Imidazo[4,5-c]pyridines as corticotropin releasing factor receptor ligands.

Article abstract of DOI:10.1016/S0960-894X(02)00833-8

A series of high affinity CRF receptor ligands with an imidazo[4,5-c]pyridine core is described. Individual analogues were synthesized and tested in vitro in rat brain receptors to determine binding affinity. The best compound was further tested in the dog N-in-1 pharmacokinetic model to assess oral bioavailability at 1 mg/kg po.

Full text of DOI:10.1016/S0960-894X(02)00833-8

Bioorganic & Medicinal Chemistry Letters 13 (2003) 125–128
Imidazo[4,5-b]pyridines as Corticotropin Releasing
Factor Receptor Ligands
Argyrios G. Arvanitis,a,* Joseph T. Rescinito,a Charles R. Arnold,a Richard G. Wilde,a,y
Gary A. Cain,a Jung Hui Sun,a Jia-Sheng Yan,a Christopher A. Teleha,a
Lawrence W. Fitzgerald,b,{ John McElroy,b Robert Zaczek,b Paul R. Hartig,b
Scott Grossman,c Stephen P. Arneric,b,{ Paul J. Gilligan,a Richard E. Olsona
and David W. Robertsona,{
aDiscovery Chemistry-Wilmington, Bristol-Myers Squibb Company, Experimental Station, Wilmington, DE 19880, USA
bCNS Diseases Research, Bristol-Myers Squibb Company, Experimental Station, Wilmington, DE 19880, USA
cDrug Metabolism and Pharmacokinetics, Bristol-Myers Squibb Company, Experimental Station, Wilmington, DE 19880, USA
Received 14 June 2002; accepted 17 September 2002
Abstract—A series of high affinity CRF receptor ligands with an imidazo[4,5-b]pyridine core is described. Individual analogues were
synthesized and tested in a rat CRF receptor binding assay. The best compounds were further tested in the dog N-in-1 pharmaco-
kinetic model to assess plasma levels at 1 mg/kg (po) and in the rat situational anxiety model to assess anxiolytic efficacy at 3 mg/kg
(po). The structure–activity relationships for good receptor binding affinity are described herein.
# 2002 Elsevier Science Ltd. All rights reserved.
Corticotropin releasing factor (CRF) is a 41-amino acid
peptide, first isolated, sequenced and characterized by
Vale and coworkers at the Salk Institute.1 It is synthe-
sized in small nuclei of the hypothalamus and acts on
7-transmembrane receptors on the anterior pituitary by
inducing the production of adenylyl cyclase. It is the
major physiologic regulator of the basal and stress-
induced release of ACTH from the pituitary, which
stimulates the release of glucocorticoids, such as corti-
sol, from the adrenal cortex. CRF also mediates beha-
vior by binding to receptors in the frontal cortex and
consequently may play a role in major neuropsychiatric
disorders, such as anxiety related disorders (panic dis-
orders), post-traumatic stress disorder and depression.2
A potent and selective CRF1 antagonist will clarify the
role of CRF antagonism as an effective therapy for the
treatment of depression and/or anxiety.
In the past we described SV030, which possessed a
purine core, as a high affinity selective CRF1 receptor
antagonist with good pharmacokinetic parameters.4
Extending our work aimed toward the discovery and
development of novel CRF receptor antagonists we
became interested in exploring the corresponding imi-
dazo[4,5-b]pyridines I as a new series with potentially
improved biological and physical properties over the
purine based series (Fig. 1).
Two CRF receptors have been identified: CRF1 and
CRF2, the former mainly expressed in the rat brain and
the later in the rat periphery. Recent experiments on
CRF1 knock-out mice implicated the involvement of
CRF1 receptors in the stress response.3
*Corresponding author. Tel.: +1-302-467-5804; fax: +1-302-467-
6705; e-mail: argyrios.arvanitis@bms.com
yPresent address: PTC Therapeutics, Inc.
{Present address: Pharmacia Corp.
Figure 1. Purine and imidazo[4,5-b]pyridine cores.
0960-894X/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00833-8

