
found, 243.0385; HPLC (tR ) 6.37 min; >99.8% purity).
2-(tert-Butyldimethylsilyloxymethyl)-4-tri-n-butylstannylthi-
azole (11). To a solution of 10 (200 mg, 0.65 mmol, 0.07 M) in
ether (10.0 mL) at -78 °C was added n-BuLi (1.6 M in hexanes,
0.78 mmol, 0.5 mL). The resulting mixture was stirred at -78 °C
for 25 min, and then tri-n-butyltin chloride (230 µL, 0.78 mmol)
was added. The solution was stirred at -78 °C for 25 min and
then slowly warmed to 25 °C over 90 min. Then the reaction
mixture was diluted with hexane (2.0 mL) and passed through silica
gel eluted with 20% EtOAc in hexanes. Flash column chromatog-
raphy (silica gel; pretreated with Et3N, 5% ether in hexanes)
furnished 11 (240 mg, 71%). Rf ) 0.36 (silica gel, 5% EtOAc in
hexanes); 1H NMR (C6D6) δ 7.08 (s, 1H), 4.98 (s, 2H), 1.75-1.57
(m, 6H), 1.44-1.31 (m, 6H), 1.26-1.09 (m, 6H), 0.94 (s, 9H),
2-(Fluoromethyl)-4-(2-(3-fluorophenyl)ethynyl)thiazole (5).
Sonogashira cross-coupling was applied to 1-fluoro-3-bromoben-
zene (245 mg; 1.4 mmol) to afford 6 (193 mg, 70%) as a white
1
powder. H NMR (CDCl3) δ 7.59 (d, J ) 7.8 Hz, 1H), 7.55 (s,
1H), 7.41 (d, J ) 7.1 Hz, 1H), 7.36 (m, 1H), 7.30 (dd, J ) 7.74
Hz, J ) 6.9 Hz, 1H), 5.58 (d, J ) 47.5 Hz, 2H, CH2); 13C NMR
(CDCl3) δ 163.2 (d, J ) 24.96 Hz), 163.2 (d, J ) 255.5 Hz), 137.4,
132.4 (d, J ) 3.55 Hz), 130.2 (d, J ) 3.0 Hz), 127.7 (d, J ) 2.05

Imaging Metabotropic Glutamate Subtype-5 Receptors
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14 3263
0.91 (t, J ) 7.0 Hz, 9H), - 0.02 (s, 6H); 13C NMR (CDCl3) δ
173.2, 159.1, 125.3, 63.5, 29.0, 27.3, 25.8, 18.3, 13.7, 10.1, -5.4.
2-(tert-Butyldimethylsilyloxymethyl)-4-iodothiazole (12). A
solution of iodine (51 mg, 0.2 mmol) in anhydrous chloroform (2.5
mL) was added to a solution of 11 (100 mg, 0.19 mmol) in
chloroform (1.0 mL) over 10 min. The mixture was stirred at room
temperature for 25 min, and the resultant dark solution was washed
with a solution of sodium hydrogen carbonate (2 mL) followed
with a solution of sodium hydrogen sulfite (3 mL). The organic
phase was dried over magnesium sulfate and evaporated to dryness
anhydrous THF (5 mL) was added a solution (1 M) of TBAF (0.58
mmol) in THF (0.58 mL). The reaction mixture was maintained
for 30 min at room temperature and then filtered through a short
cake of silica gel eluted with AcOEt. Evaporation of solvents gave
the desired alcohol 16 (150 mg). Chromatography on silica gel
(CH2Cl2-MeOH, 94:6) gave 17 (102 mg, 82%) as a white powder.
1H NMR (CDCl3) δ 7.62 (s, 1H), 7.60 (s, 1H), 7.45 (m, 1H), 7.35
(d, J ) 6.84 Hz, 1H), 4.98 (s, 2H); 13C NMR (CDCl3) δ 171.8,
161.9 (d, J ) 251.39 Hz), 136.0, 131.2 (d, J ) 3.50 Hz), 126.1 (d,
J ) 9.85 Hz), 124.6, 124.0 (d, J ) 22.8 Hz), 119.3 (d, J ) 24.73
Hz), 116.8 (d, J ) 3.48 Hz), 114.3 (d, J ) 18.18 Hz), 86.5, 85.5,
62.2; mp 127-129 °C; HRMS calcd for C9H4FN (M+ + H),-
259.0341; found, 259.0345.
