Further studies on the binding of N1-substituted tryptamines at h5-HT6 receptors

Article abstract of DOI:10.1016/j.bmcl.2006.12.089

N₁-Arylsulfonyl-substituted analogs of N,N-dimethyltryptamine bind at 5-HT₆ receptors. Replacement of the aryl moiety with similarly hydrophobic alkyl substituents results in decreased affinity, as does replacement of a benzenesulfon

Full text of DOI:10.1016/j.bmcl.2006.12.089

Bioorganic & Medicinal Chemistry Letters 17 (2007) 1691–1694
Further studies on the binding of N1-substituted tryptamines
at h5-HT6 receptors
Abner Nyandege,a Renata Kolanos,a 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, School of Medicine, Case Western Reserve University, USA
cDepartment of Psychiatry and Neurosciences, School of Medicine, Case Western Reserve University, USA
Received 24 August 2006; revised 19 December 2006; accepted 22 December 2006
Available online 4 January 2007
Abstract—N1-Arylsulfonyl-substituted analogs of N,N-dimethyltryptamine bind at 5-HT6 receptors. Replacement of the aryl moiety
with similarly hydrophobic alkyl substituents results in decreased affinity, as does replacement of a benzenesulfonyl moiety with a
benzyl group. Current findings indicate that an aryl (or substituted aryl) sulfonyl (rather than alkylsulfonyl or benzyl) moiety is opti-
mal for high-affinity binding, and further suggest that the N1-benzenesulfonyl- and their corresponding N1-benzyltryptamine coun-
terparts bind in a different fashion.
Ó 2007 Elsevier Ltd. All rights reserved.
Adenylate cyclase linked G-protein-coupled 5-HT6 sero-
tonin receptors have generated considerable recent inter-
est because of their possible involvement in obesity,
certain neuropsychiatric disorders, and cognition.1–6
Among the early 5-HT6 receptor antagonists was the
N1-arylsulfonyltryptamine MS-245 (1; Ki = 2.1 nM)
and its des-methoxy counterpart 2a (Ki = 4.1 nM).7,8
Despite structure-affinity studies by us,8 and others9–11
(reviewed5,12), a number of questions remain unan-
swered. For example, a structural feature common to
these arylsulfonyltryptamines is an ‘aryl’ moiety. Yet,
it has not been established that an aryl group is essential
for binding. That is, if the aryl group binds at the recep-
tor via a hydrophobic type of interaction, its replace-
ment by an alkyl group of similar or greater
hydrophobicity could result in retention of affinity. Con-
sequently, in this study we prepared and evaluated sev-
eral analogs of 2a (i.e., 3) where the phenyl ring is
replaced by an alkyl group. Another feature that has
not been fully explored is the necessity of the sulfonyl
group. It is generally thought that the sulfonamido por-
tion of 1 and 2a is important for binding. But, it has
been shown that the benzenesulfonyl group of 2a can
be replaced by a benzyl group (4a; Ki = 6.0 nM) with
relatively little impact on 5-HT6 receptor affinity.13
Because the 5-HT6 receptor affinities of N1-benzyltryp-
tamines have not been well investigated, we now exam-
ine several substituted analogs of 4a. In some instances,
their corresponding benzenesulfonyl counterparts were
prepared for the purpose of comparison.
H3C
H3C
H3C
CH3
CH3
N
CH3
N
N
R
N
S
N
S
N
O
O
Alkyl
O
O
1: R = OMe
2a: R = H
3:
4a
Synthesis of most of the target compounds (Scheme 1)
was achieved by sulfonylation or alkylation of the
N,N-dimethyltryptamine (5) anion, generated using
t-BuOK (for 2 and 3, except 3d and 3f) or NaH (for
3d and 3f, and generally, for 4), with the appropriate
arylsulfonyl- alkylsulfonyl- or benzyl halide (Table 1)
as previously described for the synthesis of 1 and
2a.8,14 Attempts to prepare amine analog 4d using a sim-
Keywords: Serotonin; 5-HT6 receptors; N1-Benzyltryptamines; N1-Al-
kylsulfonyltryptamines; N1-Arylsulfonyltryptamines; MS-245.
*
Corresponding author. Tel.: +1 804 828 8487; fax: +1 804 828
7404; e-mail: glennon@vcu.edu
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.12.089

