2-Substituted N-aryl piperazines as novel triple reuptake inhibitors for the treatment of depression

Article abstract of DOI:10.1016/j.bmcl.2010.05.008

Recently a class of compounds known as triple reuptake inhibitors has emerged as a new strategy for the treatment of depression. These compounds work by simultaneously inhibiting the synaptic reuptake of serotonin, norepinephrine and dopamine. In this Let

Full text of DOI:10.1016/j.bmcl.2010.05.008

Bioorganic & Medicinal Chemistry Letters 20 (2010) 3941–3945
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
2-Substituted N-aryl piperazines as novel triple reuptake inhibitors
for the treatment of depression
*
David S. Carter , Hai-Ying Cai, Eun Kyung Lee, Pravin S. Iyer, Matthew C. Lucas, Ralf Roetz,
Ryan C. Schoenfeld, Robert J. Weikert
Department of Medicinal Chemistry, 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:
Recently a class of compounds known as triple reuptake inhibitors has emerged as a new strategy for the
treatment of depression. These compounds work by simultaneously inhibiting the synaptic reuptake of
serotonin, norepinephrine and dopamine. In this Letter we describe the optimization of a novel series
of 2-substituted N-aryl piperazine based triple reuptake inhibitors.
Received 15 March 2010
Revised 4 May 2010
Accepted 5 May 2010
Available online 10 May 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Triple reuptake inhibitors
Reuptake inhibitors
Depression
Serotonin
Norepinephrine
Dopamine
It is estimated that as much as 17% of the US population 18 and
older will suffer from depression at some point in their lifetime.1
Although any age group can be effected, depression predominately
strikes 25–44 year olds. Loss of productivity from this group is esti-
mated to be more than $31 billion per year.2 Depression is character-
ized by the chronic feeling of sadness. Symptoms include depressed
mood, loss of interest, disruption in sleep patterns, fatigue and
sometimes suicidal tendencies. Not only can these symptoms them-
selves be debilitating but they often co-occur with other chronic
conditions such as cancer, stroke, heart disease, pain and HIV.3
It is hypothesized that depression results from abnormally low
levels of certain biogenic amines.4 These small molecule mono-
amine neurotransmitters are responsible for relaying and modulat-
ing signals between neurons. Of all the neurotransmitters that have
been identified to date, three have been identified as playing an
important role in depression: serotonin, norepinephrine and dopa-
mine.5 Serotonin is believed to be responsible for the feeling of well
being and is important in the perception of emotional cues.6 Norepi-
nephrine is involved in cognition, mood, and the fight or flight re-
sponse.7 Dopamine is central to the pleasure and reward system.8
Currently marketed treatments for depression rely on boosting
the levels of one or more of these neurotransmitters by selective
inhibition of transporter mediated reuptake of these amines. For
example, fluoxetine (Prozac, 1) is a selective serotonin reuptake
inhibitor (SSRI). While it does demonstrate clinical efficacy, about
30% of patients don’t respond to therapy. Furthermore, delayed on-
set of action (3–6 weeks) coupled with side effects such as insomnia
and sexual dysfunction contribute to treatment cessation.9 It has
been suggested that some patients do not respond to SSRIs because
they may have different symptomatic profiles (i.e., irritability or
anxiety in addition to depressive symptoms).10 These patients may
see a benefit from compounds like duloxetine (Cymbalta, 2) which
is a dual serotonin and norepinephrine reuptake inhibitor (SNRI).
The success of this strategy is still under investigation.
H
Cl
N
H
S
N
O
Cl
O
N
H
CF3
fluoxetine (1)
duloxetine (2)
DOV-216,303 (3)
More recently, compounds such as DOV-216,303 (3) have
emerged as an import new class of compounds for the treatment
of depression.11 The effectiveness of this class of compounds, re-
ferred to as ‘triple reuptake inhibitors’, is supported by clinical data
which demonstrates that dopamine reuptake inhibition addresses
some of the unmet needs of both SSRIs and SNRIs.12 For example
augmentation trials with buproprion, which is a NET/DAT reuptake
inhibitor, and an SSRI demonstrated improved efficacy and less
* Corresponding author. Tel.: +1 650 855 5615; fax: +1 650 855 5238.
