Development of the First Two-Pore Domain Potassium Channel TWIK-Related K+ Channel 1-Selective Agonist Possessing in Vivo Antinociceptive Activity

Article abstract of DOI:10.1021/acs.jmedchem.6b01285

The TWIK-related K⁺ channel, TREK-1, has recently emerged as an attractive therapeutic target for the development of a novel class of analgesic drugs, suggesting that activation of TREK-1 could result in pain inhibition. Here, we report the synthesis of a series of substituted acrylic acids (1-54) based on our previous work with caffeate esters. The analogues were evaluated for their ability to modulate TREK-1 channel by electrophysiology and for their in vivo antinociceptive activity (acetic acid-induced writhing and hot plate assays), leading to the identification of a series of novel molecules able to activate TREK-1 and displaying potent antinociceptive activity in vivo. Furyl analogue 36 is the most promising of the series.

Full text of DOI:10.1021/acs.jmedchem.6b01285

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Article
Development of the first Two-Pore Domain Potassium
+
Channel TREK-1 (TWIK-Related K Channel 1)-selective
agonist possessing in vivo anti-nociceptive activity
Delphine Vivier, Ismail Ben Soussia, Nuno Rodrigues, Stéphane Lolignier, Maïly
Devilliers, Franck C Chatelain, Laetitia Prival, Eric Chapuy, Geoffrey Bourdier, Khalil
Bennis, Florian Lesage, Alain Eschalier, Jérôme Busserolles, and Sylvie Ducki
J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jmedchem.6b01285 • Publication Date (Web): 04 Jan 2017
Downloaded from http://pubs.acs.org on January 5, 2017
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Development of the first Two-Pore Domain Potassium
Channel TREK-1 (TWIK-Related K+ Channel 1)-selective
agonist possessing in vivo anti-nociceptive activity
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Delphine Vivier,ꢀ Ismail Ben Soussia,ꢀ,¶ Nuno Rodrigues,†,‡ Stéphane Lolignier,#,ǁ Maïly Devilliers,#,ǁ
,†,‡
Franck C. Chatelain,¶ Laetitia Prival,#,ǁ Eric Chapuy,#,ǁ Geoffrey Bourdier,‡,§ Khalil Bennis,‡,§ Florian
Lesage,¶ Alain Eschalier,#,ǁ,$ Jérôme Busserolles,#,ǁ and Sylvie Ducki*,‡,§
†
Université Clermont Auvergne, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand,
BP10448, F-63000 CLERMONT-FERRAND.
‡ CNRS, UMR6296, ICCF, F-63171 AUBIERE
¶
Labex ICST, Institut de Pharmacologie Moléculaire et Cellulaire, UMR CNRS 7275, Université de
Nice Sophia Antipolis, F-06560 VALBONNE
§ Université Clermont Auvergne, Sigma Clermont, Institut de Chimie de Clermont-Ferrand, BP10448,
F-63000 CLERMONT-FERRAND
# Université Clermont Auvergne, Université d'Auvergne, Neuro-Dol, BP 10448, F-63000 CLERMONT-
FERRAND
ǁ Inserm, UMR1107, Neuro-Dol, F-63001 CLERMONT-FERRAND
$ CHU Clermont-Ferrand, Service de pharmacologie, F-63003 CLERMONT-FERRAND
KEYWORDS.TREK-1 channel • Pain • Antinociceptive • Caffeate ester • Structure-Activity Relationship
Study • Analgesic agents
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ABSTRACT : The TWIK-Related K+ channel, TREK-1, has recently emerged as an attractive
therapeutic target for the development of a novel class of analgesic drugs, suggesting that activation of
TREK-1 could result in pain inhibition. Here we report the synthesis of a series of substituted acrylic
acids (1-54) based on our previous work with caffeate esters. The analogues were evaluated for their
ability to modulate TREK-1 channel by electrophysiology and for their in vivo antinociceptive activity
(acetic acid-induced writhing and hotplate assays), leading to the identification of a series of novel
molecules able to activate TREK-1 and displaying potent anti-nociceptive activity in vivo. The furyl
analogue 36 is the most promising of these series.
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INTRODUCTION
Pain represents a major public health issue worldwide. Acute or chronic, pain is responsible for 90% of
medical consultations, with chronic pain affecting over 1.5 billion people worldwide.1 Today, morphine
remains the treatment of reference for severe acute or chronic pains. Its analgesic effect is often
accompanied by adverse side-effects such as constipation, respiratory depression and dependence. This
opioid drug produces its pharmacological action by interacting with the ꢀ-opioid receptor (ꢀOR).
Recently Devilliers2 reported that the beneficial and adverse effects of morphine could be dissociated by
demonstrating that TREK-1 (TWIK-Related potassium channel-1, TWIK Tandem of pore domains in a
Weak Inward rectifying K+ channel) contributed to the analgesic effect of morphine but not to its
adverse side effects. These observations suggest that direct activation of TREK-1 channel by acting
downstream from ꢀOR, might have a strong analgesic effect without opioid-ligand adverse effects.3, 4
The TREK-1 channel (KCNK2 gene),5 first isolated from mouse brain in 1996,6, 7 belongs to the two
pore-domain potassium channels family (K2P), which are responsible of potassium (K+) leak currents
and are major contributors to resting membrane potential. Human TREK-1 (hTREK-1) is highly and
broadly expressed in the central nervous system.8, 9 Human and rodent TREK-1 is also expressed in
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peripheral sensory neurons, particularly in small dorsal root ganglion neurons, that are associated with
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nociception.
The functionality of TREK-1 results from its sensitivity to a variety of stimuli.
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TREK-1 is gated by membrane stretching (mechano-activation), pH, heat, polyunsaturated fatty acids
(PUFA) such as arachidonic acid and some general volatile anesthetics (chloroform, diethyl ether, and
nitrous oxide).12 It can also be down-modulated by PKA / PKC phosphorylation pathways and tonically
inhibited by the actin cytoskeleton.13
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Structurally, TREK-1 channel (Figure 1) is a transmembrane (TM) protein with intracellular N- and C-
terminal extremity.14, 15 The channel pore results from the assembly of two dimers, containing each four
helical transmembrane segments (M1-M4) arranged in tandem with two pore domains (P1-P2) and two
extracellular cap helices (C1 and C2) following the M1 segment (Figure 1).
Figure 1. Topology of the TREK-1 subunit from the intracellular N-terminus to the intracellular C-
terminus (including M1, C1, C2, P1, M2, M3, P2 and M4 domains).
The recent discovery of the involvement of TREK-1 in pain perception3, 4 has prompted researchers to
identify molecules capable to activate the pharmacologically orphan target.5 The neuroprotective agent
riluzole (Figure 2), used in the treatment of motor neuron diseases, was reported as a non-specific
TREK-1 agonist.16, 17 This activation was reported to be transient followed by an inhibition, process
attributable to an increase in the intracellular cAMP concentration by riluzole that produces a PKA-
dependent inhibition of TREK-1. The ethacrynic acid derivative DCPIB (Figure 2), a known inhibitor of
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volume-regulated anion channels (VRAC), was reported to promote the activation of K+ conductance
mediated by K2P channel.18 The DCPIB effect occluded that of arachidonic acid, which activates
TREK-1incultured astrocytes. A high-throughput yeast screen allowed the discovery of a carbazole,
ML67 (Figure 2), as a TREK-1 activator.19 Optimization of this carbazole allowed the identification of
an acridine analogue, ML67-33 (Figure 2) as a TREK-1 activator. Finally, cinnamyl 3,4-dihydroxyl-α-
cyanocinnamate (CDC, Figure 2), derived from caffeic acid, was reported as a TREK-1 activator.20 So
far none of the reported TREK-1 activators has proven to be selective, since they modulate other K2P
channels.
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Figure 2. Reported TREK-1 activators
COOH
O
O
Cl
C4H9
F3CO
S
N
HO
HO
O
NH2
CN
O
Cl
DCPIB
CDC
Riluzole
N N
COOH
HN
N
N
O
N
HO
HO
O
CN
Cl
Cl
Cl
Cl
1
ML67
ML67-33
We have recently disclosed the synthesis of a small set of CDC analogues and identified compound 1 as
a potent TREK-1 activator possessing antinociceptive activity in vivo.21, 22 CDC and 1 displayed
equipotent activity; hence in terms of ligand efficiency, we decided to focus our attention on shorter
analogues. In the present study, we expanded our initial structure-activity relationship (SAR) study
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around compound 1 and herein report the synthesis and pharmacological evaluation of 53 analogues of
1.
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CHEMISTRY
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Considering that the skeleton of 1 is composed of three main moieties (Ar, R, X), we studied their
influence on the pharmacological activity of the analogues (Figure 3A). The target molecules 1-54 were
obtained by Knoevenagel-type condensation between the appropriately substituted aldehydes 55-69 and
an α-substituted carbonyl synthon (Figure 3B).
