Synthesis and biological evaluation of novel 6,7-dihydro-5H-cyclopenta[d]pyrimidine and 5,6,7,8-tetrahydroquinazoline derivatives as sigma-1 (σ1) receptor antagonists for the treatment of pain

Article abstract of DOI:10.1016/j.bmcl.2016.02.077

The synthesis and biological evaluation of new series of 6,7-dihydro-5H-cyclopenta[d]pyrimidine and 5,6,7,8-tetrahydroquinazoline derivatives as selective sigma-1 receptor (σ₁R) antagonists are reported. The receptor affinities of new compounds

Full text of DOI:10.1016/j.bmcl.2016.02.077

Accepted Manuscript
Synthesis and biological evaluation of novel 6,7-dihydro-5H-cyclopenta[d]pyr-
imidine and 5,6,7,8-tetrahydroquinazoline derivatives as sigma-1 (σ1) receptor
antagonists for the treatment of pain
Yu Lan, Yiyan Songyang, Lingli Zhang, Yan Peng, Jinchun Song
PII:
S0960-894X(16)30201-3
DOI:
http://dx.doi.org/10.1016/j.bmcl.2016.02.077
BMCL 23631
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
24 September 2015
17 February 2016
26 February 2016
Please cite this article as: Lan, Y., Songyang, Y., Zhang, L., Peng, Y., Song, J., Synthesis and biological evaluation
of novel 6,7-dihydro-5H-cyclopenta[d]pyrimidine and 5,6,7,8-tetrahydroquinazoline derivatives as sigma-1 (σ1)
receptor antagonists for the treatment of pain, Bioorganic & Medicinal Chemistry Letters (2016), doi: http://
dx.doi.org/10.1016/j.bmcl.2016.02.077
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis
and
biological
evaluation
of
novel
6,7-dihydro-5H-cyclopenta[d]pyrimidine
and 5,6,7,8-tetrahydroquinazoline
derivatives as sigma-1 (σ1) receptor antagonists for the treatment of pain
Yu Lan, Yiyan Songyang, Lingli Zhang, Yan Peng, Jinchun Song*
Department of Pharmacy, Renmin Hospital of Wuhan University, Wuhan 430060,
China
*
Author to whom correspondence should be addressed;
E-Mail: songjcwhu@163.com;
Tel.: +86-27 8804 7471; Fax.: +86-27 8804 7471
1

Abstract
The synthesis
and
biological
evaluation
and
of
new
series
of
6,7-dihydro-5H-cyclopenta[d]pyrimidine
5,6,7,8-tetrahydroquinazoline
derivatives as selective sigma-1 receptor (σ1R) antagonists are reported. The receptor
affinities of new compounds were evaluated in vitro in σ1 and σ2 receptor binding
assays. The structure-activity relationship study leads us to the most promising
compound:
2-(4-chlorophenyl)-4-(3-(4-methylpiperidin-1-yl)propoxy)-5,6,7,8-tetra-hydroquinazo
line (33). Compound 33 has exerted nanomolar affinity for σ1R (Kiσ1 = 15.6 nM) and
high σ1/σ2 selectivity (Kiσ2 > 2000 nM), and identified to be a σ1R antagonist. In
animal model, compound 33 exhibited dose dependent anti-nociceptive effeccts in the
formalin test. These results suggest that compound 33 could be a potent analgesic for
pain treatment.
Keywords
6,7-dihydro-5H-cyclopenta[d]pyrimidine; 5,6,7,8-tetrahydroquinazoline; sigma-1 (σ1)
receptor antagonists; analgesic
2

