The discovery and SAR of indoline-3-carboxamides - A new series of 5-HT6 antagonists

Article abstract of DOI:10.1016/j.bmcl.2010.04.085

Antagonists of the 5-HT₆ receptor have been shown to improve cognitive function in a wide range of animal models and as such may prove to be attractive agents for the symptomatic treatment of cognitive disorders such as Alzheimer's disease (AD) and schizophrenia. We report herein the identification and SAR around N-(2-aminoalkyl)-1-(arylsulfonyl)indoline-3-carboxamides - a novel chemotype of 5-HT₆ antagonists.

Full text of DOI:10.1016/j.bmcl.2010.04.085

Bioorganic & Medicinal Chemistry Letters 20 (2010) 3713–3716
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
The discovery and SAR of indoline-3-carboxamides—A new series of
5-HT6 antagonists
a
a
a
a
c
Mark Reid a, , Ian Carlyle , Wilson L. Caulfield , Tom R. Clarkson , Fiona Cusick , Ola Epemolu ,
Robert Gilfillan a, Richard Goodwin a, David Jaap a, Elise C. O’Donnell a, Jeremy Presland b, Zoran Rankovic a,
Daniel Spinks a, Gayle Spinks a, Anne M. Thomson b, Fiona Thomson b, James Strain b, Grant Wishart a
*
*
,
a Department of Chemistry, Schering-Plough Research Institute, Newhouse, Lanarkshire ML1 5SH, UK
b Department of Molecular Pharmacology, Schering-Plough Research Institute, Newhouse, Lanarkshire ML1 5SH, UK
c Department of Pharmacology, Schering-Plough Research Institute, Newhouse, Lanarkshire ML1 5SH, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Antagonists of the 5-HT6 receptor have been shown to improve cognitive function in a wide range of ani-
mal models and as such may prove to be attractive agents for the symptomatic treatment of cognitive
disorders such as Alzheimer’s disease (AD) and schizophrenia. We report herein the identification and
SAR around N-(2-aminoalkyl)-1-(arylsulfonyl)indoline-3-carboxamides—a novel chemotype of 5-HT6
antagonists.
Received 15 February 2010
Revised 19 April 2010
Accepted 19 April 2010
Available online 24 April 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
5-HT6
GPCR
Antagonist
The 5-HT6 receptor is almost exclusively expressed within the
brain and is localised in areas important for memory formation
and habituation (striatum, nucleus accumbens, olfactory tubercle,
cerebral cortex and hippocampus).1 5-HT6 antagonists have been
shown to be effective in many diverse learning and memory para-
digms in rats, including novel object recognition, passive avoid-
ance, autoshaping, social recognition and Morris water maze,
demonstrating significant improvement in memory retention, con-
solidation and spatial learning.2–4
Reversal of scopolamine-induced deficits has indicated that an
enhancement of cholinergic function may be involved in mediating
the cognitive enhancing properties of 5-HT6 antagonists.5–7 In
addition, reversal of NMDA receptor antagonist-induced deficits
by 5-HT6 antagonists has suggested the involvement of glutama-
tergic mechanisms.8,9 The 5-HT6 receptor has also been reported
to modulate other neurotransmitter systems, including dopami-
nergic and noradrenergic but these effects are less well character-
ised. As well as reversing deficits in normal adult rats, 5-HT6
antagonists have been shown to reverse age-dependant cognitive
deficits in aged rats, for example in object recognition and water
maze tasks.6
impairment and schizophrenia. As a consequence, 5-HT6 antago-
nists have attracted considerable attention within the pharmaceu-
tical industry in recent years.
Our interest began with the identification of 1 as a potent 5-HT6
antagonist during an internal cross-screening programme. The
development of 5-HT6 receptor antagonists is an increasingly com-
petitive field10 and we were encouraged to find that indoline sulfon-
amides represented a novel chemotype. This compound has high
affinity (pKi = 8.1) for the 5-HT6 receptor, has good ligand efficiency
and has physicochemical properties consistent with a CNS drug
(Fig. 1). However, the compound also displayed generally poor
N
O
N
MW = 493
PSA11 = 54.3
cLogP12 = 3.6
N
S
O
LE13 = 0.32
Thus, 5-HT6 antagonists may have utility for the treatment of
cognitive dysfunction in conditions such as AD, mild cognitive
O
1
O
* Corresponding authors. Tel.: +44 1698 736113; fax: +44 1698 736187.
E-mail addresses: mark.reid@spcorp.com (M. Reid), zoran.rankovic@spcorp.com
(Z. Rankovic).
Figure 1. (See above-mentioned references for further information.)
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.04.085

