Discovery of tetralin carboxamide growth hormone secretagogue receptor antagonists via scaffold manipulation

Article abstract of DOI:10.1021/jm0491750

A case study of rational design of an efficient, specific, and proprietary molecular scaffold based on the structure-activity relationship (SAR) information on a screening hit is described. Potent, selective, and orally bioavailable tetralin carboxamide g

Full text of DOI:10.1021/jm0491750

© Copyright 2004 by the American Chemical Society
Volume 47, Number 27
December 30, 2004
Letters
peptide with an n-octanoyl modification on Ser 34 is a
growth hormone secretagogue and may be also involved
in short- and long-term regulation of energy balance.
As an orexigenic agent, ghrelin is more potent than
melanin-concentrating hormone (MCH) but less potent
than neuropeptide Y (NPY).5 Administration of ghrelin
induces food intake (rodent6 and human7), and anti-
ghrelin IgG reduces body weight.8 Antagonizing GHS-R
with a peptide antagonist resulted in reduction of food
intake and body weight gain in diet-induced obese mice.9
A small molecule GHS-R antagonist is expected to have
similar beneficial effects in obesity therapy.
Discovery of Tetralin Carboxamide
Growth Hormone Secretagogue Receptor
Antagonists via Scaffold Manipulation
Hongyu Zhao,* Zhili Xin, Gang Liu,
Verlyn G. Schaefer, H. Douglas Falls,
Wiweka Kaszubska, Christine A. Collins, and
Hing L. Sham
Metabolic Disease Research, Global Pharmaceutical
Research and Development, Abbott Laboratories,
Abbott Park, Illinois 60064-6098
Received October 14, 2004
We have reported the discovery of isoxazole carbox-
amide-based GHS-R antagonists previously (e.g., com-
pound 1, Figure 1).10 The only other nonpeptidyl GHS-R
antagonist, a 3-amino-2,3,4,5-tetrahydro-benzo[b]azepin-
2-one derivative, was reported in 1993.11 GHS-R ago-
nists, on the other hand, are well documented in the
literature. However, most of these compounds were
optimized toward stimulating pituitary growth hormone
release rather than regulating food intake.5 An orally
active nonpeptidyl GHS-R agonist that stimulates food
consumption and adiposity in rats was reported re-
cently.12
A high throughput screening of Abbott Laboratories
compound collection identified isoxazole carboxamide 1
(Figure 1) as a GHS-R antagonist with a binding IC50
of 130 nM. Compound 1 was from a library of amides
made via a one-step amide coupling reaction between
commercially available carboxylic acids and amines.
Extensive SAR exploration led to several low nanomolar
antagonists, but no compound that demonstrated a rat
oral bioavailability (F) higher than 5% was identified
(F for 1 was 3.9%). The only significant SAR site in 1 is
the methyl group in the central isoxazole ring.10 Ana-
logues that carry certain larger substituents at this
position showed increased potency, but at the expense
of increased molecular weights and flexibility, which
may contribute to their poor pharmacokinetic (PK)
profiles.13 It was clear that modification of the skeleton
of these compounds was necessary to improve their PK
properties. Besides molecular weight and flexibility, the
Abstract: A case study of rational design of an efficient, spe-
cific, and proprietary molecular scaffold based on the structure-
activity relationship (SAR) information on a screening hit is
described. Potent, selective, and orally bioavailable tetralin
carboxamide growth hormone secretagogue receptor (GHS-R)
antagonists were discovered. Union of rational design and high
throughput synthesis provided a quick access to high quality
chemical leads.
Identification of molecular scaffolds of biological sig-
nificance is an important and difficult task in drug
discovery.1 Current approaches include direct screening,
structure-based design, ligand-based virtual screening,2
and combinatorial synthesis.3 Here we disclose a case
study of rational molecular scaffold design guided by
structure-activity relationships (SAR). Potent, selec-
tive, and bioavailable tetralin carboxamide growth
hormone secretagogue receptor (GHS-R) antagonists
were discovered using this approach.
Obesity is now recognized as a major health issue in
the developed world. Although lack of physical activity
and eating habits contribute to much of the obesity
epidemic, drug therapy is likely a realistic solution to
the problem. Medicines that are capable of reducing food
intake can be a potential way of regulating energy
balance to achieve weight loss. Ghrelin, a 28 amino acid
* To whom correspondence should be addressed: R4MC, AP-10,
Abbott Laboratories, 100 Abbott Park Road, Abbott Park, IL 60064-
6098. Tel: 847-935-4566. Fax: 847-938-1674. E-mail: hongyu.zhao@
abbott.com.
10.1021/jm0491750 CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/07/2004