The sigma (σ) receptor was first discoverd in 1976, it was originally
mischaracterized as a noval class of opioid receptor and later confused with
2
phencyclidine/N-methyl-D-aspartate (NMDA) glutamate receptor complex.1,
Currently, sigma receptors are considered as unique proteins and classified into two
subtypes by pharmacological studies and biochemical analyses, which were named as
sigma-1 (σ1) receptor and sigma-2 (σ2) receptor.3 The σ1 receptor has been cloned,
encodes a protein of 223 amino acids with no homology to any other known
mammalian receptor protein, and shares a 90% amino acid identity and a 95%
similarity across various species.4 The σ1R is widely distributed in the central nervous
system (CNS) and peripheral nervous system (PNS) in voloved in memory, emotions,
and sensory and motor functions.5 Moreover, the σ1R is highly expressed in the key
areas of pain processing such as the superficial dorsal horn, periaqueductal gray,
rostral ventromedial medulla and also in the astrocytes and microglia.6-8 The σ1R
modulates the activity of various neurotransmitter receptors and ion channels,
including NMDA recepter, potassium channels, calcium channals and sodium
channels.6, 9 The σ2R has not been cloned, but numerous works report that the σ2R
have been involved in tumor cell proliferation and death, suggesting a promising role
in cancer imaging and treatment.10, 11 Due to their different pharmacological functions,
selectivity between the σ1R and σ2R is desirable.
Currently, selective σ1R ligands have been not introduced to the market, but
many of the compounds have been tested in clinical studies as antidepressants,12
antipsychotics,13 treatments for drug abuse,14 and learning/memory enhancers.15
Moreover, numerous groups have studied the potential role of the σ1R in pain
management.16 The σ1R has been known to modulate opioid analgesia since the 1990s.
σ1R agonists inhibit antinociception induced by morphine but σ1R antagonists and σ1R
antisense oligodeoxynucleotides enhance other µ-opioid receptor agonists in the acute
nociceptive test.17, 18 The pharmacological studies using genetic σ1R knockout (KO)
mice supported a role for σ1R in modulating pain behaviors in the absence of opioids.
Intraplantar administration of formalin or capsaicin elicited pain behaviours in those
of wild-type (WT) mice, but the phenotype of pain was reduced in σ1R KO mice.19, 20
3

Pharmacological antagonism of the σ1R produced similar results. Haloperidol (1),
with an affinity and antagonism effect to the σ1R, exhibited the inhibition in
formalin-induced pain and capsaicin-induced sensitization in WT mice.21 The σ1R
antagonists BD-1063 (2) and NE-100 (3) inhibited the mechanical allodynia induced
by capsaicin.20 The leading compound in the field of σ1R antagonists, S1RA (4),
showed high affinity to σ1 receptor (Ki = 17 nM) and excellent selectivity ratio (σ1
/σ2 > 550) (Figure 1). S1RA is currently undergoing phase II clinical trials in several
neuropathic pain conditions and the results are eagerly awatied.22
Cl
Cl
N
Cl
O
O
HO
N
O
N
N
F
1. Haloperidol
3. NE-100
O
2. BD-1063
Cl
N
O
N
N
O
N
O
N
N
N
O
N
F
Cl
6. Lan-0101
4. S1RA
5. Lan-0825
Figure 1. Representative σ1 receptor ligands.
In our previous study, we have identified two new series of compounds based on
3,4-dihydro-2(1H)-quinolinone and pyrimidine scaffolds through a three-dimensional
(3D) pharmacophore model of σ1R antagonists (Figure 2).23 The candidated
compounds, Lan-0825 (5) and Lan-0101 (6), exhibited the most potent in vivo
24
antinociceptive properties in animal models.23,
Forthermore, the cyclizing
derivatives of pyrimidine scaffold also possesses moderate affinities for σ1 receptor
and low affinity for σ2 receptor.
4

Figure 2. Main features of Lan`s σ1R pharmacophore in comparison to the distances described by Glennon`s σ1R
pharmacophore model.
This finding was from the study conducted by our research group in which two
novel classses of σ1R antagonists, compounds 10-34, were developed by connecting
4-position and 5-position of pyrimidine scaffold of 6 into cycloalkanes (Figure 3).
Target compounds 10-34 have been subjected to preliminary biological evaluation to
determine their affinities for the σ1 and σ2 receptor. Among the derivatives synthesized,
compound 33 exerted nanomolar affinity for σ1R (Kiσ1 = 15.6 nM), high σ1/σ2
selectivity (Kiσ2 > 2000 nM), and identified to be a σ1R antagonist. In animal model,
compound 33 exhibited dose dependent anti-nociceptive effeccts in the formalin test.
Figure 3. Design of new 6,7-dihydro-5H-cyclopenta[d]pyrimidine and 5,6,7,8-tetrahydroquinazoline derivatives.
5

