Optimization of benzimidazole series as opioid receptor-like 1 (ORL1) antagonists: SAR study directed toward improvement of selectivity over hERG activity

Article abstract of DOI:10.1016/j.bmcl.2009.04.022

A structure-activity relationship (SAR) study on the benzimidazole series of opioid receptor-like 1 (ORL1) antagonists related to 1 is described. Optimization of 1 by introduction of a hydrophilic substituent into the thioether part resulted in identifica

Full text of DOI:10.1016/j.bmcl.2009.04.022

Bioorganic & Medicinal Chemistry Letters 19 (2009) 3100–3103
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Bioorganic & Medicinal Chemistry Letters
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Optimization of benzimidazole series as opioid receptor-like 1 (ORL1)
antagonists: SAR study directed toward improvement of selectivity
over hERG activity
Kensuke Kobayashi, Tetsuya Kato, Izumi Yamamoto, Atsushi Shimizu, Sayaka Mizutani, Masanori Asai,
Hiroshi Kawamoto, Satoru Ito, Takashi Yoshizumi, Mioko Hirayama, Satoshi Ozaki, Hisashi Ohta,
*
Osamu Okamoto
Tsukuba Research Institute, Banyu Pharmaceutical Co., Ltd, Okubo-3, Tsukuba, Ibaraki 300-2611, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A structure–activity relationship (SAR) study on the benzimidazole series of opioid receptor-like 1 (ORL1)
antagonists related to 1 is described. Optimization of 1 by introduction of a hydrophilic substituent into
the thioether part resulted in identification of potent ORL1 antagonists with high selectivity over binding
affinity for hERG and other opioid receptors.
Received 4 February 2009
Revised 27 March 2009
Accepted 1 April 2009
Available online 11 April 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
ORL1 antagonist
Nociceptin/orphanin FQ
Benzimidazole
hERG channel
Opioid receptor-like 1 (ORL1), was discovered in 1994 based on
its high amino acid sequence homology to the classical opioid
receptors.1 Despite this homology, this receptor was found not to
bind classical opioids with appreciable affinity. Soon after, its
endogenous agonist, a 17-amino acid peptide known as nociceptin
or orphanin FQ (NC/OFQ), was identified.2 Subsequently, a number
of reports have demonstrated the possible involvement of the NC/
OFQ-ORL1 system in pain regulation,3 morphine tolerance,4 learn-
ing and memory,5–7 food intake,8 anxiety,9 the cardiovascular sys-
tem,10,11 locomotor activity,12 etc.13 To date, however, the
development of non-peptidic ORL1 antagonists with good selectiv-
(SAR) investigation on the benzimidazole series to overcome this
issue. Our strategy to attenuate hERG channel binding in com-
pounds related to 1 consisted of reducing the basicity of the piper-
azine nitrogen30 and introducing a hydrophilic group to the
thioether part, as shown Figure 1.
The synthesis of benzimidazole ORL1 antagonists reported
herein is outlined in Scheme 1–3.
For SAR studies on the basicity of the piperazine ring, com-
pounds 5a–f were prepared according in Scheme 1. A chiral nitro-
aniline 229 was reduced with Fe followed by condensation with
carbon disulfide gave a benzimidazole 2-thione 3. Coupling 3 with
t-butyl alcohol and deprotection of the Boc group on the piperazine
ring in TFA provided N-unsubstituted piperazine intermediate 4.
Next, 4 was converted to compounds 5a–f by reductive amination
with the corresponding aldehyde, acylation with the related acid
anhydride followed by reduction with borane-methyl sulfide com-
plex (BMS), or alkylation with bromoacetonitrile or 3-hydroxypro-
pionitrile, respectively.
ity towards opioid receptors has resulted in only a few classes14–16
:
benzimidazolyl piperidines,17–19 4-aminoquinolines,20 and spiropi-
peridines and 4-aryl piperidines.21–28 The biological role of the
ORL1 receptor and therapeutic roles for ORL1 antagonists will be
established through the development of drug-like molecules with
good bioavailability and good brain penetration.
In a previous communication,29 we reported the discovery of
orally active and brain-penetrable ORL1 antagonist 1. However,
compound 1 showed relatively potent human ether-a-go-go re-
lated gene (hERG) K+ channel binding affinity (IC50 = 1600 nM).
Therefore, our efforts focused on removal of hERG inhibitory activ-
ity. Herein, we report an extensive structure–activity relationship
* Corresponding author. Tel.: +81 29 877 2000; fax: +81 29 877 2029.
E-mail address: osamu_okamoto@merck.com (O. Okamoto).
Figure 1. Strategy to improve the selectivity over the hERG binding affinity.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.04.022