E-mail address: david.carter5@comcast.net (D.S. Carter).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.05.008

3942
D. S. Carter et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3941–3945
sexual side-effects over an SSRI alone.13 Moreover, methylpheni-
date augmentation led to an accelerated onset of action.14 Together
these results demonstrate that compounds that act as triple reup-
take inhibitors may be advantageous in treating depression.
Recently we reported on a novel C5-aminoindole series of triple
reuptake inhibitors.15 Compounds such as 4-aminopiperidine 4
were potent reuptake inhibitors at all three transporters (SERT,
NET, and DAT),16 had good drug-like properties,17 but lacked
in vivo efficacy. Compound 4 was inactive at 30 mg/kg in the loco-
motor assay (LMA) and the tail suspension test (TST) in mice. This
was surprising since this compound was expected to have good
CNS penetration.18 Another method for predicting in vivo efficacy
is the measurement of receptor occupancy. For compound 4 the per-
centage of occupied DAT receptors in mouse (% DAT occu-
pancy = concentration in striatum/concentration in cerebellum)
was 0%. Based on this result, the lack of in vivo activity is not surpris-
ing since human clinical data from buproprion augmentation trials
demonstrated that approximately 20% DAT occupancy was required
for efficacy.19 We hypothesize that the lack of dopamine transporter
occupancy was likely due to the insufficient unbound concentration
required to occupy the dopamine transporters in the striatum.
Although not measured directly, we expect the SERT/NET occupancy
would similarly be lower than the activity threshold.
side-chain modification
- CYP2D6 inhibition
- 3A4 TDI
HN
N
indole analogs
- clearance
N
H
5
Figure 1. The optimization strategy for piperazine 5.
Based on the SAR from an earlier template,24 we suspected that
the aminoindole moiety was responsible for the high levels of NET
and DAT potency. Unfortunately, the 2,3-unsubstituted indole was
most likely responsible for the low metabolic stability. We there-
fore prepared several analogs to determine the important features
of the indole (Table 1). The phenyl analogs (6, 7) lacked sufficient
NET and DAT activity suggesting that the 3,4-disubstituted ring
system was important for activity. Furthermore, 2,3-dichloro-
phenyl 8 and N-methyl indole 9 led to a 11- and 13-fold drop in
DAT potency, respectively. These results together suggested that
the NET and DAT potency benefit from an important hydrogen
bond interaction.
With the requirement for an indole-like heterocycle estab-
lished, we turned our attention to indole analogs that were more
metabolically stable since the in vitro clearance of indole 5 in hu-
man liver microsomes (HLM)25 predicted low stability (Table 2). In
order to protect the 2,3-unsubstituted indole, we prepared two
amide analogs. Neither the 3-substituted amide 10 nor 2-substi-
tuted amide 11 showed significant NET activity. Interestingly,
amide 11 was very potent at both SERT and DAT. Next, we pre-
pared several 7-substituted analogs. While methoxyindole 12
lacked sufficient DAT activity, chloroindole 13 lacked the desired
balance between NET and DAT. Although relatively balanced at
all three transporters, fluoroindole 14 still lacked the desired high
metabolic stability. Finally, we prepared two aza-indole analogs.
While the 7-aza-indole 15 lacked sufficient NET and DAT activity,
indazole 16 displayed good potency at all three transporters. The
indazole ring also provided considerable metabolic stability as
compared to the indole ring.
