Bioisosteric replacement of the hydrazide pharmacophore of the cannabinoid-1 receptor antagonist SR141716A. Part I: Potent, orally-active 1,4-disubstituted imidazoles

Article abstract of DOI:10.1016/j.bmcl.2009.07.130

A new series of CB₁ receptor antagonists incorporating an imidazole-based isosteric replacement for the hydrazide moiety of rimonabant (SR141716) is disclosed. Members of this imidazole series possess potent/selective binding to the rCB₁

Full text of DOI:10.1016/j.bmcl.2009.07.130

Bioorganic & Medicinal Chemistry Letters 19 (2009) 5351–5354
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Bioisosteric replacement of the hydrazide pharmacophore of the
cannabinoid-1 receptor antagonist SR141716A. Part I: Potent, orally-active
1,4-disubstituted imidazoles
*
Robert L. Dow , John R. Hadcock, Dennis O. Scott, Steven R. Schneider, Ernest S. Paight, Philip A. Iredale,
Philip A. Carpino, David A. Griffith, Marlys Hammond , Paul DaSilva-Jardine
Pfizer Global Research and Development, Groton, CT 06340, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A new series of CB1 receptor antagonists incorporating an imidazole-based isosteric replacement for the
hydrazide moiety of rimonabant (SR141716) is disclosed. Members of this imidazole series possess
potent/selective binding to the rCB1 receptor and exhibit potent hCB1 functional activity. Isopropyl analog
9a demonstrated activity in the tetrad assay and was orally-active in a food intake model.
Ó 2009 Published by Elsevier Ltd.
Received 12 June 2009
Revised 25 July 2009
Accepted 28 July 2009
Available online 6 August 2009
Keywords:
Cannabinoid-1 receptor antagonist
SR-141716
Imidazole isostere
Food intake inhibition
The endocannabinoid system (ECS), and specifically the cannab-
inoid type 1 (CB1) receptor, plays a pivotal role in energy homeo-
stasis.1–3 As such, stimulation of the ECS promotes food intake
and energy storage and may be chronically overactive in obese
subjects.4–7 In contrast, blockade of the CB1 receptor in the central
nervous system decreases food intake and increases energy expen-
diture, leading to a reduction in body weight.8–11
CB1 receptor antagonists may provide effective therapy options
for the management of metabolic disorders, such as obesity. It was
hoped that CB1 receptor antagonists might provide effective ther-
apy options for the management of metabolic disorders, such as
obesity. Unfortunately, several CB1 receptor inverse agonists/
antagonists were recently withdrawn from clinical development
including the diarylpyrazole rimonabant12 (1, SR141716A), the
acyclic amide taranabant 2 (MK-0364)13 and otenabant 3 (CP-
945,598).14
ity that this chemotype could prove problematic upon longer term
exposure in humans. Our concerns were based on reports that cer-
tain hydrazide-containing drugs, such as isoniazid and iproniazid,
are hepatotoxic.15,16 Mechanistic studies on these drugs concluded
that the observed hepatotoxicity is likely driven by free radical for-
mation via oxidation of the hydrazine core. Based on this chemo-
type toxicity concern, efforts in our laboratory focused on
designs that would eliminate this potential issue.
Based on their precedent as isosteric equivalents of amides we
chose to focus on nitrogen-containing heterocycles as replace-
ments for the hydrazide functionality in 1.17 Our design approach
focused on two key pharmacophore elements present in 1. The first
of these is incorporation of a hydrogen bond acceptor feature that
could mimic the carbonyl oxygen of 1. It has been hypothesized
that a hydrogen bond between Lys192 of the CB1 receptor and this
oxygen is a significant component of the binding affinity and the
expression of inverse agonist activity of 1.18–20 The second phar-
macophore element taken into account is proper disposition of
the substituents so as to attain good overlap with the piperidinyl
substituent of 1.
In considering options for designing agents that would poten-
tially have an improved profile relative to 1, one of the elements
we chose to focus on is the hydrazide functionality. While during
its early clinical testing there were no reports of toxicity resulting
from the hydrazide functionality present in 1, there is the possibil-
From the various heterocyclic options, 1,4-disubstituted imida-
zoles 4 met the pharmacophore criteria cited above and are the fo-
cus of this communication.21 Subsequent to the studies reported
here on imidazoles, both oxadiazole22 and tetrazole23 replace-
ments for the amide functionality of 1 have been reported (Fig. 1).
An overlay of the minimized conformations of 8a and 1 suggests
that the N-3 imidazole nitrogen atom of 8a is positioned to serve
* Corresponding author. Tel.: +1 860 441 4423; fax: +1 860 441 0548.
E-mail address: robert.l.dow@pfizer.com (R.L. Dow).
Present address: GlaxoSmithKline Pharmaceuticals, Metabolic Pathways Centre
for Excellence in Drug Discovery, 709 Swedeland Road, King of Prussia, PA 19406,
USA.
0960-894X/$ - see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2009.07.130

