Indole-3-piperazinyl derivatives: Novel chemical class of 5-HT6 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2010.11.001

N₁-Arylsulfonyl-3-piperazinyl indole derivatives were designed and identified as a novel class of 5-HT₆ receptors ligands. All the compounds have high affinity and antagonist activity towards 5-HT₆ receptor. The compound 7a (K_i = 3.4 nM, functional assay IC ₅₀ = 310 nM) shows enhanced cognitive effect when tested in NORT and Morris water maze models. Synthesis, SAR and PK profile of these novel compounds constitute the subject matter of this Letter.

Full text of DOI:10.1016/j.bmcl.2010.11.001

Bioorganic & Medicinal Chemistry Letters 21 (2011) 346–349
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Indole-3-piperazinyl derivatives: Novel chemical class of 5-HT6
receptor antagonists
Ramakrishna V. S. Nirogi a, , Amol D. Deshpande a,e, Ramasastri Kambhampati a, Rajesh Kumar Badange a,
⇑
Laxman Kota a, Anand V. Daulatabad a, Anil K. Shinde a, Ishtiyaque Ahmad b, Vishwottam Kandikere c,
Pradeep Jayarajan d, P. K. Dubey e
a Discovery Research - Medicinal Chemistry, Suven Life Sciences Ltd, Serene Chambers, Road-5, Avenue-7, Banjara Hills, Hyderabad 500 034, India
b Discovery Research - In-vitro Biology, Suven Life Sciences Ltd, Serene Chambers, Road-5, Avenue-7, Banjara Hills, Hyderabad 500 034, India
c Discovery Research - Drug Metabolism and Phamacokinetics, Suven Life Sciences Ltd, Serene Chambers, Road-5, Avenue-7, Banjara Hills, Hyderabad 500 034, India
d Discovery Research - In-vivo Biology, Suven Life Sciences Ltd, Serene Chambers, Road-5, Avenue-7, Banjara Hills, Hyderabad 500 034, India
e Department of Chemistry, Jawaharlal Nehru Technological University Hyderabad, Kukatpally, Hyderabad 500 085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
N1-Arylsulfonyl-3-piperazinyl indole derivatives were designed and identified as a novel class of 5-HT6
receptors ligands. All the compounds have high affinity and antagonist activity towards 5-HT6 receptor.
The compound 7a (Ki = 3.4 nM, functional assay IC50 = 310 nM) shows enhanced cognitive effect when
tested in NORT and Morris water maze models. Synthesis, SAR and PK profile of these novel compounds
constitute the subject matter of this Letter.
Received 6 August 2010
Revised 8 October 2010
Accepted 1 November 2010
Available online 5 November 2010
Ó 2010 Published by Elsevier Ltd.
Keywords:
5-HT6 receptor ligands
Antagonists
Cognition
PK profile
5-Hydroxytryptamine6 receptor (5-HT6R), the family member
of G-protein coupled receptors (GPCR)1,2 is present in various re-
gions of the brain and is associated with learning and memory.
Blockade of their function increases acetylcholine and glutamate-
mediated neurotransmission and enhances the cognitive pro-
cesses, which amply demonstrate the therapeutic usefulness of
this receptor for CNS mediated disorders such as schizophrenia,
AD and also in obesity and eating disorders.3–12
O
O
S
NH
Cl
CH3
H
N
N
N
S
N
S
O
O
O
N
H
SB-271046
SB-742457
F
F
As it was shown by various in vivo studies that 5-HT6R active
ligands play important role in cognition and memory forma-
tion,13–20 since then the tremendous progress has been made in
identifying novel 5-HT6 receptor ligands. A number of 5-HT6 recep-
tor antagonists are in advanced stage of clinical development
CH3
N
H
O
O
CHF2
O
S
N
O
H3C
N
H
F
N
N
H
(Fig. 1). SAM-53121, GSK-742457, LY-483518 and SB-2710468,22–
Lu AE58054
SAM-531
24
are the phase II clinical candidates, where as Lu AE5805425
,
Figure 1. 5-HT6 receptor antagonists.