3-(2-(2-(Bromomethyl)thiazol-4-yl)ethynyl)-5-fluorobenzoni-
trile (17). The alcohol 16 (30 mg, 0.116 mmol) was dissolved in
benzene (1.1 mL) containing carbon tetrabromide (0.76 g, 2.30
mmol). Triphenylphosphine (0.55 mg, 0.19 mmol) was added, and
the solution was heated to reflux for 1 h after which no starting
material was observed in TLC. The reaction mixture was cooled
to room temperature and filtered through celite. Solvent was
evaporated off and chromatography on silica gel (hexane-AcOEt,
80:20) gave 17 (19 mg, 51%) as a white powder. 1H NMR (CDCl3)
δ 7.66 (s, 1H), 7.63 (t, J ) 1.24 Hz, 1H), 7.48 (ddd, J ) 8.72 Hz,
J ) 1.34 Hz, J ) 1.44 Hz, 1H), 7.37 (ddd, J ) 0.80 Hz, J ) 1.36
Hz, J ) 1.40 Hz, 1H), 4.74 (s, 2H); 13C NMR (CDCl3) δ 166.3,
161.9 (d, J ) 251.52 Hz), 136.3, 131.2 (d, J ) 3.0 Hz), 126.2,
125.9 (d, J ) 10.2 Hz), 123.2 (d, J ) 23.14 Hz), 119.4 (25.0),
116.8 (d, J ) 3.2 Hz), 114.3 (d, J ) 10.2 Hz), 100.0, 86.2, 85.6
(d, J ) 3.02 Hz); mp 102-104 °C; HRMS calcd for C13H6BrFN2S
(M+ + H), 320.9497; found, 320.9494; HPLC (tR ) 10.05 min;
>99.8% purity).
1
to give 12 as a viscous oil (230 mg, 96%). H NMR (CDCl3) δ
7.38 (s, 1H), 4.97 (s, 2H), 0.94 (s, 9H), 0.13 (s, 6H); 13C NMR
(CDCl3) δ 175.7, 122.3, 93.1, 62.9, 25.7, 18.3, -5.4; HRMS calcd
for C10H19INOSSi (M+ + H), 356.0001; found, 355.9998.
3-Fluoro-5-(2-(trimethylsilyl)ethynyl)benzonitrile) (13). 3-Bro-
mo-5-fluorobenzonitrile (0.58 g, 2.9 mmol) and 3-(trimethylsilyl)-
acetylene (0.453 g, 4.61 mmol) were placed in a double-necked
flask (25 mL) loaded with dichlorobis(triphenylphosphine) pal-
ladium (100 mg, 0.15 mmol) and CuI (28 mg; 0.15 mmol) under
argon. Anhydrous DMF (7 mL) and triethylamine (1.6 mL) were
added, and the solution was stirred at room temperature overnight.
The dark solution was filtered through celite, evaporated to dryness
under vacuum, and then chromatographed on silica gel (hexane-
AcOEt, 95:5 to 80:20) to give 13 (0.59 g, 94%) as a viscous yellow
1
oil. H NMR (CDCl3) δ 7.54 (t, J ) 1.30 Hz, 1H), 7.39 and 7.37
(2 dd, 1H, J1 ) 8.88 Hz, J2 ) 2.52 Hz), 7.31 and 7.29 (2 dd, 1H,
J1 ) 7.86 Hz, J2 ) 2.52 Hz), 0.26 (s, 9H); 19F NMR (CDCl3) δ
-109.49 (s); 13C NMR (CDCl3) δ 162.16 (d, J ) 251.5 Hz), 131.86
(d, J ) 4.02 Hz), 127.02 (d, J ) 10.06 Hz), 123.78 (d, J ) 23.14
Hz), 119.35 (d, J ) 6.16 Hz), 117.25 (d, J ) 3.0 Hz), 114.40 (d,
J ) 10.06 Hz), 101.28 (d, J ) 3.02 Hz), 99.40, 0.28; HRMS calcd
for C12H13FNSi (M+ + H), 218.0801; found, 218.0814.
Measurement of LogD and Calculation of cLogD and cLogP.
The value for the partition coefficient of [18F]3 between n-octanol
and sodium phosphate buffer (0.15 M, pH 7.4) was determined
according to the general method of Zoghbi et al.29 cLogD and cLogP
values for mGluR5 ligands were calculated with Pallas 3.0 software
(Compudrug; S. San Francisco, CA).
3-Ethynyl-5-fluorobenzonitrile (14). Compound 13 (0.23 g,
1.15 mmol) was dissolved in anhydrous THF (12 mL) under argon
within a double-necked flask (25 mL). TBAF solution (1.0 M in
THF, 1.72 mL, 1.72 mmol) was added slowly to give a deep dark
solution. TLC (hexane-ethyl acetate, 80:20) showed no trace of
reagent at the end of addition. The reaction mixture was stirred for
30 min. Hexane (15 mL) was then added, and the solution was
filtered over a short cake of silica gel. Evaporation of solvent gave
158 mg (95%) of fine volatile crystals that were used without further
Radionuclide Production. No-carrier-added (NCA) [18F]fluoride
ion was produced with the 18O(p,n)18F reaction, by irradiating 18O-
enriched water (95 atom %; 1.8 mL) with a beam of protons (14.1
MeV; 20-25 µA) from a PETtrace cyclotron (GE). Generally, this
method produces [18F]fluoride ion with a specific radioactivity
exceeding 10 Ci/µmol.
1
purification. H NMR (CDCl3) δ 7.57 (s, 1H), 7.42 (d, 1H, J1 )
8.00 Hz), 7.42 (d, 1H, J1 ) 8.00 Hz), 3.26 (s, 1H, CH); 19F NMR
(CDCl3) δ -109.08; 13C NMR (CDCl3) δ 157.78 (d, J ) 251.52
Hz), 131.69 (d, J ) 3.02 Hz), 125.83 (d, J ) 10.06 Hz), 123.75
(d, J ) 25.15 Hz), 119.53 (d, J ) 25.15 Hz), 116.75, 114.00 (d, J
) 10.06 Hz), 81.06, 80.06; mp 56-57 °C; HRMS calcd for C9H4-
FN (M+ + H), 146.0406; found, 146.0409.
