Novel brain penetrant benzofuropiperidine 5-HT6 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2011.11.050

7-Arylsulfonyl substituted benzofuropiperidine was discovered as a novel scaffold for 5HT₆ receptor antagonists. Optimization by substitution at C-1 position led to identification of selective, orally bioavailable, brain penetrant antagonists with reduced hERG liability. An advanced analog tested in rat social recognition model showed significant activity suggesting potential utility in the enhancement of short-term memory.

Full text of DOI:10.1016/j.bmcl.2011.11.050

Bioorganic & Medicinal Chemistry Letters 22 (2012) 120–123
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel brain penetrant benzofuropiperidine 5-HT6 receptor antagonists
⇑
Babu G. Sundar , Thomas R. Bailey, Derek D. Dunn, Edward R. Bacon, Joseph M. Salvino,
George C. Morton, Lisa D. Aimone, Huang Zeqi, Joanne R. Mathiasen, Amy Dicamillo, Mark J. Huffman,
Beth A. McKenna, Karla Kopec, Lily D. Lu, Rebecca Brown, Jie Qian, Thelma Angeles, Thomas Connors,
Chrysanthe Spais, Beverly Holskin, Deborah Galinis, Emir Duzic, Herve Schaffhauser, Gerard C. Rosse
Worldwide Discovery Research, Cephalon, Inc., 145 Brandywine Parkway, West Chester, PA 19380, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
7-Arylsulfonyl substituted benzofuropiperidine was discovered as a novel scaffold for 5HT6 receptor
antagonists. Optimization by substitution at C-1 position led to identification of selective, orally bioavail-
able, brain penetrant antagonists with reduced hERG liability. An advanced analog tested in rat social rec-
ognition model showed significant activity suggesting potential utility in the enhancement of short-term
memory.
Received 26 October 2011
Accepted 14 November 2011
Available online 22 November 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
5HT6R antagonist
Benzofuropiperidine
Brain penetrant
Rat in vivo activity
The 5-hydroxytryptamine-6 receptor (5-HT6R) is one of 14
known sub-types from the receptors of neurotransmitter, seroto-
nin. With the exception of 5-HT3R, which is a ligand-gated ion
channel, all of them are G-protein coupled receptors (GPCRs).1
The rat 5-HT6R was first identified in 1993 followed by the identi-
fication of human 5-HT6R in 1996.2,3 5-HT6R is positively coupled
to adenylyl cyclase, and the receptor activation leads to increased
production of cyclic adenosine monophosphate (cAMP).4 The 5-
HT6R is found to be almost exclusively expressed in the central
nervous system (CNS) of mammals and is suggested to play a post
synaptic role.5,6 Because of their distribution in cerebral cortex and
limbic areas, these receptors were proposed to be involved in cog-
nitive processes. The 5-HT6R antisense oligonucleotide studies sug-
gested potential application of 5-HT6R antagonists in cognition.7
This hypothesis was further supported by the enhanced choliner-
gic8 and glutamatergic neurotransmission9 by selective small mol-
ecule 5-HT6R antagonists in preclinical animal studies. In addition
to their application in treating cognitive impairment in Alzheimer’s
disease (AD), Schizophrenia and Depression, 5-HT6R antagonists
have demonstrated preclinical utility in treating obesity.10,11 With
the development of ligand pharmacophore and 5-HT6R homology
models12,13 over the last few years, several selective 5-HT6 receptor
antagonists have been disclosed (Fig. 1).10,11,14 A majority of these
compounds possess either indole or arylpiperazine as a core struc-
tural feature. Some of the earlier clinically investigated compounds
such as SB-271046 suffered from poor blood–brain barrier (BBB)
penetration which may be the reason for its discontinuation from
further development. Eli Lilly’s LY-483518 was licensed in by
Saegis (SGS-518) and its clinical development was discontinued
after Phase I trials for cognition in Schizophrenia.
Currently GSK’s piperazinyl quinoline, SB-742457 is in Phase II
development for AD. Also under active clinical development are
the former Wyeth compounds SAM-531 (PF-05212365) in Phase
II, and SAM-760 (PF-05212377) in Phase I for AD. Cephalon’s early
objective in this project was to discover novel, potent 5-HT6R
antagonists with acceptable selectivity, oral bioavailability, brain
penetration and establish preclinical proof of concept. By a de novo
design, we identified arylsulfonyl substituted benzofuropiperidine
O
O
H
S
Cl
N
O
NH
N
S
O
N
O
S
N
N
H
SB-271046
SB-742457
N
3
5
NH
O
S
7
R'
Ar
O
O
O
R
1
N
S
O
O
LY-483518
Benzofuropiperidine 1
⇑
Corresponding author. Tel.: +1 610 738 6348; fax: +1 610 738 6558.
