Discovery of a 3,4,5-trisubstituted-1,2,4-triazole agonist with high affinity and selectivity at the somatostatin subtype-4 (sst4) receptor

Article abstract of DOI:10.1039/c8md00388b

A series of compounds containing a 1,2,4-triazole moiety were synthesized, targeting the somatostatin receptor subtype-4 (sst₄). Compounds were developed in which the Phe⁶/Phe⁷/Phe¹¹, Trp⁸, and Lys⁹ mimetic groups were interchanged at positions 3, 4, and 5 of the 1,2,4-triazole ring. The 1,2,4-triazoles containing an 2-(imidazol-4-yl)ethyl substituent at position-3 demonstrated moderate binding affinity at sst₄. 1,2,4-Triazoles containing an (indol-3-yl)methyl substituent at position-5 lacked affinity at sst₄. The 1,2,4-triazoles containing an aminopropyl group at position-4 showed enhanced binding affinity compared to the 3-position. One compound with an 3-(imidazol-4-yl)propyl group at position-4 (compound 44) imparted high affinity and selectivity at sst₄ (sst_2A = >10000 nM; sst₄ = 19 nM), acting as an agonist (EC₅₀ = 6.8 nM). Docking 44 into a model-built structure of sst₄ pointed to differences in its binding versus the other low-affinity compounds and was also in line with one of the two previously reported binding modes. A virtual screening (VS) experiment, employing two separate docking algorithms, was able to score 44 among the top-ranked poses. In summary, compound 44 represents a novel and promising lead structure towards the development of a clinically viable sst₄ agonist for the treatment of conditions ranging from Alzheimer's disease to chronic pain.

Full text of DOI:10.1039/c8md00388b

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RESEARCH ARTICLE
Discovery of a 3,4,5-trisubstituted-1,2,4-triazole
agonist with high affinity and selectivity at the
somatostatin subtype-4 (sst4) receptor†
ab
Cite this: Med. Chem. Commun.,
2018, 9, 2083
Iman Daryaei,
Karin Sandoval,a Ken Witt,a
Maria Kontoyiannia and A. Michael Crider
a
*
A series of compounds containing a 1,2,4-triazole moiety were synthesized, targeting the somatostatin re-
ceptor subtype-4 (sst4). Compounds were developed in which the Phe6/Phe7/Phe11, Trp8, and Lys9 mimetic
groups were interchanged at positions 3, 4, and 5 of the 1,2,4-triazole ring. The 1,2,4-triazoles containing
an 2-(imidazol-4-yl)ethyl substituent at position-3 demonstrated moderate binding affinity at sst4. 1,2,4-
Triazoles containing an (indol-3-yl)methyl substituent at position-5 lacked affinity at sst4. The 1,2,4-triazoles
containing an aminopropyl group at position-4 showed enhanced binding affinity compared to the 3-posi-
tion. One compound with an 3-(imidazol-4-yl)propyl group at position-4 (compound 44) imparted high af-
finity and selectivity at sst4 (sst2A = >10000 nM; sst4 = 19 nM), acting as an agonist (EC50 = 6.8 nM).
Docking 44 into a model-built structure of sst4 pointed to differences in its binding versus the other low-
affinity compounds and was also in line with one of the two previously reported binding modes. A virtual
screening (VS) experiment, employing two separate docking algorithms, was able to score 44 among the
top-ranked poses. In summary, compound 44 represents a novel and promising lead structure towards the
development of a clinically viable sst4 agonist for the treatment of conditions ranging from Alzheimer's dis-
ease to chronic pain.
Received 6th August 2018,
Accepted 6th November 2018
DOI: 10.1039/c8md00388b
rsc.li/medchemcomm
difference in affinity, distribution, interactions, and traffick-
ing enable each receptor to mediate a variety of outcomes.
