Novel 3,3-disubstituted pyrrolidines as selective triple serotonin/norepinephrine/dopamine reuptake inhibitors

Article abstract of DOI:10.1016/j.bmcl.2008.10.025

A series of 3,3-disubstituted pyrrolidine monoamine triple reuptake inhibitors were discovered. Analogues with low nanomolar potency, good human in vitro microsomal stability and in vitro permeability, and low drug-drug interaction potential are described. One example showed in vivo anti-depressant-like effects in the mouse tail suspension assay with a minimum effective dose of 30 mg/kg ip.

Full text of DOI:10.1016/j.bmcl.2008.10.025

Bioorganic & Medicinal Chemistry Letters 18 (2008) 6062–6066
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel 3,3-disubstituted pyrrolidines as selective triple
serotonin/norepinephrine/dopamine reuptake inhibitors
Linda M. Bannwart, David S. Carter, Hai-Ying Cai, Jason C. Choy, Robert Greenhouse, Saul Jaime-Figueroa,
*
Pravin S. Iyer, Clara J. Lin, Eun Kyung Lee, Matthew C. Lucas , Stephen M. Lynch, Ann Marie Madera,
Amy Moore, Kerem Ozboya, Lubica Raptova, Ralf Roetz, Ryan C. Schoenfeld, Karin Ann Stein, Sandra Steiner,
Marzia Villa, Robert J. Weikert, Yansheng Zhai
Roche Palo Alto LLC, 3431 Hillview Avenue, Palo Alto, CA 94304, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 3,3-disubstituted pyrrolidine monoamine triple reuptake inhibitors were discovered. Ana-
logues with low nanomolar potency, good human in vitro microsomal stability and in vitro permeability,
and low drug–drug interaction potential are described. One example showed in vivo anti-depressant-like
effects in the mouse tail suspension assay with a minimum effective dose of 30 mg/kg ip.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 19 September 2008
Revised 3 October 2008
Accepted 7 October 2008
Available online 11 October 2008
Keywords:
Triple reuptake inhibitor
Anti-depressant
Serotonin
Norepinephrine
Dopamine
Monoamine transporter
Pain
Nearly all clinical antidepressants act by elevating levels of one
or more of the monoamine neurotransmitters serotonin (5-HT),
norepinephrine (NE) and dopamine (DA).1 Selective inhibitors of
5-HT and NE reuptake (SNRIs) have been shown to be effective in
the treatment of major depressive disorders (MDD). Venlafaxine
(EffexorÒ) 1 and duloxetine (CymbaltaÒ) 2 are both Food and Drug
Administration (FDA) approved SNRI drugs for the treatment of
MDD. Venlafaxine 1 has been shown to have a higher remission
rate than the class of selective serotonin reuptake inhibitors (SSRIs)
(Fig. 1).2
NHMe
S
O
HO
NMe2
MeO
1 Venlafaxine
2 Duloxetine
Figure 1. Marketed SNRIs.
While the SSRI/SNRI classes of drugs have been shown to pro-
vide a safe and effective treatment for depression there is still a
need for improvement. Triple reuptake inhibitors (TRIs) increase
DA levels in addition to 5-HT and NE.3 Elevating CNS levels of
dopamine through inhibition of the dopamine transporter (DAT)
may address the anhedonic component of depression as well as
shorten the time to onset. This could result in improved efficacy to-
ward a broader range of the depressed population.4 Duloxetine has
also been approved for the treatment of diabetic neuropathic pain
and preclinical data suggest that dopamine transporter (DAT) inhi-
bition may bring additional efficacy.5 Thus, TRIs have the potential
to be more effective in the management of depression and neuro-
pathic pain, and to clearly differentiate from the current treat-
ments. Our aim, therefore, was to generate novel small molecule
inhibitors of all three monoamine transporters that would address
the need for improved therapies and also act as tool compounds to
permit greater understanding of the role of the DAT in MDD.
