Structure-activity relationships of the 1-amino-3-(1H-indol-1-yl)-3-phenylpropan-2-ol series of monoamine reuptake inhibitors

Article abstract of DOI:10.1016/j.bmcl.2009.07.050

The SAR of a series of 1-amino-3-(1H-indol-1-yl)-3-phenylpropan-2-ols as monoamine reuptake inhibitors, with a goal to improve both potency toward inhibiting the norepinephrine transporter and selectivity over the serotonin transporter, is reported. The e

Full text of DOI:10.1016/j.bmcl.2009.07.050

Bioorganic & Medicinal Chemistry Letters 19 (2009) 5807–5810
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Structure–activity relationships of the 1-amino-3-(1H-indol-1-yl)-
3-phenylpropan-2-ol series of monoamine reuptake inhibitors
a
a
a
a
Paige E. Mahaney a, , Callain Y. Kim , Richard D. Coghlan , Stephen T. Cohn , Gavin D. Heffernan ,
Christine A. Huselton b, Eugene A. Terefenko a, An T. Vu a, Puwen Zhang a, Kevin D. Burroughs c,
Scott A. Cosmi c, Jenifer A. Bray c, Grace H. Johnston c, Darlene C. Deecher c, Eugene J. Trybulski a
*
a Chemical Sciences, Wyeth Research, Collegeville, PA 19426, United States
b Drug Safety and Metabolism, Wyeth Research, Collegeville, PA 19426, United States
c Women’s Health, Wyeth Research Collegeville, PA 19426, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The SAR of a series of 1-amino-3-(1H-indol-1-yl)-3-phenylpropan-2-ols as monoamine reuptake inhibi-
tors, with a goal to improve both potency toward inhibiting the norepinephrine transporter and selectiv-
ity over the serotonin transporter, is reported. The effect of specific substitution on both the 3-phenyl
group and the indole moiety were explored. This study led to the discovery of compound 20 which inhib-
ited the norepinephrine transporter with an IC50 value of 4 nM while exhibiting 86-fold selectivity over
the serotonin transporter.
Received 28 May 2009
Accepted 8 July 2009
Available online 25 August 2009
Keywords:
Vasomotor symptoms (VMS)
Norepinephrine reuptake inhibitors (NRI)
Monoamine reuptake inhibitors
Serotonin transporter
Ó 2009 Published by Elsevier Ltd.
The norepinephrine transporter (NET) is a protein that consists
of 12-transmembrane domains and functions to sense the concen-
tration of free norepinephrine (NE) in the synapse and transport
excess NE back into presynaptic cells where it is either recycled
and re-released or metabolized.1 NE has been shown to stimulate
areas of the hypothalamus that are important in temperature reg-
ulation.2 Furthermore, depletion of estrogens as a result of meno-
pause may produce unstable concentrations of NE resulting in
fluctuations in body temperature which manifest as vasomotor
symptoms (VMS) or hot flashes.3 Previously, we disclosed that res-
toration of NE levels in ovariectomized rats by the administration
of a non-selective norepineprine reuptake inhibitor (NRI), desipra-
mine (1), could alleviate VMS and restore normal thermoregula-
tion.4 Thus, the goal of this program was to identify and develop
novel and selective NRIs for the treatment of temperature dysreg-
ulation associated with menopause and for evaluation toward
additional conditions that have been reported to be ameliorated
by NRIs including major depressive disorder (MDD),5 attention def-
icit hyperactivity disorder (ADHD),6 and certain pain disorders
including fibromyalgia7,8 and low back pain.7
phenylpropan-2-ols 2, which were identified by combining virtual
and focused screening efforts with design techniques.9 These
efforts led to the identification of Discovery lead molecule 3, the
2R,3S-isomer of 2, which potently inhibited NET with an IC50 value
of 28 nM and exhibited modest, 13-fold selectivity over the seroto-
nin transporter (SERT). Herein, we summarize our efforts to not
only optimize the potency of this series toward inhibiting NE up-
take, but also and more importantly, to identify compounds with
excellent selectivity (target value = 100-fold) over SERT. Com-
pounds were screened in vitro for their ability to block NE uptake
in MDCK-Net6 cells that were stably transfected with the human
norepinephrine transporter (hNET). Serotonin (5-HT) uptake was
measured in a similar manner using JAR cells that were stably
transfected with the human serotonin transporter (hSERT). Finally,
affinity for the human dopamine transporter (hDAT) was assessed
via a competition radioligand-binding assay using purified CHO
membranes expressing recombinant hDAT with the cocaine ana-
log, [3H]-3b-(4-fluorophenyl)tropane-2b-carboxylic acid methyl
ester ([3H]WIN35,428),10 as the radioligand. All assay procedures
have been formerly reported.11
Our group previously reported the discovery of a novel series of
monoamine reuptake inhibitors, the 1-amino-3-(1H-indol-1-yl)-3-
Previously,9 we reported two important features that were
critical for optimal potency and selectivity within the 1-amino-3-
(1H-indol-1-yl)-3-phenylpropan-2-ol scaffold (Fig. 1). First, only
secondary amines with small alkyl substituents or a primary amine
were tolerated for hNET activity, with the secondary methylamine
providing optimal potency. And second, of the four stereoisomers,
* Corresponding author. Present address: Boehringer Ingelheim Pharmaceuticals,
Ridgefield, CT, United States. Tel.: +1 203 798 4184.