E-mail address: bsundar@cephalon.com (B.G. Sundar).
Figure 1. Clinically investigated 5-HT6R antagonists and benzofuropiperidine.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.11.050

B. G. Sundar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 120–123
121
1 as a novel patentable scaffold. Our early structure activity (SAR)
studies had demonstrated weaker binding affinity for compounds
with substitution of arylsulfone at C-6 position of the benzofuropi-
peridine core. For example, compound 1a was >15-fold weaker
than 1b in binding affinity toward human and rat receptors
(Table 1). With arylsulfonyl at C-7, aromatic substitution was
briefly examined. Meta-fluoro substitution afforded more potent
compound compared to para-fluoro substitution (1c vs 1d). Substi-
tution of electron releasing group such as OMe led to drop in
potency (compound 1e). Compound 1c, despite showing high affin-
ity (h5-HT6 Ki = 1.6 nM, r5-HT6 Ki = 6.9 nM) with potent functional
activity (IC50 = 3.6 nM),15 displayed poor metabolic stability in the
rat and dog liver microsomes (t1/2 = <5 min and 10.3 min, respec-
tively). Guided by the in vitro metabolic identification study in
liver microsomes, blocking the C1 position from metabolism was
expected to improve the stability. To test this, 1,1-gem diMe com-
pounds 1f–1h were prepared. As previously observed, meta-fluoro
substitution gave better affinity (compound 1g) than meta-OMe
(1h) or unsubstituted phenyl compound (1f). Compound 1g
retained affinity (h5-HT6 Ki = 2.9 nM, r5-HT6 Ki = 19 nM) and func-
tional activity (cAMP IC50 = 11.7 nM). Also compared to 1c, the
liver microsome stability showed overall improvement with
t1/2 (min) = 16, 32, >40 rat, dog, and human, respectively. This
was reflected in a measurable oral exposure in the rat PK (Table 3).
But both the compounds 1c and 1g inhibited the hERG channel
(IC50 of 1–3 M). To mitigate the hERG inhibition risk, a strategy of
l
reducing the secondary amine basicity was adopted. Compounds
with electron withdrawing groups (OMe, CF3 and CF2H) at the
b-position to the amine were synthesized (Table 2). The fluoroalkyl
substitutions led to significant reduction in the amine pKa and
greater separation from hERG inhibition. But unfortunately, this
also led to reduced human receptor binding affinity with the
expected increase in logD. For example, the CF3 compound (1i)
with cLogD of 3.6, had h5-HT6 Ki of 139 nM, and hERG IC50 of
>30
better binding affinity compared to compound 1k (h5-HT6
Ki = 16 nM) and good hERG selectivity (IC50 = >30 M). But still,
lM. The less lipophilic CF2H compound 1j (cLogD of 3.0) had
l
the rat 5-HT6 affinity (Ki = 183 nM) and functional potency (cAMP
IC50 = 54 nM) of 1j were weaker than those of compound 1g. As a
next step, we focused on decreasing the amine basicity and main-
tain the potency.
The initial profile of OMe compound (1k) (cLogD = 2.3,
pKa = 7.5, h5-HT6 Ki = 18 nM, r5-HT6 Ki = 113 nM), provided a
balance between potency and calculated properties. Based on this
result, a small set of cyclic ether targets was prepared. The 4-spiro-
pyran analog 1l (4-spiro THP) displayed improved binding affinity
towards human and rat receptors with a good hERG separation
(cLogD = 2.4, h5-HT6 Ki = 2.1 nM, r5-HT6 Ki = 13.9 nM, hERG IC50
Table 1
5-HT6R binding and functional activity for benzofuropiperidines
6
X
7
NH
O
R'
R
a
Compound
R/R0
X
h5-HT6Ra (Ki, nM)
r5-HT6R (Ki, nM)
h5-HT6Rb cAMP-(IC50, nM)
1a
1b
1c
1d
1e
1f
H/H
H/H
H/H
H/H
6-(PhSO2)–
7-(PhSO2)–
46
156
9.8
6.9
—
34
41
19
—
—
2.7
1.6
22
5.6
7.7
2.9
17.5
8.5
3.6
—
21
34
7-(m-F–PhSO2)–
7-(p-F–PhSO2)–
7-(m-OMe–PhSO2)–
7-(PhSO2)–
7-(m-F–PhSO2)–
7-(m-OMe–PhSO2)–
H/H
Me/Me
Me/Me
Me/Me
1g
1h
11.7
68.6
a
[3H]LSD binding in membranes prepared from cells expressing full length human or rat 5-HT6R. The Ki values are the mean of duplicate values from three experiments
with the exception of 1f (n = 1).
b
Mean of 3 functional activity experiments from human astrocytoma cell line stably expressing the human 5-HT6R.