6656 Journal of Medicinal Chemistry, 2004, Vol. 47, No. 27
Letters
Figure 2. Representative tetralin carboxamides.
group toward hydrolysis. Efforts to quaternize the amide
carbonyl R-carbon with eight different classes of sub-
stituents [R (alkyls), ROCdO, RNHCdO, RNH, RCONH,
RSO2NH, RNHCONH, and ROCONH] were thus initi-
ated. We were pleased to find that carbamate 3 dem-
onstrated a 22-fold improvement of potency (binding
IC50 120 nM, Figure 2) over 2 while the best analogue
of each of other classes showed binding activity between
700 nM to 9000 nM. Once the carbamate functional
group was found to be the optimal group to branch the
amide carbonyl R-carbon, a library of similar carba-
mates were synthesized and isobutyl carbamate 4 was
identified as a more potent compound (binding IC50 11
nM, Figure 2). Routine follow-up SAR studies were then
initiated. Noticeably, placing a chloro group at the
8-position of the tetralin moiety in 4 further increased
the potency (5, binding IC50 2 nM, Figure 2). Other small
hydrophobic groups at this position showed similar but
less favorable effects on potency. Unfortunately, the rat
oral bioavailability for compound 5 was only 3.8%.
Replacing the chloro group in 5 with a methoxy group
and methylation of the carbamate nitrogen resulted in
6, which showed good potency (IC50 16 nM) and reason-
able rat oral bioavailability (19%, Figure 2). The im-
proved clearance (CLp, 1.3 L/h‚kg) for 6 over 1 (3.7
L/hr‚kg) suggested 6 might be metabolically more stable
than 1.
Figure 1. Designed scaffolds based on SAR of 1. Conservative
atoms in B ring during the scaffold hopping are highlighted.
metabolic instability (hydrolysis) of the amide group in
compound 1 was also suspected to account for part of
its poor PK profile. We decided to modify the central
isoxazole ring (B ring, Figure 1) in 1 so that the R-carbon
of the carbonyl in the new scaffolds could be quaternized
to stabilize the amide bond toward hydrolysis. The SAR
study showed that the B ring in 1 could be replaced by
several 5- or 6-memebered aromatic ring replacements
without losing much potency, which suggested that the
role of the B ring in 1 is largely to hold the A ring and
C ring in the right orientation.14 Therefore, the B ring
appeared to be the site that would be most likely to
tolerate more dramatic skeleton changes. Several as-
pects should be balanced in the design process in order
to identify a high quality hit with a reasonable success
rate. First, the new scaffolds should “resemble” that of
compound 1 to retain GHS-R antagonism. From the
SAR study on compound 1, the dihedral angle between
the A ring and the plane of the amide group appeared
important for potency. The distance and relative orien-
tation between the A ring and the amide group were
thus the main parameters to optimize in searching for
new scaffolds. Second, the properties of designed scaf-
folds should be tunable, meaning several closely re-
sembled scaffolds should be synthetically accessible so
a continuous area in chemical space rather than a point
or several widely distributed points could be evalu-
ated. In fact, we regard the tunable nature of the de-
signed scaffolds to accommodate the imperfection of the
design hypotheses an important aspect of lead-like-
ness.15 Finally, the designed scaffolds should be suitable
for further SAR studies. We thus designed 10 closely
related scaffolds as shown in general structure i in
Figure 1. By changing the nature and size of X in i, the
distance and relative orientation between A ring and
the amide group in i were systematically evaluated and
a tetralin carboxamide was identified as an appropriate
hit (2, binding IC50 2700 nM, Figure 1).16 Although
antagonist 2 is 20-fold less active than antagonist 1, it
carries some advantageous characteristics. The tetralin
2-carboxamide scaffold has not been exploited much,
and its analogues might possess improved PK profiles.
Several sites on the tetralin ring can be readily modified
to build SAR, and these relatively complex compounds
would be less likely to interact with other biological
targets and be more selective.17
In a fluorescent calcium indicator (FLIPR) assay that
measures the compounds’ ability to inhibit a ghrelin-
induced increase in intracellular [Ca2+] in CHO-K cells,
compound 6 demonstrated potent functional antagonism
of GHS-R (IC50 ) 29 nM). This compound also showed
good selectivity over a panel of GPCR receptors (IC50
>33 µM for adrenergic, histaminergic, muscarinic, and
dopaminergic receptors) and hERG channel (IC50 6.1
µM), which demonstrated the excellent specificity this
unique, relatively complex tetralin scaffold provided.
Compounds 1-3 were synthesized from the corre-
sponding commercial carboxylic acids and N,N-dieth-
ylphenylenediamine via a 2-(1H-benzotriazol-1-yl)-
1,1,3,3,-tetramethyluronium tetrafluoroborate (TBTU)-
mediated amide formation reaction. The syntheses of
4-6 are outlined in Scheme 1. TFA removal of the tert-
butoxycarbonyl (BOC) group in 3 followed by reacting
i
with BuOCOCl produced carbamate 4. Synthesis of 5
began with known amino acid 7.18 Esterification of the
carboxylic acid in 7 in acidic methanol followed by
carbamate formation with iBuOCOCl, saponification of
the ester, and coupling to N,N-diethylphenylenediamine
gave 5. Methylation of the carboxyl group in 8 followed
by condensation with (tBuO)2CO provided bis-ester 9.
Selective hydrolysis of the less hindered methyl ester
in 9 followed by Curtius rearrangement gave carbamate
The SAR plan around this new scaffold was directed
to further improve the potency and stabilize the amide

