Substituted imidazoles as glucagon receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(01)00498-X

A modestly active, nonselective triarylimidazole lead was optimized for binding affinity with the human glucagon receptor. This led to the identification of a 2- and/or 4-alkyl or alkyloxy substituent on the imidazole C4-aryl group as a structural determinant for significant enhancement in binding with the glucagon receptor (e.g., 41, IC₅₀ = 0.053 μM) and selectivity (> 1000 ×) over p38 MAP kinase in this class of compounds.

Full text of DOI:10.1016/S0960-894X(01)00498-X

Bioorganic & Medicinal Chemistry Letters 11 (2001) 2549–2553
Substituted Imidazoles as Glucagon Receptor Antagonists
Linda L. Chang,a,* Kelly L. Sidler,a Margaret A. Cascieri,b Stephen de Laszlo,a
Greg Koch,b Bing Li,a Malcolm MacCoss,a Nathan Mantlo,a Stephen O’Keefe,c
Margaret Pang,c Anna Rolandoc and William K. Hagmanna
aDepartment of Medicinal Chemical Research, Merck Research Laboratories, Rahway, NJ 07065, USA
bDepartment of Molecular Pharmacology and Biochemistry, Merck Research Laboratories, Rahway, NJ 07065, USA
cDepartment of Inflammation and Rheumatology, Merck Research Laboratories, Rahway, NJ 07065, USA
Received 2 May 2001; accepted 11 July 2001
Abstract—A modestly active, nonselective triarylimidazole lead was optimized for binding affinity with the human glucagon
receptor. This led to the identification of a 2- and/or 4-alkyl or alkyloxy substituent on the imidazole C4-aryl group as a structural
determinant for significant enhancement in binding with the glucagon receptor (e.g., 41, IC50=0.053 mM) and selectivity (>1000ꢀ)
over p38 MAP kinase in this class of compounds. # 2001 Elsevier Science Ltd. All rights reserved.
Glucagon is a 29-amino acid peptide hormone produced
by the a-cells in the pancreas and is a major counter-
regulatory hormone to insulin in the maintenance of
glucose homeostasis.1 Binding of glucagon to its recep-
tor, which signals via G-proteins and which is primarily
located in the liver, leads to enhanced levels of intracel-
lular cyclic AMP and Ca2+. This results in increased
hepatic gluconeogenesis and glycogenolysis and attenu-
ates the ability of insulin to inhibit these processes.2
According to a bihormonal hypothesis, in Type II dia-
betes, elevated levels of circulating glucagon result in
increased rates of hepatic glucose synthesis and glyco-
gen metabolism, translating to excessive plasma glucose
levels.3 Therefore, antagonists of the glucagon receptor
have the potential to modulate the rate of hepatic glu-
cose output and improve insulin responsiveness in the
liver, resulting in a decrease in fasting plasma glucose
levels in diabetics.
A screening effort to identify small nonpeptide antago-
nists of the human glucagon receptor (hGlur) led to the
discovery of the triarylimidazole 1. This compound
exhibited an IC50 of 0.27 mM in the hGlur assay but also
registered an IC50 of 0.16 mM in a p38 mitogen-activated
protein (MAP) kinase assay. Herein, we describe the
transformation of this lead into potent and selective
triarylimidazole glucagon receptor antagonists.9
Scheme 1 shows one of several routes used to prepare
the imidazoles studied. Acylation of the anion of a pro-
tected 4-pyridinemethanol with a Weinreb amide pro-
vided
a
2-(4-pyridyl), 2-siloxyaryl ketone. This
Peptidyl antagonists of the glucagon receptor have been
well-studied and several antagonists (without any par-
tial agonism) have been reported recently,4aꢁc although
facile metabolic cleavage remains of concern.4dꢁf Recent
years have seen a considerable interest in the pursuit of
nonpeptide glucagon antagonists,5 for example, diami-
nostyryl-dichloroquinoxaline,6 substituted benzimida-
zoles,7 pyridylphenyls and biphenyls8 have been
described.
compound was heated with an aldehyde in the presence
of ammonium acetate and an oxidizing agent to form
the triarylimidazole.10 In some cases, another step is
required to reach the target compound.11 Compounds
1–21, 25–27, 30–34, and 36–39 were prepared in this
manner.
Compounds 22–23 were prepared by treatment of 21
with sodium hydride and iodomethane followed by
chromatographic separation. The structural assign-
ments were based on NOE difference spectra between
the N-CH3 and the ortho-protons of the aromatic ring.
*Corresponding author. Tel.:+1-732-594-6465; fax:+1-732-594-5350;
e-mail: linda_chang@merck.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00498-X