E-mail address: paige.mahaney@boehringeringelheim.com (P.E. Mahaney).
0960-894X/$ - see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2009.07.050

5808
P. E. Mahaney et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5807–5810
no)-3-phenylpropan-2-olamines that were reported in an earlier
communication.13 Consequently, the meta-fluoro substituent was
held constant when we examined the effect of substitution on
the indole ring (analogs 20–36).
R
CH3
(R)
N
(S)
N
H
N
N
H
N
OH
OH
NHCH3
Substitution on the indole ring, in general, was well tolerated
and resulted in only modest improvements or losses in inhibition
of NE uptake when compared to unsubstituted analog 12. Interest-
ingly, substitution on any of the 4-, 5-, 6-, or 7-positions of the in-
dole ring reduced selectivity for NET over SERT versus the
unsubstituted analog 12. Most notably, substitution on the 5-posi-
tion of indole (analogs 23, 27, 30, and 34) essentially eliminated
selectivity. In contrast, alkyl substitution on the indole 3-position
(compounds 20 and 21) improved selectivity for NET versus SERT
compared to 12. In fact, not only did 3-methylindole 20 nearly
meet our program selectivity goal of 100-fold by exhibiting 86-fold
selectivity for NET versus SERT, it was also the most potent NRI we
had made to date exhibiting an IC50 value of 4 nM.
The pharmacokinetic profile of 20 was examined in rats after a
single oral dose of 10 mg/kg versus a single intravenous dose of
2 mg/kg. Although 20 exhibited poor oral bioavailability of 6%, the
brain to plasma ratio, obtained by comparing plasma area-under-
the-curve (AUC) to brain AUC, was determined to be 3.5. Since
the plasma concentration of 20 was significantly higher than the
in vitro IC50 value over 8 h after oral administration (Cmax = 87
ng/mL or 280 nM; 8 h plasma concentration = 12 ng/mL or 40
nM), it was assessed in a telemetric rat model of ovariectomy-in-
duced thermoregulatory dysfunction using previously reported
procedures.11 The results are summarized in Table 2. Compound
20 significantly reduced tail skin temperature (TST) in a dose-
dependent manner while its duration of action was consistent with
pharmacokinetic findings.
In an effort to determine the contribution of the 2-hydroxyl
group toward NET potency and selectivity over SERT and DAT, data
from the (2R,3S)-1-amino-3-(1H-indol-1-yl)-3-phenylpropan-2-ols
were compared to selected analogs within a series of (3R)-3-(1H-
indol-1-yl)-N-methyl-3-phenylpropan-1-amines (compounds 37
and 38, Table 1), a scaffold we reported previously.14 A direct com-
parison of the unsubstituted analogs, compound 3 versus com-
pound 37, suggested that the 2-hydroxyl group does not
contribute to either NE uptake inhibition or selectivity over SERT
since the values were essentially identical (NET IC50 values of
28 nM versus 34 nM for 3 and 37, respectively, with corresponding
SERT IC50 values of 358 nM versus 491 nM). However, a direct com-
parison of the meta-fluoro analogs 12 and 38 revealed a different re-
sult. While compound 12, which incorporated the 2-hydroxy group,
was a potent inhibitor of NE uptake (IC50 = 11 nM) with good selec-
tivity over SERT of 40-fold, des-hydroxy analog 38 was eightfold less
potent at inhibiting NET versus 12 but was a potent inhibitor of 5-
HT uptake (IC50 = 14 nM) with nearly sevenfold selectivity for SERT
versus NET. Clearly, however, the 2-hydroxyl group provided re-
duced affinity for DAT versus the des-hydroxy analogs.
desipramine (1)
3
2
hNET IC50 = 28 nM
hSERT IC50 = 358 nM
hSERT/hNET = 13
Figure 1. Structures of despiramine (1), the 1-amino-3-(1H-indol-1-yl)-3-phenyl-
propan-2-ol scaffold 2 and discovery lead compound 3.
the 2R,3S-stereochemistry afforded the most potent NE uptake
inhibition along with the best selectivity over the hSERT. Conse-
quently, SAR studies reported herein focus specifically on the
2R,3S-isomer with the N-methyl amine moiety held constant.
