The 5-HT3 antagonist tropisetron (ICS 205-930) is a potent and selective alpha7 nicotinic receptor partial agonist.

Article abstract of DOI:10.1016/S0960-894X(00)00670-3

The 5-HT3 receptor antagonist tropisetron (ICS 205-930) was found to be a potent and selective partial agonist at alpha7 nicotinic receptors. Two other 5-HT3 receptor antagonists, ondansetron and LY-278,584, were found to lack high affinity at the alpha7 nicotinic receptor. Quinuclidine analogues (1 and 2) of tropisetron were also found to be potent and selective partial agonists at alpha7 nicotinic receptors.

Full text of DOI:10.1016/S0960-894X(00)00670-3

Bioorganic & Medicinal Chemistry Letters 11 (2001) 319±321
The 5-HT Antagonist Tropisetron (ICS 205-930) is a Potent and
3
Selective ꢀ7 Nicotinic Receptor Partial Agonist
John E. Macor,* David Gurley, Thomas Lanthorn, James Loch, Robert A. Mack,
George Mullen, Oahn Tran, Nicole Wright and John C. Gordon
AstraZeneca, Three Biotech, One Innovation Drive, Worcester, MA 01605, USA
Received 18 September 2000; accepted 12 November 2000
This paper is dedicated to the memory of the outstanding chemist Robert A. Mack.
AbstractÐThe 5-HT
nicotinic receptors. Two other 5-HT
3
receptor antagonist tropisetron (ICS 205-930) was found to be a potent and selective partial agonist at a7
receptor antagonists, ondansetron and LY-278,584, were found to lack high anity at the a7
3
nicotinic receptor. Quinuclidine analogues (1 and 2) of tropisetron were also found to be potent and selective partial agonists at a7
nicotinic receptors. # 2001 Elsevier Science Ltd. All rights reserved.
Both the a7 nicotinic receptor (a7 nAChR) and the 5-
We examined three 5-HT R antagonists as possible a7
3
HT receptor (5-HT R) have been implicated to play
important roles in cognition and schizophrenia.
nAChR ligands: tropisetron, LY-278,584, and ondan-
setron. Tropisetron (ICS 205-930) and LY-278,584 were
chosen because of structural similarity to a series of aryl
quinuclidinyl carbamates, which we found to be potent
3
3
1,2
Whereas the a7 nAChR receptor is a member of the
ligand-gated ion channel family of nicotinic receptors,
3
7
the ligand-gated ion channel 5-HT R is unique within
3
a7 nAChR partial agonists. Ondansetron was tested
since it is a prototypical representative of this class of
the serotonin family of receptors, since the other mem-
bers of the 5-HT family are all serpentine G-protein
8
drugs. As shown in Table 1, tropisetron is a high a-
4
coupled receptors. While there are a number of struc-
5
nity ligand for both th a7 nAChR and 5HT R. In con-
3
turally diverse 5-HT ligands, there exists a dearth of
3
trast, tropisetron has very low anity for the other
nicotinic subtypes tested (a4b2, a1b1gd, and a3, see
Table 1). Neither ondansetron nor LY-278,584 demon-
strated appreciable anity for the a7 nAChR. These
potent and/or selective ligands for the a7 nAChR.
Because of the evolutionary familial link between these
2
two cationic channels, it seemed reasonable that some 5-
HT receptor ligands might have anity and functional
results show that within the class of 5-HT receptor
3
3
activity at a7 nAChRs, and vice versa.
antagonists, potent anity for the a7 nAChR can be
found, but is not universal. Examination of the func-
tional e€ects of tropisetron at the a7 nAChR was
performed using Xenopus oocytes.9 Compared to
acetylcholine and nicotine, tropisetron was a potent, but
partial agonist at the a7 nAChR (Table 2). Since the N-
methyl quaternized analogue of tropisetron shares the
quaternized amine functionality found in acetylcholine,
it was expected to be a more ecacious a7 nAChR
agonist. However, while this compound was potent as
an a7 nAChR ligand, it was found to be less ecacious
than tropisetron.
This assumption led to a recent paper from our group
that described the functional activity of weak a7 nico-
6
tinic agonists at the 5-HT R. In the current report, we
3
describe the opposite scenario. Namely, we describe the
activity of the potent 5-HT3 antagonist tropisetron
(Fig. 1), as a potent and selective a7 nicotinic partial
agonist. However, its nicotinic activity is not general for
the class of 5-HT R antagonists, since two other high
3
anity 5-HT R antagonists, ondansetron and LY-
3
278,584, had no a7 nAChR activity.
After ®nding the potent and selective a7 nAChR activ-
ity of tropisetron, we wanted to examine the e€ect of
di€erent bicyclic amines on the nicotinic SAR. Speci®-
cally, since our interest in aryl quinuclidinyl carbamates
*
Corresponding author at present address: Bristol Myers Squibb,
Pharmaceutical Research Institute, PO Box 5400, Bldg. 21-HPW,
Princeton, NJ 08543-5400, USA. Tel.: +1-609-818-5396; fax: +1-609-
8
18-3331; e-mail: john.macor@bms.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00670-3