N
HN
N
HN
N
H
N
H
4
5
SERT NET DAT
9.1 8.1 7.3
SERT NET DAT
7.9 8.1 8.2
pKi
pKi
Utilizing a template hopping strategy, we developed a new 2-
substituted piperazine-based series. We felt that the reduced pKa
of the piperazine would help to boost the brain levels of these com-
pounds through improved permeability thereby providing in vivo
efficacy. Gratifyingly, piperazine 5 was active in both the tail sus-
pension test (TST) and locomotor assay (LMA) models (MED
10 mg/kg). Furthermore, the in vivo efficacy was confirmed by
measuring high DAT occupancy (75%) in mouse. In fact the un-
bound brain levels20 for piperazine 5 were determined to be five-
fold higher than for piperidine 4 (206 nM vs 37 nM). Since both
of these compounds were not Pgp substrates, this result suggested
that the basicity of the amine played an important role in boosting
the brain free fraction of piperazine 5.
One critical liability for this series was the inhibition of CYP2D6,
which is a typical issue for lipophilic basic compounds.26 Not
While this series generally possessed the desired level of
potency21 at all three transporters, some of the structural features
(Fig. 1: basic amine, 5-aminoindole) imparted some early safety
alerts. These include CYP2D6 inhibition and CYP3A4 time depen-
dent inactivation (TDI). In addition, we needed to address the
low metabolic stability and investigate the effect of the stereogenic
center on the aforementioned issues. This paper describes the opti-
mization strategy we undertook to address each of these liabilities
while maintaining the desired potency profile.
Br
c
a, b
BocN
N
N
H
N
TBS
Compounds such as piperazine 5 were conveniently prepared in
a four step sequence outlined in Scheme 1. First, 5-bromoindole
was treated with lithium bis(trimethylsilyl)amide in THF followed
by TBSCl to afford the protected indole in 98% yield. The indole was
coupled directly with commercially available 1-Boc-3-benzyl-
piperazine under Buchwald–Hartwig conditions22 which provided
the coupled product in good yield (74% yield).23 Treatment with
TBAF in THF followed by deprotection with TFA in DCM gave the
final compound in 69% yield (two steps).
HN
HN
N
d
N
N
TBS
N
H
Scheme 1. Synthesis of 2-substituted N-aryl piperazine 5. Reagents and conditions:
(a) LHMDS, THF, TBSCl; (b) Pd(OAc)2, tBu3P, NaOtBu, xylenes, 1-Boc-3-benzylpi-
perazine, 80 °C; (c) TFA/DCM; (d) TBAF/THF.

D. S. Carter et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3941–3945
3943
Table 1
Table 2
Key features of the indole drive NET/DAT potency
Indole analogs address potencya and microsomal stabilityb
HN
HN
N
R
N
R
Compd
R
SERTa pKi
NETa pKi
DATa pKi
Compd
R
SERT
NET
DAT
HLM (lL/
pKi
pKi
pKi
min/mg)
5
7.9
8.1
8.2
N
H
5
7.9
7.0
8.1
8.2
6.8
43.3
N
H
O
6
7
7.8
7.1
6.4
6.3
6.3
6.4
NH2
10
<5.3
—
N
H
N
H
O
NH2
Cl
11
12
8.1
7.3
6.3
7.3
8.5
5.8
—
—
8
9
7.8
8.1
7.8
7.3
7.1
6.9
N
H
Cl
N
H
N
O
a
pKi (binding) values are the geometric mean of at least three experiments mea-
sured in HEK 293 cells.
13
14
8.5
7.8
7.9
8.2
7.0
7.8
—
N
H
Cl
wanting to move forward with this potential drug–drug interaction
(DDI) liability, we initially chose to evaluate the SAR of CYP2D6 inhi-
bition through modifications of the benzylic side-chain. A series of
indazole analogs was investigated which varied size and shape of
the benzylic side-chain as well as reduced overall lipophilicity.
