Structure-activity relationship studies on tetralin carboxamide growth hormone secretagogue receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2005.02.026

The structure-activity relationship studies on a series of tetralin carboxamide growth hormone secretagogue receptor (GHS-R) antagonists are discussed. It was found that certain 2-alkoxycarbonylamino substituted tetralin carboxamides are potent, selective

Full text of DOI:10.1016/j.bmcl.2005.02.026

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1825–1828
Structure–activity relationship studies on tetralin
carboxamide growth hormone secretagogue receptor antagonists
Hongyu Zhao,* Zhili Xin, Jyoti R. Patel, Lissa T. J. Nelson, Bo Liu,
Bruce G. Szczepankiewicz, Verlyn G. Schaefer, H. Douglas Falls, Wiweka Kaszubska,
Christine A. Collins, Hing L. Sham and Gang Liu
Metabolic Disease Research, Global Pharmaceutical Research and Development, R4MC, AP-10, Abbott Laboratories,
100 Abbott Park Road, Abbott Park, IL 60064-6098, USA
Received 18 January 2005; revised 7 February 2005; accepted 8 February 2005
Abstract—The structure–activity relationship studies on a series of tetralin carboxamide growth hormone secretagogue receptor
(GHS-R) antagonists are discussed. It was found that certain 2-alkoxycarbonylamino substituted tetralin carboxamides are potent,
selective, and orally bioavailable GHS-R antagonists.
Ó 2005 Elsevier Ltd. All rights reserved.
Ghrelin is a 28 amino acid growth hormone secreta-
Cl
O
gogue (GHS) with an n-octanoyl modification on Ser
3.1 As an orexigenic agent, ghrelin may be involved in
short- and long-term regulation of energy balance.
Administration of ghrelin induces food intake in ro-
dents2 and humans,3 and anti-ghrelin IgG reduces body
weight in rats.4 Antagonizing growth hormone secreta-
gogue receptor (GHS-R) with a peptide antagonist re-
sulted in reduction of food intake and body weight
gain in diet induced obese mice.5 An orally active non-
peptidyl GHS-R agonist that stimulates food consump-
NEt2
Cl
O
OtBu
N
N
N
N
O
H
H
O
Me
1
2
IC50 0.13 µM (binding)
0.18 µM (FLIPR)
O
Me
iBuO
OMe
N
NEt2
NEt2
O
O
tion and adiposity in rats was reported recently.6
A
N
N
small molecule GHS-R antagonist is expected to sup-
press food intake and reduce body weight.
H
H
3
4
Relatively few GHS-R antagonists are known in the lit-
erature. Only one non-peptidyl GHS-R antagonist, a 3-
amino-2,3,4,5-tetrahydro-benzo[b]azepin-2-one deriva-
tive (1, Fig. 1), has been reported7 before we disclosed
the discovery of isoxazole (e.g., 2, Fig. 1)8 and tetralin
(e.g., 3 and 4, Fig. 1)9 carboxamide GHS-R antagonists.
As discussed before, the tetralin template was identified
via scaffold manipulation based on the structure–activity
relationship (SAR) studies on the isoxazole carboxam-
ide GHS-R antagonists.9 Here we report the results of
IC50 2.7 µM (binding)
1.4 µM (FLIPR)
IC50 0.016 µM (binding)
0.029 µM (FLIPR)
Figure 1. Known GHS-R antagonists.
the SAR studies on both the tetralin and phenylenedi-
amine portion of the lead compound 3.
The syntheses of the tetralin carboxamide GHS-R
antagonists are outlined in Scheme 1 (3, 4, 15–17, 23,
24, 31–33 were prepared using protocols reported be-
fore8,9). The coupling of carboxylic acid 34 and N,N-
diethylphenylenediamine mediated by 2-(1H-benzotriaz-
ole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate
(TBTU) provided antagonist 6. Cleavage of the methyl
Keywords: Ghrelin; Antagonist; SAR.
*
Corresponding author. Tel.: +1 847 935 4566; fax: +1 847 938
1674; e-mail: hongyu.zhao@abbott.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.02.026

