Letters
Journal of Medicinal Chemistry, 2004, Vol. 47, No. 26 6441
Table 2. Species Differences for Selected Compounds
algesia model. As a result, a compound’s ability to cross
the blood-brain barrier may affect the degree of ef-
ficacy. Further experiments are underway to evaluate
the CNS role of the B1 receptor in nociception.
In summary, a novel class of 2,3-diaminopyridine-
based bradykinin B1 receptor antagonists has been
elaborated in terms of potency, selectivity, and phar-
macokinetic properties. Several of these compounds
exhibit good in vivo efficacy in rabbit models of hyper-
algesia and inflammation. Moreover, they retain suit-
able pharmacokinetic and physicochemical properties
for additional study to determine their potential for
clinical evaluation.
IC50(nM)
Ki (nM)a
rat
Ki (nM)
rabbit
rabbit
compd
(FLIPR)
6b
6c
6d
6f
35.7
35.3
81
119
12
1.1
0.69
2.7
7.2
2.4
12.3
3.9
b
3.0
6g
41.5
a Values represent the numerical average of at least two
experiments. Interassay variability was ( 25% for the binding
assays (Ki, nM) and ( 30% for the FLIPR experiments (IC50, nM).
b Not determined.
Table 3. In Vivo Experiments for Compounds 6b, 6c, and 6f
Acknowledgment. We would like to acknowledge
the efforts of Mrs. Yvonne M. Leonard, Joan Murphy,
Audrey A. Wallace, Debra McLoughlin, and Bang-Lin
Wan. We are grateful to Drs. M. R. Wood and D.-S. Su
for useful discussions.
rabbit LPS
rabbit CFA
protein
shift
compda
AD50 (mg/kg)a
ID50 (mg/kg)
6b
6c
6f
0.80 ( 0.06
0.57 ( 0.04
0.28 ( 0.04
0.9 ( 0.6
2.1 ( 0.9
3.1 ( 0.2
41
58
19
a Three rabbits were used in each experiment with iv dosing.
See Supporting Information for full experimental protocols.
Supporting Information Available: 1H NMR and HRMS
data for new compounds. Experimental details for the brady-
kinin B1/B2 binding and FLIPR assays, the rabbit CFA and
blood pressure models, and rat/dog pharmacokinetic protocols.
This material is available free of charge via the Internet at
In general, the compounds in this 2,3-diaminopyridine
series are B1 receptor selective for the human and rabbit
over the rat (Table 2). For this reason, the characteriza-
tion of lead compounds using classic rodent models of
pain and inflammation proved problematic. To accom-
modate this species difference, the in vivo efficacy of key
compounds was examined in two rabbit models.
References
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depressor blood pressure response to the B1 agonist
DABK (1 µg/Kg iv).22 Antagonist dose-responses to the
depressor effects were generated by pretreatment with
rising doses of test compounds before repeated doses of
DABK. Results from selected compounds are shown in
Table 3. All compounds studied (6b, 6c, and 6f) were
potent with AD50 values of less than 1 mg/kg. With the
exception of 6f, the B1 antagonist activity is consistent
with the rabbit B1 receptor binding affinity. While 6f
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its relatively greater antagonist activity can be at-
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In support of this result, the addition of rabbit plasma
(50%) to the binding assay showed that 6f (19× shift)
indeed exhibited the lowest protein shift among these
compounds (6b and 6c, 41× and 58× shift, respectively).
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By way of comparison, morphine gave an ID50 of ∼0.5
mg/kg, indicating the potential of these bradykinin
antagonists as potent antinociceptive agents. It is
interesting to note that the rank order of potency among
6b, 6c, and 6f in the mechanical hyperalgesia model is
reversed relative to that of the blood pressure model.
For example, 6f is 2-3× more potent in the rabbit LPS
model compared to 6b, while the opposite is true for the
rabbit CFA model. This may be attributable to a possible
CNS site of action for the B1 antagonists in the hyper-