With very few commercial 2-substituted piperazines avail-
able,27 we developed a flexible synthesis for these side-chain ana-
logs (Scheme 2). Starting with 5-bromoindazole, treatment with
lithium bis(trimethylsilyl)amide and triisopropylsilyl chloride in
THF afforded the N-protected indazole. This indazole was coupled
with 1-benzylpiperazin-2-one under Buchwald–Hartwig condi-
tions.22 Enolate formation was carried out with sec-butyllithium
in THF at À78 °C.28 Alkylation with a variety of alkyl or benzyl ha-
lides afforded 2-substituted oxo-piperazines in moderate yields
(50–60%). Concomitant removal of the silyl group and reduction
of the carbonyl was accomplished by treatment of the compound
for 10 min with lithium aluminum hydride at 120 °C in a micro-
wave reactor (typically 60% for two steps). Final deprotection
was carried out utilizing transfer hydrogenation (yield 85–90%).
We began our search for compounds with less potency at
CYP2D629 by preparing two aromatic side-chain analogs with vary-
ing chain length (Table 3). 2-Phenylpiperazine 17 was sixfold less
potent at CYP2D6 than benzylpiperazine 16 though it lacked the
desired transporter potency. Conversely, phenethylpiperazine 18
was potent at CYP2D6 without significant loss of transporter po-
tency. Next, we prepared several aliphatic analogs (piperazines
19–22). Although the potency of propylpiperazine 19 was 14-fold
lower at SERT, it was a much weaker CYP2D6 inhibitor. Simply
adding one additional carbon boosted SERT and DAT but also led
to a significant increase in CYP2D6 potency (butylpiperazine 20).
Isobutylpiperazine 20 maintained transporter potency and lost
some potency at CYP2D6 (as compared to butylpiperazine 19).
While methylcyclohexyl piperazine 22 lost potency at NET and
DAT, it was the first analog with micromolar potency at CYP2D6,
suggesting that larger branched side-chains may be a good strategy
16.8
N
H
F
15
16
7.2
8.9
6.6
7.6
6.3
7.8
—
N
H
N
N
3.8
N
H
a
pKi (binding) values are the geometric mean of at least three experiments
measured in HEK 293 cells.
b
In vitro microsomal stability measured in human liver microsomes.
O
Br
c
a, b
BnN
N
N
N
H
N
N
Si(iPr)3
O
R
R
BnN
HN
N
d,e
N
N
N
N
N
H
Si(iPr)3
Scheme 2. A flexible synthesis for 2-substituted N-aryl piperazines. Reagents and
conditions: (a) LHMDS, THF, TIPSCl; (b) Pd(OAc), tBu3P, NaOtBu, xylenes, 1-benzyl-
piperazin-2-one, 80 °C; (c) sec-BuLi, THF, RCH2Br; (d) LiAlH4, THF, 120 °C, micro-
wave 10 min; (e) Pd/C, NH4OCHO, MeOH, reflux.

3944
D. S. Carter et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3941–3945
Table 3
Small aliphatic side-chains maintain transporter potencya while reducing potency at CYP2D6 and decreasing CYP3A4 TDI
R
HN
N
N
N
H
Compd
R
SERT pKi
NET pKi
DAT pKi
CYP2D6 IC50
0.05
(lM)
3A4 TDI %
49
16
8.9
7.6
7.6
17
18
7.7
8.4
6.7
7.1
6.7
7.5
0.30
0.06
—
58
19
7.8
7.5
8.2
8.4
7.4
7.5
6.8
7.1
7.2
7.6
6.1
7.5
0.58
0.35
1.9
23
24
—
(S)-19
(R)-19
20
0.04
—
21
22
8.3
7.4
7.7
5.8
7.8
6.5
0.31
1.05
—
—
23
24
6.8
7.8
6.5
6.5
6.3
6.5
4.0
0.7
6
O
O
—
25
26
8.7
8.4
6.6
6.5
6.8
7.4
0.22
8.26
—
O
O
O
O
23
(S)-26
(R)-26
7.1
9.0
7.0
6.5
7.3
6.0
9.1
4.7
—
—
a
pKi (binding) values are the geometric mean of at least three experiments measured in HEK 293 cells.