We recently disclosed a series of indolyl phenyl propylamines
as SNRIs, exemplified by 3.6 Our knowledge of the structure–activ-
ity relationships (SAR) around this template combined with that of
DOV-21947 4, a weakly active TRI that is currently undergoing
Phase II clinical trials by DOV Pharmaceuticals for the treatment
of depression,7 led us to design a series of novel inhibitors based
* Corresponding author. Tel.: +1 6508556292.
E-mail addresses: mattlucas70@hotmail.com, matthew.lucas@roche.com (M.C.
Lucas).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.10.025

L. M. Bannwart et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6062–6066
6063
man in vitro microsomal stability and strong CYP2D6 inhibition.
When the enantiomers were separated most of the potency was
found to reside in a single enantiomer, (+)-16. When tested
in vivo in the mouse tail suspension assay, an assay used to evalu-
ate anti-depressant agents, activity was observed at 30 mg/kg, ip.13
Comparative data for Duloxetine showed anti-depressant like
activity in the same assay with a minimum effective dose of
10 mg/kg ip.
HN
Cl
Cl
4 DOV-21947
R1
B
A
HN
R2
NHMe
Initial SAR investigations focused on replacing the indole ring
(Table 2 and Schemes 2, 6 and 7). Many of the replacements were
essentially equipotent with the indole, but maintaining acceptable
microsomal stability,14 permeability, and CYP2D615 and hERG inhi-
bition levels proved challenging (Table 6). Indole regioisomer 20
retained similar potency but offered no notable benefits. Benzo-
thiophene 18 was balanced and potent at all three SERT/NET/DAT
transporters but unfortunately also showed poor in vitro stability.
Introduction of polarizing groups to increase polar surface area re-
sulted in stabilization towards microsomal clearance. Indazole 17
showed a threefold improvement in metabolic stability but re-
mained a potent CYP2D6 inhibitor. 7-Azaindole 19 gave only a
moderate improvement to stability and lost significant potency.
We next studied the SAR around the hydrophobic aromatic
group (Table 3). In general, substitution was tolerated for potency
but there was no improvement in stability towards human liver
microsomes (Table 6). Replacement of the phenyl ring with a pyr-
idyl group (e.g., 22) did result in an improved CYP2D6 and hERG
profile, but the additional hydrogen bond acceptor also resulted
in P-gp mediated efflux.16 Since minor modifications of the benzyl
group appeared unlikely to yield metabolically stable compounds,
we turned our focus back to the indole ring (Schemes 3–5).
Introduction of substituents in the 2- or 3-position of the indole
ring reduced microsomal clearance and therefore increased meta-
bolic stability (Tables 4 and 6). Inhibition of the hERG channel was
notably worse in the case of 2-cyanoindole 28 despite its increased
polar surface area, and CYP2D6 inhibition remained high. Amide 29
was potent and quite balanced at all three transporters, and
showed a marked reduction in affinity to CYP2D6 and hERG. How-
ever, the two additional hydrogen bond donors resulted in strong
P-gp mediated efflux. The efflux ratio was reduced by successive
replacement of the hydrogen atoms with methyl groups, but could
not be entirely avoided (see 30 and 31). 3-Cyanoindole 32 lost po-
tency and also suffered high P-gp mediated efflux.
3,3-disubstituted
pyrrolidine template
N
H
3
Figure 2. Design of novel triple reuptake inhibitors.
Table 1
In vitro inhibition of monoamine reuptake.a
Compd
SERT
Ki(nM) NET
DAT
3
4
9
63
89
63
>1400
251
a
Monoamine reuptake Ki values are the geometric mean of at least three
experiments.
on a 3,3-disubstituted pyrrolidine motif (Fig. 2 and Table 1). Herein
we describe the synthesis and SAR for this series of triple reuptake
inhibitors.
The target compounds were all prepared using rhodium cata-
lyzed conjugate-addition of aryl boronic acids to maleimides as
the key bond forming step.8 Representative synthetic schemes
are shown (Schemes 1–7).9 Thus, the appropriate boronic acid 5
was coupled with N-benzylmaleimide to generate 1-benzyl-3-
substituted pyrrolidine-2,5-diones 6 as outlined in Scheme 1. Pro-
tection of the indole under phase transfer conditions, followed by
alkylation under mild conditions furnished the 3,3-disubstituted
pyrrolidine-2,5-diones 7. Deprotection of 7 followed by reduction
with lithium aluminum hydride yielded the corresponding 1-ben-
zylpyrrolidine 9. Finally, a hydrogenolysis yielded the desired pyr-
rolidines 10 and 11.