Preparation of [18F]3sExperimental Labeling with Low
Radioactivity. A glass V-vial (1 mL volume) was loaded with 100
µL of a pre-prepared solution of KHCO3 (0.23 g) and 18-crown-6
(0.62 g) in acetonitrile-water (15:1 v/v; 10 mL). NCA [18F]fluoride
ion (<10 mCi) in [18O]water (50-200 µL) was added to the vial
and the solvent was evaporated off with heating at 110 °C under
reduced pressure (controlled with a bleed of nitrogen). The reaction
mixture was further dried azeotropically during three cycles of
addition-evaporation of acetonitrile (0.5 mL each cycle). A solution
of the precursor 17 (1.2 to 1.5 mg) in acetonitrile (0.5 mL),
containing water (1 µL, 56 µmol) was added to the dry residue,
and the reaction vessel was heated for a set time (10 or 20 min)
and temperature (in the range 20-110 °C). The reaction mixture
was cooled for 5 min and then analyzed by radio-TLC (hexane-
EtOAc, 80:20; Rf [18F]3 ) 0.52) or radio-HPLC.
3-Fluoro-5-(2-(tert-butyldimethylsilyloxymethyl)thiazol-4-yl)-
ethynyl)benzo-nitrile (15). Compounds 12 (136 mg, 0.38 mmol)
and 14 (65 mg, 0.45 mmol) were added to DMF (5 mL) containing
CuI (7.5 mg, 39 µmol), Pd(PPh3)4 (45 mg, 39 µmol), and
triethylamine (0.44 mL, 3.12 mmol). Argon was bubbled into the
solution while it was warmed to 80 °C, and the bubbling was
maintained for a further 10 min at 80 °C. TLC analysis showed no
starting material present in solution after 3 h. The reaction mixture
was cooled to room temperature and concentrated in vacuo. The
crude residue was filtered through a short cake of silica gel eluted
with hexane-AcOEt (80:20), and the obtained solution was
evaporated under vacuum. Chromatography on silica gel (hexane-
AcOEt, 90:20 to 80:20) gave 15 (128 mg, 90%) as a yellow oil.
1H NMR (CDCl3) δ 7.62 (m, 1H), 7.57 (s, 1H), 7.47 (m, 1H), 7.35
(dd, J1) 7.84 Hz, J2) 1.36 Hz, 1H), 4.97 (s, 2H), 0.95 (s, 9H),
0.14 (s, 6H); 13C NMR (CDCl3) δ 174.1, 161.9 (d, J ) 251.50
Hz), 135.8, 131.2 (d, J ) 3.37 Hz), 126.3 (d, J ) 10.06 Hz), 124.3,
123.2 (d, J ) 22.8 Hz), 119.1 (d, J ) 24.82 Hz), 116.8 (d, J )
3.48 Hz), 114.3 (d, J ) 10.24 Hz), 86.9, 85.1 (d, J ) 3.40 Hz),
63.05, 25.7, 18.24, -5.45; HRMS calcd for C9H4FN (M+ + H),
373.1206; found, 373.1208.
Automated Production of [18F]3 for Intravenous Injection
into Monkey. A commercial automated apparatus, namely, the
TRACERlab FXF-N (GE Medical Systems), was used initially. This
apparatus is designed for performing single-step reactions with
cyclotron-produced [18F]fluoride ion and for purification and
formulation of the labeled product. In this apparatus, aqueous [18F]-
fluoride ion was dried by cycles of addition and evaporation of
acetonitrile and complexed with K+-Kryptofix 2.2.2. This complex
was reacted with precursor 17 (0.8-1.2 mg) at 88 °C for 20 min.
The radioactive product was purified by injection (5 mL sample
loop) into a semipreparative column (Luna C18, 10 µm, 10 × 250
mm; Phenomenex) eluted at 6.5 mL/min with a mixture of 10 mM
ammonium formate (A) and MeCN (B) according to the program
3-Fluoro-5-(2-(2-(hydroxymethyl)thiazol-4-yl)ethynyl)ben-
zonitrile (16). To a stirred solution of 15 (180 mg, 0.48 mmol) in

3264 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14
Sime´on et al.
25% B for 3 min, increasing to 50% B over 7 min and held at 50%
B for 20 min. The fraction containing [18F]3 (tR ) 22.2 min) was
collected in water (100 mL). The aqueous solution was passed
through a C-18 Sep-pak, and the trapped radioactive product eluted
with ethanol (1 mL) into a sterile flask loaded with saline (9 mL).
The radiochemical purity and specific radioactivity of the product
were calculated by injecting a sample of the radioactive solution
(0.2-0.5 mCi) into a reverse phase analytical column (Luna C18,
4.6 × 250 mm, 5 µm; Phenomenex) eluted with MeCN-10 mM
ammonium formate (60:40) at 2 mL/min ([18F]3, tR ) 6.2 min).