to address this liability. Finally, we hypothesized that decreasing
the lipophilicity through the introduction of a heteroatom might
further decrease the CYP2D6 affinity. We prepared several ethers
(23–28) that were direct comparators of the aliphatic analogs. As
a general trend these ether analogs tended to be less potent at
NET and DAT (typically 10-fold lower). The first compound in the
series, 2-methoxymethylpiperazine 23 lost potency at each trans-
porter but we were encouraged that such a small change let to a
sevenfold reduction in CYP2D6 potency. In all cases, the additional
oxygen reduced CYP2D6 potency as compared to the aliphatic ana-
logs possible due to the lower lipophilicity (typically 15-fold lower
calculated log P). Loss of transporter potency was ameliorated by
increasing chain length or branching as exemplified by 4-pyranyl-
piperazine 26. Although compound 26 was somewhat less potent
at NET than our desired target profile, it was intriguing to us be-
cause it was the weakest CYP2D6 inhibitor that we investigated.
Since this piperazine series contains a stereogenic center, we
wanted to know what effect stereochemistry might have on po-
tency and metabolic profile. Propylpiperazine 19 and 4-pyranyl-
piperazine 26 were selected for profiling. The enantiomers of
propylpiperazine 19 were initially separated via chiral SCF HPLC.
Subsequently, the absolute configurations were determined via
comparison of material prepared by total synthesis from commer-
cially available amino acid precursors.30 As we suspected, each
enantiomer showed a different potency and metabolic profile.31
The relatively balanced enantiomer (S)-19 was much more potent
at CYP2D6 than the SERT-selective enantiomer (R)-19. The enanti-
omers of piperazine 26 were also separated via super critical fluid
(SCF) HPLC;32 one enantiomer was balanced while the other was
SERT-selective. In this case the balanced enantiomer (S)-26 was
much less potent at CYP2D6. Based on the potency profile and
the low CYP2D6 DDI liability, compound (S)-26 was selected for
further study.
O
O
R
oxidative
metabolism
R
N
N
N
N
+
N
N
N
N
H
27 R = H
29 R = Ac
26 R = H
28 R = Ac
After identifying several compounds that were both potent
reuptake inhibitors and weak CYP2D6 inhibitors, our attention
turned to addressing the observed time dependent inhibition

D. S. Carter et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3941–3945
3945
(TDI) of CYP 3A4.33 We suspected that the TDI signal was likely due
to a reactive metabolite formed from the 5-aminoindole moiety.
We postulated that metabolic oxidation of piperazine 26 could re-
sult in a diiminoquinone-like 27 which could be covalently trapped
by CYP 3A4. Initially, we identified several indole replacements
with lower TDI (7-aza-indole 15: TDI = 10%) but none of these
compounds maintained the desired potency. Fortunately, as with
CYP3A4 inhibition, modifications to the benzylic side-chain were
successful in decreasing TDI (Table 2). The aliphatic analog propyl-
piperazine 19 had much lower TDI than the aromatic analogs (16,
18). Analogs with lower lipophilicity, such as ether 23, demon-
strated a much lower TDI risk but were not pursued due to lack
of desired transporter potency.
While attempting to moderate TDI in this series, we became
aware of an additional potential liability of the piperazine-based
scaffold. Specifically, if acetyl piperazine 28 formed in vivo (as a
metabolite of piperazine 26) leading to diiminoquinone 29, it might
carry a much higher risk of 3A4 TDI which could lead to drug–drug
interactions. Because free piperazines are typically protonated un-
der physiological conditions, the charged species may somewhat
protect the indole ring from oxidative metabolism. If acetylated,
any protective benefit from the additional charge would be lost.