1826
H. Zhao et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1825–1828
Table 1. Modifications of the tetralin portion
OMe
O
a
b
c
NEt2
6
5
7, 8
OH
R1
O
R2
N
H
34
OMe
O
Me
d
e
OMe
No. R1
R2
Binding IC50 FLIPR
(lM)
34
9
IC50 (lM)
35
O
3
H
H
H
H
H
H
2.7
9.5
1.4
5
OH
OMe
OEt
OPr
NDa
0.76
0.20
0.80
0.77
0.61
0.10
0.10
0.018
0.16
0.031
0.039
0.13
0.63
1.20
ND
NH2
O
O
NH
NEt2
O
6
0.60
0.35
1.7
f
7
OH
N
H
8
9
OMe Me
0.69
0.77
0.15
0.12
0.011
0.12
0.047
0.028
0.060
0.37
2.7
36
37
10
11
12
13
14
15
16
17
18
19
20
21
22
23
4
H
H
H
H
H
H
H
H
H
H
H
H
H
NHCO2Et
i
NHCO2 Pr
g
10-14, 18-20
tBuO
O
O
t
NHCO2 Bu
i
NHCO2 Bu
O
h
i
NHCO2Bn
t
NMeCO2 Bu
OH
OMe
i
NMeCO2 Bu
38
39
i
NEtCO2 Bu
HO
O
O
NHCONHiPr
NHSO2Bu
NEt2
j
21,22
N
NHCO(CH2)2CH(CH3)2 >10
CONH(CH2)2CH(CH3)2 2.7
H
ND
40
CO2(CH2)2CH(CH3)2
i
0.32
0.75
0.027
0.029
0.052
OMe NHCO2 Bu
i
OMe N(Me)CO2 Bu
0.018
0.016
0.002
Scheme 1. Reagents and conditions: (a) NH2C6H4NEt2, TBTU, Et3N;
(b) BBr3; (c) K2CO3, EtI (for 7) or PrI (for 8); (d) (1) K2CO3, MeI, (2)
LDA, MeI; (e) (1) LiOH, (2) NH2C6H4NEt2, TBTU, Et3N; (f) (1)
NH2C6H4NEt2, TBTU, Et3N (2) 4 N HCl, dioxane; (g) for 10–14:
corresponding chloroformates, Et3N; for 18: isopropyl isocyanate; for
19: BuSO2Cl, Et3N; and for 20: (CH3)2CHCH2CH2COCl, Et3N; (h)
(1) MeI, K2CO3, (2) LDA, BOC2O; (i) (1) LiOH, (2) NH2C6H4NEt2,
TBTU, Et3N, (3) 4 N HCl, dioxane; (j) (CH3)2CHCH2CH2NH2 (for
21) or (CH3)2CHCH2CH2OH (for 22), TBTU, Et3N.
i
NHCO2 Bu
24
Br
a ND: not determined
2.7 lM) and cellular function assay [fluorescent calcium
indicator (FLIPR), IC50 1.4 lM].10 The SAR studies on
the tetralin portion in 3 identified the 2- and 8-positions
as significant SAR sites and the results are summarized
in Table 1. Placing a hydroxy group at 8-position (5) re-
duced binding potency while a hydrophobic methoxy
group at the same position moderately improved the
activity (6). A larger ethoxy group further increased po-
tency (7), but a propoxy group (8) seemed too bulky
implying the groups at this position interact with a small
hydrophobic binding pocket on the receptor. Halogens
such as chloro9 or bromo groups (e.g., 24) appeared
optimal for binding at this position although the func-
tional (FLIPR) IC50 for 24 correlated poorly with its
binding IC50.
group in 6 mediated by BBr3 yielded antagonist 5.
Alkylation of the phenolic hydroxy group in 5 with ethyl
iodide and propyl iodide produced antagonists 7 and 8,
respectively. Methylation of the carboxyl group in 34
followed by methylation of the a-position of the result-
ing ester carbonyl group gave 35. Antagonist 9 was ob-
tained by saponification of 35 followed by a TBTU
coupling reaction with N,N-diethylphenylenediamine.
Amine 37 could be obtained by a similar TBTU cou-
pling reaction between 36 and N,N-diethylphenylenedi-
amine followed by the HCl removal of the tert-
butoxycarbonyl (BOC) group. Antagonists 10–14 and
18–20 were prepared by coupling 37 with the corre-
sponding chloroformates, isocyanate, sulfonyl chloride,
and acid chloride, respectively. Methylation of the carb-
oxyl group in carboxylic acid 38 followed by carbonyl-
ation of the a-position of the resulting ester provided
diester 39.9 Selective saponification of the methyl ester
in 39 followed by a TBTU coupling reaction with
N,N-diethylphenylenediamine and the HCl removal of
tert-butyl group gave carboxylic acid 40, which was con-
verted to antagonists 21 and 22 under TBTU coupling
reaction conditions with 3-methylbutyl amine and 3-
methylbutyl alcohol, respectively.
The study of substitutions at the 2-postion of the tetralin
template were directed to further expand the SAR to
search for more potent antagonists and to provide steric
protection for the amide group toward hydrolysis, which
was suspected as a contributing factor to the poor phar-
macokinetic (PK) profiles of the tetralin carboxamides
without any substitutions at the 2-position [e.g., the
rat oral bioavailability (F) for 6 was 0.6% (Table 3)].
The carbamate groups proved to be the most efficient
substituents at this position. While the potencies of carb-
amates 10, 11, 12, and 14 demonstrated a general posi-
tive correlation with their sizes, isobutyl carbamate 13
showed a sharp increase of both binding and FLIPR po-
tency (IC50 11 and 18 nM, respectively). A library of
ꢀ70 similar carbamates were then synthesized and as-
sayed. A variety of carbamates were tolerated but 3–6
The prototype tetralin carboxamide 3 showed only weak
antagonist activity in both receptor binding (IC50

