Synthesis of enantiomerically pure milnacipran analogs and inhibition of dopamine, serotonin, and norepinephrine transporters

Article abstract of DOI:10.1016/j.bmcl.2007.02.054

A series of Milnacipran analogs with variation in the aromatic moiety were prepared in high enantiomeric excess. Structure-activity relationships for two parallel enantiomeric series are described. The (-)-(1R,2S)-naphthyl analog (8h) showed the highest potency in the two series and is a triple reuptake inhibitor of the SERT, NET, and DAT.

Full text of DOI:10.1016/j.bmcl.2007.02.054

Bioorganic & Medicinal Chemistry Letters 17 (2007) 2834–2837
Synthesis of enantiomerically pure milnacipran analogs and
inhibition of dopamine, serotonin, and norepinephrine transporters
Heidi Roggen,a Jan Kehler,b Tine Bryan Stensbølb and Tore Hansena,*
aUniversity of Oslo, Department of Chemistry, Sem Sælands vei 26, 0315 Oslo, Norway
bH. Lundbeck A/S, Ottiliavej 9, 2500 Copenhagen-Valby, Denmark
Received 26 January 2007; revised 20 February 2007; accepted 21 February 2007
Available online 27 February 2007
Abstract—A series of Milnacipran analogs with variation in the aromatic moiety were prepared in high enantiomeric excess. Struc-
ture–activity relationships for two parallel enantiomeric series are described. The (À)-(1R,2S)-naphthyl analog (8h) showed the high-
est potency in the two series and is a triple reuptake inhibitor of the SERT, NET, and DAT.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Monoamine transporters, including the dopamine trans-
porter (DAT), the serotonin transporter (SERT), and
the norepinephrine transporter (NET), play an impor-
tant role in maintaining the concentration of biogenic
amine neurotransmitters in the central nervous system
(CNS).1 These transporters are involved in different
pathological processes leading to several neurological
disorders.2–7 While cocaine’s reinforcing and behavioral
effects are believed to be mediated via the DAT,8,9 it is
thought that dysfunctional SERT and NET systems in
the CNS play an important role in depression.10,11 Vir-
tually all effective antidepressants increase the synaptic
concentration of serotonin (5-HT) and/or norepineph-
rine (NE) by blocking the reuptake of one or both of
the neurotransmitters. This common property of antide-
pressants was discovered initially with tricyclic antide-
pressants (TCAs).12,13 However, the additional
interactions of TCAs at a variety of neurotransmitter
receptors often result in poor tolerability and toxicity
in overdose.12,14 Thus, the newer antidepressant agents
such as SSRIs (for example fluoxetine 1, Fig. 1) have be-
come popular due to less severe adverse effects.14 How-
ever, they do not represent an improvement over older
antidepressants in terms of efficacy and latency of onset.
As new approaches to antidepressant therapy continue
to be a significant area of CNS research, a new strategy
to address some of these issues has been the develop-
CF3
O
OH
MeO
NHMe
NMe2
1
2
O
H2N
NEt2
O
NHMe
S
3
4
Figure 1. Molecular structures for selected SSRI and SNRIs.
ment of dual reuptake inhibitors of serotonin and nor-
epinephrine. The class of serotonin and norepinephrine
reuptake inhibitors (SNRIs) now comprises three medi-
cations: venlafaxine (2), milnacipran (3), and duloxetine
(4) (Fig. 1). These three drugs block the reuptake of
5-HT and NE with differing selectivity. Whereas milnacip-
ran blocks 5-HT and NE reuptake with approximately
equal potency, duloxetine has a 10-fold selectivity for
5-HT and venlafaxine displays a 30-fold selectivity for
5-HT.15
Keywords: Synthesis; Milnacipran analogs; Uptake inhibition; Dopa-
mine; Serotonin; Norepinephrine.
To the best of our knowledge there are no studies pub-
lished that describe the structure–activity relationship
for enantiomerically pure milnacipran16 and derivatives
*
Corresponding author. Tel.: +47 228 55386; fax: +47 228
55441; e-mail: torehans@kjemi.uio.no
0960-894X/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.02.054

