Discovery of a cyclopentylamine as an orally active dual NK1 receptor antagonist-serotonin reuptake transporter inhibitor

Article abstract of DOI:10.1016/j.bmcl.2014.01.036

Cyclopentylamine 4 was identified as a potent dual NK₁R antagonist-SERT inhibitor. This compound demonstrated significant oral activity in the gerbil forced swimming test, suggesting that dual NK₁R antagonists-SERT inhibitors may be

Full text of DOI:10.1016/j.bmcl.2014.01.036

Bioorganic & Medicinal Chemistry Letters 24 (2014) 1611–1614
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of a cyclopentylamine as an orally active dual NK1 receptor
antagonist–serotonin reuptake transporter inhibitor
⇑
Yong-Jin Wu , Huan He, Qi Gao, Dedong Wu, Robert Bertekap, Ryan S. Westphal, Snjezana Lelas,
Amy Newton, Tanya Wallace, Matthew Taber, Carl Davis, John E. Macor, Joanne Bronson
Research and Development, Bristol-Myers Squibb Company, 5 Research Parkway, Wallingford, CT 06492, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Cyclopentylamine 4 was identified as a potent dual NK1R antagonist–SERT inhibitor. This compound
demonstrated significant oral activity in the gerbil forced swimming test, suggesting that dual NK1R
antagonists–SERT inhibitors may be useful in treating depression disorders.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 20 December 2013
Revised 10 January 2014
Accepted 14 January 2014
Available online 27 January 2014
Keywords:
NK1 receptor (NK1R) antagonist
Serotonin reuptake transporter (SERT)
inhibitor
Dual NK1 receptor antagonists–serotonin
reuptake transporter inhibitor
Serotonin reuptake inhibitors (SSRIs) such as paroxetine1 and
fluoxetine (Chart 1) have demonstrated an improved safety profile
over the first-generation tricyclic antidepressants like imipramine
(Chart 1),2 but they still suffer from a few side effects including
gastrointestinal distress, anxiety, insomnia, weight gain and sexual
dysfunction. In addition, SSRIs take weeks to months to produce a
therapeutic effect.3–5 The delay in onset for efficacy with SSRIs
presumably results from activation of the inhibitory role of seroto-
nin 1A (5-HT1A) autoreceptors. Upon administration of an SSRI,
increased synaptic serotonin (5-HT) can act at 5-HT1A receptors
to reduce the firing of serotonergic neurons, resulting in a muted
increase of synaptic 5-HT. After initial desensitization of the
5-HT1A autoreceptors with repeated SSRI treatment, the serotoner-
gic neurons resume normal firing, allowing an increase of synaptic
5-HT, thereby generating a therapeutic antidepressant response.
Indeed, a beneficial effect on the onset of action of SSRIs has been
observed in clinical studies with co-administration of 5-HT1A
antagonists.6,7 Moreover, this combination therapy has resulted
in significant improvements among SSRI-resistant patients.6,7 An-
other potential combination therapy might involve neurokinin-1
receptor (NK1R) antagonists8–12 and SSRIs. NK1R antagonists alone
may not be sufficient in treating depression in humans, but since
NK1R antagonists indirectly modulate 5-HT function13,14 and atten-
uate presynaptic 5-HT1A receptor function, NK1 receptor antago-
nism may augment the antidepressant activity of SSRIs.
Consistent with this hypothesis, Bourin et. al. showed that
GR205171 (Chart 1), a selective and brain penetrant NK1R antago-
nist, selectively potentiated the antidepressant activity of sub-effi-
cacious doses of the SSRIs, citalopram and paroxetine, in the mouse
forced swimming test (FST).15 A more recent investigation has
shown that NK1R antagonism lowers occupancy requirement for
antidepressant-like effects of SSRIs in the gerbil forced swimming
test.16 Thus, combination therapy of SSRIs with NK1R antagonists
may be viable in the treatment for depression with potential for
improved efficacy, reduced side effects and a more rapid onset of
clinical efficacy. However, combination therapy with two different
compounds may be challenging primarily because of enhanced risk
of drug–drug interactions and potentiation of dose-related or
idiosyncratic side effects and pharmacokinetic interactions. An
alternative to combination therapy of two different drugs would
be a single compound that acts as both a serotonin reuptake trans-
porter (SERT) inhibitor and an NK1R antagonist. To this end, Ryck-
mans et al. identified a series of benzyloxyphenethyl piperazine
derivatives as dual NK1R antagonists–SERT inhibitors.17,18 One of
the best compounds in the series, (S)-1-(2-(3,5-dibromobenzyl-
oxy)-1-phenylethyl)piperazine (1, Chart 1), demonstrated oral
activity in an animal model of depression sensitive to both
mechanisms. We recently reported that 4-((3,5-bis(trifluoro-
methyl)-benzyloxy)-methyl)-4-phenylpiperidine (2, Chart 1), a
potent NK1R antagonist originated from Merck Research Laborato-
ries,19 is not only a potent NK1R antagonist but also a potent SERT
inhibitor.20 Subsequent ring expansion studies led to a series of
4,4-disubstituted homopiperidines as potent dual-acting NK1R
⇑
Corresponding author. Tel.: +1 203 677 7485; fax: +1 203 677 7702.
E-mail address: yong-jin.wu@bms.com (Y.-J. Wu).
http://dx.doi.org/10.1016/j.bmcl.2014.01.036
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