Although we never observed the acetyl metabolite directly, previous
experience taught us to de-risk this series by preparing the pur-
ported acetyl metabolite 28. Unfortunately, acetyl piperazine 28
was associated with a much higher TDI risk than piperazine 26
(TDI = 77%). Based on this experimental result even piperazines with
minimal TDI risk may generate metabolites in vivo which result in
3A4 TDI. Despite successful strategies for minimizing acetylation
of piperazines by others,34 the risk of 3A4 induced drug–drug
interactions resulted in a deprioritization of this series in favor of a
different template without this liability.35
10. Sussman, N. J. Clin. Psychiatry 2003, 64, 19.
11. (a) Skolnick, P.; Kreiter, P.; Tizzano, J.; Basile, A.; Popik, P.; Czobor, P.; Lippa, A.
CNS Drug Rev. 2006, 12, 123; (b) Azford, L.; Boot, J. R.; Hotten, T. M.; Keenan, M.;
Martin, F. M.; Milutinovic, S.; Moore, N. A.; Oneil, M. F.; Pullar, F. M.; Tupper, D.
F.; Van Belle, K.; Vivien, V. Bioorg. Med. Chem. Lett. 2003, 13, 3277.
12. Chen, Z.; Skolnick, P. Expert Opin. Invest. Drugs 2007, 16, 1365.
13. (a) Fava, M.; Rush, A. J.; Thase, M.; Clayton, A.; Stahl, S. M.; Pradko, J. F.;
Johnston, J. A. J. Clin. Psychiatry 2005, 7, 106; (b) Debattista, C.; Solvanson, H. B.;
Poirier, J.; Kendrick, E.; Schatzberg, A. F. J. Clin. Psychopharmacol. 2003, 23, 27.
14. (a) Kumar, L. H. Am. J. Geriatr. Psychiatry 2001, 9, 298; (b) Stoll, A. L.; Pillay, S. S.;
Diamond, L.; Workum, S. B.; Cole, J. O. J. Clin. Psychiatry 1996, 57, 72.
15. Lucas, M. C.; Carter, D. S.; Cai, H.; Lee, E. K.; Schoenfeld, R. C.; Steiner, S. S.; Villa,
Marzia.; Weikert, V.; Weikert, R. J.; Iyer, P. Bioorg. Med. Chem. Lett. 2009, 19,
4630.
16. A scintillation proximity assay (SPA) was carried out in HEK 293 cells. In this
assay, the binding affinity (pKi) of various ligands was determined at SERT, NET
and DAT by the completion with [3H]-Citalopram, [3H]-Nisoxetine and [3H]-
Vanoxerine, respectively. The specific details of this assay are reported in
Carter, D. S.; Schoenfeld, R. C.; Weikert, R. J. WO 2008019971.
17. MW = 305, Clog P = 3.68, PSA = 27.
18. Clark, D. E. Drug Discovery Today 2003, 8, 927.
19. Meyer, J. H.; Goulding, V. S.; Wilson, A. A.; Hussey, D.; Christensen, B. K.; Houle,
S. Psychopharmacology 2002, 162, 102.
20. Liu, X.; Vilenski, O.; Kwan, J.; Apparsundaram, S.; Weikert, R. Drug Metab.
Dispos. 2009, 37, 1548.
21. We were interested in evaluating two potency profiles: relatively balanced (all
pKi’s >7.0) similar to DOV-216,303 and SERT selective based in buproprion
augmentation trials.
22. (a) Yamamoto, T.; Nishiyama, M.; Koie, Y. Tetrahedron Lett. 1998, 39, 2367; (b)
ˇ
Prashad, M.; mak, X. Y.; Liu, Y.; Repic, O. J. Org. Chem. 2003, 68, 1163; (c)
Dubbaka, S. Synlett 2005, 709.