The general strategy used to synthesize compounds 10-34 was performed in a
three step process, according to a previously reported method with slightly
modification.
As
shown
in
Sheme
1,
the
key
intermediates
or
2-aryl-6,7-dihydro-5H-cyclopenta[d]pyrimidin-4-ol
2-aryl-5,6,7,8-tetrahydroquinazolin-4-ol derivatives 8 were prepared through
cyclization reactions from 4-substituted benzimidamide 7, which reacted with ethyl
2-oxocyclopentanecarboxylate and 2-oxocyclohexanecarboxylate in methanol,
respectively. The standard alkylation procedure of 8 with 1,3-dibromopropane,
1,4-dibromobutane, 1,5-dibromopentane or 1,6-dibromohexane in acetone led to
compounds 9. Compounds 9 were then reacted with the various amines in acetonitrile
under the basic conditions. The crude products were purified by means of
chromatography to yield the target compounds 10-34 (Table 1-3).
Scheme 1. Reagents and conditions: (I) t-BuOK, MeOH, reflux; (II) Br(CH2)nBr, K2CO3, acetone, reflux; (III)
HNR1R2, Cs2CO3, acetonitrile, reflux
The design of the new 6,7-dihydro-5H-cyclopenta[d]pyrimidine and
5,6,7,8-tetrahydroquinazoline derivatives was based on the represented σ1 receptor
ligand pharmacophore model summarized by Glennon (Figure 2).25, 26 On the basis of
our previous findings, a hydrophobic group (aromatic hydrophobic group was better )
was necessary in position 2 of pyrimidine ring, and a three-atom carbon chain was the
most suitable distance to achieve good potency and selectivity. All the derivatives
synthesized were evaluated in primary σ1 receptor and σ2 receptor binding assays,
using [3H]-(+)-pentazocine and [3H]-di-o-tolylguanidine ([3H]-DTG) as radioligands,
respectively.23, 24
In this work, our initial focus was to investigate the effect of different amine
moieties for the binding affinities to σ1 and σ2 receptors. In
6

6,7-dihydro-5H-cyclopenta[d]pyrimidine derivatives, the piperidine and pyrrolidine
derivatives 10 (CAS number: 1639219-80-9) and 11 (CAS number: 1639219-81-0)
showed a moderate affinities to σ1 receptor.23 After induced a methyl group in the
4-position of piperidine moiety (12), the σ1R binding affinity was improved (Kiσ1 =
32.3 nM and Kiσ2 = 1774 nM). This improvement is possibly due to the
hydrophobicity of methyl group in piperidine moiety, which is more fitable to the
requirement of the second hydrophobic region in Glennon`s pharmacophore model.
The 1-methylpiperazine derivative (13) was also potent and selective (Kiσ1 = 4.3 nM
and Kiσ2 = 161 nM), similar to 4-methylpiperidine derivative 12, that proved this
hypothesis. However, substitution of the methyl in 1-methylpiperazine with a large
group such as phenyl (14, Kiσ1 > 2000 nM and Kiσ2 > 2000 nM) was not tolerated by
either the σ1 or σ2 receptor. The morpholine derivative was synthesized and evaluated
because its frequent appearance in σ1R ligands (such as S1RA and its derivatives),22, 27
but the receptor affinity of compound 15 was not as good as 4-methylpiperidine
derivative (Kiσ1 = 102.8 nM and Kiσ2 = 1889 nM). Substituting the oxygen atom of
the morpholino with a carbonyl group (16, Kiσ1 = 188.7 nM and Kiσ2 > 2000 nM))
decreased greatly the affinity to both the σ1R and σ2R, which suggested the polar
groups were not conducive to σ1R or σ2R binding in this position. Open chain amines
(17, 18) retained some activity, though not as much as compound 12, and had lower
selectivity.
5,6,7,8-tetrahydroquinazoline derivatives demonstrated a similar results with
6,7-dihydro-5H-cyclopenta[d]pyrimidine derivatives with a few differences. The
4-methylpiperidine moiety produced a higher affinity for σ1 receptor (21, Kiσ1 = 25.7
nM and Kiσ2 > 2000 nM) than compound 19 (CAS number: 1639219-85-4) or 20
(CAS number: 1639219-87-6),23 and the improvement of binding affinity was even
greater than compound 12, while 6,7-dihydro-5H-cyclopenta[d]pyrimidine derivatives
were thought to be more active due to the smaller ring size in our previous work.23
The reasons for these interesting results were partially because the basic amine center
played a key role in σ1R binding affinity, and the secondary hydrophobic region which
close to center amine site might produce more influences in receptor binding. The
7

