Lead optimization of the VU0486321 series of mGlu1 PAMs. Part 1: SAR of modifications to the central aryl core

Article abstract of DOI:10.1016/j.bmcl.2015.10.013

This Letter describes the lead optimization of the VU0486321 series of mGlu₁ positive allosteric modulators (PAMs). While first generation PAMs from Roche were reported in the late 1990s, little effort has focused on the development of mGlu₁ PAMs since. New genetic data linking loss-of-function mutant mGlu₁ receptors to schizophrenia, bipolar disorder and other neuropsychiatric disorders has rekindled interest in the target, but the ideal in vivo probe, for example, with good PK, brain penetration and low plasma protein binding, for robust target validation has been lacking. Here we describe the first modifications to the central aryl core of the VU0486321 series, where robust SAR was noted. Moreover, structural variants were identified that imparted selectivity (up to >793-fold) versus mGlu₄.

Full text of DOI:10.1016/j.bmcl.2015.10.013

Bioorganic & Medicinal Chemistry Letters 25 (2015) 5107–5110
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Lead optimization of the VU0486321 series of mGlu1 PAMs. Part 1:
SAR of modifications to the central aryl core
Pedro M. Garcia-Barrantes a, Hyekyung P. Cho a,b, Anna L. Blobaum a, Colleen M. Niswender a,b
,
P. Jeffrey Conn a,b, Craig W. Lindsley a,b,c,
⇑
a Vanderbilt Center for Neuroscience Drug Discovery, Vanderbilt University Medical Center, Nashville, TN 37232, USA
b Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA
c Department of Chemistry, Vanderbilt University, Nashville, TN 37232, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
This Letter describes the lead optimization of the VU0486321 series of mGlu1 positive allosteric modula-
tors (PAMs). While first generation PAMs from Roche were reported in the late 1990s, little effort has
focused on the development of mGlu1 PAMs since. New genetic data linking loss-of-function mutant
mGlu1 receptors to schizophrenia, bipolar disorder and other neuropsychiatric disorders has rekindled
interest in the target, but the ideal in vivo probe, for example, with good PK, brain penetration and
low plasma protein binding, for robust target validation has been lacking. Here we describe the first
modifications to the central aryl core of the VU0486321 series, where robust SAR was noted.
Moreover, structural variants were identified that imparted selectivity (up to >793-fold) versus mGlu4.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 24 August 2015
Revised 30 September 2015
Accepted 5 October 2015
Available online 9 October 2015
Keywords:
mGlu1
Metabotropic glutamate receptor
Positive allosteric modulator (PAM)
Schizophrenia
Structure–activity relationship (SAR)
Recently, deleterious non-synonymous single nucleotide poly-
morphisms (nsSNPS) in the GRM1 gene, which encodes metabotro-
pic glutamate receptor subtype 1 (mGlu1), were discovered that
correlated with a higher incidence of neuropsychiatric disease,
including schizophrenia and bipolar disorder.1,2 Our lab then
demonstrated that signalling downstream of these loss-of-function
mutant receptors could be partially rescued with mGlu1 PAMs;3
however, neither the first generation Roche mGlu1 PAM (1, Ro
07-11401)4 nor our chemically distinct series (2, VU0483605),3
derived from an mGlu4 PAM scaffold (3, VU0400195, ML182)5 via
‘molecular switches’6 (Fig. 1), had the requisite PK, CNS penetration
and free fraction to serve as the ideal in vivo tool compounds for
robust target validation and potential adverse effect liability
assessment.3,7 Subsequent SAR studies in our lab identified key
substituents on the phthalimide moiety that engendered improved
plasma stability, along with a critical 3-methyl furyl amide that
provided 4 (VU0486321), a potent mGlu1 PAM with moderate rat
PK (CLp = 13.3 mL/min/kg, t1/2 = 54 min), good free fraction (human
Fu = 0.05, rat Fu = 0.03) and excellent CNS penetration (Kp = 1.02).7
Despite this notable advance, we sought an in vivo proof-of-con-
cept tool compound with a longer half-life and ideally greater
selectivity versus mGlu4 (4 is 35-fold selective). In this Letter, we
describe, for the first time, SAR for modifications to the central aryl
ring of 4, and the impact on mGlu1 and mGlu4 PAM activity with
analogs 5.
