Novel, achiral aminoheterocycles as selective monoamine reuptake inhibitors

Article abstract of DOI:10.1016/j.bmcl.2009.06.076

A variety of novel aminoheterocycle scaffolds as selective monoamine reuptake inhibitors have been prepared and one of these scaffolds is achiral. The main elements responsible for hERG channel, CYP2D6 and CYP3A4 inhibition were identified.

Full text of DOI:10.1016/j.bmcl.2009.06.076

Bioorganic & Medicinal Chemistry Letters 19 (2009) 4630–4633
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel, achiral aminoheterocycles as selective monoamine reuptake inhibitors
*
Matthew C. Lucas , David S. Carter, Hai-Ying Cai, Eun Kyung Lee, Ryan C. Schoenfeld, Sandra Steiner,
Marzia Villa, Robert J. Weikert, Pravin S. Iyer
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 variety of novel aminoheterocycle scaffolds as selective monoamine reuptake inhibitors have been pre-
pared and one of these scaffolds is achiral. The main elements responsible for hERG channel, CYP2D6 and
CYP3A4 inhibition were identified.
Received 21 May 2009
Revised 19 June 2009
Accepted 22 June 2009
Available online 25 June 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
TRI
Triple reuptake inhibitor
Monoamine reuptake inhibitor
Neurotransmitter
Indole
Major depressive disorder
Depression is an often disabling condition that will affect great-
er than 15% of the general population in their lifetime.1,3 The etiol-
ogy of the disease was widely considered to be due to sub-optimal
concentrations of the monoamine neurotransmitters serotonin (5-
HT) and norepinephrine (NE) in the central nervous system (CNS).2
It is now apparent that depression is a consequence of dysfunc-
tional endocrine, immune and neurotransmitter systems.3 How-
ever, most antidepressants have been developed based on the
assumption that an imbalance in monoamine concentration is
the primary cause. Early therapeutics, which included the mono-
amine oxidase inhibitors (MAOIs, e.g., Phenelzine, 1) and the tricy-
clic antidepressants (TCAs, e.g., Amitryptyline, 2), treated a broad
array of symptoms commonly associated with depression (Fig. 1).
This broad activity was largely due to their ability to increase the
synaptic concentration of either all three (5-HT, NE and dopamine,
DA), or two (5-HT and NE) monoamine neurotransmitters respec-
tively. These therapeutics are far from ideal, however, due to side
effects caused by ‘off-target’ pharmacology and a potential to cause
drug–drug interactions.4 One breakthrough in modern anti-
depressant therapy came in the 1980s with the introduction of
the serotonin selective reuptake inhibitor (SSRI) class of drugs
(e.g., Fluoxetine, 3). The serotonin reuptake transporter (SERT) is
located on the pre-synaptic membrane and is responsible for signal
termination and recycling of 5-HT that is released into the synaptic
cleft during synapse firing. Inhibition of the serotonin transporter
increases the concentration and duration of action of 5-HT, thus
alleviating the symptoms of major depressive disorder (MDD)
associated with low levels of 5-HT (mood deregulation, anxiety
and sleep disorders). Although these therapeutics treated a smaller
range of symptoms, the high level of selectivity led to improved
safety and tolerability through a remarkably reduced side effect
profile. Unfortunately, full remission was only observed in approx-
imately one third of treated patients. To address the remaining
clinical need, serotonin norepinephrine selective reuptake inhibi-
NMe2
H
N
NH2
1, Phenelzine (Nardil), a MAOI
2, Amitriptyline (Elavil), a TCA
CF3
O
O
NHMe
NHMe
S
* Corresponding author. Tel.: +1 6508556292.
E-mail addresses: matthew.lucas@roche.com, mattlucas70@hotmail.com
(M.C. Lucas).