5352
R. L. Dow et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5351–5354
p-TolO2S
Me
Me
CHO
NHCHO
a)
N
N
N
N
Cl
Cl
Cl
Cl
76-81%
X
X
5
6
b)
64-75%
R
N
N
N
p-TolO2S
Me
Figure 1. Overlay of minimized conformations of 114 (green) and 8a (yellow).
Me
NC
c)
R'
N
N
N
Cl
Cl
the same hydrogen bond acceptor role with the CB1 receptor
hypothesized for the carbonyl oxygen of 1. Also, the N-1 substitu-
ent vector of 8a is reasonably well aligned with the piperdinyl moi-
ety of 1.
Cl
Cl
41-73%
X
X
8a-k X = Cl
9a-e
7
X = H
CF3
Scheme 1. Reagents and conditions: (a) H2NCHO, p-toluenesulfinic acid, Me3SiCl,
toluene, 50 °C; (b) POCl3, 2,6-lutidene, THF; (c) when R0 = H:RNH2, HCOCO2H,
K2CO3, DMF; when R is other than H:RNH2, R0CHO, K2CO3, DMF.
O
N
O
Me
Me
O
N
N
Me
N
Me NH
H
Me
N
N
Cl
Cl
Me
Me
CN
Me
N
a)
H
CN
N
N
N
N
Cl
Cl
2 (MK-0364)
Cl
Cl
Cl
24%
Cl
1
(SR141716)
Cl
Cl
O
NH2
R
N
N
N
10
NH
Et
11
Me
N
N
N
Scheme 2. Reagents and conditions: (a) LiN(TMS)2, THF, then Me2C(O)CH2Br.
N
N
N
Cl
X
Me
N
Y
Cl
O
N
Me
Me
N
Me
Me
4
a)
H
3
(CP-945598)
87%
N
N
N
Cl
Cl
Cl
b)
Cl
22%
The synthetic route utilized to prepare derivatives 8a–k and
9a–e employed the tosylmethyl isocyanide-based approach to
imidazole ring formation (Scheme 1).24 Pyrazole carboxaldehyde
521 was condensed with formamide in the presence of p-toluene-
sulfinic acid to afford intermediate 6, which was dehydrated to
provide tosylmethyl isocyanide 7. Imidazole ring formation was
accomplished either with the requisite amine and glyoxylic acid
in the case of the 1,4-disubstituted analogs (R0 = H) or with an
amine and an aldehyde in the case of 1,4,5-trisubstituted analogs
8 and 9 (Schemes 2 and 3).
The 2,4-disubstituted imidazole 11 was prepared by cyclization
of the imidate derived from nitrile 1021 with the requisite bromoke-
tone. Regioisomeric imidazole 13 was prepared via condensation of
the bromoketone derived from 1221 with isopropylcarboxamidine.
Rat CB1 receptor binding affinities for the imidazole-based isos-
teres are detailed in Table 1. The rat receptor assay serves as a good
surrogate for the human form given their high sequence
Cl
Cl
13
12
Scheme 3. Reagents and conditions: (a) Br2, CHCl3; (b) isopropylcarboxamidine
HCl, K2CO3, DMF, 80 °C.
homology.25 Inhibition of GTP
c [
35S] binding to the human CB1
receptor was also determined for key analogs. Though the data is
not shown, all of the imidazoles in Table 1 possess greater than
100-fold binding selectivity versus the human CB2 receptor.
Cyclohexylimidazole 8a was selected as an initial probe com-
pound to examine the feasibility of this approach. While 8a is
ꢀ10-fold less potent than 1 in binding to the rat receptor, there
is only a threefold difference in the human CB1 functional response.
While this initial result was very encouraging, there are multiple

