Indole-2-carboxamides as novel NR2B selective NMDA receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(03)00708-X

A novel series of indole-2-carboxamide derivatives was prepared and identified as NR2B selective NMDA receptor antagonists. The influence of the number and position of OH groups on the indole skeleton as well as the substitution of the piperidine ring on

Full text of DOI:10.1016/S0960-894X(03)00708-X

Bioorganic & Medicinal Chemistry Letters 13 (2003) 3859–3861
Indole-2-carboxamides as Novel NR2B Selective NMDA
Receptor Antagonists
Istvan Borza,a,* Sandor Kolok,a Aniko Gere,a Eva Agai-Csongor,a Bela Agai,b
Gabor Tarkanyi,a Csilla Horvath,a Gizella Barta-Szalai,a Eva Bozo,a Csilla Kiss,a
Attila Bielik,a Jozsef Nagy,a Sandor Farkasa and Gyorgy Domanya
aGedeon Richter Ltd., Budapest 10. POB 27, H-1475, Hungary
bBudapest University of Technology and Economics, Budapest, POB 91, H-1521, Hungary
Received 14 March 2003; revised 16 June 2003; accepted 2 July 2003
Abstract—A novel series of indole-2-carboxamide derivatives was prepared and identified as NR2B selective NMDA receptor
antagonists. The influence of the number and position of OH groups on the indole skeleton as well as the substitution of the
piperidine ring on the biological activity of the compounds was studied.
# 2003 Elsevier Ltd. All rights reserved.
The efficacy of NR2B selective NMDA receptor
antagonists in neuroprotection, anti-hyperalgesic and
anti-Parkinson animal models have attracted significant
recent interest.1,2 A novel series of indole-2-carbox-
amide derivatives was prepared and tested in our
laboratories and several members of this group of com-
pounds were found to be potent antagonists of NR2B
subunit containing NMDA receptors (NR2B selective
antagonists). The publication of a patent application3
covering similar molecules and claiming similar activity
prompted us to report on our preliminary results.
piperidine for example, in (R)-1-[2-hydroxy-3-(4-
hydroxy-phenyl)-propyl]-4-(4-methyl-benzyl)-piperidin-4-
ol Ro 25-69814 or in 6-[2-[4-(4-fluorobenzyl)piperidin-1-
yl]ethanesulfinyl]-3H-benzoxazol-2-one CI-1041 (beson-
prodil).5
Nevertheless, there are potent carboxamides as well
indicating that this basic nitrogen is not a condition of
the activity.6 A cinnamide derivative, compound 16 was
selected as starting point of our research. The IC50 value
of compound 1 for the inhibition of NMDA evoked
increase of intracellular Ca2+ level on cells expressing
recombinant NR1/NR2B receptors was 104 nM.
It was assumed that potency may be enhanced by
increasing the rigidity of this structure and at the same
time incorporating an additional H-bond donor moiety.
Our first attempt to verify this hypothesis was the
synthesis of an indole-2-carboxamide derivative (2).
Typical antagonists of competitor companies contain
basic nitrogen, in most cases as N of a 1,4-diaralkyl
*Corresponding author. Tel.:+36-1-431-5139; fax:+36-1-432-6002;
e-mail: i.borza@richter.hu
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00708-X

