Highly selective and potent μ opioid ligands by unexpected substituent on morphine skeleton

Article abstract of DOI:10.1016/j.bmcl.2009.07.119

Unexpected substituent on the well-known morphine skeleton is described to be account for highly selective and potent μ opioid ligands, which is strongly connected to substituted aromatic groups on this omitted 8α-position.

Full text of DOI:10.1016/j.bmcl.2009.07.119

Bioorganic & Medicinal Chemistry Letters 20 (2010) 418–421
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Highly selective and potent
on morphine skeleton
l opioid ligands by unexpected substituent
Wei Li a, , Yi-Min Tao b, , Yun Tang c, Xue-Jun Xu b, Jie Chen b, Wei Fu a, Xing-Hai Wang a, Bo Chao a,
b,
Wei Sheng a, Qiong Xie a, Zhui-Bai Qiu a, , Jing-Gen Liu
*
*
a Department of Medicinal Chemistry, School of Pharmacy, Fudan University, 138 Yixueyuan Road, Shanghai 200032, China
b State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China
c School of Pharmacy, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Unexpected substituent on the well-known morphine skeleton is described to be account for highly
Received 10 April 2009
Revised 15 July 2009
Accepted 27 July 2009
Available online 29 July 2009
selective and potent
l
opioid ligands, which is strongly connected to substituted aromatic groups on this
omitted 8 -position.
a
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
l
Opioid agonists
Selective ligands
Oripavines
Thebaine (Fig. 1) is a naturally occurred alkaloid isolated from
opium. Though it bears much similar structure with morphine,
thebaine is inactive as analgesic with very low affinities to opioid
receptors.1 However, it could be converted to a variety of potent
narcotics2 (e.g., Buprenorphine, Oxycodone) widely used in clinic
through several organic transformations. And the Diels–Alder reac-
tion of thebaine with dienophiles is obviously the most valuable
one among these reactions, which leads to the discovery of extre-
mely highly active analgesics than morphine.3 And due to less
selective characters1 of these so-called oripavines yielded from
the transformations of thebaine, the tritiated form of Diprenor-
phine and Etorphine has been used universally for opioid
receptors.
It is widely accepted that subtle differences in critical structural
loci of bioactive compounds may result in great changes in biolog-
ical responses. And these may provide useful information on drug-
receptor interactions and on receptor characteristics.4 Since the
greatly enhanced biological activities were suggested strongly to
be linked with the substituent on C7 position which may be
accommodated at the postulated lipophilic domain on the opioid
receptor,5 it was desirable to investigate other cycloadducts of the-
experimentally and theoretically.6 However, the 8
a-adducts have
been not considered seriously because not only are they not acces-
sible through the cycloaddition of thebaine and dienophiles to
date, but also significant decrease in analgesic potency7 and
reduced affinities8 to opioid receptors have been observed in the
known analogue in comparison to their main 7a-counterparts.
In this Letter we reported unexpected 8 -adducts yielded from
a
thebaine and styrenes which bear obviously different SARs (Struc-
ture–Activity Relationships) with other oripavines. And the substi-
tuent on the C8-position is strongly linked with
l opioid agonistic
activity and selectivity which has been omitted before.
A straightforward synthetic strategy was developed (Scheme 1),
and thebaine was preferably attacked by styrenes from the less
bulky b face9 to afford 6
a, 14a-endo-ethenotetrahydrothebaine.
In addition to known 7a-adducts yielded as main products, unex-
pected by-products were separated in case of styrene, m-nitrosty-
rene and p-nitrostyrene. Follow-up works examined compound-5
by X-ray crystallography (Fig. 2, compound-5, CCDC 696689) and
found it was a regioisomeric 8a-adduct in comparison to its main
7a-adduct (compound-4, CCDC 696688). Similar large deshielding
effect to the proximity of the tertiary amine is observed for H-8b in
1H NMR spectra of all regioisomeric adducts, as well as character-
istic resonances of H7b upfield due to absence of substitution
(Table 1). And normal 7b-adducts as were absent (Table 1).
In the absence of the polarizing methoxy functionality at C-6 of
thebaine, Diels–Alder addition to 3-buten-2-one can give a regio-
isomeric adduct in which the acetyl group is attached at C-8.7
And the Diels–Alder addition to styrenes may afford similar
baine with dienophiles besides the well investigated 7a-adducts
* Corresponding authors. Tel.: +86 21 54237595; fax: +86 21 54237264 (Z.-B.Q.);
tel.: +86 21 50807588 (J.-G.L.).
E-mail addresses: zbqiu@shmu.edu.cn (Z.-B. Qiu), jgliu@mail.shcnc.ac.cn (J.-G.
Liu).
Both authors contribute equally to this work.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.07.119