(2R,3S)-1-Amino-3-(1H-indol-1-yl)-3-phenylpropan-2-ol ana-
logs 8–36 were synthesized stereospecifically as outlined in
Scheme 1. The synthesis started with (2R,3R)-3-phenylglycidols 6
which were either obtained from commercial sources or prepared
in three steps from trans-cinnamic acids 4. Thus, trans-cinnamic
acids 4 were converted to trans-cinnamyl alcohols 5 via a DIBAL
reduction of trans-cinnamate esters which were obtain by O-meth-
ylation of the carboxylic acid group mediated by cesium carbonate.
Subsequent Sharpless epoxidation12 of trans-cinnamyl alcohols 5
afforded (2R,3R)-3-phenylglycidols 6 in excellent enantiomeric ex-
cess. Epoxide opening with substituted indoles occurred both regio-
and stereo-selectively using sodium tert-butoxide in the presence
of titanium iso-propoxide to afford propane-1,2-diols 7 which were
converted in three steps to (2R,3S)-1-amino-3-(1H-indol-1-yl)-3-
phenylpropan-2-ol analogs 8–36 as previously reported.9
An examination of the effect of substitution on the 3-phenyl
group (compounds 8–19) revealed that, in general, substitution
on the meta -position provided the most potent NE uptake
inhibition versus either ortho or para substitution. For example,
meta-chloro analog 9 potently inhibited NE uptake with an IC50
value of 16 nM versus 304 nM and 527 nM observed with
ortho-chloro and para-chloro analogs 8 and 10, respectively. With-
in the group of compounds that we examined, this pattern was
maintained regardless of the electronic nature of the substituent.
In addition, substitution on the meta-position with an electron-
withdrawing group (analogs 9, 12, 18, and 19) improved the selec-
tivity for NET versus SERT when compared to the unsubstituted
analog 3. Although analogs 9, 12, 18 and 19 exhibited similar selec-
tivity for NET over SERT (34–40-fold), meta-fluoro analog 12 pro-
vided the best combination of potent NE uptake inhibition
(IC50 = 11 nM) and drug-like properties (metabolic stability,
solubility, CYP450 inhibition profile, etc.). This observation was
consistent with trends observed in a related series of 3-(arylami-
In summary, the SAR of a series of (2R,3S)-1-amino-3-indol-1-yl)-
3-phenylpropan-2-ols was examined. In general, substitution on the
meta-position of the 3-phenyl group with electron-withdrawing
substituents afforded potent NRIs with good selectivity (34–40-
fold) over SERT and only weak affinity for DAT. Alkyl substitution
at the 3-position of indole further enhanced selectivity for NET ver-
sus SERT. Combining the meta-fluoro substituent on the 3-phenyl
group with 3-methyl indole, compound 20, provided a highly
potent NRI (IC50 = 4 nM) that exhibited 86-fold selectivity over
SERT. Compound 20 was tested in a telemetric rat model of ovari-
ectomized-induced thermoregulatory dysfunction and reduced
TST in a dose-dependent manner; however, it exhibited low oral
bioavailability in rats. Efforts to improve the pharmacokinetic
parameters within this series will be the subject of a future
communication.
O
O
(R)
R1
R1
R1
a,b
c
OH
OH
OH
R2
(R)
5
6
4
R1
R1
R2
e, f, g
d
CH3
HCl
(R)
(S)
(S)
(S)
N
N
OH
N
H
OH
OH
7
3, 8-36
Scheme 1. Synthesis of (2R,3S)-1-amino-3-(1H-indol-1-yl)-3-phenylpropan-2-ol
analogs. Reagents and conditions: (a) Cs2CO3, CH3I, acetone; (b) DIBAL, CH2Cl2,
À78°, 82–94% for two steps; (c) D-(À)-DIPT, t-BuOOH, Ti(i-PrO)4, À25 °C, 60–72%,
>99% ee; (d) indole, NaH, t-BuOH, Ti(i-PrO)4, CH2Cl2, 25–68%; (e) TsCl, pyridine, rt;
(f) CH3NH2, CH3OH; (g) HCl, ethanol, iso-propyl ether, 21–52% for three steps.

