N1-arylsulfonyl-3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole derivatives are potent and selective 5-HT6 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2004.10.064

The development of a series of N₁-sulfonyl-3-(1,2,3,6- tetrahydropyridin-4-yl)indole 5-HT₆ antagonists is described. Two analogs, 15g and 15y, had 0.4 and 3.0 nM affinity, and antagonized the production of adenylate cyclase at sub-nanomolar concentrations. A series of N₁-arylsulfonyl-3-(1,2,3,6-tetrahydropyridin-4-yl)indole derivatives was designed and synthesized. These compounds were shown to have high affinity for the 5-HT₆ receptor. Two analogs, 4-[3-(1,2,3,6-tetrahydropyridin- 4-yl)-1H-indole-1-sulfonyl]-phenylamine 15g and 4-[3-(1,2,3,6-tetrahydropyridin- 4-yl)-5-methoxy-1H-indole-1-sulfonyl]-phenylamine 15y, had 0.4 and 3.0 nM affinity, respectively, and antagonized the production of adenylate cyclase at sub-nanomolar concentrations.

Full text of DOI:10.1016/j.bmcl.2004.10.064

Bioorganic & Medicinal Chemistry Letters 15 (2005) 379–383
N1-arylsulfonyl-3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole
derivatives are potent and selective 5-HT6 receptor antagonists
Derek C. Cole,a,* John W. Ellingboe,a William J. Lennox,a Hossein Mazandarani,c
Deborah L. Smith,c Joseph R. Stock,a Guoming Zhang,c Ping Zhoub and Lee E. Schechterc
aChemical and Screening Sciences, Wyeth Research, Pearl River, NY 10965, USA
bChemical and Screening Sciences, Wyeth Research, Princeton, NJ08852, USA
cNeurosciences, Wyeth Research, Princeton, NJ08852, USA
Received 27 August 2004; revised 21 October 2004; accepted 21 October 2004
Available online 11 November 2004
Abstract—A series of N1-arylsulfonyl-3-(1,2,3,6-tetrahydropyridin-4-yl)indole derivatives was designed and synthesized. These com-
pounds were shown to have high affinity for the 5-HT6 receptor. Two analogs, 4-[3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole-1-sul-
fonyl]-phenylamine 15g and 4-[3-(1,2,3,6-tetrahydropyridin-4-yl)-5-methoxy-1H-indole-1-sulfonyl]-phenylamine 15y, had 0.4 and
3.0nM affinity, respectively, and antagonized the production of adenylate cyclase at sub-nanomolar concentrations.
ꢀ 2004 Elsevier Ltd. All rights reserved.
The serotonin (5-HT) receptors have been divided into
seven subclasses (5-HT1–7).1 The rat 5-HT6 receptor
was cloned in 1993 and found to consist of a 438 residue
peptide chain with <40% protein sequence homology
with the other 5-HT receptors.2 The human receptor
was subsequently cloned in 1996 and consists of 440
amino acid residues and shares 89% homology with
the rat.3 The 5-HT6 receptor belongs to a group, includ-
ing 5-HT4 and 5-HT7, which is positively coupled to
adenylyl cyclase.4
fonamides, Ro-04-6790 (1) and Ro-63-0563 (2) were
described (Fig. 1).7 Subsequently, a series of piperazi-
nyl-benzenesulfonamide antagonists SB-271046 (3),8
SB- 258585 (4),9 and SB-357134 (5) were disclosed.10
The agonist 2-ethyl-5-methoxy-N,N-dimethyltryptamine
(6) was reported by Glennon et al.11 Tsai and co-work-
ers first described N1-benzenesulfonyltryptamines as 5-
HT6 antagonists12 and N,N-dimethyl-N1-benzenesulfon-
yl-5-methoxytryptamine (7) was reported by Russell
et al.13 The first non-sulfonamide antagonist, 4-(2-bro-
mo-6-pyrrolidine-1-ylpyridine-4-sulfone)phenylamine
(8) was reported by Riemer et al.14 In the course of our
work we also discovered that N1-arylsulfonyltryptamine
derivatives had high affinity for the 5-HT6 receptor.12,13
Exploration of the SARled to the replacement of the
tryptamine aminoethyl group with the more rigid tetra-
hydropyridinyl group, as had previously been reported
for 5-HT1A and 5-HT2 receptors (Fig. 2).15 Herein, we
describe the synthesis and biological activities of this
Expression of the 5-HT6 receptor mRNA is exclusive to
the central nervous system, with highest density in the
cerebral cortex, nucleus accumbens, caudate-putamen
and hippocampus, with moderate densities in the thal-
amus and substantia nigra.5 Several antipsychotic agents
and antidepressants have high affinity for the 5-HT6
receptor.6 These two findings have sparked considerable
effort to understand the role of this receptor in treatment
of CNS disorders, including schizophrenia, depression,
and impaired learning and memory.
