A novel, non-substrate-based series of glycine type 1 transporter inhibitors derived from high-throughput screening

Article abstract of DOI:10.1016/j.bmcl.2006.12.109

The synthesis and structure-activity relationships (SAR) of a series of indane and tetralin inhibitors of the type 1 glycine transporter, derived from a high-throughput screening (HTS) hit, are described. Key modifications that reduced the 5HT1B receptor

Full text of DOI:10.1016/j.bmcl.2006.12.109

Bioorganic & Medicinal Chemistry Letters 17 (2007) 1675–1678
A novel, non-substrate-based series of glycine type 1 transporter
inhibitors derived from high-throughput screening
J. Lowe,* S. Drozda, W. Qian, M.-C. Peakman, J. Liu, J. Gibbs, J. Harms, C. Schmidt,
K. Fisher, C. Strick, A. Schmidt, M. Vanase and L. Lebel
PGRD Groton Laboratories, Pfizer Inc, Groton, CT 06340, USA
Received 9 November 2006; revised 17 December 2006; accepted 22 December 2006
Available online 9 January 2007
Abstract—The synthesis and structure–activity relationships (SAR) of a series of indane and tetralin inhibitors of the type 1 glycine
transporter, derived from a high-throughput screening (HTS) hit, are described. Key modifications that reduced the 5HT1B receptor
affinity of the HTS hit and the P450 2D6 inhibition of subsequent analogues are delineated. While these modifications led to potent
and selective GlyT1 inhibitors, HERG affinity and human microsomal clearance remain an issue for this series of compounds.
Ó 2007 Elsevier Ltd. All rights reserved.
A prevailing neurochemical hypothesis of schizophrenia
posits a deficit in NMDA receptor-mediated glutamater-
gic neurotransmission. For example, NMDA antago-
nists (e.g., PCP or ketamine) can reproduce the
positive, negative, and cognitive symptoms of schizo-
phrenia in healthy volunteers and exacerbate these
symptoms in patients.1,2 In addition, NMDA blockade
can reproduce the dysregulation of the subcortical dopa-
mine system observed in schizophrenia—hence the clin-
ical activity of D2 antagonists is also consistent with this
hypothesis.3 Therefore, treatments enhancing NMDA
function may be of benefit in schizophrenia. Numerous
placebo-controlled double-blind trials have reported
improvements in negative, cognitive, and positive symp-
toms following the addition of NMDA/glycine site ago-
nists such as glycine, D-serine or D-cycloserine to
ongoing therapy with typical or atypical antipsychotic
drugs.4 Limited preclinical and clinical data are also sug-
gestive of a decrease in EPS liability/tardive dyskinesia
with glycine augmentation.5
which includes transporters for dopamine, noradrenalin,
and serotonin.9 There is a rapidly growing literature
describing inhibitors of GlyT1, including substrate-
based (sarcosine series) and non-substrate-based
compounds.10 We describe herein a novel, non-sub-
strate-based series of GlyT1 inhibitors useful as reagents
for studying the effects of GlyT1 inhibition in vivo.
Our efforts began with a high-throughput screen (HTS)
using radiolabeled (R)-(À)-NPTS, a GlyT1 ligand we
described previously.11 The HTS yielded compound 1
(Table 1) as a hit, with a GlyT1 Ki value of 500 nM.
Compound 1, however, showed poor selectivity for
GlyT1, with, for example, a 5HT1B Ki value of
<1 nM.12 In addition, 1 shows binding to the HERG
channel, as measured by displacement of the HERG li-
gand dofetilide, inhibition of cytochrome P450 2D6, and
rapid degradation in the presence of human microsomes
(see Table 1). Our goals for improving compound 1 were
to achieve HERG binding in the dofetilide assay of
>1000 nM, <10% inhibition of P450 2D6, and
>60 min half-life in the presence of human liver
microsomes.
Glycine augmentation elevates NMDA receptor trans-
mission since it is a co-agonist, binding to the NR1 sub-
unit of the receptor.6 One strategy for augmenting
glycine levels is inhibition of the type 1 glycine trans-
porter (GlyT1), which regulates glycine levels in the
vicinity of the NMDA receptor.7,8 GlyT1 is a member
of the sodium/chloride-dependent transporter family,
The 5HT1B receptor activity was recognized as likely
arising from the naphthyl piperazine pharmacophore.
Compound 2 replaces the naphthalene with an indane
ring, and adds an isopropyl group to the piperazine to
afford similar binding potency at GlyT1 (Table 1).
GlyT1 potency was improved with cyclopentyl substitu-
ents, as in compounds 3 and 4 (Table 1). The hydroxy-
methyl substituent in 4 was designed to improve
*
Corresponding author. Tel.: +1 860 441 4248; fax: +1 860 686
0013; e-mail: john.a.lowe@pfizer.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.12.109

