An octahydro-cyclopenta[c]pyrrole series of inhibitors of the type 1 glycine transporter

Article abstract of DOI:10.1016/j.bmcl.2009.12.071

We describe a novel series of inhibitors of the type 1 glycine transporter (GlyT1) as an approach to relieving the glutamatergic deficit that is thought to underlie schizophrenia. Synthesis and SAR follow-up of a series of octahydro-cyclopenta[c]pyrrole derivatives afforded potent in vitro inhibition of GlyT1 as well as in vivo activity in elevating CSF glycine. We also found that a 3-O(c-pentyl), 4-F substituent may serve as a surrogate for the widely used 3-trifluoromethoxy group, suggesting its application as an isostere for future medicinal chemistry studies.

Full text of DOI:10.1016/j.bmcl.2009.12.071

Bioorganic & Medicinal Chemistry Letters 20 (2010) 907–911
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
An octahydro-cyclopenta[c]pyrrole series of inhibitors of the type 1
glycine transporter
*
John A. Lowe III , Shari L. DeNinno, Susan E. Drozda, Christopher J. Schmidt, Karen M. Ward,
F. David Tingley III, Mark Sanner, Don Tunucci, James Valentine
Pfizer Global Research & Development, Eastern Point Road, Groton CT 06340, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
We describe a novel series of inhibitors of the type 1 glycine transporter (GlyT1) as an approach to reliev-
ing the glutamatergic deficit that is thought to underlie schizophrenia. Synthesis and SAR follow-up of a
series of octahydro-cyclopenta[c]pyrrole derivatives afforded potent in vitro inhibition of GlyT1 as well as
in vivo activity in elevating CSF glycine. We also found that a 3-O(c-pentyl), 4-F substituent may serve as
a surrogate for the widely used 3-trifluoromethoxy group, suggesting its application as an isostere for
future medicinal chemistry studies.
Received 6 November 2009
Revised 16 December 2009
Accepted 17 December 2009
Available online 23 December 2009
Keywords:
Glycine
Ó 2009 Elsevier Ltd. All rights reserved.
Transporter
Inhibitor
Schizophrenia
Glycine performs a critical role as a regulator of the activity of
the major excitatory CNS neurotransmitter glutamate. Binding to
the regulatory NR1 subunit of the NMDA receptor for glutamate,
glycine controls the ‘gain’ of the receptor, and in the process regu-
lates a post-synaptic event mediated by NMDA transmission: LTP,
or long-term potentiation, which in turn regulates memory, exec-
utive function, and affect.1 These three aspects of glutamatergic
transmission are key deficits of patients suffering from schizophre-
nia, leading to the hypothesis that NMDA hypofunction underlies
the etiology of the disease.2 Elevating glycine levels by blocking
its re-uptake would restore NMDA function and potentially relieve
many, if not most, of the symptoms of schizophrenia.3
One approach to elevating glycine levels is inhibition of the type
1 glycine transporter (GlyT1), which controls glycine uptake in the
vicinity of NMDA synapses.4 We have previously reported our
efforts to discover novel GlyT1 inhibitors. Our first approach was
based on the structure of the prototypical GlyT1 inhibitor (R)-
N[3-(40fluorophenyl)-3-(40phenylphenoxy)propyl]-sarcosine, des-
ignated ALX 5407, (racemic form named NFPS) 1,5 (Fig. 1) and
resulted in the compound 2, (R)-N[3-phenyl-3-(40-(4-toluoyl)-
phenoxy)-propyl]sarcosine ((R)-NPTS).6 Radiolabeled (R)-NPTS
served as a radioligand for a binding assay to help discover our first
structurally novel GlyT1 inhibitor series exemplified by compound
3.7 This series, however, was plagued with high clearance and
affinity for the HERG-encoded IKr potassium channel, a source of
potential cardiovascular toxicity, and hence was not pursued fur-
ther. A subsequent high-throughput screen identified compound
4 as a promising lead based on its potent in vitro GlyT1 inhibition
and in vivo activity in elevating CSF glycine levels, as reported pre-
viously and outlined in Table 1.8
As a follow up to compound 4, we selected a template variation
based on the octahydro-cyclopenta[c]pyrrole system found in the
known compound 5.9 This compound had potential sites for deriv-
atization analogous to those in compound 4, and incorporated one
F
O
O
N
N
OH
OH
O
O
O
2
1
O
Cl
OH
N
N
N
N
CH3
O
N
N
O
Cl
N
.2HCl
NH2
3
4
* Corresponding author. Tel./fax: +1 860 535 4283.
E-mail address: jal3rd@gmail.com (J.A. Lowe).
Figure 1. GlyT1 inhibitors.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.12.071

