Novel 6,7,8,9-tetrahydro-5H-1,4,7,10a-tetraaza-cyclohepta[f]indene analogues as potent and selective 5-HT2C agonists for the treatment of metabolic disorders

Article abstract of DOI:10.1016/j.bmcl.2010.11.089

The discovery of a novel series of 5-HT_2C agonists based on a tricyclic pyrazolopyrimidine scaffold is described. Compounds with good levels of in vitro potency and moderate to good levels of selectivity with respect to the 5-HT_2A an

Full text of DOI:10.1016/j.bmcl.2010.11.089

Bioorganic & Medicinal Chemistry Letters 21 (2011) 34–37
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel 6,7,8,9-tetrahydro-5H-1,4,7,10a-tetraaza-cyclohepta[f]indene
analogues as potent and selective 5-HT2C agonists for the treatment
of metabolic disorders q
Heather Tye a, , Stephan G. Mueller b, Juergen Prestle c, Stefan Scheuerer e, Marcus Schindler c,
,
⇑
Bernd Nosse b, Natacha Prevost a, Christopher J. Brown a, Alexander Heifetz a, Clemens Moeller d,
Anna Pedret-Dunn a, Mark Whittaker a
a Evotec (UK) Ltd, 114 Milton Park, Abingdon, Oxfordshire OX14 4SA, UK
b Boehringer Ingelheim Pharma GmbH & Co. KG, Dept. Chemical Research, 88397 Biberach, Germany
c Boehringer Ingelheim Pharma GmbH & Co. KG, Dept. Cardiometabolic Diseases Research, 88397 Biberach, Germany
d Evotec AG, Schnackenburgallee 114, 22525 Hamburg, Germany
e Boehringer Ingelheim Pharma GmbH & Co. KG, Dept. Drug Discovery Support, 88397 Biberach, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The discovery of a novel series of 5-HT2C agonists based on a tricyclic pyrazolopyrimidine scaffold is
described. Compounds with good levels of in vitro potency and moderate to good levels of selectivity
with respect to the 5-HT2A and 5-HT2B receptors were identified. One of the analogues (7g) was found
to be efficacious in a sub-chronic weight loss model. A key limitation of the series of compounds was that
they were found to be potent inhibitors of the hERG ion channel. Some compounds, bearing polar side
chains were identified which showed a much reduced hERG liability however these compounds were
sub-optimal in terms of their in vitro potency or selectivity.
Received 21 October 2010
Revised 17 November 2010
Accepted 18 November 2010
Available online 24 November 2010
Keywords:
5HT2C agonist
Metabolic disease
hERG inhbiiton
Ó 2010 Elsevier Ltd. All rights reserved.
The serotonin 5-HT2C receptor is one of 14 serotonin receptor
sub-types. The 5-HT2 sub-family is comprised of 5-HT2A, 5HT2B
both expressed in the CNS and peripherally and 5-HT2C which is
thought to be expressed solely in the CNS. The development of
small molecule 5-HT2C receptor agonists has attracted a high level
of interest in recent years offering the potential for the treatment
of wide ranging conditions such as obesity, schizophrenia, sexual
dysfunction, urinary incontinence and, more recently, diabetes.
Efficacy in pre-clinical animal models has been reported for several
compounds some of which have progressed into human clinical
trials.1
cinogenic as well as cardiovascular side-effects.2 5HT2B agonists
have recently been shown to be responsible for cardiac valvulopa-
thy effects and are also implicated in pulmonary hypertension.3
Many of the 5-HT2C agonists described in the literature possess
a fused arylazepine motif (Fig. 1).1,4,5 Taking this feature as a start-
ing point we focussed on the development of a novel tricyclic scaf-
fold (1) containing a fused pyrazolopyrimidine as the aryl motif.
Our primary objective was to develop potent (<100 nM EC50) and
selective (>100-fold based on EC50 vs. 5-HT2A/5-HT2B or no intrinsic
agonist activity at either receptor) 5-HT2C agonists derived from
this scaffold. An additional requirement was to identify com-
pounds with acceptable PK profiles suitable for use in the treat-
ment of metabolic disorders such as obesity or diabetes.
Synthesis of the scaffold (5) was achieved in four steps starting
with the ring expansion of N-Boc piperidinone6 to give the keto-es-
ter (2) as a key cyclization precursor (Scheme 1). Reaction of (2)
with 2-aminopyrazole under acidic conditions led to the formation
of the tricyclic scaffold which was then chlorinated using POCl3.
The crude product was then re-protected using Boc2O to furnish
the building block (5) in good yield (41% over the 4 steps).
Derivatization of the scaffold (5) was readily achieved by reac-
tion with primary or secondary amines in ethanol solvent at 80 °C
followed by acid mediated de-protection to furnish the target
compounds (6–10) as their TFA salts.
One of the key issues for developing viable 5-HT2C receptor ago-
nists is the requirement for high levels of selectivity over the other
5-HT2 receptors. 5-HT2A agonism can result in CNS mediated hallu-
q
Published in part: Heather Tye, Stephan G. Mueller, Juergen Prestle, Stefan
Scheuerer, Marcus Schindler, Bernd Nosse, Natacha Prevost, Christopher J. Brown,
Alexander Heifetz, Clemens Moeller, Anna Pedret-Dunn, Mark Whittaker, Abstracts
of Papers, 239th ACS National Meeting, San Francisco, CA, United States, March
21–25, 2010 (2010), MEDI-213.
⇑
Corresponding author. Tel.: +44 (0)1235 861561.
E-mail address: heather.tye@evotec.com (H. Tye).
Present address: (OSI) Prosidion, Windrush Court, Watlington Road, Oxford OX4
6LT, U.K.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.11.089

