Pyrido[4,3- e ][1,2,4]triazolo[4,3- a ]pyrazines as selective, brain penetrant phosphodiesterase 2 (PDE2) inhibitors

Article abstract of DOI:10.1021/ml500463t

A novel series of pyrido[4,3-e][1,2,4]triazolo[4,3-a]pyrazines is reported as potent PDE2/PDE10 inhibitors with drug-like properties. Selectivity for PDE2 was obtained by introducing a linear, lipophilic moiety on the meta-position of the phenyl ring pending from the triazole. The SAR and protein flexibility were explored with free energy perturbation calculations. Rat pharmacokinetic data and in vivo receptor occupancy data are given for two representative compounds 6 and 12.

Full text of DOI:10.1021/ml500463t

Letter
pubs.acs.org/acsmedchemlett
Pyrido[4,3‑e][1,2,4]triazolo[4,3‑a]pyrazines as Selective, Brain
Penetrant Phosphodiesterase 2 (PDE2) Inhibitors
Frederik J. R. Rombouts,*,† Gary Tresadern,‡ Peter Buijnsters,† Xavier Langlois,§ Fulgencio Tovar,∥
Thomas B. Steinbrecher,⊥ Greet Vanhoof,‡ Marijke Somers,‡ Jose-Ignacio Andres,#
́
́
and Andres A. Trabanco#
́
†Neuroscience-Medicinal Chemistry, Discovery Sciences, Neuroscience-Biology, Janssen Research & Development, Janssen
‡
§
Pharmaceutica N.V., Turnhoutseweg 30, B-2340 Beerse, Belgium
∥Villapharma Research S.L., Parque Tecnolog
Alamo de Murcia, Murcia, Spain
́
ico de Fuente Alamo. Ctra. El Estrecho-Lobosillo, Km. 2,5- Av. Azul, 30320 Fuente
́
́
⊥Schrodinger GmbH, Dynamostrasse 13, D-68165 Mannheim, Germany
#Neuroscience-Medicinal Chemistry, Janssen Research & Development, Janssen-Cilag S.A., C/Jarama 75, 45007 Toledo, Spain
̈
S
* Supporting Information
ABSTRACT: A novel series of pyrido[4,3-e][1,2,4]triazolo-
[4,3-a]pyrazines is reported as potent PDE2/PDE10 inhibitors
with drug-like properties. Selectivity for PDE2 was obtained by
introducing a linear, lipophilic moiety on the meta-position of
the phenyl ring pending from the triazole. The SAR and
protein flexibility were explored with free energy perturbation
calculations. Rat pharmacokinetic data and in vivo receptor
occupancy data are given for two representative compounds 6
and 12.
KEYWORDS: Pyrido[4,3-e][1,2,4]triazolo[4,3-a]pyrazine, PDE2 inhibitor, phosphodiesterase 2 inhibitor, tricycle, selective
Scheme 1. Optimization of HTS Hit 1 into Lead Compound
3
hosphodiesterases (PDEs) control the degradation of
Psecond messengers cAMP and cGMP. Eleven PDE
subfamilies with distinct tissue distribution and varying
selectivity for cAMP and cGMP have been identified.1 PDE2
is a dual-substrate enzyme, stimulated by cGMP and degrading
both cAMP and cGMP.2 It is the most abundant PDE subtype
in the hippocampus, a brain structure playing an important role
in cognitive processes.3 Cyclic nucleotides are key signaling
molecules implicated in the regulation of neuronal plasticity
and memory.4−6 PDE2 inhibition elevates levels of these
molecules and could exert procognitive activity and restore
hippocampal function, which are altered in disease states such
as schizophrenia or Alzheimer’s disease.7 However, the lack of
selective and brain penetrant PDE2 inhibitors has hampered
validation of this hypothesis in animal models. We have
previously reported the optimization of the poorly soluble HTS
hit 1 into molecule 2 with combined PDE2/PDE10 activity
(Scheme 1). This early lead 2 was valuable for studying CNS
effects of PDE2/PDE10 inhibition in vivo.8 More recently, we
have described optimization of 2 via structure-based drug
design to 3, a selective and brain penetrant PDE2 inhibitor.9
The poor solubility of 1 was addressed in 2 and 3 by
substituting the benzo-fused ring of the triazoloquinoxaline
core with basic aliphatic amines. While solubility was greatly
increased in 2 and 3, both compounds had poor metabolic
stability (Scheme 1). In parallel introduction of pyridyl-like
nitrogens in the benzene ring of the quinoxaline moiety was
a
b
Ten percent HPβCD, pH 2. Twenty percent HPβCD, pH 3.6−3.8.