In order to access analogs 5 and survey the SAR for the two
regions depicted in blue in Figure 1, a general five step synthetic
route was developed. As shown in Scheme 1, commercial, function-
alized p-amino nitroarenes/heteroarenes 6 were bis-Boc protected
to provide 7, followed by reduction of the nitro moiety to afford 8.
Analogs 8 were then acylated with 3-methylfuran-2-carbonyl chlo-
ride to provide congeners 9. Finally, acid-mediated deprotection
liberated the free aniline which was then condensed with various
phthalic anhydrides to afford final analogs 5. The 3-methylfuranyl
amide was held constant in this campaign, as it was the lone,
standout amide from our earlier efforts,7 where SAR proved steep.
The first core modification examined focused on an aryl moiety
substituted with H, Me, Cl, CF3 and OMe and F in the 2-position,
relative to the phthalimide, analogs of 5 indicated as 10 (Table 1).
All of the analogs 10 were potent mGlu1 PAMs with EC50s less than
500 nM, and most under 100 nM, with excellent efficacy (% Glu
Max >90), a welcomed departure from the steep SAR that plagued
the discovery of 4.7 Also, all compounds showed considerable ago-
nism at concentrations above 10 lM, a feature previously observed
with parent compound 4, with the exception of the unsubstituted
phenyl ring and 2-trifluoromethyl analogs which had a pure PAM
profile. Interestingly, SAR for mGlu4 varied greatly with respect
⇑
Corresponding author.
E-mail address: craig.lindsley@vanderbilt.edu (C.W. Lindsley).
http://dx.doi.org/10.1016/j.bmcl.2015.10.013
0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.

5108
P. M. Garcia-Barrantes et al. / Bioorg. Med. Chem. Lett. 25 (2015) 5107–5110
Cl
O
O
Cl
N
N
O
O
O
H
N
O
N
O
N
N
N
H
N
H
Cl
F3C
O
O
Cl
2, VU0483605
1, Ro 07-11401
3, VU0400195 (ML182)
F
R2
O
iterative
parallel
O
R1
O
Cl
O
O
O
N
NH
N
het
NH
synthesis
O
O
4, VU0486321
5
Figure 1. Structures of reported mGlu1 PAMs 1–4, and the novel targeted analogs 5, to be accessed through iterative parallel synthesis.
Table 1
R1
R1
R1
a
b
Structures and activities for analogs 10
H2N
het
NO2
(Boc)2N
het
NO2
(Boc)2N
NH2
het
R1
R2
O
O
O
6
7
8
N
NH
O
R2
O
R1
R1
het
O
O
O
O
O
c
d
10
het
(Boc)2N
NH
N
NH
Compd R1
R2
hmGlu1 EC50
M)a [% Glu
Max SEM]
mGlu1
pEC50
( SEM)
hmGlu4
EC50
(lM) [%
Fold
(l
selective
versus
mGlu4
9
5
Glu
Maxb]
Scheme 1. Reagents and conditions: (a) Boc2O, DMAP, THF, rt, 59–97%; (b) H2, Pd/C,
EtOH, rt, 94–99%; (c) 3-methylfuran-2-carbonyl chloride, DIEA, DCM, rt, 39–98%;
(d) (i) TFA, DCM, rt (ii) phthalic anhydrides, AcOH, reflux, 53–94%.