PRX-07034, SYN-1148,22–24 are in phase I study. SUVN-502, our in
house discovered clinical candidate, has recently completed its
phase I study for cognitive impairment in Schizophrenia and AD.26
All the reported ligands have some common pharmacophoric
features like basic ionisable amine functionality and two hydro-
phobic aromatic sites, which are the basic requirements needed
for binding at 5-HT6 receptor.4 In our recent publications, we have
disclosed a series of 2-arylsulfonylmethyl-3-piperazinylmethylin-
dole derivatives27 and aryl amino sulfonamide derivatives28 as
the 5-HT6 receptor ligands. The prior series belongs to the non-sul-
fonamide class of compounds, which have mild to moderate affin-
ity towards the receptor while the latter sulfonamide class of
compounds are potent and selective 5-HT6 receptor antagonists
⇑
Corresponding author. Tel.: +91 40 23556038/23541142; fax: +91 40 23541152.
E-mail address: ramakrishna_nirogi@yahoo.co.in (R.V.S. Nirogi).
0960-894X/$ - see front matter Ó 2010 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2010.11.001

R. V. S. Nirogi et al. / Bioorg. Med. Chem. Lett. 21 (2011) 346–349
347
Table 1
with the good ADME profile. Another recent publication from the
5-HT6 receptor binding dataa
Korea research institute of chemical technology29 has highlighted
the sulfonamide compounds with 5-HT2C/5-HT6 dual antagonist
activity.
R3
N
4
Also in our earlier effort, we disclosed a series of N1-arylsulfo-
nyl-3-piperazinylmethyl indole derivatives30 as potent 5-HT6
receptor antagonists (Fig. 2). In this particular series both the nitro-
gen are highly basic in nature. In continuation to explore the SAR
study around these derivatives and to establish the impact of re-
moval of ‘methylene’ spacers in the side-chain with respect to 5-
HT6 receptor binding affinity, the new series of N1-arylsulfonyl-3-
piperazinyl indole derivatives were conceived. Thus the present
series was designed to understand the impact of reduced basicity
of one of the nitrogens of piperazine attached to the third position
of the indole nucleus on the binding affinities as well as on the
ADME profile of these molecules.
The general synthetic strategy used for the title Compounds 7a–
7t was summarized in Scheme 1. Various substituted N1-acetyl in-
doxyl derivatives 4 were synthesized according to the literature
methods.31,32 More often, the indoxyls were synthesized starting
from variously substituted anthranilic acids and chloro acetic acid
or from 2-chloro benzoic acid and glycine to obtain 2-carboxyphe-
nylglycine derivatives 2, which were further cyclized in acetic
anhydride to 1-acetyl-3-acetyloxy indole derivatives 3. Selective
hydrolysis of these derivatives resulted in indoxyl derivatives 4.
Substituted N1-acetyl indoxyl derivatives 4 were treated with
titanium isopropoxide, N-alkyl piperazines and sodium triacetoxy-
borohydride to obtain the corresponding substituted N1-acetyl-3-
piperazinyl indole derivatives 5. These indole derivatives were
then deacetylated using inorganic bases like KOH or NaOH in polar
protic solvents under reflux conditions to obtain the deacetylated
product 6. Various alternative methods were used for the N1 sulf-
onylation of indole. The best system was found to be the sodium
hydride and DMF. Alternatively, potassium hydroxide and THF
was also found to be appropriate for obtaining target compounds
7a–7t in good yields (see Supplementary data).
