Synthesis and evaluation of N-(methylthiophenyl)picolinamide derivatives as PET radioligands for metabotropic glutamate receptor subtype 4

Article abstract of DOI:10.1016/j.bmcl.2015.11.015

In recent years, mGlu₄ has received great research attention because of the potential benefits of mGlu₄ activation in treating numerous brain disorders, such as Parkinson's disease (PD). A specific mGlu₄ PET radioligand could be an important tool in understanding the role of mGlu₄ in both healthy and disease conditions, and also for the development of new drugs. In this study, we synthesized four new N-(methylthiophenyl)picolinamide derivatives 11-14. Of these ligands, 11 and 14 showed high in vitro binding affinity for mGlu₄ with IC₅₀ values of 3.4 nM and 3.1 nM, respectively, and suitable physicochemical parameters. Compound 11 also showed enhanced metabolic stability and good selectivity to other mGluRs. [¹¹C]11 and [¹¹C]14 were radiolabeled using the [¹¹C]methylation of the thiophenol precursors 20a and 20c with [¹¹C]CH₃I in 19.0% and 34.8% radiochemical yields (RCY), and their specific activities at the end of synthesis (EOS) were 496 ± 138 GBq/μmol (n = 6) and 463 ± 263 GBq/μmol (n = 4), respectively. The PET studies showed that [¹¹C]11 accumulated fast into the brain and had higher uptake, slower washout and 25% better contrast than [¹¹C]2, indicating improved imaging characteristics as PET radiotracer for mGlu₄ compared to [¹¹C]2. Therefore, [¹¹C]11 will be a useful radioligand to investigate mGlu₄ in different biological applications.

Full text of DOI:10.1016/j.bmcl.2015.11.015

Bioorganic & Medicinal Chemistry Letters 26 (2016) 133–139
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and evaluation of N-(methylthiophenyl)picolinamide
derivatives as PET radioligands for metabotropic glutamate receptor
subtype 4
Kun-Eek Kil a, Pekka Poutiainen a, Zhaoda Zhang a, Aijun Zhu a, Darshini Kuruppu b, Shilpa Prabhakar c,
Ji-Kyung Choi a, Bakhos A. Tannous c, Anna-Liisa Brownell a,
⇑
a Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, 149, 13th St., Suite 2301, Charlestown, MA 02129, USA
b Department of Surgery, Massachusetts General Hospital, Boston, MA 02114, USA
c Experimental Therapeutics and Molecular Imaging Laboratory, Department of Neurology, Neuroscience Center, Massachusetts General Hospital, Boston, MA 02114, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
In recent years, mGlu4 has received great research attention because of the potential benefits of mGlu4
activation in treating numerous brain disorders, such as Parkinson’s disease (PD). A specific mGlu4 PET
radioligand could be an important tool in understanding the role of mGlu4 in both healthy and disease
conditions, and also for the development of new drugs. In this study, we synthesized four new
N-(methylthiophenyl)picolinamide derivatives 11–14. Of these ligands, 11 and 14 showed high in vitro
binding affinity for mGlu4 with IC50 values of 3.4 nM and 3.1 nM, respectively, and suitable physicochem-
ical parameters. Compound 11 also showed enhanced metabolic stability and good selectivity to other
mGluRs. [11C]11 and [11C]14 were radiolabeled using the [11C]methylation of the thiophenol precursors
20a and 20c with [11C]CH3I in 19.0% and 34.8% radiochemical yields (RCY), and their specific activities at
Received 21 September 2015
Revised 3 November 2015
Accepted 5 November 2015
Available online 10 November 2015
Keywords:
PET
Metabotropic glutamate receptor subtype 4
(mGlu4)
Positive allosteric modulator (PAM)
the end of synthesis (EOS) were 496 138 GBq/lmol (n = 6) and 463 263 GBq/lmol (n = 4),
respectively. The PET studies showed that [11C]11 accumulated fast into the brain and had higher uptake,
slower washout and 25% better contrast than [11C]2, indicating improved imaging characteristics as PET
radiotracer for mGlu4 compared to [11C]2. Therefore, [11C]11 will be a useful radioligand to investigate
mGlu4 in different biological applications.
Ó 2015 Elsevier Ltd. All rights reserved.