H. Roggen et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2834–2837
2835
thereof with respect to their potency at the SERT, NET,
and DAT. It has, however, been demonstrated that the
aromatic group in milnacipran is essential for binding
to the serotonin transporter.17 Alkyl substitution on
the nitrogen atom leads to a decrease in the binding
affinity to the SERT. In addition to being a 5-HT and
NE reuptake inhibitor, milnacipran is also a weak
NMDA antagonist.17 The binding affinity for the
NMDA receptor is not very high, but milnacipran has
served as a lead in the search for more potent NMDA
receptor antagonist. Several analogs of milnacipran
have been synthesized and tested in vitro.18–22 However,
the main aims of the published studies have been to opti-
mize the structure with respect to the NMDA activity
and not monoamine transporter inhibition. Based on
that information we set out to synthesize both enantio-
mers of a series of milnacipran analogs varying the aro-
matic moiety and to examine their structure–activity
relationships with respect to SNRIs. In this letter, we
describe the asymmetric synthesis of a range of milna-
cipran analogs and their inhibition of the SERT, NET,
and DAT.
Synthesis and yields of the target 2-(aminomethyl)-1-
aryl-N,N-diethylcyclopropanecarboxamide hydrochlo-
rides 8 are outlined in Table 2. The bicyclic lactones
6a–j were converted by aminolysis with diethylamine
in DCM to give the corresponding hydroxyl amides.
Reaction of the hydroxyl amides with CBr4 and PPh3
followed by reaction with sodium azide gave azides
7a–j in good yields. Azides 7a–j were reduced to the cor-
responding amines with H2/Pd/C and the desired target
compounds were isolated by precipitation with HCl to
give the HCl salts 8a–j.
Table 3 summarizes the potency of inhibition of SERT,
NET, and DAT of the two enantiomeric series of milna-
cipran analogs 8.
There was a significant difference in potency between the
two enantiomers of milnacipran 8a. The (+)-(1S,2R)-
isomer showed 20 times higher potency in the inhibition
of NE uptake compared to its enantiomer. The differ-
ence in potency of inhibition of 5-HT uptake between
the two enantiomers was only 3-4-fold, but still favoring
the (+)-(1S,2R)-isomer. None of the two enantiomers
were active at the DAT. For the isomer B series, a con-
sistent trend was observed. The order of potency in
transporter inhibition was NET > SERT > DAT. The
inhibition of the DAT was very low throughout this ser-
ies. The inhibition of the NET varied only slightly with
different aromatic substituents. The only exception was
the 1-naphthyl derivative (+)-8g which experienced a
10-fold drop in potency compared to the rest of the ser-
ies. The 1-naphthyl derivative showed very low affinity
in SERT-inhibition as well. For the rest of the isomer
B series the IC50 values for the SERT-inhibition ranged
from 86 to 790 nM and no apparent systematic trend
was observed.
The general synthetic strategy for the synthesis of enan-
tiomerically enriched milnacipran analogs was based on
published procedures.17,18,21 The one-pot reaction of
various aryl acetonitriles with sodium amide and (+)-
or (À)-epichlorohydrin resulted in the synthesis of a ser-
ies of highly enantioenriched 1-aryl-3-oxa-bicyclo-
[3.1.0]hexane-2-ones as illustrated in Table 1. Both
enantiomeric series of the building blocks 6a–j were
synthesized.
Although the yields are moderate, the method has sev-
eral advantages. It is a one-pot reaction. Both enantio-
mers of epichlorohydrin and many aryl-acetonitriles
are commercially available. The building blocks are ob-
tained in >90% enantiomeric excess.
Table 2. Yields for the synthesis of azides (7) from lactones (6), and 2-
(aminomethyl)-1-aryl-N,N-diethylcyclopropanecarboxamide hydro-
chlorides (8) from azides (7)
Table 1. Yields and enantiomeric excess for the synthesis of 1-aryl-3-
oxa-bicyclo[3.1.0]hexane-2-ones (6) from arylacetonitriles (5)
Ar
NEt2
Ar
NEt2
Ar
Cl-
+
H3N
1. AlCl3, HNEt2
H2/Pd-C
1. NaNH2, PhH
Ar
N3
2. CBr4, PPh3
3. NaN3
HCl, Et2O
O
O
2. (S)-epichlorohydrin
O
O
Ar
CN
3. KOH, EtOH
4. HCl, EtOH
6a-j
7a-j
8a-j
O
O
Ar
Ph
Compound Yielda(%) Compound Yieldb(%)
5a-j
6a-j
7a
7b
7c
47
80
8a
8b
8c
8d
8e
8f
97
11c
75c
78
65
58
78
81
60
93
Compound
Ar
Ph
Yielda(%)
Eeb(%)
3-Cl-Ph
4-Cl-Ph
6a
6b
6c
6d
6e
6f
43
39
44
37
33
45
36
50
25
40
>99
98
90
97
3-Cl-Ph
4-Cl-Ph
3,4-di-Cl-Ph 7d
94
95
4-F-Ph
7e
7f
89
94
3,4-di-Cl-Ph
4-F-Ph
4-MeO-Ph
1-Naphthyl
2-Naphthyl
95
91
7g
7h
83
86
8g
8h
8i
4-MeO-Ph
1-Naphthyl
2-Naphthyl
2-Thiophene
3-Thiophene
6g
6h
6i
>99
98
2-Thiophene 7i
3-Thiophene 7j
>95
72
8j
98
>90
Only one enantiomeric series is shown.
a Isolated yield after column chromatography.
b Isolated yield after precipitation as HCl salts.
c Reduction of azide was accomplished with PPh3 in wet MeOH due to
hydrodechlorination with H2/Pd/C.
6j
Only one enantiomeric series is shown.
a Isolated yields after flash chromatography.
b Measured by chiral HPLC.

