R. A. Hartz et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1890–1894
1893
pyridyl nitrogen in a subset of the above compounds was mea-
improvement of the IC50 to 1.6 nM and also further lowering of the
pK to <2. Compound 6f was similar in potency to 6c. Replacement
sured (Table 2). Compound 6e27 with a mildly basic pyridyl nitro-
a
gen (pK
a
= 7.9) had an IC50 of 65 nM. The corresponding phenyl
recep-
of the methoxy group in compounds 6d, 6a, and 6f with a difluoro-
methoxy group led to compounds 6n, 6m, and 6o, respectively,
each with an improved binding affinity relative to the correspond-
ing methoxy analogues. Analogues with the difluoromethoxy
group had superior potency compared to other pyridyl-containing
analogues. To summarize, the results shown in Table 2 indicate
analogue (6x) had an IC50 of 1.7 nM indicating that the CRF
tor has little tolerance for a basic nitrogen in the aryl substituent.
Replacement of a methyl group in 6e with a methoxy group (6d)
lowered the pK
IC50 of this compound was 12.0 nM. Removal of one of the methyl
groups (6a) had little effect on the pK and no significant effect on
1
a
of the protonated pyridyl nitrogen to 3.8. The
a
1
that the CRF receptor shows little tolerance for basic groups in
the IC50 compared to 6d. Replacement of the remaining methyl
group in 6a with a trifluoromethyl group (6c) resulted in a further
the aryl region.
A comparison of compounds 6a, 6d and 6f versus 6m, 6n and
o, respectively, indicated that the difluoromethoxypyridyl ana-
6
logues showed improved metabolic stability in rat and human liver
microsomal incubations (Table 1) compared to the methoxypyridyl
analogues. Additional SAR studies with analogues containing the
difluoromethoxypyridyl group led to the discovery of 2, a com-
Table 2
1 a
Relationship between CRF receptor binding affinity and pyridyl nitrogen pK
1
1
pound which was selected for further development.
In conclusion, efforts to identify suitable phenyl group replace-
ments resulted in the discovery of highly potent pyridyl-containing
N
N
O
analogues. It was found that the CRF
analogues containing a pyridyl group was influenced not only by
the substitution pattern on the pyridyl group, but also by the pK
of the pyridyl nitrogen. In general, analogues with pyridyl groups
wherein the pK of the pyridyl nitrogen was low showed improved
potency, indicating that the CRF receptor appears to have limited
1
receptor binding affinity of
Cl
NH
Aryl
a
a
Compd
Aryl
IC50 (nM)
65 ± 7
pK
a
a
1
6
e
x
7.9
N
tolerance for basic groups in the pocket where the pyridyl group
binds. Analogues containing the novel 6-(difluoromethoxy)-2,5-
dimethylpyridin-3-amine group proved to be among the most po-
tent in this series of compounds.
6
1.7 ± 0.2
12 ± 0.3
NAb
3.8
Acknowledgment
a
We thank Gottfried Wenke for performing pK measurements.
6
6
6
6
6
d
a
c
f
N
References and notes
OMe
1
2
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11 ± 2
3.2
<2
N
3.
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OMe
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1.6 ± 0.4
1.5 ± 0.5
2.1 ± 0.3
N
OMe
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3.6
<2
N
1
OMe
7. Hartz, R. A.; Ahuja, V. T.; Rafalski, M.; Schmitz, W. D.; Brenner, A. B.; Denhart, D.
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n
N
8.
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OCHF2
2
010, 38, 5.
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9.
2
6
6
m
o
1.2 ± 0.3
<2
<2
N
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OCHF
2
1
1. Hartz, R. A.; Ahuja, V. T.; Zhuo, X.; Mattson, R. J.; Denhart, D. J.; Deskus, J. A.;
Vrudhula, V. M.; Pan, S.; Ditta, J. L.; Shu, Y.-Z.; Grace, J. E.; Lentz, K. A.; Lelas, S.;
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0.30 ± 0.03
N
1
2. Hartz, R. A.; Ahuja, V. T.; Arvanitis, A. G.; Rafalski, M.; Yue, E. W.; Denhart, D. J.;
Schmitz, W. D.; Ditta, J. L.; Deskus, J. A.; Brenner, A. B.; Hobbs, F. W.; Payne, J.;
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OCHF2
a
pK
a
measurements were determined using a spectrophotometric titration
28
method.
b
NA = not applicable.