Re-exploring the N-phenylpicolinamide derivatives to develop mGlu4 ligands with improved affinity and in vitro microsomal stability

Article abstract of DOI:10.1016/j.bmcl.2015.07.031

Abstract In recent years, mGlu₄ has received great attention and research effort because of the potential benefits of mGlu₄ activation in treating numerous brain disorders, such as Parkinson's disease (PD). Many positive allosteric m

Full text of DOI:10.1016/j.bmcl.2015.07.031

Bioorganic & Medicinal Chemistry Letters 25 (2015) 3956–3960
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Re-exploring the N-phenylpicolinamide derivatives to develop mGlu4
ligands with improved affinity and in vitro microsomal stability
Zhaoda Zhang a, Kun-Eek Kil a, Pekka Poutiainen a, Ji-Kyung Choi a, Hye-Jin Kang b, Xi-Ping Huang b,
Bryan L. Roth b, Anna-Liisa Brownell a,
⇑
a Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, 149, 13th Street, Suite 2301, Charlestown, MA 02129, United States
b Department of Pharmacology, University of North Carolina Chapel Hill School of Medicine, Chapel Hill, NC 27514, United States
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 attention and research effort because of the potential benefits of
mGlu4 activation in treating numerous brain disorders, such as Parkinson’s disease (PD). Many positive
allosteric modulators of mGlu4 have been developed. To better understand the role of mGlu4 in healthy
and disease conditions, we are interested in developing an mGlu4 selective radioligand for in vivo studies.
Thus, we had synthesized and studied [11C]2 as a PET tracer for mGlu4, which demonstrated some
promising features as a PET radioligand as well as the limitation need to be improved. In order to develop
an mGlu4 ligand with enhanced affinity and improved metabolic stability, we have modified, synthesized
and evaluated a series of new N-phenylpicolinamide derivatives. The SAR study has discovered a number
of compounds with low nM affinity to mGlu4. The dideuteriumfluoromethoxy modified compound 24 is
identified as a very promising mGlu4 ligand, which has demonstrated enhanced affinity, improved
in vitro microsomal stability, good selectivity and good permeability.
Received 15 June 2015
Revised 10 July 2015
Accepted 13 July 2015
Available online 17 July 2015
Keywords:
Metabotropic glutamate receptor subtype 4
(mGlu4)
Positive allosteric modulator (PAM)
Positron emission tomography (PET)
Affinity
Metabolic stability
Structure–affinity relationship (SAR)
Ó 2015 Elsevier Ltd. All rights reserved.
L
-Glutamate is the most abundant excitatory neurotransmitter
G-protein which negatively couples with adenylate cyclase to inhi-
bit 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.4,10,11 Its activation reduces neurotransmit-
ter release, a mechanism implicated in the pathophysiology of PD.
The activation of the mGlu4 receptor can be accomplished by two
in the CNS (central nerve system) of vertebrates and probably
mediates more than 50% of all synapses.1,2 Two major classes of
receptors, mGlu and iGlu, are involved in glutamate signal transfer.
The mGlu belong to Class C of the GPCR (G protein-coupled recep-
tor) super family, which are thought to exist as dimers and have a
distinct large extracellular N-terminus. This extracellular N-termi-
nal domain contains two hinged globular domains referred as the
venus flytrap domain (VFD), which is the orthosteric binding site
different mechanisms: orthosteric agonists (competing with L-glu-
tamate) or noncompetitive positive allosteric modulators (PAMs).
