Drug design and synthesis of a novel κ opioid receptor agonist with an oxabicyclo[2.2.2]octane skeleton and its pharmacology

Article abstract of DOI:10.1016/j.bmcl.2009.11.027

A conformational analysis of κ opioid receptor agonists, TRK-820 and U-50,488H indicated an active conformation of TRK-820 in which the C-ring was in the boat form with the 14-OH interacting with the amide nitrogen. Based on the obtained active conformation of TRK-820, we designed and synthesized a novel κ agonist KNT-63 with oxabicyclo[2.2.2]octane skeleton. KNT-63 showed profound antinociceptive effects via the κ receptor which were as potent as that of TRK-820.

Full text of DOI:10.1016/j.bmcl.2009.11.027

Bioorganic & Medicinal Chemistry Letters 20 (2010) 121–124
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Drug design and synthesis of a novel
j
opioid receptor agonist with
an oxabicyclo[2.2.2]octane skeleton and its pharmacology
a
a
a
a
Hiroshi Nagase a, , Akio Watanabe , Toru Nemoto , Noriyuki Yamaotsu , Kohei Hayashida ,
*
Mayumi Nakajima b, Ko Hasebe b, Kaoru Nakao b, Hidenori Mochizuki b, Shuichi Hirono a, Hideaki Fujii a
a School of Pharmacy, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan
b Pharmaceutical Research Laboratories, Toray Industries, Inc., 6-10-1, Tebiro, Kamakura, Kanagawa 248-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A conformational analysis of
formation of TRK-820 in which the C-ring was in the boat form with the 14-OH interacting with the
amide nitrogen. Based on the obtained active conformation of TRK-820, we designed and synthesized
j opioid receptor agonists, TRK-820 and U-50,488H indicated an active con-
Received 10 October 2009
Revised 4 November 2009
Accepted 6 November 2009
Available online 13 November 2009
a novel
tive effects via the
j
agonist KNT-63 with oxabicyclo[2.2.2]octane skeleton. KNT-63 showed profound antinocicep-
receptor which were as potent as that of TRK-820.
Ó 2009 Elsevier Ltd. All rights reserved.
j
Keywords:
Opioid
j
Agonist
Active conformation
Conformational analysis
Three types of opioid receptors (
l, d,
j) are now well estab-
and synthesized 4,5-epoxymorphinan derivative 1 (Fig. 1) having
lished not only by pharmacological studies but by molecular bio-
an oxabicyclo[2.2.2]octane skeleton. Herein, we report the drug
logical studies.1 Narcotic addiction is believed to be derived from
l
receptor type, and therefore d and
j
types are promising drug
receptor,
C-ring
OH
targets for analgesics without addiction. A putative
e
Cl
14
Me
N
O
which has not yet been cloned, has also been proposed as another
opioid receptor type and much pharmacological data support its
existence.2 To obtain ideal analgesics without addiction and other
N
O
N
Cl
O
O
Me
N
OH
side effects derived from the
ious kinds of naltrexone derivatives and have reported selective
ligands for , d, and putative
receptors.3–5 Quite recently, one
of our designed
selective agonists, TRK-820 (Fig. 1),3a,b was
l receptor, we have synthesized var-
TRK-820
U-50,488H
j
e
O
O
8
O
j
NR1R2
OH
O
N
launched in Japan as an antipruritic for patients undergoing dialy-
N
H
N
sis. Although many arylacetamide derivatives such as U-50,488H6
(Fig. 1) and U-69,5937 were synthesized and developed as
j ago-
O
nists, all of these derivatives were eliminated from clinical trials
as not only analgesics but also as antipruritics because of their seri-
ous side effects like psychotomimetic8 and aversive8,9 reactions.
On the other hand, TRK-820 has neither aversive nor addictive ef-
fects. We were interested in the differences in the pharmacological
effects between TRK-820 and the arylacetamide derivatives, and
carried out the conformational analyses of them and detailed SAR
investigations of TRK-820 derivatives to develop the working
hypothesis that the C-ring in TRK-820 would be in the boat form
in its active conformation.3c Based on this hypothesis, we designed
OH
OH
NS22
Designed compound 1
O
8
N
Me
OH
N
O
OH
TAN-821
* Corresponding author. Tel.: +81 3 5791 6372; fax: +81 3 3442 5707.
E-mail address: nagaseh@pharm.kitasato-u.ac.jp (H. Nagase).
Figure 1. Structures of TRK-820, U-50,488H, NS22, TAN-821, and designed
compound 1.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.11.027

