Interaction of chiral MS-245 analogs at h5-HT6 receptors

Article abstract of DOI:10.1016/j.bmcl.2005.05.092

Optically active pyrrolidinylmethylindole analogs related in structure to the benzenesulfonyltryptamine 5-HT₆ receptor antagonist MS-245 were evaluated and their R-isomers were found to bind with affinity higher than their S-enantiomers.

Full text of DOI:10.1016/j.bmcl.2005.05.092

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3510–3513
Interaction of chiral MS-245 analogs at h5-HT6 receptors
Carmen Abate,a Renata Kolanos,a Malgorzata Dukat,a Vince Setola,b
Bryan L. Rothb,c and Richard A. Glennona,*
aDepartment of Medicinal Chemistry, School of Pharmacy, Virginia Commonwealth University, Richmond, VA 23298-0540, USA
bDepartment of Biochemistry, Case Western Reserve University, USA
cDepartment of Psychiatry and Neurosciences, School of Medicine, Case Western Reserve University, USA
Received 22 April 2005; revised 24 May 2005; accepted 25 May 2005
Abstract—Optically active pyrrolidinylmethylindole analogs related in structure to the benzenesulfonyltryptamine 5-HT6 receptor
antagonist MS-245 were evaluated and their R-isomers were found to bind with affinity higher than their S-enantiomers.
Ó 2005 Elsevier Ltd. All rights reserved.
5-HT6 receptors are members of the serotonin receptor
family (5-HT1–5-HT7) and are of potential therapeutic
interest due to their possible involvement in neuropsychi-
atric disorders, such as depression and pyschosis.1–6 This
receptor population was first identified about 10 years
ago, but only in the past few years have 5-HT6 receptor
antagonists been described (reviewed5,6). We identified
MS-245 (1a; Ki ca. 2 nM) as one of the first examples
of a 5-HT6 receptor antagonist.7,8 Subsequent pharma-
cophoric studies have revealed that the 5-methoxy group
of 1a is not required for binding (i.e., 1b; Ki = 4 nM), that
the benzenesulfonyl moiety can be replaced by a benzyl
group (2; Ki = 6 nM) with retention of antagonist ac-
tion,5 and that the indole N1 nitrogen atom is not re-
quired for binding, as evidenced by the high affinity of,
for example, isotryptamine 3 (Ki = 32 nM) and indene
4 (Ki = 3 nM) for this receptor population.9
it was reasoned that chiral analogs of N1-unsubstituted
tryptamines might bind differently than N1-substituted
tryptamines and that isomeric comparisons could prove
to be informative. In an extreme case, optical isomers of
N1-substituted tryptamines might even display opposite
enantioselectivity for binding, as compared to their cor-
responding N1-unsubstituted counterparts.
Various tryptamines lacking an N1 substituent generally
bind with reduced affinity and, like 5-hydroxytryptamine
(i.e., serotonin; Ki ca. 100 nM), can display agonist char-
acter.5 On the basis of comparative structure–affinity and
receptor modeling studies,8,10 we have proposed that
tryptamine analogs, although likely utilizing a common
aspartate moiety, bind at 5-HT6 receptors differently
depending upon whether or not they possess an N1
(e.g., a benzenesulfonyl or benzyl) substituent.
If the presence or absence of an N1 substituent dictates
how tryptamine analogs interact with 5-HT6 receptors,
Keywords: Serotonin; Pyrrolidinylmethylindoles.
*
Corresponding author. Tel.: +1 804 828 8487; fax: +1 804 828
7404; e-mail: glennon@hsc.vcu.edu
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.05.092