23. The experimental procedure for a typical Buchwald coupling is as follows. To a
mixture of Pd(OAc)2 (0.054 g, 0.2 mmol), NaOtBu (0.64 g, 6.6 mmol) in 7.0 mL
xylenes in a screw capped test tube was added tBu3P (0.049 g, 0.2 mmol). After
10 min a solution of N-TBS-5-bromoindole and 3-benzyl-piperazine-1-carboxylic
acid tert-butyl ester in 15 mL xylenes was added and the mixture was warmed to
80 °C. After 30 min the mixture was cooled, taken up in EtOAc, filtered through a
pad of Celite and concentrated in vacuo to afford a crude oil. Purification via flash
chromatography (gradient: 2–20% EtOAc in hexanes) afforded 3-benzyl-4-[1-
(tert-butyl-dimethyl-silanyl)-1H-indol-5-yl]-piperazine-1-carboxylic acid tert-
butyl ester (1.65 g, 74%) as a pale yellow oil; 1H NMR 300 MHz (CDCl3) d 7.47 (d,
J = 9.0 Hz, 1H), 7.27–7.17 (m, 4H), 7.13–7.07 (m, 2H), 7.16 (d, J = 3.1 Hz, 1H), 6.96
(dd, J = 9.0, 2.4 Hz, 1H), 6.56 (dd, J = 3.1, 0.8 Hz, 1H), 3.85–3.77 (m, 1H), 3.65–3.75
(br s, overlapping, 1H), 3.30–3.10 (m, 5H) 2.80–2.60 (br s, overlapping, 1H), 2.64–
2.56 (m, 1H), 1.51 (br s, 9H), 0.93 (s, 9H), 0.63 (s, 3H), 0.60 (s, 3H).
In conclusion, we have developed a novel 2-substituted pipera-
zine-based series of triple reuptake inhibitors. These inhibitors
demonstrated activity in vivo due to their reuptake inhibitor po-
tency and improved brain free fraction over the C5-aminoindoles.
Furthermore, analogs were developed with good metabolic stabil-
ity and low risk for CYP2D6 induced DDI. Although analogs with
diminished risk of 3A4 TDI induced toxicity were identified, the po-
tential for acetyl metabolite formation lent additional risk to this
series. In the end, we deprioritized this series in favor of a related
series that did not suffer from this liability.35
24. Iyer, P.; Lucas, M. C.; Schoenfeld, R. C.; Villa, M.; Weikert, R. J. US 2007123527.
25. (a) Clearance measured in HT format: total incubation volume 250
mL human microsomal protein, reaction started by adding NADPH and
performed at 37 °C with no shaking. 40 L aliquots at four time points (0.1,
lL, 0.5 mg/
l
10, 20 and 30 min) were taken then the samples were transferred to 384 well
plates for LC/MS analysis; (b) Clearance measured in human liver microsomes
(lL/min/mg): low stability >35, medium stability 35–6, high stability <6.5.
26. Groot, J. M.; Wakenhut, F.; Whitlock, G.; Hyland, R. Drug Discovery Today 2009,
14, 964.
27. The piperazine starting material (1-Boc-3-phenyl piperazine) for compound 17
was commercially available.
28. Berkheij, M.; van der Sluis, L.; Sewing, C.; den Boer, D. J.; Terpstra, J. W.;
Hiemstra, H.; Bakker, W. I. I.; van den Hoogenband, A.; van Maarseveen, J. H.
Tetrahedron Lett. 2005, 2369.
Acknowledgements
29. CYP2D6 inhibition was measured in a fluorescence based assay where CYP
marker substrates become fluorescent upon CYP metabolism.
30. (a) (R)-2-Propylpiperazine and (S)-2-propylpiperazine were prepared from (R)-
N-Boc-norvaline and (S)-N-Boc-norvaline, respectively by the method
described in the following patent; (b) WO 03082877 A1; (c) Both piperazines
were Boc-protected and converted to the final compounds (S)-19 and (R)-19 by
the method outlined in Scheme 1.