The first compound synthesized, 16, was a potent inhibitor at all
three of the monoamine reuptake transporters (Table 2). We were
excited to find that it exhibited significantly higher potency than
DOV-21947.10 In addition, it showed favorable in vitro permeabil-
ity and hERG inhibition (Table 6).11,12 Liabilities included low hu-
We next investigated replacement of the aromatic hydrophobe
portion of our pharmacophore with an aliphatic one (Tables 5 and
6). Increasing metabolic stability by the introduction of polarizing
O
O
OH
R1
B
a
b, c
d
BnN
BnN
HO
O
O
N
N
H
N
H
CO2Me
7
5
6
O
e
R1
f
R1
R1
BnN
BnN
HN
or
g, f
O
N
H
N
N
H
R2
8
9
R2 =H, 10
R2 =CH3, 11
Scheme 1. Reagents and conditions: (a) N-benzylmaleimide, [RhOH(cod)]2 Et3N], dioxane, H2O, rt, 99%; (b) BnNEt3Br, ClCO2Me, 30% NaOH, 0 °C, 80%; (c) RX, K2CO3, DMF, rt,
77%; (d) LiOH, THF, 80 °C, 91%; (e) LiAlH4, THF, 80 °C, 90%; (f) H2, 20% Pd(OH)2/C, MeOH, 60 psi, 54%; (g) NaH, DMF, rt, then MeI, rt, 62%.

6064
L. M. Bannwart et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6062–6066
O
Ph
Ph
a
b, c
d
O
B
BnN
BnN
HN
O
O
S
S
S
S
12
13
18
14
Scheme 2. Reagents and conditions: (a) N-benzylmaleimide, [RhOH(cod)]2 Et3N], dioxane, H2O, rt, 97%; (b) BnBr, K2CO3, DMF, 45 °C; (c) LiAlH4, THF, 80 °C, 61% (2 steps); (d)
-chloroethyl chloroformate, Et3N, 1,2-DCE then MeOH, 80 °C, 15%.
a
Ph
R3
Ph
Ph
Ph
R3
R1
a - d
N
d
BnN
BnN
HN
e
R4
R4
N
N
CO2Et
or
a, b
N
N
N
N
O
O
H
H
H
R2
36
R3 =H, R2 =CH3, 38
R1 =CH3, R2 =CH3, 39
R3 =H, R4 =CH3, 30
R3 =CH3, R4 =CH3, 31
R1 =R2 =H, 27
or
R1 =Bn, R2 =Boc, 37
Scheme 3. Reagents and conditions: (a) Boc2O, DMAP, Et3N, CH2Cl2, 76%; (b) t-BuLi, THF, À78 °C, 1 h then ClCO2Et, 75%; (c) TFA, CH2Cl2, rt, 100%; (d) H2, 20% Pd(OH)2/C,
MeOH, 60 psi, 54% for compound 30; (e) R1R2NH.HCl, AlMe3, PhH, 80 °C, 32% for compound 38.
Ph
Ph
a, b, c
Ph
Ph
e
d
HN
HN
BnN
HN
CN
Boc
CN
CONH2
N
N
N
N
H
H
H
36
40
28
29
Scheme 4. Reagents and conditions: (a) Boc2O, DMAP, Et3N, CH2Cl2, 76%; (b) t-BuLi, THF, À78 °C, 1h then PhOCN, 53%; (c)
MeOH, 76%; (d) KOH, 100 °C, 18 h, 48%; (e) KOH, 100 °C, 24 h, 34%.
a
-chloroethyl chloroformate, Et3N, 1,2-DCE then
Ph
Ph
Ph
Ph
c
a
b
CHO
CN
CN
BnN
BnN
BnN
HN
N
N
N
N
H
H
H
H
36
41
42
32
Scheme 5. Reagents and conditions: (a) (COCl)2, DMF, 0 °C then NaOH, 18%; (b) MnO2, NH3, iPrOH, MgSO4, THF, 50 °C, 15 h, 73%; (c) H2, 20% Pd(OH)2/C, MeOH, 60 psi, 67%.
