N-(1,2-Diphenylethyl)piperazines: A new class of dual serotonin/noradrenaline reuptake inhibitor

Article abstract of DOI:10.1016/j.bmcl.2006.05.051

The synthesis and structure-activity relationships of a novel series of piperazine derivatives as dual inhibitors of serotonin and noradrenaline reuptake is described. Two compounds possessed comparable in vitro profiles to the dual reuptake inhibitor dul

Full text of DOI:10.1016/j.bmcl.2006.05.051

Bioorganic & Medicinal Chemistry Letters 16 (2006) 4345–4348
N-(1,2-Diphenylethyl)piperazines: A new class of dual serotonin/
noradrenaline reuptake inhibitor
M. Jonathan Fray,* Gerwyn Bish, Alan D. Brown, Paul V. Fish, Alan Stobie,
Florian Wakenhut and Gavin A. Whitlock
Sandwich Chemistry, Pfizer Global Research and Development, Sandwich, Kent CT13 9NJ, UK
Received 11 April 2006; revised 16 May 2006; accepted 16 May 2006
Available online 5 June 2006
Abstract—The synthesis and structure–activity relationships of a novel series of piperazine derivatives as dual inhibitors of serotonin
and noradrenaline reuptake is described. Two compounds possessed comparable in vitro profiles to the dual reuptake inhibitor
duloxetine.
Ó 2006 Elsevier Ltd. All rights reserved.
Release of the neurotransmitters serotonin (5-hydroxy-
F
NHMe
NHMe
tryptamine, 5-HT), noradrenaline (norepinephrine,
NA) and dopamine (DA) into the synaptic cleft results
in receptor activation, and is followed by reuptake of
the neurotransmitters by their respective cognate trans-
porter proteins. Inhibition of these transporters consti-
tutes an attractive approach to the treatment of a
number of diseases.1 For example, the selective seroto-
nin reuptake inhibitors fluoxetine (1), sertraline (2) and
paroxetine (3) are all established anti-depressants.2
Reboxetine (4), a selective NA reuptake inhibitor, is also
used to treat depression,3 and clinical studies have been
conducted in patients with seasonal affective disorder,
Parkinson’s disease, panic disorder and attention deficit
hyperactivity disorder. Dual 5-HT/NA reuptake inhibi-
tors, for example, venlafaxine (5)4 and duloxetine (6),5
have shown efficacy as anti-depressants. Duloxetine
has also shown efficacy in stress urinary incontinence.6
Inhibitors of dopamine reuptake are of interest as agents
to treat Parkinson’s disease and cocaine dependency,7
and more recently the role of neurotransmitter uptake
inhibitors has been examined in clinical trials and
animal models of chronic and inflammatory pain.8
O
NH
O
O
O
F
F
Cl
F
Cl
2 sertraline
3 paroxetine
1 fluoxetine
O
HO
NH
NHMe
NMe2
S
O
O
MeO
OEt
5 venlafaxine
6 duloxetine
4 (+)-reboxetine
selective NA and dual 5-HT/NA transporter inhibition,
according to the particular structural features.9 To iden-
tify novel leads as dual 5HT/NA reuptake inhibitors, the
Pfizer compound files were searched for compounds
sharing some of the structural features of compounds
1–6. One particularly interesting compound was 7,
which had been prepared some years previously as an
intermediate for a d-opioid ligand programme.10
It is notable that the compounds shown above are able
to span a range of activities from selective 5-HT to
NH
7
N
5-HT IC50
NA IC50
DA IC50
=
=
=
9.5 nM
14 nM
1400 nM
Keywords: Serotonin; Noradrenaline; Reuptake inhibition; Neuro-
transmitters; Depression; Incontinence.
*
Corresponding author. Tel.: +44 0 1304 649175; fax: +44 0 1304
656776; e-mail: jonathan.fray@pfizer.com
OH
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.05.051