10a
10b
10c
H
Me
Cl
H
H
H
0.048 [82 6] 7.32 0.13 >10 [—]
0.016 [108 6] 7.79 0.09 0.35 [62] 21.7
0.029 [101 14] 7.54 0.10 0.483
>208
16.7
[35]
to mGlu4 PAM potency (EC50s from 260 nM to >10 lM) and efficacy
10d
10e
10f
10g
10h
10i
F
H
F
0.044 [96 4] 7.36 0.05 0.382
8.6
(% Glu Max from 35 to 122). While lack of activity at mGlu4 is key
for an mGlu1 PAM in vivo probe, a dual mGlu1/mGlu4 PAM is
intriguing in an antipsychotic agent, as mGlu4 PAM activity can
alleviate catalepsy, a major adverse event with the standard treat-
ment of care.8 PAM 10a, with both an un-functionalized central
phenyl core and unsubstituted phthalimide moiety, proved to be
not only potent (mGlu1 EC50 = 48 nM), but also >208-fold selective
[34]
H
Me
Cl
F
0.022 [101 4] 7.65 0.14 0.265
11.9
11.0
9.5
[65]
F
0.026 [107 3] 7.58 0.10 0.287
[102]
F
0.028 [100 5] 7.56 0.10 0.264
[93]
F
0.035 [104 6] 7.46 0.12 0.674
19.3
9.9
[78]
versus mGlu4 (mGlu4 EC50 >10 lM). All other modifications to
H
Me
0.089 [105 7] 7.05 0.10 0.885
either the phenyl core or the phthalimide moiety engendered more
significant activity at mGlu4, but mGlu1 was still preferred. Relative
to 10a, the addition of substituents at either R1 or R2 provided
potent mGlu1 PAMs, but the SAR possessed little discernable tex-
ture with electron-donating, electron-withdrawing and/or more
lipophilic moieties proving effective. One notable trend was that
2-CF3 on the phenyl core was less effective, affording a diminution
in mGlu1 PAM potency relative to 10a, providing analogs such as
10o, with an mGlu1 EC50 of 530 nM.
[75]
0.021 [100 7] 7.67 0.13 0.422
[122]
0.049 [117 10] 7.31 0.20 1.01
[127]
10j
Me Me
19.8
20.5
10k
Cl
Me
10m
10n
10o
F
H
Me
CF3
0.049 [95 3] 7.31 0.12 1.10 [86] 22.4
0.123 [108 6] 6.91 0.11 6.66 [86] 54.1
Me CF3
0.530 [117 11] 6.27 0.16 5.76
10.8
[109]
0.106 [105 4] 6.98 0.08 2.47
[120]
10p
Cl
CF3
23.3
The lack of SAR texture with analogs 10 then led us to explore
heterocyclic and bicyclic congeners of 5, indicated as analogs 11
(Table 2), wherein the phenyl core was replaced with either a pyr-
idine, pyrimidine or naphthalene, while holding the amide con-
stant and surveying substituents on the phthalimide. Here, the
SAR possessed texture, with 2-pyridyl analogs 11a–d maintaining
good mGlu1 PAM potency (EC50s 35 nM to 387 nM) and a range
of selectivity versus mGlu4 (ꢀ18- to 52-fold). Installation of a 2-
pyrimidinyl core, analogs 11e–h, lost significant activity at mGlu1
10q
10r
10s
F
H
CF3
OMe
0.155 [110 8] 6.81 0.11 2.87 [80] 18.5
0.046 [96 3] 7.33 0.08 1.18 [50] 25.6
Me OMe
0.049 [94 6] 7.31 0.15 0.882
18
[107]
0.018 [93 3] 7.74 0.14 0.647
[103]
10t
Cl
F
OMe
OMe
36.1
10u
0.144 [103 6] 6.84 0.10 1.93 [73] 13.4
a
Calcium mobilization mGlu1 assays, values are average of three (n = 3) inde-
pendent experiments performed in triplicate.