Introduction
Somatostatin (somatotropin release-inhibiting factor, SRIF) is
a naturally occurring polypeptide that was originally isolated
from the hypothalamus.1 Two biologically active forms exist,
SRIF-14 (1, Fig. 1) and SRIF-28, derived from a 92-amino acid
pro-somatostatin precursor.2 Both SRIF forms demonstrate
pharmacological action via binding to structurally-related re-
ceptors, which belong to the G-protein-coupled superfamily
(Gi-coupled).3 The five receptor subtypes (sst1–sst5) fall into
two major families, which have been cloned and character-
ized.4 Structure–activity relationship (SAR) studies on SRIF
have shown that the core residues Trp8 and Lys9 of the β-turn
are necessary for receptor activation and pharmacological ac-
tivity.5,6 Furthermore, research carried out by Lewis et al.7
showed that Phe6, Phe7, and Phe11 play a role in sst2, sst3,
and sst5 binding, whereas only Phe6 was involved in sst4 bind-
ing. While the receptor signaling pathways share similarities,
Given SRIF has shown to exhibit inhibitory actions on endo-
crine and exocrine secretions, as well as numerous modula-
tory effects on neuronal activity in the periphery and central
nervous system (CNS),8 the respective receptors have become
primary therapeutic targets.
Of the five receptors, sst4 has come to the forefront as a
potential mediator of CNS pathology, inflammation, and even
pain mechanisms. Targeting sst4 has additional advantages,
having limited pituitary distribution and shown not to inhibit
glucagon, growth hormone, or insulin secretion.9 Within the
CNS, sst4 has been shown to be expressed at relatively high
levels in the hippocampus and neocortex,10 areas identified
with memory and learning, as well as Alzheimer's disease pa-
thology.11 Recent evaluations have indeed shown sst4 agonist
action improves learning and memory in rodent models of
Alzheimer's disease, which corresponded with reduced
β-amyloid levels.12–14 Furthermore, investigations have identi-
fied sst4 receptor agonism dose-dependently enhanced cued
memory,15,16 and thus may have direct cognitive enhance-
ment activity. Other research has shown sst4 coupled to K+
M-channels was capable of regulating hippocampal excitabil-
ity,17 with implications towards sst4 agonist-based treatments
for some forms of epilepsy. Interestingly, sst4 agonist action
a Department of Pharmaceutical Sciences, School of Pharmacy, Southern Illinois
University Edwardsville, Edwardsville, IL, 62026-2000 USA.
E-mail: mcrider@siue.edu
b NuvOx Pharma, 1635 18th St, Tucson, AZ, 85719 USA
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8md00388b
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Fig. 1 Schematic depiction of the rational design of 44 from SRIF-14 peptide. Residues and corresponding substituents are colored the same.
has also been linked to peripheral anti-nociceptive and anti-
inflammatory activity in association with acute and chronic
pain models in rodents.18 Recent data shows SRIF released
from capsaicin-sensitive receptor potential vanilloid type 1
(TRPV1)-expressing nociceptors acts on sst4, as well as sst2, to
produce its anti-nociceptive effects.19 The list of applications
for sst4 agonist therapeutics continues to grow, ranging from
mood disorders20 to bladder dysfunction.21 Given this signifi-
cant potential, our laboratory has worked to design and opti-
mize a clinically viable sst4 selective agonist.
The therapeutic use of SRIF is limited by poor bioavailabil-
ity and rapid breakdown by peptidases. Although longer act-
ing analogs such as lanreotide and octreotide have been de-
veloped, administration by the subcutaneous or intravenous
routes is required.22 Recently, more stable dicarba-analogs of
octreotide have been synthesized and evaluated at sst2 and
sst5.23 As an extension of this work, dicarba-analogs of
octreotide conjugated to 1,4,7,10-tetraazacyclodecane-1,4,7,10-
tetraacetic acid (DOTA) were shown to have nanomolar affin-
ity at sst2 and sst5.24 Although the development of peptide
analogs of SRIF-14 remains an active area of research, our fo-
cus has long been on the design and discovery of selective,
orally active, non-peptide sst4 agonists.