C. Abate et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3510–3513
3511
subsequently converted to their corresponding benze-
nesulfonyl analogs 5c and 5d.
Introduction of substituents a to the terminal amine of
tryptamine analogs (e.g., an a-methyl or a-ethyl group)
to create a chiral center has been shown to result in
somewhat reduced 5-HT6 receptor affinity.11 Neverthe-
less, it was thought that reduced affinity might be an
acceptable trade-off if it allowed enantiomeric potency
comparisons to be made. Rather than examining iso-
mers of a-methyl- or a-ethyltryptamines, however, we
opted to examine their cognate pyrrolidinylmethylindole
counterparts. That is, because necessary optically active
starting materials with defined absolute configuration
were readily available, and because pyrrolidinylmethy-
lindoles have been previously shown to bind at other
(i.e., 5-HT1) serotonin receptor populations12,13, we pre-
pared the optical isomers of 5 (where R = H or Me) for
binding comparisons with their N1-unsubstituted ana-
logs 6. To evaluate this hypothesis further, we also pre-
pared the isomers of isotryptamine 7 for comparison
with their (C3) unsubstituted counterparts 8.
The isomers of 7 and 8 (Table 1) were conveniently syn-
thesized by reaction of the indolyl anion (generated by
treatment with NaH in DMF) of indole or 3-benzylin-
dole16, with the known optical isomers of 2-(chloro-
methyl)-1-methylpyrrolidine hydrochloride.17
Radioligand binding18 data for the target compounds
are shown in Table 1. As with structurally simpler
N,N-dialkyltryptamines,5 introduction of an N1-benze-
nesulfonyl moiety led to enhanced affinity (comparing
pyrrolidinylmethylindoles 5a with 6a, 5b with 6b, 5c
with 6c, and 5d with 6d) (Table 1). Likewise, in the
isotryptamine series, benzyl analogs 7a and 7b dis-
played affinity higher than those of their parents, 8a
and 8b, respectively. In fact, the pyrrolidinylmethylin-
doles bind with affinities higher than those of their
simpler N,N-dimethyltryptamine counterparts; for
example, compound 5d binds with more than 10 times
the affinity of 1b and 7b binds with 3 times the affinity
of 3.
Compounds 6a and 6b were previously reported in the
patent literature as 5-HT1 ligands;12 and the synthesis
of their 5-methoxy analogs has also been described by
Macor and co-workers.13,14 Compounds 6a and 6b were
prepared using the same methodology (Scheme 1). Ke-
tones 12 were obtained by the reaction of indole (10)
with either the commercially available R- or S-isomer
of 11; isomers 12 were reduced with LiBH4 to protected
amine 13, which was deprotected via catalytic hydroge-
nation to the desired targets 6a and 6b (Table 1). In one
instance, (ꢀ)12 was subjected to direct catalytic reduc-
tion and resulted in ketone derivative S(ꢀ)9.15
In the absence of enantioselectivity reversal, it is not
possible to conclude that the N1-substituted and N1-
unsubstituted compounds are binding in a different fash-
ion. Nevertheless, there is some tantalizing evidence in
support of this possibility. For example, there is greater
enantioselectivity in the absence of the benzenesulfonyl
group (40- and 70-fold for 6a/6b and 6c/6d, respectively)
than in its presence (<6-fold for 5a/5b and 5c/5d). A sim-
ilar, but somewhat less pronounced, trend is seen with
the isotryptamines. Furthermore, N-methylation has
dissimilar effects in the unsubstituted and substituted
series; whereas N-methylation enhanced the affinity of
6a (!6c) and 6b (!6d) by about 5-fold, methylation
enhanced the affinity of 5a (!5c) and 5b (!5d) by
25-fold. Clearly, there are differences between the
N1-unsubstituted and N1-substituted series.
Both isomers of 13 were treated with sodium bis(trim-
ethylsilyl)amide to form the corresponding anions that
were allowed to react with benzenesulfonyl chloride to
afford intermediates 14; deprotection of 14 was per-
formed, as described for 13, to afford isomers 5a and
5b (Scheme 1). Direct treatment of 13 with lithium
aluminum hydride led to 6c and 6d, and these were
Scheme 1. (a) EtMgBr; (b) LiBH4, D; (c) H2, 10% Pd/C, absolute EtOH, 35–40 psi/3 h; (d) i. [(CH3)3Si]2NNa, THF, ꢀ78 °C, ii. PhSO2Cl, rt;
(e) H2, 10% Pd/C, absolute EtOH/MeOH, 35 psi/12 h; and (f) LiAlH4, THF, D/4 h.