The absorbance response was calibrated with respect to the mass
of 3.
protocols are available online for all binding assays at the NIMH-
PDSP web site (http://pdsp.med.unc.edu)
Animal Care and Use. Experiments in four rhesus monkeys
(Macacca mulatto) were conducted according to the Guide for the
Care and Use for Laboratory Animals.31 Rhesus monkeys (∼10
kg) were initially anesthetized with ketamine (10 mg/kg, i.m.) and
then induced with propofol (5 mg/kg, i.v.), intubated, and respired
with medical grade air. During the PET scanning sessions,
anesthesia was maintained by continuous administration of isof-
lurane at 1.0-4.0% in oxygen via the endotracheal tube. Body
temperature was maintained between 37 and 37.5 °C. Electrocar-
diograph, heart, and respiration rates were also monitored through-
out the experiment.
Automated production of [18F]3 was subsequently performed in
a Synthia apparatus26 adapted to use microwave irradiation for the
labeling reaction.30 A microwave reactor (model 521; Resonance
Instruments Inc., Skokie, IL) was securely mounted on a Synthia
MKII Lab System. The time and power control were located outside
a lead-shielded hot-cell and linked to the cavity through an RF
coaxial cable. The reaction V-vial (1 mL, Alltech) was equipped
with a screw-on cap and septum (Tuf-Bond Teflon/silicone; Pierce),
pierced with a vent needle that was connected to a glass vial (20
mL) to collect the solvents, and also a charcoal trap to retain any
break-through of volatile radioactive species. Liquid handling was
achieved with a Gilson Aspec auto-injector/dispenser, which forms
part of the Synthia apparatus. Other operations of the radiosynthesis
and purification procedures were controlled by a visual chemistry-
based recipe. Heating time and power input were controlled
independently. Cyclotron-produced [18F]fluoride ion in 18O-enriched
water (350-500 µL) was dried in the V-vial containing Kryptofix
2.2.2 (5 mg) and K2CO3 (0.5 mg) in acetonitrile-water (9:1 v/v,
100 µL). Microwave heating (90 W in 3 pulses of 2 min) was
applied under N2 gas flow (200 mL/min) to speed up the removal
of the azeotropic water-acetonitrile mixture. The heating cycle was
repeated twice, and each time fresh acetonitrile (500 µL) was added.
Precursor 17 (∼0.75-1.0 mg) in acetonitrile (100 µL) was
introduced into the V-vial and heated at 82-85 °C (45 W in two
pulses of 1 min). The reaction mixture was diluted with mobile
phase (0.75 mL) and injected onto Luna C18 (5 µm, 250 × 10
mm, Phenomenex) eluted at 2.5 mL/min with a mixture of A (25
mM ammonium formate) and B (acetonitrile-25 mM ammonium
formate, 75:25) according to the following program: 68% B for 2
min, then 76% B over 4 min, and finally 87% B over 30 min ([18F]-
3, tR ) 19.6 min).
In Vitro Receptor Binding Assay. The affinity of test com-
pounds was determined by displacement of [3H]MPEPy binding
to rat brain membranes. Thus, whole rat brain, minus cerebellum
and brainstem, was homogenized (1:10 w/v) in ice-cold assay buffer
(50 mM Tris-HCl-1.1% saline buffer, pH 7.5). The homogenate
was centrifuged at 39 800 g for 10 min at 4 °C, and the resulting
pellet was resuspended in buffer (6.25 mg original wet weight per
mL) and stored at -70 °C until analysis.27 Immediately before
analysis, membranes were resuspended in assay buffer (0.9% NaCl
containing 50 mM Tris-HCl) to give a final assay concentration
of 5 mg/800 µL. Each competing ligand (10 mM in EtOH) was
added in a volume of 100 µL to give a final concentration in the
range of 0.01 nM to 1 mM along with 100 µL of [3H]MPEPy. The
incubation was initiated by adding membranes (800 µL) to
constitute a total assay volume of 1 mL and was allowed to proceed
for 1 h at 22 °C. The assay was terminated by rapid filtration over
Whatman GF/B filters that had been presoaked in poly(ethylene-
imine) (0.5%) and then three washes with ice-cold saline (0.9%
NaCl; 3 mL). Radioactivity was determined by liquid scintillation
counting.
Positron Emission Tomography. PET scans were performed
in a high-resolution research tomograph (HRRT, Siemens/CPS,
Knoxville, TN). Transmission scans were performed with a 137Cs
point source following injection of 2 to 5 mCi of the PET
radioligand. Images were reconstructed with attenuation and scatter
correction using a list mode OSEM algorithm,32 resulting in an
image resolution of 2.5 mm fwhm.
Tomographic images were analyzed with PMOD 2.75 (PMOD
Technologies Ltd, Adliswil, Switzerland). For each scan, a static
PET image was obtained by summing the dynamic frames acquired
during the acquisition.