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J. Lowe et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1675–1678
Table 1. Biological profiling of compounds in the indane and tetralin series
Compound Structure
GlyT1 Ki
GlyT2
IC50
5HT1B
Ki (nM) Ki (nM)
Dofetillide CYP2D6 % hMic t1/2
I at 1.5 lM
(nM)
500
(min)
CH3
N
1
ND
ND
ND
<1
1280
75
22
O
N
N
O
Cl
N
H3C
N
CH3
N
2
394 205
31
98.1
43
29
47
94
13
O
N
O
Cl
N
H3C
CH3
N
N
3
134 103
414
1490
ND
261
205
693
7
O
N
N
O
O
Cl
N
OH
N
N
4
26.8 7.05 ND
9
O
Cl
N
N
F
5
66.8 47.9 531 nM
12
O
N
N
O
N
OH
N
F
6
61.4 33.8 31.5 nM ND
709
525
85
70
14
12
O
N
N
O
N
H
N
OH
O
N
N
7
108 80
ND
>300
O
Cl
N
OH
8
15.9 7.21 319 nM
>1000
271
96
22
O
N
N
O
Cl

J. Lowe et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1675–1678
1677
Table 1 (continued)
Compound Structure
GlyT1 Ki
(nM)
GlyT2
IC50
5HT1B
Ki (nM) Ki (nM)
Dofetillide CYP2D6 % hMic t1/2
I at 1.5 lM
(min)
N
F
OH
9
10.2 2.12 385 nM ND
11.0 2.46 401 nM ND
85.3 43.1 857 nM ND
1300
43
17
O
N
O
N
N
OH
10
410
30
24
F
O
N
O
H3C
CH3
N
11
222
84
26
F
N
H3C
N
O
N
GlyT1, measured as the Ki value SEM for displacement of tritiated NPTS11 from the human GlyT1c transporter expressed in HEK293 cells;
GlyT2, measured as the IC50 value for blockade of tritiated glycine uptake into HEK293 cells expressing the human GlyT2 transporter; 5HT1B,
measured as the Ki value for displacement of tritiated 5-hydroxy-trytamine from cloned human 5HT1B receptors expressed in HEK293 cells;
Dofetilide Ki, measured as the Ki for displacement of tritiated Dofetilide from human HERG channel (IKr) expressed in HEK293 cells; Cyp 2D6,
measured as the % inhibition of the disappearance of the 2D6 substrate aminoethyl-methoxy-methylcoumarin (AMMC) in the presence of
recombinant human cytochrome 2D6 at a concentration of 1.5 lM of the test compound, hMic t1/2, human liver microsomal clearance, measured as
the half-life in minutes for the disappearance of compound in the presence of human liver microsomes; ND, not determined.
solubility and also reduced 5HT1B binding affinity to
above 1 lM. Further SAR studies in this series identified
the ortho-fluoro analogues 5 and 6 as potent GlyT1
inhibitors.
This intermediate, which is inactive in the GlyT1 bind-
ing assay, was then hydrolyzed to the free piperazine
and elaborated by reductive amination to the final prod-
uct, 3.
The synthesis of these indanyl piperazine compounds
was carried out as shown in Scheme 1.
An alternative synthetic route was used for compound 4,
as shown in Scheme 2.
As shown in Scheme 1, the preparation of 1-(4-acetyl-
piperazinyl)-5-methoxyindane was carried out in analo-
gy with the known procedure for the corresponding
tetralin,13 followed by removal of the methyl group
using BBr3 and addition of the oxadiazole side chain.12
In the alternate synthesis, the oxadiazole side chain is
added first, followed by conversion to the indanyl chlo-
ride. The addition of the piperazine side chain follows a
literature precedent,14 ending with double alkylation
with 1-hydroxymethyl aminocyclopentane.
COCH3
N
1. NaBH4, MeOH
2. SOCl2, toluene
O
1. BBr3, CH2Cl2
62%
84%
O
N
1. AlCl3, toluene
H3CO
N
1. NaBH4, MeOH
N
O
O
H3CO
O
Cl
2. Cs2CO3, CH3CN
H3CO
3. K2CO3, CH3CN
2. K2CO3, CH3CN
2. SOCl2, toluene
O
O
N
N
H3COCN NH
100%
Cl
Cl
Cl
N
65%
N
55%
Cl
63%
H3C
CH3
COCH3
N
HO
N
1. HCl, EtOH 82%
1. (HOCH2CH2)2NH
N
N
O
N
Cl
O
N
O
N
N
O
N
O
Cl
O
N
N
2. SOCl2, ClCH2CH2Cl
3. K2CO3, CH3CN
2. NaB(OAc)3H
N
Cl
O
Cl
N
O
N
Cl
CH3
H3C
4
79%
H2N
16%
OH
3
O
Scheme 1. Synthetic scheme for indanyl piperazine analogues exem-
plified by compound 3.
Scheme 2. Synthetic scheme for indanyl piperazine analogues exem-
plified by compound 4.