4, Duloxetine (Cymbalta), an SNRI
3, Fluoxetine (Prozac), an SSRI
Figure 1. Some marketed antidepressants.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.06.076

M. C. Lucas et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4630–4633
4631
tors (SNRIs, e.g., Duloxetine, 4) emerged that were able to treat as
broad an array of symptoms as the earlier TCAs while showing lim-
ited affinity for the receptors that caused side-effects (muscarinic,
carry additional clinical safety concerns, or both. To address this,
we synthesized 4-aminopiperidine 9 (Table 1). Although a 3-amin-
opiperidine or 3-aminopyrrolidine would have offered a more
appropriate structural mimic, these possessed a stereogenic centre
and were less desirable. Unfortunately, 9 showed a decrease in po-
tency at all three transporters. Undeterred, we reasoned that per-
haps the restricted conformation might be preventing the
molecule from adopting the optimal geometry to bind to the
monoamine transporters. We were later delighted to discover that
increasing the number of rotatable bonds by inserting a methylene
linker between the phenyl hydrophobe and the C(5)-nitrogen re-
sulted in an enormous increase in potency at SERT, NET and DAT
(10, Table 1).
With this encouraging result we quickly assessed the physico-
chemical and pharmacological properties of compound 10. We
were concerned about the potential to form reactive metabolites
through the formation of a diiminoquinone-like intermediate
(Fig. 3).10 In our hands, however, low or no risk of clinically rele-
vant CYP3A4 time dependant inactivation in human liver micro-
somes was predicted for 10.11 Time dependent inactivation
would have been one indicator of reactive metabolite formation.
Reassured, we developed additional SAR.
Two potential liabilities that emerged were strong hERG chan-
nel inhibition and a potential for drug–drug interactions due to po-
tent CYP2D6 inhibition (Tables 2 and 4). Although we had been
keen to avoid generating a chiral molecule, we recognized that
the relatively high pKa of the 4-aminopiperidine 10 might be
contributing to the potent hERG interaction.12 Therefore, 3-amin-
opiperidine 11 and 3-aminopyrrolidine 12 were synthesized
(Table 2). We expected these to show a reduced pKa due to the
1,4-arrangement of polar heteroatoms. Piperidine 11 was a 10-fold
less potent inhibitor of the serotonin and norepinephrine trans-
porters and showed no reduction in hERG interaction. Although
the pyrrolidine 12 was a fairly potent and balanced TRI, it also
showed no reduction in hERG binding. It appeared that any re-
duced binding to the hERG channel due to a lowered pKa was being
counterbalanced by the increased lipophilicity. The measured log D
values for 10, 11 and 12 were 0.6, 1.20 and 1.25, respectively. The
lack of a clear advantage of these chiral scaffolds led us to return
our focus to the 4-aminopiperidine scaffold (6a).
a-adrenergic, histaminergic). This resulted in a tolerability and
safety profile comparable to SSRIs while also addressing symptoms
such as lack of energy, lack of concentration and co-morbid pain.
The SNRI class of anti-depressants are more likely to result in
remission, and some (e.g., Duloxetine, 4) are also approved to treat
other diseases such as anxiety, fibromyalgia and neuropathic pain.
Elevating DA levels in addition to 5-HT and NE would generate a
class of therapeutics referred to as broad spectrum antidepressants
or triple reuptake inhibitors (TRIs). TRIs would be expected to
show superior efficacy to the SSRIs and SNRIs.5 Inhibiting DA reup-
take may address the anhedonia, lack of motivation, and lack of
attention components of MDD that the SSRIs and SNRIs do not.6
Duloxetine (4) has been approved by the Food and Drug Adminis-
tration (FDA) for the treatment of diabetic neuropathic pain, and
drugs which are TRIs may provide a broader spectrum of pain relief
in animal models of experimental pain than single mechanism of
action of reuptake inhibitors.7 Thus, TRIs may provide the next ma-
jor advance in antidepressant and pain therapy.
During our efforts to uncover novel series of monoamine reup-
take inhibitors, we discovered that 7 (Table 1), a member of one of
our SNRI series of indolyl aryl propylamines 5 (Fig. 2),8 exhibited
weak inhibition of DAT. We considered this compound an excellent
starting point for our search for new TRI scaffolds. Herein we de-
scribe the synthesis and SAR of several novel achiral and chiral
series of monoamine reuptake inhibitors derived from the C(5)-in-
doles 5 (6a–c, Fig. 2). Lead optimization of the achiral substructure
6a provided molecules that were potent TRIs. Also described are
our attempts to minimize undesirable hERG channel inhibition
and CYP450 inhibition, and efforts to reduce the risk of forming ac-
tive metabolites.
Interestingly, the majority of monoamine reuptake inhibitors
that have been approved by the FDA possess a stereogenic centre.