Table 1
Data for GlyT1 Inhibitors
Compound
No.
GlyTl Ki (nM)
GlyT2 IC50
1150
Rat CSF
glycine
hMic t1/2
min
MDCK
AB_C
MDR
ratio
Dofetilide
Ki
HERG IC50
(nM)
CYPZD6 IC50
nM
CYP3A4 IC50
nM
c log P Polar
Motwt
347
area
4
1.79 (1.59–2.01,
n = 68)
3.79 (2.17–
7.41)
>120
0.5
1.50
3760
65,600
>10,000
>10,000
1.75
64.15
8a
8b
8c
8d
8c
8f
1.71
1.98
2.44
3.96
5.3
>641
2.5
4200 (3720–4740, n = 2)
1170
2270
2210
>10,000
NT
6250
249% @ 10
219% @ 10
NT
NT
NT
NT
240% @ 1
0.91
>120
50
2 1
1.1
0.2
2.2
0.262
0.291
21.5
14.5
ND
ND
3310
1650
>4760
ND
ND
ND
4330
13,200
NT
NT
NT
NT
NT
1540
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
7100
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
5000
1.95
1.95
2.4
1.68
2.31
2.05
2.4
50.16
59.39
59.39
59.39
59.39
68.62
50.6
410.4
408.4
>111
>120
>120
>120
30.5
>120
1.90
5.26
2.65
2.52
1.64
1.91
426.53
382.5
408.54
438.57
422.45
438.4
9a
9b
2560
4500
2.64 (2.00–3.48,
n = 5)
>3450 (18.9–6,30,000
n = 3)
1800
2.92
41.37
9c
2.90 (1.07–7.83,
n = 5)
>7290 (2650–20,100,
n = 4)
0.58
>120
13
2.07
2100
1720
>10,000
>10,000
2.4
41.37
424.4
9d
9e
9f
3.76
3.82
>10,000
3000
>10,000
181% @ 1
NT
NT
78.1
10
<3.2
10.4
30
45.8
2.80
1.54
0.80
4910
2110
>5190
NT
NT
NT
>10,000
1100
1300
>10,000
>10,000
7900
2.84
2.93
2.8
50.6
50.6
50.6
440.56
436.48
472.46
4.44 (0.277–71.0,
n = 2)
10.5
2.79
5.74
26.9
9g
3020 (1950–4700, n = 2)
NT
NT
217% @ 10
NT
<3.2
8.19
7.06
>120
25.9
8.1
17.7
0.2
1.00
3.17
2.50
1.02
4530
177
3810
>4590
NT
NT
NT
NT
100
8700
3
50.6
504.47
528.57
480.53
410.4
10a
10b
14
3880
8870
NT
1700
1100
>10,000
>10,000
>10,000
8200
3.27
2.4
1.95
70.83
70.83
50.16
NT—not tested; GlyT1 Ki values are given in nM units, with standard error of the mean and number of determinations indicated in parentheses. GlyT2 IC50 values are given in nM units. Both GlyT1 and GlyT2 values were determined
as previously described in Refs. 6,7 CSF Glycine ED200 values represent the dose that doubles endogenous (baseline) levels of glycine in rat csf following subcutaneous administration of a test compound at a 90 min time point,
with 95% confidence limits in parentheses. hMic t1/2—compound half-life in human microsomes given in minutes. MDCK_AB_C—apparent permeability through MDCK cell membranes in units of 10–6 cm/s, corrected for
background, at an initial concentration of 2 lM. MDR ratio is MDR_C_BA_AB divided by MDCK_C_BA_AB, where the ratio of permeation in the BA direction divided by the AB direction in MDR over-expressing cells is compared with
the value in MDCK cells not over-expressing MDR. Dofetilide Ki values are given in nM units for displacement of radiolabeled dofetilide from HERG IKr channels expressed in HEK293 cells. HERG IC50 values given in nM units for
block of HERG current in patch clamp measurement. c Log P, Polar area (calculated topological polar surface area), and Mol wt (molecular weight).