1612
Y.-J. Wu et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1611–1614
F
Ph
Ph
O
Ar
Ar
Ph
O
O
Ar
O
CF3
NH2
ii,iii
O
25
26
N
N
O
O
O
Me
O
Me
N
N
H
N
H
Paroxetine (PaxilTM
)
Me
Fluoxetine (ProzacTM
)
Ph
i
Imipramine
NH2
23
( )-
O
Ar
SmithKline Beecham
Lilly
+
MeO
CF3
N
OH
Ph
Ph
22
3
O
Ar
Ar
Ph
N
N
Br
O
Ar
O
1
NH
N
1
N
N
O
NH2
27
N
Br
ii iii
CF3
O
H
N
H
Ar =
O
GR-205171
Glaxo Wellcome
.
( )-24
CF3
NH2
4
CF3
CF3
3
O
O
Scheme 2. Reagents and conditions: (i) Ph3P, DEAD, phthalimide, 0 °C, 2 h, 28% ( )-
23; 25% ( )-24; (ii) chiral HPLC, AD column for ( )-23 and OJ column for ( )-24,
20 Â 250 mm, 95% heptane/5% ethanol, 10 mL/min flow rate; (iii) hydrazine,
toluene, 80 °C, 85–90%. Ar: 3,5-bis(trifluoromethyl)phenyl.
( )n
N
1
NH2
CF3
CF3
H
4
2 (n = 1); 3 (n = 2)
( )-23 and ( )-24 in 28% and 24% yields, respectively. Separation
of the enantiomers of ( )-23 and ( )-24 by HPLC on a chiral column
furnished four enantiomerically pure stereoisomers: (1R,3S)-23,
(1S,3R)-23, (1S,3S)-2421 and (1R,3R)-24. These four isomers were
individually subjected to phthalimide removal by treatment with
hydrazine in hot toluene to give four cyclopentylamines: 25, 26,
27 and 4.22 To establish the relative and absolute stereochemistry
of these isomers, we converted (1R,3S)-25 and (1S,3S)-4 to their
tartaric acid salts which were recrystallized from ethanol to give
single crystals for X-ray diffraction analysis (Fig. 1).
The cyclopentyl- and cyclohexylamine analogs in Table 1 were
tested as mixtures of diastereomers to get an initial assessment
of activity. Although it was appreciated that individual enantio-
mers might have different profiles for NK1 antagonism and SERT
inhibition, this approach allowed us to efficiently identify pre-
ferred amine substituents. Indeed, these initial results showed that
small alkyl amines were preferred for dual NK1R/SERT activity,
with SERT inhibition being particularly sensitive to steric bulk.
Thus compounds 6, 13, 14, 16, and 17 retained dual activity while
compounds such as 7–12 and 18–21 had diminished activity
against SERT even though NK1R antagonist potency was retained
in some cases.
Chart 1.
antagonists–SERT inhibitors as exemplified by compound 3.20 As
part of our efforts in the area, we explored cyclopentyl- and cyclo-
hexylamine analogs to assess the impact of modifying the amine
part of the pharmacophore. This exercise culminated in the discov-
ery of the cyclopentylamine 4 (Chart 1) with balanced dual NK1R
antagonism and SERT inhibiton. This Letter describes the SAR lead-
ing to 4, along with the synthesis and in vivo studies of this
compound.
Scheme 1 describes the synthesis of cyclopentyl- and cyclohex-
ylamine analogs as mixture of diastereomers. The readily available
cyclic ketones 5a/b20 underwent titanium(IV) isopropoxide-pro-
moted reductive amination with various primary and secondary
amines to provide secondary and tertiary amines 6–21 as mixtures
of diastereomers (Table 1).
Our initial attempts to make cyclopentyl primary amines 4 and
25–27 through reductive amination of cyclopentanone 5a with
ammonia failed under various conditions. Therefore, we developed
an alternative Mitsunobu reaction route involving cyclopentanol
22 as shown in Scheme 2. Cyclopentanol 22 as a mixture of two
diastereomers was readily prepared from methyl 2-phenylacetate
in five steps following the procedures recently disclosed from this
laboratory.20 Mitsunobu reaction of 22 with phthalimide under
typical conditions gave a mixture of two diastereomers, which
were readily separated by silica gel chromatography to provide
CF3
CF3
O
( )n
O
( )n
i
N
CF3
R2
CF3
O
R1
5a/b
a: n = 1; b: n = 2
6-21
Scheme 1. Reagents and conditions: (i) R1R2NH, Ti(Oi-Pr)4, 65 °C, 2 h; NaBH4, EtOH,
rt, 1 h, 25–85%.
Figure 1. Thermal ellipsoid plots (35% ellipsoids) of the tartaric acid salts 25 (left)
and 4 (right).