As the most abundant excitatory neurotransmitter in the
central nervous system (CNS) of vertebrates, -glutamate mediates
several brain disorders, such as Parkinson’s disease (PD).5–7 As a
group III mGluR, mGlu4 interacts with the G subunit of G-pro-
L
a
i/o
more than 50% of all synapses.1,2 Metabotropic glutamate recep-
tors (mGluRs) and ionotropic glutamate receptors (iGluRs) are
two major classes of glutamate receptors. The mGluRs belong to
the class C G protein-coupled receptors (GPCR) superfamily, which
have a distinct large extracellular N-terminus. The mGluRs can be
further divided into three subgroups including eight known recep-
tor subtypes (group I: mGlu1 and mGlu5, group II: mGlu2 and
mGlu3, and group III: mGlu4, mGlu6, mGlu7 and mGlu8) based on
their structural similarity, ligand specificity, and preferred cou-
pling mechanisms.3 The mGluRs have distinctive biodistribution
in CNS depending on subtypes and subgroups and are involved
in glutamate signaling in almost every excitatory synapse in CNS.4
In recent years, mGlu4 has received a lot of research attention
because of the potential benefits of mGlu4 activation in treating
tein which negatively couples with adenylate cyclase to inhibit
cAMP dependent signal pathways.8,9 The mGlu4 is expressed at
multiple synapses throughout the basal ganglia, mainly localized
presynaptically and expressed in the striatum, hippocampus, tha-
lamus, and cerebellum.3,10,11 Its activation reduces neurotransmit-
ter release, a mechanism implicated in the pathophysiology of PD.
The activation of mGlu4 can be accomplished by two different
mechanisms: orthosteric agonists (competing with L-glutamate at
the extracellular N-terminal’s Venus Flytrap Domain) or noncom-
petitive positive allosteric modulators (PAMs). Previous orthosteric
ligands of mGlu4 lack clear subtype selectivity and blood–brain
barrier (BBB) penetration. However, noteworthy examples of selec-
tive and brain penetrant orthosteric agonists such as LSP4-2022
exist.12,13
Much recent effort has been focused on the development of
allosteric modulators, which target the seven-transmembrane
spanning domain. In particular, the allosteric modulation of mGlu4
⇑
Corresponding author. Tel.: +1 617 726 3807.
E-mail address: abrownell@mgh.harvard.edu (A.-L. Brownell).
http://dx.doi.org/10.1016/j.bmcl.2015.11.015
0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.

134
K.-E. Kil et al. / Bioorg. Med. Chem. Lett. 26 (2016) 133–139
has prompted intense interest after (À)-PHCCC (1), a partially
selective mGlu4 PAM, was discovered and demonstrated activity
in models of neuroprotection and PD.14 There has been substantial
progress in identifying PAMs for mGlu4, and hundreds of mGlu4
PAMs have been reported and/or patented since 2009.5,15,16
Figure 1 shows representative mGlu4 PAMs.5,15,17–20 Subsequent
results with mGlu4 PAMs have further validated the antiparkinso-
nian activity in animal models of PD.11,19,21–24 This approach has
opened a new avenue for developing nondopaminergic treatments
for PD and for identifying novel disease modifying therapeutics.
As a noninvasive medical and molecular imaging technique and
a powerful tool in neurological research, positron emission tomog-
raphy (PET) offers a possibility to visualize and analyze the target
receptor expression under physiological and pathophysiological
conditions. PET has mostly been used to detect disease-related bio-
chemical changes before the disease-associated anatomical
changes can be found using standard medical imaging modalities.
Moreover, PET radiotracers serve as invaluable biomarkers during
the development of potential therapeutic drugs. Thus, extensive
research effort has been directed towards the development of
PET radioligands suitable for probing mGlu1 and mGlu5, in which
many PET radiotracers have been reported and several of which,
Recently, we reported a carbon-11 labeled PET radioligand [11C]230
and a fluorine-18 labeled PET radioligand [18F]831 (Fig. 2). These
compounds exhibited some favorable features as PET radioligands
such as fast uptake into the brain and specific accumulation in
mGlu4-rich regions of the brain. However, in comparison to one
of the best mGlu5 PET radiotracers [18F]FPEB,26,27 these compounds
showed decreased retention time in the brain, which may affect
the quality of imaging. The results indicate that the affinity and
metabolic stability of this class of radiotracers need further
optimization.
