H. Liu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 4885–4891
4891
eas 6 with cyclic anhydrides 10 (wherein R3 and R5 optionally
forms a ring) in DMF provided pyrazolyl-phenylureas 11/12 with
a carboxylic acid moiety as the ‘hook’ group. These compounds
More thorough SAR studies on the ‘hook’ and linker regions,
accompanied by fine tuning of the pyrazole-urea anchor are de-
sired, as they will provide more insightful understanding of the
pocket that interacts with the linker and ‘hook’ moiety, facilitate
the development of a SBDD (structure-based drug design) plat-
form, and hence delineate a path forward for applying this strategy
to inhibitors for other kinases. These results will be published in
the due course.
were evaluated in the p38
Compound 11a, which has a flexible oxobutanoic acid side
chain, has an IC50 of 0.037 M against unactive p38 , and an IC50
of 0.393 M against active p38 (Table 4, entry 1). Substitutions
at the -carbon of the acid moiety are also well tolerated (11b,
a cascade assays (Table 4).
l
a
l
a
a
Table 4, entry 2). More conformational restrained analogues, such
as 11c and 11d (Table 4, entries 3 and 4), retained their inhibitory
activity. On the other hand, cyclohex-3-enecarboxylic acid deriva-
tive 11e, which has an unsaturated carbocycle (Table 4, entry 5),
has slightly improved potency. Furthermore, it is interesting to no-
tice that switching from a phenyl urea to a naphthyl urea resulted
Acknowledgment
The authors thank Dr. Richard Nugent for helpful suggestions
and discussions.
in 12, which has an IC50 of 0.005
l
M against unactive p38
a, and an
References and notes
IC50 of 0.006 M against active p38
is caused by an increased lipophilic occupancy in the specific bind-
ing area.
Some of the representative compounds from Table 4 were fur-
ther tested for their selectivity against other kinases at 10 lM. As
expected, remarkable selectivity is again observed for phenyl-pyr-
azole-urea acids 11a, 11b, 11c and 11e against a panel of 40 ki-
nases (Fig. 3).
It is also interesting to notice that the naphthyl derivative 12,
which is highly potent against both unactive and active p38
(Table 4, entry 6), showed a similar selectivity profile at 10
against a slightly different panel of 35 kinases (Fig. 4).
Pyrazole-urea acids 11a, 11b, 11c and 11e were also tested in
in vitro ADME and safety assays, and the results are summarized
in Table 5. As expected, these acids show good solubility and im-
proved stability in human liver microsome assays, when compared
with neutral inhibitors, such as 8d (Table 3, entry 1). On the other
hand, these acids demonstrate a range of permeability, as deter-
mined in the PAMPA model, with introduction of lipophilic groups
(11e, Table 5, entry 4) or steric hindrance around the acid moiety
(11b, Table 5, entry 2) restoring the permeability to an excellent le-
vel. Furthermore, all compounds show clean profiles in the CYP
P450 enzymes inhibition assays, and in the dofetilide binding
assay.
In conclusion, we demonstrated that by simultaneously target-
ing the conventional ‘DFG-out’ pocket, and an extended region of
p38a, we synthesized several pyrazole-urea based p38a inhibitors,
which demonstrated good potencies and superior selectivity. These
pyrazolyl-ureas differ from the well known ‘DFG-out’ type inhibi-
tors, such as Imatinib, Sorafenib and BIRB796, in that they have
no direct interactions with the adenine binding site, yet have se-
lected linker and ‘hook’ moieties attached to the pyrazole-urea an-
chor, which allows them to achieve desired potencies, selectivity,
and physical properties.
l
a
(Table 4, entry 6). This likely
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14. The p-38a co-crystals with this class of compounds are often partially resolved
for the ligands. Nevertheless, based on some of the partially resolved
structures, it is clear that these ureas bind to the ‘DFG-out’ conformation of
the protein.