Design, synthesis and pharmacological evaluation of novel polycyclic heteroarene ethers as PDE10A inhibitors: Part i

Article abstract of DOI:10.1016/j.bmcl.2014.03.054

We report analogue-based rational design and synthesis of two novel series of polycyclic heteroarenes, pyrrolo[3,2-b]quinolines and pyrido[2,3-b]indoles, tethered to a biaryl system by a methyl-, ethyl- or propyl ether as PDE10A inhibitors. A number of analogues were prepared with variable chain length and evaluated for their ability to block PDE10A enzyme using a radiometric assay. Detailed SAR analyses revealed that compounds with an ethyl ether linker are superior in potency compared to compounds with methyl or propyl ether linkers. These compounds, in general, showed poor metabolic stability in rat and human liver microsomes. The metabolic profile of one of the potent compounds was studied in detail to identify metabolic liabilities of these compounds. Structural modifications were carried out that resulted in improved metabolic stability without significant loss of potency.

Full text of DOI:10.1016/j.bmcl.2014.03.054

Bioorganic & Medicinal Chemistry Letters 24 (2014) 2073–2078
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and pharmacological evaluation of novel polycyclic
heteroarene ethers as PDE10A inhibitors: Part I
Sanjib Das, Rajendra L. Harde, Dnyaneshwar E. Shelke, Neelima Khairatkar-Joshi, Malini Bajpai,
Ratika S. Sapalya, Harshada V. Surve, Girish S. Gudi, Rambabu Pattem, Dayanidhi B. Behera,
⇑
Satyawan B. Jadhav, Abraham Thomas
Glenmark Research Centre, Glenmark Pharmaceuticals Ltd, MIDC, Mahape, Navi Mumbai 400 709, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We report analogue-based rational design and synthesis of two novel series of polycyclic heteroarenes,
pyrrolo[3,2-b]quinolines and pyrido[2,3-b]indoles, tethered to a biaryl system by a methyl-, ethyl- or pro-
pyl ether as PDE10A inhibitors. A number of analogues were prepared with variable chain length and
evaluated for their ability to block PDE10A enzyme using a radiometric assay. Detailed SAR analyses
revealed that compounds with an ethyl ether linker are superior in potency compared to compounds
with methyl or propyl ether linkers. These compounds, in general, showed poor metabolic stability in
rat and human liver microsomes. The metabolic profile of one of the potent compounds was studied in
detail to identify metabolic liabilities of these compounds. Structural modifications were carried out that
resulted in improved metabolic stability without significant loss of potency.
Received 25 October 2013
Revised 6 March 2014
Accepted 18 March 2014
Available online 28 March 2014
Keywords:
PDE10A inhibitors
Pyrrolo[3,2-b]quinolines
Pyrido[2,3-b]indoles
Met-ID studies
Ó 2014 Elsevier Ltd. All rights reserved.
Phosphodiesterases (PDEs) consist of a family of 11 distinct en-
zymes with several subtypes or isoforms that are encoded by 21
genes and implicated in a plethora of cellular functions.1 They
are responsible for the regulation of intracellular 30,50-cyclic aden-
osine monophosphate (cAMP) and/or 30,50-cyclic guanosine mono-
phosphate (cGMP) by hydrolyzing one of the phosphodiester bonds
to form the corresponding inactive phosphate. While some PDEs
hydrolyze either cAMP or cGMP, PDE10A, being a dual substrate
specific enzyme, hydrolyzes both the cyclic nucleotides.2 PDE10A
is highly expressed in mammalian medium spiny neurons (MSNs)
of both the dorsal and ventral striata, a part of the basal ganglia
system. It is also expressed, albeit to a lesser extent, in other brain
areas like the cortex, the hippocampus and the pineal gland.3 The
recent studies have shown that PDE10A could modulate both
dopaminergic and glutamatergic signaling pathways of the stria-
tum.4 Thus, PDE10A inhibitors have the potential to address the
unmet medical need in treating CNS disorders such as schizophre-
nia and Huntington’s disease.
as PF-02545920 had been evaluated in phase I and II clinical trials
for the treatment of schizopherenia.6 However, this study has now
been discontinued and the molecule is currently being evaluated
for the treatment of Huntington’s disease. Omeros has successfully
completed phase I clinical trials for their proprietary molecule
(OMS-824) and initiated a phase II program as a monotherapy
and in combinations with other antipsychotic medications.7 Am-
gen (AMG-579),8 Takeda (TAK-063)9 and Roche (RG-7203)10 also
have PDE10A inhibitors in phase I clinical trials. Several other com-
panies like Merck,11 EnVivo,12 Lundbeck,13 Janssen,14 Biotie15 and
Glenmark16 are also actively working on this target and molecules
are in various stages of preclinical studies.