1-methylpiperazine derivative exerted some potent and selective (22, Kiσ1 = 27.3 nM
and K iσ2 = 1922 nM), but 1-phenylpiperazine derivative still lost the actives in both
receptors. Amino Groups with oxygen atom or polar group (morpholine and
piperidin-4-one) decreased the σ1R binding, and open chain amines derivatives (26,
27) were less active than compound 21.
Together the receptor binding affinity data in Table 1 and our structure-active
relationship (SAR) studies summerized, it revealed that 5,6,7,8-tetrahydroquinazoline
derivatives have greater potential than 6,7-dihydro-5H-cyclopenta[d]pyrimidine
derivatives. Above of these derivatives, compound 21 exerted the highest σ1R binding
affinity and good selectivity to σ2R, which encouraged further exploration.
Table 1. Binding affinities for the σ1 and σ2 Receptor of Compounds 10-27.
Selectivity
Compound
m
NR1R2
Ki σ1 (nM)a
Ki σ2 (nM)b
(σ2/σ1)
10
1
44.2 8.2 c
1626 187
36.7
11
12
1
1
58.9 7.6
32.3 6.4
1896 227
1774 193
32.1
54.9
13
14
15
1
1
1
31.9 6.1
>2000
1821 184
>2000
57.1
– –
102.8 13
1889 203
18.5
8

16
17
1
1
188.7 46
120.8 21
>2000
>2000
– –
– –
18
19
20
1
2
2
62.4 5.6
138.5 20
111.8 16
1493 155
>2000
23.9
– –
1916 227
17.1
21
2
25.7 5.6
>2000
– –
22
23
24
2
2
2
27.3 6.2
>2000
1922 243
>2000
70.4
– –
119.6 17
>2000
– –
25
26
27
2
2
2
166.2 39
137.8 26
66.3 5.7
>2000
>2000
– –
– –
1546 168
23.3
a Affinities were determined in guinea pig brain using [3H]-(+)-pentazocine.
b
Affinities were determined in guinea pig brain using [3H]-DTG in the presence of (+)-SKF-10047 to block
sigma-1 receptors.
c The values are means SEM of three experiments performed in duplicate.
After identification 4-methylpiperidine as the most potent amino substituent,
several analogs with different chain lengths between the 5,6,7,8-tetrahydroquinazoline
scaffold and 4-methylpiperidine moiety were prepared. Compound 28, which
9

contained only one additional carbon atom in the linker compared to compound 21,
decreased binding affinity and selectivity for the σ1R over the σ2R (Kiσ1 = 238.2 nM
and K σ2 = 1856 nM). Further elongation of the straight carbon linker
i
5,6,7,8-tetrahydroquinazoline scaffold and 4-methylpiperidine moiety provided
derivatives 29-30, which had lower σ1R and σ2R affinities than propylene counterparts.
The results showed in Table 2 supported our previous finding that a three-atom
carbon chain between basic amino groups and primary hydrophobic region offered the
most fitable distance for receptor binding.
Table 2. Binding affinities for the σ1 and σ2 Receptor of Compounds 21, 28−30.
Compound
n
Ki σ1 (nM)a
Ki σ2 (nM)b
Selectivity (σ2/σ1)
21
28
29
30
3
4
5
6
25.7 7.6 c
238.2 77
353.9 85
>2000
>2000
1856 219
>2000
– –
7.79
– –
>2000
– –
a Affinities were determined in guinea pig brain using [3H]-(+)-pentazocine.
b
Affinities were determined in guinea pig brain using [3H]-DTG in the presence of (+)-SKF-10047 to block
sigma-1 receptors.
c The values are means SEM of three experiments performed in duplicate.
Compound 21 exhibited significant affinity to the σ1R and σ1/σ2 selectivity and
were thus used for further investigation of the effect of various substituted phenyl
groups. As shown in Table 3, the aromatic group on the 2-position of
5,6,7,8-tetrahydroquinazoline was improtent for activity and selection. Either an
electron-donating group (methyl) or electron-withdrawing group (trifluoromethyl)
could change slightly (increase or decrese) in receptor binding affinities, but after
10