Most orthosteric ligands of mGlu4 made in the past lack clear sub-
type selectivity and BBB (blood–brain barrier) penetration, but
notable examples exist of selective and brain penetrant orthosteric
agonists, such as LSP4-2022.12,13 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 has spurred intense interest after
(À)-PHCCC (1, N-phenyl-7-(hydroxyimino)-cyclopropa[b] chro-
men-1a-carboxamide), a partially selective mGlu4 PAM, was dis-
covered and demonstrated activity in models of neuroprotection
and PD. Since then there has been substantial progress in identify-
ing PAMs for mGlu4.6,14,15 Figure 1 shows some representative
mGlu4 PAMs.6,14,16–20 Subsequent results with PAMs of mGlu4 have
further validated the antiparkinsonian activity in animal models of
PD,11,17,21–24 in which this approach has opened a new avenue for
for the endogenous ligand, L
-glutamate.3 The mGlu can be further
divided into three subgroups including eight known receptor sub-
types (group I: mGlu1 and mGlu5, group II: mGlu2 and mGlu3, and
group III: mGlu4, mGlu6, mGlu7 and mGlu8) based on their struc-
tural similarity, ligand specificity, and preferred coupling mecha-
nisms.4 The mGlu are involved in glutamate signaling in almost
every excitatory synapse in CNS, and they have distinctive biodis-
tribution in CNS depending on subtypes and subgroups.5 In recent
years, mGlu4 has received great attention and research effort
because of the potential benefits of mGlu4 activation in treating
numerous brain disorders, such as Parkinson’s disease (PD).6,7
As a group III mGlu, mGlu4 interacts with the G
subunit of
a
i/o
⇑
Corresponding author. Tel.: +1 617 726 3807.
E-mail address: abrownell@partners.org (A.-L. Brownell).
http://dx.doi.org/10.1016/j.bmcl.2015.07.031
0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.

Z. Zhang et al. / Bioorg. Med. Chem. Lett. 25 (2015) 3956–3960
3957
OH
HN
N
O
N
N
NH
NH
Cl
O
O
F
N
O
N
N
N
N
H
O
N
N
N
H
O
F
S
N
N
H
4
S
N
N
H
5
3
1
2
ADX88178
(-)-PHCCC
ML-128
MW: 294.30
tPSA: 70.92
CLogP: 2.72
MW: 284.34
tPSA: 73.5
CLogP: 2.32
MW: 272.33
tPSA: 73.5
CLogP: 2.01
MW: 262.69
tPSA: 50.69
CLogP: 2.80
MW: 264.23
tPSA: 50.69
CLogP: 2.67
hEC50 = 240 nM
rEC50 = 110 nM
hEC50 = 340 nM
rEC50 = 80 nM
hEC50 =4 nM
rEC50 = 9 nM
hEC50 =9 nM
EC50 = 1400 nM
O
N
O
N
O
O
S
O
N
O
O
O
H
Cl
N
N
N
O
N
O
N
H
Cl
N
Cl
N
H
N
H
Cl
Cl
H
9
8
6
7
ML-292
ML-182
VU0364439
4-PAM-1
MW: 393.83
tPSA: 78.84
CLogP: 1.91
hEC50 =291 nM
rEC50 = 376 nM
MW: 232.67
tPSA: 41.46
MW: 391.81
tPSA: 78.84
CLogP: 3.62
MW: 422.28
tPSA: 87.63
CLogP: 3.74
CLogP: 3.05
hEC50 =1196 nM
rEC50 = 330 nM
hEC50 =20 nM
hEC50 = 43.6 nM
Figure 1. Some representative mGlu4 PAMs.
developing nondopaminergic treatments for PD and for identifying
a novel disease modifying therapeutics.
[
[
18F]FPEB (3-[18F]fluoro-5-(2-pyridinylethynyl)benzonitrile),27,28
11C]2 showed the decreased retention time in the brain, which
To better understand the role of mGlu4 in healthy and disease
conditions, we are interested in developing an mGlu4 selective
radioligand for in vivo study. As a noninvasive medical imaging
technique and a powerful tool in neurological research, positron
emission tomography (PET) offers a possibility to visualize and
analyze the target receptor expression under physiological and
pathophysiological conditions. PET is being applied more often to
detect disease-related biochemical changes before the disease-as-
sociated anatomical changes could be found by standard medical
imaging modalities. Moreover, PET tracers serve as invaluable
biomarkers during the development of potential therapeutic drugs.
