Structure-activity relationship studies of carboxamido-biaryl ethers as opioid receptor antagonists (OpRAs). Part 1

Article abstract of DOI:10.1016/j.bmcl.2007.08.008

A structurally unique and new class of opioid receptor antagonists (OpRAs) that bear no structural resemblance with morphine or endogenous opioid peptides has been discovered. A series of carboxamido-biaryl ethers were identified as potent receptor antagonists against mu, kappa and delta opioid receptors. The structure-activity relationship indicated para-substituted aryloxyaryl primary carboxamide bearing an amine tether on the distal phenyl ring was optimal for potent in vitro functional antagonism against three opioid receptor subtypes.

Full text of DOI:10.1016/j.bmcl.2007.08.008

Bioorganic & Medicinal Chemistry Letters 17 (2007) 5349–5352
Structure–activity relationship studies of carboxamido-biaryl
ethers as opioid receptor antagonists (OpRAs). Part 1
Kumiko Takeuchi,* William G. Holloway, Jamie H. McKinzie, Todd M. Suter,
Michael A. Statnick, Peggy L. Surface, Paul J. Emmerson, Elizabeth M. Thomas,
Miles G. Siegel, James E. Matt, Chad N. Wolfe and Charles H. Mitch
Lilly Research Laboratories, A Division of Eli Lilly and Company, Indianapolis, IN 46285, USA
Received 9 July 2007; revised 5 August 2007; accepted 7 August 2007
Available online 11 August 2007
Abstract—A structurally unique and new class of opioid receptor antagonists (OpRAs) that bear no structural resemblance with
morphine or endogenous opioid peptides has been discovered. A series of carboxamido-biaryl ethers were identified as potent recep-
tor antagonists against mu, kappa and delta opioid receptors. The structure–activity relationship indicated para-substituted aryloxy-
aryl primary carboxamide bearing an amine tether on the distal phenyl ring was optimal for potent in vitro functional antagonism
against three opioid receptor subtypes.
Ó 2007 Elsevier Ltd. All rights reserved.
In recent years obesity among the general public is
increasing at an alarming rate and in an epidemic pro-
portion especially among children and teenagers.1 The
frequent association of serious clinical conditions such
as diabetes and cardiovascular diseases with obesity
has become a serious concern in the medical community,
and therefore a pharmacological intervention for reduc-
ing weight in obese subjects is warranted. Opioid recep-
tor (mu, kappa, and delta) agonists are known to
stimulate, while antagonists against opioid receptors in-
hibit, food consumption in preclinical obesity models.2
Obesity and fasting elevates endogenous opioid peptides
in animal models and man.3 Reduced consummatory
behaviors appear to be most pronounced when animals
are obese or fed a palatable cafeteria diet containing
large amounts of a preferred macronutrient.4 A current
hypothesis is that opioid receptor antagonists produce
their effects on food intake by preventing central reward
mechanisms that occur when overeating a preferred or
palatable diet and/or by preventing the craving associ-
ated with dieting/abstinence. In this manner, opioidergic
control of appetite for palatable energy-dense foods may
share common neural substrates responsible for the
development of nicotine, alcohol, and narcotic depen-
dence and addiction. Human clinical studies with nal-
trexone, an opioid receptor antagonist, demonstrated
short-term effects on food intake, albeit at doses that
were higher than needed to precipitate opiate with-
drawal.5 Hepatotoxicity limited the dose of naltrexone
utilized in chronic studies and failed to demonstrate con-
sistent long-term effects on body weight. Therefore, the
clinical hypothesis evaluating the efficacy of an opioid
receptor antagonist (OpRA) as a treatment for obesity
has yet to be fully investigated.
Previous efforts at Lilly have demonstrated that
LY255582 is a nonselective OpRA that is very potent
and efficacious in rat obesity models.6 This effect ap-
pears to be mediated via antagonizing all three mu, kap-
pa, and delta receptor subtypes. LY255582 was found to
have substantially better anti-obesity activity in rats
than the clinically approved OpRAs such as naloxone
or naltrexone. Unfortunately, the poor oral bioavail-
ability and unacceptable margin of safety resulting from
irritation at the site of drug administration of LY255582
via various routes precluded its clinical development. In
the course of our efforts to find a new class of OpRAs,
we discovered carboxamido-biaryl ethers that were
structurally unrelated to the opiate morphine (Fig. 1).
We then launched structure–activity relationship
(SAR) studies to develop an orally efficacious OpRA
with activity comparable to that of LY255582 with few-
er side effects for the treatment of obesity. In this paper
Keywords: Opioid receptor antagonists (OpRAs); Carboxamido-biaryl
ethers; Obesity.
*
Corresponding author. Tel.: +1 317 276 6771; fax: +1 317 433
0715; e-mail: ktak@Lilly.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.08.008