W. Li et al. / Bioorg. Med. Chem. Lett. 20 (2010) 418–421
419
H3C
N
H3C
N
N
8
C(CH3)2OH
7
O
O
O
H3CO
OCH3
HO
OH
HO
Diprenorphine
OCH3
Thebaine
Morphine
CH3
N
8
CH3
CH2CH2CH3
OH
OCH3
7
O
HO
Etorphine
Figure 1. The structures of thebaine, morphine, diprenorphine and etorphine.
CH3
CH3
N
N
xylene, reflux
CH=CH2
+
H3CO
O
H3CO
O
thebaine
R
H3CO
H3CO
R
R
8α-adducts
7α-adducts
2
R= H
1
o-NO2
3
4
6
m-NO2
p-NO2
5
7
Scheme 1. The Diels–Alder reactions of thebaine and styrenes.
Figure 2. X-ray crystal structures of compound-4(left) and -5(right).
regioisomeric 8
a
-adducts, since these dienophiles are also devoid
heterotrimeric G protein, and has been used as a functional mea-
sure for determination of potencies and efficacies of agonists. Thus,
the agonistic activities of these compounds were determined by
of polarizing functionality in their structures which are different
to other dienophiles such as 3-buten-2-one and acrylonitrile.
Although other factors such as solvent, harsh conditions may take
effects, the trend of Diels–Alder reactions forwarded to main 7
adducts has not changed in all of these cases.
regulation of the binding of [35S]GTP
c
S (Table 3). Since most com-
pounds have no undetectable affinities for d- and -opioid recep-
tors at the concentrations up to 10 M, only opioid receptor
was employed in the functional assays. The binding profiles of
these compounds for three types of opioid receptors ( , d, ) have
been shown in Table and the agonistic activity of these
compounds for opioid receptors has been shown in Table 3.
a
-
j
l
l
Affinities of these compounds to opioid receptors were deter-
mined by competitive inhibition of [3H] diprenorphine binding
l
j
(Table 2). Ligand regulation of the binding of [35S]GTP
c
S is one of
2
the most widely used methods to measure receptor activation of
l