122
H. Nagase et al. / Bioorg. Med. Chem. Lett. 20 (2010) 121–124
design and synthesis of the target compounds possessing the
oxabicyclo[2.2.2]octane skeleton and their pharmacological
properties.
O
H
O
O
N
N
Me
O
H
N
OH
H
TRK-820 contains a tyrosine–glycine moiety that is a common
structural unit for endogenous opioid peptides. The representative
exogenous opioid morphine also has the same functionality, but
the arylacetamide derivatives like U-50,488H have no tyrosine–
glycine moiety. To clarify the common overlapping structural units
between TRK-820 and a typical arylacetamide U-50,488H, we car-
ried out computer analyses.10 The analyses indicated that both li-
gands had a basic nitrogen and an amide side chain as common
overlapping pharmacophores (Fig. 2). The former would interact
with an aspartic acid residue in the third transmembrane region
in the receptor,11 while the latter would play a role in recognition
O
OH
Figure 4. Comparison between active conformation of TRK-820 and designed
compound (R1 = Ph, R2 = H; KNT-63 (1a)). The side chain in TRK-820 or the designed
compound was indicated by red or blue color, respectively.
OH
OH
N
N
CO2Et
6'
j receptor (so-called j
address site).12 However, U-50,488H did
i)
of
O
O
O
O
not contain a phenolic structure, which is generally thought to be
an important pharmacophore in opioid ligands.13 It is worthwhile
noting that the C-ring in TRK-820 was in the boat conformation in
its active conformation, thereby elevating the amide side chain,
OMe
OMe
2
3a (6'R) 48%
3b (6'S) 21%
that is, the
conformation, we were unable to define any common overlapping
structural units between the two agonists. The obtained active
TRK-820 conformation was also supported by detailed SAR investi-
gations of TRK-820 derivatives and used for a drug design of new
agonist NS22 (Fig. 1).3c Although NS22 showed dose-dependent
antinociceptive effects via the receptor in hot-plate and tail-flick
j
address.14 Until we proposed the active hypothetical
ii)
complex mixture
3a
j
iii)
recovery of 3a
j
Scheme 1. Reagents and conditions: (i) NaH, ClCH2CO2Et, THF, rt; (ii) NaH, DMF,
90 °C; (iii) CSA, DMF, 110 °C.
j
tests, the analgesic effects induced by NS22 were less potent than
for TRK-820. A careful comparison of structures between TRK-820
and NS22 led us to find that the position of the amide side chain in
TRK-820 (red color), which was held by an interaction between the
14-OH and the amide nitrogen, was oriented a bit upper than that
of NS22 (blue color, Fig. 3). The spatial difference of the amide side
O
7
O
N
H
N
OH
i)
3
O
chain’s orientation may have significant influence on
j potency
OR
and selectivity. Based on this consideration, we designed an oxabi-
cyclo[2.2.2]octane derivative 1 whose amide side chain seemed to
be located at almost the same position as that of the active confor-
mation of TRK-820 (Fig. 4).
We attempted to synthesize an intermediate 3a to afford de-
signed compound 1 using Darzens reaction (Scheme 1). Naltrexone
4a (7S, R = Me) 60%
4b (7R, R = Me) 80%
ii)
ii)
1a (KNT-63: 7S, R = H) 72%
1b (KNT-62: 7R, R = H) 39%
Scheme 2. Reagents and conditions: (i) n-BuLi or NaH, PhNH2, THF, reflux; (ii) BBr3,
CH2Cl2, rt.
methyl ether 214c,15 was treated with NaH and ethyl chloroacetate
in THF to give epoxyesters 3 as an R and S mixture (2:1) at the 60
position.16 We attempted to convert the resulting 60R-epoxyester
3a into the oxabicyclo[2.2.2]octane derivative by intramolecular
cyclization. However, this approach gave either a complex product
mixture under basic conditions (NaH in DMF) or led to recovery of
starting epoxyester 3a under acidic conditions (camphor sulfonic
acid (CSA) in DMF). These results were probably derived from the
activation of the ester group by the inductive effect of the epoxy
ring under basic conditions or were due to difficulty of forming a
carbonium ion at 60 position by the inductive effect of the ester
group under acidic conditions. We next tried to transform the ester
group into a more inactive amide group before cyclization. Fortu-
nately, the amidation of 3a with aniline and n-BuLi in THF under
reflux directly afforded the objective cyclized amide 4a in 60%
yield. 60S-Isomer 3b was also transformed to the corresponding
amide 4b under the same reaction conditions in 80% yield (except
NaH was used instead of n-BuLi). In these transformations, no epi-
merizations were observed.16 Amides 4a and 4b were demethylat-
ed with BBr3 in CH2Cl2 to provide 1a (KNT-63) and 1b (KNT-62),
respectively (Scheme 2).
Figure 2. Superimposition of active conformations of TRK-820 (white) and U-
50,488H (orange).
O
H
O
N
O
Me
H
O
N
O
N
H
O
OH
The affinities of both compounds KNT-62 (1b) and KNT-63 (1a)
Figure 3. Comparison between active conformation of TRK-820 and NS22. The side
chain in TRK-820 or NS22 was indicated by red or blue color, respectively.
for j and l opioid receptors were stronger than that of naltrexone