Thus, extensive research efforts have been directed toward the
development of PET radioligands suitable for probing mGlu such
as mGlu1 and mGlu5.15
may affect the quality of the imaging. The results indicate that
the affinity and metabolic stability of this class of tracers need
further optimization. We report here the synthesis and struc-
ture–affinity relationship study of new N-phenylpicolinamide
derivatives to develop mGlu4 ligands with improved affinity and
metabolic stability.
We have modified and synthesized a series of new N-phenylpi-
colinamide derivatives for SAR study, in which the syntheses are
shown in Schemes 1–3 (see Supplementary data). Three most
active known compounds in this series (2, 3, and 10) were also syn-
thesized and evaluated as the reference compounds for optimiza-
tion. On the basis of previous SAR results,16 we modified
compound 2 at three positions (3- or 4-phenyl, 6-pyrindyl) as illus-
trated in Figure 2. It is known that the SAR of this series was
tight,6,16 so we started with minor modifications based on the
reported data. As shown in Table 1, the modifications include the
isosteric replacement of hydrogen by fluorine or deuterium, oxy-
gen by sulfur, methoxy by cyano group and change for different
halogen atoms. The radiolabeling strategy was also considered in
lead optimization design to generate the facile labeling positions
for either C-11 or F-18 tracer.
Since poor BBB permeability and high nonspecific binding (NSB)
are among the most frequent causes for failure in CNS PET ligand
development, it is necessary to consider some important physico-
chemical parameter such as MW, ClogP and tPSA at the design
stage. It has been recently proposed that more desirable ranges
for CNS drugs are ClogP <3, MW <360 and 40 < tPSA < 90.29 As
shown in Table 1, all compounds except 12 and 22 possess the
favorable physicochemical parameters, making them ideal candi-
dates for CNS ligand development.
Recently, we have reported a carbon-11 labeled PET ligand
[
11C]2 (N-(4-chloro-3-[11C]methoxyphenyl)picolinamide),25 which
was based on a reported mGlu4 PAM 2.16 In 2009, two research
groups at Addex Pharma26 and Vanderbilt University16 have inde-
pendently disclosed a series of small arylamide compounds as a
new class of mGlu4 PAMs. Engers et al. found from a high-through-
put screening there were a number of small arylamide compounds
having mGlu4 PAM activity. They reported the SAR study, in vitro
pharmacokinetic (PK) parameters and in vivo rat PK, which
included the SAR results for sixteen N-phenylpicolinamide deriva-
tives.16 Compounds 2 and 3 were the most potent mGlu4 PAMs in
this series and showed some potentially suitable properties for PET
tracer development, which include: (1) rapid penetration into rat
brain following intraperitoneal injection (Tmax for brain: 0.5 h);
(2) high brain/plasma (B/P) partition coefficients for both com-
pounds (B/P = 4.1 for 2 and 9.9 for 3), in which B/P was determined
by AUC0–8h,Brain/AUC0-8h,Plasma; (3) good in vitro potency and effi-
cacy for both human and rat mGlu4 compared to previous reported
mGlu4 PAM; (4) good selectivity over other mGlu subtypes; (5)
compound 2 was the first mGlu4 PAM to demonstrate efficacy in
a preclinical rodent model of motor impairments associated with
PD.6 Thus, we had synthesized and studied [11C]2 as a PET tracers
for mGlu4. This compound demonstrated some promising features
as a PET radioligand such as the fast uptake into brain and the
specific accumulation in mGlu4-rich regions of the brain.
However, in comparison to one of the best mGlu5 PET tracer
a or b or c
53-97%
O
O
R'
R'
+
H2N
N
H
R
X
R
Scheme 1. Synthesis of the N-phenylpicolinamide derivatives. Reagents and
conditions: (a) for carboxylic acids, EDCÁHCl, HOBtÁH2O, DIPEA, dioxane; (b) for
carboxylic acids, (1) thionyl chloride, benzene, reflux for 2 h; (2) TEA, THF, 40 °C,
1 h; (c) for acid chloride, DIPEA, CH2Cl2, 4 h.