In a baseline experiment, [18F]3 (3.99 mCi; specific radioactivity,
1.93 Ci/µmol) was injected intravenously into the monkey (14.7
kg) as a bolus in physiological saline (5 mL) containing ethanol
(10%). For a pretreatment experiment on a separate day, MTEP
(2-5 mg/kg) in physiological saline containing ethanol (10%) was
injected intravenously into the same monkey at 15 min before
injection of [18F]3 (3.82 mCi; specific radioactivity, 2.02 Ci/µmol).
Regions of interest were drawn on the coregistered images in
striatum (caudate and putamen), thalamus, hippocampus, occipital
cortex, cerebellum, and bone (mandible). These regions were
transferred onto the dynamic scans to obtain the corresponding
time-activity curves. Radioactivity was expressed as standardized
uptake value (SUV), which normalizes for injected activity and
body weight.
%SUV ) % injected activity/cm3 brain × body weight (g)
MRI and Image Fusion. All monkeys had T1-weighted
magnetic resonance imaging (TR/TE/x ) 24 ms/3 ms/300), acquired
on a 1.5-T GE Horizon Instrument (General Electric Medical
Systems, Waukesha, WI). PET and NMR images were coregistered
with SPM2 (Wellcome Department of Cognitive Neurology,
London, U.K.).
Emergence of Radio-Metabolite in Monkey Blood In Vivo.
Heparinized blood samples (1 mL) were drawn at 1 min intervals
for 5 min and then at 10, 25, 40, 60, 90, 120, and 180 min after
injection of [18F]3 (2.2. mCi) into an anesthetized monkey. They
were placed immediately on ice to retard ex vivo metabolism.
Measured aliquots (by pipetting; about 1 mL) of blood were then
centrifuged at 1800 g for 1.5 min and plasma separated. The cellular
and the plasma fractions were then counted in a γ-counter. The
distribution of radioactivity was calculated for each sample. The
total radioactivity in the plasma and the cells represented the amount
in whole blood.
Stability of [18F]3 in Monkey and Human Whole Blood and
Plasma In Vitro. In these experiments, the initial radiochemical
purity of [18F]3 and subsequent measures of stability were
determined by reverse phase chromatography on a Novapak C-18
column (100 × 8 mm; Waters Corp., Milford, MA) using a Radial-
Pak compression module RCM-11 with a sentry precolumn and a
mobile phase of MeOH-H2O-Et3N (70:30:0.1) at a flow rate of
2.0 mL/min.
Phosphoionisitol Hydrolysis Assay. Ligands 3 and F-MTEB
were tested in a CHO cell-based assay by the National Institute of
Mental Health Psychoactive Drug Screening Program for their
potency to inhibit phosphoionistol hydrolysis.4
Receptor Screening of 3. Ligand 3 was screened for binding to
a wide range of receptors and transporters by the National Institute
of Mental Health Psychoactive Drug Screening Program. Detailed
To test for the stability of [18F]3 in monkey whole blood,
anticoagulated (EDTA) blood (8 mL) was drawn from an anesthe-
tized monkey. [18F]3 (15 µCi) was added to whole blood (1.5 mL),
which was then sampled (50 µL) at 0.5, 1, 2, 3, 4, 5, 10, 15, 30,
60, and 90 min. Each sample was added immediately to acetonitrile

Imaging Metabotropic Glutamate Subtype-5 Receptors
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14 3265
References
(300 µL) that had been spiked with 3. The mixture was then
agitated, diluted with water (50 µL), and mixed. The mixture was
counted for radioactivity in a γ-counter and centrifuged at 9400 g
for 1 min. The clear supernatant liquid was analyzed with HPLC
and the precipitate counted for radioactivity in a γ-counter.
(1) Bra¨uner-Osborne, H.; Egebjerg, J.; Nielsen, E. O.; Madsen, U.;
Krogsgaard-Larsen, P. Ligands for glutamate receptors: Design and
therapeutic prospects. J. Med. Chem. 2000, 43, 2609-2645.
(2) Conn, P. J.; Pin, J. P. Pharmacology and functions of metabotropic
glutamate receptors. Ann. ReV. Pharmacol. Toxicol. 1997, 37, 205-
237.
(3) Pin, J.-P.; Acher, F. The metabotropic glutamate receptors: Structure,
activation mechanism, and pharmacology. Curr. Drug Targets: CNS
Neurol. Disord. 2002, 1, 297-317.
(4) Gasparini, F.; Lingenhohl, K.; Stoehr, N.; Flor, P. J.; Heinrich, M.;
Vranesic, I.; Biollaz, M.; Allgeier, H.; Heckendorn, R.; Urwyler, S.;
Varney, M. A.; Johnson, E. C.; Hess, S. D.; Rao, S. P.; Sacaan, A.
I.; Santori, E. M.; Velicelebi, G.; Kuhn, R. 2-Methyl-6-(phenylethy-
nyl)-pyridine (MPEP), a potent, selective and systemically active
mGlu5 receptor antagonist. Neuropharmacology 1999, 38, 1493-
1503.
(5) Cosford, N. D. P.; Roppe, J.; Tehrani, L.; Schweiger, E. J.; Seiders,
T. J.; Chaudary, A.; Rao, S.; Varney, M. A. [3H]-Methoxymethyl-
MTEP and [3H]-methoxy-PEPy: Potent and selective radioligands
for the metabotropic glutamate subtype 5 (mGlu5) receptor. Bioorg.