Letters
Journal of Medicinal Chemistry, 2004, Vol. 47, No. 27 6657
Scheme 1. Synthesis of Antagonists 4-6a
Bemis, G. W.; Murcko, M. A. The Properties of Known Drugs.
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a
i
Reagents and conditions: (a) 1. TFA; 2. BuOCOCl, Et3N;
(b) 1. MeOH, HCl; 2. iBuOCOCl, Et3N; 3. NaOH(aq); 4.
p-NH2C4H6NEt2, TBTU, Et3N; (c) 1. MeI, K2CO3; 2. LDA, BOC2O;
i
(d) 1. LiOH; 2. (Ph)2(PdO)N3; 3. BuOH; (e) 1. MeI, NaH; 2.TFA;
3. p-NH2C4H6NEt2, TBTU, Et3N.
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In conclusion, based on the SAR information on a hit
compound (1) synthesized in one step from commercially
available materials, we have identified a more complex,
specific, efficient, and proprietary scaffold. Potent, selec-
tive, and bioavailable GHS-R antagonists were discov-
ered. The key to this approach was to use the SAR
information as guidance to conceive a cluster of closely
resembled and synthetically accessible scaffolds that are
suitable for further SAR studies. This approach exhib-
ited a good success rate and synergized the molecular
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Acknowledgment. We thank Dr. David W. A. Beno
for determining rat oral bioavailability, Mr. Gilbert Diaz
and Ms. Sandra Leitza for running the hERG assay, and
Ms. April Miao of Millennium Pharmaceuticals, Inc., for
generating the GPCR selectivity data.
(16) Other scaffolds will be disclosed elsewhere.
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Supporting Information Available: Experimental pro-
cedures and spectral characterization of compounds 1-6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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References
(1) It has been shown that shapes of 50% of marked drugs could be
described by the 32 most frequently occurring frameworks.
JM0491750