4346
M. Jonathan Fray et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4345–4348
Compound 7 potently inhibited both [3H]-5-HT and
[3H]-NA reuptake in HEK293 cells transfected with
the human amine transporters with 100-fold selectivity
over the dopamine transporter.11 We therefore under-
took to explore the structure–activity relationships of
analogues of compound 7.
CF3O (16), which is often considered a good isostere
of chlorine, were about 2-fold less potent. The trifluoro-
methyl analogue 17 was similar, with a 3-fold difference
between 5-HT and NA activities, but had >100-fold
selectivity versus the DA transporter. In contrast, the
methyl and ethyl analogues, 18 and 19, possessed good
potency versus 5-HT reuptake but were 3.5- to 4.5-fold
less potent than duloxetine versus NA reuptake. The
alkoxy analogues 20–23 showed a slight trend to
decreasing potency with increasing size. Thus, in this
series, the methoxy (20) and ethoxy (21) analogues were
the best, and exhibited 2-fold lower potency than
duloxetine versus 5-HT reuptake, but comparable
potency to duloxetine versus NA reuptake and superior
selectivity versus DA reuptake. The structure–activity
relationships for substituents at the 2-position are rela-
tively flat with both electron-withdrawing and elec-
tron-donating substituents giving good potency as long
as they were not too large. Introduction of more polar
substituents, as in 24–26, led to a significant loss of
potency.
Compounds 8–26 were conveniently prepared using mod-
ifications of Katritzky’s amine synthesis (Scheme 1).12,13
Thus, condensation of equimolar quantities of benzalde-
hyde, N-Boc-piperazine and benzotriazole in refluxing
toluene with azeotropic removal of water afforded 27,
which was not isolated, but added directly as a solution
in toluene to a solution of the relevant benzylic
Grignard reagent (2 equiv) or zinc reagent.14 The result-
ing N-Boc piperazines were then deprotected under stan-
dard conditions to give 8–10 and 12–24.15 Compounds
11 and 25 were prepared by hydrolysis of the
corresponding nitriles 10 and 23 (3 equiv KOH,
t-BuOH, reflux).16 Compound 26 was prepared by the
reaction of 27 with (2-methanesulfonyl)benzyl zinc
bromide (from (2-methanesulfonyl)benzyl bromide17
and activated zinc dust), followed by Boc-deprotection.
We have found that in this series of N-substituted pip-
erazines it is possible to obtain dual 5-HT/NA reuptake
inhibition similar to duloxetine (e.g., compounds 7 and
14) or at slightly lower potency (e.g., compounds 8, 15–
17) or with slightly modified 5-HT:NA reuptake inhibi-
tion ratios (duloxetine = 3:1, compounds 20 and
21 = approx 1:1). In the context of treating urinary
incontinence, duloxetine, with a 5-HT:NA reuptake
inhibition ratio of 3:1, has been shown to be effica-
cious, however, it is as yet unclear whether this ratio
is optimal. It was, however, not possible to achieve
very high potency (<10 nM) versus NA reuptake. It
is of interest that an ortho substituent in one of the
phenyl rings (optimally ethoxy, as in compound 22)
was necessary to obtain potent activity versus the nor-
adrenaline transporter, cf. reboxetine, a highly selective
NA reuptake inhibitor, yet potent blockade of the 5-
HT transporter was also achieved. Of the compounds
screened versus DA reuptake, most possessed very
low potency. A recent paper18 describes diaryl tropane
derivatives as selective DA reuptake inhibitors, which
would at first glance appear to have some similarity
with the compounds described here, although the dis-
tance between the phenyl rings and the basic centre
seems shorter. Such observations show just how subtle
the dependence of activity is on structure.
Compounds 8–27 were assayed for their ability to inhib-
it the uptake of [3H]-5-HT and [3H]-NA in HEK293 cells
expressing a single human amine transporter.11 Refer-
ence data were also measured for fluoxetine, reboxetine
and duloxetine. In addition, selected compounds were
also assayed against the dopamine transporter in
HEK293 cells. Results are shown in Table 1.
Compound 7 has comparable potency to duloxetine ver-
sus both the 5-HT and NA transporters, with at least
100-fold selectivity over the DA transporter. The unsub-
stituted derivative 8 was marginally less potent against
all three transporters. Of the other analogues bearing a
single substituent in the 3-position (9–13), most retained
good potency versus 5-HT reuptake, but were less po-
tent than 7 versus NA reuptake, with the 3-carboxamide
11 derivative being particularly weak. This may indicate
a severe steric restriction at this position.
A series of close 2-position analogues was then exam-
ined (compounds 14–26). The chloro-substituted ana-
logue 14 was comparable in potency and selectivity to
duloxetine, whereas the bromo analogue 15 and the
O
O
OBut
N
OBut
b, c
NH
N
a
N
N
N
HN
N
4
R
2
N
3
27
8-10, 12-24, 26
Scheme 1. Reagents and conditions: (a) benzaldehyde, benzotriazole, toluene, Dean–Stark apparatus, reflux, 18 h; (b) ArCH2MgHal, THF, À78 °C
to 0 °C, 30 min or ArCH2ZnHal, THF, 20 °C, 3 h, 35–75%; (c) 50% TFA, CH2Cl2, 20 °C, 80–95%.