In order to discover novel, structurally distinct PDE10A inhibi-
tors, we evaluated the structural features of several known PDE10A
inhibitors reported in the literature. We observed that, many of
these reported inhibitors share similar structural and topographi-
Several groups are currently exploring the possibility of using
small molecule PDE10A inhibitors as potential therapeutic agents
for the treatment of neurological disorders particularly schizophre-
nia and Huntington’s disease.5 Pfizer has extensively studied two
molecules, MP-10 (1) and TP-10 (2) (Fig. 1). MP-10, also known
N
N
N CH CF
N CH
2
3
3
N
N
N
N
O
O
1
2
hPDE10A IC : 0.37 nM
hPDE10A IC : 1.41 nM
50
50
⇑
Corresponding author. Tel.: +91 226772 0000; fax: +91 22 2778 1206.
E-mail address: abraham_thomas@glenmarkpharma.com (A. Thomas).
Figure 1. Structures of PDE10A inhibitors MP10 (1) and TP10 (2).
http://dx.doi.org/10.1016/j.bmcl.2014.03.054
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

2074
S. Das et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2073–2078
Br
O
4 was first coupled with 2-fluoro-3-bromopyridine to give the 3-
Me
Me
c
CHO
Me
d
bromopyridine ether 5 in good yield. Suzuki coupling reaction of
5 with pyridine-4-boronic acid gave the desired compound 7. Sim-
ilarly, Suzuki coupling reaction of 5 with pyrimidine-5-boronic
acid under identical conditions gave 8.
N
N
OH
N
N
N
a
N
b
N
3
4
5
The inhibitory activity of compounds was measured using a
scintillation proximity assay with [3H]-cAMP as the substrate by
measuring hydrolysis of cAMP to AMP using recombinant human
PDE10A enzyme and the results are shown in Table 1.18 Com-
pounds 6, 7 and 8 with a methyl ether linker showed poor PDE10A
inhibitory activity, suggesting very weak binding of these mole-
cules with the enzyme, while the comparator quinoline methyl
ether derivatives 1 and 2 showed excellent potency of 0.51 and
0.98 nM, respectively. These values are in very good agreement
with the reported IC50 values of 0.37 and 1.41 nM, respectively.6b
Concurrently, another novel tricyclic core pyrido[2,3-b]indole
was also prepared and linked via a methyl ether linker to an appro-
priate pyrazine (e.g. 12) or pyrimidine (e.g. 13a) core as shown in
Scheme 2. Thus, pyrido[2,3-b]indol-3-yl nitrile 919 on N-methyla-
tion followed by nitrile group reduction with DIBAL afforded for-
myl derivative 10. NaBH4 reduction of 10 gave the desired
alcohol 11. The alcohol 11 was coupled with 2,3-dichloropyrazine
and then subjected to Suzuki coupling reaction with pyrimidine-5-
boronic acid to provide 12. Cesium carbonate mediated reaction of
11 with 2-chloro-4-(4-fluorophenyl)pyrimidine and 2-chloro-4-(5-
pyrimidinyl)pyrimidine afforded 13a and 13b, respectively.
As in the pyrroloquinoline (6-6-5-fused ring) derivatives 6–8,
the pyridoindole (6-5-6-fused ring) derivatives 12, 13a–13b also
showed poor enzyme inhibition. Thus, both the tricyclic cores
when coupled with pyrazine, pyridine or pyrimidine, with a 1,2-
substitution (7, 8 and 12) or 1,3-substitution (6, 13a–b) pattern
through a methyl ether resulted in poor PDE10A activity.