class of 5-HT6 receptor ligands16,17
.
In 1998, the first 5-HT6 antagonists, a series of bismeth-
ylaminopyrimidinyl- and bismethylaminopyridinyl-sul-
1. Chemistry
The synthesis of N1-arylsulfonyl-3-(1,2,3,6-tetrahydro-
pyridin-4-yl)-1H-indole compounds 12, 15, and 18 was
carried out as shown in Scheme 1. Condensation of an
indole with a 4-methyl- or 4-benzyl-piperidone in the
presence of potassium hydroxide in refluxing methanol
Keywords: 5-HT6 antagonists; 5-Hydroxytryptamine6 antagonists.
*
Corresponding author. Tel.: +1 845 602 4619; fax: +1 845 602
5561; e-mail: coledc@wyeth.com
0960-894X/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.10.064

380
D. C. Cole et al. / Bioorg. Med. Chem. Lett. 15 (2005) 379–383
afforded 3-(1-methyl- and 3-(1-benzyl-1,2,3,6-tetrahyd-
MeHN
N
NHMe
NH
H
ropyridinyl)-indole, 9 and 10, respectively.15 Sulfonyl-
ation by reaction with the appropriate sulfonyl chloride
in THF in the presence of potassium tert-butoxide gave
the desired N1-sulfonylindoles 12 and 13. The N1-benz-
ylindole, N1-benzoylindole, and N1-phenylindolecarbox-
amide compounds (12i–k; L = CH2, CO, CONH) were
prepared by coupling the indole 9 with benzyl bromide,
benzoyl chloride, and phenylisocyanate in the presence
of potassium tert-butoxide.
Ar
N
N
X
S
O
O
O
O
OMe
NH
S
Cl
3: Ar =
S
NH2
4: Ar = 4-iodophenyl
5: Ar = 2,5-dibromo-3-fluorophenyl
1: X = N
2: X = CH
N
Br
N
S
N
The N–H tetrahydropyridinyl compounds (15) were pre-
pared by coupling the substituted indoles with N-Boc-
piperidinone, to give the 3-(N-Boc-1,2,3,6-tetrahydropy-
ridin-4-yl)-1H-indoles 11, followed by sulfonylation and
removal of the Boc group by stirring with HCl in diox-
ane. When 4-acetylaminophenylsulfonyl chloride was
used, treatment with HCl to remove the Boc group led
to simultaneous removal of the acetyl group affording
the corresponding 4-aminophenylsulfonyl derivatives.
Hydrogenation of 3-(N-Boc-1,2,3,6-tetrahydropyridin-
4-yl)-1H-indole 11 over palladium on carbon in ethanol
gave the reduced 3-(N-Boc-piperidin-4-yl)-1H-indole 17,
which was sulfonylated and deprotected under the con-
ditions described above. The N-acyl piperidine com-
pound 16 was prepared by acylation of 15a with acetyl
chloride.
MeO
R2
O
O
N
R1
6: R1 = H; R2 = Et
7: R1 = PhSO2; R2 = H
NH2
8
Figure 1. Structures of 5-HT6 antagonists (1–5, 7–8) and agonist (6).