c
Percent metabolized after incubating 1 μM solution 15 min with rLM
or hLM at 37 °C.
explored, as these could increase solubility while maintaining
high ligand efficiency (LE) and good overall physicochemical
and ADME properties.
Prior to synthesis, the four possible isomers bearing a
nitrogen atom in the quinoxaline benzene ring (5−8, Chart 1)
were docked into the PDE2 protein using the SP mode of
Received: November 8, 2014
Accepted: January 15, 2015
© XXXX American Chemical Society
A
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ACS Medicinal Chemistry Letters
Letter
20% HPβCD at pH 3.9), which can be attributed to the higher
solvent exposure and hydrogen bonding capability of the
pyridine-like nitrogen in 6. This is also reflected by their
respective experimental pKa values (5, pKa = 1.0; 6, pKa = 2.7).
Hence, 6 was selected for further optimization.
Chart 1. Triazoloquinoxaline 4 and Aza-Analogues 5-8
The in vitro selectivity data of 6 versus other PDE’s along
with in vitro ADME and physicochemical data are summarized
in Table 1. Molecule 6 inhibited PDE2 and PDE10,
Table 1. In Vitro PDE Selectivity Profile, in Vitro ADME, and
Physicochemical Properties of 6
Glide10 with default protein preparation. A docking protocol
with enhanced ligand sampling reproduced the crystal structure
binding mode of four different PDE2−inhibitor complexes with
good accuracy (see Supporting Information (SI)). Docking
helped prioritize 5, 6, and 8, but 7 was not targeted as it
presents an unfavorable interaction between the pyridyl
nitrogen and Met847 in PDE2. The proximity of this atom
to Met847 is revealed by the short 4.3 Å distance between the
closest heavy atoms of 9 and Met847 (Figure 1, vide infra). For
baseline comparison the [1,2,4]triazolo[4,3-a]quinoxaline 4
was also synthesized.
ADME and
physicochemical properties
PDE selectivity profile
subtype
IC50 (nM)
a
hPDE1A
hPDE2A
hPDE3B
>10000
29
hLM
7
a
rLM
13
>10000
kinetic solubility at
pH 7.4 (μM)
>1000
b
c
hPDE4D
hPDE5A
hPDE6AB
5890
pKa
2.7
4
d
>10000
>10000
formulatability (mg/mL)
CYP450
(>50% inhibition @ 5 μM)
none
e
f
hPDE7A
hPDE9A
rPDE10A
hPDE11A
>10000
>10000
480
AMESII
negative
6920
a
Percent metabolized after incubating 1 μM solution 15 min with rLM
b
or hLM at 37 °C. Compound concentration after 4 h incubation in
buffer pH 7.4. Determined by potentiometric titration (25 °C). 20%
HPβCD at pH > 3.5. Enzymatic inhibition of CYP1A2, 2C9, 2D6,
c
d
e
f
2C19, and 3A4. Adaptation of the colorimetric AMESII Mutagenicity
Assay kit from Xenometric. Compound concentration 125 μg/mL.
respectively, with an IC50 value of 29 and 480 nM. This is in
line with the previously reported values for compound 2. In
addition 6 did not show significant inhibition of a panel of
CYP450 enzymes (CYP1A2, 2C9, 2D6, 2C19, and 3A4). The
compound was also inactive up to a concentration of 125 μg/
mL in a bacterial mutagenicity assay (AMESII). The in vitro
metabolic stability of 6 was greatly improved both in human
(hLM) and rat (rLM) liver microsomes when compared to 2.