Glu Max is expressed as % of PHCCC response, which is used as a normalization
control as the % max values vary on a daily basis for the mGlu4 assay.
b
(EC50s 200 nM to >5
lM), and selectivity versus mGlu4 was eroded.
Indeed, both 11e and 11h, were equipotent mGlu1 and mGlu4

P. M. Garcia-Barrantes et al. / Bioorg. Med. Chem. Lett. 25 (2015) 5107–5110
5109
Table 2
Structures and activities for analogs 11
R1
O
O
O
N
het
NH
O
11
Compd
11a
R1
H
Het/bicycle
hmGlu1 EC50
(
l
M)a [% Glu Max SEM]
mGlu1 pEC50 ( SEM)
6.41 0.11
hmGlu4 EC50
>10 [—]
(l
M) [% Glu Maxb]
Fold Selective versus mGlu4
N
0.387 [80 4]
0.095 [105 6]
0.035 [91 8]
0.198 [93 3]
>25.8
52.7
31.9
17.9
N
N
N
N
11b
Me
Cl
F
7.02 0.12
5.00 [100]
1.113 [44]
3.55 [19]
11c
7.46 0.09
11d
6.70 0.08
11e
11f
H
4.61 [89 4]
0.751 [111 5]
0.208 [84 5]
5.33 [99 6]
5.33 0.22
6.12 0.22
6.68 0.09
5.27 0.19
4.67 [23]
4.47 [104]
2.53 [45]
>10 [24]
1.01
N
N
Me
Cl
F
5.95
N
N
11g
11h
12.2
N
N
>1.88
17.4
N
11i
H
0.020 [99 8]
0.016 [88 6]
0.023 [92 8]
0.020 [90 3]
7.69 0.14
7.80 0.10
7.65 0.13
7.71 0.09
0.353 [54]
0.429 [103]
0.513 [102]
0.809 [83]
11j
Me
Cl
F
27.3
22.8
41.1
11k
11m
a
Calcium mobilization mGlu1 assays, values are average of three (n = 3) independent experiments performed in triplicate.
Glu Max is expressed as % of PHCCC response.
b
PAMs. Replacement of the phenyl core with a naphthyl bicyclic
ring system, for example, 11i–m, afforded potent mGlu1 PAMs
(EC50s 16 nM to 23 nM), irrespective of the phthalimide sub-
stituent, with modest selectivity versus mGlu4 (17.4- to 41-fold).
Thus, from these two efforts, while mGlu1 potency and efficacy
were attractive, the overall profile and selectivity versus mGlu4
were not appropriate for an in vivo proof of concept tool com-
pound. However, the overall SAR developed with analog libraries
10 and 11 was insightful and overcame the steep SAR previously
encountered for this series.
Based on these data, we elected to survey the impact of regioi-
somers of 10 and 11, a region of chemical space not previously
explored, wherein the substituents were in the 3-position with
respect to the phthalimide, for example, analogs 12. Analogs 12
were accessed as shown in Scheme 1, and these analogs afforded
the greatest selectivity versus mGlu4 to date (>793 fold), despite
possessing an unsubstituted phthalimide moiety. Here, the fluoro
analog, 12a, was a potent mGlu1 PAM (EC50 = 12.6 nM, 84% Glu
as no activity at mGlu5. Interestingly, the CF3 analog 12d was inac-
tive, as were the regioisomeric pyridine and pyrimidine analogs
12f and 12g, further highlighting the non-obviousness and chal-
lenges in allosteric modulator SAR (Table 3).