Thus, we have carried out the structure–affinity relationship
(SAR) study of a series of new N-phenylpicolinamide derivatives.32
It was then discovered that the 3-methylthio group was superior to
the 3-methoxy group for mGlu4 affinity by comparing compounds
9 (IC50 = 13.7 nM) and 10 (IC50 = 4.9 nM), showing a 2.8 fold
enhancement in affinity. Metabolic stability was considered one
of the major issues for ML128 (2), in which the 3-methoxy group
was recognized as the metabolic soft group. As the N-methylthio-
phenyl derivatives may have a different metabolic profile from
the corresponding N-methoxyphenyl analogs, it is interesting to
understand its effect on the metabolic stability of the picolinamide
derivatives. Based on these reasons, we extended our SAR study to
four new N-(methylthiophenyl)picolinamide derivatives 11–14
(Fig. 3).
such as [18F]FIMX25 for mGlu1, and [18F]FPEB,26,27 18F]SP20328
[
and [11C]ABP68829 for mGlu5, have been advanced for human clin-
ical trials.
Although many PET probes have been developed for mGlu1 and
mGlu5, mGlu4 still lacks useful PET radioligands for clinical study. A
specific mGlu4 PET radioligand could be an important tool for
understanding the role of mGlu4 in healthy and disease conditions,
and also for the development of new drugs targeting this receptor.
Compounds 11 and 12 have a Cl- or F-substitution at the 4-phe-
nyl position, respectively, in which the same substitutions have
enhanced the affinity for N-(3-methoxyphenyl)picolinamide and
they may have a similar effect for N-(3-(methylthio)phenyl)picoli-
namide. The 4-phenyl position of the N-phenylpicolinamide was
tolerated with a relatively large substitution as demonstrated in
Figure 1. Representative mGlu4 PAMs.

K.-E. Kil et al. / Bioorg. Med. Chem. Lett. 26 (2016) 133–139
135
ammonium thiocyanate to give 23 in 65% yield. Picolinic acid was
converted to acid chloride and then coupled with 23 to form amide
24 in 88% yield. The nitrile group in 24 was removed by sodium
borohydride to give 20c in 75% yield. The methylation of 20c was
carried out using CH3I and K2CO3 to give 14 in 28% yield (Scheme 1c).
Compounds, 2 and 9–14, were evaluated with competitive bind-
ing studies using mGlu4 transfected CHO cells by increasing the
concentration of test materials from 0.01 nM to 10 lM in the pres-
Figure 2. The PET radiotracers for mGlu4.
ence of 2 nM of [3H]2.31 As shown in Table 1, the IC50 values of 11
and 14 are 3.4 and 3.1 nM, respectively, in which the affinity is
enhanced by 1.5–1.6 fold compared to 2 (IC50 = 5.1 nM). Compound
10 (IC50 = 4.9 nM) has a similar affinity to 2, while compounds 12
and 13 show reduced affinity. Compounds 10–14 were further
tested for their metabolic stability in microsome from rat liver
extract. Compound 2 was also examined at the same time as a ref-
erence. As Table 1 shows, the metabolic stabilities are in the follow-
ing order: 11 > 2 > 10 > 12 > 13 > 14. Although the half-life
difference between compounds 2 and 11 is not significant, we antic-
ipate that the difference of the half-life in the brain between these
two compounds could be more significant. It is interesting to note
that as the regioisomers of 10 and 11, compounds 13 and 14 display
a substantial decreased trend in metabolic stability. The results
show that compound 11 has the most promising enhanced affinity
and/or increased in vitro microsomal stability compared to other
compounds (significance p <0.05, t-test) in this series.
compounds 6 and 7, so we synthesized compounds 13 and 14,
which are the regioisomers of 10 and 11, respectively. Further-
more, these compounds can be easily radiolabeled by carbon-11
on the thiomethyl group.
Since poor BBB permeability and high nonspecific binding (NSB)
are among the frequent causes for failure in CNS PET radioligand
development, it is necessary to consider some important structural
parameters such as MW, cLogP, tPSA, and HBD at the design stage. It
has been proposed that more desirable ranges for CNS drugs are
tPSA < 90; HBD < 3; 2 < cLogP < 5; and MW < 450.33 As shown in
Figure 3, these compounds possess favorable physicochemical
parameters, making them good candidates for CNS ligand
development.
The syntheses of compounds 11–14 are illustrated in Scheme 1.