Figure 3. A. Structure of analogues 1-54. B. Strategy for the synthesis of analogues 1-54
A
R =
Ar =
-OH (2, 4-6, 14-17, 21-27, 36-37, 46)
3,4-catechol (1-13)
phenyl (14-20)
ethoxy (1, 3, 18, 28-30, 32, 34, 38, 47)
O
t-butoxy (10, 39, 48)
3,4-dichlorophenyl (21)
i-pentyloxy (11, 40, 49)
3,4-dimethylphenyl (22)
Ar
R
12, 19, 31, 33, 35, 41, 50
cyclohexyloxy (
dodecyloxy (
-NH2 (
morpholino ( )
ethylamino (
cyclohexylamino (
phenylamino (45, 54)
)
23
4-hydroxyphenyl (
)
13, 42, 51
)
X
24
)
4-fluorophenyl (
25-26
)
8, 20
)
3-thienyl (
2-pyridyl (
9
43, 52
27
)
29
1-54
)
)
44, 53
28
2-furyl (
4-oxazolyl (
2-pyrrolyl (30-31)
)
)
4-imidazolyl (32-33)
naphtyl (24-35)
3-furyl (36-45)
X=
-CN (
+
1-2, 4, 8-14, 18-25, 27-36, 38-54
-COOH (6, 16), -F (7, 17)
3 5, 15, 26, 37
), -H ( ), -CH2NH3 ( ),
2-indyl (46-54)
B
Knoevenagel
condensation
O
O
O
OH
R
Ar CHO
+
Ar
R
X
X
X
carbonyl
synthon
aldehydes
(55-69)
1-54
The preparation of esters (R = ethoxy, t-butoxy, i-pentyloxy, cyclohexyloxy, docecyloxy), amides (R =
morpholino, ethylamino, cyclohexylamino, phenylamino) and acids (R = OH) bearing a nitrile at the α-
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position (X = CN) is depicted on scheme 1. Esters (1, 10-13, 18-19, 28-35, 38-42, 47-51) were prepared
by reacting the appropriate aldehyde and the α-cyanoacetate esters 71-75. Amides (9, 43-45, 52-54)
were obtained by reaction of an aldehyde with the α-cyanoacetate amides 76-79. Finally acids (2, 14,
21-27, 36, 46) were prepared by saponification of the t-butyl esters (10, 39, 48, 80-86).
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Scheme 1. Preparation of Esters (1, 10-13, 18-19, 28-35, 38-42, 47-51), Amides (9, 43-45, 52-54) and
acids (2, 14, 21-27, 36, 46)
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i or ii
or iii or iv
O
O
O
v or vi or vii
Ar
R
OH
R
CN
70
CN
CN
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1, Ar = 3,4-catechol, R = ethoxy (v, 55%)
9, Ar = 3,4-catechol, R = morpholino(vi, 74%)
10, Ar = 3,4-catechol, R = t-butoxy (v, 52%)
11, Ar = 3,4-catechol, R = i-pentyloxy (v, 66%)
12, Ar = 3,4-catechol, R = cyclohexyloxy (v, 71%
13, Ar = 3,4-catechol, R = dodecyloxy (v, 76%)
18, Ar = phenyl, R = ethoxy (v, 84%)
71, R = ethoxy (i. 66%)
72, R = t-butoxy (i. 45%)
73, R = i-pentyloxy (i. 48%)
74, R = cyclohexyloxy (i. 77%)
75, R = dodecyloxy (i. 87%)
76, R = ethylamino (ii. 30%)
77, R = cyclohexylamino (iii. 34%)
78, R = phenylamino (iv. 94%)
79, R = morpholino (i. >99%)
19, Ar = phenyl, R = cyclohexyloxy (v, 73%)
28, Ar = 2-furyl, R = ethoxy (vii, 16%)
29, Ar = 4-oxazoyl, R = ethoxy (vii, 41%)
30, Ar = 2-pyrrolyl, R = ethoxy (v, 66%)
31, Ar = 2-pyrrolyl, R = cyclohexyloxy (v, 51%)
32, Ar = 4-imidazolyl, R = ethoxy (v, 77%)
33, Ar = 4-imidazolyl, R = cyclohexyloxy (v, 63%)
34, Ar = 2-naphtyl, R = ethoxy (v, 65%)
35, Ar = 2-naphtyl, R = cyclohexyloxy (v, 73%)
38, Ar = 3-furyl, R = ethoxy (v, 33%)
CHO
X
R2
R1
CHO
Y
55, R1 = R2 = OH
56, R1 = R2 = H
57, R1 = R2 = Cl
58, R1 = R2 = CH3
59, R1 = = OH, R2 = H
60, R1 = F, R2 = H
61, X = CH, Y = S
64, X = N, Y = O
66, X = N, Y = NH
68, X = CH, Y = O
39, Ar = 3-furyl, R = t-butoxy (v, 42%)
40, Ar = 3-furyl, R = i-pentyloxy (v, 32%)
41, Ar = 3-furyl, R = cyclohexyloxy (v, 33%)
42, Ar = 3-furyl, R = dodecyloxy ( v, 35%)
43, Ar = 3-furyl, R = ethylamino (v, 46%)
44, Ar = 3-furyl, R = cyclohexylamino (v, 43%)
45, Ar = 3-furyl, R = phenylamino (v, 11%)
47, Ar = 2-indyl, R = ethoxy (v, 61%)
CHO
X
48, Ar = 2-indyl, R = t-butoxy (v,42%)
CHO
49, Ar = 2-indyl, R = i-pentyloxy (v, 27%)
50, Ar = 2-indyl, R = cyclohexyloxy (v, 98%)
51, Ar = 2-indyl, R = dodecyloxy (v, 39%)
52, Ar = 2-indyl, R = ethylamino (v, 70%)
53, Ar = 2-indyl, R = cyclohexylamino (v, 57%)
54, Ar = 2-indyl, R = phenylamino (v, 74%)
80, Ar = phenyl, R = t-butoxy (v, 56%)
81, Ar = 3,4-dichlorophenyl, R = t-butoxy (v, 51%)
82, Ar = 3,4-dimethylphenyl, R = t-butoxy (v, 50%)
83, Ar = 4-hydroxyphenyl, R = t-butoxy (v, 71%)
84, Ar = 4-fluorophenyl, R = t-butoxy (v, 80%)
85, Ar = 3-thienyl, R = t-butoxy (v, 66%)
86, Ar = 3-pyridyl, R = t-butoxy (v, 58%)
63, X = O
65, X = NH
N
62
CHO
CHO
N
H
69
67
O
O
viii.
Ar
OH
Ar
OtBu
CN
CN
10, 39, 48, 80-86
2, Ar = 3.4-catechol (90%)
14, Ar = phenyl (90%)
21, Ar = 3,4-dichlorophenyl (90%)
22, Ar = 3,4-dimethylphenyl 57%)
23, Ar = 4-hydroxyphenyl (35%)
24, Ar = 4-fluorophenyl (90%)
25, Ar = 3-thienyl (57%)
27, Ar = 3-pyridyl (80%)
36, Ar = 3-furyl (90%)
46, Ar = 2-indyl(78%)
Conditions. i. RH, DCC, DMAP, DCM, 0°C to RT, 24h; ii. a. (COCl)2, DCM/DMF, 0°C to RT, 2h b. EtNH2
(2M in THF), NEt3, DCM, 0°C to RT, 3h. iii. cyclohexanamine, EtONa, EtOH, RT, 72h. iv. a. PCl5, DCM,
reflux, 30 min b. aniline, DCM, reflux, 12h. v. ArCHO 55-69, piperidine, DCM, RT 24h. vi. ArCHO 55,
piperidine, AcOH, toluene reflux, 6h. vii. ArCHO 63-64, H2O, RT, 72h. viii. TFA, DCM, RT, 1h.
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Scheme 2. Preparation of analogues 7-8, 15-17, 20, 26, 37.
1
2
3
4
5
6
7
8
9
O
O
O
P
O
O
ii.
i.
+
EtO
EtO
Ar
OH
NH3+ TFA-
Ar
OEt
OEt
NHBoc
Ar
H
NHBoc
88, Ar = phenyl (60%)
89, Ar = 3-thienyl (not isolated)
90, Ar = 3-furyl (53%)
56, 61, 68
87
15, Ar = phenyl (46%)
26, Ar = 3-thienyl (71%)
37, Ar = 3-furyl (46%)
10
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O
O
O
O
R
R
+
iii.
OR'
H
BuOt
OtBu
R
O
OR'
R
91, R = OMOM
92, R = OMOM, R' = OtBu (51%)
iv.
6, R = R' = OH (68%)
O
O
O
O
vi
v.