2836
H. Roggen et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2834–2837
Table 3. Effects of selected compounds on 5-HT, NE, and DA uptake inhibition
Ar
Ar
NEt2
Cl-
+
Cl-
+
H3N
H3N
NEt2
O
O
(-)-(1R, 2S)-isomer A
(+)-(1S, 2R)-isomer B
Compound
Ar
5-HTa
NEa
DAa
5-HTa
NEa
DAa
8a
8b
8c
8d
8e
8f
Ph
3-Cl-Ph
420d
170
190
84
200d
38
10,000d
10,000
3700
120d
86
7d
33
10,000d
9000
4-Cl-Ph
3,4-di-Cl-Ph
4-F-Ph
140
10
790
440
210
510
7100
130
250
190
44
26
10,000
1700
540
320
230
1200
18
500
140
200
5
10,000
10,000
5300
63
35
10,000
10,000
10,000
1400
4-MeO-Ph
1-Naphthylb
2-Naphthyl
2-Thiophenec
3-Thiopheneb
8g
8h
8i
370
29
140
970
520
250
1500
410
65
19
10,000
10,000
8j
10,000
IC50 values for the synaptosomal uptake inhibition of two enantiomeric series of 2-(aminomethyl)-1-aryl-N,N-diethylcyclopropanecarboxamide
hydrochlorides (8) with the 5-HT, NE, and DA transporters. Values are given in nM. Data shown are means of a minimum of two values. IC50 values
were determined using drug concentrations covering three decades.
a The IC50 values in the biological assays were measured using the procedures described in Ref. 23.
b The sign of optical rotation is reversed compared to the rest of the series.
c Absolute stereochemistry is (1R,2R) for isomer A and (1S,2S) for isomer B.
d Owen et al. have reported IC50 values of 151, 61, and >100,000, respectively, on SERT, NET, and DAT for ( )-milnacipran. See Ref. 24.
The inhibition of DAT uptake was also consistently
lower than uptake inhibition of SERT and NET in the
isomer A series. Most of the compounds in the isomer
A series showed very low inhibition of the DAT. The
selectivity for the NET over the SERT was not consis-
tent throughout the series. Both thiophene analogs (8i
and 8j) and the 4-fluoro derivative (8e) displayed higher
inhibition of the SERT over the NET. Two compounds
displayed significantly higher inhibition of both the
SERT and NET over the others. The 3,4-dichloro-phe-
nyl derivative (À)-8d had IC50 values of 84 and 10 nM
to the SERT and NET respectively, which were similar
to the IC50 values for (+)-milnacipran. The major differ-
ence in binding profile between the two was the differ-
ence in inhibition of the DAT. Compound (À)-8d was
moderately potent, while (+)-milnacipran displayed no
uptake inhibition of the DAT. The naphthyl derivative
(À)-(1R,2S)-8h was the most potent analog of those
tested in both enantiomeric series with IC50 values of
5, 18, and 140 nM to the NET, SERT, and DAT, respec-
tively. The inhibition potency of (À)-8h to the SERT
was similar to (+)-8a. Compound (À)-8h displayed
slightly higher potency to the SERT compared to (+)-
8a, giving a NET/SERT selectivity ratio which was
slightly lower for (À)-8h compared to (+)-8a. To the lim-
it of our awareness, compound (À)-8h is the first exam-
ple of a milnacipran analog which displays significant
inhibition of DAT as well as to the SERT and NET.
Thus, compound (À)-8h can be considered a triple reup-
take inhibitor. The development of triple reuptake
inhibitors for the treatment of depression has only re-
cently been pursued.25 Combination studies have indi-
cated that addition of some inhibition of the DA
uptake site may have some clinical benefits.26–28 The ser-
endipitous discovery that compounds within this series
can be triple reuptake inhibitors merits further study.
In conclusion, these studies describe novel milnacipran
analogs with potent in vitro dual reuptake inhibition.
The (À)-(1R,2S)-isomer of the 1-naphthyl derivative 8h
is a novel example of a triple reuptake inhibitor. Further
SAR studies on the development of triple reuptake
inhibitors will be reported in due course.
Acknowledgments
Financial support by the Chemistry Department, Uni-
versity of Oslo, and the Norwegian Research Council
is gratefully acknowledged.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2007.02.054.
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