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M. Reid et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3713–3716
Table 1
pharmacokinetic properties (e.g., human microsomal Clint
>270
l minÀ1 mgÀ1 protein). Hence, a limited exploratory pro-
l
gramme was initiated to rapidly explore SAR around this novel
chemotype as well as to identify an analogue with a PK profile suit-
able for preliminary validation studies in in vivo pre-clinical models.
The importance of the indoline core was confirmed by the
synthesis of the corresponding indole analogue, which showed
significantly lower affinity for the 5-HT6 receptor (pKi = 6.9).
Replacement of the sulfonamide group with either amide or ben-
zylic linkers was not tolerated in this series (data not shown).
Therefore, attention was then focused on variation of the diamine
amide portion and variation of the naphthalene sulfonamide
group.
The synthetic route used to prepare analogues with initial var-
iation of the diamine moiety is shown in Scheme 1. Thus racemic
indoline-3-carboxylic acid was prepared by reduction of the corre-
sponding indole-3-carboxylic acid. Sulfonamide formation was
then achieved by reaction with 4-methoxynaphthyl sulfonyl chlo-
ride. Amide coupling with various diamines (and deprotection
where necessary) was used to furnish the desired compounds
2a–l.
Table 1 shows selected examples of variation of the diamine
unit. Example 2a shows that a secondary amide is tolerated. Simi-
larly, increased steric bulk on the basic amine by incorporation of a
benzyl group (2b) can be achieved with little further loss in affinity
for the 5-HT6 receptor. Replacement of the piperidinyl amide with
bio-isosteres 2c–e generally gave moderate affinity and offered lit-
tle improvement with respect to 1. However, replacement of the
piperidinyl amide could be achieved with piperazine to give the
equipotent compound 2f. Furthermore, expanding the ring size of
the diamine unit from 6 to 7 afforded a marked increase in 5-HT6
receptor affinity (2h). Investigation of further open chain replace-
ments were also carried out (2i–l). This generally resulted in less
potent ligands although interestingly, incorporation of the imidaz-
ole fragment in 2l was achieved with no loss in binding affinity
compared to 2j, suggesting that attenuation of the pKa of the basic
nitrogen atom may be possible for optimisation of the physico-
chemical properties of the series.
Compound
Amine
5-HT6 pKi14
N
N
1
8.1
N
2a
2b
7.6
7.3
N
H
N
H
N
H
H
N
2c
6.8
7.1
N
N
H
N
2d
H
N
2e
7.5
N
H
H
N
N
2f
8.1
7.2
N
2g
O
N
2h
2i
8.9
7.7
7.1
N
NH
N
Variation of the sulfonamide portion was achieved via the syn-
thetic sequence shown in Scheme 2. The indoline carboxylic acid
was firstly reacted to form protected indoline amide 3, which
was then reacted with a variety of sulfonyl chlorides to afford
the desired sulfonamides in moderate yield.
N
H
H
N
2j
N
Table 2 shows a number of examples of aryl sulfonyl replace-
ments for 1. Generally, the 4-methoxy group on the naphthyl ring
could be replaced with the 5-Cl naphthyl group while maintaining
the affinity of 1 (4a). Removal of the substituents from the naph-
thyl ring was tolerated although resulted in a reduction of 5-HT6
affinity (4b and 4c) as did isosteric replacements for the naphthyl
ring system (4d and 4e). The requirement for a large lipophilic
group in this region of the molecule was suggested by the reduc-
tion in affinity when more polar heterocycles were introduced
(4f and 4g). Furthermore, replacement of the naphthyl ring with
H
N
H
N
O
N
2k
2l
7.5
7.1
N
N
either phenyl (4i) or thiophene (4j) resulted in a marked reduction
in affinity, although some of this affinity may be recovered by sub-
stitution on the phenyl ring (4k–q).
O
O
O
O
OH
OH
OH
NRR'
a
c
b
N
N
N
SO2
N
SO2
H
H
2
O
O
Scheme 1. Reagents and conditions: (a) Na, n-BuOH, reflux, 99%; (b) DIPEA, 4-methoxylnaphthyl sulfonyl chloride, DMF/H2O, rt, 94%; (c) amine, HATU, DIPEA, rt, 54–87%.

M. Reid et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3713–3716
3715
N
N
O
O
O
N
OH
N
a
b
N
N
N
H
H
SO2
4
3
R
Scheme 2. Reagents and conditions: (a) EDCI, HOBT, DIPEA, methyl-(1-methyl-piperidin-4-yl)-amine, DCM, rt, 58%; (b) RSO2Cl, DIPEA, DCM, rt, 32–64%.
Table 2
Table 2 (continued)
Compound
R =
5-HT6 pKi
Compound
R =
5-HT6 pKi
4l
7.7
7.6
7.0
1
8.1
OCF3
OMe
4m
4n
4a
8.3
CF3
Cl
4b
4c
7.5
7.6
O
4o
7.0
4d
4e
7.5
7.3
O
O
4p
4q
6.9
7.5
Ph
O
N
O
Ph
4f
6.5
6.9
7.2
N
N
Following the initial exploratory libraries, compounds were
profiled in a number of in vitro assays including solubility, micro-
somal stability and hERG binding affinity, on which basis 2h was
selected for further in vitro functional testing and in vivo profiling.
Thus 2h was separated into its constituent enantiomers using
chiral HPLC to afford (R)-2h and (S)-2h (Fig. 2).15
4g
4h
S
Cl
S
4i
4j
6.5
6.5
NH
O
N
MW = 465
PSA11 = 63.8
S
cLogP12 = 3.6
N
S
LE13 = 0.37
O
O
4k
7.6
(R)-2h
CF3
O
Figure 2. (See above-mentioned references for further information.)