R1
NR1R2
N
double / single
bond
R2
N
N
O
O
Linker
SO2, CO, CH2
CONH
S
L
R3
Ar
2. Biology
Figure 2. Design of N1-arylsulfonyl-(1,2,3,6-tetra-hydropyridin-4-yl)-
1H-indoles.
All of the 3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-
indoles were evaluated for their ability to displace
[3H]-LSD from cloned human 5-HT6 receptors stably
expressed in HeLa cells.18,19 In general, the N1-aryl-
sulfonylindoletetrahydropyridinyl N–Me and N–H
derivatives (12, 15) had excellent affinity for the 5-HT6
receptor and higher affinity than the N1-unsubstituted
indole derivative (9; Ki = 98nM). The larger N-benzyl
derivative (13a; Ki = 246nM) had lower affinity, also
the requirement for a basic amine was confirmed by
the low affinity of the N-Ac (14a; Ki = 54nM) and
N-Boc (16; 64% inhibition at 1lM) derivatives. The
R1
R1
N
N
9: R1 = Me
10: R1 = Bn
11: R1 = Boc
O
R2
R2
a
N
N
H
H
R1
N
N1-phenylsulfonyl-3-piperidin-4-yl-1H-indole
deriva-
tives (18a and 18f; Ki = 12 and 20nM) had about 6-fold
lower affinity than the corresponding unsaturated
3-(tetrahydropyridin-4-yl) derivatives (15a and 15f;
Ki = 2 and 2.4nM) while in the 4-aminophenylsulfonyl
series the saturated analogs (18h and 18w; Ki = 3 and
2nM) had similar affinity to the unsaturated analogs
(15h and 15w; Ki = 3 and 1nM). Substitution of the
sulfonyl linker with a methylene (12i; Ki = 10nM), acyl
(12j; Ki = 99nM) or amido (12k; Ki = 118nM) group
led to 5-, 50-, and 60-fold reductions in affinity.
Similar outcomes were observed in the N1-phenylsulfon-
yltryptamine series where replacement of the phenyl-
sulfonyl moiety with benzyl and benzoyl groups
resulted in 3- and 11-fold loss in affinity, respectively13,20
(Table 1).
12: R1 = Me
13: R1 = Bn
14: R1 = Boc
R2
c
b
N
15: R1 = H
L
R3
H
Boc
N
N
11
N
b,c
N
d
SO2
H
Ph
17
18
In general, unsubstituted phenyl, and 2-, 3- and 4-
fluoro- and chlorophenylsulfonyl analogs had high
affinity, as did the 5-chlorothienyl derivative (15l;
Scheme 1. Reagents and conditions: (a) KOH, MeOH, 60ꢁC; (b)
R3SO2Cl, BnBr, PhCOCl, or PhNCO, t-BuOK, THF, rt; (c) 2N HCl,
dioxane, rt; (d) H2, EtOH, palladium on carbon.

