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1659
Table 4
simplicity of the heuristic model guided the rapid design of new
molecular entities during the lead optimization process.
The results of shifting the aryl substituent to remove the stereogenic centers
Having restored the potency with the pyrazole bioisostere in
place, a brief SAR survey of the nitrogen substituent was con-
ducted. Substitution on the b-carbon with a hydroxyl (10) lost 2-
to 4-fold potency relative to 9 and substitution with a ketone
(11) resulted in a more substantial decrease in inhibitory activity.
Removal of a methylene from 8 afforded analog 12 with a 2-fold
loss in potency. The trifluoropropyl analog 13 lost 11- and 25-fold
relative to 9. Trifluoroethyl analog 14 was substantially less potent.
The progressive loss in potency for 13 and 14 is likely attributable
to inductive effects reducing the basicity of the nitrogen rather
than steric effects because the tert-butyl (15) and isopropyl (16)
analogs were more potent.24 The results for compounds 13–16
gave additional credence to the importance of a basic nitrogen in
the pharmacophore. Finally, the secondary amine 17 had an IC50
Compd
PrCP IC50
h,ma (nM)
X
F
18 (X = Cl)
19 (X = F)
105,—
30, 23
N
N
N
N
N
N
N
N
HN
20
21
22
7.0, 3.6
300,—
N
Me
F
F
value of 1 lM.
As mentioned earlier, the position of the p-fluorophenyl aro-
matic of 9 relative to the pharmacophore appears to be less re-
stricted to variations. To test this hypothesis, a series of analogs,
with the aromatic shifted by one carbon atom, was prepared
(Table 4). Compound 18 lost potency relative to the closest analog
9; however, substitution of the chloro- with a fluoro-group affor-
ded a modest improvement with compound 19. Methylation of
the pyrazole afforded 20 and 21, which demonstrated a greater
than 40-fold difference in potency against human PrCP and may
indicate the preferred pyrazole tautomer of 19. Finally, replace-
ment of the phenyl in 20 with a pyridyl afforded 22, which is the
most potent compound in the series.
N
Me
N
3.3, 0.7
N
Me
a
Compounds were tested as the trifluoroacetate salt; values are based on one or
two experiments, each in triplicate; h = human, m = mouse.
and the piperidine core shown in Table 2 would balance the linear
distance between the basic nitrogen and the HBA. The changes
would appear as a one bond frameshift of the original pharmaco-
phore that is in 1 and 2. Importantly, envisioning this model re-
quires selecting the proper tautomeric form of the pyrazole. To
test the heuristic model, the racemic pyrazole hybrid 8 was pre-
pared and demonstrated a greater than 10-fold improvement in
potency (Table 3). Replacing the p-chlorophenyl with a phenyl re-
stored the potency to the level exhibited by the initial leads 1 and
2. Thus, moving the nitrogen to an endocyclic position and
exchanging the chlorine atom for a hydrogen atom afforded potent
inhibitors of PrCP that incorporated a pyrazole as a replacement of
the amide in the lead structures. In addition, the results for the
stereoisomers of 9 suggested that the exact positioning of the
p-fluorophenyl aromatic relative to the core structure is less influ-
ential on the PrCP inhibition than the stereogenicity at the pyrazole
center.22,23 In general, additional geometric or electronic perturba-
tions could also explain differences in binding affinities, but the
The effect of the bioisosteric replacement on the mouse phar-
macokinetics was also evaluated (Table 5). Both amides 1 and 2
demonstrated high plasma protein binding (PPB) with the morpho-
line in 1 imparting a measurable free-fraction. The pharmacokinet-
ics of 1 and 2 reveal the differences in PPB because 1 has higher
total clearance and a consequently shorter half-life compared to
2. Compounds 9 and 12, like compound 2, were highly plasma pro-
tein bound and the pharmacokinetics parameters are generally
comparable to 2, with long half-life and improved oral bioavailabil-
ity. Similarly, compounds 15 and 16 also had improved oral bioav-
ailabilities relative to 1 and 2. Finally, compounds 18–22, with the
aromatic group shifted by one carbon atom thus removing the ste-
reogenic carbons, demonstrated high total clearances and modest
oral bioavailabilities in the mouse. The results in Table 5 demon-
strated that the pyrazole bioisostere can have mouse pharmacoki-
netic parameters that are comparable to those of the original
amides 1 and 2.
Table 5
Mouse pharmacokinetic parameters for selected compounds
PPBb
Mouse pharmacokinetics (C57Bl/6)c
Compda
h
m
Cl (mL/min/kg)
Vd (L/kg)
t
(h)
AUCNpo
(l
Mhkg/mg)
%F
½
1d
2
2
<1
3
<1
—
—
3
7
<1
1
4
72.2 5.5
31.8 8.0
28.9 7.5
15.4 0.7
44.7 4.0
50.1 3.0
59.2 13.1
77.1 6.6
51.9 4.3
74.4 16.3
10.8 1.5
12.1 3.5
13.3 3.3
9.4 0.7
18.1 0.5
18.3 4.3
11.4 3.3
17.2 4.2
19.9 2.8
14.1 4.0
1.7 0.2
4.4 2.0
4.8 0.4
7.7 0.8
5.4 0.5
4.6 1.3
1.8 0.7
2.7 0.7
4.7 0.5
2.0 0.3
0.037 0.011
0.276 0.131
0.413 0.088
0.877 0.229
0.313 0.100
0.453 0.043
0.057 0.014
0.043 0.013
0.163 0.017
0.076 0.016
7.8
28
33
39
36
54
8.7
12
22
14
<1
<1
<1
—
9e
12
15
16
18
19
20
22
—
<1
<1
<1
4
a
b
c
Compounds were tested as the trifluoroacetate salt.
Plasma protein binding data are reported as % unbound in 100% plasma; h = human, m = mouse.
Unless otherwise noted, mixture PK dosing (4 compounds + 1 standard, n = 3): iv 0.5 mg/kg (0.1 mg/mL in EtOH/PEG/water for each compound); po 1 mg/kg, (0.1 mg/mL
in EtOH/PEG/water for each compound).
d
Single compound PK dosing (n = 3): iv 2 mg/kg (1 mg/mL in EtOH/PEG/water); po 10 mg/kg, (1 mg/mL in EtOH/PEG/water).
Single compound PK dosing (n = 3): iv 1 mg/kg (0.2 mg/mL in EtOH/PEG/water); po 2 mg/kg, (0.2 mg/mL in EtOH/PEG/water).
e
Graham, Thomas H.