The PK properties of compound 6 were studied in rats after
2.5 mg/kg i.v. and 10 mg/kg p.o. administration (Table 2).
After i.v. administration, a rapid clearance was observed (t1/2
=
0.47 h), which was not expected based on the in vitro metabolic
stability in rLM (Table 1). Interestingly, 6 showed much slower
clearance after p.o. administration (t1/2 = 2.36 h), resulting in
good bioavailability and a maximum plasma concentration
(Cmax) of 997 ng/mL.
The good in vitro profile and acceptable PK properties of 6
prompted us to explore the possibility of increasing the
selectivity of this chemical series versus PDE10. Introducing a
lipophilic group to the meta-position of the phenyl on the
triazole ring can induce an open Leu770 protein conformation
allowing the ligand to access a hydrophobic roof pocket,
increasing selectivity versus PDE10.9 Hence a focused set of
analogues 9−18 bearing an additional substituent on the 2-
chlorophenyl group in 6 was synthesized, and their in vitro
potency and selectivity were determined (Table 3).
Analogues having a meta-substituted phenyl (9−15) or
pyridyl (18) on the pyrido[4,3-e][1,2,4]triazolo[4,3-a]pyrazine
core displayed increased PDE2 selectivity versus PDE10. For
instance, introduction of a n-butoxy group as in 9 resulted in a
Figure 1. Molecule 9 (cyan color) docked into PDE2A crystal
structure solved with 3 (PDB 4D08). Viewed from the entrance to the
active site (A) and top down (B). Important amino acids are
highlighted in green, and catalytic Zn2+ and Mg2+ ions are shown,
along with active site water molecules (red spheres). Distance between
9 and Met847 highlighted in orange.
Of the three isomers prepared, 5 (IC50 = 15 nM) and 6 (IC50
= 29 nM) showed comparable inhibitory potency for PDE2 to
the carbon analogue 4 (IC50 = 14 nM), whereas isomer 8 was
13-fold less potent (IC50 = 190 nM).11 Formulatability of 5
(<0.5 mg/mL with 20% HPβCD at pH 2) was similar to that of
the HTS hit 1; however, the regioisomeric 6 was significantly
more soluble (>1 mM in buffer at pH 7.4 and 4 mg/mL with
B
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ACS Medicinal Chemistry Letters
Letter
a
typical H-bond interaction with conserved Gln859. As seen in
the previously reported series,9 the triazolo ring forms two
water mediated H-bonds to Tyr655 and Gln812. The n-butoxy
group binds in the lipophilic pocket formed by hydrophobic
residues Leu770, Leu809, Ile866, and Ile870. The close
proximity of Met847 to the ligand at the site entrance can be
seen.
Table 2. Pharmacokinetic Properties of 6 in Rats
The binding mode of 9 helps to rationalize the observed
SAR. The PDE2 activity of 2-methoxyethoxy derivative 10 is
16-fold lower than for 9, suggesting that polar moieties in the
chain are not preferred in the lipophilic roof pocket. The
morpholine in 14 is also detrimental for activity likely due to
the polarity and suboptimal steric fit. Shifting the ether to the
benzylic position as in 11−13 is allowed, with 12 being the
most potent selective PDE2 inhibitor identified in this
exploration (PDE2 IC50 = 3 nM, PDE10 IC50 = 2450 nM).
Interestingly, the trifluoromethyl group in 15 results in a 5-fold
decrease in potency (IC50 = 150 nM) when compared to 6,
which may be due to an orthogonal orientation of this group.
Pyridyls with small substituents, 16 and 17, were significantly
lower in activity. Combination of the n-butoxy group in 9 with
the distal pyridyl reduced PDE2 activity: molecule 18 was 9-
fold less potent than 9. Of note is that, similar to 6, analogues
9−18 displayed high selectivity over other tested PDEs (see
SI).
mean i.v.
s.d.