EtO
OEt
Ph
OEt
COOEt
93 (50%)
Ph
OH
COOH
16 (43%)
O
O
O
vii.
viii
R
OH
F
F
OEt
OEt
F
R
TMS
17, R = H (91%)
95 (91%)
94
96, R = OMOM (37%)
7, R = OH (52%)
ix.
O
O
O
R
R
R
x.
xi.
NHBoc
NH2
NC
NHBoc
97
CN
CN
R
98, R = OH (38%)
99, R = H (38%)
8, R = OH (99%)
20, R = H (80%)
Conditions: i. NaH, THF, RT, 2 days. ii. a. LiOH 1M, MeOH, RT, 18h. b. TFA, DCM, RT, overnight. iii.
Piperidine, toluene, reflux, 2h. iv. TFA, DCM, RT, 12h. v. ArCHO 56, piperidine, EtOH, reflux, 12h. vi. a.
NaOH 1.5 M, MeOH, RT, 12h. b. H2SO4, Et2O, RT, 4 days. vii. a. LDA, THF, -30 °C, 30 min. b. TMSCl,
THF, -78 °C to RT over 4h. c. Tartaric acid, Et2O, RT, 12h. viii. a. ArCHO (56 or 91), LDA, THF, -78 °C,
30 min. b. NaOH 1.5 M, MeOH, RT, 2h. ix. TFA, DCM, RT, 12h. x. a. EtOH, DCC, DCM, RT, 1h. b.
ArCHO 55-56, piperidine, DCM, RT, 12h. xi. TFA, DCM, RT, 1h.
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Table 1. Biological evaluation of analogues 1-54. Ar, R and X refer to the substitution of the scaffold
molecule as presented in figure 3. (a) RTREK-1 is the ratio of currents measured after and before the
application of the compounds (concentration in ꢀM). TREK-1 current was measured in (b) HEK293
cells by whole-cell patch-clamp technique at +60mVor in (c) Xenopus oocytes by the double
microelectrode technique at 0 mV. "% antinociceptive effect" refers to (d) the antinociceptive effect of
compounds (10 mg/kg, s.c.) against acetic acid-induced abdominal writhing in CDI mice (n = 8) and (e)
the antinociceptive effect of compounds (60 mg/kg, s.c.) on hotplate model in CDI mice (n=8), 30 min /
45 min after injection of modulators. T-test: *≤0.05, **≤0.01, ***≤0.001.
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56
57
58
59
60
% antinociceptiveeffect
a
Cpds
Ar
R
X
RTREK-1
AcOHd
Hotplatee
185.0 15.0% ***/
159.7 23.3%***
Morphine (3 mg/kg)
77.4 18.7%*** /
36.6 13.9%
8.56 2.24 (100) b
ML-67
10.30 2.68 (100) b
2.64 0.95 (20) c
34.8 14.5% /
30.5 12.5%
3,4-catechol
3,4-catechol
Cinnamyloxy-
-CN
-CN
50.8 4.5%**
57.8 7.4 %*
31.8 14.5%21
CDC
1.54 0.1 (100) b
2.76 0.72 (20) c
1.0 (40)20
34.6 17.2% /
21.8 11.3%
-OEt
1
1.23 0.08 (20) c
1.5 0.2 (100) b
1.0 (40) 20
21.4 18.9 % /
59.4 10.6 %**
3,4-catechol
3,4-catechol
3,4-catechol
3,4-catechol
-OH
-OH
-OEt
-OH
-CN
-H
2
3
4
5
34.6 3.9%23*
52.8 7.9 %*
ND
ND
ND
-H
1.14 0.12 (20) c
2.87 0.32 (20) c
58.1 4.4%%**23
50.1 5.4%***
+
-CH2NH3
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3,4-catechol
3,4-catechol
3,4-catechol
-OH
-OH
-NH2
-COOH
-F
1.76 0.14 (20) c
2.4 0.2 (20) c
-2.8 11.0%
-5.8 12.3%
6.4 21.0%
ND
ND
ND
6
7
8
1
2
3
4
1.05 0.08 (20) c
5
-CN
6
7
8
9
28.7 8.9%* /
16 9.1 %
ND
3,4-catechol
Morpholino-
-CN
3.80 0.86 (100) b
40.0 5.8%**
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3,4-catechol
3,4-catechol
tButoxy-
-CN
-CN
2.52 0.54 (100) b
4.07 0.57 (100) b
43.4 10.4 %
27.2 17.1 %
10
11
iPentyloxy-
ND
25.5 9.7 %* /
34.2 12.1 %*
3,4-catechol
Cyclohexyloxy-
-CN
5.78 0.64 (100) b
55.3 4.5%**
12
3,4-catechol
Phenyl
Dodecyloxy-
-OH
-CN
-CN
2.7 0.37 (20) b
1.29 0.12 (20) c
0.95 0.19 (20) c
1.37 0.18 (20) c
1.44 0.12 (20) c
45.2 12.5%
43.1 13.0%*
-2.4 16.8%
ND
ND
ND
ND
ND
13
14
15
16
17
+
Phenyl
-OH
-CH2NH3
-COOH
-F
Phenyl
-OH
29.4 15.8 %*
34.0 5.9%***
Phenyl
-OH
55.7 16.5%* /
45.1 9.8%*
Phenyl
-OEt
-CN
1.80 0.11 (100) b
30.8 7.2%
18
Phenyl
Phenyl
Cyclohexyloxy-
-NH2
-CN
-CN
1.17 0.22 (100) b
1.7 0.03 (20) c
1.62 0.09 (20) c
1.70 0.05 (20) c
1.50 0.13 (20) c
1.60 0.22 (20) c
1.20 0.15 (20) c
1.05 0.06 (20) c
1.50 0.27 (20) c
7.1 8.7%
18.0 9.6%
ND
ND
ND
ND
ND
ND
ND
ND
ND
19
20
21
22
23
24
25
26
27
3,4-dichlorophenyl
3,4-dimethylphenyl
4-hydroxyphenyl
4-fluorophenyl
3-thienyl
-OH
-CN
7.5 19.2 %
-6.1 11.3%
24.1 23.1 %
17.8 15.2 %
1.8 20.4 %
-13.5 10.6%
-39.8 12.3% *
-OH
-CN
-OH
-CN
-OH
-CN
-OH
-CN
+
3-thienyl
-OH
-CH2NH3
-CN
3-pyridyl
-OH
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Journal of Medicinal Chemistry
2-furyl
-OEt
-OEt
-CN
-CN
-CN
-CN
2.36 0.65 (100) b
2.56 0.36 (100) b
1.89 0.23 (100) b
2.62 0.47 (100) b
23.3 11.4%
28.9 14.0%
25.8 14.7%
10.0 13.6%
ND
ND
ND
ND
28
1
2
3
4
5
6
7
8
4-oxazolyl
2-pyrrolyl
2-pyrrolyl
29
-OEt
30
Cyclohexyloxy-
31
9
10
11
12
13
14
145.2 22.3%*** /
43.2 13.0%**
4-imidazolyl
4-imidazolyl
Naphtyl
-OEt
-CN
-CN
-CN
-CN
1.80 0.16 (100) b
1.50 0.21 (100) b
1.59 0.23 (100) b
2.60 0.61 (100) b
33.0 9.4%
50.0 13.9%**
44.7 16.8%
30.8 8.0%
32
27.0 17.1 % /
62.4 23.5%
15
Cyclohexyloxy-
-OEt
33
16
17
18
93.2 34.4%* /
67.5 34.8%
19
34
20
21
22
23
70.3 13.3%* /
50.9 12.6%
Naphtyl
Cyclohexyloxy-
35
24
25
26
27
4.53 0.46 (100) b
1.43 0.17 (20) c
1.49 0.08 (20) c
56.4 14.4%* /
119.5 21.9%**
28
3-furyl
3-furyl
3-furyl
-OH
-OH
-OEt
-CN
-CH2NH3
-CN
30.7 5.0%*
-1.5 16.8%
55.9 7.5%***
36
29
30
31
32
33
34
+
ND
37
28.4 10.5% /
11.9 10.6%
20.8 12.7% /
-1.3 4.0%
6.5 6.9% /
25.5 14.7%
2.09 0.4 (100) b
35
38
36
37
38
39
3-furyl
tButoxy-
-CN
6.09 1.24 (20) b
81.8 1.8%***
39
40
41
42
43
3-furyl
3-furyl
iPentyloxy-
-CN
-CN
2.80 0.41 (100) b
1.79 0.16 (100) b
82.2 9.1 %**
91.2 3.6%***
44
40
45
46
47
48
18.4 14.2 % /
Cyclohexyloxy-
41
49
96.7 20.8%***
50
51
52
53
54
55
21.8 15.1 % /
22.2 13.6%
3-furyl
3-furyl
Dodecyloxy-
-NH-Ethyl
-CN
-CN
2.33 0.23 (100) b
1.28 0.13 (100) b
53.8 9.7%***
31.0 17.7%
42
56
43
57
ND
58
59
60
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-NH-
1
2
3
4
5
6
7
8
3-furyl
3-furyl
2-indyl
2-indyl
2-indyl
2-indyl
-CN
-CN
-CN
-CN
-CN
-CN
1.07 0.08 (100) b
4.64 0.61 (20) b
0.0 9.0%
48.0 15.8%*
25.8 6.8%
ND
44
45
46
47
48
49
Cyclohexyl
107.7 23.6%*** /
148.6 41.3%**
-NH-Phenyl
-OH
9
1.18 0.16 (100)b
2.44 0.34 (20)c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ND
33.3 13.7%* /
32.7 15.3%
-OEt
4.71 1.85 (20) b
4.07 0.56 (20) b
4.01 1.7 (20) b
88.4 5.3 %**
51.3 15.3%***
46.6 15.3%
40.8 11.4%* /
39.7 16.5%*
tButoxy-
iPentyloxy-
21.8 9.1% /
37.8 15.9%*
65.7 17.5%** /
52.6 13.3%*
2-indyl
2-indyl
Cyclohexyloxy-
-CN
-CN
1.98 0.24 (10) b
2.19 0.24 (100) b
83.6 8.1 %***
54.1 11.6%***
50
51
23.2 18.7% /
20.3 13.7%
ND
Dodecyloxy-
-NH-Ethyl
2-indyl
2-indyl
-CN
-CN
1.26 0.10 (100) b
1.52 0.17 (100) b
45.0 7.5%*
45.0 16.5%*
52
53
-NH-
67.5 14.1 %*** /
35.4 13.9%
Cyclohexyl
49.5 18.6%* /
49.5 29.7%
2-indyl
-NH-Phenyl
-CN
2.16 0.53 (10) b
52.0 16.7%*
54
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Journal of Medicinal Chemistry
The preparation of analogues substituted at the α-position of the carbonyl function with various X
1
2
3
4
+
groups [(ammonium salts (X = -CH2-NH3 15, 26, 37), malonic acids (R = -COOH 6, 16) and the α-
5
6
7
8
fluoro acrylic acid (R = F 7, 17)] is presented in scheme 2. The ammonium salts were prepared as
previously described21through a Wadsworth-Horner olefination between the appropriate benzaldehyde
(56, 61 or 68) and the phosphonate 87 followed by a deprotection in trifluoroacetic acid.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
A Knoevenagel condensation between benzaldehydes (56 and 91) and malonate esters afforded, after