5
R1
N
3
6
2
7
N1
6'
S O
O
5'
4'
R2
2'
3'
Compound
R1
R2
R3
5-HT6R Ki (nM)
7a
7b
7c
7d
7e
7f
7g
7h
7i
H
H
H
H
H
H
5-Br
5-Br
5-Br
5-Br
5-Br
5-Br
5-Cl
5-Cl
5-Cl
6-Cl
6-Cl
6-Cl
6-Cl
6-Cl
40-iPr
40-F
CH3
CH3
CH3
CH3
CH3
C2H5
CH3
CH3
C2H5
CH3
3.4
11.8
10.8
15.2
15.9
8.6
30-Cl
20-Br
H
40-iPr
30-Cl
40-F
7.8
9.2
40-F
10.6
23.2
30.1
10.2
10.7
20.3
30.5
82.3
55.3
63.5
25.5
30.2
7j
7k
7l
40-iPr
40-iPr
20-Br
40-F
20-Br
40-iPr
40-OMe
40-iPr
C2H5
CH3
7m
7n
7o
7p
7q
7r
7s
7t
C2H5
C2H5
C2H5
C2H5
CH3
C2H5
C2H5
CH3
2,5-di-OMe
20-Naphthyl
20-Br
Displacement of [3H]-LSD binding to cloned human 5-HT6 receptors stably
expressed in HeLa cells, Ki values were determined in duplicates and mean values
a
are reported.
All the synthesized compounds were tested in a standard radi-
oligand-binding assay using human-cloned 5-HT6 receptors stably
transferred in HeLa cells to determine affinity for 5-HT6 receptor. Ki
values are summarized in Table 1. For the initial SAR study, the
unsubstituted indole moiety was undertaken first. Different substi-
tuted sulfonyl chlorides were used for sulfonylation at N1 of indole
nucleus. Among the derivatives synthesized, compound 7a, which
is 40-isopropyl benzene sulfonyl derivative, was found to have the
highest affinity (Ki = 3.4 nM), while its unsubstituted sulfonyl
analog (7e, Ki = 15.9 nM) has shown fivefold decrease in binding
affinity at 5-HT6 receptor. Replacement of isopropyl group in
compound 7a with fluoro (7b, Ki = 11.8 nM) reduced the binding
affinity by three fold. The analogs 7c (Ki = 10.8 nM) and 7d
(Ki = 15.2 nM), with halo substitution in the benzene sulfonamide
ring, show reduction in the binding affinities by 4–5-fold as com-
pared to 7a. Replacement of methyl group at piperazinyl nucleus
R3
R3
N
N
N
N
R1
R1
N
S
N
S
O
O
O
O
R2
R2
Figure 2. Genesis of ligands.
COOH
COONa
O
OCOCH3
R1
R1
a
b
c
R1
R1
NH2
N
COONa
50 %
N
N
80%
60 %
H
1
2
O
CH3
O
CH3
3
4
R3
R3
R3
N
N
N
d
N
N
e
N
f
R1
R1
R1
50-70%
80%
N
N
N
S
70-85%
H
O
CH3
O
O
6
R2
5
7a-7t
Scheme 1. Synthesis of compounds 7a–7t. Reagents and conditions: (a) ClCH2COOH, water, NaOH, heat; (b) acetic anhydride, sodium acetate, H2O, DMF, heat; (c) Na2SO3, abs
ethanol, heat; (d) titanium isopropoxide, N-alkyl piperazine, NaBH(OAc)3, ethanol, rt; (e) KOH, ethanol, reflux; (f) NaH, DMF, ArSO2Cl, rt.

348
R. V. S. Nirogi et al. / Bioorg. Med. Chem. Lett. 21 (2011) 346–349
with alkyl groups. Compounds 7g (R2 = 30-Cl), 7h (R2 = 40-F) and 7l
Table 2
Functional activity dataa
(R2 = 20-Br) had Ki values 7.8, 9.2 and 10.2 nM, respectively, while
compound 7j (R2 = 40-iPr) has a Ki value of 23.2 nM. Almost similar
pattern was observed for the 5-chloro and 6-chloro substituted in-
dole derivatives 7m, 7n, 7o and 7t.