Co (ng/mL)
t1/2 (h)
3021
0.47
388
0.08
0.55
0.10
6.5
Vdz (L/kg)
1.61
Vdss (L/kg)
1.02
Cl (mL/min/kg)
AUC0‑inf (ng·h/mL)
38.7
1042
184
s.d.
mean p.o.
Cmax (ng/mL)
tmax (h)
997
241
1.17
2.36
4229
100
0.76
0.46
1892
t1/2 (h)
This exploration and our previous work9 reveal an interesting
SAR originating from the phenyl/pyridyl group, its substitution,
orientation, and ability to access the Leu770 lipophilic roof
pocket. Induced binding and dynamic effects are not easily
studied with conventional docking approaches. Therefore, to
investigate these effects more sophisticated modeling based on
FEP+ free energy calculations were employed (see SI for details
and ΔG0 values).13,14 Seven compounds (6, 9, 10, 12, 15, 16,
and 17) were studied covering a range of activity and
substitution patterns while having sufficient similarity to permit
reliable structural perturbations between pairs of molecules.
The computational results had good agreement with
experimental values (R2 = 0.79, mean unsigned error 0.54
kcal/mol) suggesting the approach captures the molecular
details of the ligand enzyme interactions.
Compounds 9 and 12 were correctly predicted to be the
most potent inhibitors with approximately equivalent predicted
binding free energies. Compounds 16 and 17 were also
predicted to be nearly equipotent but with the least favorable
binding free energies. These two compounds were more mobile
during the simulations, indicating a less tight fit into the binding
site. Compounds 10 and 15 were predicted to have nearly
equipotent binding free energies and were correctly ranked
with intermediate level of potency. Hence, the more hydro-
philic linear side chain in the 5-position of 10 as well as the
smaller trifluoromethoxy group of 15 produce weaker inhibitors
compared to the more lipophilic chain of, e.g., 9. The binding
free energy for 6 deviated most from experiment being
underpredicted by ca. 2 kcal/mol. This is understandable as
the unsubstituted 2-chlorophenyl binds optimally to a different
PDE2 protein conformation compared to that used to start this
set of FEP calculations (see SI for further discussion).
Molecular dynamics (MD) calculations also shed further light
on the conformational flexibility of Leu770 depending on the
bound ligand structure. We found that on a simulation time
scale of 30 ns, ligands with a large, linear side chain in the
phenyl ring 5-position induce a more open form of the
lipophilic roof pocket, while for ligands with no 5-substituent
AUC0‑inf (ng·h/mL)
bioavailability (%)
a
Male Sprague−Dawley rat fed (n = 3 per time point); i.v., 2.5 mg/kg;
p.o., 10 mg/kg.
Table 3. In Vitro PDE2 and PDE10 Inhibition and Metabolic
Stability Data for Compounds 6 and 9−18
hPDE2 IC50
rPDE10 IC50
a
a
b
b
R
(nM)
(nM)
rLM
hLM
6
H
29
480
13
98
18
98
77
98
53
11
7
9
On-Bu
6
5500
9120
3890
2450
6030
>10000
8910
n.m.
34
5
10
11
12
13
14
15
16
17
18
O(CH2)2OMe
CH2On-Pr
CH2OCH2CF3
CH2Oi-Pr
morpholine
OCF3
95
21
78
12
57
14
3
3
10
310
150
5750
2450
54
5-Cl
n.m.
n.m.
46
n.m.
n.m.
6
4-Me
n.m.
5-On-Bu
3800
a
b
Average value of at least two independent experiments. Percent
metabolized after incubating 1 μM solution 15 min with rLM or hLM
at 37 °C.
very potent PDE2 inhibitor with 917-fold selectivity over
PDE10. Docking of 9 in the active site of PDE2 (Figure 1)
confirmed a similar binding mode as seen in the X-ray structure
of 3 (PDB 4D08)9 and also that of a similar tricyclic molecule
binding to PDE10 (PDB 3SNI).12 The molecule sits in the
hydrophobic clamp between Phe862 and Phe830 and forms the
C
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Letter
the crucial Leu770 dihedral angle exhibits higher flexibility (see
SI for details). Given these promising results we are further
studying the FEP+ approach for the design of PDE2 inhibitors.