420
W. Li et al. / Bioorg. Med. Chem. Lett. 20 (2010) 418–421
Table 1
Isolated yields and characteristic resonances of H7b upfield and H8b downfield of 8
a
-adducts in comparison to 7
a-adducts
Compound entry
Dienophilea
7a
-Adductb -Adductb
8
a
dH7b ppm
dH8b ppm
1
2
40.9
83.6
3.04
2.16
3.32–3.27
4.28
CH=CH2
1.3
CH=CH2
NO2
3
3.71
3.46
CH=CH2
4
5
6
75.6
71.7
3.18–3.14
2.25–2.20
3.40–3.34
4.44
O2N
O2N
2.7
3.0
3.16
3.38
4.41
CH=CH2
7
2.24–2.17
a
The ratio of dienophile versus diene (thebaine) was 3:1.
(Isolated yield)%.
b
(e.g., compound-3, 4) almost abandoned activities against all opi-
oid receptors. However, same substituents on 8 -position show
different pharmacological profiles on opioid receptor with en-
Table 2
a
Affinity values for the binding of compounds to
l
-, d- and
j
-opioid receptorsa
l
Ki (nM)
hanced affinities and selectivity which becomes even more signif-
icant when a nitro-group is introduced. And the Ki values of
compound-5 and compound-7 are 22.0 8.6 and 674.0
117.1 nM, respectively. Although compound-5 bears similar affini-
l
d
j
1
2
4
5
6
7
>5000
>5000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
76.9 16.3
ties with morphine,10 it is highly selective to
while morphine reserves certain affinities to
l
opioid receptor
>10,000
22.0 8.6
>10,000
674.0 117.1
6.9 1.3
l
and j-opioid recep-
tors. Follow-up functional assays demonstrated that both com-
pounds bear similar potencies and efficacies with morphine with
the EC50 values to be 302.3 7.5 (compound-5) and 751.0
95.0 nM (compound-7) and Maximal stimulation values of
221 2.8 (compound-5) and 248 8.6 (compound-7), respectively.
Morphine
116.8 15.4
a
Membrane from CHO cell expressing
l
-, d- and
j-opioid receptors were incu-
bated with varying concentrations of compounds in the presence of 0.45 nM
[3H]diprenorphine as described under ‘Experimental section’ in Supplementary
data. Data are expressed as Mean SEM for at least three determinations performed
in triplicate.
Compound-5 is potent and highly selective
based on these pharmacological assays. The substituent on 8
sition accounting for opioid selectivity and activity of this
compound is different from that on 7 -position producing non-
l
opioid agonist
a-po-
l
a
Table 3
selective opioids although these two positions are sterically close
to each other. And this difference in SAR results seemed to be con-
Stimulation of [35S]GTP S binding to membrane receptors by compoundsa
c
[
35S]GTP
c
S
nected to the substituted aromatic substituent on 8
which may selectively increase the agonistic activities against
opioid receptor, but not d- or -opioid receptors. Further investiga-
tions on this specific position will be reported later.
a-position
l
EC50 (nM)
Maximum (% of basal)
j
1
2
4
5
6
7
>10,000
>10,000
NDb
175 6.0
174 5.2
NDb
Morphine is a standard therapeutic agent of the treatment of
moderate-to-severe pain. However, its clinical use is greatly lim-
ited due to adverse side effects such as tolerance and dependence.
Lack of exclusive selectivity for individual receptor subtype might
be linked to its high potential to develop tolerance and depen-
dence.11 Thus, the findings of the present study may have potential
importance for the development of new opioid analgesics with less
liability to develop tolerance and dependence.
302.3 7.5
221 2.8
NDb
NDb
751.0 95.0
123.5 30.5
248 8.6
209 15.9
Morphine
a
Assays were performed in membranes prepared from the cell expressing
l
opioid receptor with varying concentrations of compounds in the presence of
0.1 nM [35S]GTP
c
S as described under ‘Material and methods’. Data are expressed
as the Mean SEM for at least three experiments performed in triplicate.
b
In conclusion, unexpected 8a-adducts from thebaine and dieno-
ND: not determinable, which means that the compounds nearly have no ability
to stimulate [35S]GTP
cS binding to membrane receptors (inactive).
philes were reported, and it was proposed that the appearance of
these by-products may be connected to the structural features of
dienophiles. Furthermore, compound-5 is potent and highly selec-
Unlike typical oripavines (e.g., diprenorphine and etorphine)
with flexible acyclic hydrophobic substituent on the 7 -position,
more compact and rigid aromatic groups attached directly to
-adducts result in reduced affinities to all opioid receptors.
Furthermore, more bulky nitro-group substituted analogues
tive
l opioid agonist, which is connected to the substituent on the
a
8a-position on morphine skeleton. Further investigation on this
omitted position may be valuable to provide useful information
for drug-receptor interactions since it is still in darkness for
detailed interactions between opioid receptors and opioids, and
7a