M. Jonathan Fray et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4345–4348
4347
Harv. Rev. Psychiatry 1999, 7, 69; (c) Finley, P. R. Ann.
Pharmacother. 1994, 28, 1359.
Table 1. Inhibition of Human amine reuptake by N-substituted
piperazines
3. (a) Page, M. E. CNS Drug Rev. 2003, 9, 327; (b) Svensson,
T. H. Acta Psychiatr. Scand. Suppl. 2000, 402, 18; (c)
Tanum, L. Acta Psychiatr. Scand. Suppl. 2000, 402, 37.
4. (a) Einarson, T. R.; Addis, A.; Mittmann, N.; Iskedjian,
M. Can. J. Clin. Pharmacol. 1998, 5, 205; (b) Dubovsky,
S. L. J. Clin. Psychiatry 1994, 55(Suppl.), 34; (c) Brunello,
N.; Racagni, G. Human Psychopharmacol. 1998, 13,
S13.
NH
N
4
R
2
3
5. (a) Boot, J. R.; Brace, G.; Delatour, C. L.; Dezutter, N.;
Fairhurst, J.; Findlay, J.; Gallagher, P. T.; Hoes, I.;
Mahadevan, S.; Mitchell, S. N.; Rathwell, R. E.; Richards,
S. J.; Simmonds, R. G.; Wallace, L.; Whatton, M. A.
Bioorg. Med. Chem. Lett. 2004, 14, 5395; (b) Bymaster, F.
P.; Beedle, E. E.; Findlay, J.; Gallagher, P. T.; Krushinski, J.
H.; Mitchell, S. N.; Robertson, D. W.; Thompson, D. C.;
Wallace, L.; Wong, D. T. Bioorg. Med. Chem. Lett. 2003,
13, 4477; (c) Schatzberg, A. F. J. Clin. Psychiatry 2003,
64(Suppl. 13), 30; (d) Detke, M. J.; Lu, Y.; Goldstein, D. J.;
McNamara, R. K.; Demitrack, M. A. J. Psychiatr. Res.
2002, 36, 383; (e) Karpa, K. D.; Cavanaugh, J. E.; Lakiski, J.
M. CNS Drug Rev. 2002, 8, 361.
Compound
R
IC50 (nM)
NA
5-HT
9.5
14
9.4
DA
7
3-OH
H
14
39
1400
>4000
NT
8
9
10
3-Cl
3-CN
54
4
210
25
300
>400
130
>400
22
NT
NT
11
12
3-CONH2
3-OMe
3-CF3
2-Cl
>4000
NT
13
14
13
5.4
1300
NT
15
16
2-Br
15
13
35
38
2-CF3O
2-CF3
2-Me
NT
>4000
NT
6. Bump, R. C.; Norton, P. A.; Zinner, N. R.; Yalcin, I.
Obstet. Gynecol. 2003, 102, 76.
17
18
10
12
31
90
7. Yu, P. Curr. Opin. Invest. Drugs 2000, 1, 504; Preti, A.
Curr. Opin. Invest. Drugs 2000, 1, 241.
19
20
2-Et
16
18
72
22
>4000
11,000
>4000
NT
2-MeO
2-EtO
2-n-PrO
8. (a) Mochizucki, D. Human Psychopharmacol. 2004,
19(Suppl. 1), S15; (b) James, C. K.; Peters, S. C.; Shannon,
H. E. J. Pharm. Exp. Ther. 2005, 312, 726; (c) Goldstein,
D. J.; Lu, Y.; Detke, M. J.; Hudson, J.; Iyengar, S.;
Demitrack, M. A. Psychosomatics 2004, 45, 17.
9. Gundertofte, K.; Bogeso, K. P.; Liljefors, T. In Computer-
Assisted Lead Finding and Optimization: Current tools for
Medicinal Chemistry; Van de Waterbeemd, H., Testa, B.,
Folkers, G., Eds.; Wiley-Verlag: Weinheim, 1997; pp 445–
459.
21
22
13
22
16
31
23
24
2-CF3CH2O
2-CN
38
87
61
73
>4000
NT
NT
25
26
2-CONH2
2-SO2CH3
>400
>400
16
>400
270
5200
11
NT
Fluoxetine
Reboxetine
Duloxetine
4400
>25,000
870
590
6.0
19
Notes: IC50 values are a geometric mean of at least n = 4. A difference
of <2-fold should not be considered significant.
NT, not tested.
10. (a) Natsuka, K.; Nishikawa, Y.; Nakamura, H. Chem.
Pharm. Bull. 1999, 47, 1790; (b) Natsuka, K.; Nakamura,
H.; Negoro, T.; Uno, H.; Nishimura, H. J. Med. Chem.
1978, 21, 1265.
11. The assays were a modification of those described by
Blakely, R. D.; Clark, J. A.; Rudnick, G.; Amara, S.