H. Tye et al. / Bioorg. Med. Chem. Lett. 21 (2011) 34–37
35
H
H
N
R
S
Cl
N
H
N
H
N
Athersys
Lorcaserin
Vabicaserin
H
R
N
R
N
N
N
N
H
N
H
N
H
R
N
NPS Allelix
Forest Labs
1
Figure 1. 5-HT2C agonists containing a fused arylazepine motif.
O
N
O
O
H
O
O
Cl
N
O
O
N
N
N
b
c
N
a
N
N
N
H
N
O
O
O
O
2
3
4
1R
R2
Cl
N
N
N
O
O
N
N
d
N
e, f
N
N
N
H
5
6-10
Scheme 1. Synthesis of pyrazolopyrimidine azepine analogues. Reagents and conditions: (a) ethyl diazoacetate, BF3ÁEt2O ; (b) aminopyrazole, AcOH, 80 °C ; (c) POCl3, iPr2EtN,
90 °C; (d) Boc2O, Et3N, DCM; (e) R1R2NH, EtOH, iPr2EtN, 80 °C; (f) TFA/DCM.
The agonist activity of target compounds was evaluated in vitro
using a FLIPR based assay employing recombinant CHO cells
expressing the human 5-HT2A, 5-HT2B or 5-HT2C receptors.7
A preliminary exploration of simple amine analogues (Table 1)
led to the identification of azetidine derivative (6a) as a potent
5-HT2C agonist with acceptable selectivity over 5-HT2A (>100-fold,
partial agonist) but poor selectivity over 5-HT2B (<20-fold, full ago-
nist). Taking this compound as a starting point we explored a num-
ber of substituted azetidines which led to the discovery of phenyl
ether (7a) which retained a good level of potency and 5-HT2A
selectivity and had a reduced level of activity at 5-HT2B (partial
agonist).
We then embarked on the synthesis of a library of aryl ether
analogues. Data for a sub-set of compounds (7a–g) from the library
is shown in Table 2. Aryl groups bearing electron withdrawing
groups (e.g., F) tended to retain good potency whereas compounds
possessing electron donating groups (e.g., OMe) tended to be less
potent. Whilst all analogues retained a good level of 5-HT2A selec-
tivity the position of the substituent on the aryl ring had varying
effects on the 5-HT2B selectivity, substitution at the 3- or 4-posi-
tion giving the best results.
Compounds from this first library were profiled in additional
in vitro ADME-Tox assays including permeability (Caco-2) and
hERG assays. Whilst all of the analogues tested showed good levels
of Caco-2 permeability they exhibited strong levels of hERG
inhibition (>80% at 10 lM). This finding was not entirely surprising
considering the compounds have structural features which match
quite well with established hERG pharmacophores8 and their
lipophilicity is in an undesirable range (c Log P >3). This is also con-
sistent with data published recently by Fish et al. who reported
high levels of hERG inhibition for compounds bearing a related
azetidine aryl ether motif.9
In an attempt to address the hERG liability for this series a sec-
ond library of compounds was prepared incorporating polar groups
on the aryl ring or replacing the aryl ring with heteroaryls in order
to reduce lipophilicity (Table 2). 2-Pyridyl analogue (8a) retained
potency and showed a slight reduction in hERG inhibition, how-
ever the 3 and 4-pyridyl analogues (8b, 8c) gave a significant loss
in potency. The best results were achieved with acetamide (9) and
indolone (10) which showed much reduced hERG activity.
Unfortunately these compounds exhibited a drop in potency or
selectivity and, in the case of the acetamide (9), a significant drop