W. Li et al. / Bioorg. Med. Chem. Lett. 20 (2010) 418–421
421
opioids with novel pharmacological profiles from the classical
References and notes
structure of analgesics.
1. Aldrich, J. V.; Vigil-Cruz, S. C.. In Burger’s Medicinal Chemistry & Drug Discovery;
Abraham, D. J., Ed.; John Wiley and Sons Inc.: New York, 2003; Vol. 6, pp 329–482.
2. Synthesis of Essential Drugs; Vardanyan, R. S., Hruby, V. J., Eds.; Elsevier, 2006;
pp 19–55.
Acknowledgments
3. (a) Bently, K. W.; Hardy, D. G. J. Am. Chem. Soc. 1967, 89, 3267; (b) Bently, K. W.;
Hardy, D. G.; Meek, B. J. Am. Chem. Soc. 1967, 89, 3273; (c)Opioid analgesics:
Chemsitry and receptor; Casy, A. F., Parfitt, R. T., Eds.; Plenum Press: New York
and London, 1986.
4. Höltje, H. D. In The Practice of Medicinal Chemistry; Wermuth, C. G., Ed.; Elsevier:
London, 2003; pp 387–401.
5. Loew, G. H.; Berkowitz, D. S. J. Med. Chem. 1979, 22, 603.
6. (a) Coop, A.; Grivas, K.; Husbands, S.; Lewis, J. W.; Porter, J. Tetrahedron Lett.
1995, 36, 1689; (b) Baas, J. M. A.; Woudenberg, R. H.; Maat, L. Liebigs Ann. Recl.
1997, 13; (c) Shults, E. E.; Shakirov, M. M.; Tolstikov, G. A.; Kalinin, V. N.;
Schmidhammer, G. Russ. J. Org. Chem. 2005, 41, 1132; (d) Jeong, I. H.; Kim, Y. S.;
Cho, K. Y.; Kim, K. J. Bull. Korean Chem. Soc. 1991, 12, 125; (e) Lewis, J. W.;
Readhead, M. J.; Selby, I. A.; Smith, A. C. B.; Young, C. A. J. Chem. Soc., C 1971,
1158; (f) Michne, W. F. J. Org. Chem. 1976, 41, 894.
This work was supported by Doctoral Fund of the Ministry of
Education of China 20070246088, research project from Shanghai
Municipal Health Bureau 2007089, and Shanghai Natural Science
Foundation 08ZR1401500; and was also supported by the Na-
tional Basic Research Program grant from the Ministry of Science
and Technology of China G2003CB515400, National Science Fund
for Distinguished Young Scholar from the National Natural Sci-
ence Foundation of China 30425002, fund provided by Chinese
Academy of Sciences (To J.-G. Liu). We appreciate Miss Jing-mei
Wang and Professor Min-Qin Chen in Research Center for Analysis
& Measurement, Fudan University for their works on X-ray
crystallography.
7. Knipmeyer, L. L.; Rapoport, H. J. Med. Chem. 1985, 28, 461.
8. Maat, L.; Woudenberg, R. H.; Meuzelaar, G. J.; Linders, J. T. M. Bioorg. Med. Chem.
1999, 7, 529.
9. Linders, J. T. M.; Maat, L. Bull. Soc. Chim. Belg. 1989, 98, 265.
10. Neilan, C. L.; Husbands, S. M.; Breeden, S.; Ko, M. C.; Aceto, M. D.; Lewis, J. W.;
Woods, J. H.; Traynor, J. R. Eur. J. Pharmacol. 2004, 499, 107.
11. (a) Abdelhamid, E. E.; Sultana, M.; Portoghese, P. S.; Takemori, A. E. J. Pharmacol.
Exp. Ther. 1991, 258, 299; (b) Miyamoto, Y.; Portoghese, P. S.; Takemori, A. E. J.
Pharmacol. Exp. Ther. 1993, 265, 1325.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2009.07.119.