3512
C. Abate et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3510–3513
Table 1. Radioligand binding data for reference compounds 1–3, and physicochemical properties and 5-HT6 receptor affinities of the target
compounds
R
R0
Isomer
Melting point (°C)
Empirical formulaa
h5-HT6 Ki, nM ( SEM)b
1a
1b
2
OMe
H
SO2Ph
SO2Ph
CH2Ph
CH2Ph
H
—
—
—
—
—
2
–
—
4
H
—
—
—
—
6
32
3
—
—
6a
6b
5a
5b
6c
6d
5c
5d
8a
8b
7a
7b
H
S(+)
R(ꢀ)
S(+)
R(ꢀ)
S(ꢀ)
R(+)
S(ꢀ)
R(+)
S(+)
R(ꢀ)
S(+)
R(ꢀ)
203–205c
204–205c
182–184c
179–181c
216–218c
216–217c
145–147c
145–147c
157–158e
157–158e
124–125e
124–125e
C13H16N2 HCl
C13H16N2 HCl
C19H20N2SO2 C2H2O4
C19H20N2SO2 C2H2O4
C14H18N2 HCl
C14H18N2 HCl
C20H22N2SO2 C2H2O4
C20H22N2SO2 C2H2O4
C14H18N2 HCl
C14H18N2 HCl
C21H24N C4H4O4
C21H24N C4H4O4
2400 ( 400)
60 ( 10)
46 ( 6)
7.8 ( 1.4)
640 ( 110)
9.1 ( 0.8)
1.7 ( 0.3)
0.3 ( 0.1)
8750 ( 2200)
550 ( 110)
29 ( 5)
9.9 ( 1.3)
H
H
H
SO2Ph
SO2Ph
H
d
d
H
Me
Me
Me
Me
—
H
SO2Ph
SO2Ph
H
—
—
H
CH2Ph
CH2Ph
—
a All compounds analyzed within 0.4% of theory for C, H, and N. C2H2O4, oxalate salt; C4H4O4, maleate salt.
b Ki values were determined, at least in triplicate, as previously described.18,22 Binding data for 1–3 have been previously reported5,8,9 and are shown
here for comparison.
c Recrystallization solvent = anhydrous MeOH/absolute EtOH.
d Crystallized with 0.25 mol H2O.
e Recrystallization solvent = CH2Cl2/Et2O.
It was shown that R-pyrrolidinylmethylindoles bind
with higher affinity than their S-enantiomers and that
5d (Ki = 0.3 nM) binds with nearly 10-fold higher affin-
ity than MS-245 (1). Interestingly, a recent review of
the patent literature revealed that 5d had been previ-
ously described as a 5-HT6 antagonist; although the
compound was not physically characterized and
though no enantiomeric potency comparison was pro-
vided (because its S-enantiomer was not prepared),
5d was reported to bind at 5-HT6 receptors with a
Ki < 10 nM.19
preparation, Holenz et al.20 showed that 5-arylsulfon-
amido analogs of tryptamines (e.g., 15) bind at 5-
HT6 receptors, and that some are 5-HT6 antagonists,
whereas others are agonists. At this time, however, it
is not known how 15 binds relative to the compounds
described herein. Nevertheless, Cole et al.,21 coinciden-
tally, reported on the pyrrolidinylmethylindole coun-
terparts of such agents (i.e., 16). In the latter
investigation, there was a trend for the R-isomers of
16 (where R = Me) to bind with about 4-fold higher
affinity, relative to the corresponding NH analogs,
and even more fascinating is that the S-enantiomers
generally behaved as 5-HT6 antagonists, whereas the
R-isomers displayed agonist action.21
Acknowledgment
The present work was supported in part by NIMH MH
60599.
Although the present results cannot be used as evi-
dence for different modes of binding, they are not
inconsistent with the concept that N1-substituted trypt-
amine-related analogs might bind differently than their
N1-unsubstituted counterparts. The results further
identify R-pyrrolidinylmethylindole analogs of MS-
245 (1a) as binding with affinity higher than their
S-enantiomers. While this manuscript was under
References and notes
1. Hoyer, D.; Hannon, J. P.; Martin, G. R. Pharmacol.
Biochem. Behav. 2002, 71, 533.
2. Humphrey, P. P. A.; Hartig, P. R.; Hoyer, D. Trends
Pharmacol. Sci. 1993, 14, 233.