R. L. Dow et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5351–5354
5353
Table 1
solubility (<5 lg/mL-kinetic) and high clearance associated with
these highly lipophilic analogs.
Biochemical profiles for compounds 1, 8a–k, 9a–e, 11 and 13
In order to explore the vector off the imidazole 5-position (R0 in
8) a small set of trisubstituted analogs were prepared. Incorpora-
tion of a methyl group (8i) has no impact on the biochemical pro-
file. This result was somewhat surprising based on the negative
impact of N-methylation of the N–H of a homolog of 126 and an
expectation that the methyl group of 8i should induce a signifi-
cantly altered conformation around the pyrazole-imidazole bond.
Attempts to incorporate larger substituents (8j) or polar function-
ality (8k) resulted in diminished activity against the CB1 receptor.
Because the CB1 receptor antagonist pharmacophore apparently
requires highly lipophilic chemical matter (1/8d, clog D7.4 = 5.8/
5.9), modifications to 8 that reduced lipophilicity were of interest.
Replacement of the pyrazole methyl group of 8d with a hydrogen
resulted in a sixfold reduction in CB1 inhibitory activity and signif-
icantly lower aqueous solubility.28 An alternative approach, based
on other CB1 chemical series identified in our laboratories, is to re-
move the para chlorine atom from the N-1 pyrazole phenyl substi-
tuent. Comparison of des-chloro analogs 9a and 9b with 8d and 8h,
respectively, confirmed the expectation that there would be little
or no impact on their in vitro profiles. This modification results
in a one log unit reduction in the clog D7.4 relative to their struc-
tural counterparts in series 8.
Compd
R
R0
rCB1 Kia
(nM)
hGTPc[
35S]
Kib (nM)
1
0.9
8.6
10
14
4.9
20
25
14
1.8
5.7
38
1.6
4.7
5.0
NDc
9.0
ND
ND
ND
0.1
9.0
ND
ND
7.6
0.1
7.9
3.8
ND
5.2
9.5
8a
8b
8c
8d
8e
8f
8g
8h
8i
Cyclohexyl
Cyclobutyl
t-Butyl
i-Propyl
Ethyl
4-Tetrahydropyranyl
Benzyl
C(Me)2Ph
i-Propyl
i-Propyl
i-Propyl
H
H
H
H
H
H
H
H
Methyl
Ethyl
CH2OH
H
H
H
8j
8k
9a
9b
9c
9d
9e
11
13
19
i-Propyl
5.3
1.1
7.4
19
31
9.5
7.2
C(Me)2Ph
Cyclopropyl
Phenyl
H
H
i-Propyl
a
Rat brain tissue preparation used for CB1 receptor assay.14 These data were
obtained from a single determination run in triplicate.
b
Human CB1 receptor stably transfected into CHO cells.14 These data were
obtained from a single determination run in triplicate.
c
ND = Value not determined.
Since significant structure–activity analysis has been carried
out on series 8 and the initial potency trends between 8 and 9 were
comparable, further efforts with 9 focused on a small number of
analogs directed towards improving potency. Given the potential
for cyclopropyl groups to serve as enhanced replacements29 for di-
methyl or isopropyl substituents, compound 9c was prepared, but
did not show any potency advantage. Analog 9d was prepared to
determine whether a phenyl group could mimic the interactions
imidazole regioisomers that could potentially fulfill the CB1 recep-
tor pharmacophore and it was important to optimize this feature
before investigating finer structure–activity relationships.
For the lead CB1 receptor antagonist 1 there is the potential for a
hydrogen bond to be formed between the N–H of the hydrazide and
N-2ofthepyrazolering. If thisarrangementisanimportantdetermi-
nate of conformational stabilization resulting in enhanced receptor
binding or alternatively the N–H provides a key interaction with
the CB1 receptor, incorporation of a similar hydrogen bond donor
feature could provide enhanced affinity for the imidazole series.26
This led to the preparation of the regioisomeric 2,4-disubstituted
imidazoles 11 and 13. Both of these imidazoles contain a free N–H
and would thus have the potential to mimic the hydrazide N–H of
1. However, when compared to the corresponding lead imidazole
8d, neither 11 nor 13 provide a potency advantage. In addition,
in vitro human microsomal analysis suggested series 8 (8d,
hCLint = 14 mL/min/kg) would have a threefold lower hepatic clear-
ance potential relative to the two N–H containing imidazole series
(11/13, hCLint = 42,50 mL/min/kg, respectively). Based on these re-
sults further optimization efforts were focused on the 1,4-substi-
tuted imidazoles.
Since the overlap of the cyclohexyl ring of 8a extends beyond
the space occupied by the piperidinyl group of 1 in the respective
minimized structures, analogs possessing smaller N-1 alkyl sub-
stituents were evaluated. Reducing the steric bulk of the N-1 sub-
stituent by incorporation of smaller carbocyclic (8b) or simple
branched alkyl (8c–e) substituents has less than a twofold impact
on CB1 binding/functional activities. Introduction of heteroatoms
(e.g., 8f) as a means of further improving the clearance profile of
this series was successful (hClint <7 mL/min/kg), though in all
cases reduced CB1 potency is observed. While N-benzyl analog 8g
is significantly less potent than 1, findings from related CB1 series
of the
a,a-dimethylbenzyl substituent present in 9b. This modifi-
cation leads to an approximate 30-fold loss in functional activity
relative to 9b. The final modification pursued is that in which the
isopropyl substituent of 9a is moved into the 5-position of the
imidazole ring, producing the free N–H imidazole 9e. In analogy
to the earlier studies on series 8, substitution in the 5-position
led to a significant reduction in activity.
Based on a balance of in vitro CB1 receptor antagonist potency
and physical properties, 9a was selected for in vivo profiling. Phar-
macokinetic analysis of the crystalline hydrochloride salt of 9a in
the rat revealed moderate clearance, half-life and volume of distri-
bution (21 mL/min/kg, 1.5 h, 2.5 L/kg, 2 mg/kg, iv), in addition to
moderate bioavailability (42%, 5 mg/kg, po). Based on this profile
9a was advanced to testing in in vivo models of functional CB1 antag-
onism.27 In the hot plate/hypothermia legs of the tetrad assay, 9a
(10 mg/kg; ip) partially reversed the effects of the CB agonist CP-
55940, 56% (p <0.01) and 33% (p <0.01), respectively. In comparison,
SR141716 is able to completely reverse the agonism induced by CP-
55940 at 3 mg/kg, ip (p <0.01). In a rat model of CB1 receptor-medi-
ated fast-inducerefeeding, 9a (10 mg/kg;po) producesa statistically
significant reduction (44%, p <0.01) in food intake over a 4 h test per-
iod, when compared to SR141716 at 3 mg/kg (ꢁ31%, p <0.01).
Though our expectation was that the 1,4-disubstitution pattern
of the imidazole ring would mitigate any CYP inhibition risk asso-
ciated with the imidazole functionality present in 9a, inhibition
studies employing the major isoforms were carried out. Compound
pursued in our laboratory (unpublished data) suggested that
a,a-
9a showed no significant inhibitory activity (IC50’s >10 lM) against
dimethylbenzyl analog 8h would have improved potency. This
trend did translate between series with 8h being approximately
sevenfold more potent than 8g and potently inhibits GTP binding
(0.1 nM). However, the hydrochloride salt of 8h fails to exhibit sig-
nificant in vivo responses in either the tetrad or fasted-induced
refeeding assays.27 Pharmacokinetic analysis revealed very low
free plasma and brain exposures, which is driven by the low
the CYP isoforms 3A4, 1A2, 2C9, 2C19 and 2D6.
In summary, isosteric replacement of the hydrazide pharmaco-
phore of 1 with a 1,4-disubstituted imidazole moiety has been
shown to be a viable isosteric substitution. Analogs from this imid-
azole series exhibited potent CB1 antagonism both in vitro and in
vivo. Conformational restriction of these and related heterocyclic
isosteres will be the subject of a future publication.

