Triazoloquinazolines as a novel class of phosphodiesterase 10A (PDE10A) inhibitors

Article abstract of DOI:10.1016/j.bmcl.2011.04.067

Novel triazoloquinazolines have been found as phosphodiesterase 10A (PDE10A) inhibitors. Structure-activity studies improved the initial micromolar potency which was found in the lead compound by a 100-fold identifying 5-(1H-benzoimidazol-2-ylmethylsulfan

Full text of DOI:10.1016/j.bmcl.2011.04.067

Bioorganic & Medicinal Chemistry Letters 21 (2011) 3738–3742
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Triazoloquinazolines as a novel class of phosphodiesterase 10A
(PDE10A) inhibitors
Jan Kehler a, , Andreas Ritzen a, Morten Langgård a, Sebastian Leth Petersen a, Mohamed M. Farah b,
⇑
Christoffer Bundgaard a, Claus Tornby Christoffersen c, Jacob Nielsen d, John Paul Kilburn a
a H. Lundbeck A/S, Department of Discovery Chemistry & DMPK, 9 Ottiliavej, DK-2500 Valby, Denmark
b Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
c H. Lundbeck A/S, Department of Molecular Pharmacology, 9 Ottiliavej, DK-2500 Valby, Denmark
d H. Lundbeck A/S, Department of Synaptic Transmission 2, 9 Ottiliavej, DK-2500 Valby, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel triazoloquinazolines have been found as phosphodiesterase 10A (PDE10A) inhibitors. Structure–
activity studies improved the initial micromolar potency which was found in the lead compound by a
100-fold identifying 5-(1H-benzoimidazol-2-ylmethylsulfanyl)-2-methyl-[1,2,4]triazolo[1,5-c]quinazo-
line, 42 (PDE10A IC50 = 12 nM) as the most potent compound from the series. Two X-ray structures
revealed novel binding modes to the catalytic site of the PDE10A enzyme.
Received 24 February 2011
Revised 12 April 2011
Accepted 15 April 2011
Available online 30 April 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
PDE10A
Phosphodiesterase
Psychosis
Schizophrenia
Structure
Triazoloquinazoline
The cyclic nucleotides cyclic adenosine monophosphate (cAMP)
and cyclic guanosine monophosphate (cGMP) are ubiquitous sec-
ond messengers responsible for mediating the biological response
of a variety of extracellular signals, including hormones, light, and
neurotransmitters and they influence a vast array of processes, for
example, muscle contraction, learning, differentiation, and apopto-
sis as well as metabolic processes.1 In neurons, this includes the
activation of cAMP- and cGMP-dependent kinases and subsequent
phosphorylation of proteins involved in acute regulation of synap-
tic transmission as well as in gene expression and neuronal differ-
entiation and survival.2,3 The phosphodiesterases (PDEs) are a
family of bimetallic hydrolase enzymes that inactivate cAMP/cGMP
by catalytic hydrolysis of the 30-ester bond, forming the inactive 50-
monophosphate thereby terminating the signaling.1,3 PDE10A is a
relatively newly identified phosphodiesterase with a distinct brain
localisation of the protein with predominant expression in the
medium spiny neurons of the striatal complex.4–6 Based on this un-
ique localisation, research has focused extensively on using
PDE10A modulators as a novel therapeutic approach for dysfunc-
tion in the basal ganglia circuit including Parkinsons’ disease,
Huntingtons disease, schizophrenia, addiction and obsessive
compulsive disorder.7 Notably, MP-10 (Fig. 1) is currently being
evaluated in clinical trials for the treatment of schizophrenia.7,8
Preclinical evidence suggests that a PDE10A inhibitor could pro-
vide anti-psychotic, pro-cognitive and negative symptom
efficacy.7,8 In a HTS campaign at H. Lundbeck A/S we identified
2-amino[1,2,4]triazolo[1,5-c]quinazolines 5a–d (Fig. 1) and the
(2-methyl-[1,2,4]triazolo[1,5-c]quinazolin-5-ylsulfanyl)-acetic
acid ethyl ester 6 (Fig. 1) to be low-micromolar PDE10 inhibitors.