induced chlorine atom in 4-position of phenyl group in compound 21, it showed
higher affinity to σ1R and better σ1/σ2 selectivity (33, Kiσ1 = 15.6 nM and Kiσ2 > 2000
nM). 6,7-dihydro-5H-cyclopenta[d]pyrimidine derivatives with 1-methylpiperidine
and 4-chlorophenyl groups were also synthesized and assessed, which maintained the
activity of σ1 receptors and selectivity to σ2R (34, Kiσ1 = 21.1 nM and Kiσ2 > 2000
nM).
Table 3.
Binding affinities for the σ1 and σ2 Receptor of Compounds 12, 21, 31-34.
Selectivity
Compound
Structure
Ki σ1 (nM)a
Ki σ2 (nM)b
(σ2/σ1)
12
21
31
32
33
34
32.3 6.4 c
1774 193
54.9
25.7 7.6
24.1 6.5
29.4 7.2
15.6 3.3
21.1 4.8
>2000
1961 235
>2000
– –
81.3
– –
>2000
– –
>2000
– –
11

a Affinities were determined in guinea pig brain using [3H]-(+)-pentazocine.
b
Affinities were determined in guinea pig brain using [3H]-DTG in the presence of (+)-SKF-10047 to block
sigma-1 receptors.
c The values are means SEM of three experiments performed in duplicate.
The phenytoin-media shift on σ1R ligand affinity and fluorescence resonance
energy transfer (FRET) based biosensor of σ1R ligand were mainly used to categorize
compounds into agonists and antagonists recently.28, 29 Phenytoin, a low-potency
allosteric modulator of the σ1 receptor, shifts σ1R agonists to significantly higher
affinities (Ki ratios without phenytoin vs. with phenytoin > 1), while σ1R antagonists
show no effect or a very little effect on lowering the affinity values (Ki ratios without
phenytoin vs. with phenytoin ≤ 1). In this work, compound 33 produced a small shift
lowering the affinity when incubated in the presence of phenytoin (Ki ratios without
phenytoin vs. with phenytoin = 0.92), which exhibited antagonist properties on the σ1
receptor.
Due to the marked effects on the σ1R and σ2R with its antagonist property,
compound 33 was selected as a promising candidate and subjected to further
pharmacological evaluation. As a classic model of acute and chronic pain, the
formalin test was representative and frequently used for the evaluation of the
anti-nociceptive effect. Intraplantar injection of formalin solution into the hind paw
produces a biphasic pain response, a brief, acute phase that caused by direct activation
of C-fibers (phase I) followed by a longer-lasting tonic phase that reflects
inflammation (phase II).30 The time spent licking or biting the paw after injection is
measured as an indicator of the pain response in mice.
Recent research have reported that σ1R antagonists could reduce both phases of
formalin-induced paw licking/biting behavior in mice, especially in the delayed phase
II, which appears to be dependent on the combination of an inflammatory reaction in
the peripheral tissue and functional changes in the spinal cord, involving both
peripheral and central sensitization.23, 24, 27 As shown in Figure 4, pretreatment with
compound 33 (80 mg/kg, i.p.) inhibited pain responses to a similar degree as
compound 4 (S1RA), reducing licking and biting time to 23.7 5.2 s in phase I and
64.2 8.3 s in phase II. Compound 4 at the same dose reduced licking and biting time
12

