Bradykinin B1 antagonists: SAR studies in the 2,3-diaminopyridine series

Article abstract of DOI:10.1016/j.bmcl.2005.05.133

SAR study of the biphenyl region of 2,3-diaminopyridine bradykinin B ₁ antagonists was investigated with non-aromatic carbo- and heterocyclic rings. A piperidine ring was found to be a good replacement for the proximal phenyl ring while replacement of the distal phenyl was optimal with a cyclohexyl group leading to a dramatic improvement in affinity for the B ₁ receptor.

Full text of DOI:10.1016/j.bmcl.2005.05.133

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3925–3929
Bradykinin B1 antagonists: SAR studies
in the 2,3-diaminopyridine series
Scott D. Kuduk,a,* Ronald K. Chang,a Christina Ng,a Kathy L. Murphy,b
Richard W. Ransom,b Cuyue Tang,c Thomayant Prueksaritanont,c Roger M. Freidinger,a
Douglas J. Pettiboneb and Mark G. Bocka
aDepartment of Medicinal Chemistry, Sumneytown Pike, PO Box 4, West Point, PA 19486, USA
bDepartment of Neuroscience Drug Discovery, Sumneytown Pike, PO Box 4, West Point, PA 19486, USA
cDepartment of Drug Metabolism, Merck Research Laboratories, Sumneytown Pike, PO Box 4, West Point, PA 19486, USA
Received 24 February 2005; accepted 25 May 2005
Available online 1 July 2005
Dedicated to Professor Iwao Ojima on the occasion of his 60th birthday.
Abstract—SAR study of the biphenyl region of 2,3-diaminopyridine bradykinin B1 antagonists was investigated with non-aromatic
carbo- and heterocyclic rings. A piperidine ring was found to be a good replacement for the proximal phenyl ring while replacement
of the distal phenyl was optimal with a cyclohexyl group leading to a dramatic improvement in affinity for the B1 receptor.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
role for the B1 receptor has been implied based on evi-
dence that it is constitutively expressed in the central
nervous systems of rats and mice.6–8 Thus, bradykinin
B1 receptor antagonists are considered good prospects
as novel therapeutic agents for the treatment of chronic
pain and inflammation.9
Bradykinin (BK) peptides are rapidly formed in plasma
after tissue injury and exert a variety of physiological
effects, such as pain and inflammation.1 Two known
G-protein-coupled receptors, designated as B1 and B2,
regulate these effects.2 The constitutively expressed B2
receptor is believed responsible for the immediate
acute pain response following tissue injury and is medi-
ated by the peptides, bradykinin (Arg-Pro-Pro-Gly-
Phe-Ser-Pro-Phe-Arg) and kallidin (Lys-BK). Their
corresponding metabolites, [des-Arg9]bradykinin and
[des-Arg10]kallidin, are agonists for the B1 receptor,
which is induced in the hours following the injury.3
We recently disclosed the preparation and biological eval-
uation of a series of non-peptide, 2,3-diaminopyridine
bradykinin B1 antagonists (Fig. 1).10 These compounds
displayed excellent selectivity and exhibited good human
bradykinin B1 antagonist activity. The purpose of this
communication is to report the results of an expanded
SAR study to identify alternative biphenyl ÔisosteresÕ that
exhibit enhanced receptor affinities and improved physi-
cal properties relative to lead compound 1.
Bradykinin B1 receptor agonists have been shown in ani-
mal models to produce hyperalgesia, which is blocked
by peptide-derived B1 antagonists.4,5 Also, transgenic
B1 receptor knockout mice exhibit reduced sensitivity
to painful stimuli, while appearing normal in all other
respects. In addition to peripheral B1 receptors, a central
Keywords: Bradykinin; Diaminopyridine.
*
Corresponding author. Tel.: +2156525147, fax: +2156523971; e-mail:
scott_d_kuduk@merck.com
Figure 1. Lead diaminopyridine bradykinin B1 antagonist 1.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.05.133