deprotection, the desired 2-arylidenemalonic acids 6 and 16. The α-fluoroacrylic acids (7 and 17) were
obtained through a Peterson olefination between the benzaldehydes (56 and 91) and the fluorinated
compound 95.
Finally the two amides (R = NH2, 8 and 20) were obtained after deprotection of the carbamates 98-99,
the latter being formed through a Knoevenagel condensation between the benzaldehydes 55-56 and the
2-cyanoacetylcarbamate 97.
BIOLOGICAL EVALUATION
The analogues (1-54) were first evaluated for their ability to enhance K+ currents in TREK-1
transfectedHEK293 cells and/or in TREK-1 injected Xenopus oocytes using whole-cell patch-clamp
technique and double microelectrodes technique respectively, in voltage-clamp configuration(Table 1).6
In these electrophysiological experiments, ML-67 and CDC were assayed for comparative purposes.
The results are expressed as the ratio of currents measured after and before the application of the
compounds (RTREK-1 = Imax/I0). Currents were measured at +60mV for HEK293 cells and at 0mV for
oocytes.
The analogues (1-54) were then assessed for their in vivo antinociceptive activity using the acetic acid
(AcOH by i.p.)-induced-writhing test in mice (Table 1).24, 25 In this assay, the compounds or vehicle are
administered into the subcutaneously (s.c.) to the animal 15 minutes before the AcOH injection into the
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peritoneal cavity (i.p.). This activates nociceptors directly and/or produces inflamed viscera (sub-
diaphragmatic organs) and subcutaneous (muscle wall) of tissus.3 The number of induced abdominal
writhes was determined during 20 min after the injection of acetic acid. The results are expressed as a
percentage of antinociceptive effect (Table 1) determined according to the following equation:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
( nb writhes control − nb writhes test)
% ꢀꢁꢂꢃꢁꢄꢅꢃꢅꢆꢇꢂꢃꢈꢆ ꢆꢉꢉꢆꢅꢂ (ꢀꢅꢊꢋ) =
x100
nb writhes control
Where “nb writhes control” is the mean number of writhes in the control group (n=8) and “nb writhes test” is the mean number
of writhes in the test group (n=8).
For the compounds that showed an ability to activate TREK-1 (RTREK-1>1.5 at 100 µM) and a
percentage of analgesic activity in the AcOH writhing test higher than 30% (10 mg/kg), the in vivo
analgesic activity was confirmed with a second screening, the hotplate assay. This model is more
discriminating; it measures the response to a non-inflammatory acute nociceptive input and is used to
study central antinociceptive activities. The withdrawal response latency evoked by exposing mice paws
to a thermal stimulus (hotplate set to 52 °C) determines the antinociceptive reaction. Response was
defined by the animal either licking the forepaws, hindpaws or flicking the hindpaws. To avoid tissue
damage, animals were exposed to the hot plate (Ugo Basile model) for a maximum of 30 s (cutt-off
time). Withdrawal latencies were measured before and 30 / 45 minutes after injection of vehicle,
morphine (3 mg/kg) or analogues (60 mg/kg). The results are expressed as a percentage of
antinociceptive effect (Table 1) determined according to the following equation:
(postdrug latency − predrug latency)
% ꢀꢁꢂꢃꢁꢄꢅꢃꢅꢆꢇꢂꢃꢈꢆ ꢆꢉꢉꢆꢅꢂ (ꢌꢄꢂꢇꢍꢎꢂꢆ) =
x100
predrug latency
Where "predrug latency" is the withdrawal latency before drug injection and "postdrug latency" is the withdrawal latency 30 and 45 min
after drug injection
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Journal of Medicinal Chemistry
1
2
3
4
5
6
7
8
In order to confirm the role of TREK-1 in the analgesic response of the most potent compounds, their
sensitivity to thermal stimuli (hotplate) was evaluated in TREK-1-/- mice (KO) and compared with the
nociceptive behavior of wild type (WT) mice (Table 3).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Finally, the most promising drug candidates were evaluated for their selectivity to TREK-1 by testing
them on others K2P channels (Table 4).
RESULTS AND DISCUSSION
Structurally diverse molecules were prepared sharing the same prop-2-en-1-one scaffold (Figure 3A)
and bearing an aromatic moiety at the 3-position (Ar), a carboxy derivative (R), and various α-
substituents (X). In each case, we chose groups that differed in nature: size/volume (Ar = Phenyl vs
naphthyl), hydrophobicity (R = OC2H5 vs OC12H25), hydrogen bond ability (Ar = 3,4-catechol, 4-
hydroxyphenyl, phenyl or X = F, COOH), electronic properties (Ar = 3.4-catechol, 3,4-dimethylphenyl,
3,4-dichlorophenyl). Since compounds featuring catechol moieties can be qualified as pan assays
interference compounds (PAINS) since they can react with biomolecules and degrade or inactivate
them, we aimed to prepare analogues bearing (hetero)aromatic moieties (Ar) that are not classified as
PAINS such as Ar = phenyl, naphthyl, furyl, imidazolyl, pyrrolyl. Analogues (1-54) were prepared in
two to four steps in moderate (20%) to excellent (85%) overall yields. Most of the compounds are new
and have not been reported before.
The analogues (1-54) were evaluated for their ability to enhance K+ currents in TREK-1 transfected
HEK293 cells and/or Xenopus oocytes (Table 1). Most compounds were able to enhance TREK-1
current (RTREK-1> 1) and the activation was rapidly reversed following washout of the tested compounds.
The latter reveals that the compounds are not covalently bound to the target (reversible interaction),
suggesting the molecules are not Michael acceptors. We note that the activities are ranging from inactive
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to active compounds which comfort us in the fact that the activity of the compounds does not solely rely
on the presence of the α,β−unsaturated carbonyl/nitrile. Compounds that can enhance the TREK-1
current by at least 2-fold (RTREK-1 ≥2) can be considered as TREK-1 activators, which is the case for 24
compounds. Among these, eight analogues (11-12, 36, 39, 45, 47-49), all bearing a cyano- group at the
α-position (X=CN), were particularly strong activators (RTREK-1 ≥4). These compounds contained either
the 3,4-catechol, the 3-furyl or the 2-indyl aromatic moieties (Ar), which is why we concentrated our
efforts on the synthesis of several analogues in these families. Substitution of the carbonyl function to
give carboxylic acids (R = OH), esters (R = alkoxy) or amides (R = amines) was well tolerated and the
ability of these analogues to activate TREK-1 varied quadratically with the size/hydrophobicity of the R
substituent, with an optimal R size (vertex) for the t-butyloxy group (R = OtBu). When we took into
account both the aromatic ring (Ar) and the carboxyl substituent (R), we observed a trend suggesting
that the volume of the molecule should be smaller than 230 Ǻ3 and the molecule should contain an
aromatic ring with at least one hydrogen-bond donor group (N, O).