D. C. Cole et al. / Bioorg. Med. Chem. Lett. 15 (2005) 379–383
381
Table 1. 5-HT6 binding affinity of 3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indoles
R1
N
S/D
R2
N
L
R3
a
5-HT6 Ki (nM)
Compd
R1
S/Db
L
R2
R3
9
Me
Me
Me
Me
Me
Me
Me
Me
Me
Bn
Boc
H
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
S
Bond
SO2
SO2
SO2
SO2
SO2
CH2
CO
H
H
Ph
98
12a
12b
12c
12d
12e
12i
H
2
10
10
1
0.1
1
H
2-Cl–Ph
2-F–Ph
3-Cl–Ph
4-F–Ph
Ph
H
1
H
1
H
5
1
H
10
99
1
12j
H
Ph
Ph
2
12k
13a
14a
15a
15b
15c
15f
15g
15h
15l
CONH
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
H
118 7.7
246 26
54 1.4
H
Ph
Ph
H
H
Ph
2 1
0.4 0.1
H
H
2-Cl–Ph
2-F–Ph
Ph
H
H
1
2.4
2
0.2
0
H
5-F
H
H
4-NH2–Ph
4-NH2–Ph
5-Cl-thiophen-2-yl
4-F–Ph
4-Cl–Ph
4-CF3-Ph
4-MeO-Ph
1-Naphthyl
4-NH2–Ph
4-Benzo[1,2,5]thiadiazolyl
2-Naphthyl
2-Naphthyl
n-Bu
0.4
1
H
6-Cl
H
3
H
3
0.3
0.1
1
15m
15n
15o
15p
15q
15r
15s
15t
15u
15v
15w
15x
15y
15z
15aa
15ab
16
H
5-F
5-F
H
3
H
14
20
18
4
H
2
H
H
3
H
5-F
6-NO2
H
0.3
2
H
31
5
H
1
H
H
27
19
6
H
5-MeO
H
5
H
77 2.4
H
5-F
5-Br
5-MeO
5-BnO
5-NO2
4-NH2–Ph
4-NH2–Ph
4-NH2–Ph
4-NH2–Ph
4-NH2–Ph
4-NH2–Ph
Ph
1
7
3
0.1
1
H
H
1
H
50 2.4
H
29
1
64%c
12
20
3
2
0.4
H
5,6-Methylenedioxy
H
Ac
H
18a
18f
18h
18w
H
Ph
Ph
1
H
S
5-F
6-Cl
5-F
1
H
S
4-NH2–Ph
4-NH2–Ph
0.2
0.2
H
S
2
a Displacement of [3H]-LSD binding to cloned h5-HT6 receptors stably expressed in HeLa cells.18,19 Mean of three determinations.
b S/D = single/double bond.
c % inhibition @ 1lM.
Ki = 3nM), while larger groups, for example, 4-trifluoro-
methyl, and 4-methoxy (15o–p; Ki = 20 and 18nM) had
weaker affinity. Replacement of the N1-phenylsulfonyl
group with 1-naphthylenesulfonyl (15q; Ki = 4nM) and
4-benzo[1,2,5]thiadiazolylsulfonyl (15s; Ki = 5nM) led
to compounds with similar affinity, while the 2-naph-
thylenesulfonyl (15t, 15u; Ki = 27, 19nM) and n-but-
Published data for the N1-arylsulfonyltrpytamine series
show the 4-amino analogs bind with between 1- and
15-fold enhanced affinity, relative to the phenylsulfonyl
parent, and a proposed binding mode places the amino
group proximal to Asp303 on TM7.21 In this N1-aryl-
sulfonyl-3-(tetrahydropyridin-4-yl)-1H-indole series the
4-aminophenylsulfonyl derivatives bind with similar
affinity as the phenylsulfonyl parents (15g and 12a;
Ki = 2nM).
ylsulfonyl
(15v;
Ki = 77nM)
derivatives
had
significantly lower affinity. This is in contrast to the
N1-sulfonyltryptamine series where the 1- and 2-naph-
thylenesulfonyl derivatives had similar affinity to the
N1-phenylsulfonyl analogs.12,21
The effect of indole substitution can be observed in the
4-aminophenylsulfonyl series (15g; Ki = 2nM). Halo

382
D. C. Cole et al. / Bioorg. Med. Chem. Lett. 15 (2005) 379–383
Table 2. Antagonism of cAMP production
affinity to their phenylsulfonyl parents, they are signifi-
cantly more potent antagonist. In particular, 15g and
15y showed complete inhibition of cAMP production
at sub-nanomolar concentrations.
Compd
cAMP assay
IC50 (nM)
Imax (%)
12a
12b
12c
12d
12e
12i
13.8 2.6
0.7 0.1
3.9 1.6
9.4 0.4
42.7 8.9
14.8 0.1
100
83
0
2
1
2
1
0
1
1
0
0
0
1
1
2
2
1
0
1
0
1
1
1
0
The published literature does not contain enough data
on the functional activity of 4-aminophenylsulfonyl
derivatives relative to their phenylsulfonyl parents to
indicate that the 4-amino moiety generally enhances
antagonist activity. However, it is interesting to note
the 4-aminophenylsulfonyl moiety is present in the lead
compounds (1, 2, and 8) in several different series.