F
e
N
N
N
N
Me
Me
Me
N
N
N
O
N
O
N
O
N
N
N
N
6
7
8
Scheme 1. Reagents and conditions: (a) NaBH4, MeOH, 0 °C, 30 min, 79%; (b) 3-
bromo-2-fluoropyridine, Cs2CO3, DMSO, 80 °C, 16 h, 70%; (c) 2-chloro-4-(4-fluoro-
phenyl)pyrimidine, Cs2CO3, DMSO, 80 °C, 9 h, 58%; (d) pyridine-4-boronic acid,
Pd(dppf)Cl2ꢁCH2Cl2, Na2CO3, DMSO, water, 80 °C, 16 h, 75%; (e) pyrimidine-5-
boronic acid, Pd(dppf)Cl2ꢁCH2Cl2, Na2CO3, DMSO, water, 80 °C, 16 h, 72%.
CN
CHO
CH2OH
N
a
N
N
b
HN
N
N
Me
Me
9
10
11
d
c
N
N
N
N
N
N
O
N
R
N
O
Me
N
Me
N
13a, R = 4-fluorophenyl
13b, R = pyrimidin-5-yl
We next explored the possibility of improving the potency by
attaching the pyrroloquinoline core via an ethyl ether linker to
the central core as shown in Scheme 3. Wittig homologation reac-
tion of 3 with methyltriphenylphosphonium bromide afforded the
olefin intermediate which on hydroboration followed by alkaline
hydrogen peroxide treatment gave the desired alcohol 14 in 14%
overall yield. The coupling reaction of 14 with 2,3-dichloropyr-
azine using Cs2CO3 followed by Suzuki coupling with pyrimidine-
5-boronic acid afforded ethyl ether derivative 15.
12
Scheme 2. Reagents and conditions: (a) (i) CH3I, K2CO3, DMF, 0 °C to rt, 5 h, 67%; (ii)
DIBAL, CH2Cl2 ꢀ78 °C to rt, 1 h, 67%; (b) NaBH4, MeOH, 0 °C, 30 min, 87%; (c) (i) 2,3-
dichloropyrazine, Cs2CO3, DMSO, 80 °C, 12 h, 74%; (ii) pyrimidine-5-boronic acid,
Pd(dppf)Cl2ꢁCH2Cl2, Na2CO3, DMSO, water, 80 °C, 16 h, 58%; (d) 2-chloro-4-substi-
tuted pyrimidine, Cs2CO3, DMSO, 80 °C, 12 h, 65–70%.
cal features. In our design, we intended to incorporate pyrido[2,3-
b]indole and pyrrolo[3,2-b]quinoline and hoped to achieve MP10-
and TP10-like binding interactions with PDE10A. Herein we de-
scribe the synthesis and detailed SAR analysis of these classes of
PDE10A inhibitors.
In our initial design, the two tricyclic cores mentioned above
were linked to a biaryl system via a methyl ether group to result
in polycylic heteroarene ethers 6–8, 12, 13a–b (Schemes 1 and
2). It may be noted that these novel molecules are devoid of the
pyrazole and the quinoline ring present in MP10, TP10 and their
derivatives reported in the literature. However, these molecules
may be viewed as analogues wherein the quinoline ring is replaced
by a pyrrolo[3,2-b]quinoline (6, 7, 8) or a pyrido[2,3-b]indole (12,
13aꢀb) ring and the pyrazole ring by a pyridine, pyrimidine or pyr-
azine connected via a methyl ether linker instead of a phenoxy
methyl ether linker.
The strategy employed for the synthesis of hitherto unknown 1-
methyl-1H-pyrrolo[3,2-b]quinoline-3-yl)methanol 4 and its link-
ing to the pyrimidine (6) or pyridine (7 and 8) core is shown in
Scheme 1. Thus, 1-methyl-1H-pyrrolo[3,2-b]quinoline-3-carboxal-
dehyde 317 on formyl group reduction with sodium borohydride
gave the desired alcohol 4. Alcohol 4 was then coupled with 2-
chloro-4-(4-fluorophenyl)pyrimidine in the presence of cesium
carbonate to furnish pyrimidin-2-yl ether 6. The pyridine-2-yl
ethers 7 and 8 were prepared in two steps using alcohol 4. Alcohol
The compound 15 with a 6-6-5-fused tricyclic core connected
through an ethyl ether linker showed greatly improved potency
of 123.2 nM compared to the methyl ether derivative 8. This was
based on the assumption that potency is not significantly affected
by swapping the central pyridine ring with a pyrazine ring.