5350
K. Takeuchi et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5349–5352
OH
OH
OH
OH
O
O
O
N
OH
N
R
OH
N
H
Naloxone
R =
Morphine
LY255582
Naltrexone
R =
Antagonist from structural
simplification of morphine
Opioid antagonists
derived from morphine
O
O
()n
()m
H
N
NH2
R
N
H
A2
NH2
O
A1
O
N
Carboxamido-biaryl ethers
structurally unrelated to morphine
2
Figure 1. Opioid receptor antagonists.
we report initial findings of our SAR study efforts on the
series of novel carboxamido-biaryl ether opioid receptor
antagonists.
para-orientation between the aryloxy group and the car-
boxamide on the pyridine ring (2) was required for the
opioid receptor antagonism at the three receptor
subtypes.
In our early SAR study, we identified 6-(4-(2-benzylami-
noethyl)phenoxy)nicotinamide (2) as an initial lead hav-
ing good binding affinities at mu, kappa, and delta
opioid receptors (Table 2). We then explored the SAR
surrounding the lead structure 2. We first examined
the regiochemical effect of the carboxamide functional-
ity. The compounds were readily prepared by methods
as shown in Scheme 1. The results in Table 1 show that
We then sought a replacement of the carboxamide by
converting the 6-aryloxynicotinonitrile intermediate to
various functionalities R (Scheme 2 and Table 2). The
results in Table 2 indicate that a primary amide (2 and
9) is optimal for antagonizing the three opioid receptors.
The substituents on the amide nitrogen were not well
tolerated especially at kappa and delta receptors (12–
1) K2CO3, DMF, 100 o
2) (For X,Y = CN only)
30% H2O2, K2CO3, DMSO
3) TFA,:CH2Cl2
4) benzaldehyde, MeOH
3Å MS, NaBH4, rt
C
Y
W
Cl
N
O
N
O
N
+
O
N
X
O
N
O
Compound A
X,Y = H, W = CONH2
Y,W = H, X = CN
X,W = H, Y = CN
Scheme 1.
Table 1. Effects of carboxamide regiochemistry on the opioid receptor binding affinities
H
N
O
NH2
O
N
Compound
Amide position
Receptor binding affinity at high Na, Ki (nM)7
Mu
Kappa
Delta
1
2
3
4
2
3
4
5
643.47 45.10
7.44 2.94
>2500
>5000
>5000
70.21
136.26 59.55
>5000
>5000
>5000
>5000
>2500

K. Takeuchi et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5349–5352
5351
O
N
( )m
1) TFA:CH2C2
( )m
H2N
1) (BOC)2O, THF
2) K2CO3, DMF,
6-chloronicotinonitrile
O
N
H
2) Aldehyde, MeOH,
3Å MS, NaBH4
OH
O
N
N
( )n
R
( )m
( )
( )m
n N
H
N
H
O
N
O
N
Scheme 2.
Table 2. Effects of bioisosteric replacement of the carboxamide on the opioid receptor binding affinities
R
( )
( )m
n N
H
O
N
Compound
m
n
R
Receptor binding affinity at high Na, Ki (nM)7
Mu
Kappa
Delta
2
5
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
CONH2
CN
7.44 2.94
1076.86 179.58
>2500
136.26 59.55
>5000
>5000
70.21
>5000
6
CH2NH2
CO2H
>5000
>5000
7
>2500
>2500
>5000
>5000
a
8
SO2NH2
CONH2
C(NH)NH2
>5000
1.16 0.23
214.73 6.35
9
0.17 0.02
20.55 1.93
234.53 15.41
52.29 2.20
57.15 1.07
52.27 13.74
389.49 61.60
8.38 2.30
1147.5 145.98
>5000
10
11
12
13
14
15
C(NH)NHMe
CONHMe
CONHEt
3458.36 244.32
718.45 33.98
673.58 27.70
2414.41 38.48
>5000
>5000
2808.74
>5000
>5000
CONHi-Pr
COPipb
a Benzenesulfonamide was prepared from 4-fluorobenzenesulfonyl chloride and [2-(4-hydroxyphenyl)ethyl]carbamic acid tert-butyl ester as starting
materials.
b Pip = 1-piperidine.
15). Particularly detrimental seems to be a tertiary or a
cyclic amide (15). Replacement of the amide with ami-
dine (10 and 11) also suffered significant loss in binding
affinities at kappa and delta receptors.
tween the terminal and middle phenyl rings. Interest-
ingly, the phenethylaminomethyl compound 9 (where
m = 1 and n = 2) exhibited an order of magnitude
greater affinities at all three receptors than the benzyla-
minoethyl compound 2 (where m = 2 and n = 1). This
prompted us to examine further the optimal position
of the secondary amine nitrogen in the in vitro func-
tional antagonism (Table 3). By fixating the linker
Along with this SAR we also examined the positioning
of secondary amine nitrogen of the amine tether by
interchanging the carbon chain length (Cm and Cn) be-
Table 3. Effects of the position of amine tether of the 6-phenoxynicotinamide on the in vitro functional antagonism at the opioid receptors
O
( )
( )m
n N
H
NH2
O
N
Compound
m
n
GTPcS functional antagonism, Kb (nM)8
Mu
Kappa
Delta
2
9
2
1
2
2
3
3
0
1
1
2
2
3
1
0
3
1
2.25 0.47
0.07 0.02
16.56 2.49
18.47 3.34
22.26 0.33
>1000
18.41 7.99
1.15 0.24
23.34 1.93
24.15 1.87
45.83 1.83
>1000
23.14 6.88
0.97 0.14
128.07 52.19
211.63 83.06
495.99 12.24
>1000
16
17
18
19
20
21
>1000
4.09 1.1.44
>1000
2.47 0.36
>1000
54.15 16.31