We found the triazoloquinazoline scaffold to be an attractive
starting point for ligand design, because the triazoloquinazoline
makes up a rigid template, which is ideally placed within the
CNS drugability space due to its molecular properties (MW ꢀ300,
TPSA ꢀ70 Å2, c log P ꢀ2.5).12 However, initial SAR-evaluation of
the 2-amino triazoloquinazolines revealed a relatively flat SAR
(for a few representative examples see Table 1).
The flat SAR of the amino-triazoloquinazolines appeared prob-
lematic to us and we therefore turned our attention to explore
the SAR around the 2-methyltriazoloquinazoline thioether 6. The
compounds were synthesized using the three-step protocol out-
lined in Scheme 1.
Commercially available substituted 2-amino-benzonitriles, 7,
were converted to isothiocyanates, 8, by treatment with thiophos-
gene in methylene dichloride and water, and stirring at room tem-
perature for 4–20 h. The organic phase was extracted and
evaporated to yield crystalline solids that were used without
⇑
Corresponding author.
E-mail address: jke@lundbeck.com (J. Kehler).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.04.067

J. Kehler et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3738–3742
3739
purification in the following step. Alkyl acid hydrazide
9
N
N
N
N
(1.3 equiv) was added, and the materials dissolved in ethanol and
heated to reflux and evaporated to give the substituted 6H-
[1,2,4]triazolo[1,5-c]quinazoline-5-thiones 10. The reactions were
followed by LC–MS, and except in one case the evaporated reaction
mixture was used as starting material for the following alkylation
without preceding purification. The sulphur was selectively alkyl-
ated by different alkylating reagents to give final products 11–51.
Initial SAR evaluation focused on the in vitro potency for
PDE10A measured in a PDE10A enzyme inhibition assay. First we
investigated the SAR around the substituent on the thioether and
the methyl group in the 2-position (R1 = H) (Table 2).
N
O
O
O
N
N
N
O
O
O
O
N
N
O
3, MP-10
1, papaverine
2, PQ10
NH2
The SAR shown in Table 2 indicate that a substituent on the sul-
phur is critical for PDE10A affinity, but there seem to be some tol-
erability to the type of thioether since both methyl, ethyl,
acetamide, ethylene-morpholine give rise to equipotent com-
N
N
N
N
N
N
N
N
S
pounds with affinities in the 1–2 lM range. However, the acetoni-
O
N
N
N
O
trile derivative is favored with 10-fold increase in potency
compared with the original ester hit 6. A few small variations of
the R2 group were explored with the R3 group kept constant as
acetonitrile, but no improvement in potency beyond R2 = Me was
achieved, see compounds 17–21 in Table 2.
N
R
OEt
5a-e
6
4
Figure 1. Some published PDE10A inhibitors 1,7d 2,8b 3,9 4,10 and the novel
triazoloquinazoline leads 5a–d and 6.
In order to better understand the SAR of this compound series
we carried out a number of docking studies, which demonstrated
that several different binding modes could be possible. To clarify
the binding mode more precisely we obtained a crystal of the com-
plex between 14 and the PDE10A enzyme for which the X-ray dif-
fraction data was consistent with the binding mode shown in
Figure 2. The crystal structure of 14 revealed a novel binding mode
compared with previously published crystal structures of PDE10A
inhibitors.8d From a structural point of view, there are two major
contributions to binding, one predominantly polar and the other
predominantly hydrophobic. The nitrogen in the 1-position of the
triazole part of the triazoloquinazoline core accepts a hydrogen
bond from the conserved Gln-726 and the aromatic part of the core
Table 1
IC50 of compounds 1–8 for the PDE10A enzyme
a
Compound
PDE10A IC50 (nM)
1, papaverine
2, PQ10 (Pfizer)
3, MP10 (Pfizer)
4, (Wyeth–Elion)
5a, R = 2-Br
5b, R = 4-Br
5c, R = 3,4-MeO
5d, R = 2-Cl
5e, R = H
210
64
2.6
32
1000
31% inh@10,000
4500
41% inh@10,000
27% inh@10,000
3000
interacts through hydrophobic and
p-stacking aromatic interac-
6
tions with Phe-726 and Ile-692 in the central part of the binding
pocket, which has been termed ‘the clamp’. The thiomethylene acts
as a linker reaching backwards in the binding pocket into what has
been termed ‘the Q1-pocket’.8 Here, the acetonitrile functions as a
hydrogen bond acceptor to the hydroxyl group of Ser-667 with a
distance to the to the hydroxyl group of Ser-667 of 1.62 Å. The
a
Values are means of three experiments, and
typical standard deviation was 30%.