3958
Z. Zhang et al. / Bioorg. Med. Chem. Lett. 25 (2015) 3956–3960
10 fold loss of activity.16 Our initial effort was directed at 3-cyano
I
Br
SnMe3
O
O
O
substitution, in which 11C-cyanation may be carried out through a
palladium-mediated cyanation or the Rousenmund-von Braun
reaction.33 Five 3-cyanophenyl compounds (18–22) with different
4-phenyl substitutions were evaluated. The results show that the
3-cyano-4-chloro-analog 20 give a similar affinity compared to
the 3-methoxy-4-chloro-analog 2. The affinity values of 18–22
are depending on 4-phenyl substitution and in the following order:
Cl < F < H < Br < I, which shows different substitution effect com-
pared to 3-methoxy analogs 2 and 10–13. We then replaced 3-
methoxy with 3-fluoromethoxy for two reasons: first, since both
3-methoxy- and 3-difluoromethoxy-analogs exhibited the activity,
fluoromethoxy should be also active; second, it generates a posi-
tion for fluorine-18 labeling. Fluorine-18 is often the radionuclide
of choice for both its physical and nuclear characteristics. Its
half-life is long enough to carry out relatively extended imaging
protocols when compared to what is possible with carbon-11.
This facilitates kinetic studies and high-quality metabolic and
plasma analysis. However, fluorine-18 labeling is normally limited
to chemical structures already containing a fluorine atom and the
possible labeling strategies are limited for the preparation of radio-
tracers of high specific radioactivity. The result shows that 3-fluo-
romethoxy compound 23 has an improved affinity (3.2 nM)
compared to that (5.1 nM) of 2, which improves 1.6 fold.
However, 3-fluoromethoxy group may not be metabolically stable,
since the 3-methoxy and 3-difluoromethoxy groups were metabol-
ically unstable in compounds 2 and 3. On the other hand, 3-triflu-
oromethoxy analog of 3 was significantly more stable but lack
activity.16 Hence, we had applied a 3-dideuteriumfluoromethoxy
group to replace 3-fluoromethoxy group as shown in compounds
24 and 25. Deuterium isotope effects have been used to reduce
in vivo metabolic rates. For example, Zhang et al. reported that a
deuterium-substituted analog (with 18FD2CO) as a radioligand for
peripheral benzodiazepine receptor (PBR) had remarkably pro-
longed the half-life (T1/2) in mice brain.34 The deuterium substitu-
tion may reduce the rate of defluorination initiated by cleavage of
the C–H bond without altering the binding affinity to mGlu4. The
result shows that the 3-dideuteriumfluoromethoxy modified com-
pounds 24 and 25 have excellent affinity.
a
b
N
N
N
N
H
N
H
N
H
R
R
R
12 (R = MeO): 78.4%
22 (R = CN): 85.1%
R = MeO: 60.4%
R = CN: 5.4%
Scheme 2. Synthesis of the N-phenylpicolinamides 12 and 22. Reagents and
conditions: (a) (SnMe3)2, Pd(PPh3)4, Toluene, reflux, 8.5 h; (b) I2, CH2Cl2, 2 h.
R
O
X
X
X
X
I
a
X
X
X
F
X
b
c
N
N
H
O
F
TsO
TsO
I
OTs
X = H: 50.8%
X = D: 49.2%
X = H: 41.3%
X = D: 48.4%
23 (X = H; R = Cl): 40.2%
24 (X = D: R = Cl): 80.4%
25 (X = D; R = H): 44.0%
Scheme 3. Synthesis of the N-(3-fluoromethoxyphenyl)picolinamides 23–25.
Reagents and conditions: (a) Ag(OTs), MeCN, reflux, overnight; (b) CsF,
HO(CH2O)6H, reflux, 3.5 h; (c) N-(4-R-3-hydroxyphenyl)picolinamide, K2CO3, 40–
50 °C, 3 days.
6-Pyridyl
4-Phenyl
Cl
O
H
N
3-Phenyl
N
H
O
2
Figure 2. Modificatons on compound 2.