J. A. Lowe et al. / Bioorg. Med. Chem. Lett. 20 (2010) 907–911
909
O
Cl
O
H
N
N
N
H2N
Na(OAc)3BH
O
N
X
X
X
N
CH3
N
H3C
H
H
H
H
H
H
(i-Pr)2NEt, CH3CN
84.5%
AcOH, ClCH2CH2Cl
91-100%
N
N
N
O
O
O
O
O
O
5
7
6
O
O
N
N
N
N
X
X
RCHO
HBr
N
N
H3C
H3C
HOAc
77%
H
H
H
Na(OAc)3BH
AcOH, ClCH2CH2Cl
50-70%
H
N
R
N
H
8
9
Et3N
O
8a, X = 3-CF3, 4-F
8b, X = 3-OCF3
R1
R2
EtOH
67-73%
O
9a, R = CH3, X = 3-OCF3
N
9b, R = CH2CH3, X = 3-CF3, 4-F
9c, R = CH3, X = 3-CF3, 4-F
9e, R = CH2CH3, X = 3-OCF3
9f, R = CH2CHF2, X = 3-OCF3
N
X
N
H3C
H
H
N
9g, R = CH2CH2CF3, X = 3-OCF3
10a, R1 = H, R2 = Ph, X = 3-OCF3
10b, R1, R2 = CH3, X = 3-OCF3
HO
R1
R2
10
Scheme 1. Preparation of compounds 8–10.
of the nitrogens into a five-membered ring. In addition, we hoped
its increased lipophilicity would address the poor permeability of
4, indicated by its MDCK_C_AB value of 0.5 (where a value >10 de-
notes good permeability to facilitate brain penetration). We began
by attaching the requisite benzylamine and imidazole amide func-
tions as shown in Scheme 1. We chose benzylamines substituted in
the meta-position as in 4. In some cases, the synthesis required
addition of an appropriately substituted aldehyde to the 5-ami-
no-(octahydro-cyclopenta[c]pyrrole) ring system, as shown in
Scheme 2. We focused on 3-alkoxy-substituted benzylamines to
enable expansion of the SAR. The initial N–H compounds, 8, gener-
ated by deblocking compounds 7, have potent GlyT1 inhibitory
activity, as well as good selectivity over the type 2 glycine trans-
porter (GlyT2), as shown in Table 1. In order to achieve this potent
GlyT1 inhibitory activity, electron-withdrawing substituents are
favored, but not required. For example, addition of a fluorine at po-
sition 4 to the 3-O(c-pentyl) compound 8e to afford compound 8c
improves GlyT1 affinity. Addition of a 4-methoxy substituent, as in
compound 8f, on the other hand, abolishes activity. We prepared
the endo isomer, compound 14, in low yield from the minor isomer
formed in the first two steps in Scheme 1, as shown in Scheme 3,
and found it to be far less active than the exo isomer 8a.
Compounds 8a–f, however, are poorly permeable through
MDCK cell membranes (MDCK_C_AB values <2.5). The two most
potent compounds reported here, 8a and 8b, show only modest
in vivo activity in elevating CSF glycine, giving an estimated
ED200 dose, the dose required to double CSF glycine, of a little un-
der 10 mg/kg administered subcutaneously (sc). CSF glycine levels
are a measure of glycine that spills over from the CNS compart-
ment upon GlyT1 block at NMDA synapses. In order to achieve clin-
ically relevant doses, we sought compounds with ED200 values
less than 1 mg/kg sc. The Pgp transporter, which effluxes drug from
the brain compartment, plays only a limited role with these com-
pounds, as indicated by MDR ratios generally less than three as
shown in Table 1. In addition, compounds 8a–f show good stability
in the presence of human microsomes (hMic t1/2 generally
>100 min), and weak inhibition of the HERG-encoded IKr potassium
channel or cytochrome P450 2D6 or 3A4.
Addition of an alkyl group to the ring nitrogen via reductive
amination afforded much better MDCK_C_AB values (generally
>10), leading to an improvement in in vivo efficacy. For example,
compound 9a shows over 200% elevation of CSF glycine at a dose
of 1 mg/kg sc, and compounds 9b and 9c show ED200 values of
0.91 and 0.58 mg/kg sc, meeting the goal we had set for this activ-
ity. This improvement came at the cost of increased HERG-encoded
IKr block for compounds 9b and 9c, and inhibition of cytochrome
P450 2D6 and 3A4 for 9b. Compound 9d, with the larger O-cyclo-
pentyl substituent, reversed this trend, but showed weaker CSF
glycine elevation. We tried to decrease IKr block by reducing the
basicity of the ring nitrogen using fluorine substitution, in com-
pounds 9f and 9g. While this modification achieved reduced IKr
block, and increased permeability in MDCK cells, it resulted in cat-
astrophic loss of microsomal stability and increased inhibition of
cytochrome P450 2D6 and 3A4. Finally, we tried addition of an
alcohol substituent by reaction of the N–H compound with an
epoxide, expecting decreased basicity and increased polarity to re-
duce IKr block. These expectations were unfulfilled, however, in
that compound 10a shows potent binding to IKr, possibly due to
the phenyl ring in proximity to the basic ring nitrogen. Compound
10b shows only modest activity in elevating CSF glycine, and both
compounds show instability in the presence of human microsomes
and inhibition of cytochrome P450 2D6.