K. Kobayashi et al. / Bioorg. Med. Chem. Lett. 19 (2009) 3100–3103
3101
Table 1
Binding affinity and functional activity to ORL1, binding affinity to hERG, and
calculated pKa of 5a–f
ORLl bindinga ORL1
IC50 (nM)
antagonisma
hERG binding pKa
IC50 (nM)
b
Compd
R
IC50 (nM)
1
Hydroxyethyl 2.6
0.65
2.9
1600
990
>10,000
7.24
7.74
4.64
3.62
2.48
4.97
4.04
5a
5b
5c
5d
5e
5f
Et
9.2
23
630
61
CH2CHF2
CH2CF3
CH2CN
CH2CH2CN
>10,000
>10,000
>10,000
Scheme 1. Reagents and conditions: (a) Fe, NH4Cl, THF–MeOH–H2O, reflux; (b) CS2,
aq NaOH, EtOH, 80 °C, 92% (2 steps); (c) t-BuOH, TFA, rt, 94%; (d) 5a, 5f: MeCHO or
AcCHO, Zn(BH3CN)2, MeOH, 70% (for 5a), 26% (for 5f); 5b, 5c: (CF2HCO)2O or TFAA,
NEt3, THF; then BMS, THF, 23% (for 5b), 23% (for 5c); 5d: BrCH2CN, K2CO3, DMF,
39%; 5e: HOCH2CH2CN, DIAD, PPh3, THF, 19%.
27
6.4
41
*
CH2COCH3
38
*
Racemate.
n = 1 (Ref. 35).
a
Table 2
Binding affinity and functional activity to ORL1 and binding affinity to hERG of 7a–f
Scheme 2. Reagents and conditions: (a) tertiary alcohols, TFA, rt-80 °C, then
separation by Chiral HPLC (for 7d–n).
Compd
R
Bindinga
IC50 (nM)
Antagonisma
IC50 (nM)
hERG binding
IC50 (nM)
Compounds 7a–n, which have hydrophilic functionality in the
thioether part, were prepared according to Scheme 2. Analogues
were derived from key intermediate 631 via a coupling reaction
with the corresponding tertiary alcohol in TFA. The stereoisomers
of cyclohexane or cyclopentane derivatives (7d–n) were separated
using an appropriate chiral HPLC.
Analogues 7o–q were prepared according to Scheme 3; cycliza-
tion of ester 8, which was prepared by the procedure described in
Scheme 2, with N-hydroxyacetamidine gave 1,2,4-oxadiazole
derivative 7o. Preparation of hydrazide from 8 with hydrazine hy-
drate followed by cyclization with the corresponding ortho ester
gave 1,3,4-oxadiazole derivatives 7p and 7q.
7a
22
7b
7c
7d
7e
21
8.6
1.1
2.1
0.72
46
4.3
9.5
2.4
16
3900
1500
3500
2000
Analogues were tested for their inhibitory effects on ligands to
the human ORL1 receptor, and on GTPcS binding to proteins using
membrane fractions of CHO cells expressing ORL1. Binding affini-
ties for ORL1 were determined by displacement of [125I]Tyr14-NC/
OFQ, and agonist/antagonist activities were measured by the
[
35S]GTP S binding method.32 Binding affinity to the hERG K+
c
channel was measured by displacement of [35S]-radiolabeled
MK499 in membranes derived from HEK 293 cells stably transfec-
ted with the hERG gene and expressing the IKr channel protein.33,34
First, the effects of substituents at the piperazine nitrogen were
investigated. In the previous SAR study,31 it was found that intro-
duction of bulky substituents at the piperizine nitrogen in our benz-
imidazole class decreased the binding affinity for ORL1. N-
Hydroxyethyl and N-ethyl analogues showed potent binding affin-
7f
*
Racemate.
a
n = 1 (Ref. 35).
Scheme 3. Reagents and conditions: (a) TFA, 80 °C; (b) 7o: N-hydroxyacetamidine, NaH, THF, reflux, 11%; 7p, 7q: hydrazine hydrate, EtOH, reflux, 83%, HC(OEt)3 or
MeC(OEt)3, reflux, 33% (for 7p), 45% (for 7q).