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S. D. Kuduk et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3925–3929
Scheme 4. Reagents and conditions: (a) Ph3PCHCO2Me, toluene,
80 °C; (b) butadiene, toluene, 180 °C, 3–7 days; (c) RaNi, H2, NH3–
MeOH.
Scheme 1. Reagents and conditions: (a) TEA, THF, 70 °C; (b) SnCl2,
MeOH, 60 °C; (c) cyanoacetic acid, EDC, HOBt, DMF, TEA.
Scheme 1 outlines the general route employed for pre-
paring the 2,3-diaminopyridine derivatives highlighted
in this report. Displacements of the chlorine in 2-chlo-
ro-3-nitro-4-methylpyridine with the appropriate amines
2a–23a (Schemes 2–7), were carried out in THF in the
presence of TEA. Reduction of the nitro group was
Scheme 5. Reagents: (a) Pd(OAc)2, Ph3P, K2CO3, THF; (b) RaNi,
NH3–MeOH, H2.
Scheme 2. Reagents and conditions: (a) Pd(Ph3P)4, 2-IZn-meth-
ylbenzoate, THF, 70 °C; (b) (i)—NaOH, THF; (ii)—BH3; THF,
0 °C; (c) (i)—MsCl, TEA, CH2Cl2; (ii)—NaN3, DMF; (iii)—Ph3P,
THF-H2O; (d) Pd/C, EtOAc, H2.
Scheme 6. Reagents and conditions: (a) 4-Fluorobenzonitrile, NaH,
DMSO, 60 °C; (b) 4-fluorobenzonitrile, K2CO3, DMSO, 120 °C; (c)
RaNi, NH3–MeOH, H2.
Scheme 7. Reagents: (a) Pd(OAc)2, 2-bromopyridine, Ph3P, K2CO3,
THF; (b) RaNi, NH3–MeOH, H2; (c) Boc2O, CH2Cl2; (d) EtOH,
Pd(OH)2, H2 (60 psi); (e) ClCO2Me, TEA, CH2Cl2; (f) HCl (g) EtOAc.
Scheme 3. Reagents and conditions: (a) (i)—2-Fluoromethyl benzoate,
K2CO3, DMSO, 120 °C; (ii)—TPAP–NMO, CH2Cl2; (b) TMSCN,
ZnI2, DMF; (c) RaNi, NH3–MeOH, H2; (d) (i)—MsCl, TEA, CH2Cl2;
(ii)—NaN3, DMF; (iii)—Ph3P, THF-H2O; (e) (i)—TFAA, TEA,
CH2Cl2; (ii)—LiHMDS, MeI, THF, 0 °C.
accomplished with SnCl2 in MeOH followed by an
EDC-mediated coupling of the resultant amine with
cyanoacetic acid to afford test compounds 2–23.

S. D. Kuduk et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3925–3929
3927
Table 1. Human bradykinin B1 receptor affinities and physical
properties for B-ring phenyl replacements
Scheme 2 shows the preparation of amines in which
the B-ring phenyl is replaced with cycloalkane rings.
Negishi coupling of triflate 24 with the corresponding
zinc reagent afforded 25. Selective hydrolysis of the
ethyl ester was followed by reduction to the alcohol
with borane. Subsequent conversion of 26 to the amine
and reduction of the olefin produced the requisite
amines.
Compounds bearing a B-ring piperidine in place of
phenyl were prepared as shown in Scheme 3. Heating
2-fluoromethyl benzoate with 4-piperidin-ol followed
by oxidation afforded 27. Conversion to the cyanohy-
drin 28a (R = OH) and reduction with Raney Nickel
afforded 4a. In a similar fashion, piperidines 29 and 30
could be transformed to the desired amines.
Compounds
X
h Ki (nM)a
log P
PBb (%)
99
1
11
2.5
2
13
2.1
nd
93
nd
65
90
99
nd
Two routes were employed for preparation of com-
pounds in which the C-ring phenyl is replaced with
cycloalkyl derivatives. The first general example is
shown in Scheme 4. A thermal Diels–Alder reaction of
cinnamate 31 with butadiene afforded the necessary
trans-4,5-cyclohexene 32. Reduction of the nitrile with
Raney Nickel in NH3–MeOH produced the desired
amine 14a without reduction of the olefin.11 Employing
the cis-cinnamate afforded the cis-cyclohexene, while
cyclopentadiene or cyclohexadiene were used as the
other dienes to afford the bicyclic analogs (not shown
for amines 15a, 19a–22a).
3
130
>5000
3500
25
4
2.1
2.6
2.5
2.7
5
6
7c
385
The second route employed a Suzuki coupling of the
appropriate triflates 33a and b with 4-cyanophenylbo-
ronic acid followed by reduction to produce 12a and
13a (Scheme 5).
a Values represent the average of three experiments; standard deviation
is 25%.
b Protein binding measured using 10% human serum.
c Approximately 2:1 mixture of cis:trans isomers.
In Scheme 6 is outlined the synthesis of the amines used
to prepare compounds 8–10 in which the C-ring phenyl
is replaced with an amine heterocycle. Nucleophilic dis-
placement of 4-fluorobenzonitrile with the appropriate
nitrogen nucleophile was followed by reduction to afford
the desired amines 8a–10a.
The piperidine plays a key role as replacement of 2 with
a cyclohexene (6) led to ꢀ2-fold reduction in human B1
receptor affinity, while the cyclohexane 7 led to a >30-
fold reduction. Overall, piperidine 2 was the best
replacement for phenyl as it provided an equipotent
compound with more desirable physical properties.
The reduced protein binding is of particular interest as
significant protein shifts have complicated in vivo
studies.10b
Scheme 7 illustrates the preparation of the amine
used for the synthesis of 11. Suzuki coupling of 2-
bromopyridine with 4-cyanophenyl boronic acid was
followed by reduction of the nitrile and Boc protec-
tion to yield biaryl 33. The HCl salt of the pyridine
was then reduced with PearlmanÕs catalyst to provide
piperidine 34. The piperidine nitrogen was acylated
with methyl chloroformate and Boc removal afforded
11a.
The exchange of the C-ring with alternative ring systems
was examined (Table 2). The aryl C-ring proved to be
less amenable to replacement. For example, although a
piperidine ring (2) proved to be an effective replacement
for the phenyl, compound 8 bearing a C-ring piperidine
in place of the phenyl was devoid of B1 receptor binding
affinity. The related pyrrolidine derivative 9 was also de-
void of potency.
The central phenyl B-ring in 1 was replaced with several
alternative ring systems as shown in Table 1. Replace-
ment of the phenyl with a 4-piperidine ring afforded 2,
which exhibited equivalent bradykinin B1 receptor affin-
ity and lower log P and human plasma protein binding
compared to 1. The piperidine proved very sensitive to
further modification as all other derivatives which were
tested had reduced potency. For example, introduction
of an olefin (3) led to a 10-fold loss in affinity while addi-
tion of a hydroxy or methyl group on the piperidine fur-
ther decreased receptor binding affinity (entries 4–5).
However, pyrrole 10 retained substantial potency indi-
cating that a planar or aromatic ring is preferred. Mov-
ing the piperidine to the 2-position as in 11 also afforded
an inactive compound. It appears that conformational
requirements of the C-ring are more stringent than the
B-ring.