To confirm the effect of chain length on potency, pyrido[2,3-
b]indol-3-yl derivatives 19a–d and 20a–d were also prepared as
shown in Scheme 4. As described previously, aldehyde 10 on Wit-
tig reaction with methyltriphenylphosphonium bromide followed
by hydroboration and oxidation gave alcohol 16 in 48% overall
yield. The coupling reaction of 16 with 1,2-dichloropyrazine fol-
lowed by Suzuki reaction with appropriate aryl boronic acid gave
19a–d in good yields. The coupling reaction of 16 with 3-bromo-
N
N
N
N
OH
Me
CHO
Me
Me
N
N
N
b
a
O
N
N
N
3
14
15
Scheme 3. Reagents and conditions: (a) (i) Ph3PCH3Br, (CH3)3COK, THF, 0 °C, 1 h,
34%; (ii) 9-BBN, THF, 0 °C to rt, 16 h, then 30% H2O2, 3N NaOH, 0 °C to rt, 3 h, 39%;
(b) (i) 2,3-dichloropyrazine, Cs2CO3, DMSO, 80 °C, 12 h, 75%; (ii) pyrimidine-5-
boronic acid, Pd(dppf)Cl2ꢁCH2Cl2, Na2CO3, DMSO, water, 80 °C, 16 h, 60%.

S. Das et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2073–2078
2075
Me
N
Me
Me
Me
N
N
N
N
N
N
N
Cl
N
N
N
a
b
b
a
OH
10
CO2Et
10
O
OH
21
22
c
17
16
Me
d
N
c
N
R
Me
N
Me
N
23a, R = 2-F-3-pyridyl
23b, R = 5-pyrimidyl
23c, R = morpholin-1-yl
O
N
Br
O
R
O
N
N
N
23
N
18
19
d
Scheme 5. Reagents and conditions: (a) [(Ethoxycarbonyl)methylene] triphenyl-
phosphorane, toluene, 60 °C 5 h, 87%; (b) (i) H2, 10% Pd-C, EtOH, RT 2 h, 100%; (ii)
LAH, THF, 0 °C, 3 h, 82%; (c) (i) 3-bromo-2-fluoropyridine, Cs2CO3, DMSO, 80 °C,
12 h, 72%; (ii) boronic acid, Pd(dppf)Cl2ꢁCH2Cl2, Na2CO3, DMSO, water, 80 °C, N2,
16 h (for 23a and 23b), or morpholine, Pd2(dba)3, xantphos, (CH3)3CONa, toluene,
100 °C, 12 h (for 23c), 40–50%.
Me
N
19a 20a
,
,
, R = 4-fluorophenyl
, R = 4-chlorophenyl
R
O
N
19b 20b
N
19c, 20c, R = pyridin-4-yl
20
19d, 20d, R = pyrimidin-5-yl
Scheme 4. Reagents and conditions: (a) (i) Ph3PCH3Br, (CH3)3COK, THF, 0 °C, 1 h,
62%; (ii) 9-BBN, THF, 0 °C, 16 h, then 30% H2O2, 3N NaOH, 0 °C to rt, 3 h, 78%; (b) 2,3-
dichloropyrazine, Cs2CO3, DMSO, 80 °C, 12 h, 66%; (c) 3-bromo-2-fluoropyridine,
Cs2CO3, DMSO, 80 °C, 16 h, 68%; (d) boronic acid, Pd(dppf)Cl2ꢁCH2Cl2, Na2CO3,
DMSO, water, 80 °C, 16 h, 55–75%.
3-bromo-2-fluoropyridine gave the bromo ether, which on Suzuki
coupling with appropriate boronic acid afforded 23a–b. The above
bromo ether intermediate on Buchwald coupling with morpholine
gave 23c.
The 6-5-6-fused derivatives 23a–c (entries 16–18) with ex-
tended linkers showed significant reduction in inhibitory activity
compared to the corresponding ethyl ether linked compounds.
For example, the propyl-linked 5-(pyridine-3-yl)-pyrimidine deriv-
ative 23b was around 130-fold less potent than the corresponding
ethyl-linked compound 20d. The results given in Table 1 suggest
that elongation of the ether linker from methyl to ethyl ether lead
to improvement in potency while further extension to propyl ether
resulted in significant drop in potency.
2-fluoropyridine gave bromo ether 18. The bromo ether 18 on Su-
zuki coupling reaction with appropriate aryl boronic acids gave
20a–d in good yield.