H. Nagase et al. / Bioorg. Med. Chem. Lett. 20 (2010) 121–124
123
tent than morphine).3a,b These results support our working
hypothesis that the position of the amide side chain in KNT-63
Table 1
Binding affinity of KNT-62 (1b), KNT-63 (1a), TRK-820, and naltrexone for opioid
receptorsa
(1a) would play an important role for
j agonist activity and selec-
b
c
Compound
Ki (
l
)
(nM)
Ki (j)
(nM)
Ki (d)d (nM)
tivity. The pharmacological effects of KNT-63 (1a) support our
hypothesis that the C-ring in an active TRK-820 conformation
may be in the boat form with an interaction between the 14-OH
and the amide nitrogen.
KNT-62 (1b)
KNT-63 (1a)
TRK-820
0.205
0.212
0.582
0.335
0.152
0.111
0.225
0.373
3.23
2.73
96.5
20.7
Naltrexone
The location of the amide side chain in KNT-63 (1a), a bit upper
a
Binding assay was carried out in duplicate using homogenate of guinea-pig
than that of NS22, would lead to more
j selectivity and more
brain (j: cerebellum, l and d: forebrain).
potent analgesic activity for KNT-63 (1a). Detailed SARs of KNT-
63 (1a) derivatives are now under investigation.
b
[3H]DAMGO was used.
[3H]U-69,593 was used.
[3H]NTI was used.
c
d
We also reported that putative
the bicyclo[2.2.2]octene skeleton showed no
antinociceptive effect induced by TAN-821 was not antagonized
by either nor-BNI, antagonist b-FNA, or NTI. Only b-endorphin
[1–27], which is a prototypical putative antagonist, antagonized
the analgesia produced by TAN-821, which means TAN-821 would
be a putative agonist not a , and d agonist. Other TAN-821
derivatives having the bicyclo[2.2.2]octene skeleton also showed
putative selectivity. The structures of KNT-63 (1a) and TAN-821
e
agonist TAN-821 (Fig. 1) with
j
selectivity.5 The
(Table 1). KNT-63 (1a) showed more
(Ki = 0.111 nM for and Ki = 0.212 nM for
(1b) (Ki = 0.152 nM for
ated the antinociceptive effect induced by sc-administered KNT-63
(1a) using the acetic acid writhing test (AAW test). A sc-adminis-
tration of KNT-63 (1a) produced a strong antinociceptive effect
(ED50 = 0.0063 mg/kg) in a dose-dependent manner (Fig. 5A). This
j
l
selective affinity
l
j
) than did KNT-62
e
j
and Ki = 0.205 nM for l). We then evalu-
e
j, l
e
differ by the type of the atom at the 8 position, that is, O in
analgesia was antagonized by
antagonist naloxone (NLX) and d antagonist NTI (Fig. 5B). NS22
showed the same affinity for (Ki = 0.134 nM) and receptors
(Ki = 0.135 nM) and antinociceptive effect (ED50 = 0.1 mg/kg)
through
receptor.3c As a result, KNT-63 (1a) showed slightly
higher selectivity for the receptor and a 16-fold improved anal-
gesic activity in comparison to NS22.
The analgesic activity of KNT-63 (1a) was almost the same as
that of TRK-820 (ED50 = 0.0033 mg/kg) in AAW test (100-fold po-
j antagonist nor-BNI, but not by l
KNT-63 (1a) and C in TAN-821. The 14-OH in TRK-820 also played
an important role for
effect of the atom (C, O, or N) at the 8 position on
j
selectivity.3b,c We are now investigating the
l
j
j
selectivity.
In conclusion, a novel 4,5-epoxymorphinan derivative with an
oxabicyclo[2.2.2]octane skeleton, KNT-63 (1a) was designed and
synthesized on the basis of an active TRK-820 conformation, the
boat conformation, with interaction between the 14-OH and the
amide nitrogen. The designed KNT-63 (1a) showed stronger anal-
j
j
gesia than that of the previously reported
j agonist NS22 whose
amide side chain was positioned a bit lower than that of KNT-63
(1a). The antinociceptive effect of KNT-63 (1a) (ED50 = 0.0063
mg/kg) was almost as potent as that of TRK-820.
Acknowledgments
We acknowledge the financial supports from Shorai Foundation
for Science and the Uehara Memorial Foundation. We also
acknowledge the Institute of Instrumental Analysis of Kitasato Uni-
versity, School of Pharmacy for its facilities.
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*
**
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**
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H. Nagase et al. / Bioorg. Med. Chem. Lett. 20 (2010) 121–124
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OH
OH H
H
H
N
H
O
N
CO2Et
H
O
CO2Et
O
O
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OMe
3a
3b
14. The C-ring in 4,5-epoxymorphinan derivatives would be usually in chair-like
form. Therefore, the 6b-substituent may be in quasi-equatorial position to
extend horizontally. In the active conformation of TRK-820, the C-ring would
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O
O
H
N
H
H
OH
H
O
N
H
OH
O
H
H
H
N
N
O
O
OMe
OMe
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4a
4b