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The in vivo antinociceptive activity of the analogues (1-54) was assessed in the acetic acid (AcOH)-
induced-writhing test in mice (Table 1).
+
Overall, the influence of the α-substitution (X=CN, CH2NH3 , COOH, F) was the most significant with
a neat improvement in activity when the analogues bore an α-cyano substitution (15-30 times more
active when X = CN versus X = F, COOH). Changing this substitution for an ammonium salt (X =
+
CH2NH3 ) was unfavorable leading to compounds (26 and 37) which even caused hyperalgesia.
Variation of the carbonyl function to give carboxylic acids (R = OH), esters (R = alkoxy) or amides
(R=amines) was well tolerated although it did not match the tendency observed in vitro. The best
analgesic effects were observed for the esters bearing a cyclohexyloxy chain (41 and 50) able to inhibit
80-90% of AcOH-induced writhes. Presumably the esters are hydrolyzed in vivo and the parent
carboxylic acids are exerting their activity. In order to confirm this hypothesis, we studied the metabolic
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Journal of Medicinal Chemistry
stability of compounds within the catechol / 3-furyl / 2-indyl series (R= OH 2/36/46, R = ethoxy 1/38/47
and R = cyclohexyloxy 12/41/50) in the presence of rat liver microsomes and the cofactor NADPH,
hence simulating phase I metabolism. Under these conditions, the compounds were monitored over 24
hours by HPLC to determine their half-life (t1/2). While the ethyl esters 1/38/47 were rapidly degraded
into the corresponding carboxylic acids 2/36/46 (t1/2 <4 min), the cyclohexyloxyanalogues 12/41/50
were more resistant to hydrolysis (t1/2 = 25-45 min). Presumably the steric hindrance of the cyclohexyl
ring protects the ester against rapid hydrolysis. This could explain why these compounds are potent both
in vitro and in vivo.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
We next examined the influence of the aromatic moiety (Ar) where the compounds bearing 3-furyl or 2-
indyl series stood out with analgesic effects reaching the 80-90%.While the 3,4-catechol (1-5 and 9-13),
4-imidazoyl (32-33), naphthyl (34-35), 3-furyl (36-45 except 37 and 44) and 2-indyl (46-54) rings were
well tolerated, the substituted phenyl (15-24), 3-thienyl (25), and 2-pyrrolyl (30-31) rings proved
detrimental for the analgesic activity. We attempted to explain this observation by looking at the nature
of the aryl moieties (size, hydrogen-bond acceptor/donor capacities, hydrophobicity) but none of the
parameters seemed to correlate with the observed activity. We note that some heterocyclic rings even
elicited hyperalgesia like the 3-thienyl bearing molecule 26 or more markedly the 3-pyridyl analogue
27.
In order to validate the in vivo analgesic activity, the analgesic hits (% inhibition > 30% in writhing
assay) were evaluated in the hotplate test (Table 1). While the first assay relied on chemically-induced
nociception (AcOH), this assay relies on a thermal stimulus to induce pain. This assay is known to
predict the clinical potency of opioid analgesics, such as morphine, but is usually not sensitive for non-
opiate analgesics. To our delight, half of the hit molecules (16/32) caused a significant increase (>50%)
in their action latencies (12-18 seconds compared to 8 seconds for the vehicle) against thermal stimuli.
The most significant results were obtained for the 3-furyl analogues 36 (X = CN, R = OH) and 45 (X =
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CN, R = anilino-) which presented higher antinociceptive activity at 45 minutes than at 30 minutes
(lasting effect).
1
2
3
4
5
6
7
Considering the results obtained from the in vitro and in vivo assays, we grouped the molecules in
different categories according to their in vitro/in vivo potencies (Table 2). The molecules possessing
both a good ability to activate TREK-1 currents (RTREK-1>4 at 100 µM or >2 at 20 µM) and good in vivo
analgesic properties (> 30% inhibition in AcOH-induced writhing and >50% inhibition in hotplate
assay) belonged to group A (3 molecules). Group B comprised 12molecules possessing a good ability to
activate TREK-1 currents but no or little in vivo analgesic properties. Presumably these compounds are
metabolized into inactive species that do not activate TREK-1 in vivo or are not bioavailable enough to
reach their target. Group C included ten analgesic molecules which are not TREK-1 activators, hence
may involve other pathways. Finally 29inactive molecules fitted into group D.
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2. Groups A-D. (a) TREK-1 activator if RTREK-1 > 4 at 100 µM or >2 at 20 µM. (b) not TREK-1
activator if RTREK-1 < 4 at 100 µM or <2 at 20 µM. (c) Analgesic if % antinociceptive effect AcOH >
30% and hotplate >50%. (d) Not analgesic if % antinociceptive effect AcOH< 30% or/and hotplate <
50%.
Groups
TREK-1 activatora
Not TREK-1 activatorb
A
36, 45, 54
B
C
Analgesicc
2, 18, 32-35, 41, 50, 52-53
D
Not analgesicd
1, 5, 7, 9, 11-13, 39, 46-
49
3-4, 6, 8, 10, 14-17, 19-31, 37-38, 40, 42-
44, 51
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Journal of Medicinal Chemistry
In order to confirm the role of TREK-1 in the analgesic response of the compounds that belong to group
A, the sensitivity to thermal stimuli (hotplate) of the most promising compounds (36, 45, 54) was
evaluated in TREK-1-/- mice (KO) and compared with the nociceptive behavior of wild type (WT) mice.
We also evaluated the naphthyl analogue 34 as a negative control since it does not activate TREK-1
(RTREK-1< 2). The results (Table 3) show that compound 36 conserves its analgesic activity in WT mice,
while KO mice showed an increase of sensitivity to heat, demonstrating that TREK-1 is a major effector
of this compound’s induced anti-nociceptive effect. On the other hand, compound 45, as well as
compound 54 and morphine, displayed less analgesic activity in this strain of mice (C57Bl/6J for Table
3 vs CD1 for Table 1). Strain differences in the analgesic effect of compounds such as morphine have
previously been observed26 and could be attributable to genetic differences in these animals. Future
work will be carryout out to assess TREK-1 expression in different strains of mice and the specific
pharmacological blockade of the TREK-1 channel to ascertain our conclusions. Finally compound 34
was expectedly inactive on both WT and KO mice, showing no dependence of TREK-1.
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Finally the drug candidates 36, 45 and 54 were evaluated for their selectivity against TREK-1 by testing
them on other K2P channels (TREK-2, TRAAK and TASK-3).TREK-2 and TRAAK are a part from
TREK subfamily and show high structure similarities with TREK-1 (78% homology with 63% identity
for TREK-2 and 69% homology with 51% identity for TRAAK).27 TASK-3 is a member of TASK
subfamily and shares 50% of homology with TREK-1 with only 30% identity. The results (Table 4)
show that only compound 36 is TREK-1-specific since its effects on other channels are weak (TREK-2)
or absent (TRAAK, TASK-3). This difference in activity suggests that the compound is not PAINS. The
other two compounds (45 and 54) show no selectivity between TREK-1 and TREK-2.
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Table 3. Analgesic effect of compounds34, 36, 45, 54 (60 mg/kg, s.c.) on hotplate model in WT and KO
black mice (n=7), 30 and 45 min after injection of modulators. % inhibition is the difference between
the latency time observed after and before administration of compounds (60 mg/kg, s.c.). T-test: *≤0.05,
**≤0.01, ***≤0.001.
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Cpds
34
TREK-1+/+
TREK-1-/-
%Analgesic effect due to TREK-1
0% / 0%
17.4 8.8 %/31.1 8.8%*
121.2 26.1%**/93.5 19.4%**
24.8 10.8% */22.1 6.4%
27.9 10.2%*/39.2 13.4%**
98.6 22.5%/124.5 15.6%
18.0 7.9 %/34.4 14.7 %
78.1 22.2%** /75.4 21.1%**
29.2 11.5%/35.1 14.4%
18.7 8.4%/51.2 16.1 %*
86.7 27.7%/94.6 18.5%
36.6 25.1 % / 19.0 25.7%
0 % / 0%
36
45
33 28.3% / 0%
54
12.6 17.9 %/24.3 14.1%
Mor-
phine
Table 4. Modulation of K2P channels by group A compounds. RK2P is the ratio of currents measured
after and before the application of the compounds (concentration in ꢀM). TREK-1, TREK-2, TRAAK
and TASK-3 channels were expressed in HEK293 cells and currents were measured by whole-cell
patch-clamp technique at +60 mV.