Med. Chem. Lett. 2003, 13, 351-354.
(6) Pietraszek, M.; Nagel, J.; Gravius, A.; Scha¨fer, D.; Danysz, W. The
role of group I metabotropic glutamate receptors in schizophrenia.
Amino Acids 2006, 32, 173-178.
(7) Tsai, V. W. W; Scott, H. L.; Lewis, R. J.; Dodd, P. R. The role of
group I metabotropic glutamate receptors in neuronal excitotoxicity
in Alzheimer’s disease. Neurotoxic. Res. 2005, 7, 125-141.
(8) Brodkin, J.; Busse, C.; Sukoff, S. J.; Varney, M. A. Anxiolytic-like
activity of the mGluR5 antagonist MPEP: A comparison with
diazepam and buspirone. Pharmacol., Biochem. BehaV. 2002, 73,
359-366.
The effect of sodium azide on stability of [18F]3 in whole monkey
blood was also assessed. Thus, whole blood (1 mL) was mixed
well with a solution (20 µL) of sodium azide (6 mg). [18F]3 was
added and the solution incubated at room temperature for 190 min
before centrifugation at 1800 g for 4 min. The supernatant plasma
was treated with acetonitrile and analyzed with HPLC. The stability
of [18F]3 in monkey plasma was assessed as follows. Whole monkey
blood (2.0 mL) was centrifuged at 1800 g for 5 min. Supernatant
plasma (1.0 mL) was added to [18F]3 (5 µCi) and the solution
incubated at RT for 168 min. A sample was then analyzed by
HPLC. To test the stability of [18F]3 in whole human blood, [18F]3
(2 µCi) was added to human whole blood (2.0 mL), incubated at
23 °C, and sampled (50 µL) at 10, 15, 20, 30, 40, 60, and 90 min.
Each sample was placed in acetonitrile (50 µL) that had been spiked
with 3 and mixed well. Then potassium fluoride solution (0.5 M;
50 mL) was added to each sample before mixing again, counting
for radioactivity in a γ-counter, and centrifugation at 10 000 g for
1 min. Each supernatant liquid was injected onto HPLC for analysis.
The stability of [18F]3 in aqueous solution was also checked.
Thus, solutions of [18F]3 in water alone and in saline (0.9%)
containing phosphoric acid (0.15 mM) were prepared and sampled
at set times (34, 48, and 159 min for water solution and 54 and 71
min for salt solution) for immediate HPLC analysis.
(9) Cosford, N. D. P.; Tehrani, L.; Roppe, J.; Schweiger, E.; Smith, N.
D.; Anderson, J. J.; Bristow, L.; Brodkin, J.; Jiang, X. H.; McDonald,
I.; Rao, S.; Washburn, M.; Varney M. A. 3-[(2-Methyl-1,3-thiazol-
4-yl)ethynyl]-pyridine: A potent and highly selective metabotropic
glutamate subtype 5 receptor antagonist with anxiolytic activity J.
Med. Chem. 2003, 46, 204-206.
(10) (a) Walker, K.; Reeve, A.; Bowes, M.; Winter, J.; Wotherspoon, G.;
Davis, A.; Schmid, P.; Gasparini, F.; Kuhn, R.; Urban, L. mGlu5
receptors and nociceptive function II. mGlu5 receptors functionally
expressed on peripheral sensory neurones mediate inflammatory
hyperalgesia. Neuropharmacology 2001, 40, 10-19. (b) Walker, K.;
Bowes, M.; Panesar, M.; Davis, A.; Gentry, C.; Kesingland, A.;
Gasparini, F.; Spooren, W.; Stoehr, N.; Pagano, A.; Flor, P. J.;
Vranesic, I.; Lingenhoehl, K.; Johnson, E. C.; Varney, M.; Urban,
L.; Kuhn, R. Metabotropic glutamate receptor subtype 5 (mGlu5)
and nociceptive function I. Selective blockade of mGlu5 receptors
in models of acute, persistent and chronic pain. Neuropharmacology
2001, 40, 1-9.
Stability of [18F]3 in Monkey and Human Brain Homogenates
In Vitro. Monkey brain tissue (3.34 g), previously stored for 1
year at -70 °C, was thawed slowly at room temperature and then
homogenized with [18F]3 (∼100 µCi). The tissue suspension was
ice-cooled between each pass of homogenization. Brain homogenate
was sampled after incubation at 37 °C in an oscillating water bath
at 5, 10, 15, 30, 60, and 90 min.
Human brain tissue (1.76 g), previously stored for a few weeks
at -70 °C, was thawed slowly at room temperature and then
homogenized with ice-cold saline (3 mL). The tissue suspension
was ice cooled between each pass of homogenization. [18F]3 (∼50
µCi) in ethanol-water (1:1 v/v; 450 µL) was slowly mixed into to
the cold homogenate. This mixture was incubated at 37 °C in an
oscillating water bath and sampled (50 µL) after 5, 10, 15, 30, 45,
60, 90, 120, and 180 min.