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S. D. Kuduk et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3925–3929
Table 2. Human bradykinin B1 receptor affinities and physical
properties for C-ring phenyl replacements
Table 2 (continued)
Compounds
X
h Ki (nM)a log P PBb (%)
21 ( trans)
54
3.4
nd
22 ( trans)
730
nd
nd
Compounds
X
h Ki (nM)a log P PBb (%)
a Values represent the average of three experiments, standard deviation
is 25%.
b Protein binding measured using 10% human serum.
1
11
2.5
2.7
nd
99
78
nd
81
nd
96
nd
96
96
95
nd
nd
nd
nd
The most promising results were obtained with the C-
ring carbocyclic derivatives. The 1,2-cyclohexene deriva-
tive 12 showed ꢀ3-fold drop in potency compared to 1,
and the related 1,2-cyclopentene was even less potent
indicating the preference for the six-membered ring.
Relocation of the olefin to provide the trans-4,5-cyclo-
hexene analog 14 led to a dramatic increase in potency.
Racemic compound 14 exhibited a Ki of 0.83 nM,
approximately 10-fold greater than 1. The closely relat-
ed racemic cis-cyclohexene was less potent with a Ki of
7 nM. Reduction of trans-olefin 14 produced saturated
trans-cyclohexane 16 with a Ki of 2.2 nM. Both trans-cy-
clohexane 16 and trans-cyclohexene 14 exhibit higher
affinity for the B1 receptor than the original biphenyl
lead compound 1. Nonetheless, the physical properties
of this series of carbocyclic C-ring replacements were
not substantially different from 1.
8
>5000
>5000
250
>5000
35
9
10
2.4
nd
11
12
3.0
nd
The resolution of racemic cyclohexene 14 afforded 17
and 18. Both compounds have similar receptor affinity
indicating that this segment of the scaffold appears not
to have any stereochemical influence on the Ki. In an ef-
fort to rigidify the carbocyclic C-rings, bicyclic cyclohex-
ane derivatives were prepared (19–22), but all proved
significantly less potent than the parent compounds 14
and 16. Overall, carbocyclic replacements for the C-ring
proved to be the optimum. In particular, the trans-iso-
mers were preferred with cyclohexene 14 and cyclohex-
ane 16 as the best substitution for the C-ring phenyl in
terms of providing a significant improvement in poten-
cy, with a modest reduction of protein binding.
13
126
0.83
7
14 ( trans)
15 ( cis)
16 ( trans)
17 (+)
2.7
3.3
2.7
2.3
2.4
nd
2.2
2. Pharmacokinetic studies
1.3
To evaluate the pharmacokinetic attributes of the 2,3-
diaminopyridines bearing alternative B- and C-ring
systems, the pharmacokinetic properties for selected
compounds were determined in the rat as summarized
in Table 3. Compound 2 in which the piperidine re-
placed the central B-ring, led to an increase in t1/2 with
a reduced clearance. Coupled with the good human bra-
dykinin B1 affinity of 2, this improvement in PK pro-
vides support for further inspection of the piperidine
as a phenyl replacement in the 2,3-diaminopyridine ser-
ies. Both trans-cyclohexene 14 and cyclohexane 16
showed poor oral bioavailability and were rapidly
cleared in the rat compared to 1. We speculate, given
18 (À)
1.0
19 ( trans exo)
20 ( trans endo)
38
142
3.1