126
A. G. Arvanitis et al. / Bioorg. Med. Chem. Lett. 13 (2003) 125–128
Scheme 1. (a) HNO3, Á; (b) cycloheptyl amine, MeOH, Á; (c) POCl3, 25 ꢀC: (d) Ag2CO3, BzlBr, C6H6 60 ꢀC, 5 h; (e) ArB(OH)2, Pd(PPh3)2Cl2,
Ba(OH)2, DME/water, Á, 5–16 h; (f) CF3CO2H, 25 ꢀC, 16 h; (g) POCl3, reflux, 6 h; (h) RNH2, 2–3 equiv, MeCN, reflux, 16–36 h; (i) Na2S2O4,
dioxane/water/NH4OH; (j) R1CO2H or R1CO2H/ R1C(OEt)3, Á, or EtCO2H/C(OMe)4, 48 h.
The synthesis started from the commercially available
2,4-dihydroxypyridine 1a which was nitrated to 2a
under the previously described conditions5 (Scheme 1).
The 4-hydroxy group was selectively converted to the
4-chloro by treatment with cycloheptyl amine to form
the salt, which was isolated, dissolved in POCl3 and
stirred at 25 ꢀC for 24 h. The reaction mixture was
poured into ice/water and the precipitated product was
filtered off and dried. Attempts to effect a Suzuki or
Stille coupling6 with arylboronic acids or aryltin
reagents on 3a were unsuccessful, presumably because
of the acidity of the OH proton. The pyridine 3a was
therefore converted to the corresponding O-benzyl ether
4a utilizing known conditions7 in good to excellent
yields. Aryl chloride 4a was treated with a variety of
substituted arylboronic acids to give the corresponding
coupled products 5a, which were purified by column
chromatography on silica gel. The benzyl group was
removed by treatment with CF3CO2H at 25 ꢀC8 and
after evaporation of CF3CO2H under reduced pressure
the crude product was converted to the corresponding
2-chloropyridine 6a, which was purified by column
chromatography on silica gel. The 2-chloro substituent
was displaced with a branched primary amine to give
7a, the 3-nitro group was reduced to the corresponding
amine and the 2,3-diamino-4-aryl pyridines were con-
verted to the desired product I by heating at reflux in
propionic acid for 48 h. In the case of the presence of
acid sensitive substituents (1-cyclopropyl alkyl chains)
the 2,3-diamino-5-arylpyridine intermediates were
heated at reflux in triethylorthopropionate in the pre-
sence of a catalytic amount EtCO2H (20% mol).9 The
2-methoxy imidazo[4,5-b]pyridines I (R1=OMe) were
synthesized by heating at reflux the 2,3-diamino-4-aryl-
pyridine intermediates with tetramethyl orthocarbonate
in the presence of a catalytic amount of EtCO2H.
Modifications were made on the side chains, 2-sub-
stituents and aryl groups based on previous SAR
developed in the purine series. Individual compounds
were tested in vitro in the rat receptor binding assay as
previously described10 and the data are summarized in
Table 1.
Table 1. Chains on the 1-position of the imidazo[4,5-b]pyridine core
Data on Table 1 indicate that the best R groups for
optimal receptor interaction are small lipophilic alkyl
groups (1–8), however polar groups on the alkyl chains,
Table 2. The 2-position group on the imidazopyridine core
Compd
Ra
R0
Ki (nM)b
SD
1
CH(cPr)Me
CH(cPr)Et
CH(cPr)Et
CH(cBu)Me
CH(nPr)Me
CHEtMe
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
0.6
0.2
0.4
1.0
0.5
1.3
1.0
1.5
2.4
4.7
0.7
7.6
0.3
—
—
2(R)c
20(S)c
3
4
5
6
7
8
9
10
0.3
0.3
0.3
0.2
0.7
0.9
1.1
0.3
0.8
CHEt2
CH(cPr)C2H4Ome11
CH(cBu)C2H4OMe
CH(Et)CH2OMe
CH(Et)C2H4OMe
a-Helical CRF(9-41)
Compd
R
X
R1
Kia
SD
Cl
CF3
CF3
11
12
13
14
CH(Et)C2H4OMe
CH(Et)C2H4OMe
CH(nPr)Me
OMe
cPr
CHF2
CH2OMe
OCF3
CF3
Cl
1.9
20.8
15.8
82.7
0.1
5.9
2.6
48
aRacemic mixtures.
CH(Et)C2H4OMe
CF3
bValues are means of two or more experiments. Receptor binding
affinity for all compounds was determined using rat cortical homo-
genates. SD, standard deviation.
aValues are means of two or more experiments. Receptor binding
affinity for all compounds was determined using rat cortical homo-
genates. SD, standard deviation.
cSingle determination.