Compound
Kb (nM)
IC50 (nM)
7a
0.53
0.5
0.6
3.0
3.5
310
292
322
1528
1783
7b
7h
7g
7m
Replacement of R3 methyl group with ethyl, in 5-halo and 6-
halo substituted indole sulfonamide derivatives, shows marginal
reduction or no effect on binding affinity as can be seen by compar-
ing the Ki values of compounds 7h (Ki = 9.2 nM) and 7j
(Ki = 23.2 nM) with 7i (Ki = 10.6 nM) and 7k (Ki = 30.1 nM),
respectively.
a
Kb values were determined using non-radioactive
cell-based assay. IC50 and Kb values are the mean of two
experiments.
By comparing the SAR data of these derivatives, we came to the
conclusion that in unsubstituted indole derivatives, electron
donating groups in benzene sulfonamide ring (R2) were more pre-
ferred for 5-HT6R binding over the halogens, while the reverse was
the case for the 5-halo and 6-halo substituted indole derivatives.
The most potent compounds7a, 7b, 7h, 7g and 7m from the ser-
ies, were further evaluated for their functional activity at 5-HT6
receptor using non-radioactive CHO cell based assay. All the tested
compounds have shown antagonist activity by inhibiting the 5-HT
stimulated cAMP accumulation as can be seen from the IC50 and Kb
values (Table 2).
These compounds were further profiled for their selectivity
against a panel of receptors, inclusive of other 5-HT receptor sub-
types, dopamine D2, histamine H1 and the transporters like SERT
and NET. All the compounds are 50–100-fold selective over all
the tested receptors (data not shown).
Table 3
Human cytochrome P450 inhibitory data and microsomal metabolic stability
Compound
IC50
CYP2D6
(l
M)
% Metabolism in liver microsomes
CYP3A4
Human
Rat
7a
7b
7h
39.2
26.4
28.8
5.54
4.77
4.61
80
85
96
95
99
99
The cytochrome P450 inhibitory potential was determined using isoform-selective
assays and heterologously expressed human CYP2D6 and CYP3A4. These values are
the mean of duplicate determinations. Microsomal metabolic stability in Wistar rat
and human at 0.5 h, Concn 2.5 lM.
with ethyl (7f, Ki = 8.6 nM) resulted in twofold decrease in binding
affinity.
Substitutions at 5 and 6-position of indole nucleus with differ-
ent halo groups like bromo and chloro were well tolerated. In the
case of 5-bromo substituted derivatives, the compounds with halo
substitution at ortho, meta -and para-position in benzene sulfon-
amide ring have shown better affinity compared to the compounds
Compounds 7a, 7b and 7h were further evaluated for their met-
abolic stability and CYP liabilities. All the three compounds have
extensively metabolized in rat and human liver microsomes. The
values were summarized in Table 3. The effect of compounds 7a,
7b and 7h on cytocrome 450 enzymes were determined in liver
Table 4
PK profile in male Wistar ratsa
Compound 7a
AUC (ng h/mL)
Route
n
Dose (mg/kg)
Cmax (ng/mL)
t1/2 (h)
Vz (L/kg)
Cl (mL/min/kg)
F (%)
8 0.7
Oral
iv
3
3
5
5
40 30
365 90
59 25
674 257
2.35 0.21
4.47 0.3
436 203
51.8 19.7
2200 1100
132.8 44.2
a
Fasted male wistar rats, vehicle used: water for injection for both oral and iv routes. Dosing volumes: 10 mL/kg for oral and 2 mL/kg for iv.
Familiar object vehicle (1 mL/kg, p.o.)
Novel object vehicle (1 mL/kg, p.o.)
Familiar object (compound 7a 10 mg/kg, p.o.)
Novel object (compound 7a 10 mg/kg, p.o.)