We were particularly excited about the prospect of finding and
developing a novel achiral series. We had been frustrated by diffi-
cult enantioselective syntheses and inefficient enantioselective
separations for advanced compounds with structures related to
5. Thus, we sought to replace the C(5)-carbon of 7 with a nitrogen
atom to form tertiary amine 8 (Table 1). We were disappointed to
find that 8 was only weakly active. Nevertheless, we felt we would
be able to develop this series of TRIs further. Our past experience
with N-methyl amino-containing monoamine reuptake inhibitors9
suggested that a significant metabolic pathway for this series
would be N-demethylation. Our concern was that the resulting
metabolites might be active and complicate future in vivo studies,
We rapidly established that although several alternatives to the
indole heteroaryl group were possible, a compelling replacement
did not present itself (Table 2). Indazole (e.g., 13) was tolerated
for potency, but despite an increase in polar surface area its affinity
for the hERG channel was even stronger. The N(1)-indole/indazole
hydrogen bond donor seemed to be one element contributing to
both hERG and CYP inhibition. Unfortunately, benzothiophene 14
Table 1
In vitro inhibition of monoamine reuptakea
N
N
N
N
H
N
H
HN
HN
N
H
N
H
N
H
N
H
7
8
9
10
Compound
pKi SERT
pKi NET
pKi DAT
hERG IC20 (lM)
4
7
8
9
10
9.3
7.5
6.4
6.0
9.1
8.1
8.0
6.7
5.9
8.1
6.6
6.7
6.3
<5.8
7.3
<0.3
<1
—
—
1
a
pKi values are the geometric mean of at least three experiments.

4632
M. C. Lucas et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4630–4633
R
N
R
N
R
R
N
N
H
B
B
B
N
H
A
A
HN
N
H
N
H
A
5
6a
6b
6c
Figure 2. Examples of our novel monoamine reuptake inhibitors.
Table 3
In vitro inhibition of monoamine reuptakea and hERG12
Metabolic activation
R
N
N+
N
HN
HN
N
H
N
HN
N
H
Figure 3. Postulated pathway to form a reactive diiminoquinone-like intermediate.
Compd
R
pKi SERT
pKi NET
pKi DAT
hERG IC20 (lM)
10
16
17
18
19
20
21
22
23
24
25
26
27
CH2Ph
CH(CH3)Ph
C(O)Ph
CH2CH2Ph
CH2(4-THP)
CH2(2-F-Ph)
CH2(3-F-Ph)
CH2(4-F-Ph)
CH2(2-CN-Ph)
CH2(3-CN-Ph)
CH2(4-CN-Ph)
CH2(3-SO2NH2-Ph)
CH2(3-MeO-Ph)
9.1
7.4
7.6
8.2
7.4
8.4
8.9
8.7
8.4
9.6
8.6
8.2
8.8
8.1
5.8
6.1
7.3
7.4
7.9
7.9
6.6
8.2
8.0
6.0
7.1
8.4
7.3
5.8
5.4
6.8
6.3
6.9
7.1
6.6
6.3
7.5
6.1
7.6
7.1
1
—
—
—
—
—
—
—
—
<1
—
>10
1.0
Table 2
In vitro inhibition of monoamine reuptakea
N
[
]
m
B
HN
[
]
n
A
Compd
n
m
A
B
pKi SERT pKi NET pKi DAT hERG IC20 (lM)
10
11
12
13
14
15
2
3
2
2
2
2
2
1
1
2
2
2
NH
NH
NH
NH
S
CH 9.1
CH 8.2
CH 7.7
8.1
7.2
8.0
7.8
6.7
7.6
7.3
7.4
7.5
7.3
6.7
7.1
1
a
<1b
<1
<1
1
pKi values are the geometric mean of at least three experiments.
N
8.9
CH 7.9
Table 4
NCH3 CH 8.4
5.1
In vitro human liver microsomal stability,14 CYP2D6 and CYP3A4 inhibition15 and
CACO-2 efflux ratio16 data for selected compounds
a
pKi values are the geometric mean of at least three experiments.