Table 2
5-HT6R and hERG activity of 1,1-disubstituted benzofuropiperidines
NH
F
S
R'
O
O
R
O
Compound
R/R0
h5-HT6Ra (Ki, nM)
r5-HT6Ra (Ki, nM)
h5-HT6Rb cAMP-(IC50, nM)
hERGc (IC50
,
lM)
pKa/cLogDd
1i
1j
1k
1l
1m
1n
2
Me/CF3
Me/CF2H
Me/CH2OMe
4-THP
( )3-THF
( )3-THP
(ent-1)-3-THP
(ent-2)-3-THP
139
16
18
2.1
22.3
6.5
4.4
130
—
—
54
—
6.8
32
—
9.8
—
>30
>30
—
>30
—
13.6
12
9.6
5.7/3.6
6.2/3.0
7.5/2.3
7.4/2.4
7.5/1.8
7.5/2.1
7.5/2.1
7.5/2.1
183
113
14
131.5
38.3
16.9
—
3
a
[3H]LSD binding in membranes prepared from cells expressing full length human or rat 5-HT6R. The Ki values are the mean of duplicate values from three experiments
with the exception of 1i (n = 1), 2 and 3 (n = 2).
b
Mean of 3 functional activity experiments on human astrocytoma cell lines stably expressing the human 5-HT6R.
Determined by the patchXpress assay at MDS Pharma Services.
Calculated values from ACD software.
c
d

122
B. G. Sundar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 120–123
Table 3
Compound 2 was further profiled for in house serotonin sub-
type selectivity, hERG and receptor selectivity and cytochrome
Rat pharmacokinetic data for select benzofuropiperidines
P450 (CYP) inhibition. Compound 2 with MDSP patchXpress hERG
Rat pharmacokinetic data
1g
1l
2
IC50 of 12
ity (Ki = 4.4 nM). In the CEREP lead profile testing, compound was
highly selective, showing >50% inhibition at 10 M for only 8 out
of 95 tested targets (5-HT2A, 5-HT2B, 5-HT6, a2A 2B, D2S, D3 and
rat -type Ca-channel). The CYP IC50 was >30 M for all isoforms
(1A2, 2C19, 2D6, 3A4) except for CYP2C9 (IC50 = 1.9 M). Com-
pound 2, tested for serotonin subtype selectivity in the counter
screen binding assays was found to be very selective. Considering
the h5-HT6 Ki of 4.4 nM for compound, the binding selectivity
against the closely related serotonin sub-types were determined
lM showed >2700-fold separation from its h5-HT6 affin-
i.v. (1 mg/kg)
t
(h)
0.9 0.2
5.5 0.2
82 16
0.9 0.2
1.4 0.2
1.4 0.4
½
Vd (L/kg)
CL (mL/min/kg)
4
32
1.3
4
l
17
3
,
a
p.o. (5 mg/kg)
Cmax (ng/mL)
6 h-AUC (ng/h/mL)
% F
L
l
35
149
15
3
9
1
84
7
210 25
974 119
l
368 31
6
36
4
i.p. (5 mg/kg)
1 h-Brain conc. (nM/g)
Brain/plasma
1625 604
7.7 0.4
979 79
3.3 0.1
1659 350
0.5
2
to be 258-fold versus 5-HT2A
>10,000-fold versus 5-HT2C. Follow up testing for 5-HT2B functional
activity at CEREP showed that compound 2 was an antagonist with
, 100-fold versus 5-HT2B and
i.v. Vehicle: 3% DMSO, 30% Solutol, 67% PBS.
p.o. Vehicle: Tween 80:propylene carbonate:propylene glycol (5:4:1).
an IC50 of 1.5 lM.
Following the pharmacokinetic and the selectivity profiling,
compound 2 advanced to establishing preclinical in vivo proof of
concept. Based on previous published work, rat social recognition
model17 was chosen for this project for evaluation of short-term
memory. In this model, compound 2 produced a significant activity
at doses between 0.1 and 3.0 mg/kg i.p. compared to the vehicle.
Detailed biological characterization and in vivo testing of this novel
5-HT6 antagonist 2 will be described in due course.