30
*
25
20
15
10
5
0
Compound 7a 10 mg-kg
Vehicle
Data represents mean SEM of exploration time (Paired ‘t’ test), *P<0.05). Vehicle-
PEG 400 50% v/v; 1 mL/kg, p.o. n=6-9 /group, p.o., dosing vehicle/drug: 60 min
prior to test (p.o.).
Figure 3. Novel object recognition test for compound 7a in male Wistar rats.

R. V. S. Nirogi et al. / Bioorg. Med. Chem. Lett. 21 (2011) 346–349
349
Vehicle (1mL/kg, p.o.) + Scopolamine (0.4 mg/k g, s.c.)
70000
60000
50000
40000
30000
Donepezil (3mg/kg, s.c.)+Scopolamine (0.4 mg/kg, s.c.)
Compound-7a (10 mg/kg, p.o.)+Scopolamine (0.4 mg/kg, s.c.)
*
*
*
*
1
2
3
4
Day
Figure 4. Effect of acute treatment of compound 7a in Morris water maze for Latency to target in rats. Data represents Mean SEM of latency to target, *p <0.05, (One Way
ANOVA, Dunnett’s post hoc analysis).
microsomes, the IC50 values for all three compounds were found to
be >25 M for 2D6, while for 3A4 it was nearly 5 M (Table 3).
References and notes
l
l
1. Hoyer, D.; Clarke, D. E.; Fozard, J. R.; Harting, P. R.; Martin, G. R.; Saxena, P. R.;
Humphrey, P. Pharmacol. Rev. 1994, 46, 157.
2. Glennon, R. A. J. Med. Chem. 2003, 46, 2795.
3. Sebben, M.; Ansanay, H.; Dumuis, A. Neuroreport 1994, 5, 2553.
4. Holenz, J.; Pauwels, P. J.; Diaz, J. L.; Merce, R.; Codony, X.; Buschmann, H. Drug
Discovery Today 2006, 11, 283.
5. Davies, S. L.; Silvestre, J. S.; Guitart, X. Drugs Future 2005, 30, 479.
6. Fisas, A.; Codony, X.; Romero, G.; Dordal, A.; Giraldo, J.; Merce, R.;
Holenz, J.; Heal, D.; Buschmann, H.; Pauwels, P. J. Br. J. Pharmacol. 2006,
148, 973.
7. Roth, B. L.; Craig, S. C.; Choudhary, M. S.; Uluer, A.; Monsma, F. J., Jr.; Shen, Y.;
Meltzer, H. Y.; Siblev, D. R. J. Pharmacol. Exp. Ther. 1994, 268, 1403.
8. Werner, J. G.; Cornelis, J. V. Curr. Top. Med. Chem. 2008, 8, 1035.
9. Reavill, C.; Rogers, D. C. Curr. Opin. Invest. Drugs 2001, 2, 104.
10. Christopher, N. J.; Mahmood, A.; Neil, D. M. Curr. Opin. Drug Discov. Dev. 2008,
11, 642.
11. Geldenhuys, W. J.; Schyf, C. V. Exp. Rev. Neurother. 2009, 9, 1073.
12. Witty, D.; Ahemad, M.; Chuang, T. T. Prog. Med. Chem. 2009, 48, 163.
13. Sleight, A. J.; Boess, F. G.; Bos, M.; Levet-Trafit, B.; Riemer, C.; Bourson, A. Br. J.
Pharmacol. 1998, 124, 556.
Compound 7a was further selected for determining its pharma-
cokinetics profiling in male Wistar rats. Compound 7a displayed
lower oral exposure with AUC 59 ng h/mL, moderate half-life (t1/
2 = 2.35 h), high clearance (2200 mL/min/kg) with large volume of
distribution (436 L/kg), which resulted in moderate bioavailability
(%F) of 8% following the oral administration of 5 mg/kg dose (Table
4). The low oral bioavailability of these compounds may be be-
cause of their poor metabolic stability in rat liver microsomes.