A. G. Arvanitis et al. / Bioorg. Med. Chem. Lett. 13 (2003) 125–128
127
such as OMe can be accomodated in high receptor affi-
nity ligands (e.g., 7–10).
Data in Table 3 indicate that binding affinity is pri-
marily dependent on the size than polarity of the phenyl
substituents. The 2-aryl position has very stringent
steric requirements and replacement of the 20- Cl with
an acetyl led to drastic reduction in affinity (21). The 40-
aryl position can tolerate a larger number of sub-
stituents with various functionalities such as OMe,
COMe, SO2Me, SMe and CH(OH)Me.
Substituents were introduced on the 2-position using
procedures described in Scheme 1. Data are listed in
Table 2 and indicate that the only good replacement for
an ethyl chain is a methoxy group (11). Small increases
in size to a cyclopropyl or difluoromethyl (12 and 13)
led to drastic reduction in affinity. Further increases in
size to a methoxymethyl group gave a compound with
modest affinity (14).
The best compounds with respect to binding affinity
and physical properties (ClogP) were tested in the dog
N-in-1 pharmacokinetics at 1 mg/kg po and in the rat
situational anxiety model at 3 mg/kg po to assess
anxiolytic efficacy. Data for three compounds are listed
in Table 4.
After exploring the 2-position, we turned our efforts on
the optimization of the substitution on the 7-aryl ring.
We wanted to introduce substitution that would main-
tain receptor affinity, while improving physical proper-
ties such as reducing lipohilicity.
Table 4 shows that two of the compounds showed good
plasma levels in the dog N-in-1 study. Plasma con-
centration was dependent on substitution and the
4-methoxyphenyl group seemed to give the lowest
exposure. All of the compounds showed efficacy in the
rat situational anxiety model at 3 mpk po.
Table 3. The SAR of various aryl groups
In conclusion the 4-arylimidazo[4,5-b]pyridine series
represent a new series of potent CRF receptor ligands.
Additionally, certain analogues possessed good oral
bioavailability in dog at 1 mg/kg po and showed efficacy
in the rat situational anxiety model.
Compd
R1
R2
R3
Ki (nM)a
SD
Acknowledgements
15
16
17
18
19
20
21
22
23
24
25
26
27
Cl
CF3
Me
CF3
Me
Cl
CF3
OMe
OMe
SMe
OMe
H
H
H
H
Me
F
H
H
H
H
H
H
H
1.1
0.7
0.8
1.4
0.6
2.4
95.6
1.0
5.1
24.0
3.5
6.8
2.3
0.7
0.0
0.3
0.1
0.2
0.6
—
0.2
1.1
0.4
0.2
1.3
0.1
The authors wish to acknowledge Walt Meredith, Jan
Hyterk, Jim Piecara, Christine Tabaka-Blom of the
Chemical Synthesis group for large scale synthesis of
2-benzyloxy-4-chloro-3-nitropyridine and several aryl-
boronic acid intermediates and Anne Marshall, Sue
Keim, Deborah Conklin, and Carol Krause for provid-
ing primary screen data.
F
Cl
COMe12
Cl
Cl
COMe13
CH(OH)Me
C(OH)Me2
CH(OMe)Me
SO2Me
C(Me)NOMe
Cl
Cl
CF3
Cl
References and Notes
aValues are means of two or more experiments, except 21. Receptor
binding affinity for all compounds was determined using rat cortical
homogenates. SD, standard deviation.
1. Vale, W.; Spiess, J.; Rivier, C.; Rivier, J. Science 1981, 213,
1394.
2. (a) Nemeroff, C. B. Neuropsycophamcacology 1992, 6, 69.
(b) Owens, M. J., Nemeroff, C. B., Eds. Corticotropin-Rea-
leasing Factor: Basic and Clinical Studies of a Neuropeptide.
CRC: Boca Raton, 1990.
Table 4. Dog N-in-1 pharmacokinetic parameters14 at 1 mg/kg and
rat situational anxiety model efficacy for selected compounds
3. (a) Heinrichs, S. C. Curr. Opin. Drug Disc. Dev. 1999, 2,
491. (b) Gilligan, P. J.; Robertson, D. W.; Zaczek, R. J. Med.
Chem. 2000, 43, 1641. (c) Gilligan, P. J.; Hartig, P. R.;
Robertson, D. W.; Zaczek, R. In Annu. Rep. Med. Chem.;
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4. Wilde, R. G.; Klaczkiewicz, J. D.; Carter, K, L.; Gilligan,
P. J.; Olson, R. E.; Frietze, W. E.; Buckner, W. H.; Beck, J. P.;
Curry, M. A.; Robertson, D.W.; Sun, J. H.; Arneric, S. P.;
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5. Koeglet, F.; Van Der Want, G. M.; Salemink, C. A. Recl.
Trav. Chim. Pays-Bas 1948, 67, 29.
6. For a recent review of the various cross coupling methods
see: Stanforth, S. P. Tetrahedron 1998, 54, 263.
7. Hoffman, J. M.; Smith, A. M.; Rooney, C. S.; Fisher, T. E.;
R
R1
R2
Ki
AUC Cmax T1/2
Situational
(nM) (nM) (nM) (h) anxiety ED50
CH(nPr)Me CF3 OMe 0.7
CH(nPr)Me Cl CF3 1.1
CH(cBu)Me Cl OCF2 0.7
182
604
795
62
159
185 10
5.9
4.9
3 mpk (po)
3 mpk (po)
<3 mpk (po)

