Novel delta opioid receptor agonists with oxazatricyclodecane structure

Article abstract of DOI:10.1021/ml400491k

We synthesized compounds 4a,c-f,h,i containing the oxazatricyclodecane structure from a novel rearrangement reaction product 2a. All the prepared compounds 4a,c-f,h,i exhibited full agonistic activities for the δ opioid receptor (DOR). Among them, the N-methyl derivative 4c was highly selective, and the most effective DOR agonist in functional assays. Subcutaneous administration of 4c produced dose-dependent and NTI (selective DOR antagonist)-reversible antinociception lacking any convulsive behaviors in the mice acetic acid writhing tests. The N-methyl derivative 4c is expected to be a promising lead compound for selective DOR agonists with a novel chemotype.

Full text of DOI:10.1021/ml400491k

Letter
pubs.acs.org/acsmedchemlett
Novel Delta Opioid Receptor Agonists with Oxazatricyclodecane
Structure
Hideaki Fujii,*,† Kohei Hayashida,† Akiyoshi Saitoh,‡ Akinobu Yokoyama,§,∥ Shigeto Hirayama,†
Takashi Iwai,† Eriko Nakata,⊥ Toru Nemoto,† Yuka Sudo,§,∥ Yasuhito Uezono,§ Mitsuhiko Yamada,‡
and Hiroshi Nagase†,#
†School of Pharmacy, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan
‡Department of Neuropsychopharmacology, National Institute of Mental Health, National Center of Neurology and Psychiatry, 4-1-1
Ogawahigashimachi, Kodaira, Tokyo 187-8553, Japan
§Division of Cancer Pathophysiology, National Cancer Center Research Institute, 5-1-1 Tsukiji, Tokyo 104-0045, Japan
∥Faculty of Pharmaceutical Science, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 274-8510, Japan
⊥Discovery Research Laboratories, Nippon Chemiphar Co., Ltd., 1-22 Hikokawado, Misato, Saitama 341-0005, Japan
S
* Supporting Information
ABSTRACT: We synthesized compounds 4a,c−f,h,i containing the
oxazatricyclodecane structure from a novel rearrangement reaction
product 2a. All the prepared compounds 4a,c−f,h,i exhibited full
agonistic activities for the δ opioid receptor (DOR). Among them, the
N-methyl derivative 4c was highly selective, and the most effective
DOR agonist in functional assays. Subcutaneous administration of 4c
produced dose-dependent and NTI (selective DOR antagonist)-
reversible antinociception lacking any convulsive behaviors in the
mice acetic acid writhing tests. The N-methyl derivative 4c is expected
to be a promising lead compound for selective DOR agonists with a
novel chemotype.
KEYWORDS: Opioid, DOR, oxazatricyclodecane structure, CellKey
he δ opioid receptor (DOR) is one of the three opioid
receptor types (μ (MOR), DOR, and κ (KOR)), and
more selectively than the DOR homomer.16 It is not yet clear
why the various DOR agonists mentioned above elicit different
pharmacological responses, but the structure of the DOR
agonist may account, in part, for their distinct activities. For
example, a structural feature of DOR agonists may influence the
induction of convulsive behaviors: the DOR agonists that do
not cause convulsion had a structure distinct from diary-
lmethylpiperazine and its related structures such as BW373U86
and SNC80.3 However, diarylmethylpiperazine derivative
AZD2327 (Figure 1) reportedly produced no convulsion.17
The synthesis and pharmacological characterization of DOR
agonists with different chemotypes will help to better
understand the different pharmacological profiles of distinct
DOR agonists. We have recently reported the synthesis and
binding affinities for the MOR, DOR, and KOR of an
oxazatricyclodecane derivative 2a, which was obtained from
endoethanotetrahydrothebaine derivative 1 by a novel rear-
rangement reaction18 (Scheme 1). This new compound
exhibited moderate affinities for the opioid receptors (Ki
(MOR) = 47.7 nM, Ki (DOR) = 174.6 nM, and Ki (KOR)
T
activation of this receptor is associated with various
pharmacological effects such as antinociceptive, antidepressive,
anxiolytic, and cardioprotective effects.1−3 In contrast to the
undesirable effects mediated by the MOR such as dependence,
constipation, emesis, and respiratory depression or the aversive
effects mediated by the KOR,4,5 the DOR is a promising
medical target because it seems to induce neither addictive nor
aversive effects. Since the first nonpeptidic DOR agonist TAN-
676,7 (Figure 1) emerged,3 various nonpeptidic DOR agonists
have been reported.1−3 Several investigations revealed that the
DOR agonists like BW373U868 and SNC809 (Figure 1)
exerted convulsive behaviors.3 However, some DOR agonists
such as ADL574710 and KNT-12711,12 (Figure 1) have recently
been reported to induce no convulsion. Although SNC80 has
been reported to induce the internalization of the DORs and to
develop tolerance toward the analgesic, locomotor, and
anxiolytic effects, ARM39013 (Figure 1) induced hardly any
internalization of the DORs and showed tolerance to analgesia
but not to locomotor or anxiolytic responses.14,15 Thus, a
distinct DOR agonist interacting with the same DOR
sometimes exerted different pharmacological responses.