Y.-J. Wu et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1611–1614
1613
Table 1
IC50 values of diastereomeric cyclopentyl- and cyclohexylaminesa,b,c
CF3
O
( )n
R
CF3
hNK1R (nM)
Compd
R
n
hSERT (nM)
6
7
Me2N
i-Pr2N
1
1
8.0
470
57
>1000
H
8
1
16
20
530
N
Figure 2. Effects of various oral doses of (1S,3S)-4 (3.0, 10, 30 mg/kg) and fluoxetine
(10 mg/kg, PO) in gerbil FST. Results are expressed as mean immobility time SEM.
⁄P < 0.01. T-30: 30 min pretreatment time.
N
9
1
>1000
O
10
11
12
N
1
1
1
14
4.7
45
>1000
>1000
>1000
results were expressed as the immobility time during the 6-minute
test period (mean SEM) for the 10 gerbils tested in each group.
Figure 2 shows the effect of various oral doses of compound 4
and fluoxetine (positive control) at 10 mg/kg (PO). The lower oral
doses of 4 (3.0 and 10 mg/kg) did not modify the immobility times
in gerbils when compared to the vehicle-treated control group.
However, 4 at 30 mg/kg (PO) decreased immobility time. We also
tested 4 in a locomotor activity assay to determine whether the re-
sults obtained in the gerbil FST were impacted by a possible in-
crease in spontaneous locomotor activity and found that
compound 4 at 30 mg/kg (PO) produced no significant changes in
locomotor activity as compared with the vehicle-treated control
group. At the 30 mg/kg dose, the plasma concentration of 4 was
300 nM and the brain concentration was 7400 nM, indicating that
4 had good brain uptake, but that high brain concentrations were
required for efficacy. Clearly, free fraction data and/or CSF concen-
trations are needed to better understand the PK-PD relationship.
Compound 4 was identified as a balanced dual NK1R antagonist
and SERT inhibitor with oral activity in the gerbil FST. Because of
its oral activity, excellent brain penetration, and low molecular
weight, this compound represents a potential lead compound in
the search for potent dual NK1R antagonists–SERT inhibitors as no-
vel anti-depressant candidates.
N
O
N
13
14
MeNH
EtNH
2
2
16
14
190
150
H
15
2
74
680
N
16
17
Me2N
Et2N
2
2
48
38
250
280
18
19
20
21
N
2
2
2
2
120
93
>1000
970
N
O
N
N
350
120
>1000
520
N
a
b
c
All values are the mean of at least two separate assay determinations.
For assay conditions, see Ref. 19.
Mixture of diastereomers.
Prompted by these findings, we decided to focus on primary
amines and to invest the effort in preparing individual enantiomers
in the cyclopentane series (Scheme 2 and Table 2). All four cyclo-
pentylamine stereoisomers exhibited good NK1R binding activity,
but surprisingly, SERT activity was significantly better for 4 com-
pared with the other stereoisomers (Table 2). Overall, 4 was the
most potent dual NK1R antagonist and SERT inhibitor.
The antidepressant effect of 4 was evaluated in the gerbil forced
swimming test (FST), in which immobility (i.e., no swimming or
struggling) is seen as a measure of despair.23 Gerbils were used
in the forced swim test because mice and rat NK1 receptors are
not highly homologous with the human NK1 receptor, whereas gui-
nea pigs and human NK1Rs are highly homologous. This behavioral
test is one of the most widely used preclinical paradigms for pre-
dicting antidepressant activity of drug candidates. The compound
was administered by oral gavage 30 min prior to testing, and the
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Table 2
IC50 values of cyclopentylaminesa,b
Compd
hNK1R (nM)
hSERT (nM)
4
7.5
31
19
11
4.7
27
25
26
210
350
130
a
All values are the mean of at least two separate assay determinations.
For assay conditions, see Ref. 19.
b