Data for (+)-11.
b
Compd
HLM
L/min/mg)
CYP2D6 IC50
M)
CYP3A4 IC50
M)
ER (BA/AB)
(l
(l
(l
(cm/s  10À6
)
4
7
8
38
11
—
0.6
3
2
26
0.1
0.3
2
0.5
19
5
17
5
40
14
3
0.6 (9/15)
—
—
—
1 (6/6)
—
—
—
0.3 (0.4/1.4)
0.4 (4/7)
—
3.6 (14/4)
4.2 (3/0.7)
favoured the serotonin transporter and also carried a high risk of
forming reactive intermediates,13 and although the N(1)-methyl
indole 15 showed a good in vitro profile and significantly reduced
hERG affinity, we were concerned that it might generate pharma-
cologically active metabolites via N-demethylation.
9
—
10
11
12
13
14
15
19
24
26
17
23a
36
—
18
26
—
5
2
Turning our attention to the benzyl side chain (Table 3), we
found that an unsubstituted methylene linker was optimal for
TRI potency. Substitution (e.g., 16) or formation of the amide
(e.g., 17) was not tolerated for potency, and generated weakly ac-
tive SSRIs. Extension of the linker, in contrast, generated a rela-
tively good SNRI (e.g., 18). Although replacement of the phenyl
ring with an aliphatic group (e.g., tetrahydro-4-pyran 19) was
poorly tolerated for potency, it also led to reduced CYP2D6 and
CYP3A4 binding (Table 4). Thus, it became clear that the aromatic
hydrophobe was a strong contributor to the drug–drug interaction
potential of this scaffold, likely due to its increased lipophilicity.
Having established that a methylene linked aromatic was opti-
mal for potency, we next sought to optimize drug-like properties
through appropriate substitution on the phenyl ring (Table 3, only
selected examples shown for clarity). We discovered that ortho-
substitution tended to generate balanced SNRIs (e.g., 20, 23),
meta-substitution tended towards our desired TRI profile (e.g.,
21, 24, 27), and para-substitution tended to generate potent SSRIs
(e.g., 22, 25).
0.4
35
>50
27
0.5
3
>50
2
2
13
2
a
Data for (+)-11.
cess. For example, while nitrile 24 maintained excellent potency,
there was minimal effect on hERG binding, little improvement to
the stability in human liver microsomes, and CYP3A4 inhibition
grew stronger (Table 3). Sulfonamide 26 did show excellent stabil-
ity towards human liver microsomes and no detectable hERG chan-
nel binding (Table 3), but unfortunately a combination of reduced
membrane permeability and increased number of hydrogen bond
donors resulted in moderate P-gp efflux. We had determined
through in vitro and in vivo studies that even low levels of P-gp ef-
flux (>2) led to significantly reduced CNS penetration in this series
(data not shown). In addition, during our regular screening for the
potential to form reactive intermediates, we were dismayed when
24 and 26 displayed moderate potential for clinically relevant
CYP3A4 time dependent inactivation (data not shown). This data
led us to conclude that for this scaffold it would be challenging
We therefore concentrated our efforts on finding the optimal
meta-substituent. We expected introduction of polarizing hetero-
atoms to reduce inhibition of CYP450 enzymes and the hERG ion
channel simultaneously, while also improving stability towards
human liver microsomes. This approach was met with some suc-

M. C. Lucas et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4630–4633
4633
Boc
N
H
N
H2N
a
b, c
HN
R
N
N
H
N
H
N
H
27
28
29
Scheme 1. Reagents and conditions: (a) 4-oxo-piperidine-1-carboxylic acid tert-butyl ester, NaBH(OAc)3, AcOH, 1,2-DCE, rt, 18 h; (b) RCH2Br, Et3N, DMAP, sealed tube,
100 °C, 18 h; (c) TFA, CH2Cl2, rt, 1 h.
5. Augmentation of citalopram with sustained-release bupropion in a clinical
to combine CNS drug-like properties, potent TRI activity, and low
setting showed certain advantages, including
a greater reduction in the
potential to form reactive metabolites in a single molecule.
In general, compounds of substructure 6a–c were synthesized
in an expedient three step sequence represented by Scheme 1.
Thus, heterocyclic amine 27 underwent reductive amination with
the appropriate N-Boc protected aminoketone to form 28, and this
was followed by N-alkylation and a final deprotection to furnish
29. Compounds 6b and 6c were synthesized in a similar fashion,
starting from the N-Boc-protected piperidin-3-one or pyrrolidin-
3-one, respectively.
number and severity of symptoms and fewer side effects and adverse
events. See: Trivedi, M. H.; Fava, M.; Wisniewski, S. R.; Thase, M. E.;
Quitkin, F.; Warden, D.; Ritz, L.; Nierenberg, A. A.; Lebowitz, B. D.; Biggs,
M. M.; Luther, J. F.; Shores-Wilson, K.; Rush, A. J. N. Engl. J. Med. 2006,
354, 1243.