Recently, SNC80, a well-known representative selective DOR
agonist, was reported to activate the MOR/DOR heteromer
Received: November 29, 2013
Accepted: January 27, 2014
Published: January 27, 2014
© 2014 American Chemical Society
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ACS Medicinal Chemistry Letters
Letter
a
Scheme 3. Synthesis of 4h and 4i
Figure 1. Structures of DOR agonists, TAN-67, BW373U86, SNC80,
ADL5747, KNT-127, ARM390, and AZD2327.
a
Reagents and conditions: (a) 12 M NH3aq, EtOH, rt, 73%; (b) t-
BuOK, t-BuOH, reflux, quant.; (c) CH2ClBr, K2CO3, DMF (0.0004
M), rt, a solution of 2g and 7g or 2h and 7h in DMF was added
portion-wise. 73−93%; (d) 60% NaH, PhCH2CH2Br, DMF, rt, 83%;
(e) BBr3, CH2Cl2, 0 °C, 61−89%.
Scheme 1. Potential Opioid Ligand 2a
Table 1. Binding Affinities of 4a,c−f,h,i for the Opioid
a
Receptors
Ki (nM)
selectivity
b
c
d
compd
SNC80
MOR
DOR
KOR
MOR/DOR
KOR/DOR
695
1.04
175
>1000
248
668
0.27
10.0
12.0
26.6
55.8
17.7
8.6
962
1.4
a
e
Scheme 2. Synthesis of 4a,c−f
2a
47.7
3.14
23.3
186
4a
4c
4d
4e
4f
0.313
1.94
7.00
1.23
2.59
0.534
1.16
5.14
200
16.4
103
17.0
46.2
227
3.2
119
68.4
45.9
4.61
1.75
56.6
588
4h
4i
1.69
1.94
1.5
1.7
a
Binding assays were carried out in duplicate using mouse whole brain
without cerebellum membranes for MOR and DOR or guinea pig
b
c
cerebellum membranes for KOR. [3H] DAMGO was used. [3H]
d
e
DPDPE was used. [3H] U-69,593 was used. Ref 18.
=248.1 nM). The potential opioid ligand 2a was expected to
lead to other ligands selective for an opioid receptor type with a
unique core structure. Herein, we report the synthesis of novel
DOR agonists 4a,c−f,h,i with oxazatricyclodecane structure
derived from 2a and their pharmacological properties.
a
Reagents and conditions: (a) Troc-Cl, K2CO3, 1,1,2,2-tetrachloro-
The synthesis of the objective compounds 4a,c−f com-
menced with compound 2a18 (Scheme 2). The treatment of 2a
with 2,2,2-trichloroethyl chloroformate (Troc-Cl) in the
presence of K2CO3 and the subsequent zinc/AcOH treatment
gave norcompound 2b.19 Various N-substituents were intro-
duced by reductive alkylation of 2b or the alkylation of 2b with
an alkyl bromide to provide 2c−f. Compound 2a reacted with
CH2Br2 in the presence of K2CO3 under high dilution
ethane, 150 °C; (b) Zn, AcOH, rt, 80% from 2a; (c) aldehyde, AcOH,
NaB(OAc)3H, 1,2-dichloroethane, rt, 74%-quant. (for R = Me, 2-
phenethyl); (d) alkyl bromide, NaHCO3, DMF, rt, 48−92% (for R =
allyl, i-Bu); (e) CH2Br2, K2CO3, DMF (0.0005 M), rt, 66%; (f)
CH2ClBr, K2CO3, DMF (0.0004 M), rt, a solution of 2c−f in DMF
was added portion-wise. 69−98%; (g) BBr3, CH2Cl2, 0 °C, 76−95%.
