N-Benzyl-N-(tetrahydro-2H-pyran-4-yl)pyrrolidin-3-amines as selective dual serotonin/noradrenaline reuptake inhibitors

Article abstract of DOI:10.1016/j.bmcl.2007.01.020

A series of N-benzyl-N-(tetrahydro-2H-pyran-4-yl)pyrrolidin-3-amine monoamine reuptake inhibitors are described. Selective dual 5-HT and NA reuptake inhibition was achieved, and analogues with weak CYP2D6 inhibition, good human in vitro metabolic stabilit

Full text of DOI:10.1016/j.bmcl.2007.01.020

Bioorganic & Medicinal Chemistry Letters 17 (2007) 2022–2025
N-Benzyl-N-(tetrahydro-2H-pyran-4-yl)pyrrolidin-3-amines as
selective dual serotonin/noradrenaline reuptake inhibitors
Paul V. Fish, M. Jonathan Fray, Alan Stobie, Florian Wakenhut and Gavin A. Whitlock*
Sandwich Chemistry Department, Pfizer Global Research and Development, Sandwich Labs,
Ramsgate Road, Sandwich, Kent CT13 9NJ, UK
Received 6 December 2006; revised 3 January 2007; accepted 7 January 2007
Available online 19 January 2007
Abstract—A series of N-benzyl-N-(tetrahydro-2H-pyran-4-yl)pyrrolidin-3-amine monoamine reuptake inhibitors are described.
Selective dual 5-HT and NA reuptake inhibition was achieved, and analogues with weak CYP2D6 inhibition, good human in vitro
metabolic stability and wide ligand selectivity, such as 12b, were identified.
Ó 2007 Elsevier Ltd. All rights reserved.
Selective inhibition of serotonin (5-HT) and noradrena-
NH
NHMe
line (NA) reuptake (SNRI) has been shown to be an
attractive dual pharmacology approach for the treat-
ment of a number of diseases. For example, venlafaxine
1 and duloxetine 2 are approved drugs for the treatment
of depression.1,2 Efficacy in clinical trials for neuropath-
ic pain3,4 and stress urinary incontinence (SUI)5 has
also been demonstrated for dual 5-HT/NA reuptake
inhibitors.
S
N
HO
O
NMe2
MeO
1 venlafaxine
Figure 1.
R
3R = Cl
2 duloxetine
4R = OEt
We recently described the discovery of N-substituted
piperazine derivatives, for example, 3 and 4, that acted
as dual inhibitors of 5-HT and NA reuptake with good
selectivity over dopamine (DA) reuptake (Fig. 1).6
H
N
NH
R1
N
Scaffold-hopping
N
Scaffold-hopping from the piperazine template led to a
series of amino-pyrrolidines, and we investigated this
area for selective dual 5-HT/NA reuptake inhibitors
(SNRI’s) with attractive drug-like properties (Fig. 2).7
3 R = Cl
4 R = OEt
R
R
Amino-pyrrolidine template
Compounds 8-21
The target compounds were readily prepared in a 3-step
sequence from enantiomerically pure BOC-protected 3-
amino-pyrrolidine 5 (Scheme 1). Standard reductive
alkylation conditions introduced R1 (via NaBH(OAc)3
for aldehyde substrates and hydrogenation conditions
[H2, Pd/C catalyst] for ketone substrates). The benzylic
group was then installed by alkylation with a benzylic
Figure 2.
halide. Finally BOC deprotection with trifluoroacetic
acid gave the required pyrrolidines 8–21.
Initial SAR investigations focused on variation of the
R1 group, with the benzylic R substitution as 2,3 di-Cl
or 2,3 di-Me (Table 1). Cyclopropyl methyl and pyr-
idylmethyl analogues 8a,b and 9a,b gave balanced and
potent SNRI activity with moderate selectivity over
dopamine transport. Replacement with a more polar
Keywords: Serotonin reuptake; Noradrenaline reuptake; Metabolic
stability; hERG; SNRI.
*
Corresponding author. Tel.: +44 1304 649174; fax: +44 1304
651987; e-mail: gavin.whitlock@pfizer.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.01.020