Unfortunately, the most potent compounds 9, 12, and 13
were metabolically unstable when tested in vitro against hLM
and mainly rLM. An improvement in rat metabolic stability
profile was obtained for pyridyl analogue 18. Prediction of
human CYP3A4, 2D6, and 2C9 related metabolism using
StarDrop 515 and analysis of physicochemical properties as well
as structural elements associated with metabolism revealed a
nice correlation between predicted compound site liabilities
and in vitro metabolism with hLM. Especially aliphatic chains
pending from the distal phenyl (Table 3, R-group) were
predicted to introduce metabolic liabilities, with alkoxy-
methylene groups correctly being predicted inferior to alkoxy
groups (see SI for a comparison between 6, 9, 13, and 18). The
fact that increasing polarity (as in 18) or fluorination of the side
chain (as in 12 and 15) effectively improves hLM metabolism
in vitro supports these predictions.
Compounds 6, 9−12, and 18 were assessed for their
potential to cross the blood−brain barrier in rats after 10 mg/
kg s.c. administration. All tested compounds showed good
formulatability with 10 to 20% HPβCD at pH > 3.5. Brain
exposure for 9−12 and 18 was lower compared to 6;
nevertheless, the brain concentration for these compounds
after 1 h administration was in the range of 370−895 ng/g with
high brain free fractions and brain/plasma ratios (Table 4).
Figure 2. In vivo dose/PDE2 occupancy graphs for compounds 6 and
12 (rats, n = 3/dose).
Leu770 lipophilic roof pocket. Two representative compounds
from this chemical class have been evaluated in vivo. Compared
to our previous reports these molecules offer an alternative
tricyclic scaffold with improved metabolic stability in some
cases permitting target engagement by oral administration.
More specifically, compound 6, which is orally bioavailable,
occupied PDE2 with an ED50 of 21 mg/kg, a significant
improvement over previously reported compound 2. In
addition the highly selective PDE2 inhibitor 12 showed high
occupancy after s.c. administration with an ED50 of 3.6 mg/kg.
These compounds contribute valuable probes to further study
the role of PDE2 and PDE10 in vivo across a variety of CNS
disorders.
Table 4. Total Brain Level Comparison between 6, 9−12,
a b
,
and 18
ASSOCIATED CONTENT
* Supporting Information
■
cmpd
6
9
10
11
12
18
S
c
fu, brain (rat)
>0.30
2730
0.65
0.026
807
0.17
801
0.44
0.10
895
0.98
0.10
370
1.1
>0.30
789
Computational docking, FEP and Molecular Dynamics
calculation protocols, protocols for measuring inhibition of
PDEs in vitro, selectivity data for 9−15 and 18, in vivo
occupancy protocols and occupancy data for 18, CYP3A4
metabolism predictions for 6, 9, 13, and 18, and the
experimental details of the synthesis of key intermediates and
compounds 6 and 9−18. This material is available free of
charge via the Internet at http://pubs.acs.org.
brain 1 h (ng/g)
B/P 1 h
0.69
0.61
a
b
c
Male Sprague−Dawley rat fed (n = 2 per time point) 10 mg/kg s.c.
Compounds were formulated with 20% HPβCD at pH 3.5−4.
Estimated unbound drug fraction to rat brain homogenates after 5 h
incubation at 37 °C using a Rapid Equilibrium Dialysis device.
Target engagement of pyrido[4,3-e][1,2,4]triazolo[4,3-a]-
pyrazines 6, 12, and 18 was evaluated in an in vivo occupancy
study in rats using tritiated 3 as radioactive tracer (10 μCi i.v.).