3102
K. Kobayashi et al. / Bioorg. Med. Chem. Lett. 19 (2009) 3100–3103
Table 4
Off-target activities (
ity. Therefore, we selected relatively small substituents with the
ability to lower the pKa of the basic nitrogen for our initial approach
(Table 1).35 Introduction of a difluoroethyl group resulted in a sig-
nificant decrease in hERG activity; however, the binding affinity
for ORL1 also decreased by 10-fold (compound 5b). The trifluoro-
ethyl analogue 5c, whose calculated pKa is lower than 5b, showed
very low ORL1 affinity. The other derivatives (5d–f) also
showed>10-fold decreased intrinsic potency, despite their attenu-
ated hERG binding. As extensive precedent for the circumvention
of hERG activity, the reduction of basicity of the molecule worked
well in this series, but this approach impaired ORL1 binding affinity.
We then shifted our attention to another approach, the intro-
duction of a hydrophilic substituent onto the thioether portion.
We selected N-ethyl piperazine as a template, as shown in Table 2.
Compound 7b with a carbamate group at the acyclic thioether
part was equipotent to a t-butyl analogue 7a. Encouraged by the
observation that introduction of the hydrophilic substituent has
l-,
j
-opipid receptors) of 1 and 7h
Binding IC50 (nM)
Compd
ORL1
la
ja
1
7h
2.6
0.81
1900
840
120
160
a
Displacement of a [3H]diprenorphin (
l
) and [3H]U69593 (
j
) binding to CHO
cells stably expressing cloned human l-, and j-opioid receptors, respectively.
little impact on the intrinsic potency, further exploration of the thi-
oether part was performed. As a result, piperidine analogue 7c
showed increased ORL1 activity. Moreover, when a carbamate
group was installed at the 4-position of the 1-meth-
ylcyclohexylthioether, trans-isomer 7e exhibited significantly en-
hanced binding affinity and antagonistic activity for ORL1. On the
other hand, ring contraction of the cyclohexane ring to a cyclopen-
tane ring resulted in a decrease in ORL1 affinity (7f).
By modifying the thioether substituent, ORL1 activity was en-
hanced. Interestingly, however, the hERG inhibitory activity re-
mained high. We next focused on optimization of the 4-
substituent on the 1-methylcyclohexane ring. Table 3 summarizes
the results from SAR studies on the potency of ORL1 and hERG bind-
ing affinity. Replacement of the carbamate group with the other
nitrogen substituents, such as amide or sulfonamide (7g–j), resulted
in negligible hERG inhibitory activity (IC50 > 10,000 nM) while
retaining ORL1 potency. This result can be attributed to the in-
creased hydrophilicity of 7g–j (log D7.4 = 2.5, 2.7, 2.5 and 2.9, respec-
tively) in comparison to 7e (log D7.4 = 3.4).30 Analogues with
carbamoyl (7k) and sulfone (7l) also showed good selectivity over
hERG binding. We then incorporated heterocycles into this position.
Derivatives having five-membered heterocycles (7m–q) exhibited
good ORL1 antagonistic activity, with 2-methyl-1,3,4-oxadiazole
7q showing negligible binding affinity for hERG K+ channels.
Representative compound 7h36 from this series was evaluated
for selectivity over other opioid receptors (Table 4). This compound
Table 3
Effects of 4-substitution on the 1-methylcyclohexane ring
ORLl bindinga
IC50 (nM)
Antagonisma
IC50 (nM)
hERG
binding
IC50 (nM)
Log D7.4
b
Compd
R
7e
NHC02Me
2.4
1.9
0.81
2.1
1.4
1.0
6.9
2.2
7.4
1.7
1.1
1.8
0.72
1.1
3500
3.4
7g
7h
7i
NHAc
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
4400
2.5
2.7
2.5
2.9
N(Me)Ac
NHS02Me
1.3
had good selectivity over
l and j receptors, similarly to lead com-
pound 1.
1.8
In conclusion, we optimized the benzimidazole lead 1 for the
removal of hERG activity. The results of the SAR study led to the
identification of potent ORL1 antagonists, which exhibited high
selectivity over binding affinity for hERG. In addition, the selected
7j
0.55
1.1
compound 7h showed good selectivity over
l and j receptors. Fur-
ther SAR studies of this series are currently underway.
7k
71
7m
7n
7o
7p
CONHMe
S02Me
Acknowledgments
We would like to acknowledge the contributions of the follow-
ing scientists to this work: T. Azuma-Kanoh, H. Nambu, N. Sakai, T.
Inoue, D. Ichikawa, S. Okuda, N. Ami, M. Fukushima and M. Nishino.
4.0
0.90
3.7
3.1
2.3
3.8
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