P. V. Fish et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2022–2025
2023
O
H
O
O
N
O
O
N
O
R1
Aldehyde,
NaBH(OAc)3, THF, RT
Benzylic halide,
K2CO3 , THF, RT
N
N
TFA, CH2Cl2, RT
R
N
R1
N
R1
or Ketone, H2, Pd/C
catalyst, EtOAc, RT
H2N
R
N
H
5 (R) and (S) enantiomers
7
8-21
6
Scheme 1. Synthesis of amino-pyrrolidine target compounds.
Table 1. In vitro inhibition of monoamine reuptake,a,b human microsomal stabilityc and CYP2D6 inhibition for compounds 8–12
H
N
R1
N
R
Compound
R
R1
Stereochemistry
5-HT IC50
(nM)
NA IC50
(nM)
DA IC50
(nM)
HLM T1/2
(min)
CYP2D6 IC50
(lM)
8a
2,3 di-Cl
(R)
4
15
44
>120
>120
NT
61
NT
0.03
0.5
8b
2,3 di-Cl
2,3 di-Cl
2,3 di-Cl
2,3 di-Cl
2,3 di-Me
2,3 di-Me
2,3 di-Me
2,3 di-Me
—
(S)
(R)
(S)
(S)
(R)
(S)
(R)
(S)
6
9
206
N
9a
3
5
125
N
9b
5
3
122
0.1
O
10
10
15
8
5
89
NT
24
4.8
N
N
11a
8
1470
1600
>4000
727
NT
NT
9.4
11b
12
144
7
14
O
O
12a
43
9
>120
>120
NT
12b
5.9
Duloxetine
—
6
19
870
NT
NT denotes not tested.
a See Ref. 8 for description of assay conditions.
b Monoamine reuptake IC50 values are geometric means of at least three experiments.
c Maximum measurable half-life was 120 min.
branched 4-tetrahyropyranyl (4-THP) group in 10
retained very potent SNRI activity and similar selectivity.
It was found that by replacing 2,3 di-Cl with 2,3 di-Me,
dopamine selectivity could be improved, for example,
compound 12b had dopamine selectivity of 80- to 100-
fold compared to 10 with 8- to 18-fold window.
Conscious of the fact that CYP2D6 inhibition was
prevalent in the monoamine reuptake inhibition
area,9 we collated in vitro CYP2D6 inhibition data
and human in vitro metabolic stability data in
parallel with primary pharmacology and selectivity
screening.

2024
P. V. Fish et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2022–2025
Compounds 8 and 9 gave very potent CYP2D6 inhibi-
tion, whereas the 4-THP analogues 10 and 12 had signif-
icantly reduced activity. By following this parallel
screening approach we quickly showed the 4-THP ana-
logues 12 combined excellent metabolic stability with
weak P450 inhibition. An assessment of stereochemistry
showed that both enantiomers possessed SNRI activity,
with the (S) enantiomer exhibiting slightly better
potency.
cytes, with examples 12b and 15 being the most
metabolically robust. It was also interesting to note that
replacement of the 3-Me of 12b with a CF3 group in 15
improved stability, again indicating that the 3-position
may be a site of metabolism. This trend was also seen
when comparing regioisomers 13 and 14, with the
2-Me 3-Cl isomer conferring more stability than 2-Cl
3-Me.
We also assessed the series for its potential to block the
hERG channel, as activity at this ion channel has been
linked to QT prolongation and cardiac arrhythmia in
man.12 We were pleased to see that the 4-THP amino-
pyrrolidine series, in general, possessed weak affinity
for the hERG channel (Table 2),13 however some inter-
esting SAR trends were observed. It was found that
hERG activity appeared to be more sensitive to struc-
tural changes on the aromatic ring rather than lipophil-
icity.14 For example, the 2,3 di-methyl substituted
example 12b showed the weakest hERG activity,
whereas the more polar 2-methyl analogue 18 had a
greater than 5-fold increase in hERG potency. In anoth-
er example, the 2-CF3 analogue 19 had the most potent
hERG activity, even though it was isolipophilic with
example 12b.
Having identified the 4-THP as a key group, we then
investigated substitution on the benzylic aryl ring in
more detail in the (S) enantiomeric series (Table 2). A
range of different substitution patterns were investigat-
ed, with 2,3 and 2,4 disubstitution affording excellent
dual serotonin/noradrenaline reuptake inhibition, with
good selectivity over dopamine reuptake activity. Inter-
estingly, a single 2-Ph substituent, that is, compound 20,
demonstrated the series could also deliver selective
inhibitors of the noradrenaline transporter.10 The
majority of 4-THP analogues had excellent metabolic
stability, the exception being compound 16 which had
a half-life of less than 120 min. This may be due to
O-dealkylation of the 3-alkoxy group resulting in more
rapid metabolic turnover. The 4-THP series generally
exhibited weak CYP2D6 inhibition, the exception being
the 2-Ph analogue 20 which was a potent CYP2D6
inhibitor.
With the optimal balance of pharmacological and
ADME properties, example 12b was assessed in a panel
of 150 receptors and enzymes, including adrenergic,
dopaminergic, nicotinic and opiate receptors, CYP450’s
and a range of sodium/calcium/potassium channels. We
were pleased to find that 12b demonstrated >200-fold
selectivity for serotonin and noradrenaline reuptake
inhibition over all targets.
In order to rank compounds which had human liver
microsomal half-lives of >120 min still further, a num-
ber of analogues were assessed in a human hepatocyte
assay (Table 2).11 This assay demonstrated that com-
pounds could indeed be differentiated in human hepato-
Table 2. In vitro inhibition of monoamine reuptake,a,b human microsomal stability,c CYP2D6 inhibition, human hepatocyte stability,d hERG
channel activitye and clogP calculations for compounds 12b–21
H
N
O
N
R
Compound
R
5-HT IC50
(nM)
NA IC50
(nM)
DA IC50
(nM)
HLM T1/2
(min)
CYP2D6 IC50
(lM)
Hepatocyte
T1/2 (min)
hERG IC50
(lM)
clogP
12b
13
14
15
16
17
18
19
20
21
2,3 di-Me
2-Cl 3-Me
2-Me 3-Cl
2-Me 3-CF3
2-Me 3-OMe
2-CF3 3-F
2-Me
9
5
7
4
727
115
>120
>120
>120
>120
94
5.9
15.7
NT
5.5
8.2
30
144
71
14.4
NT
NT
2.9
2.4
2.7
2.7
2.9
1.9
2.6
2.0
2.4
3.1
1.8
11
13
11
24
29
21
400
15
5
196
>4000
968
104
>240
NT
NT
NT
NT
83
20
11
13
7
NT
1.6
>4000
1200
>4000
>4000
1190
>120
>120
>120
>120
NT
30
30
2.4
1.1
2-CF3
2-Ph
2,3,4 tri-F
9
12
40
0.1
6.8
NT
1.2
NT
NT denotes not tested.
a See Ref. 8 for description of assay conditions.
b Monoamine reuptake IC50 values are geometric means of at least three experiments.
c Maximum measurable half-life was 120 min.
d Maximum measurable half-life was 240 min.
e Values are geometric means of at least two experiments.