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J. Lowe et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1675–1678
OH
isomers, but also the affinity for dofetilide displace-
O
N
O
N
1. CH2(CO2t-Bu)2,
82%
N
N
Cl
ment from the HERG channel is similar, and
hence this structural feature did not offer any
opportunity to separate GlyT1 affinity from HERG
affinity.
O
KOtBu
O
Cl
Cl
2. H3O+, dioxane
3. BH3.SMe2
23%
CN
O
N
1. DIBAL, toluene
1. SOCl2
95%
O
Cl
N
Despite the structural diversity of these series, the com-
bination of a basic head group with a long lipophilic tail,
required for GlyT1 binding activity, predisposes these
compounds to HERG binding and rapid degradation
by human microsomes. These structural motifs may be
a consideration for chemists when they triage hits from
high-throughput screening.
2. HNR2, Na(OAc)3BH
2. Et4N+CN-, CH3CN
20%
75%
H
O
N
N
N
O
Cl
OH
7
Scheme 3. Synthetic scheme for indanyl propylamine analogues
exemplified by compound 7.
While the indane ring system resolves the selectivity
issue, affording potent GlyT1 binding activity, it does
not address either the HERG channel affinity or the
clearance in human microsomes. The next structural
modification was to replace the piperazine with an acy-
clic propylamine chain. The synthesis of this series of
analogues is shown in Scheme 3.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.
2006.12.109.
References and notes
The key step is the condensation of di-t-butyl malonate
with the indanyl chloride prepared in Scheme 2. Follow-
ing hydrolysis and decarboxylation, conversion to the
chloride and condensation with cyanide afforded
the nitrile. Reduction to the aldehyde and reductive
amination completed the synthesis of compound 7.
Compounds 8–10 were prepared in a similar manner
starting from the corresponding tetralin.
1. Lindsley, C. W.; Shipe, W. D.; Wolkenberg, S. E.;
Theberge, C. R.; Williams, D. L., Jr.; Sur, C.; Kinney,
G. G. Curr. Top. Med. Chem. 2006, 6, 771.
2. Coyle, J. T.; Tsai, G.; Goff, D. Ann. N. Y. Acad. Sci 2003,
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3. Laruelle, M.; Frankle, W. G.; Narendran, R.; Kegeles, L.
S.; Abi-Dargham, A. Clin. Ther. 2005, 27, S16.
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7. Heresco-Levy, U.; Javitt, D. C. Eur. Neuropsychopharma-
col. 1998, 8, 141.
Although this series introduces further structural novel-
ty, it unfortunately does not resolve the HERG and
clearance issues. Changing the oxadiazole for a pyridine,
in compound 10, does reduce 2D6 inhibition, but does
not reduce HERG binding. A final attempt at structural
diversity was the preparation of compound 11, from
3-hydroxybenzaldehyde by straightforward methods.
This approach also did not resolve the HERG or clear-
ance issues.
8. Heresco-Levy, U.; Javitt, D. C.; Ermilov, M.; Mordel, C.;
Silipo, G.; Lichtenstein, M. Arch. Gen. Psychiatry 1999,
56, 29.
9. Aragon, C.; Lopez-Corcuera, B. Eur. J. Pharmacol. 2003,
479, 249.
Many of the compounds depicted herein are stereo-
isomeric, and all are indicated as mixtures of enan-
tiomers and/or diastereomers. To examine the role
of stereochemistry in the SAR of this series, com-
pound 8 was separated by chiral HPLC into its
four isomeric components (all data cited are for
N = 1): isomer 8A, aD = +3.5°, GlyT1 IC50 = 38 nM,
Dofetilide Ki = 381 nM; isomer 8B, aD = +6.5°,
GlyT1 IC50 = 24 nM, Dofetilide Ki = 214 nM, isomer
8C, aD = À6.1°, GlyT1 IC50 = 31 nM, Dofetilide
10. Slassi, A.; Egle, I. Exp. Opin. Ther. Pat. 2004, 14, 201.
11. Lowe, J. A.; Drozda, S. E.; Fisher, K.; Strick, C.; Lebel,
L.; Schmidt, C.; Hiller, D.; Zandi, K. S. Bioorg. Med.
Chem. Lett. 2003, 13, 1291.
12. Howard, H. R., Jr.; Chenard, B. L.; Macor, J. E.; Shenk,
K. D.; Desai, K. A. US Patent 6,166,020, 2000, Chem.
Abs., 2000, 134, 56692.
13. Perrone, R.; Berardi, F.; Tortorella, V.; Govoni, S.;
Lograno, M. D.; Daniele, E. Eur. J. Med. Chem. Chim.
Ther. 1991, 26, 869.
14. Mutulis, F.; Yahorava, S.; Mutule, I.; Yahorau, A.;
Liepinsh, E.; Kopantshuk, S.; Veiksina, S.; Tars, K.;
Belyakov, S.; Mishnev, A.; Rinken, A.; Wikberg, J. E. S.
J. Med. Chem. 2004, 47, 4613.
Ki = 219 nM;
isomer
8D,
aD = À4.3°,
GlyT1
IC50 = 26 nM, Dofetilide Ki = 206 nM. Thus not
only does the GlyT1 affinity differ little between