S. D. Kuduk et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3925–3929
3929
Table 3. Rat pharmacokinetics for selected compounds
employing these new ring systems will be reported in
due course.
Compound
F (%)a
t1/2
Clb
4.3
2.2
29
Vdss
b
b
1
4
18
22
5
1.9
3
0.36
0.24
0.55
0.38
0.47
Acknowledgments
14
16
23
1.0
0.5
1.2
4
18
30
17
We thank Yvonne Leonard for rat PK experiments and
Bang-Lin Wan, Matt Zrada, Ken Anderson, and Kristi
Hoffman for analytical measurements. We are grateful
to Carl Homnick for chiral HPLC separations and
analyses.
a Sprague–Dawley rats. Dose: 2 mg/kg i.v.; 10 mg/kg p.o.
b Cl in ml/min/kg, t1/2 in h, Vdss is in L/kg.
References and notes
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Figure 2. Cyclohexene analog 23 containing an oxadiazole.
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Hanson, W. L.; Zuzack, J. S. Can. J. Physiol. Pharmacol.
1997, 75, 719.
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that the absence of conjugation with an aromatic ring,
that the methyl ester of the cyclohexene ring is more
prone to hydrolysis than the ester in 1. Accordingly,
an ester isostere of 14 was prepared in the form of 3-
methyloxadiazole 23. Indeed, the PK of 23 was im-
proved over 14 and gave similar values in terms of rat
t1/2 and clearance compared to 1. Consequently, the
highly potent C-ring trans-cyclohexenes also appear to
have suitable pharmacokinetic properties and qualify
as new lead structures in the 2,3-diaminopyridine series
(Fig. 2).
9. Bock, M. G.; Hess, J. F.; Pettibone, D. J. In Annual
Reports in Medicinal Chemistry; Doherty, A. M., Ed.;
Elsevier Inc.: USA, 2003; 38, p 111.
In summary, we have successfully identified alternative
ring systems as suitable replacements for the biphenyl
motif in a series of 2,3-diaminopyridine-based bradyki-
nin B1 receptor antagonists. A 4-piperidine ring replace-
ment for the B-ring phenyl provided compound 2, which
proved to be equipotent with lead compound 1 and
exhibited improved pharmacokinetic and physical prop-
erties. Concerning the C-ring, a trans-cyclohexene ring
led to a 10-fold increase in receptor binding affinity
and provided a compound with equivalent PK proper-
ties relative to 1 through employment of an ester isoste-
re. Additional studies of bradykinin B1 antagonists
10. (a) Feng, D. M.; Wai, J. M.; Kuduk, S. D.; Ng, C.;
Murphy, K. L.; Ransom, R. W.; Reiss, D.; Chang, R. S.-L.;
Harrell, C. M.; Tang, C.; Prueksaritanont, T.; Freidinger,
R. M.; Pettibone, D. J.; Bock, M. G. Bioorg. Med. Chem.
Lett. 2005, 15, 2385; (b) Kuduk, S. D.; Ng, C.; Feng, D. M.;
Wai, J.; Chang, R. S. L.; Harrell, C. M.; Murphy, K. L.;
Ransom, R. W.; Reiss, D.; Prueksaritanont, T.; Tang, C.;
Mason, G.; Boyce, S.; Freidinger, R. M.; Pettibone, D. J.;
Bock, M. G. J. Med. Chem. 2004, 47, 6439.
11. The olefin in the C-ring of 14a was later reduced to the
cyclohexane derivative (Raney Nickel) after the pyridine
was attached to give 14b as shown in Scheme 1.