3
20
J. E. Macor et al. / Bioorg. Med. Chem. Lett. 11 (2001) 319±321
Figure 1.
Table 1. Binding anitiesa,b
Compound
a7 nAChR K
i
(nM) 5-HT R K
3
i
(nM) a4b2 nAChR K
i
i i
(nM) a3 (PC12) nAChR K (nM) a1b1gd nAChR K (nM)
(
Tropisetron
Quaternized tropisetron
LY-278,584
Ondansetron
� )-Nicotine
480 Æ 30 (6)
6.9 Æ 2.4 (3)
6.4 Æ 1.2 (3)
>10,000 (3)
>3000 (3)
71,000 Æ 25,000 (3)
5.3 Æ 3.0 (3)
7.2 Æ 1.7 (3)
24 Æ 11 (3)
2.3 Æ 0.2 (6)
220 Æ 20 (4)
5500 Æ 440 (13)
27,000 (2)
3000 (1)
55,000 Æ 28,000 (3)
7800 Æ 1000 (3)
38,000 (2)
16,000 Æ 2400 (3)
2300 Æ 520 (3)
67,000 (1)
Not done
Not done
Not done
Not done
Not done
Not done
Not done
12 Æ 6 (3)
46,000 (2)
2600 (2)
14,000 (2)
1
2
2.3 Æ 0.6 (3)
1.1 Æ 0.2 (3)
10 Æ 4 (3)
8.8 Æ 1.7 (3)
a
Number of determinations in parentheses.
SEM for n ꢀ 3.
b
led us to test tropisetron at a7 nAChR, we examined the
replacement of the azabicyclo[3.2.1]heptane amine in
tropisetron with the azabicyclo[2.2.2]hexane amine,
quinuclidine. The syntheses of these analogues are
shown in Scheme 1. Activation of indole-3-carboxylic
acid with carbonyldiimidazole, followed by treatment
of the imidazolyl amide with 3-quinuclidinol using
DMAP as a catalyst a€orded the quinuclidin-3-yl
indole-3-carboxylate (1).
replacement for the azabicyclo[3.2.1]heptane found in
tropisetron. However, the exact regiomeric analogue of
tropisetron (i.e., attachment of the carboxylate at C3 of
the indole) using quinuclidine a€orded an a7 nAChR
agonist (1) with greater intrinsic agonist activity than
tropisetron. This compound represents an important
pharmacological tool for the study of a7 nAChRs.
In conclusion, we have found that the 5-HT R antago-
3
nist tropisetron is a potent and selective partial agonist
for the a7 nAChR. Replacement of the azabicy-
clo[3.2.1]heptane found in tropisetron with quinuclidine
a€orded a ligand (1), which was as potent and selective
in binding and more ecacious as a partial agonist than
tropisetron at a7 nAChRs. Given the paucity of selec-
tive ligands for the a7 nAChRs, these compounds
should prove to be useful as tools to study that ligand-
gated ion channel.
Using methodology we developed for the regioselective
acylation of the indole nitrogen,10 we also prepared the
quinuclidin-3-yl indole-1-carboxylate (2). This prepara-
tion also involved the use of carbonyldiimidazole, but it
was used directly on indole itself in the presence of a
catalytic amount of DMAP to form the imidazol-1-yl
indol-1-yl urea, which when reacted with quinuclidin-3-
ol a€orded 2.
Table 1 summarizes the in vitro pharmacology of these
analogues of tropisetron. Clearly quinuclidine sub-
stitutes well for the azabicyclo[3.2.1]heptane found in
tropisetron. Also, the position of attachment of the
carboxylate function (i.e., C3 for 1 and tropisetron, N1
for 2) did not seem to have any e€ect on the anity of
these analogues for the a7 nAChR. In addition, there
was little (if any) stereogenic di€erentiation between the
enantiomers of 1 and 2 although in both cases the (S)-
enantiomer seemed to be slightly more potent than the
It should be noted that tropisetron was studied exten-
sively in human clinical trials and is available in Europe
for the treatment of emesis. Its unremarkable safety
pro®le12 can now be interpreted as demonstrating the
11
safety of both a 5-HT receptor antagonist and an a7
3
nicotinic receptor partial agonist. Furthermore, the dis-
covery that tropisetron is also a potent partial agonist
for a7 nAChRs gives cause for the re-examination of
both clinical and preclinical ®ndings with this com-
(R)-enantiomer (data not presented). The only di€er-
Table 2. Functional activity
ence seen in this series of tropisetron analogues was that
indole-3-carboxylate (1) was a slightly more ecacious
agonist than the indole-1-carboxylate (2, Table 2).
Whereas one could describe both tropisetron and 2 as
weak partial agonists, 1 appeared to be a true partial
agonist for the a7 nAChR. We are presently using these
compounds in in vivo models to see if this di€erence can
be measured and observed. In summary, quinuclidine
of either stereogenicity is minimally an equivalent
Compound
a7 nAChR Ecacy (%)
a7 nAChR EC50 (nM)
Acetylcholine
� )-Nicotine
100
230,000 Æ 4000
38,000 Æ 5000
1300 Æ 200
(
Tropisetron
53 Æ 6
36 Æ 2
13 Æ 1
Quaternized
Tropisetron
1
2
280 Æ 40
42 Æ 1
17 Æ 7
580 Æ 80
4800 Æ 3000