The authors wish to thank Marc Cummings for analytical and
preparatory chiral HPLC support and R. Than Hendricks for edito-
rial suggestions.
References and notes
31. (S)-19 Rat SDPK (10 mpK PO): %F = 34, Vdss = 4.3 mL/kg, t1/2 = 0.5 h,
AUC = 406 ng-h/mL.; (R)-19 Rat SDPK (10 mpK PO): %F = 53, t1/2 = 0.8 h,
AUC = 182 ng-h/mL.
32. (a) The absolute configurations of (S)-19 and (R)-19 were tentatively assigned
based on comparing data from (S)-19 and (R)-19.; Super critical fluid
chromatography: (b) Banu, B.; Bharathi, K.; Babu, G. M. J. Pharm. Res. 2009, 2,
148.
33. (a) Zhou, S.; Chan, S. Y.; Goh, B. C.; Chan, E.; Duan, W.; Huang, M.; Mcleod, H. L.
Clin. Pharmacokinet. 2005, 44, 279; (b) Kalgutkar, A. S.; Soglia, J. R. Exp. Opin.
Drug Metab. Toxicol. 2005, 1, 91; (c) Human liver microsomes are preincubated
with test compound and diluted at various time points (3, 6, 12 min). The
remaining enzyme activity is measured (kobs) and the data is plotted as a % of
1. (a) Thase, M. E. J. Clin. Psychiatry 2006, 61, 17; (b) Keesler, R. C.; Berglung, P.;
Demler, O.; Jin, R.; Loretz, D.; Merikangas, K. R.; Rush, A. J.; Walters, E. E.; Wang,
P. S. J. Am. Med. Assoc. 2003, 289, 3095.
2. Stewart, W. F.; Ricci, J. A.; Chee, E.; Hahn, S. R. J. Am. Med. Assoc. 2003, 289, 3135.
3. (a) Blumenthal, J. A.; Sherwood, A.; Rogers, S. D.; babyak, M. A.; Doraiswamy, P.
M.; Watkins, L.; Hoffman, B. M.; O’Connell, C.; Johnson, J. J.; Patidar, S. M.;
Waugh, R.; Hinderlitter, A. Clin. Trials 2007, 4, 548; (b) Moussavi, S.; Chatterji, E.
M.; Tandon, A.; Patel, V.; Ustun, B. Lancet 2007, 370, 851; (c) Munce, E. E.;
Stansfeld, S. A.; Blackmore, E. R.; Stewart, D. E. J. Occup. Environ. Med. 2007, 49,
1206.
4. Leonard, B. E. Neurochem. Int. 1982, 4(339), 5.
5. (a) Fava, M. J. Clin. Psychiatry 2003, 64, 26; (b) Nutt, D. J. J. Clin. Psychiatry 2007,
64, 3.
6. Harmer, C. J.; Bhagwagar, Z.; Völlm, B. A.; Cowen, P. J.; Goodwin, G. M.
Neuropsycopharmacology 2003, 28, 148.
control (no inhibitor). In this manuscript 3A4 TDI is reported as
a % of
t = 12 min. Compounds with a greater rate of inactivation than verapamyl are
categorized as high, while compounds with a rate less than ethynylestradiol are
categorized as low.
34. Rawal, J.; Jones, R.; Payne, A.; Gardner, I. Xenobiotica 2008, 38, 1219.
35. Iyer, P.; Lin, C.; Lucas, M.; Lynch, S.; Madera, A. M.; Ozboya, K.; Schoenfeld, R.;
Weikert, R. WO 2009153178.
7. Dremencov, E.; el Mansari, M.; Blier, P. Curr. Drug Targets 2009, 10, 1061.
8. Esch, T.; Stefano, G. B. Neuroendocrinol. Lett. 2004, 25, 235.
9. Modell, J. G.; Katholi, C. R.; Modell, J. D.; DePalma, R. L. J. Clin. Pharm. Ther. 1997,
61, 478.