cally depicts the rationale for the design of compound 44. The
two core residues of SRIF-14 (1), Trp8 and Lys9, thought to be
critical for binding to all sst receptors are highlighted, along
with Phe7. Using previous structure–activity relationships, in-
dole, dichlorobenzyl, and imidazole moieties were selectively
chosen as substituents corresponding to Trp8, Phe6/7/,11 and
Lys9, respectively. The cis-peptide bond is replaced by a triazole
for a peptidomimetic design; it also positions the aforemen-
tioned amino acid residues, mimicking the endogenous sub-
strate optimally within the binding pocket, using the structure-
based design of our earlier work.27 The thiourea (NNC 26-9100,
2, Fig. 1) was shown to have over 100-fold selectivity at sst4
compared to other ssts, and in a functional assay using inhibi-
tion of forskolin-induced cyclic 3,5-adenosine monophosphate
(cAMP), NNC 26-9100 was shown to be a full agonist at sst4.25
Unfortunately, thioureas have been associated with adverse ef-
fects thought to result from reactive sulfur oxidation products
catalyzed by cytochrome P450 and flavin monooxygenases
(FMO).28,29 Thus, our program has been refocused towards the
development of non-thioureas sst4 agonists. In this regard, the
1,2,4-triazole nucleus (3, Fig. 2) was pursued as a principle scaf-
fold, given it: 1) represents a highly viable scaffold that is
Earlier work from our laboratory focused on a thiourea moi-
ety in the design and discovery of non-peptide sst4 agonists. Re-
garding the thiourea scaffold, we focused on the following
three side chain mimetics: a) a heteroaromatic nucleus (Trp8
mimetic), b) a non-heteroaromatic group (Phe7 mimetic), and
c) a basic moiety (Lys9 mimetic).25,26 The same reasoning di-
rected our efforts towards the triazole derivatives. Fig. 1 specifi-
Fig. 2 Structures of the 3,4,5-trisubstituted-1,2,4-triazole moiety (3)
and the 3-thio-1,2,4-triazole moiety (4).
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readily synthesized in a variety of functional forms, 2) is a
cis-amide bond isostere30 that is found in natural peptides and
is not subject to peptidase action, 3) has three points of diver-
sity making it more easily modifiable via modular parallel
chemistry, and 4) does not contain the potentially problematic
thiourea. Moreover, the related 3-thio-1,2,4-triazole moiety (4,
Fig. 2) has been previously utilized in the targeting of sst2/sst5
ligands,31 providing further validation to our approach.
Herein, our goal was to develop ligands based on the
1,2,4-triazole ring with high affinity and selectivity at sst4.
Three series of compounds were proposed in which the Phe6/
Phe7/Phe11, Trp8, and Lys9 mimetic groups were interchanged
at positions 3, 4, and 5 of the 1,2,4-triazole ring. The choice
of side chains was guided by previous SAR studies on sst4 ag-
onists.25,26,32 Using a recently generated homology model of
27
sst4, docking assessments were conducted to delineate prin-
Fig. 3 Hydrazides 5–8 used in the synthesis of 1,2,4-triazoles.
ciple binding attributes of respective compounds. A VS exper-
iment was also carried out to affirm docking findings.
propionic acid (10) using a mixed anhydride coupling proce-
dure.36 The 3,4-dichlorophenylacetamide derivative 25 was pre-
pared from tryptamine and thionyl chloride in the presence of
triethylamine. All amides could be thionated using Lawesson's
reagent in good to excellent yields (18–24 and 26,
Scheme 2).37,38 Since each equivalent of Lawesson's reagent
produces two equivalents of active thionating agent, only 0.5–
0.6 equivalent of Lawesson's reagent was required for thio-
amides 18–24 and 26. Due to solubility issues with amides 16
and 17, 1.0–1.1 equivalent of Lawesson's reagent was required
for the synthesis of thioamides 23 and 24. Conversion of am-
ides to thioamides results in a significant downfield shift in
the 13C NMR spectrum. The presence of a peak in the 13C NMR
spectrum at 200–210 ppm and the absence of the amide peak
at 160–170 ppm were indicative of thioamide formation.