Compounds 11 and 12 were synthesized in six steps (Scheme 1a).
5-Amino-2-halo-thiophenol (17a and 17b) were prepared by the
chlorosulfonylation of 4-halo-nitrobenzene (15a and 15b) followed
by tin(II) chloride mediated reduction in acidic media. To prevent
nucleophilic attack on the thiol group during the amide coupling
reaction, 17a and 17b were oxidized to form a disulfide bond. The
resulting amino-dimer 18a and 18b were coupled with picolinic
acid or its acid chloride under the corresponding amide coupling
conditions to give 19a and 19b. The disulfide bond in 19a and 19b
was reduced using either sodium borohydride or tris(2-car-
boxyethyl)phosphine hydrochloride (TCEPÁHCl) to give the corre-
sponding thiophenol derivatives, 20a and 20b, respectively. 20a
and 20b were methylated using iodomethane (CH3I) in presence
of potassium carbonate (K2CO3) or diisopropylethylamine (DIPEA)
to give 11 and 12, respectively. Compound 13 was synthesized by
an amide coupling reaction between acid chloride of picolinic acid
and aniline 21 in 60% yield (Scheme 1b). Compound 14 was synthe-
sized from 22 in four steps. Compound 22 was derivatized with
The selectivity of 11 to other mGluRs including group I (mGlu1
and mGlu5), II (mGlu2) and III (mGlu6 and mGlu8) were checked
(Supplementary material). The assays were carried out at eight dif-
ferent concentrations between 0.1 nM and 30
indicated that compound 11 did not have agonist and antagonist
activities (>30 M) toward mGlu1, mGlu5, mGlu2 and mGlu6, but
displayed a weak agonist activity to mGlu8 (EC50 = 14
M).34,35
lM of 11. The results
l
l
The selectivity data supports compound 11 as the most promising
imaging agent candidate.
Since compounds 11 and 14 displayed the most promising
affinity, they were selected for in vivo evaluation as potential
mGlu4 radiotracers and were compared to [11C]2 to understand
how their structure, affinity, and metabolic stability affect PET
imaging.
Carbon-11 labeling was performed by methylation of the
thiophenol precursors (20a and 20c) using [11C]CH3I and K2CO3
in acetone at 50 °C for 3 min (Scheme 2). All radiosyntheses took
Figure 3. The N-phenylpicolinamide derivatives for SAR study.

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K.-E. Kil et al. / Bioorg. Med. Chem. Lett. 26 (2016) 133–139
Scheme 1. Syntheses of 11–14 and thiophenol precursors, 20a and 20c. Reagents and conditions: (i) ClSO3H, 120 °C; (ii) SnCl2, HCl, reflux; (iii) DMSO, 80 °C; (iv) EDCÁHCl,
DIPEA, HOBt, dioxane, 50 °C; (v) picolyl chloride, DIPEA, DMF, rt; (vi) NaBH4, EtOH, reflux; (vii) TCEPÁHCl, DMF, rt; (viii) K2CO3, Cs2CO3, MeI, rt; (ix) DIPEA, MeI, CH2Cl2, rt; (x)
(1) picolinic acid, SOCl2, benzene, reflux; (2) TEA, THF, 40 °C; (ix) NH4SCN, I2, MeOH, rt; (xii) NaBH4, erythritol, MeOH, 0 °C; (*) yield was based on 15b.
Table 1
In vitro properties
unlabeled compound on a radio-HPLC. As shown in Figure 4b, the
retention time of [11C]11 was the same as that of coinjected nonra-
dioactive standard 11. The QC analysis of [11C]14 was also con-
ducted in the same way. The radiochemical purities of [11C]11
and [11C]14 determined by radio-TLC were 98.0 0.5% (n = 7),
and 97.5 1.0% (n = 4), respectively. Similar results were also
observed in the radio-HPLC analyses.
Affinitya
Stabilityb
Compound IC50 (nM) Log(IC50 SEM) T1/2 (min) jc (minÀ1
)
SEM (j)
2 (ML 128)
5.1
13.7
4.9
3.4
7.8
À8.29 0.09e
À7.86 0.10e
À8.31 0.07e
À8.46 0.07
À8.11 0.10
À7.88 0.09
À8.50 0.09
5.99
n.d.d
5.83
6.54
5.73
4.14
3.86
0.116
n.d.
0.007
n.d.