Cpds TREK-1 TREK-2 TRAAK
TASK-3
4.5 (100) 1.8 (100) 1.1 (100) 1.0 (100)
36
45
54
4.6 (20)
2.2 (10)
3.0 (20)
1.6 (10)
1.0 (20)
0.9 (10)
0.5 (20)
1.2 (10)
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Journal of Medicinal Chemistry
CONCLUSION
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We have conducted a focused SAR study to understand the influence of three main moieties (Ar, R, X),
which allowed us to prepare 53 analogues of the lead compound 1. In general, compounds bearing a
cyano- group at the α-position (X = CN), either the 3-furyl or the 2-indyl as aromatic moieties (Ar), and
acids (R = OH) displayed favorable activities both in vitro and in vivo. We also noted that the volume of
the molecule should be below 230 Ǻ3. This SAR allowed us to identify the furyl analogue 36 for further
evaluation due to potent in vitro and in vivo activities. Compound 36 demonstrates improved activities
compared to the lead compound 1 and excellent selectivity for TREK-1 in the K2P channels panel.
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Recent studies of the TREK-1 channel have already exposed its central role in pain perception,3, 28, 29
and a number of pharmacologically active compounds have recently been reported to enhance the
activity of TREK-1 channels.5 However most, if not all, of these compounds are not selective for
TREK-1 channels. We herein report the first TREK-1-selective agonist that displays analgesic activity in
vivo. Thus, compound 36 represents a new tool for studying the effects of TREK-1 agonism in vitro and
in vivo and should allow a better understanding of the implication of TREK-1 in the pain perception.
EXPERIMENTAL SECTION
Biological evaluation
Electrophysiology
Cell culture and transfection. HEK-293 cells were purchased from American Type Culture Collection
(Manassas, VA, USA). The coding sequence of the human TREK-1 (kcnk2) gene (GenBank Acc. No.
NM_014217) was cloned into pIRES2-eGFP vector (Invitrogen, Cergy-Pontoise, France). In order to
obtain a cell line that stably expresses the TREK-1 channel, the plasmid was transfected (25 ng) into
HEK-293 cells using the Calcium Phosphate Method as described by the manufacturer protocol in the
presence of 1.5 mg/mL Geneticin (G418) (Invitrogen, Cergy-Pontoise, France). After 20–30 days of
culture in G418 selection medium, individual colonies of resistant cells were isolated by using cloning
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cylinders (Sigma Aldrich, Saint-Quentin Fallavier, France).The human-TREK-1/HEK-293 (h-TREK-
1/HEK) cells, which are a HEK-293 cells stably expressing the TREK-1 channel, were cultured in
Dulbecco’s modified Eagle’s medium (Gibco, Life Technologies, Saint Aubin, France) supplemented
with 10 % fetal calf serum (Hyclone, Thermo Fisher Scientific GMBH, Ulm, Germany), 1 % penicillin-
streptomycin (Gibco, Life Technologies, Saint Aubin, France), and were grown in the presence of a h-
TREK-1/HEK cell selection antibiotic (G418) (Gibco, Life Technologies, Saint Aubin, France) at 0.5
mg/mL, in a humidified incubator at 37 °C and 5 % CO2. Cells were passaged using 0.25 % trypsin-
EDTA (Gibco, Life Technologies, Saint Aubin, France) after reaching a maximal confluence of 80 %
and were used for up to 10 consecutive passages after thawing.
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Patch-clamp recording. Electrophysiological experiments were performed on h-TREK-1/ HEK- cells
plated on 35-mm dishes with a density of 30,000 cells/dish and used between 4 and 48 h after plating.
Dishes were placed in the patch-clamp chamber and continuously perfused with the control bath
solution containing (in millimol per litre): 140 NaCl, 10 TEA-Cl, 5 KCl, 3 MgCl2, 1 CaCl2, 10 HEPES
and adjusted to pH 7.4 with NaOH. The pipette solution contained (in millimol per litre): 155 KCl, 3
MgCl2, 5 EGTA, 10 HEPES and adjusted to pH 7.2 with KOH. All experiments were performed at room
temperature (21–22 °C). TREK-1 current was observed using RK 400 patch clamp amplifier (Bio-Logic
Science Instruments), low-pass filtered at 3 kHz and digitized at 10 kHz using a 12-bit analog-to-digital
converter Digidata-1322 (Axon Instrument, Sunnyvale, CA, USA). For current visualization and
stimulation protocol application, we used commercial software (Clampex 8.2). The patch pipettes were
double-step-pulled from haematocrit-capillaries (Hirschmann Laborgeraete, Germany) using a vertical
puller (PC-10, Narishige International, London, UK). Filled pipettes had resistances of 1–2 Mꢀ. Whole-
cell patch-clamp configuration was obtained at a holding potential of −80 mV. Voltage ramp protocols
consisted of a step to −100 mV (20 ms in duration) followed by a ramp going from −100 to +60 mV
over 400 ms. Ramps were applied every 5 s. Current amplitudes were expressed as current densities
(pA/pF, picoAmper per picoFarad) and results are presented as mean standard error of the mean (SEM).
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Journal of Medicinal Chemistry
Current was recorded under control bath solution perfusion and under bath solution perfusion
supplemented with different test molecules. The evaluation of the molecules effects was obtained by the
ratio between currents after molecules application and currents under control perfusion. The ratio values
were calculated at +60mV of the current/voltage (I/V) curves. Molecules were prepared as stock
solutions in DMSO (Sigma Aldrich, Saint-Quentin Fallavier, France) (20 mM) and were used at a final
concentration of 100µM.
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Animals
Male CD1 mice (20-25 g) were purchased from Elevage Janvier. Homozygous TREK-1 (-/-) and wild-
type (+/+) littermates were generated from crosses between heterozygous animals. The detailed
experimental methodology used to generate and genotype TREK-1 knockout mice has been previously
described.30 Animals were housed under controlled environmental conditions (22 °C; 55% humidity)
and kept under a 12/12 h light/dark cycle, with food and water ad libitum for a week before start the
experiments to acclimatize.The behavioural experiments were performed blind to the treatment, in a
quiet room, by the same experimenter taking great care to avoid or minimize discomfort of the animals.
Animals were randomly divided in groups of 8 mice each and each animal was used only once per
compound and euthanized. Animal care and experiments were performed in accordance with the
Committee for Research and Ethical Issues of the International Association for the Study of Pain
(Zimmermann, 1983).All animal procedures were approved by the local Animal Ethics Committee of
Auvergne (Comité Régional d'Ethique en Matière d'Expérimentation Animale Auvergne).
Behavioral testing in mice. Acetic acid writhing assay. Mice were pretreated with the compound (10
mg / kg, s.c.) or vehicle (5% Tween 80 in saline (0.9%)) 15 minutes before injection of acetic acid
(0.6% solution, 10 mL/kg) into the peritoneal cavity of the animal. This activates nociceptors directly
and / or produces inflamed viscera (sub-diaphragmatic organs) and subcutaneous (muscle wall) of
tissues. The number of induced abdominal writhes was determined during 20 min after the injection of
acetic acid. The inhibition of abdominal writhes (analgesic effect) is determined by determining the
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percentage of the number of cramps induced in the presence of compound reported to the number of
cramps induced in the presence of vehicle. Hot-plate test. After acclimatization for 5 min on a plate at
room temperature, mice were placed on a plate set at 52 °C until they started licking their forepaws
(Latencies) (cut-off time: 30 s). After having obtained two consecutive stable latencies, treatment effects
were assessed after 30 and 45 min.
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Microsomal stability. Aliquots contained 30 µL of rat liver microsomes (20 mg/mL), 237 µL
Phosphate buffer (pH 7.4) and 30 µL of NADPH solution (40 mM) were prepared and mixed
thoroughly. 3 µL drug solution (40 mM in DMSO) was added to each aliquot, the samples was vortexed
and incubated at 37°C with stirring. Aliquots (300 µL) were stopped at specific time points (2, 5, 15, 30,
60 min and 24 hours), and protein were precipitated in equal volume of cold acetonitrile. The mixture
was then filtered with microfilter (0.45µm) prior to HPLC analysis (wavelength: 254.4 nm).