From these efforts, 12a and 12b emerged as mGlu1 PAMs with
the requisite potency and selectivity versus mGlu4 to serve as
in vivo proof-of-concept tools; thus, they were profiled in a battery
of in vitro and in vivo Drug metabolism and pharmacokinetic
(DMPK) assays9 to further assess their potential. In terms of
in vitro predicted hepatic clearance measures, both 12a and 12b
display moderate hepatic clearance in both human (CLHEP = 9.65
mL/min/kg and 9.25 mL/min/kg, respectively) and rat
(CLHEP = 48.5 mL/min/kg and 48.7 mL/min/kg, respectively) hepatic
microsomes. Unfortunately, both compounds suffered from high
plasma protein binding (Fu <1% in human and rat) as well as high
rat brain homogenate binding (Fu <1%). CNS penetration varied sig-
nificantly between the two PAMs. In our standard rat plasma:brain
level (PBL) cassette study (0.2 mg/kg, 10%EtOH:40% PEG 400:50%
DMSO, 15 min time point), 12a displayed a Kp of 1.57, while 12b,
the Cl-congener, showed very poor CNS penetration (Kp = 0.12).
While CNS exposure of 12a was attractive, the low free fraction
precluded it from further consideration or advancement as an
in vivo mGlu1 PAM tool compound.
Max) with >793-fold selectivity versus mGlu4 (EC50 >10 lM), and
12b, the chloro congener, was similarly potent (mGlu1 PAM
EC50 = 29.1 nM, 68% Glu Max) with >344-fold selectivity versus
mGlu4 (EC50 >10
lM). Also, these compounds act like pure PAMs
and did not show any observable agonistic effect in mGlu1 as well

5110
P. M. Garcia-Barrantes et al. / Bioorg. Med. Chem. Lett. 25 (2015) 5107–5110
Table 3
(>344-fold selective vs mGlu4). DMPK properties hindered these
Structures and activities for analogs 12
analogs’ utility as in vivo probes, but lessons learned will be
applied to other scaffolds, and these results will be reported in
due course.
O
N
O
O
O
het
NH
R1
Acknowledgments
12
We thank William K. Warren, Jr. and the William K. Warren
Foundation who funded the William K. Warren, Jr. Chair in
Medicine (to C.W.L.). P.M.G. would like to acknowledge the VISP
program for its support. This work was funded by the William K.
Warren, Jr. Chair in Medicine and the NIH (U54MH084659).
Compd R1 or Het/
hmGlu1 EC50
M)a [% Glu
mGlu1
pEC50
( SEM)
hmGlu4
EC50 M)
Fold
(
l
(
l
Selective
versus
mGlu4
Max SEM]
[% Glu
Maxb]
12a
12b
12c
F
0.013
[84 2]
0.029
[68 5]
0.015
[90 2]
>10 [—]
0.330
7.90 0.08 >10 [—]
7.54 0.09 >10 [—]
>793
>344
References and notes
Cl
Me
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2011, 6, e19011.
7.82 0.09 0.609 [52] 9.8
12d
12e
CF3
OMe
>5.00
>10 [—]
—
>30.3
6.48 0.19 >10 [—]
[80 6]
2. Ayoub, M. A.; Angelicheva, D.; Vile, D.; Chandler, D.; Morar, B.; Cavanaugh, J. A.;
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12f
>10 [—]
>10 [—]
>5.00
>5.00
>10 [—]
>10 [—]
—
—
N
N
12g
N
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a
Calcium mobilization mGlu1 assays, values are average of three (n = 3) inde-
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b
Glu Max is expressed as % of PHCCC response.
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In conclusion, our first exploration of central core SAR of the
VU0486321 series of mGlu1 positive allosteric modulators (PAMs)
provided a combination of both steep and tractable SAR, unex-
pected findings and new avenues for optimization. Interestingly,
analogs where the halogen on the central phenyl core was located
adjacent to the 3-furyl amide eliminated activity at mGlu4, provid-
ing analogs such as 12a (>793-fold selective vs mGlu4) and 12b
7. Garcia-Barrantes, P. M.; Cho, H. P.; Niswender, C. M.; Byers, F. W.; Locuson, C.
W.; Blobaum, A. L.; Xiang, Z.; Rook, J. M.; Conn, P. J.; Lindsley, C. W. J. Med. Chem.
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