With the exception of 5, all required hydrazides (Fig. 3)
were known compounds and were prepared by literature
methods. Coupling of the appropriate hydrazide with a thio-
amide (Scheme 3) in tetrahydrofuran in the presence of ei-
ther mercuric acetate or silver benzoate as the thiophile
afforded the 3,4,5-trisubstituted-1,2,4-triazoles. Flash chroma-
tography was required to remove the precipitated Ag or Hg
from the thiophile. After cyclization, the trityl or Boc-
protecting group was removed with 1 N hydrochloric acid in
ethanol or methanol. The target 1,2,4-triazoles 27–44
(Scheme 3) were purified by flash chromatography, recrystal-
lization or salt formation. Yields of the target 1,2,4-trizoles
over two steps ranged from low (11%, compound 34) to good
(62%, compound 44). All 1,2,4-triazoles exhibited two charac-
teristic peaks in the 13C NMR spectra at 150–160 ppm corre-
sponding to the ring carbons at positions 3 and 5.
Results and discussion
Chemistry
The general synthetic pathway for the synthesis of 3,4,5-
trisubstituted-1,2,4-triazoles is shown in Scheme 1. This
method utilizes a literature procedure in which a thioamide
(I) is reacted with a hydrazide (II) to yield an intermediate
acylamidrazone (III). Cyclization to the desired 3,4,5-
trisubstituted-1,2,4-triazoles (IV) was facilitated by use of ei-
ther mercuric acetate or silver benzoate as the thiophile.33,34
Compared to mercuric acetate, the use of silver benzoate as
the thiophile generally decreased reaction times and afforded
cleaner reaction products.
The required hydrazides (5–8, Fig. 3) were prepared from
either tert-butyloxycarbonyl (Boc) or trityl-protected methyl or
ethyl esters in refluxing hydrazine hydrate. Methyl esters
which were not commercially available were prepared from
either amino acids or carboxylic acids using trimethylsilyl
chloride (TMS-Cl).35
Thioamides required for the synthesis of the 1,2,4-triazole
moiety were prepared as shown in Scheme 2. Amides 11–17
were synthesized from either indoleacetic acid (9) or indole-
Receptor binding assay
The direct interaction of the target 3,4,5-trisubstituted-1,2,4-
triazoles with human sst2A and sst4 receptors were evaluated
using in vitro radioligand binding displacement studies.
Scheme 1 General pathway for the synthesis of 3,4,5-trisubstituted-
1,2,4-triazoles.
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Scheme 2 Synthesis of amides (11–17, 25) and thioamides (18–24, 26). Reagents: (a) i. NNM/IBCF/THF, ii. RNH2; (b) Lawesson's reagent/THF; (c) i.
SOCl2/CH2Cl2, ii. tryptamine/THF.
Membrane Target™ Systems (Perkin Elmer) for sst4 (ES-524-
M400UA) and sst2A (ES-521-M400UA) were used in these studies
as previously described by Wang et al.39 The data from the
competition binding studies using 125I-Tyr-SRIF as the
radioligand is summarized in Table 1. Previously reported non-
peptide sst4 agonists and octreotide are included in Table 1 for
reference. Several conclusions can be drawn from the binding
studies: 1) none of the target 1,2,4-triazoles exhibited any ap-
preciable affinity for sst2A, 2) 1,2,4-triazoles containing an
(indol-3-yl)methyl substituent at position-5 (compounds 27–32)
lacked affinity at sst4, 3) 1,2,4-triazoles containing an
2-(imidazol-4-yl)ethyl substituent at position-3 demonstrated
moderate binding affinity at sst4 (compounds 34, 37, 39, and
41), 4) 1,2,4-triazoles containing an aminopropyl group at
position-4 showed enhanced binding affinity compared to the
3-position (compare compounds 36 and 43), and 5) an
3-(imidazol-4-yl)propyl group at position-4 (compound 44)
imparted high affinity and selectivity at sst4 (sst1 = 4750 nM;
sst2A = >10000; sst3 = 6628 nM; sst4 = 19 nM; sst5 = >10000).