O
Ph
Ph
Br
O
B
a
b, c
d, e
HN
BnN
O
N
N
O
N
TBDMS
N
N
N
N
N
H
TBDMS
TBDMS
43
44
45
19
Scheme 6. Reagents and conditions: (a) bis(pinacolato)diborane, Pd(dppf)Cl2. KOAc, dioxane, 95 °C, 90 min 47%; (b) N-benzylmaleimide, [RhOH(cod)]2ÁEt3N], dioxane, H2O,
rt, 52%; (c) BnBr, K2CO3, DMF, rt, 67%; (d) LiAlH4, THF, 80 °C, 67%; (e) H2, 20% Pd(OH)2/C, MeOH, 60 psi, 76%.
O
Ph
Ph
Ph
O
B
a, b
c, d
e
BnN
HN
BnN
O
N
N
N
N
O
N
N
N
N
H
H
THP
THP
46
47
48
17
Scheme 7. Reagents and conditions: (a) N-benzylmaleimide, [RhOH(cod)]2 Et3N], dioxane, H2O, rt, 80%; (b) BnBr, K2CO3, DMF, rt, 77%; (c) LiAlH4, THF, 80 °C, 63%; (d) 2 N HCl,
MeOH, 50 °C, 63%; (e) H2, 20% Pd(OH)2/C, MeOH, 60 psi, 53%.
groups had always come at the expense of in vitro permeability
(vide supra). Our earlier discovery that replacing the benzyl group
with a pyridine had minimized CYP2D6 and hERG binding led us to
infer that the benzyl group played a major role in binding to these
‘off-targets’. Loss of potency was observed with methyl compound
33, indicating that the pharmacophore required a significant

L. M. Bannwart et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6062–6066
6065
Table 2
Table 5
In vitro inhibition of monoamine reuptake.a
In vitro inhibition of monoamine reuptake.a
R
HN
N
H
HN
Ar
Compound
R
Ki (nM)
Compd
Ar
Ki (nM)
SERT
NET
DAT
SERT
NET
DAT
11
11
81
115
16
741
43
33
34
35
Me
nPr
nBu
871
66
16
288
71
9
550
363
60
16
5-indolyl
10
6
54
1
1
1
17
5
3
41
1
(+)-16
(-)-16
17
18
19
5-indolyl
5-indolyl
5-indazolyl
5-benzothiophenyl
5-(7-azaindolyl)
6-indolyl
NT denotes not tested.
a
7
Measured at 30 °C.
41
13
8
20
(+)-20
(-)-20
21
6-indolyl
6-indolyl
3-indolyl
1
21
190
23
78
60
447
1260
Table 6
In vitro human microsomal stability, CYP2D6 inhibition, CACO-2 efflux ratio, and
a
hERG inhibitiona for selected compounds.
Monoamine reuptake Ki values are the geometric mean of at least three
experiments.
Compound HLM (
l
L/min/ CYP 2D6 IC50
ER (BA/AB) (cm/s
hERG IC50
(lM)
mg)
(lM)
Â10À6
)
Table 3
3
4
915
47
100
154
28
198
69
NT
NT
NT
NT
86
3.1
7
NT
1
10
9
In vitro inhibition of monoamine reuptake.a
0.7 (2.1/2.9)
0.8 (14/17)
0.5 (9.9/21)
2.2 (22/10)
NT
NT
NT
NT
NT
16
(+)-16
17
18
19
0.5
0.3
0.4
NT
11.5
2.6
0.9
1.6
1.8
3.5
0.1
0.1
0.3
0.1
0.6
0.1
4.1
4.8
2.9
0.8
8.7
7.5
3.4
R
NT
NT
11
NT
NT
NT
NT
NT
>30
9
HN
N
H
20
(+)-20
(À)-20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Compound
R
Ki (nM)
SERT
NET
DAT
NT
22
23
24
25
Pyridin-2-ylmethyl
4-F-Bn
3-MeO-Bn
53
15
34
13
8
7
11
16
62
79
39
58
5.6 (18/3.2)
NT
NT
0.4 (4.5/11.4)
NT
1.0(3.4/3.5)
1.0 (7.8/7.9)
28.3 (17/0.6)
16.0 (19/1.4)
5.9 (24/4.0)
11.3 (25/2.2)
NT
84
77
NT
NT
7
CH2CH2Ph
106
106
47.8
0
1.4
11.3
12.8
18.8
NT
a
Monoamine reuptake Ki values are the geometric mean of at least three
NT
1.4
>10
>10
NT
NT
NT
NT
4.4
experiments.