G.; Anal. Biochem. 1991, 194, 302. HEK293 cells
expressing a single human amine transporter protein
(7500 cells/well in Millipore 96-well filter bottom plates)
were pre-incubated at 25 °C for 5 min with assay buffer
containing vehicle (DMSO in water) or test compound.
Uptake of neurotransmitter into the cells was initiated
by the addition of tritiated 5-HT (50 nM), NA (200 nM)
or DA (200 nM) substrates, the samples were shaken in
an incubator at 25 °C for 5 min (5-HT, DA) or 15 min
(NA). The assays were stopped by an ice-cold buffer
wash followed by filtration. The filters were then dried
before measuring the amount of radioactivity taken up
into the cells by scintillation counting. Potency of test
compounds was quantified as IC50 values, that is,
concentration required to inhibit the specific uptake of
radiolabelled substrate into the cells by 50% relative to
maximum (vehicle only) over a 10-point dose response
range. A minimum of four measurements of the IC50
were made. Typically, the IC50s did not fit a normal
distribution. Thus, geometric means were used to
minimise the distorting effect of outliers. From valida-
tion data sets on all three transporter assays, we
established that the 95% confidence intervals were
1.3-fold (5-HT, DAT) to 1.5-fold (NA) of the IC50,
that is, IC50 = 4 nM would have 95% confidence inter-
vals of 1.4–6.6 nM.
These results were very encouraging and we therefore
prepared further analogues, which are the subject of
the following communication.
Acknowledgments
We wish to acknowledge the contributions of Drs.
Stephen Phillips and Donald Newgreen and their teams
(Discovery Biology Department) for screening data, and
Miles Tackett, Arnaud Lemaitre and Bhairavi Patel for
compound synthesis.
References and notes
1. (a) Reith, M. E. A., Ed.; Neurotransmitter Transporters:
Structure, Function and Regulation, 2nd ed.; Humana:
Totowa, New Jersey, 2002; (b) Iversen, L. Mol. Psychiatry
2000, 5, 357; (c) Schatzberg, A. F. J. Clin. Psychiatry 2000,
61(Suppl. 11), 9; (d) Barker, E. L.; Blakely, R. D. In
Psychopharmacology: The Fourth Generation of Progress;
Bloom, F. E., Kupfer, D. J., Eds.; Raven Press: Philadel-
phia, 1995; p 21.
2. (a) Pacher, P.; Kohegiji, E.; Kecskemeti, V.; Furst, S.
Curr. Med. Chem. 2001, 8, 89; (b) Masand, P. S.; Gupta, S.

4348
M. Jonathan Fray et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4345–4348
12. Katritzky, A. R.; Yannakopoulou, K.; Lue, P.; Rasala,
D.; Urogdi, L. J. Chem. Soc., Perkin Trans. 1 1989, 225.
13. Katritzky, A. R.; Strah, S.; Belyakov, S. A. Tetrahedron
1998, 54, 7167.
14. The organometallic reagents used were either obtained
commercially or generated from the commercial benzylic
chloride or bromide immediately prior to use.
15. All test compounds were prepared and screened as
racemates.
16. Hall, J.; Gisler, M. J. Org. Chem. 1976, 41, 3769.
17. Kidd, D. A. A.; Wright, D. E. J. Chem. Soc. 1962, 1420.
18. Kotturi, S. V.; Jiang, S.; Chang, A.-C.; Abraham, P.;
Navarro, H. A.; Kuhar, M. J.; Carroll, F. I. J. Med. Chem.
2005, 48, 7437.