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L. L. Chang et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2549–2553
compounds reported herein [e.g., for 1: hGlur
IC50=0.27 mM (ꢁMg2+); IC50=2.9 mM (+Mg2+)].9,12,13
Assays were run in duplicate and the inter-assay
variability was ꢂ50%.
As a counterscreen, the inhibition of human p38a MAP
kinase (p38) by the test compounds was assessed as
described previously.14 Assays were run in duplicate and
the inter-assay variability was ꢂ3-fold.
Scheme 1. (a) LDA, THF, ꢁ30ꢃ!0 ꢃC, 50–75%; (b) R1CHO,
Cu(OAc)2, NH4OAc, HOAc, 115 ꢃC, 4 h, 22–80%; (c) (n-Bu)4NF,
THF, ꢁ60 ꢃC, (4-MeS)C6H4-COCl, 30%; (d) NH4OAc, HOAc, 90 ꢃC,
3 h, 40%.
Initially, the screening protocol was run in the absence
of divalent magnesium ions.12 Indeed, the discovery of
the lead compound (1) and the preliminary development
of the SAR of this class of glucagon receptor antago-
nists relied on this version of the assay. Table 1 shows
that among C2-aryl substituted imidazoles, a 4-sub-
stituent on the aryl ring is preferred (1–4) The 4-bromo
derivative (1) was the most active among 4-halo, 4-
alkyl, and 4-aryl substitutions studied (1 and 5–10).
Excessive steric bulk was not tolerated (9 vs 10). Ana-
logues with a heteroatom directly attached to the 4-
position of the imidazole C2-aryl moiety fared poorly
(11–13). Those with a distal heteroatom or polar group
also had moderate potency (14–15). Compounds 1 and
16–18 demonstrate the importance of the directionality
of the ring substituent and the deleterious effect of a
polar heteroatom in the ring (16 vs 18). Data from the
a-naphthyl (19) and the phenethyl (20) derivatives show
the effect of relaxing the structural constraint of the
phenyl ring. Taken together, these data suggest that the
binding space of these triaryl imidazoles with the gluca-
gon receptor at the 2-position of the imidazole is largely
hydrophobic, best accommodated by a 4-substituted
aryl group, rather narrow, and quite directional towards
the 4-position of the phenyl ring. As shown in Table 1,
no separation from p38 activity was found in this series.
The oxazole 24 was prepared via the requisite ester as
shown in Scheme 1.
In the dione route (Scheme 2), acylation of the anion of
4-picoline with the requisite Weinreb amide provided
the corresponding ketone, which was oxidized to the
dione and then transformed to triarylimidazoles 40 and
41. For the synthesis of 35, subsequent to imidazole
formation, palladium(0)-mediated biaryl coupling reac-
tion between the 3-bromo functionality of the triar-
ylimidazole and the corresponding phenylboronic acid
provided the desired analogue.
A modified version of Scheme 2 was used to prepare
compounds 28 and 29. As shown in Scheme 3, the
requisite substituted pyridyl moiety was prepared from
deprotonation of the corresponding lutidine. Sub-
sequent oxidation and imidazole formation provided
the desired compound.
The glucagon receptor binding affinity of the com-
pounds examined was determined by measuring the
reduction in binding of 125I-glucagon to hGlur expres-
sed in CHO cell membranes in the presence of the test
compound, as previously described.12 In the presence of
physiological concentrations of Mg2+ (5 mM), a reduction
of 2- to 25-fold in binding affinity was observed for the
Table 2 shows the crucial role of the basic imidazole
hydrogen. Comparing the data from 21 versus 22–24, it
is clear that the NH is required for reasonable glucagon
binding.
The significance of the 4-pyridyl group on the 5-position
of the imidazole was studied. Data from compounds
25–27 (Table 3) show that this 4-pyridyl nitrogen cannot
be substituted by a carbon or an exocyclic oxygen atom
(e.g., the OH of phenol in 27). Alkyl substitutions
around the pyridyl ring were also studied (28–29). These
all gave compounds with inferior binding to the gluca-
gon receptor compared to the unsubstituted 4-pyridyl
derivative 5.
Scheme 2. (a) 4-Picoline, LDA, THF, ꢁ40ꢃ!0ꢃC, 81%; (b) SeO2,
HOAc, 50%; (c) (4-Cl)C6H4CHO, NH4OAc, HOAc, 65%; (d)
Pd(PPh3)4, NaOH, C6H5B(OH)2, toluene, EtOH, 70%.
For the compounds presented above, the glucagon
receptor affinity, where they are significant, are invari-
ably accompanied by submicromolar activities in the
MAP kinase assay. A breakthrough in the separation of
this kinase activity from glucagon receptor binding
came when the SAR of the 4-position of the imidazole
was examined.
Studies on the 4-halo substituted 4-aryl moiety (5 and
30–31) suggested that larger substituents might increase
glucagon receptor binding and could decrease p38
Scheme 3. (a) NaN(TMS)2, THF, 0 ꢃC, 20%; (b) SeO2, Ac2O, 75 ꢃC,
24%; (c) (4-Cl)C6H4CHO, NH4OAc, HOAc, 60%.