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M. Reid et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3713–3716
Table 3
Shows selected in vitro data for (R)-2h
Compound
Microsomal stability
(Clint
l minÀ1 mgÀ1 protein)
Hepatic stability
(Clint
l minÀ1 106 cellsÀ1
hERG binding
assay16
Caco-2 permeability
(nm/s)
Plasma protein
binding human/rat
,
l
,
l
)
(R)-2h
Rat Clint = 105
Human Clint = 69
Rat Clint = 11
Human Clint = 7
pKi = 5.9
A–B <22
B–A = 47
99.4%/97.1%
Table 4
allowing us to quickly establish the SAR around this novel chemo-
type and resulted in the identification of (R)-2h. Homopiperazine
(R)-2h was profiled in a range of in vitro and in vivo assays and
was found to have an improved PK profile compared to compound
1 and demonstrated suitable pharmacokinetics for progression into
pre-clinical behavioural models, the results of which will be re-
ported elsewhere in due course.
Oral bioavailability of (R)-2h in male Wistar rats (dosed at 10 mg/kg po and 1 mg/kg
iv, vehicle: 5%, DMA: 95% saline)
Pharmacokinetic
parameters (plasma)
IV dose
(mean SD)
PO dose
(mean SD)
Elimination half life (t1/2 elim., h)
1.9 0.4
1.9 0.2
Volume of distribution at
steady state (Vss, l kgÀ1
)
Clearance (Cl, ml minÀ1 kgÀ1
Bioavailability (%)
)
16.5 1.2
23.5
Acknowledgements
Estimated fraction of the
dose absorbed (fabs, %)
29.0
We also acknowledge Brian Montgomery and the separation
science team for the separation of 2h into the constituent enantio-
mers using chiral HPLC.
Testing of the separated enantiomers in the 5-HT6 binding assay
showed a clear preference for the R-enantiomer ((R)-2h) (pKi = 9.2)
with the S-enantiomer ((S)-2h) (pKi = 7.5) being two orders of mag-
nitude less potent. Furthermore (R)-2h shows an increased ligand
efficiency compared to 1.
Compound (R)-2h was evaluated for stability in both rat and
human microsomes and hepatocytes and showed improved
in vitro PK properties compared to compound 1 in terms of both
human and rat data (Table 3). No significant cytochrome P450 inhi-
bition was detected across the major isoforms. Blockade of the
hERG channel was assessed in a [3H]dofetilide16 competition bind-
ing assay to give a pKi = 5.9 (>1000-fold less than the affinity for
the 5-HT6 receptor). Wider selectivity was assessed by testing
compound (R)-2h against a panel of 33 GPCRs, ion channels, en-
zymes and transporters. Excellent binding selectivity (>100-fold)
against all targets tested was noted including against the closely
related 5-HT2A, 5-HT2B and 5-HT2C receptor sub-types.
Chemical stability of (R)-2h was also explored due to initial
concerns about the potential of (R)-2h to oxidise to afford the cor-
responding indole. Aqueous stability was assessed over 24 h at
both pH 1.5 and 7.4 with no instability detected. Furthermore,
(R)-2h was found to be stable over a number of months on the
bench without any need for storage at sub-ambient temperature
or under inert atmosphere.
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B.; Selzer, P. J. Med. Chem. 2000, 43, 3714.
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14. Compounds were tested for their ability to inhibit [3H] LSD binding to human
5-HT6 receptors stably expressed in CHO cells, with data expressed as pKi
values, n P 2 in all cases.
In a functional assay17 (R)-2h was shown to be an antagonist at
the 5-HT6 receptor with a pEC50 = 7.5. In vivo, (R)-2h has a moder-
ate clearance and oral bioavailability in rat of 23.5% (Table 4). The
percentage of the dose absorbed was estimated to be 29% which is
likely to be a consequence of the compound’s poor permeability, as
indicated by Caco-2 data (Papp <22 nm/s; Table 3).
15. Absolute stereochemistry of (S)-2h was determined by X-ray crystallography.
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to displace tritiated dofetolide (a class III antiarrhythmic and potent hERG
blocker) in membrane homogenate from HEK-293 cells expressing the hERG
channel.
17. Compounds were evaluated for antagonistic activity at the human 5-HT6
receptor using CHO cells stably transfected with human 5-HT6 receptor and
In summary, compound 1 was identified by cross-screening.
Although having high affinity for the 5-HT6 receptor, compound
1 lacked the PK profile suitable for progression into pre-clinical
behaviour models. A rapid chemical exploration was undertaken
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on a FLIPR 384 (fluorometric imaging plate reader). Increased fluorescence in
response to agonist (5-HT) was inhibited by the presence of 5-HT6 antagonists.
Fluorescence readout was calculated as max–min responses and data
expressed as EC50 values.