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ACS Medicinal Chemistry Letters
Letter
a
Table 2. Functional Activities of 4a,c−f,h,i for the Opioid Receptors Assessed by [35S]GTPγS Binding Assays
MOR
DOR
KOR
b
c
d
compd
EC50 (nM)
Emax (%)
EC50 (nM)
Emax (%)
EC50 (nM)
Emax (%)
e
e
e
e
SNC80
4a
NT
NT
1.9
100
92.8
112
96.4
94.6
115
118
103
NT
NT
2.8
113
223
2.7
2.3
9.0
2.1
13.7
110
8.4
1.1
80.5
478
760
231
69.1
83.6
65.6
74.0
4c
11
4d
15.6
6.5
4e
5.4
f
f
4f
83.0
25.6
19.7
9.2
ND
ND
42.9
51.2
4h
0.98
0.41
6.5
4i
3.9
a
b
[35S]GTPγS binding assays were carried out in duplicate using human MOR, DOR, or KOR expressed CHO cells. Emax was calculated as the % of
c
d
the response obtained with DAMDO. Emax was calculated as the % of the response obtained with SNC80. Emax was calculated as the % of the
response obtained with U-69,593. Not tested. Not determined.
e
f
a
Table 3. Functional Activities of 4a,c−f,h,i for the Opioid Receptors Assessed by CellKey Assays
MOR
DOR
KOR
b
c
d
compd
EC50 (nM)
Emax (%)
EC50 (nM)
Emax (%)
EC50 (nM)
Emax (%)
SNC80
4a
0.14
1.8
6.8
1.7
100
88.7
138
130
77.2
108
123
90.6
5264
39.8
307
5.9
9.6
1.54
141
140
2.0
50.5
79.2
57.9
4c
1350
400
5.1
36.1
12.2
11.3
40.2
8.8
4d
333
e
e
4e
ND
ND
4f
639
1.2
20.5
0.39
0.62
12530
1.2
22.6
80.4
75.8
4h
4i
5.2
4.8
2.4
a
b
CellKey assays were carried out in duplicate using human MOR, DOR, or KOR expressed HEK293 cells. Emax was calculated as the % of the
c
d
response obtained with DAMGO. Emax was calculated as the % of the response obtained with SNC80. Emax was calculated as the % of the response
obtained with (−)-U-50,488H. Not determined.
e
Figure 2. (a) Antinociceptive effect of 4c administered subcutaneously in the mice acetic acid writhing tests. The statistical significance of differences
between the groups was assessed with one-way ANOVA followed by Bonferroni’s test. *p < 0.05 and ***p < 0.001 versus saline treated mice. (b)
Effects of opioid receptor antagonists on the antinociception induced by subcutaneous treatment of 4c in the mice acetic acid writhing tests. The
statistical significance of differences between the groups was assessed with one-way ANOVA followed by Bonferroni’s test. ***p < 0.001 versus
saline treated mice. #p < 0.05 versus 4c treated mice.
conditions (0.0005 M) to provide dioxymethylene compound
3a in 66% yield concomitantly with a dimer in 30% yield in
which two 2a units were tethered with a methylene group (see
the Supporting Information for details). A portion-wise
addition of a solution of 2c−f markedly improved the yields
of 3c−f and prevented formation of the dimer. Finally, the O-
methyl group in 3a,c−f was removed by a treatment with BBr3
to give 4a,c−f. Compounds 4h and 4i with respective phenyl
and 2-phenethyl groups as the lactam nitrogen substituents
were prepared as shown in Scheme 3. After a conversion of
ester 5 into 6, the treatment of 6 with t-BuOK in t-BuOH
provided an equilibrium mixture of 2g and 7g. An equilibrium
mixture of 2h and 7h was prepared from 5 by a previously
reported method.18 The mixture of 2g and 7g or 2h and 7h was
reacted with CH2ClBr in the same manner shown in Scheme 2
to afford dioxymethylene compounds 3g,h. The 2-phenethyl
group was introduced on the lactam nitrogen in 3g by
alkylation to give 3i.