The lead compounds 2 and 3 were identified as mGlu4 PAMs by
using functional assays (calcium mobilization assays for human
mGlu4 and thallium flux assays for rat mGlu4) and characterized
with EC50, the maximum response and the fold shift values.16 It
is known that the EC50 value may not always correlated closely
to the affinity value for PAM.30 It is very important to study the
binding affinity for developing PET ligands. Thus, we prepared
the tritium-labeled compound 2 ([3H]2, N-(4-chloro-3-(methoxy-
t3)phenyl)picolinamide) for competitive binding assay.31 The syn-
thesized compounds were characterized with competitive binding
studies using mGlu4 transfected CHO cells by increasing the con-
On the basis of the affinity of these picolinamide derivatives, we
subsequently determined the in vitro microsomal stability of the
selected compounds that include 2–4 and 23–24 (Table 2).
Compound 4 (ADX88178) is one of a most potent mGlu4 PAM to
date and was shown to be orally active in a number of preclinical
in vivo PD models.14,22 As Table 2 shows, the dideuteriumfluo-
romethoxy-compound 24 (T1/2 = 7.4 min) is more stable than the
corresponding fluoromethoxy-analog 23 (T1/2 = 5.8 min) and the
methoxy-analog 2 (T1/2 = 4.9 min). It was reported that the cleav-
ing rate of the C–H bond was about 6.7 times faster than that of
C–D bond at 25 °C.34 On the other hand, the half time and the dif-
ference of the metabolic rates of the dideuteriumfluoromethoxy
analog and the fluoromethoxy analog depended on the level of
the enzyme. In developing the PET ligand for PBR, Zhang et al.
found that the half time (T1/2) in the plasma was 2.575 min for
the deuterium-substituted analog (with 18FD2CO) and 2.367 min
for the non-deuterated analog. However, the half time (T1/2) of
the deuterium-substituted analog in the brain was >60 min,
whereas that of for the non-deuterated analog was only
2.227 min.34 We anticipate that the difference of the half times
in the brain between compounds 24 and 23 as well as 2 could be
more significant. Compared to 4, compound 24 has the same affin-
ity and a similar in vitro microsomal stability. It is clear that com-
pound 24 has both enhanced affinity and improved in vitro
microsomal stability compared to 2.
centration of test materials from 0.01 nM to 10 lM in presence
of 2 nM of [3H]2, in which the binding affinities to mGlu4 were
described as IC50 values (Table 1).32
In structure–affinity study, we first evaluated the substitutions
at the 4-phenyl position by keeping the 3-methoxy group constant.
The 4-phenyl position of N-phenylpicolinamide was tolerated with
some substitutions as demonstrated in known compounds 6–8, in
which compounds 6 and 7 were reported very potent but poor
brain penetration.20 Thus we limited the 4-phenyl substitutions
for different halogens. The results show that the 4-chloro substitu-
tion give the best affinity, in which the affinity values of 2 and 10–
13 are in the following order: Cl < H < F < I < Br. Larger halogen
substitutions such as iodine and bromine led to substantial loss
in affinity. It was then found that the 3-methylthio group was
superior to the 3-methoxy group by comparing compounds 13
and 14, showing a 2.8 fold enhancement in affinity.
On the other hand, compounds 15–17 had been incorporated a
fluorine atom at 6-pyrindyl position of N-phenylpicolinamide,
which can have a relatively facile fluorine-18 labeling. Compared
to 2 and 3, the affinity of 15 and 16 was not significantly reduced.