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K. Takeuchi et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5349–5352
Table 4. Effects of the replacement of pyridyl ring with other aryl rings9 on the opioid receptor antagonism
O
NH2
N
H
A2
A1
O
Compound
A1
A2
Receptor binding affinity at high Na, Ki (nM)7
GTPcS functional antagonism, Kb (nM)8
Mu
Kappa
Delta
Mu
Kappa
Delta
9
22
N
C
C
C
C
N
0.17 0.02
0.29
8.38 2.30
15.04
1.16 0.23
1.88 0.17
8.54 1.40
4.82 0.49
16.31 0.28
0.07 0.02
0.15 0.01
0.44 0.12
0.043 0.008
0.59 0.02
1.15 0.24
1.22 0.33
9.60 1.14
0.32 0.03
2.99 0.10
0.97 0.14
0.92 0.11
2.86 0.30
1.19 0.21
11.06 0.32
23
LY255582
Naltrexone
1.77 0.16
0.15 0.01
0.87 0.02
69.27 22.70
4.68 0.83
5.28 0.12
atom numbers to four in the chain, the nitrogen was
moved along the chains from m = 0–3 (2, 9, 19, and
20). The position of nitrogen benzylic from the middle
phenyl ring (9 where m = 1 and n = 2) was confirmed
optimal for the in vitro antagonism, as the same trend
seen with the binding affinities. The loss of antagoniz-
ing activity with an aniline link either at the terminal
phenyl (19) or the middle phenyl (20) ring indicates
that a basic nitrogen is required for the receptor antag-
onism. We also changed the number of carbon linkers
n = 1–3 while maintaining m = 2 (2, 16, and 17). As
the chain length increased, functional antagonism
decreased especially at the delta receptor. These results
suggest that the four-atom linker between the two phe-
nyl rings may be optimal for the in vitro antagonism
(also seen with 2 vs. 18). This was also confirmed by
the shorter linker compound 21 (where m = n = 1)
which was less potent than 9 at all three receptors, par-
ticularly at delta.
References and notes
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With these findings, we then explored other aryl amides
as a potential replacement for the nicotinamide (Table
4). Benzamido- (22) and 2-pyridinecarboxamido- (23)
biaryl ethers exhibited comparable binding affinities
at the three opioid receptors as the nicotinamide 9,
albeit the 2-pyridinecarboxamide 23 was much less
potent at the kappa receptor. However, all the carbox-
amido-biaryl ethers possessed excellent in vitro func-
tional antagonist activities at all three receptors,
which were several fold more potent than the binding
affinities.
7. SPA binding affinity for cloned human mu and kappa
(defined by [3H]-diprenorphine binding), and delta (defined
by [3H]-bremazocine binding) opioid receptors expressed in
CHO cells under high sodium conditions (n P 2). Under low
sodium binding conditions the affinity was reduced at the
receptor subtypes, consistent with findings on the prototype
Lilly 4-phenylpiperidine opioid antagonist series: see Emm-
erson, P. J.; McKinzie, J. H.; Surface, P.; Suter, T. M.; Mitch,
C. H.; Statnick, M. A. Eur. J. Pharmacol. 2004, 494, 121.
8. In vitro functional assay, inhibiting agonist stimulated G-
protein activation (measured using GTPcS binding) in
CHO membranes expressing the cloned human mu, kappa,
and delta receptors (n P 2). In the present study, opioid
ligand binding and GTPcS functional assays were con-
verted to a homogeneous SPA permitting the development
of simpler assays with dramatically increased throughput.
Optimization in the presence of sodium chloride was
designed to bias these assays toward the detection of
opioid antagonists: see Rodgers, G.; Hubert, C.; McKinzie,
J.; Suter, T.; Statnick, M.; Emmerson, P.; Stancato, L.
Assay Drug Dev. Technol. 2003, 1, 627.
In conclusion, we have discovered potent opioid
receptor antagonists in vitro, which are structurally
unrelated to the typical opiate morphine. Further
SAR exploration of each series of carboxamido-biaryl
ethers shown above will be reported separately in due
course.
9. 5-Fluoro-2-pyridinecarboxamide was prepared from 2-
amino-5-fluoropyridine via Sandmeyer reaction followed
by ester/amide formation or direct amidation of 2-bromo-5-
fluoropyridine, the Sandmeyer product, with CuCN in
DMF.
Acknowledgment
We thank scientists at LOB labs at Lilly Research Lab-
oratories for the SPA binding assay data generation.