Compound 7a was further evaluated for its cognitive potential
in novel object recognition (NORT) paradigm and Morris water
maze. Compound 7a has shown improvement in cognitive perfor-
mance at 10 mg/kg oral dose in NORT model (Fig. 3) while in Mor-
ris water maze model, it has significantly reversed the scopolamine
induced memory deficit at 10 mg/kg ip dose, which was apparent
from lesser target latency (Fig. 4).
In summary, a novel series of 3-piperazinyl-N1-indolesulfona-
mide derivatives were designed and identified as 5-HT6 receptor
antagonist ligands. Compound 7a has shown pro-cognitive poten-
tial when tested in NORT and Morris water maze models. Most of
these derivatives are 50–100-fold selective over the other tested
receptors. High metabolism of these compounds could be one of
the reasons for low bioavailability. So the further efforts are in pro-
gress towards the prediction of metabolism sites and blocking
them with appropriate substitutions so as to improve upon the
metabolism and thereby make these molecules more bioavailable
while maintaining the affinity towards 5-HT6 receptor.
14. Boess, F. G.; Riemer, C.; Bos, M.; Bentley, J.; Bourson, A.; Sleight, A. J. Mol.
Pharmacol. 1998, 54, 577.
15. Rogers, D. C.; Hagan, J. J. Neuropsychopharmacol. 2001, 158, 114.
16. Perez, G. G. Pharmacol. Biochem. Behav. 2005, 81, 673.
17. Meneses, A. Drug News Perspect. 2001, 14, 396.
18. Marcos, B.; Chuang, T. T.; Gil-Bea, F. J.; Ramirez, M. J. Br. J. Pharmacol. 2008, 155,
434.
19. Costa, V. D. S.; Duchatelle, P.; Boulouard, M.; Dauphin, F.
Neuropsychopharmacology 2009, 34, 488.
20. Upton, N.; Chuang, T.; Hunter, A. J.; Virley, D. J. Neurotherapeutics 2008, 5, 458.
21. Robichaud, A. J. 239th ACS National Meeting and Exposition, Mar 21–25, 2010;
San Francisco, CA. MEDI-34.
22. Heal, D. J.; Smith, S. L.; Fisas, A.; Buschmann, H. Pharmacol. Ther. 2008, 117, 207.
23. Liu, K. G.; Robicchaud, A. J. Drug Dev. Res. 2009, 70, 145.
24. Rosse, G.; Schaffhauser, H. Curr. Top. Med. Chem. 2010, 10, 207.
25. Arnt, J.; Andersen, B. B.; Bymaster, F. P.; Chohen, M. P. 2nd Biennial
Schizophrenia international Research Conference (SIRS) April 10–14th, 2010;
Florence, Itlay.
Acknowledgments
26. Nirogi, R.; Kambhampati, R.; Shinde, A.; Kandikere, V.; Mudigonda, K.;
Bhyrapuneni, G.; Jayarajan, P.; Abraham, R.; Mulla S.; Jasti, V. ICAD 2009,
Vienna, Austria.
27. Nirogi, R.; Deshpande, A. Int. J. Pharm. Tech. Res. 2010, 2(3), 2090.
28. Nirogi, R.; Daulatabad, A. Bioorg. Med. Chem. Lett. 2010, 20, 4440.
29. Park, C. M.; Kim, S. O. Bioorg. Med. Chem. Lett. 2010, 20, 5221.
30. Nirogi, R. et.al. WO2004/048330.
The authors thank Analytical department and acknowledge the
support received from Mr. Venkateswarlu Jasti, CEO, Suven Life
Sciences Ltd, Hyderabad, India.
Supplementary data
31. Conway, S. C.; Gribble, G. W. Heterocycles 1990, 30, 627.
32. Su, H. C. F.; Tsou, K. C. J. Am. Chem. Soc. 1960, 82, 1187.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.11.001.