Next we turned our attention to the 3-phenyl position. It is con-
sidered that the metabolic stability was one of major issues for ML-
128 (2), in which the 3-methoxy group was identified as the soft
group. The 3-phenyl position was also very sensitive with substitu-
tions. It was reported a simple change of 3-difluoromethoxy in
compound 3 to 3-trifluoromethoxy group imparted a more than
The selectivity of compound 24 was also determined among the
various mGlu subtypes, in which the functional assays were carried

Z. Zhang et al. / Bioorg. Med. Chem. Lett. 25 (2015) 3956–3960
3959
Table 1
SAR of N-phenylpicolinamide derivatives
Compd
R1
H
R2
MW
ClogP
tPSA
Affinity IC50 (nM)
5.1
Log(IC50 SE)
2
262.69
2.80
50.69
À8.29 0.09
À8.34 0.08
À7.50 0.09
3
H
H
264.23
246.24
2.67
2.26
50.69
50.69
4.6
10
31.6
11
H
307.15
2.95
50.69
322
À6.49 0.08
12
13
14
H
H
H
354.15
228.25
244.31
3.16
2.22
2.81
50.69
50.69
41.46
146
13.7
4.9
À6.84 0.10
À7.86 0.10
À8.31 0.07
15
16
17
18
19
F
280.68
282.22
264.23
223.23
241.23
2.96
2.85
2.42
1.89
2.06
50.69
50.69
50.69
65.25
65.25
7.3
6.7
À8.13 0.11
À8.18 0.10
À7.05 0.10
À7.98 0.05
À8.13 0.04
F
F
89.2
10.4
7.4
H
H
20
21
H
H
257.68
302.13
2.50
2.78
65.25
65.25
5.3
47
À8.28 0.04
À7.33 0.08
22
23
24
H
H
H
349.13
280.68
282.70
3.04
2.97
2.97
65.25
50.69
50.69
172
3.2
3.2
À6.77 0.08
À8.49 0.07
À8.49 0.11
25
H
248.25
2.39
50.69
3.7
À8.43 0.03
Table 2
In vitro properties of the selected compounds
out on mGlu1, mGlu2, mGlu5, mGlu6 and mGlu8. Compound 24
showed little activity against these mGlu (Supporting information).
In addition, the permeability values of 2–4 and 23 were mea-
sured using BBB PAMPA model at pH 7.4, which characterized
the rate across the BBB due to passive diffusion. The determined
effective permeability (Pe) values are summarized in Table 2, in
which the Pe results for internal highly and low permeable stan-
dards are 160 for propranolol and <2.8 for atenolol, respectively.
This result indicates that compounds 23 and 24 have good BBB per-
meability. Although high BBB passive permeability does not
Compd Affinity IC50 (nM) ja
(n = 3)
SEM(
(n = 2)
j)
T1/2
(min)
Avg. Pe
(10À6 cm/s)
2
3
4
23
24
5.1
4.6
3.2
3.2
3.2
0.141 0.010
0.132 0.008
0.099 0.005
0.120 0.010
0.093 0.005
4.9
5.2
7.0
5.8
7.4
256
214
257
272
a
The decay constant that is slope of logconcentration versus time profile
(T1/2 = Ln2/ ).
j

3960
Z. Zhang et al. / Bioorg. Med. Chem. Lett. 25 (2015) 3956–3960
8. Fettagutti, F.; Balani-Guerra, B.; Corsi, M.; Nakanishi, S.; Corti, C. Eur. J. Neurosci.
1999, 11, 2073.
9. Niswender, C. M.; Conn, P. J. Annu. Rev. Pharmacol. Toxicol. 2010, 50, 295.
10. Corti, C.; Aldegheri, L.; Somogyi, P.; Ferraguti, F. Neuroscience 2002, 110, 403.
11. Valenti, O.; Mannaioni, G.; Seabrook, G. R.; Conn, P. J.; Marino, M. J. J. Pharmcol.
Exp. Ther. 2005, 313, 1296.
12. Goudet, C.; Vilar, B.; Courtiol, T.; Deltheil, T.; Bessiron, T.; Brabet, I.; Oueslati,
N.; Rigault, D.; Bertrand, H.-O.; McLean, H.; Daniel, H.; Amalric, M.; Acher, F.;
Pin, J.-P. FASEB J. 2012, 26, 1682.
13. Cajina, M.; Nattini, M.; Song, D.; Smagin, G.; Joergensen, E. B.; Chandrasena, G.;
Bundgaard, C.; Toft, D. B.; Huang, X.; Acher, F.; Doller, D. ACS Med. Chem. Lett.
2014, 5, 119.
14. Lindsley, C. W.; Hopkins, C. R. Expert Opin. Ther. Pat. 2012, 22, 461.
15. Zhang, Z.; Brownell, A.-L. In Neuroimaging – Clinical Applications Bright; InTech
– Open Access Publisher: Rijeka, Croatia, 2012; p 499.