L. L. Chang et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2549–2553
2551
Table 1. In vitro binding potencies of triarylimidazoles for the human glucagon receptor and p38 MAP kinase: imidazole-C2 variations
Compound
R
hGlur IC50 (ꢁMg, mM) p38 IC50 (mM) Compound
R
hGlur IC50 (ꢁMg, mM) p38 IC50 (mM)
1
2
3
4
5
6
7
8
9
10
(4-Br)Ph
C6H5
0.27
>20
na1
0.16
0.13
0.09
0.15
0.08
0.10
0.10
0.09
0.28
0.30
11
12
13
14
15
16
17
18
19
20
(4-NH2)Ph
(4-OH)Ph
(4-OMe)Ph
(4-CN)Ph
(4CO2Me)Ph
(5-Br)-2-thienyl
(4-Br)-2-thienyl
(5-Br)-2-furyl
a-naphthyl
2.0
naa
13
8.0
8.7
2.2
2.8
naa
1.5
naa
0.07
0.08
0.10
0.21
0.30
0.11
0.10
0.19
0.34
0.13
(2-Br)Ph
(3-Br)Ph
(4-Cl)Ph
(4-F)Ph
(4-I)Ph
(4-Me)Ph
(4-iPr)Ph
(4-Ph)Ph
1.4
0.40
2.0
0.51
1.3
0.70
10
phenethyl
ana=<20% inhibition at 2 mM under the assay conditions.
Table 2. In vitro binding potencies of various compounds for the human glucagon receptor and p38 MAP kinase
Compound
21
22
23
24
hGlur (ꢁMg) IC50 (mM or % inh. @ 2mM)
p38 IC50 (mM)
0.49
0.12
10%
0.52
38%
0.86
ꢁ8%
0.93
Table 3. In vitro binding potencies of triarylimidazoles for the human glucagon receptor and p38 MAP kinase: imidazole-C5 derivatives
Compound
R1
R2
R3
hGlur IC50 (ꢁMg, mM)
p38 IC50 (mM)
25
26
27
28
29
(4-Br)Ph
(4-Br)Ph
(4-Br)Ph
(4-Cl)Ph
(4-Cl)Ph
C6H5
C6H5
C6H5
(4-F)Ph
(4-F)Ph
4-pyridyl
C6H5
(4-OH)Ph
0.782
37% inh.a
24% inh.a
1.1
0.04
2.2
0.10
0.02
0.05
3-Me(4-pyridyl)
2-Me(4-pyridyl)
38% inh.a
a% inhibition at 2 mM.
activity. Several 4-aryl and 4-alkyl derivatives were pre-
pared. As seen on Table 4, this approach indeed allowed
the identification of more selective glucagon receptor
antagonists (32–34), the most active being the 4-n-butyl
derivative (34).
25-fold, as shown in Table 4. Compound 34 again stood
out in its submicromolar glucagon binding affinity in
the +Mg2+ assay and selectivity over p38 kinase.
A number of analogues at all positions of the aryl ring
were prepared in attempts to identify a selective com-
pound possessing higher binding affinity with the glu-
cagon receptor in the presence of Mg2+ (IC50 < 100
nM) while maintaining selectivity over p38 kinase.
Compounds 35–41 are representative of this effort. As
A glucagon receptor binding assay incorporating physio-
logical concentrations (5 mM) of Mg2+ ions was
developed. When tested in this version, the activity of
our glucagon-selective compounds decreased by 2- to

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L. L. Chang et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2549–2553
Table 4. In vitro binding potencies of triarylimidazoles for the human glucagon receptor and p38 MAP kinase: imidazole-C4 analogues
Compound
R
hGlur IC50 (ꢁMg, mM)
p38 IC50 (mM)
hGlur IC50 (+Mg, mM)
5
4-F
4-Cl
4-I
4-Ph
4-t-Bu
4-n-Bu
3-Ph
0.40
0.19
0.13
0.14
0.080
0.023
0.14
3.3
3.2
2.2
0.75
3.8
3.4
0.15
>3
0.29
0.15
0.11
0.11
0.12
0.053
30
31
32
33
34
35
36
37
38
39
40
41
0.13
>10
>10
0.074
0.061
0.0065
0.013
0.027
0.0085
0.013
0.0065
0.59
0.15
0.22
0.25
2-OPh
3-OPh
4-OPh
2-O-n-Bu
2,4(O-n-Pr)2
2,4(O-n-Bu)2
>1
2.4
20% inh. @ 40 mM
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positions of the central imidazole ring, a nonselective
triarylimidazole glucagon receptor antagonist lead, 1
(hGlur IC50=2.9 mM, p38 IC50=0.16 mM) was trans-
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The key discovery was the identification of a glucagon
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phenyl ring of imidazole-C4, which separated the glu-
cagon receptor affinity from the kinase activity and
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in the management of glucose homeostasis.
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nichny, J. V.; Colwell, L.; Koch, G. E.; Cascieri, M. A.; Hag-
mann, W. K. Bioorg. Med. Chem. Lett. 1999, 9, 641. (b) Kim,
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Acknowledgements
We thank Dr. George Doss for NOE spectral analysis
and Ms. Amy Bernick for mass spectral analysis.
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