5354
R. L. Dow et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5351–5354
13. Lin, L. S.; Lanza, T. J., Jr.; Jewell, J. P.; Liu, P.; Shah, S. K.; Qi, H.; Tong, X.; Wang, J.;
Acknowledgments
Xu, S. S.; Fong, T. M.; Shen, C. P.; Lao, J.; Xiao, J. C.; Shearman, L. P.; Stribling, D.
S.; Rosko, K.; Strack, A.; Marsh, D. J.; Feng, Y.; Kumar, S.; Samuel, K.; Yin, W.;
Van der Ploeg, L. H.; Goulet, M. T.; Hagmann, W. K. J. Med. Chem. 2006, 49, 7584.
14. Griffith, D. A.; Hadcock, J. R.; Black, S. C.; Iredale, P. A.; Carpino, P. A.; DaSilva-
Jardine, P.; Day, R.; DiBrino, J.; Dow, R. L.; Landis, M. S.; O’Connor, R. E.; Scott, D.
O. J. Med. Chem. 2009, 52(2), 234.
We thank Shawn Black, Denise Gautreau, Jeffrey Lizano, Dawn
Kelly-Sullivan, Rebecca O’Connor, Meihua Tu and Karen Ward for
their contributions to this work.
15. Timbrell, J. A. Drug Metab. Rev. 1979, 10, 125.
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