9
10
11
12
13
14
0.119
0.106
0.121
0.168
0.179
0.009
0.005
0.008
0.013
0.016
We determined the LogD7.4 values for [11C]11 and [11C]14 as
3.15 0.04 (n = 4) and 3.00 0.05 (n = 4) using octanol and phos-
phate buffered saline (PBS, pH 7.4), which lie in the range normally
considered favorable for a PET radioligand. The calculated cLogP
values of 11 and 14 are both 3.63 (Fig. 3), which is higher than
the experimental values.
13.3
3.1
a
b
c
Binding inhibition assay.
Microsomal stability assay (rat).
The decay constant that is slope of logconcentration versus time profile (t1/
).
Not determined.
Ref. 32.
2 = ln2/
j
PET imaging studies were conducted using male Sprague Daw-
ley rats. The in vivo uptake and kinetics were examined using
small-animal PET. The PET images (Fig. 5) indicated that [11C]11
and [11C]14 crossed the BBB quickly and were mainly accumulated
in the thalamus, hippocampus, cerebellum, and striatum, which
were reported as the mGlu4-rich regions of the rat brain.10,36–38
We also acquired PET images of [11C]2 in the same time frames
for comparison. To confirm the corresponding brain regions,
anatomical templates of rat brains were used for the images with
d
e
35–40 min from the end of bombardment (EOB) to the end of
synthesis (EOS). The carbon-11 labeled compounds [11C]11 and
[
11C]14 were obtained in 19.0 7.7% (n = 7) and 34.8 8.0%
(n = 4) RCYs from [11C]CO2, respectively. Their specific activities
were 496 138 GBq/
at EOS, respectively.
lmol (n = 6) and 463 263 GBq/lmol (n = 4)
[
11C]11 (Fig. 5). The results suggested that [11C]11 gave the best
Figure 4a indicates that the difference in retention times
between the precursors (20a and 20c) and the corresponding radi-
olabeled products ([11C]11 and [11C]14) was big enough to purify
the radiotracers as a baseline separation. The HPLC profile of
contrast among the three radiotracers and displayed better images
in the hippocampus, striatum, thalamus and cerebellum. Using the
mGlu4-negligible region in the brain as a reference, we calculated
the uptake ratio (contrast) of radioactivity between the mGlu4-rich
and the reference regions (areas immediately outside the cortical
areas) from the reconstructed PET images. The maximum ratio
[
11C]14 was similar to that of [11C]11. The identities of [11C]11
and [11C]14 were confirmed by coinjection with the corresponding

K.-E. Kil et al. / Bioorg. Med. Chem. Lett. 26 (2016) 133–139
137
Scheme 2. Radiosyntheses of [11C]11 and [11C]14.
Figure 4. The UV and radioactivity profiles (a) the reaction mixture was purified by a semi-preparative HPLC and (b) the QC analysis of the purified [11C]11 was carried out by
an analytical HPLC. (a) Gemini-NX C18 semi-preparative column (Phenomenex, 250 mm  10 mm, 5 m), ammonium formate solution (0.1 M): acetonitrile = 40:60, 4 mL/min
(b) Alltima C18 analytical column (150 mm  4.6 mm, 5 m), 0.1% TFA solution: acetonitrile = 35:65, 1 mL/min.
l
l
Figure 5. Color-coded PET images of [11C]11, [11C]14, and [11C]2 at different brain levels (striatum, thalamus, hippocampus, and cerebellum). The images represent
distribution of radioligand from 5 to 10 min after administration. All the images were normalized to the highest voxel value (% ID/cm3) of the whole data set. These images
demonstrate improved imaging characteristics of [11C]11 compared to [11C]14 and [11C]2 based on the high accumulation and slow washout. The thickness of each slice was
0.625 mm.

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K.-E. Kil et al. / Bioorg. Med. Chem. Lett. 26 (2016) 133–139
was 1.67 0.08 (n = 30; 10 images in 3 baseline studies) for [11C]2
and 2.10 0.25 (n = 30) for [11C]11. The results show that the sig-
nal to noise ratio of [11C]11 was enhanced up to 25% compared
to [11C]2.