a
a)
Cl
S
R1
N
R1
N
C
Table 2
Cl
IC50 of compounds 10–21 for the PDE10A enzyme
rt/CH2Cl2
NH2
a
N
Compound
R2
R3
PDE10A IC50 (nM)
7
C
8
10
11
12
13
14
CH3
CH3
CH3
CH3
CH3
H
CH3
Ethyl
–CH2C(O)NH2
–CH2CN
>10000
1800
2400
1300
310
S
H
N
R2
H
2N
9
O
b)
O
O
µw 130°C
EtOH
CH2
N
N
15
CH3
1300
2500
O
R2
2
R2
1
N
R1 10
9
N
R1
16
CH3
L
N
S
3
N
S
CH2
R3
N
N
17
18
19
H
–CH2CN
–CH2CN
–CH2CN
1300
>10,000
57% inhib@10000
8
–CH2OH
–CH2CH2OH
N
R3
c)
N
5
7
H
OH
20
21
–CH2CN
–CH2CN
960
CH
CH
N
N
11-51
10
O
>10,000
Scheme 1. Reagents and conditions: (a) thiophosgene in methylene dichloride and
water, and stirring at room temperature for 4–20 h; (b) microwave heating, 130 °C,
for 4–20 h; (c) NaH (60%), toluene, microwave heating, 135 °C. L is a leaving group.
a
Values are means of three experiments, and a typical standard deviation was
30%.

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J. Kehler et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3738–3742
a
a
b
TYR
693
GLN 726
TYR
693
b
O
GLN 726
H2N
Q1
O
H2N
OH
Q1
SER 667
OH
S
N
HO
C
N
N
N
N
N
N
S
N
Q2
Q2
N
N
N
H
Water
Mg+2
Zn+2
Water
Mg+2
Zn+2
Figure 3. (a) X-ray structure of compound ligand 42; (b) Cartoon showing binding
interactions and pharmacophore of ligand 42 with the catalytic pocket in the
PDE10A enzyme.
Figure 2. (a) X-ray structure of compound ligand 14; (b) Cartoon showing binding
interactions and pharmacophore of ligand 14 with the catalytic pocket in the
PDE10A enzyme.
Compound 33 was screened in a panel of seven other PDEs and
showed greater than 300-fold selectivity towards PDE1b, PDE2,
PDE3A, PDE7b, PDE9. However, compound 33 had only about a
100-fold selectivity over PDE4d6 (IC50 = 2200 nM) and PDE5a
(IC50 = 5700 nM). Preliminary screening of a few other compounds
in this series indicated that selectivity towards PDE4 pose a prob-
lem for some of these compounds, since the PDE4 potency of com-
pound 32 (PDE4/PDE10 IC50 = 3000/1900 nM), compound 14
(PDE4/PDE10 IC50 = 1300/310 nM) and compound 38 (PDE4/
PDE10 IC50 = 640/790 nM) were in the same range as the potency
at PDE10A.