36
H. Tye et al. / Bioorg. Med. Chem. Lett. 21 (2011) 34–37
Table 1
5-HT2A, 5-HT2B, and 5-HT2C agonist activity for compounds 6a–e
R2
1R
N
N
N
N
H
N
6a-e
R1
R2
5-HT2C
5-HT2A
5-HT2B
a
a
a
Compound
EC50 (nM)
Emax (%)
EC50 (nM)
Emax (%)
EC50 (nM)
Emax (%)
6a
6b
6c
6d
6e
Azetidine
Pyrrolidine
Piperidine
5.4
103
110
139
105
104
2348
2310
585
nd
51
15
10
nd
62
94
2242
2848
842
97
36
28
69
97
764
4564
362
63
Me
H
Et
Et
138
602
a
Values reported as an average of n = 2 measurements (in triplicates) where SEM is typically below 20%.
Table 2
5-HT2A, 5-HT2B, and 5-HT2C agonist activity and in vitro ADMET data for compounds 7–10
H
Me
R
N
N
O
O
R
O
O
N
O
O
N
N
N
N
N
N
N
N
N
N
N
N
H
N
H
N
H
N
H
N
N
N
N
10
9
8a-c
7a-g
hERGb
c Log Pc
Caco-2d
a
a
a
Compound
R
5-HT2C
5-HT2A
5-HT2B
group
(% inhib @ 10 lM)
EC50 (nM)
Emax (%)
EC50 (nM)
Emax (%)
EC50 (nM)
Emax (%)
(A–B) Â 10À6 cm/s
Efflux ratio
7a
7b
7c
7d
7e
7f
7g
8a
8b
8c
9
H
2-F
3-F
4-F
2-MeO
3-MeO
4-MeO
2-Pyridyl
3-Pyridyl
4-Pyridyl
—
18
5
25
23
84
65
95
37
211
574
188
8.4
95
85
78
95
84
88
76
93
99
102
96
99
7774
648
32
22
15
5
8
48
9
21
29
94
27
35
86
218
94
13
58
69
23
43
89
43
31
71
73
79
60
82
81 (5)
nd
3.1
3.2
3.4
3.4
2.8
3.2
3.2
2.5
2.1
2.1
2.2
1.8
13.3
nd
7.9
nd
2.3
nd
1.6
nd
nd
2.1
1.3
2.2
nd
nd
6.9
nd
93 (1)
95 (2)
88 (1)
97 (1)
93 (1)
78 (4)
nd
nd
5 (1)
11 (2)
3
163
379
215
503
706
1478
1158
1184
73
2676
40
2634
5165
63
1227
2046
762
nd
10.1
10.4
10.3
nd
nd
0.62
nd
10
—
a
Values reported as an average of n = 2 measurements (in triplicates) where SEM is typically below 20%.
Whole-cell potassium currents were investigated electrophysiologically by means of the patch-clamp technique in CHO cells using PatchXpress. Average leak- and run-
b
down-corrected current inhibitions, measured upon application of the test compound at 10
l
M, (n = 2–4, SD is given in brackets).
c
c Log P values calculated using BioByte software.
d
Caco-2 A–B flux values reported as an average of two measurements with SD typically in the range of 30% or less. Efflux ratio expressed as a ratio of the B–A/A–B values.
Table 3
Rat PK data for compound 7g (iv—male Wistar rats (n = 2), dose 10 mg/kg; po—fasted male Wistar rats (n = 3), dose 30 mg/kg)
Compound
Cl
Vss
(L/kg)
MRT (iv, h)
MRT
(po, h)
AUC
dn (po, nMh)
CSF/Plu
ratioa
(ml/min/kg)
7g
14
11
13
>24
720
0.15
a
CSF and plasma levels measured at 2 h post dose after 30 mg/kg po administration. Plu is fraction unbound in plasma based on % unbound in rat plasma as determined by
equilibrium dialysis method (data not shown).