87
95
83
76
15a
15b
15c
15g
15h
15l
43
2
96
97
8.8 0.4
2.7 0.7
0.8 0.1
11.2 3.7
14.1 1.8
100
97
105
83
The binding affinity of selected compounds for a panel
of receptors is shown in Table 3. In general, this
class of 5-HT6 antagonists displayed low affinity for
5-HT1A, 5-HT1B, 5-HT1D, 5-HT1F, 5-HT2A, 5-HT2c,
5-HT7, dopamine D2, D3, and D4 receptors.
15m
15q
15s
15u
15w
15x
15y
15ab
18a
18h
18w
31
9.0 1.4
169
4
87
85
6
65
97
4.1 0.6
9.7 0.4
33.7 1.2
0.4 0.1
1.7 0.1
19.5 2.5
8.5 1.1
1.3 0.2
100
102
100
99
In conclusion, an interesting class of N1-arylsulfonyl-
tryptamine analogs in which the aminoethyl chain is rig-
idified into a tetrahydropyridinyl ring were designed.
Synthesis and biological evaluation indicate they are
selective 5-HT6 receptor antagonists. The N1-40-amino-
phenylsulfonyl moiety appears to be important for
enhanced functional activity, with 4-[3-(1,2,3,6-tetra-
hydro-pyridin-4-yl)-1H-indole-1-sulfonyl]-phenylamine
15g and 4-[3-(1,2,3,6-tetrahydro-pyridin-4-yl)-5-meth-
oxy-1H-indole-1-sulfonyl]-phenylamine 15y having
sub-nanomolar inhibition of adenylate cyclase
production.
54
98
100
Inhibition of 5-HTstimulated cAMP production in HeLa cells
stably transfected with human 5-HT6 receptors. Mean of three
determinations.
(15h, 15w and 15x; Ki = 3, 1, and 7nM) or methoxy
(15y; Ki = 3nM) groups are tolerated in the 5- and 6-
positions, as is the 5,6-methylenedioxy group (15ab;
Ki = 1nM). Larger groups such as 5-benzyloxy (15z;
Ki = 50nM) and 5- and 6-nitro (15aa and 15r; Ki = 29
and 31nM) led to lower affinity. Similar results were re-
ported for the N1-benzenesulfonyltryptamine series.12,13
References and notes
1. Hoyer, D.; Clarke, D. E.; Fozard, J. R.; Hartig, P. R.;
Martin, G. R.; Mylecharane, E. J.; Saxena, P. R.;
Humphrey, P. P. Pharmacol. Rev. 1994, 46, 157–203.
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Compounds which had high affinity for the 5-HT6
receptor were tested in the cyclase assay to determine
if they could modulate 5-HT6 function. None of the
compounds tested showed any agonist activity.22
Many N1-arylsulfonyl-3-(tetrahydropyridin-4-yl)-1H-
indole compounds inhibited 5-HT stimulated cAMP
production at low nanomolar concentrations, suggesting
that they are antagonists (Table 2).22 Interestingly,
although 4-aminophenylsulfonyl derivatives had similar
4. Sebben, M.; Ansanay, H.; Bockaert, J.; Dumuis, A.
Neuroreport 1994, 5, 2553–2557.