Due to the limited selectivity towards PDE4 we decided to revi-
sit structural variations of and expand the SAR-study around the
thioether moiety using compound 11 as a lead-structure. Starting
from compound 10 and using the alkylation reactions shown in
Scheme 1, we prepared compounds 39–51 shown in Table 4. The
SAR indicates that the PDE10 potency is very sensitive to the type
of sulphur substituent, and the methylene acetophenone-deriva-
tive 39 completely lost activity at the PDE10 enzyme. However,
the introduction of heterocycles like pyridyl (compound 40) and
quinolinyl (compound 47) resulted in an order of magnitude high-
er PDE10 potency compared with the thiomethyl lead 11. The most
potent heterocycles turned out to be the benzimidazole (42 and
43) and imidazopyridine (46) which had a 100-fold higher PDE10A
potency compared with the lead-compounds. Ring-transformation
X-ray therefore nicely is consistent with the relatively high affinity
of compound 14 and especially the particular role of the cyano-
group. However, it is very unlikely that the observed (scaffold)
binding mode for 14 can be adapted by all structures in Table 2.
In particular it is clear, that the bigger morpholine groups of 15
and 16 will force a reorientation of the core scaffold in the binding
site similarly to what is observed for compound 42 in Figure 3
(vide infra).
We next attempted to optimise the potency by exploring the
SAR of different substituents at positions 7–10 of the triazolo-qui-
nazoline scaffold, while keeping the acetonitrile (R3) and 2-methyl
(R2) substituents constant. The resulting potency data are shown
in Table 3. The SAR described in Table 3, show that the the substit-
uents in the positions 7–10 of the triazoloquinazoline ring are
important for PDE10A affinity. It seems that highest potency is ob-
tained with relatively small, lipophilic substituents at the 9-posi-
tion. Bromine gives the highest potency, approximately 7- to 10-
fold higher than methyl, fluorine and methoxy, and 3-fold higher
than chlorine. This SAR is consistent with the scaffold binding
mode shown in Figure 2, which predicts a relatively lipophilic area
in the clamp region of the binding pocket (see above) where the
triazoloquinazoline binds. Relatively large substituents like phenyl
and morpholine were not tolerated at position 9. The most potent
compound, the 9-bromo-derivative 33, had an IC50 of 35 nM.

J. Kehler et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3738–3742
3741
Table 3
Table 4
IC50 of compounds 14, and 22–38 for the PDE10A enzyme
IC50 of compounds 39-51 for the PDE10A enzyme
Compound
R3’
PDE10A
IC50, nMa
2
N
R1 10
9
Cl
N
Cl
N
C
N
39
>10000
8
N
S
*
7
22-38
O
a
40
41
380
990
N
Compound
R1
PDE10A IC50 (nM)
*
*
14
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
H
10-F
9-F
8-F
7-F
310
1600
460
320
1500
170
460
1100
450
1000
2300
1900
35
H
N
N
H
N
10-Me
9-Me
8-Me
10-CF3
10-Br
8-Br
8-CF3
9-Br
9-Cl
9-MeO
9-Ph
9-(1-Morpholinyl)
8,9-DiMeO
42
43
12
14
*
N
N
*
*
*
N
O
44
45
110
55
130
250
>10,000
8900
790
N
N
N
N
N
a
Values are means of three experiments, and a typical standard deviation was
46
47
48
15
*
*
30%.
N
N
N
160
120
from the benzimidazole 43 to phenyl imidazoles (49–51) gave a 5-
to 10-fold drop in PDE10A potency compared with the fused bicy-
clic benzimidazoles.
When revisiting the X-ray structure of compound 14, it became
clear from docking-studies, that the heterocycles in compounds
39–51 would not fit in the same binding mode due to lack of space
in the binding pocket as visualised in Figure 2.
S
*
*
N
N
N
49
50
51
190
150
65
N
In order to better understand the SAR and binding mode of this
series of compounds, we obtained a high-resolution X-ray struc-
ture of the catalytic site of the PDE10A enzyme with the most po-
tent compound (42) in (Fig. 3).11 Notably, structural differences
between the two crystal structures of the catalytic site of the
PDE10A enzyme are negligible and overlaps consistently with ear-
lier published X-ray structures of PDE10A.