In this experiment, MP-10 (2.5 mg/kg s.c.),16 a potent and
selective PDE10 inhibitor was predosed since it was found to
greatly increase the signal-to-noise ratio of the tracer. This can
be explained by the fact that PDE10 inhibition increases
intracellular cGMP, which is known to activate PDE2 through
its GAF domain.17 Using this protocol we could demonstrate
that the PDE2/PDE10 inhibitor 6 occupies PDE2 with an ED50
of 21 mg/kg (Figure 2) after p.o. administration. For the highly
selective PDE2 inhibitor 12 no occupancy was seen up to 10
mg/kg after p.o. dosing, most probably due to poor oral
bioavailability. However, when 12 was dosed s.c., high PDE2
occupancy was observed, with an ED50 of 3.6 mg/kg.
Interestingly compound 18, despite its good potency and
brain exposure, gave a poor occupancy of 23% at the highest
tested dose of 10 mg/kg s.c. (see SI).
AUTHOR INFORMATION
Corresponding Author
*Phone: +32-14608216. E-mail: frombout@its.jnj.com.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Ilse Lenaerts for performing the occupancy
experiments. We also thank Geert Pille for measuring the pKa
of 5 and 6. Finally, we thank Dr. Gerhard Gross for performing
and interpreting the StarDrop metabolic site predictions.
ABBREVIATIONS
In summary we have described the design, synthesis, and
pharmacological characterization of a novel series of selective
and brain penetrant PDE2 inhibitors. Also, we have shown a
promising application of free energy perturbation and
molecular dynamics calculations to predict the SAR of the
■
AUC0‑inf, area under the curve until infinite time; Co, zero
concentration; B/P, brain-to-plasma ratio; cAMP, cyclic
adenosine monophosphate; cGMP, cyclic guanosine mono-
phosphate; Cl, clearance; FEP, free energy perturbation;
D
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ACS Medicinal Chemistry Letters
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Schmidt, C. J.; Suiciak, J. A.; Liras, S. Discovery of a novel class of
phosphodiesterase 10A inhibitors and identification of clinical
candidate 2-[4-(1-methyl-4-pyridin-4-yl-1H-pyrazol-3-yl)-phenoxy-
methyl]-quinoline (PF-2545920) for the treatment of schizophrenia.
J. Med. Chem. 2009, 52, 5188−96.
(17) Zoraghi, R.; Corbin, J. D.; Francis, S. H. Properties and
functions of GAF domains in cyclic nucleotide phosphodiesterases and
other proteins. Mol. Pharmacol. 2004, 65, 267−78.
HPβCD, (2-hydroxypropyl)-beta-cyclodextrin [128446-35-5];
hLM, human liver microsomes; i.v., intravenous; n.m., not
measured; PDE, phosphodiesterase; p.o., per os (oral); s.c.,
subcutaneous; s.d., standard deviation; rLM, rat liver micro-
somes; t1/2, half-life; tmax, time at maximum concentration; Vdss,
steady-state volume of distribution; Vdz, apparent volume of
distribution
REFERENCES
■
(1) Francis, S. H.; Blount, M. A.; Corbin, J. D. Mammalian cyclic
nucleotide phosphodiesterases: molecular mechanisms and physio-
logical functions. Physiol. Rev. 2011, 91, 651−90.
(2) Rosman, G. J.; Martins, T. J.; Sonnenburg, W. K.; Beavo, J. A.;
Ferguson, K.; Loughney, K. Isolation and characterization of human
cDNAs encoding a cGMP-stimulated 3′,5′-cyclic nucleotide phospho-
diesterase. Gene 1997, 191, 89−95.
(3) Lakics, V.; Karran, E. H.; Boess, F. G. Quantitative comparison of
phosphodiesterase mRNA distribution in human brain and peripheral
tissues. Neuropharmacology 2010, 59, 367−74.
(4) Gomez, L.; Breitenbucher, J. G. PDE2 inhibition: potential for
the treatment of cognitive disorders. Bioorg. Med. Chem. Lett. 2013, 23,
6522−27.