to 16.8 3.4 s and 55.4 7.7 s during phase I and II, respectively; the vehicle had no
effect relative to treatment with formalin alone. To better characterize the
antinociceptive effects of 33, a wide range of doses was tested (20-160 mg/kg).
Compound 33 produced dose dependent anti-nociception in both phases; the ED50
values were 51.8 5.3 and 57.5 4.6 mg/kg for phase I and II, respectively.
Figure 4. Anti-nociceptive effect of compound S1RA (4) and 33 in phase I (0 – 5 min) and phase II (15 – 45 min)
of the mice formalin test at the dose of 80 mg/kg. Each column and vertical line represents mean SEM of the
values obtained in at least ten animals. Statistically significant differences: ## p<0.01 VS vehicle; ** p<0.01 VS
vehicle+formalin (two-way ANOVA followed by Newman-Keuls test).
A detailed SAR investigation of 6,7-dihydro-5H-cyclopenta[d]pyrimidine and
5,6,7,8-tetrahydroquinazoline derivatives has shown that several factors influence the
binding affinity of these compounds to σ1 and σ2 receptors: (1) a basic amine was
necessary matching the known σ1R pharmacophoric model, piperidine and
4-methylpiperidine were favored; (2) a straight three-carbon chain alkyl between the
pyrimidine ring and the amino moiety was preferred over other linkers; and (3)
introduction of a halogen (chloro) on the 5-position (R3) showed moderate affinities to
σ1R and increased selectivity to σ2R.
In summary, we described the synthesis and pharmacological evaluation of series
of 6,7-dihydro-5H-cyclopenta[d]pyrimidine and 5,6,7,8-tetrahydroquinazoline
derivatives as novel selective σ1 receptor antagonists for pain treatment. The most
promising compound, compound 33, exhibited the high affinity for σ1R (Kiσ1= 15.6
13

nM) and good σ1/σ2 selectivity (Kiσ2 = 1922 nM), and was identified to be a σ1R
antagonist. In the formalin test, compound 33 exerted clear dose-dependent
antinociceptive effects, which suggested that compound 33 may facilitate the
development of a novel class of drugs for pain treatment.
Conflict of Interest
There is no conflict of interests among the authors.
Supplementary data
Supplementary data (experimental procedures and analytical data of new compounds)
associated with this article can be found, in the online version, at
Abbreviations
σ1R, sigma-1 receptor; σ2R, sigma-2 receptor; NMDA, N-methyl-D-aspartate; CNS,
central nervous system; PNS, peripheral nervous system; KO, knockout; WT,
wild-type; [3H]-DTG, [3H]-di-o-tolylguanidine; SAR, structure-active relationship;
FRET, fluorescence resonance energy transfer; i.p., intraperitoneal injection.
References
1. Martin, W. R.; Eades, C. G.; Thompson, J. A.; Huppler, R. E.; Gilbert, P. E. J.
Pharmacol. Exp. Ther. 1976, 197, 517.
2. Mendelsohn, L. G.; Kalra, V.; Johnson, B. G.; Kerchner, G. A. J. Pharmacol. Exp.
Ther. 1985, 233, 597.
3. Quirion, R.; Bowen, W. D.; Itzhak, Y.; Junien, J. L.; Musacchio, J. M.; Rothman,
R. B.; Su, T. P.; Tam, S. W.; Taylor, D. P. Trends Pharmacol. Sci. 1992, 13, 85.
4. Hanner, M.; Moebius, F. F.; Flandorfer, A.; Knaus, H. G.; Striessnig, J.; Kempner,
E.; Glossmann, H. Proc. Natl. Acad. Sci. USA 1996, 93, 8072.
5. Wolfe, S. A. Jr.; Culp, S. G.; De Souza, EB. Endocrinology 1989, 124, 1160.
6. Maurice, T.; Su, T. P. Pharmacol. Ther. 2009, 124, 195.
7. Roh, D. H.; Kim, H. W.; Yoon, S. Y.; Seo, H. S.; Kwon, Y. B.; Kim, K. W.; Han,
H. J.; Beitz, A. J.; Na, H. S.; Lee, J. H. Anesthesiology 2008, 109, 879.
8. Sanchez-Fernandez, C.; Montilla-Garcia, A.; Gonzalez-Cano, R.; Nieto, F. R.;
14