C. Abate et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3510–3513
3513
3. Kroeze, W. K.; Kristiansen, K.; Roth, B. L. Curr. Top.
Med. Chem. 2002, 2, 507.
18. The h5-HT6 radioligand binding assay was performed, as
previously described.22 In brief, h5-HT6 cDNA was
transiently expressed in HEK-293 cells using Fugene6
according to the manufacturerꢀs recommendations; 24 h
after transfection, the medium was replaced, and 24 h
later, medium containing dialyzed serum (to remove 5-
HT) was added. At 72 h after transfection, cells were
harvested by scraping and centrifugation. Cells were then
washed by centrifugation and resuspension in phosphate-
buffered saline (pH 7.40; PBS), and frozen as tight pellets
at ꢀ80 °C until use. Binding assays were performed at
room temperature for 90 min in binding buffer (50 mM
Tris–Cl, 10 mM MgCl2, and 0.1 mM EDTA, pH 7.40)
with [3H]LSD (1 nM final concentration) using 10 lM
clozapine for nonspecific binding. Concentrations of
unlabeled test agent (1–10,000 nM) were used for Ki
determinations with Ki values calculated using the pro-
gram GraphPad Prism (V4.0). Specific binding represent-
ed 80–90% of total binding. Ki values are the result of
triplicate determinations.
4. Meltzer, H. Y.; Li, Z.; Kaneda, Y.; Ichikawa, J. Prog.
Neuropsychopharmacol. Biol. Psychiatry 2003, 27, 1159.
5. Glennon, R. A. J. Med. Chem 2003, 46, 2795.
6. Woolley, M. L.; Marsden, C. A.; Fone, K. C. F. Curr.
Drug Top. 2004, 3, 59.
7. Glennon, R. A.; Lee, M.; Rangisetty, J. B.; Dukat, M.;
Roth, B. L.; Savage, J. E.; McBride, A.; Rauser, L.;
Hufesien, L.; Lee, D. K. H. J. Med. Chem. 2000, 43, 1011.
8. Tsai, Y.; Dukat, M.; Slassi, A.; MacLean, N.; Demchy-
shyn, L.; Savage, J. E.; Roth, B. L.; Hufesein, S.; Lee, M.;
Glennon, R. A. Bioorg. Med. Chem. Lett. 2000, 10, 2295.
9. Kolanos, R.; Siripurapu, U.; Pullagurla, M.; Riaz, M.;
Setola, V.; Roth, B. L.; Dukat, M.; Glennon, R. A.
Bioorg. Med. Chem. Lett. 2005, 15, 1987.
10. Pullagurla, M.; Westkaemper, R. B.; Glennon, R. A.
Bioorg. Med. Chem. Lett. 2004, 14, 4569.
11. Glennon, R. A.; Bondarev, M.; Roth, B. L. Med. Chem.
Res. 1999, 9, 108.
12. Macor, J. E.; Whytes, M. J. U. S. Patent 5,607,951, 1997.
13. Macor, J. E.; Blake, J.; Fox, C. B.; Jonhson, C.; Koe, B.
K.; Lebel, L. A.; Morrone, J. M.; Ryan, K.; Schmidt, A.
W.; Schulz, D. W.; Zorn, S. H. J. Med. Chem. 1992, 35,
4503.
19. Slassi, A.; Edwards, L.; OꢀBrien, A.; Xin, T.; Tehim, A. U.
S. Patent 6,100,291, 2000.
20. Holenz, J.; Merce, R.; Diaz, J. L.; Guitart, X.; Codony,
X.; Dordal, A.; Romero, G.; Torrens, A.; Mas, J.;
Andaluz, B.; Hernandez, S.; Monroy, X.; Sanchez, E.;
Hernandez, E.; Perez, R.; Cubi, R.; Sanfeliu, O.; Busch-
mann, H. J. Med. Chem. 2005, 48, 1781.
14. Macor, J. E.; Chenard, B. L.; Ronald, J. P. J. Org. Chem.
1994, 59, 7496.
15. Compound S(ꢀ)9 was characterized as its hydrogen
oxalate salt, mp 205–206 °C, following recrystallization
from methanol and analyzed within 0.4% of theory for
C13H14N2OÆC2H2O4. S(ꢀ)9 was found to bind
(Ki = 2700 600 nM) with an affinity similar to that of 6a.
16. Swaminathan, S.; Ranganathan, S.; Sulochana, S. J. Org.
Chem. 1958, 23, 707.
21. Cole, D. C.; Lennox, W. J.; Lombardi, S.; Ellingboe, J.
W.; Bernotas, R. C.; Tawa, G. J.; Mazandarani, H.;
Smith, D. L.; Zhang, G.; Coupet, J.; Schechter, L. E. J.
Med. Chem. 2005, 48, 353.
22. Kohen, R.; Metcalf, M. A.; Khan, N.; Druck, T.;
Huebner, K.; Lachowicz, J. E.; Meltzer, H. Y.; Sibley,
D. R.; Roth, B. L.; Hamblin, M. W. J. Neurochem. 1996,
66, 47.
17. Floyd, D. M. J. Med. Chem. 1992, 35, 756.