Given the enhanced affinity and selectivity, compound 44
was further evaluated in a functional assay based on the
forskolin-induced cAMP inhibition in recombinant Chinese
hamster ovary (CHO-K1) cells expressing sst4. Compound 44
demonstrated approximately 4-fold greater efficacy (EC50
=
6.8 nM) compared to NNC 26-9100 (2, EC50 = 27.91 nM) and
was shown to be a full agonist in this assay.
Docking studies
Docking studies were undertaken to delineate binding modes
of respective compounds (Fig. 4). The objective was two-fold:
1) predict and explain differences in activities by performing
docking experiments and evaluating binding modes, and 2)
carry out a VS experiment in hopes of assessing whether
docking algorithms can single out the active among the top-
ranked poses. Regarding the second goal, it is apparent we
were pushing the ability of VS in that it has been proposed
that at 2.5% fraction of an entire compound collection half
of all actives are retrieved.43 In other words, we were asking
for a “needle in a haystack”, which is the very idea of VS,
however, a higher number of actives are typically included in
those experiments.
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Scheme 3 Synthesis of 3,4,5-trisubstituted-1,2,4-triazoles. Reagents and conditions: (a) CH2Cl2, AgOBz or HgIJOAc)2, HOAc, THF, RT; (b) 1 N HCl,
EtOH, reflux.
GOLD44 was able to explain observed activities for all mol-
ecules in line with our previous work on sst4 binding.27 Com-
pounds 45–48 are placed in the binding pocket with favorable
hydrophobic and pi-stacking interactions, but none featured
all electrostatics. Specifically, the secondary amine (45) or the
pyridine nitrogen (47, 48) hydrogen bond with Gln243. Aryl–
aryl interactions are observed between the pyridine and
Phe239 and Phe175, however, no pose displays all ionic inter-
actions involving Trp171, His258, and Asp90. Similarly, the best
pose of compound 39 has the indole N–H hydrogen bonding
with Gln243 and imidazole forming a hydrogen bond with
Tyr18. π–π Interactions are observed between indole and
Trp171 and Phe239. Analysis of compound 41 reveals that the
indole is interacting with Asp90 and the triazole hydrogen
bonds with the amide N–H. In compound 43, however, the
indole forms a hydrogen bond with Gln243, while the primary
amine points towards Asp90. The extent of hydrophobic and
pi-stacking interactions are limited in both poses. Finally, the
predicted binding mode of the most active 44 reveals that im-
idazole forms a hydrogen bond with Gln243 and Tyr240, the in-
dole N–H is positioned near Asp90, and the backbone amide
hydrogen of Trp271 hydrogen bonds with triazole. An edge-to-
face arrangement is observed between His258 and triazole,
while the imidazole makes a π–π interaction with Phe239. The
chlorine of chloro-benzyl forms a halogen bond with the car-
bonyl oxygen of Asp253. Table 2 shows a summary of the best
poses, respective ranking, and scoring function energies. VS
with GOLD ranked the active 44 in the top 10% of the library
(1005 compounds, with 996 decoys). Compound 44 was
ranked 78 with GoldScore.