The affinities of the prepared compounds 4a,c−f,h,i were
evaluated by competitive binding assays (Table 1). All the
compounds 4a,c−f,h,i bound to the opioid receptors. The
phenolic hydroxy group at the 3-position appeared to play an
important role in improving the binding affinities for the opioid
receptors compared to the parent compound 2a.20 Except for
N-(2-phenethyl)lactam 4i, compounds 4a,c−f,h showed
selectivities for the DOR, suggesting that the phenyl group of
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ACS Medicinal Chemistry Letters
Letter
Present Address
the substituent on the lactam nitrogen would function as a
DOR address such as the phenyl moiety in NTI.21,22The
binding affinities of 4a and 4h for the DOR were better than
that of SNC80. Compounds 4c and 4f with respective N-
methyl and N-(2-phenethyl) substituents were over 100-fold
more selective for the DOR as compared to the KOR. The
functional activities of 4a,c−f,h,i were determined by
[35S]GTPγS binding and CellKey assays (Tables 2 and 3).23
The CellKey system utilizes impedance biosensors for
detection of cell behaviors and is a functional cell-based assay
technology enabling label-free analysis of cell surface receptor
activity.24,25 It is noteworthy that the [35S]GTPγS and CellKey
assays differed in the observed output, even though giving
similar results. All the compounds 4a,c−f,h,i were full agonists
for the DOR. The agonistic activities for the DOR of 4c,f,h
were more efficacious than that of SNC80 in both of the
functional assays. Compounds 4h and 4i were also potent KOR
agonists, whereas compounds 4c and 4f exhibited agonistic
activities for the MOR. Although N-methyl derivative 4c had
moderate to high efficacy for the MOR and KOR, the potencies
for these receptors were poor, which suggested that 4c was
highly selective and the most efficacious DOR agonist among
the tested compounds. Derivatives 4a,e,f with respective
cyclopropylmethyl (CPM), allyl, and 2-phenethyl substituents
on the basic nitrogen were more potent agonists for the DOR
than N-methyl derivative 4c in both functional assays; however,
their functional selectivities for the DOR were lower than that
of 4c in [35S]GTPγS binding assays and lower or comparable to
that of 4c in CellKey assays. Therefore, the N-methyl
substituent on the basic nitrogen appeared to be the optimal
group among the tested compounds.
We next assessed the antinociceptive effects of 4c in mice by
acetic acid writhing tests. Subcutaneously administered 4c
significantly exhibited antinociception in a dose-dependent
manner and its EC50 value was 5.26 mg/kg (Figure 2a). No
convulsive behaviors were observed. The antinociceptive effects
induced by 4c were attenuated by the selective DOR antagonist
NTI but not by the selective MOR antagonist β-FNA or the
selective KOR antagonist nor-BNI (Figure 2b). Taken together,
these results indicate that compound 4c could be a promising
lead compound for selective DOR agonists with a novel
chemotype, the oxazatricyclodecane structure
In conclusion, we synthesized novel DOR agonists 4a,c−f,h,i
with oxazatricyclodecane structure. Among the synthesized
compounds, N-methyl derivative 4c was highly selective and
the most effective DOR agonist. Subcutaneous administration
of 4c produced dose-dependent and NTI-reversible antinoci-
ception without any convulsive behaviors. N-Methyl derivative
4c is expected to be a promising lead compound for selective
DOR agonists containing the novel oxazatricyclodecane
structure.
#(H.N.) International Institute for Integrative Sleep Medicine,
University of Tsukuba, 1−1−1 Tennodai, Tsukuba, Ibaraki
305−8577, Japan.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
This work was partly supported by JSPS KAKENHI
(23590133) [Grant-in-Aid for Scientific Research (C)] (H.F)
and Adaptable and Seamless Technology Transfer Program
through target-driven R&D, JST (AS2315040G) (to H.F, K.H,
S.H, T.I, E.N., T.N., and H.N.).
Notes
The authors declare no competing financial interest.
ABBREVIATIONS
■
Bn, benzyl; CHO, chinese hamster ovary; CPM, cyclo-
propylmethyl; DAMGO, [D-Ala2, N-Me-Phe4, Gly-ol5]-enke-
phalin; DOR, δ opioid receptor; DPDPE, [D-Pen2, D-Pen5]-
enkephalin; β-FNA, β-funaltrexamine; HEK, human embryonic
kidney; KOR, κ opioid receptor; MOR, μ opioid receptor; nor-
BNI, nor-binaltorphimine; NTI, naltrindole; Troc, 2,2,2-
trichloroethoxycarbonyl
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures for the synthesis and characterization
of the compounds, the in vitro activity assay, the in vivo mice
acetic acid writhing assay, and the spectral data of the reported
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*(H.F.) E-mail: fujiih@pharm.kitasato-u.ac.jp.
■
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ACS Medicinal Chemistry Letters
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