A dramatic improvement in potency was observed when the
methylene bridge was replaced by an ethylene bridge as shown
in the case of 19a–d and 20a–d (Table 1, entries 8–15). The 2-(pyri-
din-4-yl)pyrazine derivative 19c (11.2 nM) was very close in po-
tency to 3,40-bipyridine derivatives 20c (10.4 nM) suggesting that
the central pyrazine core can be replaced by a pyridine core with-
out significant loss of potency. The pyrido[2,3-b]indole derivative
19d is 2-fold more potent than the pyrroloquinoline derivative
15 suggesting that 6-5-6-fused tricyclic core is better than the 6-
6-5-core.
Having explored the synthesis of methyl- and ethyl ether linked
compounds, we next turned our attention towards the synthesis of
propyl ether derivatives 23a–c using 6-5-6-fused pyrido[2,3-b]
indole core as shown in Scheme 5. Wittig reaction of 10 with (eth-
oxycarbonylmethylene)triphenylphosphorane followed by cata-
lytic hydrogenation and LAH mediated ester group reduction
gave the propanol derivative 22. The alcohol 22 on coupling with
As a part of the study, selected potent compounds listed in Ta-
ble 1 were screened against a broader panel of PDEs to assess the
selectivity of this class of compounds and the results are given in
Table 2. The PDE selectivity assay of compounds 19a, 19c, 20c,
20d and 33 was carried out using human recombinant PDEs 1–5,
7–9 and 11 at 1.0 and 10.0
However, only 1.0 M values are shown in the table. The com-
pounds, in general, were highly selective against all other PDEs
tested, except for PDE2. The PDE2 inhibitory values at 10.0
l
M concentrations of the compounds.18
l
lM
concentrations were higher (60–86%) for all the five compounds
and are therefore considered as less selective. However, these com-
pounds were estimated to have minimum 100-fold selectivity on
PDE2, based on the PDE10A IC50 values.
Having achieved good selectivity over related PDEs, we ex-
plored the in vitro metabolic stability of 15, 19a, 19c, 20c and
20d and the results are shown in Table 3. The in vitro metabolism
studies were carried out in rat and human liver microsomes using
the standard procedure. Briefly, the compounds were incubated at
Table 1
Binding affinity and IC50 values of compounds
Entry
Compound
% Inhib. at 1.0
l
Ma
h IC50 SD (nM)b
1
2
3
4
5
6
7
8
6
7
8
12
13a
13b
15
19a
19b
19c
19d
20a
20b
20c
20d
23a
23b
23c
29
12.1
24.6
30.7
10.9
23.9
23.6
82.1
85.4
87.2
97.2
83.4
88.5
75.7
95.8
92.5
48.9
43.1
19.1
54.7
96.3
ND
ND
ND
ND
ND
ND
a concentration of 1.0 lM with 1.0 mg/mL protein at 37 °C for
60 min. All these compounds (entries 1–5) showed poor metabolic
stability with less than 10% remaining in both rat and human liver
microsomes. We studied the metabolic liabilities of this class of
compounds in more detail and the results are discussed below.
123.2 35.5
41.5 8.2
188.2 13.4
11.2 2.1
67.4 14.5
68.5 6.6
592.1 102.8
10.4 1.6
24.2 2.1
5936.8 393.2
3217.2 402.9
ND
9
Table 2
10
11
12
13
14
15
16
17
18
19
20
Selectivity profile of compounds 19a, 19c, 20c, 20d, and 33
PDEs
Percentage inhibition at 1.0 l
M of compoundsa
19a
19c
20c
20d
33
PDE1
PDE2
PDE3
PDE4
PDE5
PDE7
PDE8
PDE9
PDE11
13.5
11.9
0.1
0.7
5.9
18.3
0.0
1.6
30.1
34.5
0.2
11.9
13.6
27.0
2.0
40.9
41.4
0.0
10.7
67.2
0.0
25.8
23.9
20.7
0.0
14.0
40.6
0.4
0.6
3.8
971.2 60.5
33.2 3.2
36.5
40.5
11.2
1.3
16.7
20.8
2.1
5.3
15.8
33
a
Percentage inhibitions are measured at 1.0 and 10.0
trations of the compound and were run in duplicate.
l
M (not shown) concen-
4.3
21.7
9.8
12.4
18.6
17.4
b
IC50 values are mean SD of two independent experiments using 7 to 9-point
concentration-response curve. ND refers to ‘not determined’.
a
Values are mean of two independent experiments run in duplicate.