In regards to docking with Glide,45,46 the best poses of 45
and 46 form interactions with either Asp90 or Gln243, but not
with both. Specifically, the secondary amine is close to Asp90
in 45, with Trp171 and Phe239 forming a hydrophobic pocket
around the pyridine moiety. However, no interactions are ob-
served with His258 or Gln243. A hydrogen bond interaction be-
tween the secondary amine of 46 and the carbonyl oxygen of
Gln243 is predicted, while the carbonyl oxygen of the central
2-thiohydantoin ring hydrogen bonds with His258. The reason
for the different interactions of the secondary amine is that
46 is positioned deeper in the pocket with the amino-
pyridine moiety rotated away from Asp90 and pointing to-
wards Gln243; this also results in pyridine no longer forming
π–π interactions with Trp171. The methoxy-substituent in 45
is exposed to the extracellular solvent while the indole groups
in both compounds are facing the extracellular domain. The
best poses of compounds 47 and 48 are similar to 46 in that
the 2-aminopyridine forms a hydrogen bond with Gln243, and
the carbonyl oxygen of the 2-thiohydantoin interacts with the
backbone amide nitrogen of Trp171. It should be noted that
neither the indole nor the dichlorophenyl and naphthalene
groups form any interactions. The predicted binding mode of
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Table 1 In vitro binding of 1,2,4-triazoles at sst2A and sst4 receptorsa
Table 2 Best GOLD rankings and respective scoring functions
Compound GoldScore (rank number) ChemScore (rank number)
b
Compound
sst2A (nM)
sst4c (nM)
45
46
47
48
39
41
43
44
60.3 (10)
—
18.5 (10)
—
27
28
29
30
31
32
33
34
>10 000
>10 000
—
—
—
>10 000
>10 000
—
>10 000
>10 000
>10 000
>10 000
>10 000
>10 000
<10 000
4471
66.6 (5)
69.3 (4)
67.5 (2)
64.3 (10)
67.5 (8)
73.1 (1)
36.3 (1)
34.5 (2)
27.2 (4)
29.1 (2)
35.9 (4)
27.4 (4)
35
36
37
38
39
40
41
42
>10 000
>10 000
>10 000
>10 000
>10 000
—
>10 000
>10 000
1426
>10 000
2032
>10 000
4803
>10 000
2380
interacts with His258. The indole nitrogen forms a hydrogen
bond with the backbone carbonyl oxygen of Ser70. In contrast,
the binding mode of 39 has the reverse orientation with in-
dole forming a hydrogen bond with Asp90, and imidazole's
Nε hydrogen bonding with the backbone carbonyl oxygen of
Ser70. The benzene ring mediates a π–π stacking interaction
with Trp76, and hydrophobic interactions with Val86 and
Leu87. The only interaction observed for compound 41 is be-
tween imidazole and Asp90 with an overall conformation that
seems strained. This in turn explains its very low affinity. A
salt bridge is observed between the amine of compound 43
and Asp90, while the indole nitrogen forms a hydrogen bond
with the backbone carbonyl oxygen of Ser70. The dichloro-
benzyl forms a π–π interaction with Trp171 and Phe239 and
the triazole hydrogen bonds with His258. Contrary to the
above binding modes, compound 44 encompasses electro-
statics and hydrophobic interactions with the amino acids we
predicted in prior publication as being critical.27 Specifically,
the binding mode analysis of 44 reveals a hydrogen bond be-
tween the indole and Gln243, while another hydrogen bond is
formed between imidazole and Asp90. Triazole hydrogen
>10 000
—
43
44
>10 000
>10 000
2621
19
NNC 26-9100
148
621d
6d
J-2156
>5000e
4720f
0.4–2.1g
0.68
1.2e
L-803087
Octreotide
SRIF-28
0.7f
66–2100g
2.41
a
Competitive radioligand binding experiments were performed
using Membrane Target Systems (Perkin Elmer, Boston, MA) for
human sst4 (ES-524-M400UA) and sst2A (ES-521-M400UA) receptors as
previously described by Wang et al.39 Binding assays were performed
in triplicate using 125I-Tyr-SRIF (Perkin Elmer) dissolved in 1 mM
b
HCl. Concentration of radioactivity = 0.075 nM, KD = 0.075 nM.
c
Concentration of radioactivity
=
0.2625 nM, KD
= 1.05 nM.