P. E. Mahaney et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5807–5810
5809
Table 1
Characterization of (2R,3S)-1-amino-3-(1H-indol-1-yl)-3-phenylpropan-2-ol analogs compared to (3R)-3-(1H-indol-1-yl)-N-methyl-3-phenylpropan-1-amines at the human
norepinephrine, serotonin and dopamine transportersa
R1
R1
7
7
R2
CH3
CH3
(R)
(S)
(R)
N
N
H
N
H
N
4
4
OH
3
3
37-38
3, 8-36
Compd
R1
R2
hNET uptake IC50b nM
(SD)
hSERT uptake IC50c nM
(SD)
Ratio of hSERT uptake IC50/hNET
uptake IC50
hDAT binding %inhibition @
1 l
Me (%)
d
3
8
9
H
H
H
H
H
H
H
H
H
H
H
H
H
28 (11)f
304
16 (1)f
527
92 (35)f
11 (3)f
84
358 (108)f
1333
554 (8)f
748
13
4
34
1
19
7
20
16
4
23
11
11
8
0
0
13
14
8
0
12
6
9
0
4
0
7
3
o-Cl
m-Cl
p-Cl
o-F
m-F
p-F
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
473
5
441 (7)f
1366
266
40
16
1
8
3
o-CH3
m-CH3
p-CH3
m-OCH3
m-Br
m-CF3
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
m-F
H
232
49
117
111
14
377
398
882
547
8
39
40
86
83
3
1
9
2
15
2
29
26
4
31
25
4
1
8
5
14
0.2
H
54 (8)f
4 (1)f
10 (3)f
64
2184 (383)f
343 (50)f
834 (109)f
221
3-CH3
3-CH3CH2
4-Cl
5-Cl
6-Cl
7-Cl
4-CH3
5-CH3
7-CH3
4-F
5-F
6-F
7-F
4-Br
5-Br
6-Br
7-Br
N/A
N/A
60
92
21
15
21
18
79
865
48
228
48
527
16 (5)f
22
417 (75)f
78
9
15
8
24
10
12
0
44
52
20 (7)f
7 (2)f
52
626 (45)f
175 (38)f
207
60
124
21
34
91
52
1003
113
491
14
F
a
Data is the average of at least three triplicate runs; standard error (SE) 6 30% of the mean, unless otherwise indicated. NT = not tested.
Inhibition of NE uptake in MDCK-Net6 cells, stably transfected with hNET. Desipramine (IC50 = 3.4 + 1.6 nM) was used as a standard.
Inhibition of serotonin uptake in JAR cells, stably transfected with human SERT. Fluoxetine (IC50 = 9.4 3.1 nM) was used as a standard.
Unitless value as a ratio in which higher numbers represent relatively greater NET selectivity. A value of 1 represents no selectivity.
b
c
d
e
f
Inhibition of [3H]WIN35,428 binding to membranes from CHO cells expressing recombinant hDAT. Mazindol (22.1 6.5 nM) was used as a standard.
Data is the average of between 2 and 6 independent experiments, each run in triplicate.
Table 2
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A.; Gaszner, P.; Aguglia, E.; Batista, R.; Castillo, A., et al Hum. Psychopharmacol.
1998, 13, S29–S39.
Oral activity of 20 in a telemetric rat model of ovariectomized-induced thermoreg-
ulatory dysfunctiona
Dose
(mg/kg)
Onset of
activity (h)
Duration of
action (h)
Mean reduction
Maximum reduction
in TSTb (°C)
in TSTb (°C)
10
30
1
0.5
5
10.5
À1.5
À3.8
À2.3
À7.0
a
Compound was dosed orally (n = 16 rats) via 2% Tween-80/0.5% methylcellulose
in water vehicle.
b
6. Bymaster, F. P.; Katner, J. S.; Nelson, D. J.; Hemrick-Luecke, S. K.; Threlkeld, P. J.;
Heiligenstein, J. H.; Morin, S. M.; Gehlert, D. R.; Perry, K. W.
Neuropsychopharmacology 2002, 27, 699–711.
TST = tail skin temperature.
7. Krell, H. V.; Leuchter, A. F.; Cook, I. A.; Abrams, M. Psychosomatics 2005, 46,
379–389.
8. Bergan, T. Can. J. Psychiatry 2004, 49, 499–500.
Acknowledgement
9. Kim, C. Y.; Mahaney, P. E.; McConnell, O.; Zhang, Y.; Manas, E.; Ho, D. M.;
Deecher, D. C.; Trybulski, E. J. Bioorg. Med. Chem. Lett. 2009, 19, 5029–5032.
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Chem. 2008, 51, 4038.
The authors thank the Wyeth Discovery Analytical Chemistry
group for spectral data and compound analysis.
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