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Table 3. Binding affinity for other serotonin and dopamine receptors
Compd
Ki (nM)
5-HT1A
5-HT1B
5-HT1D
5-HT1F
5-HT2A
5-HT2C
5-HT7
D2
D3
D4
15b
15c
15g
15y
18h
>2000
875 16
1670 191
12%a
11%a
33%a
27%a
30%a
6%a
9%a
NT
32%a
10%a
2%a
53%a
NT
599 59
50%a
NT
383 18
57%a
NT
NT
NT
>2000
>2000
NT
627 39
741 22
NT
>5000
>5000
NT
1555
>5000
>2000
9
97 6
115 15
NT
16%a
NT
65%a
NT
15%a
NT
10%a
NT
7%a
NT
7%a
Receptors were all human clones stably expressed in CHO cells (5-HT receptors) or CHO-K1 Cells (D receptors). Radioligands were as follows:
5-HT1A: DPAT; 5-HT1B, 5-HT1D, 5-HT1F, 5-HT2C: [3H]-5-HT; 5-HT2A: [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ([125I]DOI); 5-HT6,
5-HT7: [3H]LSD; dopamine D2, D3, and D4: [3H]spiperone].
NT = not tested.
a % inhibition @ 1lM.

D. C. Cole et al. / Bioorg. Med. Chem. Lett. 15 (2005) 379–383
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operation is repeated once more. The collected cells are
then homogenized in 10 volumes of 50mM Tris–HCl,
pH7.4 and 0.5mM EDTA. The homogenate is centrifuged
at 900 · g for 10min and the supernatant collected. The
supernatant is then centrifuged at 40,000 · g for 30min.
The pellet is resuspended in 10 volumes of Tris–HCl buffer
and recentrifuged at the same speed. The final pellet is
suspended in a small volume of Tris–HCl buffer and the
tissue protein content is determined in aliquots of 10–
25lL volumes. The volume of the suspended cell mem-
branes is adjusted to give a tissue protein concentration of
1.0mg/mL of suspension. The prepared membrane sus-
pension (10 times concentrated) is aliquoted in 1.0mL
volumes and stored at ꢀ70ꢁC until used in subsequent
binding experiments.
19. 5-HT6 binding assay: total bound well: 80lL binding
buffer, 20lL of 3lM [3H]-LSD, 100lL membrane protein
(20–50lg protein). Non-specific well: 60lL buffer, 20lL
of 1lM cold LSD, 20lL of 3lM [3H]-LSD, 100lL of
membrane protein (20–50lg protein). Compound well:
containing 60lL buffer, 20lL test compounds at different
concentrations, 20lL of 3lM [3H]-LSD, 100lL mem-
brane protein (20–50lg protein). Incubate at room tem-
perature for 2h and harvest reaction mixture by using
Packard 96 well harvest system followed by counting the
plate in Top Count machine (Packard).
20. Pullagurla, M. R.; Dukat, M.; Setola, V.; Roth, B.;
Glennon, R. A. Bioorg. Med. Chem. Lett. 2003, 13, 3355–
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18. 5-HT6 membrane preparation: cultured HeLa cells
expressing h5-HT6 receptors are harvested and centri-
fuged at low speed (1000 · g) for 5min to remove the
culture media. The harvested cells are suspended in 1
volume of fresh physiological phosphate buffered saline
(PBS) solution and recentrifuged at the same speed. This
22. Determination of 5-HT6 compound intrinsic activity using
cAMP accumulation: intracellular cAMP levels are meas-
ured using 24-well plates containing the human 5-HT6
receptor stably transfected into HeLa cells. Upon initia-
tion of the assay, the media from the cell maintenance is
aspirated and the cells are preincubated at 37ꢁC for 15min
in KREBS buffer. Following this primary incubation, the
buffer is aspirated and an additional incubation is
performed at 37ꢁC for 5min in KREBS buffer containing
500lM IBMS (3-isobutyl-1-methylxanthine). Subse-
quently, the cells are incubated with the test compound
at concentrations ranging from 10ꢀ5 to 10ꢀ10 M for 10min
at 37ꢁC (antagonist assay require a second incubation
with the addition of 100nM 5HT). The assay is terminated
by the addition of 0.5M percloric acid. Intracellular
cAMP levels were determined by radioimmunoassay
through the cAMP SPA screening kit. Data were analyzed
graphically with GraphPad Prism (GraphPad Software,
San Diego, CA).