P. V. Fish et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2022–2025
2025
(c) Millard, R. J.; Moore, K.; Rencken, R.; Yalcin, I.;
Bump, R. C. BJU Int. 2004, 93, 311.
In summary, scaffold-hopping from piperazines 3 and 4
to the amino-pyrrolidine structure has delivered a series
of compounds with excellent dual serotonin and nor-
adrenaline reuptake inhibition and 100-fold selectivity
over dopamine reuptake inhibition. By determining
potency, selectivity, CYP2D6 inhibition and human
metabolic stability in parallel we were quickly able to
optimize the drug-like properties of the series. Example
12b was found to have a large selectivity window over
hERG channel activity, and showed greater than 200-
fold selectivity for 5-HT and NA activity over a wide
panel of receptors, enzymes and ion channels. This
investigation has also shown that small substitution
changes can have subtle effects on the ratio of 5-HT to
NA reuptake inhibition. This may prove useful in deter-
mining the optimal ratio of activities in a number of dis-
ease areas such as depression, neuropathic pain and
stress urinary incontinence. Further information on this
series, including pharmacokinetic and in vivo efficacy
data, will be reported in the near future.
6. (a) Fray, M. J.; Bish, G.; Brown, A. D.; Fish, P. V.; Stobie,
A.; Wakenhut, F.; Whitlock, G. A. Bioorg. Med. Chem.
Lett. 2006, 16, 4345; (b) Fray, M. J.; Bish, G.; Fish, P. V.;
Stobie, A.; Wakenhut, F.; Whitlock, G. A. Bioorg. Med.
Chem. Lett. 2006, 16, 4349.
7. After we completed this series, researchers at Eli Lilly
independently published their own work in this area,
Beadle, C. D.; Cases-Thomas, M. J.; Clark, B. P.;
Gallagher, P. T.; Masters, J. J.; Timms, G. H.; Walter,
M. W.; Whatton, M. A.; Wood, V. A.; Gilmore, J.
WO2005000811.
8. 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 neuro-
transmitter 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 three
experiments were made.
Acknowledgments
We wish to thank Drs. Stephen Phillips and Donald
Newgreen and their teams (Discovery Biology Depart-
ment) for screening data, and Debbie Lovering, Edward
Pegden, Carol Bains and Kerry Paradowski for com-
pound synthesis. We are also grateful to Anthony Har-
rison, Nicola Lindsay and Ian Gurrell of the
pharmacokinetics and metabolism department for
ADME studies.
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K.; Knadler, M. P.; Gonzales, C. R.; Yeo, K. P.;
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Pharm. Ther. 2003, 73, 170; (b) Ereshefsky, L.; Riesen-
man, C.; Lam, Y. W. Clin. Pharmacokinet. 1995, 29(S1),
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NA reuptake inhibition.
11. The human hepatocyte assay was able to accurately
determine half-lives of up to 240 min, whereas the
maximum measurable half-life in human liver microsomes
was 120 min.
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