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Y.-J. Wu et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1611–1614
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21. (1S,3S)-24: HPLC retention time: 8.43 min (AD column, 4.6 Â 250 mm, flow
rate 1 mL/min, 90% hexanes/10% ethanol). 1H NMR (CDCl3, 400 MHz) d 2.2 (3H,
m), 2.5 (2H, m), 2.59 (1H, dd, J = 10.8, 10.4 Hz), 3.51 (1H, d, J = 8.8 Hz), 3.56 (1H,
d, J = 8.8 Hz), 4.48 (1H, d, J = 13.2 Hz), 4.53 (1H, d, J = 13.2 Hz), 4.95 (1H, m),
7.31 (5H, m), 7.59 (2H, s), 7.68 (5H, m). 13C NMR (100 MHz, CDCl3) d 28.40,
33.82, 38.38, 49.66, 51.45, 71.75, 77.98, 123.32 (q, J = 271 Hz), 121.32, 123.09,
126.30, 126.80, 126.99, 128.07, 131.56 (q, J = 32 Hz), 132.06, 133.89, 141.23,
146.88, and 168.28. HRMS m/z calcd for C29H22F6NO3 (MÀH)À 546.1504, found
546.1493. (1R,3R)-8: HPLC retention time: 7.64 min (AD column,
4.6 Â 250 mm, flow rate 1 mL/min, 90% hexanes/10% ethanol). The NMR data
is identical to that of its (1S,3S)-7. HRMS m/z calcd for C29H22F6NO3 (MÀH)À
546.1504, found 546.1494.
22. Compound 4: 1H NMR (CDCl3, 400 MHz) d 1.46 (1H, m), 1.60 (2H, broad s), 1.74
(1H, dd, J = 6.0, 12.8 Hz), 2.09 (2H, m), 2.19 (1H, m), 2.48 (1H, dd, J = 6.8,
12.8 Hz), 3.40 (1H, d, J = 8.8 Hz), 3.44 (1H, d, J = 8.8 Hz), 3.53 (1H, quintet,
J = 7.2 Hz), 4.43 (2H, s), 7.25 (5H, m), 7.55 (2H, s), and 7.73 (1H, s). HRMS m/z
calcd for C21H22F6NO (M+H)+ 418.1607, found 418.1625.
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Neuropsychopharmacology 2008, 33, 1919.