Romero, L.; Artacho-Cordon, A.; Montes, R.; Fernandez-Pastor, B.; Merlos, M.;
Baeyens, J. M.; Entrena, J. M.; Cobos, E. J. J. Pharmacol. Exp. Ther. 2014, 348,
32.
9. Su, T. P. J. Pharmacol. Sci. 2015, 127, 1.
10. Mach, R. H.; Zeng, C.; Hawkins, W. G. J. Med. Chem. 2013, 56, 7137.
11. Huang, Y. S.; Lu, H. L.; Zhang, L. J.; Wu, Z. Med. Res. Rev. 2014, 34, 532.
12. Bermack, J. E.; Debonnel, G. J. Pharmacol. Sci. 2005, 97, 317.
13. Ovalle, S.; Andreu, F.; Perez, M. P.; Zamanillo, D.; Guitart, X. Neuroreport 2002,
13, 2345.
14. Hayashi, T.; Su, T. P. Life Sci. 2005, 77, 1612.
15. Meunier, J.; Gue, M.; Recasens, M.; Maurice, T. Br. J. Pharmacol. 2004, 142,
689.
16. Zamanillo, D.; Romero, L.; Merlos, M.; Vela, J. M. Eur. J. Pharmacol. 2013, 716,
78.
17. King, M.; Pan, Y. X.; Mei, J.; Chang, A.; Xu, J.; Pasternak, G. W. Eur. J.
Pharmacol. 1997, 331, R5.
18. Chien, C. C.; Pasternak, G. W. J. Pharmacol. Exp. Ther. 1994, 271, 1583.
19. Cendan, C. M.; Pujalte, J. M.; Portillo-Salido, E.; Montoliu, L.; Baeyens, J. M.
Eur. J. Pharmacol. 2005, 511, 73.
20. Entrena, J. M.; Cobos, E. J.; Nieto, F. R.; Cendan, C. M.; Gris, G.; Del Pozo, E.;
Zamanillo, D.; Baeyens, J. M. Pain 2009, 143, 252.
21. Cendan, C. M.; Pujalte, J. M.; Portillo-Salido, E.; Baeyens, J. M.
Psychopharmacology (Berl) 2005, 182, 485.
22. Diaz, J. L.; Cuberes, R.; Berrocal, J.; Contijoch, M.; Christmann, U.; Fernandez,
A.; Port, A.; Holenz, J.; Buschmann, H.; Laggner, C.; Serafini, M. T.; Burgueno,
J.; Zamanillo, D.; Merlos, M.; Vela, J. M.; Almansa, C. J. Med. Chem. 2012, 55,
8211.
23. Lan, Y.; Chen, Y.; Cao, X.; Zhang, J.; Wang, J.; Xu, X.; Qiu, Y.; Zhang, T.; Liu,
X.; Liu, B. F.; Zhang, G. J. Med. Chem. 2014, 57, 10404.
24. Lan, Y.; Chen, Y.; Xu, X.; Qiu, Y.; Liu, S.; Liu, X.; Liu, B. F.; Zhang, G. Eur. J.
15

Med. Chem. 2014, 79, 216.
25. Glennon, R. A.; Ablordeppey, S. Y.; Ismaiel, A. M.; el-Ashmawy, M. B.; Fischer,
J. B.; Howie, K. B. J. Med. Chem. 1994, 37, 1214.
26. Glennon, R. A. Mini Rev. Med. Chem. 2005, 5, 927.
27. Diaz, J. L.; Christmann, U.; Fernandez, A.; Torrens, A.; Port, A.; Pascual, R.;
Alvarez, I.; Burgueno, J.; Monroy, X.; Montero, A.; Balada, A.; Vela, J. M.;
Almansa, C. J. Med. Chem. 2015, 58, 2441.
28. Cobos, E. J.; Baeyens, J. M.; Del Pozo, E. Synapse 2005, 55, 192.
29. Gomez-Soler, M.; Fernandez-Duenas, V.; Portillo-Salido, E.; Perez, P.; Zamanillo,
D.; Vela, J. M.; Burgueno, J.; Ciruela, F. J. Med. Chem. 2014, 57, 238.
30. Rosland, J. H.; Tjolsen, A.; Maehle, B.; Hole, K. Pain 1990, 42, 235.
16

Graphical abstract
17