Samples from incubations of [18F]3 with monkey or human brain
homogenate were added to acetonitrile (700 µL), which had been
spiked with 3 and mixed well. Then potassium fluoride solution
(0.5 M; 50 µL) was added, and the solution was mixed well again.
Each sample was counted in a γ-counter and centrifuged at 10 000
g for 1 min. The supernatant liquids were analyzed with HPLC.
The precipitates were counted in a γ-counter to enable calculation
of the recovery of radioactivity into acetonitrile.
(11) (a) Tessari, M.; Pilla, M.; Andreoli, M.; Hutcheson, D. M.; Heidbre-
der, C. A. Antagonism at metabotropic glutamate 5 receptors inhibits
nicotine- and cocaine-taking behaviours and prevents nicotine-
triggered relapse to nicotine-seeking. Eur. J. Pharmacol. 2004, 499,
121-133. (b) Chiamulera, C.; Epping-Jordan, M. P.; Zocchi, A.;
Marcon, C.; Cottiny, C.; Tacconi, S.; Corsi, M.; Orzi, F.; Conquet,
F. Reinforcing and locomotor stimulant effects of cocaine are absent
in mGluR5 null mutant mice. Nat. Neurosci. 2001, 4, 873-874.
(12) Kenny, P. J.; Markou, A. The ups and downs of addiction: Role of
metabotropic glutamate receptors. Trends Pharmacol. Sci. 2004, 25,
265-272.
(13) (a) Olive, F. M.; Mcgeehan, A. J.; Kinder, J. R.; McMahon, T.;
Hodge, C. W.; Janak, P. H.; Messing, R. O. The mGluR5 antagonist
6-methyl-2-(phenylethynyl)pyridine decreases ethanol consumption
via a protein kinase C-epsilon-dependent mechanism. Mol. Phar-
macol. 2005, 67, 349-355. (b) Ba¨ckstro¨m, P.; Bachteler, D.; Koch,
S.; Hyytia¨, P.; Spanagel, R. mGluR5 antagonist MPEP reduces
ethanol-seeking and relapse behavior. Neuropsychopharmacology
2004, 29, 921-928.
(14) (a) Kokic, M.; Honer, M.; Ametamey, S. M.; Gasparini, F.; Andres,
H.; Bischoff, S.; Flor, P. J.; Heinrich, M.; Vranesi, I; Spooren, W.;
Kuhn, R.; Schubiger, P. A. Radiolabeling and in vivo evaluation of
11C-M-MPEP as a PET radioligand for the imaging of the metabo-
tropic glutamate receptor 5 (mGLuR5). J. Labelled Compd. Radiop-
harm. 2001, 44 (Suppl. 1), S231-S232. (b) Musachio, J. L.; Ghose,
S.; Toyama, H.; Kozikowski, A. P.; Klaess, T.; Mukhopadhyaya, J.
K.; Ichise, M.; Hong, J.; Zoghbi, S.; Liow, J.-S.; Innis, R. B.; Pike,
V. W. Two potential mGuR5 PET radioligands [11C]M-MPEP and
[11C]methoxy-PEPysSynthesis and initial PET evaluation in rats and
monkeys. Mol. Imaging Biol. 2003, 5, 168.
Acknowledgment. This research was supported by the
Intramural Research Program of the National Institutes of Health
(NIMH). We thank the NIH PET Department for fluorine-18
production and successful completion of the scanning studies,
PMOD Technologies for providing the image analysis software,
and the NIMH Psychoactive Screening Program (PDSP) for
performing assays. The PDSP is directed by Bryan L. Roth,
Ph.D., with project officer Jamie Driscol (NIMH) at the
University of North Carolina at Chapel Hill (Contract No.
NO1MH32004).
Supporting Information Available: HPLC chromatograms
verifying the purities of target compounds 3-7 and precursor 17
and data from a PET displacement experiment in monkey with [18F]-
3. This material is available free of charge via the Internet at http://
pubs.acs.org.

3266 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14
Sime´on et al.
(15) (a) Hamill, T. G.; Seiders, T. J.; Krause, S.; Ryan, C.; Sanabria, S.;
Gibson, R. E.; Patel, S.; Cosford, N.; Roppe, J.; Yang, J.; King, C.;
Hargreaves, R. J.; Burns, H. D. The synthesis and characterization
of mGLuR5 receptor PET ligands. J. Labelled Compd. Radiopharm.
2003, 46 (Suppl 1), S184. (b) Krause, S. M.; Hamill, T. G.; Seiders,
T. J.; Ryan, C.; Sanabria, S.; Gibson, R. E.; Patel, S.; Cosford, N.
D.; Roppe, J. R.; Hargreaves, R. J.; Burns, H. D. In vivo character-
ization of PET ligands for the mGluR5 receptor in rhesus monkey.
Mol. Imaging Biol. 2003, 5, 166.
(16) Ametamey, S. M.; Kessler, L. J.; Honer, M.; Wyss, M. T.; Buck,
A.; Hintermann, S.; Auberson, Y. P.; Gasparini, F.; Schubiger, P.