H. Zhao et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1825–1828
1827
Table 3. The rat PK data for selected GHS-R antagonists
carbon long aliphatic carbamates typically demon-
strated better affinity. Methylation of the carbamate
nitrogen in 12 led to a more potent antagonist 15, but
the same operation on 13 resulted in a weaker antagonist
(16) indicating more potent compounds like 13 might in-
duce receptor conformation changes. Increased size of
this N-alkyl group further reduced potency (17).
NEt2
OMe
O
R1
N
H
R2
CLp
No. R1
R2
F
Binding
FLIPR
(%) (L/hkg) IC50 (lM) IC50 (lM)
6
H
H
H
H
0.6 3.52
0.60
0.76
A variety of other substituents at the 2-position were
exploited and the most active antagonist in each class
is shown in Table 1. For the alkyl substituents, a methyl
group was best tolerated (9) while larger alkyl groups
tend to reduce potency. An optimized urea (18) showed
moderate improvement while even the best sulfonamides
(e.g., 19), amides (e.g., 20), and reversed amides (e.g., 21)
studied showed neutral or negative effects on the
activity.
4
iBuOCONMe
iBuOCONH
iBuOCONH
19
18
1.32
1.72
1.29
1.35
0.016
0.018
0.031
0.010
0.029
0.027
0.020
0.033
23
32
33
Me 15
iBuOCONMe Me 21
Antagonist 31 also lacks an electron rich N,N-dialkyl-
1,4-phenylenediamine group, which could be a metaboli-
cally unstable fragment in most other antagonists dis-
cussed in this report.
The beneficial effects of the substituents at the 2- and 8-
positions of the tetralin template are not independent.
For example, compound 23 showed almost comparable
potency as 13 in both binding and FLIPR assays, which
again suggests tight binding groups might induce recep-
tor conformation changes.
The study of tetralin carboxamide GHS-R antagonists
was initiated in response to the poor pharmacokinetic
profiles of the first generation isoxazole carboxamides.9
One strategy was to quaternize the a-position of the
amide carbonyl group so that the amide is more resis-
tant toward metabolic hydrolysis. Although oral bio-
The SAR results on the phenylenediamine portion are
summarized in Table 2. The ortho-methylation (25,
and 32, 33 from Table 3) was tolerated while N,N-
diethyl group was fairly sensitive toward modifications.
A larger N,N-dipropyl analog 26 barely registered in the
binding assay and a yet larger N,N-dibutyl analog 27
was still 10 times weaker than its N,N-diethyl counter-
part 13. A closely related N,N-diisobutyl analog (28)
was completely inactive. Close analogs such as conforma-
tionally restrained N-pyrrolidinyl and N-morpholinyl
compounds 29 and 30 were inactive in binding assay
at the highest concentration tested (10 lM). In sharp
contrast to the SAR observed in isoxazole carboxamide
GHS-R antagonists,11 trans-cyclohexyldiamine analog
31 showed 100-fold loss of potency compared to 13.
However, antagonist 31 should be more water-soluble
than 13 due to its increased basicity and flexibility.12
availability is
a difficult parameter to rationally
improve,13 this strategy appeared to workreasonably
well. For example, a compound that obeys LipinskiÕs
rules14 but lacks a quaternized a-carbon to the amide
carbonyl group (6) showed only a 0.6% rat oral bioavail-
ability (Table 3). In contrast, larger compounds with
more hydrogen bond donors and acceptors but contain-
ing a quaternized a-carbon to the amide carbonyl group
demonstrated significantly improved rat oral bioavail-
abilities (Table 3). The reduced clearance of these com-
pounds over that of 6 (Table 3) suggests the bulky
substituents at the a-carbon to the amide carbonyl
group might play a role in stabilizing these molecules.
Several potent antagonists (4, 13, and 24) were subjected
to a GPCR selectivity study and they showed only weak
activity toward a panel of receptors (IC50 > 33 lM for
adrenergic, histaminergic, muscarinic, and dopaminer-
gic receptors). These compounds were also tested for
hERG channel blockade and they all demonstrated
Table 2. Modifications of the phenylenediamine portion
O
O
weakaffinity in this assay (IC
for 4, 13, and 24, respectively).
6.1, >10, and 7.7 lM
50
R1
iBuO
NH
R3
O
NH
NEt2
O
O
N
H
N
H
In conclusion, a series of potent and selective tetralin
carboxamide GHS-R antagonists were identified and
studied. Some potent GHS-R antagonists also demon-
strated reasonable rat oral bioavailability.
R2
25-30
31
No.
R1
R2
R3
Binding
IC50 (lM)
FLIPR
IC50 (lM)
tBu
iBu
iBu
iBu
tBu
tBu
—
Me
H
NEt2
NPr2
NBu2
NiBu2
0.092
9.1
0.078
ND
0.12
ND
ND
ND
5.6
Acknowledgements
25
26
27
28
29
30
31
H
0.19
>10
>10
>10
1.4
We thankDr. David W. A. Beno for determining rat
oral bioavailability, Mr. Gilbert Diaz and Ms. Sandra
Leitza for running hERG assay, and Ms. April Miao
of Millennium Pharmaceuticals, Inc. for generating the
GPCR selectivity data.
H
H
N-pyrrolidinyl
N-morpholinyl
—
H
—

1828
H. Zhao et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1825–1828
6. Lugar, C. W.; Clay, M. P.; Lindstrom, T. D.; Woodson,
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