6. It has been suggested that dopamine agonists may treat anhedonia. See:
Lemke, M. R.; Brecht, H. M.; Koester, J.; Reichmann, H. J. Neuro. Sci. 2006, 248,
266.
7. Pedersen, L. H.; Nielsen, A. N.; Blackburn-Munro, G. Psychopharmacology 2005,
182, 551.
8. Iyer, P.; Lucas, M. C.; Schoenfeld, R. C.; Villa, M.; Weikert, R. J. US20070123527,
2007.
In conclusion, several novel scaffolds 6a–c were discovered that
were excellent inhibitors of monoamine reuptake. Scaffold 6a was
achiral, an uncommon characteristic amongst anti-depressant
drugs developed since the early tricyclic antidepressants. We
found that subtle modifications to the phenyl ring, quite remote
from the suspected C(5)-aminoindole toxicophores, could still ef-
fect the propensity to form reactive metabolites. Key elements
responsible for CYP2D6, CYP3A4 inhibition and hERG channel
interaction were also found. Although scaffold 6a did not deliver
in a single molecule our desired target product profile, the key lear-
nings described herein have been used to develop improved scaf-
folds that will be reported in due course.17
9. (a) Greenhouse, R.; Jaime-Figueroa, S.; Raptova, L.; Reuter, D. C.; Stein, K. A.;
Weikert, R. J. WO2005118539, 2005.; (b) Greenhouse, R.; Jaime-Figueroa, S.;
Lynch, S. M.; Raptova, L.; Stein, K. A.; Weikert, R. J. US2007123535, 2007.
10. Neither the postulated electrophilic diiminoquinone-like intermediate or its
derivatives were identified in metabolite ID studies.
11. TDI was measured as the percentage of CYP3A4 inactivation after pre-
incubation of the test compound for 25 min with human liver microsomes.
The potential risk was ranked low if the measured value was less than that
observed for ethynylestradiol (no clinical effects observed due to TDI), high
if it was greater than that observed for verapamil (drug–drug interactions
observed in the clinic), and moderate if it lay between ethynylestradiol and
verapamil.
12. hERG IC20 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 (0.3 lM, 1 lM, or 1 lM, 10 lM).
13. (a) Evans, C. A.; Fries, H. E.; Ward, K. W. Chem. Biol. Interact. 2005, 152, 25; (b)
Mansuy, D.; Valadon, P.; Erdelmeier, I.; Lopez-Garcia, P.; Amar, C.; Girault, J. P.;
Dansette, P. M. J. Amer. Chem. Soc. 1991, 113, 7825.
Acknowledgements
The authors thank Dr. Michael P. Dillon for his interest in this
work and John Chiang of the Analytical Separations Group for HPLC
purification of libraries.
14. CLint measured in HT format: total incubation volume 250
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
lL, 0.5 mg/mL
l
30 min) were taken then the samples transferred to 384 well plates for LC/
MS analysis.
15. CYP2D6 and CYP3A4 inhibition was measured in
a fluorescence based
References and notes
assay where CYP marker substrates become fluorescent upon CYP
metabolism.
16. 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.
1. (a) Doris, A.; Ebmeier, K.; Shajahan, P. The Lancet 1999, 354, 1369; (b) Paykel, E.
S.; Brugha, T.; Fryers, T. Eur. Neuropsychopharmacol. 2005, 15, 411.
2. Leonard, B. E. Drugs Today 2007, 43, 705.
3. Belmaker, R. H.; Agam, G. N. Engl. J. Med. 2008, 358, 55.
4. Baldessarini, R. J. In The Pharmacological Basis of Therapeutics, 10th ed.;
Hardman, J. G., Limbird, L. E., Gilman, A. G., Eds.; McGraw-Hill, 2001, pp 451–
483.
Substrate concentration 10
lM.
17. Carter, D. S.; Schoenfeld, R. C.; Weikert, R. J. WO2008019971, 2008.