128
A. G. Arvanitis et al. / Bioorg. Med. Chem. Lett. 13 (2003) 125–128
Wai, J. S.; Thomas, C. M.; Bamberger, D. L.; Barnes, J. L.;
Williams, T. M.; Jones, J. H.; Oslon, B. D.; O’Brien, J. A.;
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8. Ducharme, Y.; Wuest, J. D. J. Org. Chem. 1988, 53, 5787.
9. In the case of electron rich aryl substituted pyridines the
corresponding imidate was formed under these reaction con-
ditions. This could be converted to the imidazo[4,5-b]pyridine
by heating it at reflux in toluene with 20% mol EtCO2H.
10. DeSouza, E. B. J. Neurosci. 1987, 7, 88.
11. Synthesized from the oxime methyl ether of cyclo-propyl
methy lketone by deprotonation with nBuLi followed by
alkylation with bromomethylmethylether according to the
protocol developed by: Shatzmiller, S. Dolitzki, B-Z. Liebigs
Ann. Chem. 1991, 189. The alkylated oxime methylether was
purified by distillation under reduced pressure and reduced
with LiAlH4 in refluxing THF.
12. Synthesized from dichlorophenyl 3 by a Stille coupling
with ethoxyvinyl tri-n-butyl tin using 2.5% Pd(PPh3)4 and
2.5% Pd(PPh3)2Cl2 in refluxing toluene, followed by acid
hydrolysis. Coupling of the 4-Cl was not observed, pre-
sumably because of the directing effect of the imidazole N.
13. Synthesized from intermediate 6a (Ar=2,4-Cl2Ph) using
the same conditions as ref 12. In this case the coupling of the
4-Cl was selective (ꢁ6:1 over 2-Cl) presumably because of the
weaker complexing ability of the nitro group.
14. Formulation: iv: 10% (v/v) N,N-dimethylacetamide, 3%
(v/v) ethanol, 65% (v/v) propylene glycol in water; po: 8% (v/
v) ethanol, 2% (v/v) N,N-dimethylacetamide in Labrafil 1944
CS.