910
J. A. Lowe et al. / Bioorg. Med. Chem. Lett. 20 (2010) 907–911
H
H2N
N
H2N
Na(OAc)3BH
O
H2, Pd/C
EtOH
H
H
H
H
H
H
AcOH, ClCH2CH2Cl
91%
N
N
N
O
O
O
O
O
O
11
5
6
O
Cl
OHC
O
X
N
N
N
X
N
X
N
CH3
Na(OAc)3BH
N
H3C
H
H
H
H
AcOH, ClCH2CH2Cl
71.5% overall
(i-Pr)2NEt, CH3CN
81%
N
N
O
O
O
O
6
7
O
O
N
N
N
X
N
HCl
RCHO
X
N
N
H3C
EtOAc
H3C
Na(OAc)3BH
AcOH, ClCH2CH2Cl
80-90%
H
H
H
H
94%
N
N
H
R
9
8
8c, R = H, X = 3-O(c-pentyl), 4-F
8d, R = H, X = 3-O(i-Pr)
8e, R = H, X = 3-O(c-pentyl)
8f, R = H, X = 3-O(c-pentyl), 4-OCH3
9d, R = CH3, X = 3-O(c-pentyl), 4-F
Scheme 2. Preparation of compounds 8–9.
O
Cl
O
CF3
H
CF3
F
N
N
N
N
N
CH3
N
F
H3C
H
H
H
H
(i-Pr)2NEt, CH3CN
N
N
O
O
O
O
13
12
O
CF3
F
N
N
N
H3C
HBr
HOAc
H
H
N
H
14
Scheme 3. Preparation of compound 14.
In summary, compound 9d represents the best balance of prop-
methoxy group, a substituent widely used by medicinal chemists.
It only affords a modest addition of molecular weight (18 mass
units) and lipophilicity (0.44 c Log P units). In the case of com-
pounds 9a and 9d, there is a modest decrease in MDCK cell perme-
erties in this series, although its in vivo activity in elevating CSF
glycine is somewhat less than desired. It also demonstrates that
3-O(c-pentyl), 4-F may serve as a surrogate for the 3-trifluoro-

J. A. Lowe et al. / Bioorg. Med. Chem. Lett. 20 (2010) 907–911
911
3. Kinney, G. G.; Sur, C. Curr. Neuropharmacol. 2005, 3, 35.
ability and an increase in MDR ratio. While these results may vary
in particular cases, it nonetheless may serve as an isostere for the
trifluoromethoxy group for future medicinal chemistry studies.
4. (a) Atkinson, B. N.; Bell, S. C.; De Vivo, M.; Kowalski, L. R.; Lechner, S. M.;
Ognyanov, V. I.; Tham, C.-S.; Tsai, C.; Jia, J.; Ashton, D.; Klitencik, M. A. Mol.
Pharmacol. 2001, 60, 1414; (b) Aubrey, K. R.; Vandenberg, R. J. Br. J. Pharmacol.
2001, 134, 1429.
5. Bergeron, R.; Meyer, T. M.; Coyle, J. T.; Greene, R. W. Proc. Natl. Acad. Sci. U.S.A.
1998, 95, 15730.
Supplementary data
6. Lowe, J. A., III; Drozda, S. E.; Fisher, K.; Strick, C.; Lebel, L.; Schmidt, C.; Hiller, D.;
Zandi, K. S. Bioorg. Med. Chem. Lett. 2003, 13, 1291.
7. Lowe, J.; Drozda, S.; Qian, W.; Peakman, M. C.; Liu, J.; Gibbs, J.; Harms, J.;
Schmidt, C.; Fisher, K.; Strick, C.; Schmidt, A.; Vanase, M.; Lebel, L. Bioorg. Med.
Chem. Lett. 2007, 17, 1675.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2009.12.071.
8. Lowe, J. A., III; Hou, X.; Schmidt, C.; Tingley, F. D.; McHardy, S., III; Kalman, M.;
DeNinno, S.; Sanner, M.; Ward, K.; Lebel, L.; Tunucci, D.; Valentine, J. Bioorg. Med.
Chem. Lett. 2009, 19, 2974.
References and notes
1. Coyle, J. T. Cell. Mol. Neurobio. 2006, 26, 365.
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