Table 4
In vitro inhibition of monoamine reuptake.a
2.3
38
1.5 (24/16.5)
2.0 (19/9.4)
NT denotes not tested.
HN
a
R
Measured at 30 °C.
N
H
Compound
R
Ki (nM)
bility suffered slightly, the overall in vitro profile of this compound
was significantly improved.
SERT
NET
DAT
In summary, we were able to combine two discrete sub-struc-
tures to arrive at a novel series of potent triple serotonin, norepi-
nephrine and dopamine reuptake inhibitors. We were not only
able to optimize this novel chemical series for both potency and
selectivity, but also we modulated CYP2D6 and successfully im-
proved metabolic stability in human liver microsomes. Overall,
we have discovered that subtle chemical modifications in the
structure lead to an enormous effect on these properties. Addi-
tional information on the progress of this scaffold will be reported
in due course.
26
27
28
29
30
31
32
1-(N)-Me
2-CO2Et
2-CN
2-CONH2
2-CONHCH3
2-CON(CH3)2
3-CN
4
4
0.4
2
13
22
66
1
19
10
9
151
10
5
10
38
83
18
331
104
724
a
Monoamine reuptake Ki values are the geometric mean of at least three
experiments.
hydrophobe for tight binding to the monoamine reuptake trans-
porters. The propyl-substituted compound 34 recovered potency
and demonstrated a 40-fold increase in stability upon incubation
with human liver microsomes (HLMs). CYP2D6 inhibition also con-
currently improved approximately 15-fold. We were also able to
increase potency at the target transporters further with the butyl
compound 35, and although CYP2D6 binding and microsomal sta-
Acknowledgments
The authors thank in particular Marquis Cummings Jr. of the
Analytical Separations Group for the timely separation of enantio-
mers and Francisco Talamas for his invaluable contributions to this
work.

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8944; (f) Hayashi, T.; Takahashi, M.; Takaya, Y.; Ogasawara, M. J. Am. Chem. Soc.
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the compound set.
10. DOV-21947 was prepared in house and submitted to assay: pKi (nM) SERT/
NET/DAT = 63/63/251.
11. Permeability was measured using Caco-2 cells at 21 days of culture. Efflux
ratios are measured using the bi-directional (Papp A to B and B to A) values.
Substrate concentration 10 lM.
12. hERG IC50 values calculated using the PatchXpress 7000A (MDS, Inc., Toronto,
Canada) to carry out electrical recording of K+ currents in CHO cells expressing
the human ERG channel. Recordings were made at 30 °C with test compounds
at two concentrations (1
13. Steru, L.; Chermat, R.; Thierry, B.; Simon, P. Psychopharmacology 1985, 85, 67.
14. CLint measured in HT format: total incubation volume 250 L, 0.5 mg/mL
microsomal protein, reaction started by adding NADPH and performed at 37 °C
with no shaking. 40 L aliquots at four time-points (0.1, 10, 20 and 30 min)
lM, 10 lM).
l
l
were taken then the samples transferred to 384 well plates for LC/MS analysis.
15. CYP2D6 inhibition was measured in a fluorescence based assay where CYP
marker substrates become fluorescent upon CYP metabolism.
16. P-gp inhibition was confirmed by addition of 2
in the Caco-2 permeability assay.
lM elacridar as P-gp inhibitor