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M. Radi et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1207–1211
loop is in a closed position.7 This characteristic reduces
or abrogates drug effectiveness when Abl TK is stabi-
lized in its active conformation in consequence of auto-
phosphorylation events or mutations. An important step
forward has been made with the recent approval of
Dasatinib (SprycelTM, Fig. 1), a multiply targeted tyro-
sine kinase inhibitor, active against 14 of the 15 clini-
cally relevant Gleevec resistant mutants.8 Acquired
resistance is becoming a common issue in targeted ther-
apy of malignant diseases and, as a consequence, there is
a growing interest in developing novel TK inhibitors
able to target Imatinib-resistant form of CML.9
latter compounds was split in three portions and reacted
with three different benzoyl chlorides (in the presence of
the same scavenger) thus obtaining, after removal of the
resin, a small library of 21 compounds (6a–u). These fi-
nal compounds were then screened, in a cell-free assay,
against recombinant human c-Src and Abl using Imati-
nib as a reference compound (Table 1). Interestingly,
several of the tested derivatives were characterized by
high affinity toward the isolated Abl with an inhibitory
activity (expressed as Ki values) comparable to that of
Imatinib. A significative improvement has been there-
fore reached with respect to the parent compounds 1–
3. In addition, the metabolic stability of compounds
6a–u in the presence of CYP3A4 was modeled using
molecular interaction field descriptors calculated in Vol-
Surf.14 As a result, the thiadiazole derivatives showed a
metabolic stability comparable to or, sometimes, even
better than Imatinib (Fig. 2).
A computational protocol, based on docking/MD simu-
lations and a pharmacophore-based database search,
has been recently developed by our research group lead-
ing to the identification of commercially available com-
pounds showing moderate inhibitory activity toward
both Src and Abl kinases.10 Among these new molecular
scaffolds (original in the field of both Src and Abl inhib-
itors), the thiadiazole derivatives 1–3 attracted our
attention due to their ability to inhibit both Src and
Abl (dual inhibitors) or alternatively only one of the
two kinases, depending on the substitution pattern on
the benzamido moiety (Fig. 1). To fully understand
the SAR of these new Src/Abl inhibitors, we planned
to synthesize a small library of thiadiazoles (analogues
of 1–3) bearing a wide range of substituents on both aro-
matic rings. Herein we describe the synthesis, anticancer
activity, and docking studies of novel benzoylamino-2-
[(4-benzyl)thio]-1,3,4-thiadiazoles 6a–u. For the most
active compound (6a), a preliminary study on its cyto-
differentiating properties on HL-60 cells is also de-
scribed.11 A two-step parallel solution phase approach
has been used to generate the combinatorial library of
compounds 6a–u (Scheme 1).
The substitution pattern on the two aromatic rings of
6a–u plays therefore an important role in determining
the affinity for the enzyme Abl. However, while it was
evident that the less active compounds 6r–u were charac-
terized by the presence of an ortho-chloro substituent on
the benzamide moiety, the rationalization of the SAR
for the remaining derivatives required the intervention
of 3D considerations. Accordingly, after validation of
the computational protocol,16 compounds 6a–u were
docked into the ATP binding site of Abl17 both in its
inactive conformation (PDB code: 1IEP, cocrystallized
with Imatinib)18 and active conformation (PDB code:
Table 1. c-Src/Abl tyrosine kinase inhibitory activities exerted by
compounds 6a–u
Compound
R1
R2
Activitya (Ki, lM)
c-Src
Abl
In the first step, the commercially available 5-amino-
1,3,4-thiadiazole-2-thiol (4) was partitioned into seven
separate reaction vessels and then reacted with seven dif-
ferent benzyl bromides using the resin AmberliteÒ IRA-
67 both as a base and as a scavenger for the hydrobro-
mic acid released in the reaction medium as a side prod-
uct.12 Once all the reactions were completed (24 h), the
resin was removed by parallel filtration to give the inter-
mediates 5a–g in more than 95% purity.13 Each of the
6a
6b
6c
p-F
p-F
0.354
0.217
0.464
0.195
0.219
0.221
0.165
0.200
0.569
0.199
0.263
0.170
0.064
0.522
0.718
0.247
0.334
0.900
0.169
1.137
0.272
31
0.044
0.047
0.070
0.073
0.083
0.089
0.092
0.104
0.167
0.189
0.195
0.210
0.217
0.225
0.272
0.369
0.400
0.406
0.760
0.920
1.260
0.013
p-Br
H
p-Cl
p-F
6d
6e
m-Cl
p-NO2
p-Br
p-NO2
p-F
p-Cl
o-Cl
p-F
6f
6g
6h
6i
p-Cl
p-Cl
p-F
m-F
6j
p-OCH3
p-OCH3
p-NO2
m-F
p-Cl
p-F
6k
6l
p-F
R1
6m
6n
6o
6p
6q
6r
p-Cl
o-Cl
o-Cl
p-F
N
N
i.
N
N
p-Br
m-F
S
H2N
SH
H2N
S
S
m-Cl
H
5a-g
4
p-Cl
o-Cl
o-Cl
o-Cl
o-Cl
p-F
ii.
6s
m-Cl
H
R1
6t
O
R2
N
N
6u
Imatinib
p-OCH3
S
N
S
H
a Values are means of at least two experiments and were calculated
according to the following equation: Ki = ID50/{E0 + [E0 (Km(ATP)/
S0)]}/E0, where E0 and S0 are the enzyme and the ATP concentration
(0.125 and 0.160 pmol, respectively, for Src; 0.022 and 0.016 lM,
respectively, for Abl).
6a-u
Scheme 1. Reagents and conditions: (i) benzyl bromides, AmberliteÒ
IRA-67, CH3CN, 24 h, rt; (ii) benzoyl chlorides, AmberliteÒ IRA-67,
THF, 24 h, 60 °C.