Interestingly, this X-ray structure revealed yet another novel
bidentate binding mode. Here the 1-nitrogen of the triazoloquinaz-
oline core accepts a hydrogen bond from Gln-726 and the aromatic
part of the core participates with hydrophobic and aromatic inter-
actions in the clamp with Phe-729 and Ile-692. The thiomethylene
acts as a linker reaching into the (unique) Q2-pocket, which among
the PDEs is found only in PDE10A. The benzimidazole fills out this
lipophilic Q2-pocket and the benzimidazole nitrogen functions as a
hydrogen bond acceptor to the hydroxyl group of Tyr-693. The X-
ray therefore nicely is consistent with the high potency of com-
pound 42.
The 5-(1H-Benzoimidazol-2-ylmethylsulfanyl)-2-methyl-
[1,2,4]triazolo[1,5-c]quinazoline, 42, was screened in a panel of
six other PDEs and showed greater than 1000-fold selectivity to-
wards PDE1b, PDE2, PDE3A, PDE4d6, PDE5a, PDE7b, PDE9 and
about 100-fold selectivity towards PDE11. Compound 42, there-
fore, looked like an attractive candidate for in vivo studies, how-
ever, we disappointingly found that compound 42 did not have
any brain exposure with a brain/plasma ratio (B/P) <0.04 (for some
experimental details, for example, animal species and exposure see
Table 5). Due to the high number of heteroatoms, we hypothesised
that compound 42 could be a substrate for pgp.12,13 However,
Cl
*
*
N
N
N
O
a
Values are means of three experiments, and a typical standard deviation was
30 %.
Table 5
Permeability across MDR1–MDCK cells, PSA and in vivo B/P-ratio for selected
compounds (60 min following drug administration)
Cmpd.
MDR1-hum
MDR1
ratio
PSA
(Å2)
B/P
(r)
Plasma (ng/ml)/brain
conc (ng/g)
(ꢁ10ꢂ6 cm/s)
42
43
44
45
46
47
43
32
35
41
40
11
0.57
0.46
0.43
0.84
0.49
0.4
72
61
69
73
61
56
<0.04
1.3
n.d.
0.08
0.2
629/<25 (po)
221/286 (po)
—
1492/118 (po)
180/35 (sc)
139/113 (po)
0.86
(r) = rat.
testing the permeability using the pgp-overexpressing MDR1–
MDCK cells revealed that, for example, compounds 42, 44 and 45
had medium permeability with no signs of active efflux (ratio <3,
see Table 5).
We noted, that many of the compounds have high polar surface
area (PSA>70 Å2). In order to test if this physical–chemical param-

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J. Kehler et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3738–3742
eter could influence the B/P-ratio, we lowered the PSA by either
removing one or more heteroatoms, for example, like in the quin-
oline compound 47 or by removing the hydrogen bond donor in
the 1-H-benzimidazole 42, for example, to give the N-methylated
benzimidazole compound 43. These compounds had better brain
exposure with B/P-ratios close to one.
In conclusion, novel triazoloquinazolines have been found as
selective PDE10A inhibitors. Structure–activity studies improved
the initial lM potency found in the lead by a 100-fold identifying
For this work we want to thank Phil Leonard and Alison Ritchie for
their expertise and great help.
References and notes
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olo[1,5-c]quinazoline, 42 (PDE10A IC50 = 12 nM), as the most po-
tent compound from the series. Problems with low brain
penetration of some of the compounds were solved by lowering
the polar surface area either by removing a hydrogen bond donor
or by removing heteroatoms. X-ray structures revealed novel
bidentate binding modes to the catalytic site of the PDE10 enzyme.
Depending on the pattern of chemical substituents, the molecules
can be triggered to display at least two different binding modes,
even though they are based on the same chemical scaffold. In this
respect, X-ray structures are a very nice illustration of a general
point to be taken in medicinal chemistry, namely that different
compounds although based on the same chemical scaffold, can
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pocket and thereby bring different pharmacophoric interactions
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inhibitors toward the cAMP/cGMP-binding site and will facilitate
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Acknowledgments
All work regarding the production of protein, growth of crystals,
and solving the X-ray structures was done at BioFocus, Chesterford
Research Park, Little Chesterford, Saffron Walden, Essex CB10 1XL.