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H. Jin et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5229–5232
O
O
F
OH
OH
N
N
NH2
O
NH2
CF3
N
N
N
H
NH2
1
2
Figure 1. Chemical structures of compounds 1 and 2.
say. As shown in Table 1, introduction of the 2-amino triazine re-
sulted in compound 6a having an IC50 of 0.5 M against TPH1,
which is slightly more potent than the HTS hit, pyrazine 2
(IC50 = 0.67 M). The effect of varying substitutions on the distal
sis by introducing a bulky fused ring system. Adamantane analog
6l proved highly potent (IC50 = 0.019 M) and demonstrated an
l
l
additional threefold improvement in cell potency compared to 6k.
As illustrated in Table 2, we continued to look at sizeable hydro-
phobic substitutions on the left part of the molecule. To this end,
we decided to introduce a naphthalene moiety. Not surprisingly,
naphthalene analog 7a (Table 2) showed a good potency of
l
phenyl ring was evaluated. A fourfold in vitro potency increase
was observed when either electron withdrawing groups, such as
a trifluoromethyl group (6b), or electron donating groups, such
as methyl groups (6c) were substituted on the phenyl ring. How-
ever, these changes did not improve the cell potency. Replacement
of the phenyl ring with cyclohexyl gave analog 6d which was equi-
potent to the substituted phenyl derivatives. Using the published
apo-structure of human TPH1, a homology model was constructed
which suggested that the binding pocket could accommodate lar-
ger groups on the left side of the molecule. Biphenyl analogs were
designed to test the hypothesis. Compared to 6a, the para-biphenyl
0.024
at the benzylic position. The (R)-2-methyl amino isomer 7b
(IC50 = 0.026 M) demonstrated much better cellular potency than
lM against TPH1. We next investigated the stereochemistry
l
(S) isomer 7c but maintained similar primary potency. Further-
more, we evaluated modulating the nature of the linker of 7b. As
shown in Table 2, 7d (IC50 = 0.055 lM, EC50 = 1.7 lM) with a gem-
inal dimethyl group exhibited a twofold decrease in enzyme po-
tency and a fivefold decrease in cell potency compared with 7b.
6e (IC50 = 0.06
fold in the biochemical assay. Moving the distal phenyl ring to
the ortho-position resulted in 6f (IC50 = 0.007 M, EC50 = 2.2 M),
which further improved the primary potency. meta-Biphenyl 6g
(IC50 = 0.028 M, EC50 = 1.4 M) improved primary potency by
twofold compared with 6e. Interestingly, these biphenyl analogs
improved the cellular potency more than sixfold compared with
the corresponding phenyl analogs. Electronic effects on the biphe-
nyl system were also investigated. Introduction of electron donat-
l
M, EC50 = 1.8
l
M) increased the activity by eight-
Trifluoromethyl substituted alkoxy compound 7e (IC50 = 0.32
EC50 = 9.6 M) and trifluoromethyl substituted amine 7f
(IC50 = 0.33 M, EC50 = 6.3 M) both proved detrimental to the en-
zyme and cellular potencies. N-methylation of 7b resulted in 7g
(IC50 = 0.05 M, EC50 = 0.48 M), which showed a twofold decrease
in primary potency and had little effect on cell potency. Changing
the -methyl group to ethyl, isobutyl, and isopropyl (7h, 7i, and 7j,
respectively) resulted in loss of cell potency. Efforts toward modi-
fying the triazine substitution are also summarized in Table 2.
Morpholino triazine 7k, 2-N-methyl triazine 7l and des-amino tri-
azine 7m all had reduced potencies.
An X-ray crystal structure of 7b co-crystallized with TPH1 was
obtained (Fig. 2) after our SAR exploration on the triazine series.9
This crystal structure revealed that the carboxylate group of com-
pound 7b interacted with Arg257 through a charge–charge interac-
tion and accepted two hydrogen bonds: one from the backbone
nitrogen of Thr265 and another one from the hydroxyl group on
lM,
l
l
l
l
l
l
l
l
l
a
ing groups, such as
EC50 = 2.3 M) retained potency while electron withdrawing
groups (e.g., 6j, IC50 = 0.16 M, EC50 = 4.1 M) led to decreased
a methyl group (6i, IC50 = 0.014 lM,
l
l
l
in vitro potency. Phenoxyphenyl analog 6h displayed fourfold re-
duced in vitro potency relative to 6g. The addition of a benzylic
a-methyl group as in 6k (IC50 = 0.024 lM, EC50 = 2.5 lM) increased
in vitro potency by more than twofold compared with the corre-
sponding unsubstituted analog 6e. We further tested our hypothe-
R2
R1
R3
X
N
Y
Cl
Cl
N
Cl
a
R2
X
+
R3
R1
N
N
N
N
Y
3
4
5
4-1 Y=NH2
b
4-2
4-3
Y=H
Y=NHCH3
O
4-4
N
O
Y=
OH
R2
R1
R3
X
N
Y
NH2
N
N
6a-l
7a-m
Scheme 1. General procedure for the synthesis of triazine analogs. Reagents and conditions: (a) 1,4-dioxane, K2CO3, reflux, 95%; (b)
(PPh3)2PdCl2, 1 M Na2CO3, acetonitrile, microwave, 150 °C, 10 min, 92%; all compounds were characterized by 1H NMR, LC/MS, and HPLC.
L-phenylalanine-boronic acid,