1692
A. Nyandege et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1691–1694
H3C
H3C
H3C
CH3
N
CH3
CH3
N
N
a
N
N
N
b
a or b
b
N
H2C
N
N
H
X
H2N
S
O
Aryl or Alkyl)
(Bn)2N
O2N
12
7
13
O
5
2 and 3
c
4
H3C
CH3
N
NO2
Scheme 1. Reagents and conditions: (a) i—NaH, DMF, 100 °C;
ii—ArSO2Cl; (b) t-BuOK, 18-crown-6, THF, RCH2X, rt.
CHO
e
d
N
N
N
ilar approach (i.e., utilizing a 4-nitro- or 4-acetamido-
substituted benzyl halide) were unsuccessful. Subse-
quently, compounds 4d and 714 were prepared via a
common route from 1215 as shown in Scheme 2.
NH3
(Bn)2N
H2N
(Bn)2N
15
14
4d
Human 5-HT6 receptor binding data are shown in Table 1.
Several analogs 3 were prepared where the alkyl group
ranged in length from n-propyl (3b; Ki = 280 nM) to n-oc-
tyl (3e; Ki = 440 nM); some analogs possessed branched
alkyl groups such as i-propyl analog 3a (Ki = 590 nM)
and cyclohexyl analog 3f (Ki = 210 nM). Yet, none of
the alkylsulfonyl analogs displayed the affinity of the
simple benzenesulfonyltryptamine 2a (Ki = 4.1 nM).
The results show that an arylsulfonyl group at the
tryptamine N1-position is preferred, relative to an
alkylsulfonyl group, for 5-HT6 receptor affinity. Even
cyclohexyl analog 3f, which is simply a reduced version
of 2a, binds with 50-fold lower affinity than 2a itself.
Scheme 2. Reagents and conditions: (a) SnCl2Æ2H2O, EtOH; (b) BnBr,
NEt3, CH2Cl2; (c) POCl3/DMF; (d) MeNO2, AcONH4; (e) i—LiAlH4,
THF; ii—NaBH3CN, H2C = O; iii—Pd/C, HCOONH4, MeOH.
A small series of N1-benzyl analogs 4 was examined.
Substituents selected for evaluation included several
electron-donating and electron-withdrawing groups.
Only 4-substituted benzyl analogs were examined to
reduce any potential complications in data interpreta-
tion that might arise from rotameric binding. 5-HT6
receptor affinities ranged from 20 to >400 nM. But,
none of the benzyl analogs retained the affinity of the
Table 1. Physicochemical properties and h5-HT6 receptor affinities for target compounds
H3C
CH3
N
N
Z
Compound
Z
Melting pointa (°C)
Recryst solvent
Empirical formulaa
Kib, nM ( SEM)
3a
3b
3c
3d
3e
3f
–SO2-i-propyl
–SO2-n-propyl
175–178
144–145
162–163
167–169
138–139
173–175
—
MeOH/Et2O
MeOH/Et2O
MeOH/Et2O
MeOH/Et2O
MeOH/Et2O
MeOH/Et2O
—
C15H22N2O2S (COOH)2
C15H22N2O2S (COOH)2
C16H24N2O2S (COOH)2
C17H26N2O2S (COOH)2
C20H32N2O2S (COOH)2
C18H26N2O2S (COOH)2
—
590 (110)
280 (40)
135 (20)
220(30)
440(65)
210(30)
4.1
–SO2-n-butyl
–SO2-n-amyl
–SO2-n-octyl
–SO2-cyclohexyl
–SO2-phenyl
2a
2b
2c
2d
2e
2f
–SO2-(4-Me)phenyl
–SO2-(4-OMe)phenyl
–SO2-(4-NH2)phenyl
–SO2-(4-CF3)phenyl
–SO2-(4-Cl)phenyl
–CH2-phenyl
191–193
164–166
—
Acetone
Acetone
C19H22N2O2S (COOH)2
C19H22N2O3S (COOH)2
—
2.5(0.6)
13 (3)
0.8
—
Acetone
176–179
180–182
—
C19H19F3N2O2S (COOH)2
C19H22ClN2O2S (COOH)2
—
1.9 (0.4)
94 (25)
6.0
Acetone
—
4a
4b
4c
4d
4e
4f
–CH2-(4-Me)phenyl
–CH2-(4-OMe)phenyl
–CH2-(4-NH2)phenyl
–CH2-(4-CF3)phenyl
–CH2-(4-Cl)phenyl
179–182
172–173
157–159
162–163
170–172
MeOH/Et2O
MeOH/Et2O
MeOH/Et2O
MeOH/Et2O
MeOH/Et2O
C20H22N2 (COOH)2
C20H22N2O (COOH)2
C19H24N3Æ2HClÆ0.5H2O
C20H21F3N2 (COOH)2
C19H21ClN2 (COOH)2
29 (3)
132 (25)
44 (7)
445 (50)
20 (5)
a Compounds were homogeneous to thin layer chromatography, analyzed within 0.4% of theory for C, H, and N, and assigned structures are
consistent with 1H NMR spectra.
b Ki values ( SEM for new results) were determined at least in triplicate25 as previously described.26 SEM are not shown for previously reported
binding data. Binding data for 2a, 2d, and 4a have been previously published from our laboratories.13