A. Radiosynthesis and preclinical evaluation of 11C-ABP688 as a
probe for imaging the metabotropic glutamate receptor subtype 5. J.
Nucl. Med. 2006, 47, 698-705.
(17) Ametamey, S. M.; Treyer, V.; Streffer, J.; Wyss, M. T.; Schmidt,
M.; Blagoev, M.; Hintermann, S.; Auberson, Y.; Gasparini, F.;
Fischer, U. C.; Buck, A. Human PET studies of metabotropic
glutamate receptor subtype 5 with 11C-ABP688. J. Nucl. Med. 2007,
48, 247-252.
(18) Hamill, T. G.; Krause, S.; Ryan, C.; Bonnefous, C.; Govek, S.;
Seiders, T. J.; Cosford, N. D. P.; Roppe, J.; Kamenecka, T.; Patel,
S.; Gibson, R. E.; Sanabria, S.; Riffel, K.; Eng, W.; King C.; Yang,
X.; Green, M. D.; O’Malley, S. S.; Hargreaves, R.; Burns, H. D.
Synthesis, characterization, and first successful monkey imaging
studies of metabotropic glutamate receptor subtype 5 (mGluR5) PET
radiotracers. Synapse 2005, 56, 205-216.
(23) Waterhouse, R. N. Determination of lipophilicity and its use as a
predictor of blood-brain barrier penetration of molecular imaging
agents. Mol. Imaging Biol. 2003, 5, 376-389.
(24) Iso, Y.; Grajkowska, E.; Wroblewski, J. T.; Davis, J.; Goeders, N.
E.; Johnson, K. M.; Sanker, S.; Roth, B. L.; Tueckmantel, W.;
Kozikowski, A. P. Synthesis and structure-activity relationships of
3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine analogues as potent,
noncompetitive metabotropic glutamate receptor subtype 5 antago-
nists; search for cocaine medications. J. Med. Chem. 2006, 49, 1080-
1100.
(25) Briard, E.; Pike V. W. Substitution-reduction: An alternative process
for the [18F]N-(2-fluoroethylation) of anilines. J. Labelled Compd.
Radiopharm. 2004, 47, 217-232.
(26) See http://www.uppsala.imanet.se/research-synthia.asp for a descrip-
tion of similar apparatus.
(27) Patel, S.; Krause, S. M.; Hamil, T.; Chaudhary, A.; Burns, D. H.;
Gibson, R. A. In vitro characterization of [3H]methoxyPEPy, an
mGluR5 selective radioligand. Life Sci. 2003, 73, 371-379.
(28) Scheibye, S.; Pedersen, B. S.; Lawesson, S.-O. Studies on organo-
phosphorus compounds XXI. The dimer of p-methoxythionophos-
phine sulfide as a thiation reagent. A new route to thiocarboxamides.
Bull. Soc. Chim. Belg. 1978, 87, 229-237.
(29) Zoghbi, S. S.; Baldwin, R. M.; Seibyl, J.; Charney, D. S.; Innis, R.
B. A radiotracer technique for determining apparent pKa of receptor
binding ligands. J. Labelled Compd. Radiopharm. 1997, 41, 136-
138.
(19) Sonogashira, K.; Tohda, Y.; Hagihara, N. A convenient synthesis of
acetylenes: Catalytic substitutions of acetylenic hydrogen with
bromoalkenes, iodoarenes, and bromopyridines. Tetrahedron Lett.
1975, 50, 4467-4470.
(30) Lazarova, N.; Sime´on, F. G.; Musachio, J. L.; Lu, S.; Pike, V. W.
Integration of a microwave reactor with Synthia to provide a fully
automated radiofluoridation module. J. Labelled Compd. Radiopharm.
2007, 50, 1-3.
(31) Clark, J. D.; Baldwin, R. L.; Bayne, K. A.; Brown, M. J.; Gebhart,
G. F.; Gonder, J. C.; Gwathmey, J. K.; Keeling, M. E.; Kohn, D. F.;
Robb, J. W.; Smith, O. A.; Steggerda, J.-A. D.; Van de Berg, J. L.
Guide for the Care and Use of Laboratory Animals; National
Academy Press: Washington DC, 1996.
(20) Dondoni, A.; Perrone, D. Total synthesis of (+)-galactostatin. An
illustration of the utility of the thiazole-aldehyde synthesis. J. Org.
Chem. 1995, 60, 4749- 4754.
(21) Nicolaou, K. C.; King, N. P.; Finlay, M. R. V.; He, Y.; Roschangar,
F.; Vourloumis, D.; Vallberg, H.; Sarabia, F.; Ninkovic, S.; Hepworth,
D. Total synthesis of epothilone E and related side-chain modified
analogues via a Stille coupling based strategy. Bioorg. Med. Chem.
1999, 7, 665-697.
(22) Pike, V. W. Positron-emitting radioligands for studies in vivosProbes
for human psychopharmacology. J. Psychopharmacol. 1993, 7, 139-
158.
(32) Carson, R. E.; Barker, W. C.; Liow, J.-S.; Johnson, C. A. Conference
Record of the IEEE Nuclear Science Symposium and Medical
Imaging Conference; Portland, Oregon, 2003.
JM0701268