d
Ankersen et al.25 e Engstrom et al.40 f Rohrer et al.41 g Patel.42
compound 34 reveals a hydrogen bond between the imidaz-
ole and the carbonyl oxygen of Gln243, while the Nδ nitrogen
Fig. 4 Structures of compounds docked. aWang et al.39
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selective, full agonist (EC50 = 6.8 nM) at sst4. This compound
has several advantages over sst4 agonists reported from our lab-
oratory in that it: 1) contains a 1,2,4-triazole moiety that is a
cis-amide bond isostere, 2) is not subject to peptidase activity,
3) exhibits high selectivity at sst4 over other ssts, and 4) does
not contain a thiourea group that can form potentially reactive
metabolites. Additionally, compound 44, exhibited greater af-
finity at sst4 than the previously reported thiourea, NNC 26-
9100 (2).25 Docking 44 into a recently model-built sst4 structure
of sst4 (ref. 27) confirmed that this compound forms extensive
interactions with key residues such as Asp90, Gln243, Trp171
,
and His258, contrary to the other synthetic targets reported
herein. Continued optimization of the 1,2,4-triazole moiety to-
wards enhancement of pharmacokinetic parameters, while
maintaining affinity and selectivity at sst4, will provide a viable
therapeutic with vast potential.
Fig. 5 Docking of 44 with Glide.
Experimental
Table 3 Summary of rankings by Glide
The detailed procedures for the chemical synthesis, receptor
binding assays, and docking studies are described in the
ESI.† All intermediates and final products were purified by re-
crystallization or flash chromatography and were character-
Compound
Rank number
Emodel
45
46
47
48
34
39
41
43
44
2
1
1
1
1
1
6
1
3
−74.5
−83.9
−79.9
−94.2
−72.9
−63.0
−53.0
−64.0
−73.8
1
ized by H and 13C NMR, HRMS, and/or elemental analyses.
Conflicts of interest
No Conflicts of Interest to report by any of the authors.
Acknowledgements
bonds with Trp171 and His258 and a halogen bond is formed
between the meta-chlorophenyl and the carbonyl oxygen of
Ser70. Aryl–aryl interactions are also predicted between the in-
dole and Trp171 and Phe239 (Fig. 5). In summary, all com-
pounds with the exception of the active 44 suffer from lim-
ited interactions with the binding site residues; most
“Research reported in this publication was supported by the
National Institute On Aging of the National Institutes of
Health under Award Number R01AG047858. The content is
solely the responsibility of the authors and does not necessar-
ily represent the official views of the National Institutes of
Health.” Additional financial assistance is gratefully acknowl-
edged through a SIUE New Directions Grant (A. M. C.).
noticeably hydrogen bonds with Asp90, His258 and Trp171
,
along with hydrophobic interactions. Furthermore and based
on previous findings,27 we believe that 44 binds similarly to
binding mode II agonists. A summary of the rankings for
each of the compounds docked is presented in Table 3. The
VS experiment with Glide did not dock all compounds in the
library. However, compound 44 was ranked at 132 of a total
of 969 successful dockings.
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5 A. M. Crider, Mini-Rev. Med. Chem., 2002, 2, 507–517.
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Conclusions
In this report, we describe the synthesis of eighteen 3,4,5-
trisubstituted-1,2,4-triazoles and their binding affinities at sst2
and sst4. The triazole 44, substituted with an 3-(imidazole-4-
yl)propyl group at position-4, demonstrated high affinity and
selectivity at sst4 over other ssts with selectivities: sst4/sst1
250-fold, sst4/sst2 = >500-fold, sst4/sst3 = 350-fold, sst4/sst5
=
=
>500-fold. On the basis of a functional assay to access inhibi-
tion of forskolin-induced adenosine cyclic 3′,5′-monophosphate
(cAMP) accumulation, the triazole 44 was shown to be a highly
7 I. Lewis, W. Bauer, R. Albert, N. Chandramouli, J. Pless, G.
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