A. Nyandege et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1691–1694
1693
unsubstituted benzyl analog 4a (Ki = 6.0 nM) or its ben-
zenesulfonyl counterpart 2a (Ki = 4.1 nM).
H3C
CH3
N
One explanation for the reduced affinity of 4b–f, com-
pared with 4a, is that the receptor does not tolerate sub-
stituents on the benzylic nucleus. This seems unlikely
because certain analogs of 1 bearing benzenesulfonyl sub-
stituents have been previously shown to bind with affini-
ties comparable to that of 1 itself.8 However, with
regard to these latter compounds, each possesses a meth-
oxy group on the indolic ring. In order to make a more
strict comparison, the benzenesulfonyl counterparts of
4b–f (i.e., 2b–f) were prepared and examined. The results
(Table 1) show that 5-HT6 receptors tolerate aryl substit-
uents, and that the 4-methyl, 4-amino, and 4-trifluoro-
methyl analogs (2b, 2d, 2e; Ki = 2.5, 0.8, and 1.9 nM,
respectively) bind at least as well as their unsubstituted
parent 2a. The results indicate that even though N1-ben-
zyl-substituted analogs bind at 5-HT6 receptors, they bind
with reduced affinity relative to their corresponding
N1-benzenesulfonyl counterparts, and further suggest
that the two series might bind in a somewhat different
fashion. It is commonly held that two series of compounds
might be binding in a similar manner when parallel struc-
tural changes result in parallel shifts in affinity. However,
a comparison of the affinities of the series 4 compounds
with the series 2 compounds (r2 = 0.048; n = 6) shows lit-
tle correspondence between the two.
H3CO
N
N
X
X
NH2
8 X = SO2
9 X = CH2
10 X = SO2
11 X = CH2
General findings of the present investigation are that
replacement of the benzenesulfonyl group of MS-245-
like (i.e., 1-like or 2a-like) benzenesulfonyltryptamines
with either an alkylsulfonyl group or a benzyl group re-
sults in diminished affinity for h5-HT6 receptors. The
alkylsulfonyl derivatives 3 differed with respect to chain
length, shape, and hydrophobicity, but none retained
the affinity of the simplest MS-245-like compound 2a.
It would seem that electronic or p–p interactions better
account for the binding of these compounds than do
simple hydrophobic interactions.
The N1-benzyl-substituted compounds 4 bind at 5-HT6
receptors but typically do so with affinities somewhat low-
er than their benzenesulfonyl counterparts 2. Evidence
suggests that the two series (i.e., 2 and 4) are probably
binding differently and, because the only structural differ-
ence between the two series is a sulfonyl versus methylene
group, it would seem that the sulfonyl group determines
the manner of binding. That is, the presence of the sulfo-
nyl group results in a somewhat higher affinity. The phen-
yl–SO2–N bond angle of N-benzenesulfonylpyrrole18 and
related benzenesulfonylindoles19,20 (ca. 105–106°) is only
slightly less than the bond angle of a tetrahedral carbon
atom. Furthermore, the N1–S bond length (ca. 1.6–
As a further test to determine if the N1-benzyl- and ben-
zenesulfonyl analogs behave in a similar manner, we
compared 6 with 7. It has been shown, when an amino
group is present at the 4-position of the benzenesulfonyl
moiety, that the N,N-dimethylaminoethyl portion of the
tryptamines can be removed with only a slight decrease
in affinity.14,16 For example, compound 6 (Ki = 10 nM)16
binds with an affinity similar to that of 2a. Interestingly,
compound 7 (Ki = 8200 800 nM) was found to bind
with >1000-fold lower affinity than its tryptamine coun-
terpart 4a. Here, then, is another example of where a
parallel structural change resulted in a dissimilar effect
on 5-HT6 receptor affinity.
˚
1.7 A) found in such compounds is only slightly longer
˚
than the N1–C bond length (ca. 1.5 A) of N1-benzylin-
doles.18–23 So, it is unlikely that geometry plays a substan-
tial role in the affinity differences observed between the
benzenesulfonyltryptamines and their benzyltryptamine
counterparts.24 Thus, although it cannot be concluded
that the sulfonyl group is essential for binding, it would
appear that its presence is optimal when similarly substi-
tuted pairs of compounds are examined, and that the oxy-
gen atoms might form an anchoring interaction with some
receptor-associated feature. An alternative explanation
for the observed affinity differences between the two series
involves the electronic effects of benzenesulfonyl versus
benzyl substituents. That is, in the benzenesulfonyl series,
the N1-substituent is conjugated with the indole nucleus
and benzenesulfonyl substituents might exert a greater
effect on the electronic character of the indole ring than
they would if appended to a non-conjugated N1-benzyl
moiety. This remains to be further examined. The results
also suggest, if the two series are binding differently, that
structure-affinity findings from the N1-benzenesulfonyl
series cannot be extrapolated to the N1-benzyl series
(e.g., compare 2e and 4e). Hence, because N1-benzyltryp-
tamines bind at 5-HT6 receptors, additional studies will be
required to optimize their affinity.
N
N
S
O
NH2
H2C
NH2
O
6
7
The present results help explain some findings previously
published from our laboratory.17 A (partially) conforma-
tionally constrained analog of 1 (i.e., 8; Ki = 1.5 nM)
binds at 5-HT6 receptors with high affinity. However,
replacement of the benzenesulfonyl group with a benzyl
group (i.e., 9; Ki = 136 nM) resulted in decreased affini-
ty—results that were difficult to explain at that time given
the similar affinity of 2a and 4a. Furthermore, whereas 10
(Ki = 29 nM) binds, its benzyl counterpart 11
(Ki = 6000 nM) displayed much lower affinity.17 In this
respect, the earlier results are consistent with the present
findings. It would appear, then, that the similar affinity
of 2a and 4a might be merely coincidental.