H. Tye et al. / Bioorg. Med. Chem. Lett. 21 (2011) 34–37
37
Figure 2. Data for compound (7g) and Lorcaserin in the 3 day cafeteria diet model.
in Caco-2 permeability compared to earlier analogues. Conse-
quently these analogues were not considered to be suitable for
further profiling.
At this stage we were looking to identify a suitable tool com-
pound to establish whether compounds of this structural class
would be efficacious in vivo. Despite its high level of hERG inhibi-
hERG ion channel. Whilst some compounds, bearing polar side
chains did show a much reduced hERG liability this came at the ex-
pense of potency and permeability. Further research efforts are fo-
cussed on identifying compounds which are not limited by such off
target activity issues.
tion (0.4 lM in manual patch-clamp) 4-MeO analogue (7g) showed
References and notes
the best overall profile. It met the potency and selectivity criteria
we had set based on its in vitro profile (<100 nM 5-HT2C, weak
partial agonism at 5-HT2A and 5-HT2B) and it was found to be
neutral at 5-HT2A and an antagonist at 5-HT2B in ex vivo tissue
assays.10,11 In addition this compound exhibited an excellent rat
PK profile (Table 3) having low clearance and long exposure after
oral administration (MRT >24 h). Based on the good CSF/Plasma-
unbound ratio of 0.15 we expected this compound to be brain pe-
netrant and therefore effective in vivo.
Compound (7g) was found to efficacious in a 3 day weight loss
experiment where the compound was administered orally to rats
fed on a so-called ‘cafeteria’ chocolate based diet. Statistically sig-
nificant weight loss of 3.3% (P = 0.0014) relative to the cafeteria fed
control group was observed at a dose of 100 mg/kg which com-
pared well to the 3.2% (P = 0.06) weight loss observed with Lorcas-
erin dosed at 30 mg/kg (Fig. 2).
Female Wistar rats (ex-breeder; n = 6 each group) were fed a
cafeteria diet for three consecutive days. The diet consisted of dif-
ferent chocolate bars and cookies add libitum on top of standard
chow diet. Animals were randomized to body weight at day one.
Compound 7g, Lorcaserin, and vehicle control (0.5% natrosol) was
applied once daily (po application) and body weight was assessed
every day. Control chow received standard chow diet only.
In summary we have identified a novel series of 5-HT2C agonists
based on a tricyclic pyrazolopyrimidine scaffold. Compounds with
good levels of in vitro potency and moderate to good levels of
selectivity with respect to the 5-HT2A and 5-HT2B receptors were
identified. One of the analogues (7g) was found to be efficacious
in a sub-chronic weight loss model. However a key limitation of
the series of compounds was found to be their inhibition of the
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