16. Engers, D. W.; Niswender, C. M.; Weaver, C. D.; Jadhav, S.; Menon, U. N.;
Zamorano, R.; Conn, P. J.; Lindsley, C. W.; Hopkins, C. R. J. Med. Chem. 2009, 52,
4115.
17. Jones, C. K.; Engers, D. W.; Thompson, A. D.; Field, J. R.; Blobaum, A. L.; Lindsley,
S. R.; Zhou, Y.; Gogliotti, R. D.; Jadhav, S.; Zamorano, R.; Bogenpohl, J.; Smith, Y.;
Morrison, R.; Daniels, J. S.; Weaver, C. D.; Conn, P. J.; Lindsley, C. W.; Niswender,
C. M.; Hopkins, C. R. J. Med. Chem. 2011, 54, 7639.
18. Kalinichev, M.; Le Poul, E.; Bolea, C.; Girard, F.; Campo, B.; Fonsi, M.; Royer-
Urios, I.; Browne, S. E.; Uslaner, J. M.; Davis, M. J.; Raber, J.; Duvoisin, R.; Bate, S.
T.; Reynolds, I. J.; Poli, S.; Celanire, S. J. Pharmacol. Exp. Therap. 2014, 350, 495.
11.
necessary translate to sufficient unbound drug concentration in the
brain because of potential intrinsic clearance and efflux transport,
it is beneficial for CNS drug candidates.
In summary, we have modified, synthesized and evaluated a
series of new N-phenylpicolinamide derivatives. Our research fur-
ther demonstrated that N-phenylpicolinamide is a good template
to develop mGlu4 ligands, which has offered extensive SAR results
by us and other labs.16,26 The SAR study has discovered a number of
compounds with good affinities (<10 nM) to mGlu4. The dideuteri-
umfluoromethoxy modified compound 24 is identified as a very
promising mGlu4 ligand, which has demonstrated enhanced affin-
ity, improved in vitro microsomal stability, good selectivity and
good permeability. Compound 24 is considered as an attractive
candidate for future labeling with fluorine-18 as an mGlu4 PET tra-
cer. Since a number of compounds have good affinity we are study-
ing their PAM activity to mGlu4 and potential therapeutic
applications.
Acknowledgments
19. Hong, S.-P.; Liu, K. G.; Ma, G.; Sabio, M.; Uberti, M. A.; Bacolod, M. D.; Peterson,
J.; Zou, Z. Z.; Robichaud, A. J.; Doller, D. J. Med. Chem. 2011, 54, 5070.
20. Lindsley, C. W.; Niswender, C. M.; Engers, D. W.; Hopkins, C. R. Curr. Top. Med.
Chem. 2009, 9, 949.
21. Valenti, O.; Marino, M. J.; Wittmann, M.; Lis, E.; DiLella, A. G.; Kinney, G. G.;
Conn, P. J. J. Neurosci. 2003, 23, 7218.
22. Le Poul, E.; Bolea, C.; Girard, F.; Poli, S.; Charvin, D.; Campo, B.; Bortoli, J.; Bessif,
A.; Luo, B.; Koser, A. J.; Hodge, L. M.; Smith, K. M.; DiLella, A. G.; Liverton, N.;
Hess, F.; Browne, S. E.; Reynolds, I. J. J. Pharmacol. Exp. Ther. 2012, 343, 167.
23. Bennouar, K.-E.; Uberti, M. A.; Melon, C.; Bacolod, M. D.; Jimenez, H. N.; Cajina,
M.; Kerkerian-Le Goff, L.; Doller, D.; Gubellini, P. Neuropharmacology 2013, 66,
158.
24. Iderberg, H.; Maslava, N.; Thompson, A. D.; Bubser, M.; Niswender, C. M.;
Hopkins, C. R.; Lindsley, C. W.; Conn, P. J.; Jones, C. K.; Cenci, M. A.
Neuropharmacology 2015. Ahead of Print.