The benzofuropiperidines were prepared as described in Scheme
1.18 The commercially available iodosalicylate was O-alkylated with
ethylbromo acetate and the resultant ether product was subjected
to Claisen condensation to provide the benzofuranone 4. Horner–
Emmons olefination followed by borane reduction of the vinylnitrile
intermediate afforded the key furan-3-ethylamine intermediate 5
in 50% overall yield. Pictet–Spengler cyclization with 5 and parafor-
maldehyde was readily performed in aq. HCl. Cyclization with
ketones required stronger conditions such as warm trifluroacetic
acid.18 Boc-protection of tricyclic amines followed by copper
catalyzed thiolation reaction resulted in the thioether derivatives
7. Oxidation of the sulfide group was accomplished with m-CPBA
in methylene chloride and the boc-group was removed using HCl
in dioxane. Sterically hindered Pictet–Spengler products with
weakly basic amines were subjected to thiolation, oxidation se-
quence without the N-boc-protective group.
>30 lM). The 3-spirotetrahydrofuran 1m (3-spiroTHF) had binding
affinities (h5-HT6 Ki = 22.3 nM, r5-HT6 Ki = 131.5 nM), comparable
to those of the acyclic ether (1k). The 3-spirotetrahydropyran 1n
(3-spiroTHP) analog exhibited superior binding affinity (h5-HT6
Ki = 6.5 nM, r5-HT6 Ki = 38.3 nM). Anticipating potential activity
differences between the enantiomers, the racemate spiroether 1n
was resolved by chiral supercritical fluid chromatography.16 As
illustrated in Table 2, the two 3-spiroTHP enantiomers displayed
a large difference in binding affinity (2, h5-HT6 Ki = 4.4 nM and 3,
h5-HT6 Ki = 130 nM) with an eudismic ratio of 29.5. Based on the
balanced potency, hERG separation and physical properties, the
compounds 1l and 2 were selected for further profiling. The liver
microsome stability for 1l (t1/2 (min) = 25, >40, 19 rat, dog, and hu-
man) was comparable with that of 2 (t1/2 (min) = 25, >40, 30 rat,
dog, and human) except that 1l had relatively inferior human met-
abolic stability.
Due to their overall favorable attributes, compound 1l and 2 ad-
vanced to rat pharmacokinetic experiments (Table 3) After a single
i.v. bolus dose of 1 mg/kg, compound 1l had a t of 0.9 h with mod-
½
erate clearance (17 mL/min/kg) and volume of distribution (1.4 L/
kg). Oral administration of a single 5 mg/kg dose showed low oral
bioavailability (F = 6%) based on 6hAUC (386 ng/h/mL) and Cmax
(84 ng/mL). On the other hand, compound 2 displayed an i.v. half
life of 1.4 h, with acceptable plasma clearance (32 mL/min/kg)
and volume of distribution of 4.0 L/kg. In the p.o. arm of the rat
pharmacokinetic study, after an oral dose of 5 mg/kg, compound
2 had superior plasma exposure with an estimated oral bioavail-
ability of 36% based on 6hAUC (974 ng/h/mL) and Cmax (210 ng/
mL). Compound 2 also had BBB penetration with a brain to plasma
ratio (b/p) of 2.0. Based on its overall superior rat PK profile, com-
pound 2 progressed to further characterization.
In summary, we have disclosed here a novel fused piperidine
scaffold for 5-HT6 receptor antagonists. Scaffold optimization with
1,1-gem disubstitution resulted in compounds with improved
pharmacokinetic profile. Further elaboration of 1,1-disubstitution
into spirocyclic ethers provided an overall balance in physical
properties, potency, ADME and hERG selectivity profile. A potent,
selective functional antagonist from this study, compound 2
O
CO2Me
R''
NH2
N
(a)
(b)
(c)
(d)
(e)
(f)
(g)
R'
I
OH
O
O
O
R
I
I
I
4
5
6a R'' = H
6b R'' = boc
NH
R'
R''
(h)
(i)
N
R'
Ar
O
R
O
S
R
ArS
O O
7a R'' = H
1 (a-n)
7b R'' = boc
Scheme 1. Preparation of benzofuropiperidines. Reagents and conditions: (a) BrCH2CO2Et, K2CO3, KI, acetone, 50 °C, 5 h, 90–95%; (b) (i) LiHMDS, toluene; 0–25 °C, 2 h; (ii) aq.