Chemistry
General. Solvents were distilled prior to use. Other reagents were used as received. Products in organic
solutions were dried over dry sodium sulphate prior to evaporation of the solvent under reduced
pressure. Thin layer chromatography (TLC) was performed on pre-coated glass backed silica Duracil 25
UV 254 (Macherreynagel). Spots were visualized using UV light (254 nm) before using an ethanolic
solution of phosphomolybdic acid (heating). Purifications by column chromatography were carried out
on silica gel (70-230 mesh) or on C18 (30µm). Melting points were measured by a Reichert plate-
1
heating microscope. H NMR and 13C NMR spectra were recorded on a Brücker Avance 400
spectrometer at 400.13 and 100.61 MHz respectively. Chemical shifts δ are reported in ppm relative to
solvent residual signal (1H, 13C). The coupling constants J are given in Hertz (Hz). Electron Impact
Mass Spectra (EI-MS) were obtained on a spectrometer Hewlett Packard 5989B at 70 eV. High
Resolution Electro-spray Ionisation Mass Spectra (HR-ESI-MS) were obtained from the “Service de
Spectrométrie de masse du service UBP-START, (Université Blaise Pascal)”. GC/MS analysis
conditions used for progress of the reaction; column: UB 1701 (14% cyanopropylphenyl)-
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Journal of Medicinal Chemistry
methylpolysiloxane, injector temperature : 250 °C; oven temperature : 50 °C for 2 min then heating 50
°C/min until 290 °C. High performance liquid chromatography (HPLC) analysis conditions used to
determine purity of the product ; column: Poroschell 120 EC-C18 2.7 µm (3.0 x 50 mm), temperature
column: 40 °C; injection: 10 µL, 1 mL/min.; solvents used A- water with 0.1% formic acid and B-
methanol (95:5 to 5:95, method A) or A-water and B-acetonitrile (95:5 to 5:95, method B), Method UV
using 254.4 nm and 280.5 nm). On the basis of LC-MS, all final compounds were >95% pure unless
otherwise stated.
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Procedure A - Steglich esterification. Under an argon atmosphere, cyanoacetic acid 70 (1 to 1.1
equiv.) was dissolved in dry dichloromethane and cooled to 0 °C. At this temperature, DMAP (cat.) is
added to the mixture, followed by a dropwise addition of the appropriate alcohol (1.0 to 1.3 equiv.).
After 5 minutes of stirring, DCC (1.0 equiv.) is added in portion to the mixture. The mixture is allowed
to reach room temperature and stirred overnight. The white precipitate is filtered and the resulting
solution was concentrated in vacuo. Chromatography purification of the residue on silica gel with
cyclohexane / ethyl acetate gave the desired ester.
Procedure B - Fischer esterification. To a solution of cyanoacetic acid 70 (1.0 equiv.) and alcohol (1.5
equiv.) in toluene were added a few drops of concentrated H2SO4. The mixture was stirred at reflux for
24 hours, using a Dean-Stark apparatus to remove water. The reaction mixture was concentrated to half
its initial volume under reduced pressure, diluted with ethyl acetate (20 mL), and washed with saturated
NaHCO3 solution (3 x 8 mL), water and brine. The crude product was purified by flash chromatography
(cyclohexane/ ethyl acetate) on silica gel to give the desired.
Procedure C - Knoevenagel reaction. To a solution of 3,4-dihydroxybenzaldehyde 55 (1.1 equiv.) in
dry DCM (60 mL) under argon atmosphere were added 4-methylpentyl-2-cyanoacetate (1.0 equiv.) and
piperidine (0.9 equiv.). The reaction was carried out at room temperature for 20 hours. The mixture was
then quenched by the addition of a saturated aqueous solution of NH4Cl (20 mL) and extracted with
ethyl acetate (3 x 8 mL). The combined organic fractions were dried (Na2SO4). After removal of the
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solvent under reduced pressure, flash chromatography purification of the residue on silica gel
(cyclohexane - ethyl acetate: 1:0 to 0:1, v/v) gave the desired product.
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8
Procedure D - Hydrolysis reaction with TFA. The t-butyl ester (1.0 equiv.) was dissolved in 3.6 mL of
trifluoroacetic acid. The resulting solution was stirred for 1 h 30 at room temperature before removal of
the TFA under reduced pressure. The residue was triturated with ether and the solvent was evaporated
under reduce pressure. This work up was repeated three times to give the desired acid.
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Procedure E - Horner-Watsworth-Emmons Reaction. A solution of phosphonate (1.3 eq) in dry THF
was added dropwise to a stirred suspension of NaH (50% in oil, 1.4 eq) in dry THF under Ar
atmosphere at 0 °C. The reaction mixture was stirred for 30 min at 0 °C and a solution of aldehyde (1
eq) in dry THF was added. The reaction was carried out at room temperature until total consumption of
aldehyde (TLC). The reaction mixture was quenched with water. The aqueous phase was extracted with
EtOAc (3 x 20 mL) and the combined organic layers were dried (Na2SO4). After removal of the solvent
under reduced pressure, the purification of the residue by column chromatography on SiO2 using
cyclohexane/EtOAc as eluent gave the desired alkene.
(E)-2-Cyano-3-(3,4-dihydroxyphenyl)acrylic acid 2. Scheme 1, step i : Tert-butyl 2-cyanoacetate 72 was
prepared following procedure A using tert-butanol (1.86 mL, 19.40 mmol, 1.1 equiv.), cyanoacetic acid
70 (1.5 g, 17.63 mmol, 1.0 equiv.) and DCC (3.64 g, 17.63 mmol, 1.0 equiv.) in dry dichloromethane
(20 mL). Chromatography purification of the residue on silica gel with 9:1, v/v, cyclohexane / ethyl
acetate gave the desired product 72 as a pale liquid (1.13 g, 45%). Rf = 0.36 (SiO2, ethyl acetate/
1
13
cyclohexane, 1:9, v/v). H NMR (400 MHz, CDCl3): δ 3.36 (s, 2H), 1.48 (s, 9H). C NMR (101 MHz,
CDCl3): δ161.9, 113.6 (CN), 84.3, 27.8, 25.9. CAS Registry Number: 1116-98-9. Scheme 1, step v :
(E)-tert-butyl-2-cyano-3-(3,4-dihydroxyphenyl)acrylate 10 was prepared following procedure C using
3,4-dihydroxybenzaldehyde 55 (1.11 g, 8.00 mmol, 1.0 equiv.), tert-butyl-2-cyanoacetate 72 (1.13 g,
8.00 mmol, 1.0 equiv.) and piperidine (710 µL, 7.20 mmol, 0.9 equiv.) in anhydrous DCM (150 mL).
After removal of the solvent under reduced pressure, flash chromatography purification of the residue
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Journal of Medicinal Chemistry
on silica gel (cyclohexane - ethyl acetate: 1:0 to 0:1, v/v) gave the desired product 10as a pale yellow
1
2
3
1
solid (1.09 g, 52%). m.p: 211 °C. Rf = 0.20 (SiO2, ethyl acetate/ cyclohexane, 4:6, v/v). H NMR (400
4
5
6
7
8
MHz, MeOD-d4): δ 8.03 (s, 1H), 7.63 (d, J = 2.2 Hz, 1H), 7.36 (dd, J = 8.4, 2.2 Hz, 1H), 6.88 (d, J = 8.3
Hz, 1H),1.56 (s, 9H). 13C NMR (101 MHz, MeOD-d4): δ 163.6, 155.6, 152.9, 147.1, 127.8, 125.1,
117.7, 117.5 (CN), 116.7, 100.2, 84.1, 28.3. HR-ESI-MS calcd for C14H15NO4 (M + Na+) = 284.0899;
found 284.0905. HPLC: 254.4 nm: 97% (H2O/MeOH) tR = 18.681 min. Scheme 1, step viii : (E)-2-
Cyano-3-(3,4-dihydroxyphenyl)acrylic acid 2 was prepared following procedure D using (E)-tert-butyl
2-cyano-3-(3,4-dihydroxyphenyl)acrylate 10 (75.5 mg, 0.29 mmol, 1.0 equiv.), TFA (1 mL) to give the
desired product 2as a yellow powder (53.5 mg, 90%). m.p: 261 °C. Rf = 0 (ethyl acetate/ cyclohexane,
2:8, v/v). 1H NMR (400 MHz, MeOD-d4): δ 8.11 (s, 1H), 7.65 (s, 1H), 7.37 (2 x d, J = 8.0 Hz, 2H). 13C
NMR (101 MHz, MeOD-d4): δ 166.3, 156.4, 153.1, 147.3, 128.0 - 116.9 (C-ar), 99.4, 85.2. HPLC:
254.4 nm: 100% (H2O/MeOH), tR = 12.422 min. (Method A). HR-ESI-MS calcd for C10H7NO4 (M +
Na+) = 228.0273; found 228.0263. CAS Registry Number: 122520-79-0.