2076
S. Das et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2073–2078
Table 3
N
N
H
N
Metabolic stability data of 15, 19a, 19c, 20c, 20d, 29 and 33
H
N
N
N
N
N
+ [O]
Entry
Compound
% Remaining (rat)a
% Remaining (human)a
O
N
O
N
OH
1
2
3
4
5
6
7
15
0.27 0.1
3.19 0.4
1.03 0.2
0.77 0.3
1.93 0.1
17.5 7.0
41.2 11.6
0.11 0.1
9.92 0.6
0.72 0.1
0.63 0.2
0.45 0.1
19.6 3.8
51.1 5.6
M6
(m/z: 368.1)
M3
(m/z: 384.1)
N
19a
19c
20c
20d
29
- CH2
20d
Me
+ [O]
- H2O
N
N
N
(m/z: 382.1)
O
N
+ 2[O]
- H2O
33
M9
(m/z: 380.1)
+ 2[O]
- H2O
a
Values are mean SD of three independent experiments.
N
O
N
N
N
Me
Me
N
N
N
N
Metabolite identification (Met-ID) studies were carried out
O
N
O
N
O
using 20d as a tool compound to identify the predominant site(s)
of metabolism. Extensive metabolism was observed for 20d that
is consistent with the poor metabolic stability in both rat and hu-
man liver microsomes (Table 3, entry 5). After 60 min incubation,
nine major UV active metabolites (M1–M9) were observed along
with small amounts of 20d (retention time: 14.51 min) as shown
in Figure 2. The metabolites were characterized by LCMS/MS
analysis.
M4
M5
(m/z: 396.1)
(m/z: 396.1)
Scheme 6. Metabolic pathways for M3, M4, M5, M6 and M9.
O
N
N
N
O
N
Me
H
N
N
N
N
O
N
O
N
The potential structures were assigned on the basis of mass
fragmentation pattern of metabolites. Dealkylation, hydroxylation,
and hydroxylation followed by dehydration were the major meta-
bolic pathways. Chemical structures for all metabolites were pro-
posed and are given in Schemes 6 and 7. The formation of
metabolites M3, M4, M5, M6 and M9 have been rationalized in
Scheme 6. The major metabolite M9 (retention time: 11.81 min)
was formed by oxidation (+16 amu) followed by loss of water
(ꢀ18 amu), which we believe is hydroxylation at methylene posi-
tion followed by dehydration to an olefinic compound. The metab-
olite M6 was formed by demethylation (ꢀ14 amu) of N-methyl
group of pyrido[2,3-b]indole moiety. The metabolite M3 was
formed by demethylation (ꢀ14 amu) and hydroxylation (+16
amu) at the active methylene group. The metabolites M4 and M5
showed [M+H]+ ion with m/z 396.1 with different fragmentation
pattern (M4: m/z 223.2 and M5: m/z 207.2) suggesting N-oxide for-
mation at different sites of the molecule. The metabolites M4 was
formed by N-oxide formation at the pyrido[2,3-b]indole ring fol-
lowed by hydroxylation at one of the methylene groups and finally
in situ dehydration. The metabolites M5 having the same molecu-
lar ion peak, but a different fragmentation pattern was formed by
N-oxide formation at pyridine nitrogen followed by hydroxylation
and dehydration as described in the case of M4.
M1
M2
(m/z: 396.1)
(m/z: 384.1
)
+ 2[O]
+ [O]
- CH2
20d
- H2O
+ [O]
(m/z: 382.1
)
N
N
+ [O]
- H2O
O
N
N
Me
N
N
N
N
O
N
N
Me
OH
(m/z: 398.1
M8
M7
)
(m/z: 380.1)
Scheme 7. Metabolic pathways for M1, M2, M7 and M8.
molecular ion and fragmentation pattern (384.1/211.2). We believe
that M1 was formed by oxidation at the pyridine nitrogen of pyrid-
oindole moiety. Similarly, metabolites M2 and M4 also showed the
same molecular ion and fragmentation (396.1/223.2) suggesting
that oxidation is again at the pyridoindole ring. Thus, N-oxide
formation at indole nitrogen was proposed for M2. Metabolites
M7 and M9 showed same molecular ion at m/z 380.1 and the
structure for M7 is tentatively assigned as cis-olefin formed by
hydroxylation and loss of water by E2-elimination. The Met-ID stud-
ies suggest that the major metabolic soft spots of 20d were the
methylene group and the N-methyl group of pyridoindole moiety.