3860
I. Borza et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3859–3861
The modification of the cinnamide moiety resulted in a
fivefold increase in potency (IC50: 19.9 nM) so we deci-
ded to explore the SAR of the indole-2-carboxamides
and a series of derivatives (3–11) was prepared.
assay using cells expressing recombinant NR1/NR2A
receptors and none of the compounds exhibited
significant activity up to 10 mM concentration.
Monohydroxy derivatives 2, 3 and 4 showed good
activities while the 7-hydroxy analogue (5) was less
active. One can assume that the orientation of this OH
and/or the interaction between the hydroxy group and
the indole NH prevented the sufficient interaction with
the receptor. The fact that the 6-hydroxy (2) and the 4-
hydroxy (4) derivatives had similar activity suggested
that there might be more than one site in the receptor
where H bond donor moieties on the indole can attach.
This assumption was further supported by the observa-
tion that the 4,6-dihydroxy analogue (6) had increased
activity compared to either that of 2 or 4. Actually
compound 6 is one of the most potent antagonists of the
NR2B subunit containing NMDA receptors prepared
so far. At the same time the 5,7-dihydroxy analogue (7)
was less active probably due to the destabilizing effect of
the OH at position 7.
Compounds 2–11 were synthesized in a straightforward
manner coupling indole-2-carboxylic acids with piper-
idine derivatives using O-benzotriazol-1-yl-N,N,N0,N0-
tetramethyluronium hexafluorophosphate (HBTU) and
triethylamine (TEA) in N,N-dimethylformamide
(DMF) as solvent (Scheme 1). The indole-2-carboxylic
acids and piperidines used as starting materials are
either commercially available or can be prepared by
standard procedures like the Hemetsberger–Knittel
indole synthesis following the literature.12,13 Dihydroxy
derivatives 6 and 7 were prepared as their O,O-dibenzyl
derivatives and catalytic hydrogenolysis afforded the
end-products (Scheme 2).7
Biological activity of the prepared compounds was
measured in a binding assay using tritiated Ro 25-6981
as radioligand4,8,9 and in a functional assay where the
inhibition of NMDA-evoked increase of intracellular
Ca2+ level was determined on cells expressing recombi-
nant NR1/NR2B receptors10 (Table 1). Baseline and
NMDA-evoked changes of intracellular Ca2+ were
monitored with fluorimetry using a Ca2+-selective
fluorescent dye (Fluo-4/AM) and a plate reader fluori-
meter.10 Selectivity towards NR2A subunit containing
NMDA receptors was tested by the same functional
Another sensitive point of 2 is the spacer between the
terminal phenyl group on the right-hand side of the mole-
cule and position 4 of the piperidine. Either shortening (8)
Scheme 2. Synthesis of compounds 6 and 7. Reagents and conditions:
(i) methyl azidoacetate, NaOMe, MeOH; (ii) (1) heating in xylene; (2)
KOSiMe3; (3) HCl; (iii) 4-benzylpiperidine, HBTU, TEA, DMF;
(iv) H2/Pd/C, MeOH.
Scheme 1. Synthesis of compounds 2–5 and 8–11. Reagents and con-
ditions: (i) 4-substituted piperidine, HBTU, TEA, DMF.
Table 1. Receptor binding and functional assay results for compounds 2–11
a,b
i
R4
R5
R6
R7
X
Y
[3H]Ro-25,6981 bindinga
IC50 (nM)
n
NMDA-evoked Á[Ca2+
IC50 (nM)
]
n
2
3
4
5
6
7
8
9
10
H
H
OH
H
OH
H
H
H
H
H
H
OH
H
H
H
OH
H
H
H
H
OH
H
H
H
OH
H
OH
OH
OH
OH
H
H
H
OH
H
OH
H
H
H
H
CH2
CH2
CH2
CH2
CH2
CH2
—
CH2
O
CH2
—
—
—
—
—
—
—
CH2
CH2
O
12ꢀ2
31ꢀ10
18ꢀ5
380ꢀ141
6ꢀ1
4
3
4
3
3
3
20ꢀ5
40ꢀ8
39ꢀ4
77ꢀ6
10ꢀ1
52ꢀ11
>8000
24
46ꢀ7
19ꢀ2
104ꢀ12
57ꢀ5
8ꢀ1
3
4
2
4
3
2
1
3
3
3
2
4
4
106ꢀ19
n.d.
359ꢀ93
47ꢀ2
19ꢀ1
196ꢀ43
6ꢀ1
3
3
3
3
3
3
11
1
Ro 25-6981
CI-1041
4ꢀ1
aValues represent the meansꢀSEM The number of experiments (n) is indicated.
bNMDA-evoked changes of intracellular Ca2+
.

I. Borza et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3859–3861
3861
or lengthening with an additional methylene group (9)
of this part resulted in dramatic decrease in activity.
J. N. C.; Mohacsi, E.; Heitz, M. P.; Kemp, J. A. J. Pharmacol.
Exp. Ther. 1997, 283, 1285.
5. Wright, J. L.; Kesten, S. R.; Upasani, R. B.; Lan, N. C.
PCT Int. Appl. WO2000000197-A1. 2000; Chem. Abstr. 2000,
132, 64255.
The lengthening of the methylene group by an oxygen
atom (10 and 11), however, resulted in similar activities
to that of the parent compound 2 and not to that of the
isosteric phenylethyl analogue 9, providing another
example of the Friedman’s ether oxygen-methylene
group paradox.11
6. Tamiz, A. P.; Cai, S. X.; Zhou, Z.; Yuen, P.; Schelkun,
R. M.; Whittemore, E. R.; Weber, E.; Woodward, R. M.;
Keana, J. F. W. J. Med. Chem. 1999, 42, 3412.
1
7. The IR, H NMR, 13C NMR and MS spectra for all inter-
mediates and final compounds were consistent with the
assigned structures. All final compounds gave satisfactory C,
H and N analyses.
8. Mutel, V.; Buchy, D.; Klingelschmidt, A.; Messer, J.;
Bleuel, Z.; Kemp, J. A.; Richards, J. G. J. Neurochem. 1998,
70, 2147.
Results of further SAR studies and biological data will
be published under separate cover.
9. Richards, J. G.; Messer, J.; Buchy, D.; Klingelschimdt, A.;
Mutel, V. Br. J. Pharmacol. 1997, 120 (Proc. Suppl, 39P).
˜
10. Nagy, J.; Boros, A.; Dezso, P.; Kolok, S.; Fodor, L.
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