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2-(3,4-Dihydroxybenzylidene)malonic acid 6. Scheme 2, step iii : To 3,4-dihydroxybenzaldehyde 55
(800 mg, 5.97 mmol, 1 eq) in anhydrous DMF (7 mL) was added a solution MOMCl (951.7 ꢁL, 12.53
mmol, 1.1 eq), followed by i-PrNEt2 (6.24 mL, 35.82 mL 6 eq). After stirring for 5 h at room
temperature, a solution of 2 N NH4OH was added slowly and the mixture was extracted with Et2O (3 x
50 mL). The combined organic phases were washed with brine, dried (Na2SO4) and concentrated under
reduced pressure. The residue was purified by flash chromatography (SiO2, 20% EtOAc/cyclohexane) to
obtain 3,4-bis(methoxymethoxy)-benzaldehyde 91 as a clear liquid obtained (917.4 mg, 68%). Rf = 0.55
(20% EtOAc/cyclohexane). 1H NMR (400 MHz, CDCl3): δ 9.85 (s, 1H, CHO), 7.66 (d, J = 2.0 Hz, 1H),
7.50 (dd, J = 8.0, 2.0 Hz, 1H), 7.27 (d, 1H, J = 8.0 Hz), 5.32 and 5.31 (2 x s, 4H), 3.55 and 3.54 (2 x s,
6H). 13C NMR (101 MHz, CDCl3): δ 190.8 (CHO), 152.6, 147.5, 131.1, 126.3, 116.0, 115.3, 95.3, 95.0,
56.5, 56.3. CAS Registry Number: 6515-06-6. Scheme 2, step iv : A mixture of 3,4-
bis(methoxymethoxy)benzaldehyde 91 (300 mg, 1.33 mmol, 1 eq), di-tert-butyl malonate (311.1 µL,
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Journal of Medicinal Chemistry
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1.39 mmol, 1.05 eq), piperidine (22.3 µL, 0.23 mmol, 0.17 eq) and benzoic acid (18.0 mg, 0.15 mmol,
0.11 eq) in toluene (2 mL) was heated under reflux using a Dean-Stark trap. After 20 h, toluene was
evaporated off and the reaction mixture was dissolved in EtOAc (10 mL). This solution was washed
with H2O (10 mL). The organic phase was dried (MgSO4), filtered and evaporated under reduced
pressure. The residue was purified by column chromatography (SiO2, 20% EtOAc/cyclohexane) to lead
to di-tert-butyl-2-(3,4-bis(methoxymethoxy)benzylidene)-malonate 92 as a clear oil (286.1 mg, 51%).
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Rf = 0.41 (20% EtOAc/cyclohexane). H NMR (400 MHz, CDCl3): δ 7.45 (s, 1H), 7.31 (d, J = 2.0 Hz,
1H), 7.19 (dd, J = 8.5, 2.0 Hz, 1H), 7.12 (d, J = 8.5 Hz, 1H), 5.25 (s, 2H), 5.21 (s, 2H), 3.50 (2 x s, 6H),
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1.54 and 1.52 (2 x s, 18H). C NMR (101 MHz, CDCl3) δ 166.1, 163.8, 149.1, 146.9, 139.5, 127.6 -
115.9 (C-Ar), 95.6, 95.1, 82.2, 81.9, 56.3, 56.2, 28.1, 28.0. 2-(3,4-Dihydroxy-benzylidene)malonic acid
6 was prepared according to procedure D with TFA (4.5 mL) and di-tert-butyl 2-(3,4-
bis(methoxymethoxy)-benzylidene)malonate 92 (286.1 mg, 0.67 mmol, 1.0 equiv.) in 4.5 mL of dry
DCM. Acid 6 was obtained as a pale green powder (102.6 mg, 68%). m.p: 208 °C. Rf = 0 (ethyl acetate/
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cyclohexane, 6:4, v/v). H NMR (400 MHz, MeOD-d4): δ 7.50 (s, 1H), 7.08 (d, J = 2.1 Hz, 1H), 6.97
(dd, J = 8.3, 2.1 Hz, 1H), 6.77 (d, J = 8.3 Hz, 1H). C NMR (101 MHz, MeOD-d4): δ 171.2, 167.5,
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149.7, 146.5, 142.4, 125.7, 124.5, 124.4, 117.1, 116.2. HPLC: 254.4 nm: 98% (H2O/MeOH), tR = 0.879
min (Method A). HR–ESI-MS calcd for C10H7O6 (M + H+) = 223.0243; found 223.0256.
(Z)-3-(3,4-Dihydroxyphenyl)-2-fluoroacrylic acid 7. Scheme 2, step vii : To a solution of freshly
distilled of diisopropylamine (2.6 mL, 19.3 mmol, 4.1 equiv.) in dry THF (8 mL), was added 1.6 M of
n-BuLi (11.7 mL, 18.84 mmol, 4.0 equiv.) at -30 °C. The solution was stirred for 30 min at -30 °C, then
transferred into the prepared solution of ethyl 2-fluoroacetate 94 (500 mg, 4.71 mmol, 1.0 equiv.) and
TMSCl (3.6 mL, 28.3 mmol, 6.0 equiv.) in 45 mL of dry THF at -78 °C. The reaction mixture was
stirred then warmed to 0 °C over a period of 4 h. The reaction was quenched with a saturated aqueous
solution of NaHCO3 (100 mL) at 0 °C. The aqueous phase was extracted with ether (3 x 50 mL). The
organic phase was dried (MgSO4), filtered and concentrated (20 mL). The organic phase was washed
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Page 29 of 48
Journal of Medicinal Chemistry
with a solution of tartaric acid (50 mL) and was stirred overnight at room temperature. The aqueous
layer was extracted twice with ether (3 x 20 mL). The organic layers were combined, dried (MgSO4)
and concentrated under reduced pressure. The residue was purified by chromatography (Et2O/pentane,
1:9, v/v) to lead to Ethyl 2-fluoro-2-(trimethylsilyl)acetate 95 as colourless oil (800 mg, 91%). Rf = 0.68
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(ethyl acetate/ cyclohexane, 5:95, v/v). H NMR (400 MHz, CDCl3): δ 4.95 (d, J = 48.1 Hz, 1H), 4.20
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(q, J = 7.0 Hz, 2H), 1.28 (t, J = 7.0 Hz, 3H), 0.18 (s, 9H).The 1H RMN is agreement with the literature.
CAS Registry Number: 128822-96-8. Scheme 2, step viii :To a solution of freshly distilled of
diisopropylamine (436.8 µL, 3.37 mmol, 1.5 equiv.) in dry THF (13 mL), was added 1.6 M of n-BuLi
(2.11 mL, 3.37 mmol, 1.5 equiv.) at -30 °C. The solution was stirred for 30 min at -30 °C and then
allowed to cool to -78 °C. Ethyl 2-fluoro-2-(trimethylsilyl)acetate 95 (400 mg, 2.25 mmol, 1.0 equiv.)
dissolved in dry THF (2 mL) was added to the LDA solution and stirred for 40 min at -78 °C, followed
by addition of 3,4-bis(methoxymethoxy)-benzaldehyde 91 (523.2 mg, 2.31 mmol, 1.03 equiv.) in dry
THF (2 mL). The reaction was carried out at -78 °C until total consumption of 95 by GC-MS. The
reaction mixture was quenched with a saturated aqueous solution of NH4Cl (30 mL) at 0 °C. The
separated aqueous layer was extracted with EtOAc (3 x 20 mL). The combined organic layers were
dried (MgSO4), filtered and evaporated. The residue was purified by silica gel chromatography (20%
EtOAc/cyclohexane) giving (Z)-ethyl 3-(3,4-bis(methoxymethoxy)phenyl)-2-fluoroacrylate as a
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colourless oil (263.3 mg, 37%). Rf = 0.31 (ethyl acetate/ cyclohexane, 2:8, v/v). H NMR (400 MHz,
CDCl3): δ 7.38 (s, 1H), 7.19 – 6.80 (m, 2H), 6.79 (d, J = 35.1 Hz, 1H), 5.17 and 5.15 (2 x s, 4H), 4.24
(q, J = 7.2 Hz, 2H), 3.43 and 3.41 (2 x s, 6H), 1.28 (t, J = 7.2 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ
161.5 (d, J = 34.0 Hz), 148.6, 147.6, 146.0 (d, J = 263.0 Hz), 125.5 (d, J = 7.1 Hz), 125.4 (d, J = 4 Hz),
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118.7 (d, J = 8.5 Hz), 117.2 (d, J = 4.5 Hz), 116.2, 95.5, 62.1, 56.3, 14.7. F NMR (376 MHz, CDCl3):
δ -127.6. A solution of (Z)-ethyl 3-(3,4-bis(methoxy-methoxy)phenyl)-2-fluoroacrylate (263.3 mg,
0.838 mmol, 1.0 equiv.) in MeOH (10 mL) and 1 M NaOH (1.68 mL, 1.68 mmol, 2.0 equiv.) was then
stirred at room temperature for 2 h. After removing the solvent under reduced pressure, the residue was
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