1694
A. Nyandege et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1691–1694
16. Siripurapu, U.; Kolanos, R.; Dukat, M.; Roth, B. L.;
Acknowledgment
Glennon, R. A. Bioorg. Med. Chem. Lett. 2006, 16,
3793.
17. Chang-Fong, J.; Rangisetty, J. B.; Dukat, M.; Setola, V.;
Raffay, T.; Roth, B.; Glennon, R. A. Bioorg. Med. Chem.
Lett. 2004, 14, 1961.
The present work was supported in part by National
Institute of Mental Health Grant MH 60599.
18. Grossie, D. A.; Malwitz, D. J.; Ketcha, D. M. Acta Cryst.
E 2006, 62, o980, [with corrected system numbering as per
D. A. Grossie (personal communication)].
19. Sonar, V. N.; Parkin, S.; Crooks, P. A. Acta Cryst. C
2004, 60, o659.
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24. It might be noted, however, that the presence of the
sulfonyl oxygen atoms might influence rotation about
the S–C bond. For example, in the solid state, the N–S–
C
1Ar–C2Ar torsion angle of an N1-benzenesulfonylindole
is about À92°, whereas for its N1-benzyl counterpart the
corresponding angle was found to be À155°.20,23
25. The h5-HT6 radioligand binding assay was performed as
previously described.26 In brief, h5-HT6 cDNA was
transiently expressed in HEK-293 cells using Fugene6
according to the manufacturer’s 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
À80 °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 cloza-
pine for non-specific binding. Concentrations of unlabeled
test agent were used for Ki determinations with Ki values
calculated using the program GraphPad Prizm (V4.0).
Specific binding represented 80–90% of total binding. Ki
values are the result of triplicate determinations.
26. 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.