The regional time–activity curves (TACs) of PET with [11C]2,
[
11C]11, and [11C]14 in the hippocampus, striatum, thalamus and
cerebellum of rat brains are given in Figure 6. The % ID/cc values
at each time point obtained from the different PET images were
averaged for each radiotracer and converted into TACs. As shown
in the TACs, the radioactivity of [11C]2, [11C]11, and [11C]14 quickly
increased in the brain within 2 min after a tail vein injection and
then washed out within 20 min. The maximum radioactivity
uptake depends on the region in the brain. The highest uptake
for [11C]11 was seen in the thalamus, followed by the striatum,
hippocampus, and cerebellum. The results indicate that [11C]11
has better brain uptake than [11C]2 and [11C]14 and is an improved
PET radioligand for mGlu4.
A blocking experiment for [11C]11 was carried out 2 h after the
baseline scans to compare blocking images with baseline images.
An mGlu4 PAM (3ÁHCl) was used as an mGlu4 blocking agent. The
blocking agent was administered intravenously 1 min before
[
11C]11 injection.
The mGlu4 blocking effects with compound 3 were 19–24% for
Figure 7. Blocking studies of [11C]11 with an mGlu4 PAM (3ÁHCl, 10 mg/kg, iv)
during first 5 min after injection of [11C]11. Each rat was scanned for baseline and
individually normalized (whole brain was set as 1.0). The blocking study was done
with same instrumentation and settings. The blocking results were normalized to
corresponding baseline results. Integrals were calculated over 0–5 min time
periods. (n = 4 blotted with SD and significances were calculated using t-test,
**p <0.01).
first 5 min in selected regions of interest (ROIs). (Fig. 7) The
incomplete blocking effect by compound 3 might be attributed to
its relatively weak affinity and fast kinetics.17,32
In this study, we synthesized four new N-(methylthiophenyl)
picolinamide derivatives 11–14. Of these ligands, 11 and 14
showed high in vitro binding affinity for mGlu4 together with suit-
able structural parameters. Compound 11 also showed enhanced
metabolic stability and good selectivity to other mGluRs. [11C]11
and [11C]14 were radiolabeled via the [11C]methylation of the
thiophenol precursors, 20a and 20c, with [11C]CH3I in reliable RCYs
and specific activities. The PET studies showed that [11C]11
accumulated fast into the brain and had higher uptake, slower
washout and 25% better contrast than [11C]2, indicating improved
imaging characteristics as a PET radiotracer for mGlu4 compared to
[
11C]2. [11C]11 will be a useful radioligand to investigate mGlu4 in
different biological applications.
Figure 6. Brain regional TACs of PET with [11C]2, [11C]11, and [11C]14 in (a) hippocampus, (b) striatum, (c) thalamus and (d) cerebellum of rat brain. The % ID/cc values at each
time point were averaged from different studies for constructing TACs.

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139
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A.; Fujita, M.; Pike, V. W.; Innis, R. B. J. Nucl. Med. 2008, 49, 2042.
29. Ametamey, S. M.; Treyer, V.; Streffer, J.; Wyss, M. T.; Schmidt, M.; Blagoev, M.;
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Funding sources
NIBIB-R01EB012864 and NIMH-R01MH91684 to A.-L. Brownell.
Acknowledgements
The authors would like to thank the technical support of the
PET/MRI and radiochemistry facility at the Athinoula A. Martinos
Center for Biomedical Imaging at Massachusetts General Hospital
for the operation of cyclotron and synthesis modules. The authors
are also grateful to Drs. Tanabe and Nakanishi’s laboratory
(Osaka Bioscience Institute, Osaka, Japan) for providing the vector
of mGlu4 from the rat as
a gift. The following supporting
instrument Grants (1S10RR029495-01, 1S10RR026666-01, and
1S10RR023452-01) are also appreciated. The authors appreciate
the generous help of the NIMH PDSP program (Contract # HHSN-
271-2008-00025-C) led by Dr. Bryan L. Roth MD, PhD at the
University of North Carolina at Chapel Hill and the Project Officer
Jamie Driscol at NIMH, Bethesda MD, USA. Finally, the authors
would like to express their appreciation of the financial support
for P.P. from The Orion Farmos Research Foundation, Saastamoinen
Foundation, Sigrid Juselius Foundation, Osk Huttunen Foundation
and Kuopio University Foundation.
Supplementary data
Supplementary data (experimental procedures, spectroscopic
characterization, HPLC data, in vitro selectivity data, and TACs for
mGlu4 blocking studies) associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.bmcl.2015.11.
015.
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