(5) Schmidt, C. J. Phosphodiesterase inhibitors as potential cognition
enhancing agents. Curr. Top. Med. Chem. 2010, 10, 222−30.
(6) Sanderson, T. M.; Sher, E. The role of phosphodiesterases in
hippocampal synaptic plasticity. Neuropharmacology 2013, 74, 86−95.
(7) Maurice, D. H.; Ke, H.; Ahmad, F.; Wang, Y.; Chung, J.;
Manganiello, V. C. Advances in targeting cyclic nucleotide
phosphodiesterases. Nat. Rev. Drug Discovery 2014, 13, 290−314.
́
(8) Andres, J.-I.; Buijnsters, P.; De Angelis, M.; Langlois, X.;
Rombouts, F.; Trabanco, A. A.; Vanhoof, G. Discovery of a new series
of [1,2,4]triazolo[4,3-a]quinoxalines as dual phosphodiesterase 2/
phosphodiesterase 10 (PDE2/PDE10) inhibitors. Bioorg. Med. Chem.
Lett. 2013, 23, 785−90.
́
(9) Buijnsters, P.; Andres, J.-I.; De Angelis, M.; Langlois, X.;
Rombouts, F.; Sanderson, W.; Tresadern, G.; Ritchie, A.; Trabanco,
A.; Vanhoof, G.; Van Roosbroeck, Y. Structure-based design of a
potent, selective, and brain penetrating PDE2 inhibitor with
demonstrated target engagement. ACS Med. Chem. Lett. 2014, 5,
1049−1053.
(10) Friesner, R. A.; Banks, J. L.; Murphy, R. B.; Halgren, T. A.;
Klicic, J. J.; Mainz, D. T.; Repasky, M. P.; Knoll, E. H.; Shelley, M.;
Perry, J. K.; Shaw, D. E.; Francis, P.; Shenkin, P. S. Glide: A new
approach for rapid, accurate docking and scoring. 1. Method and
assessment of docking accuracy. J. Med. Chem. 2004, 47, 1739−1749.
(11) Synthetic protocols are provided as SI. Compounds 6 and 7
have been previously reported in: Joergensen, M.; Bruun, A. T.;
Rasmussen, L. K. Preparation of pyridine compounds useful for
treating neurological and psychiatric disorders. PCT Int. Appl., WO
2013034761 A1, 2013.
(12) Malamas, M. S.; Ni, Y.; Erdei, J.; Stange, H.; Schindler, R.;
Lankau, H.-J.; Grunwald, C.; Fan, K. Y.; Parris, K.; Langen, B.;
Egerland, U.; Hage, T.; Marquis, K. L.; Grauer, S.; Brennan, J.;
Navarra, R.; Graf, R.; Harrison, B. L.; Robichaud, A.; Kronbach, T.;
Pangalos, M. N.; Hoefgen, N.; Brandon, N. J. Highly potent, selective,
and orally active phosphodiesterase 10A inhibitors. J. Med. Chem.
2011, 54, 7621−7638.
(13) Jorgensen, W. L. Efficient drug lead discovery and optimization.
Acc. Chem. Res. 2009, 42, 724−733.
(14) Chodera, J. D.; Mobley, D. L.; Shirts, M. R.; Dixon, R. W.;
Branson, K.; Pande, V. S. Alchemical free energy methods for drug
discovery: Progress and challenges. Curr. Opin. Struct. Biol. 2011, 21,
150−160.
(15) StarDrop Optibrium, Ltd.: Cambridge, U.K.; http://www.
optibrium.com (accessed Dec 19, 2014).
(16) Verhoest, P. R.; Chapin, D. S.; Corman, M.; Fonseca, K.;
Harms, J. F.; Hou, X.; Marr, E. S.; Menniti, F. S.; Nelson, F.;
O’Connor, R.; Pandit, J.; Proulx-Lafrance, C.; Schmidt, A. W.;
E
DOI: 10.1021/ml500463t
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