NaOH, EtOH, 0–80 °C, 1.5 h; (iii) aq. HCl, 25–60 °C, 2.5 h (85%, 3 steps); (c) NaH, CNCH2PO(OEt)2, THF, 15–25 °C, 1 h; (d) BH3-THF, 0–5 °C, 20 h (70–80% 2 steps); (e) carbonyl
compound, 1 N aq. HCl, 70 °C, 2 h or DCE, TFA, 80 °C, 15 h, 50–85%; (f) (Boc)2O, Et3N, CH2Cl2 or THF; (g) ArSH, NaOtBu, CuI, Neocuproine, DMF, 100 °C, 15 h (60–90% 2 steps);
(h) m-CPBA, CH2Cl2, 0–25 °C, 15 h or oxone, H2O, MeOH, rt, 3 h; (i) 4 M HCl-dioxane (50–95%).

B. G. Sundar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 120–123
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expressing h5-HT6, receptor. Briefly, 18
cells per well were plated to white 384-well OptiPlates (Cat
PerkinElmer Life Sciences, Boston, MA). 100 nL of test compounds were added
to cells and the plate was pre-incubated at room temperature for 10 min (final
l
L of cells in the cell density of 5000
600727,
#
DMSO concentration was 0.5%). The cells were then stimulated with 6
HT at its EC80 concentration of 10 nM for 30 min at room temperature. The
reaction was stopped by the addition of the 12 L of cAMP-d2 in lysis buffer,
followed by addition of 12 L of anti cAMP-cryptate in lysis buffer. The plate
lL of 5-
l
l
was incubated at room temperature for 1 h and read on multi-labeled plate
reader EnVision 2102 or 2104 (Perkin Elmer Life Sciences, Boston, MA). The
fluorescence ratio (665 nm/590 nm ꢀ 104) was calculated, which was inversely
proportional to the level of cAMP in the sample.
16. Supercritical fluid chromatography chiralseparation conditions: Chiral Pak AS-H
column (10 ꢀ 250 mm), 6.0 ml/min 80% CO2/20% iPrOH/0.1% Et2NH, k = 220 nm,
compound 2(ent-1): 20 min, compound 3(ent-2): 26 min. Compound 2: 7-[(3-
fluorophenyl)sulfonyl]-3,4,50,60-tetrahydro-2H,40H-spiro[1-benzofuro[2,3-c]py
ridine-1,30-pyran].
11. (a) Heal, D. J.; Smith, S. L.; Fisas, A.; Codony, X.; Buschmann, H. Pharmacol. Ther.
2008, 117, 207; (b) Holenz, J.; Pauwells, P.; Diaz, J. L.; Merce, R.; Codony, X.;
Buschmann, H. Drug Discovery Today 2006, 11, 283.
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Medchem. 2004, 12, 3815; (b) Sikazwe, D.; Bondarev, M. L.; Dukat, M.;
Rangisetty, J.; Roth, B. L.; Glennon, R. A. J. Med. Chem. 2006, 49, 5217.
13. (a) Bromidge, S. M. Special Publication-Roy. Soc. Chem. 2001, 264, 101; (b)
Lopez-rodrigez, M. L.; Benhamu, M. L.; de La Fuente, T.; Sanz, A.; Pardo, L.;
Campillo, M. J. Med. Chem. 2005, 48, 4216; (c) Pullaguria, M. R.; Westkaemper,
R. B.; Glennon, R. A. Bioorg. Med. Chem. Lett. 2004, 14, 4569; (d) Dukat, M.;
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Carlyle, I.; Caulfield, W. L.; Clarkson, T. R.; Cusick, F.; Epemolu, O.; Gilfillan, R.;
1H NMR (400 MHz, CDCl3): d 1.7 (d, J = 12.6 Hz, 1H), 1.83 (d, J = 14.2 Hz, 1H), 2.0
(m, 2H), 2.3 (m, 1H), 2.7 (m, 2H), 3.2 (m, 2H), 3.6 (m, 1H), 3.7 (d, J = 11.5 Hz, 1H),
3.85 (d, J = 12 Hz, 1H), 4.0 (d, J = 10.6 Hz, 1H), 7.2 (m, 2H), 7.4 (m, 1H), 7.57 (d,
J = 8.1 Hz, 1H), 7.66 (d, J = 8.1 Hz, 1H), 7.76 (d, J = 8.1 Hz, 1H), 7.82 (d, J = 8.2 Hz,
1H), 8.08 (s, 1H), MS (m/e): 403 (M+H); m.p. (HCl salt) = 300–320 °C dec.
17. (a) Schaffhauser, H.; Mathiasen, J. R.; Dicamillo, A.; Huffman, M. J.; Lu, L. D.;
McKenna, B. A.; Qian, J.; Marino, M. J. Biochem. Pharmacol. 2009, 78, 1035; (b)
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