J. E. Macor et al. / Bioorg. Med. Chem. Lett. 11 (2001) 319±321
321
campal membranes, 0.1 mg/mL BSA, bu€er (120 NaCl, 5 KCl,
ꢁ
1
MgCl , 2 CaCl , 50 Tris, pH 7.4; mM), 2 h, 20 C. (b)
2
2
3
a4b2nAChR: 3 nM [ H]nicotine, rat cortical membranes, buf-
fer, 1 h, 4 C. (c) a3nAChR: 0.1 nM [ H]epibatidine, PC12 cell
membranes, 1 h, 20 C. (d) a1b1gdnAChR: 1 nM [ I]a-bun-
gartoxin, BC H1 cell membranes, bu€er, 2 h, 37 C. (e) HT R:
0
1
9
ꢁ
3
ꢁ
125
ꢁ
3
3
3
.5 nM [ H]zacopride, rat small bowel muscularis membranes,
ꢁ
50 mM NaCl, 50 mM Tris, pH 7.4, 1 h, 37 C.
. Potency and intrinisic activity values were determined by
measuring current activation in Xenopus oocytes expressing
rat a7 nAChRs (Fig. 2). Oocytes were injected with cRNA
coding for nAChR a7. Oocytes were used 3±10 days post-
injection. 100% intrinsic activity was de®ned for each egg by
current elicited by 3 mM acetylcholine. Currents were mea-
sured from baseline to peak. Oocytes were washed for 5 min
after each application of agonist to allow receptors to fully
recover from desensitization. Data from individual experi-
ments were ®t by the logistic equation to estimate EC50 and
ecacy. Data in Table 2 are averages of three or more experi-
ments ÆSEM.
Scheme 1.
pound, particularly when tropisetron displayed phar-
macological e€ects di€erent from other 5-HT R
3
antagonists. In one notable study, it was reported that
tropisetron and ondansetron had qualitatively di€erent
results in learning and memory paradigms in rats.13
Thus, the results reported in our paper give reason to re-
examine such studies with tropisetron to ascertain whe-
ther the results were the e€ect of 5-HT antagonism or
3
a7 nicotinic partial agonism.
Acknowledgements
The mouse nAChR a7 clone was kindly provided by
Dr. Avi Orr-Urtreger at the Tel-Aviv Sourasky Medical
Center, Tel-Aviv, Israel.
References and Notes
Figure 2. The e€ect of acetylcholine and tropisetron on nAChR a7.
Top panel, representative traces of current elicited in oocytes expres-
sing mouse nAChR a7 (see above). Traces shown are from the same
oocyte; superfusion of acetylcholine and tropisetron begins at arrow
(5 min between agonist applications). Bottom panel, concentration±
response curve to acetylcholine and tropisetron. Data are ®t by the
logistic equation.
1
2
1
3
4
5
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. (a) SEM for values determined ꢀ3 times. Binding assay
8
1
25
conditions: a7nAChR: 5 nM [ I]a-bungartoxin, rat hippo-