Funding was provided by NIBIB-R01EB012864 and NIMH-
R01MH91684 to A.-L.B. Authors would like to acknowledge
Supporting Grants for the instrumentation 1S10RR029495-01,
1S10RR026666-01, and 1S10RR023452-01. The mGlu functional
data was generously provided by the National Institute of Mental
Health’s Psychoactive Drug Screening Program, Contract # HHSN-
271-2013-00017-C (NIMH PDSP). The NIMH PDSP is Directed
by Bryan L. Roth MD, PhD at the University of North Carolina at
Chapel Hill and Project Officer Jamie Driscoll at NIMH, Bethesda
MD, USA. Financial support for PP from The Orion Farmos
Research Foundation, Kuopio University Foundation, and Sigrid
Juselius Foundation, Finland is gratefully acknowledged.
25. Kil, K.-E.; Zhang, Z.; Jokivarsi, K.; Gong, C.; Choi, J.-K.; Kura, S.; Brownell, A.-L.
Bioorg. Med. Chem. 2013, 21, 5955.
26. Bolea, C. Preparation of amido derivatives and their use as positive allosteric
modulators of metabotropic glutamate receptors. 2008-EP59043 2009010454,
20080710, 2009.
Supplementary data
27. Patel, S.; Hamill, T.; Connolly, B.; Jagoda, E.; Li, W.; Gibson, R. Nucl. Med. Biol.
2007, 34, 1009.
28. Wang, J.; Tueckmantel, W.; Pellegrino, D.; Brownell, A.-L. Synapse 2007, 61,
951.
29. Zhang, L.; Villalobos, A.; Beck, E. M.; Bocan, T.; Chappie, T. A.; Chen, L.;
Grimwood, S.; Heck, S. D.; Helal, C. J.; Hou, X.; Humphrey, J. M.; Lu, J.; Skaddan,
M. B.; McCarthy, T. J.; Verhoest, P. R.; Wager, T. T.; Zasadny, K. J. Med. Chem.
2013, 56, 4568.
Supplementary data (experimental procedures and spectro-
scopic characterization of all new compounds) associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.bmcl.2015.07.031.
References and notes
30. Kenakin, T.; Onaran, O. Trends Pharmacol. Sci. 2002, 23, 275.
31. Kil, K.-E.; Poutiainen, P.; Zhang, Z.; Zhu, A.; Choi, J.-K.; Jokivarsi, K.; Brownell,
A.-L. J. Med. Chem. 2014, 57, 9130.
32. Poutiainen, P.; Kil, K.-E.; Zhang, Z.; Kuruppu, D.; Tannous, B.; Brownell, A.-L.
Neuropharmacology 2015.
33. Miller, P. W.; Long, N. J.; Vilar, R.; Gee, A. D. Angew. Chem., Int. Ed. 2008, 47,
8998.
34. Zhang, M.-R.; Maeda, J.; Ito, T.; Okauchi, T.; Ogawa, M.; Noguchi, J.; Suhara, T.;
Halldin, C.; Suzuki, K. Bioorg. Med. Chem. 2005, 13, 1811.
1. Conn, P. J. Ann. N. Y. Acad. Sci. 2003, 1003, 12.
2. Watkins, J. C.; Jane, D. E. Br. J. Pharmacol. 2006, 147, S100.
3. Hampson, D. R.; Rose, E. M.; Antflick, J. E. In The Glutamate Receptors; Gereau, R.
W., Swanson, G. T., Eds.; Human Press: Totowa, 2008; p 363.
4. Conn, P. J.; Pin, J. P. Annu. Rev. Pharmacol. Toxicol. 1997, 37, 205.
5. Riedel, G.; Platt, B.; Micheau, J. Behav. Brain Res. 2003, 140, 1.
6. Robichaud, A. J.; Engers, D. W.; Lindsley, C. W.; Hopkins, C. R. ACS Chem.
Neurosci. 2011, 17, 433.
7. Amalric, M.; Lopez, S.; Goudet, C.; Fisone, G.; Battaglia, G.; Nicoletti, F.; Pin, J. P.;
Acher, F. C. Neuropharmacology 2013, 66, 53.