We then evaluated the oral PK of 19a (Mol. Wt.: 398.4; cLog P:
4.92 0.95; PSA 52.83) in male SD rats (5 mg/kg). Compound 19a
was selected for the study as it showed marginally better meta-
bolic stability compared to other 5 compounds (Table 2, entries
1–5). The systemic exposures (Cmax of 103 ng/mL and a mean
AUC0–t of 190 ng h/mL) were very low with quick elimination of
19a from plasma, probably because of high metabolic clearance
as reflected from the poor in-vitro metabolic stability. The concen-
trations of 19a were found to be below the limits of quantification
(<5.4 ng/mL) in all brain homogenate samples collected after 1.0 h
post dose.
The formation of metabolites M1, M2, M7 and M8 have been
rationalized in Scheme 7. Metabolite M1 and M3 showed same
Having understood the metabolic liabilities of 20d in more de-
tail, we explored the possibilities of blocking the predominant
metabolic sites by chemical intervention, while maintaining the
PDE10A potency. Our efforts in this direction are depicted in
Schemes 8 and 9. Since hydroxylation at the methylene group is
one of the major pathways identified in the Met-ID studies, we de-
signed and synthesized a compound of the formula 29, wherein the
labile methylene group is quaternized by introduction of a cyclo-
propyl ring as shown in Scheme 8. The bromination of aminopyri-
Figure 2. HPLC chromatogram of rat microsomomal incubation of 20d at 0 min
(faint line) and 60 min (bold line).

S. Das et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2073–2078
2077
N
In summary, we have demonstrated that fused heteroarenes
such as pyrrolo[3,2-b]quinolines and pyrido[2,3-b]indoles linked
to a biaryl system act as PDE10A inhibitors with good potency
and selectivity. However, these classes of compounds suffered
from the severe limitation of poor metabolic stability. The problem
was addressed to a major extent by blocking the major sites of
metabolism. Further structural modifications are in progress to
improve potency, metabolic stability and physicochemical proper-
ties and the results of this study will be disclosed in an upcoming
publication.
N
Br
Br
H2N
O
O
b
a
O
O
25
N
24
N
H
Br
Br
N
O
O
d
c
O
O
26
27
N
Me
Me
N
N
N
N
N
e
OH
O
N
28
29
Acknowledgments
Scheme 8. Reagents and conditions: (a) (i) NBS, THF, 0 °C 1 h, 63%; (ii) C6H5CH2-
N(C2H5)3Br, tert-butyl nitrite, CH2Br2, rt, 16 h, 64%; (b) NaH, DMF, 0 °C, 5 min, then
1,2-dibromoethane, N2, 1 h, 71%; (c) (i) aniline, xantphos, Pd(OAc)2, Cs2CO3, 1,4-
dioxane, 100 °C, 16 h, 64%; (ii) Pd(OAc)2, DBU, DMA, 160 °C, 10 h, 40%; (d) (i) CH3I,
NaHMDS, DMF-THF, 0 °C to rt, 2 h, 84%; (ii) LAH, THF, reflux, 4 h, 82%; (e) (i) 3-
bromo-2-fluoropyridine, Cs2CO3, DMSO, 80 °C, 16 h, 65%; (ii) pyrimidine-5-boronic
acid, Pd(dppf)Cl2ꢁCH2Cl2, Na2CO3, DMSO, water, 80 °C, 12 h, 62%.
We are grateful to Dr. Srinivas Gullapalli for helpful discussions
and suggestions during preparation of the manuscript. We thank
Ms. Aarti Sawant for her excellent technical assistance during man-
uscript preparation.
Supplementary data
F
F
F
F
Supplementary data (chemical synthesis, analytical data,
in vitro assay procedure and scanned 1H NMR spectra of selected
compounds) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.bmcl.2014.03.054.
H
N
N
N
N
N
N
a
b
CN
CN
OH
31
30
9
F
N
F
F
F
N
N
N
References and notes
Br
O
N
N
c
d
O
N
N
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32
33
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dine derivative 2420 with NBS followed by diazotization of the ami-
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derivative 25.21 The reaction of 25 with 1,2-dibromoethane using
sodium hydride gave the desired cyclopropyl derivative 26.22
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