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E. Venkateswararao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4523–4527
Table 2
In vitro anti-proliferative activity of benzoxathiole derivatives 1a–n
a
Compound No.
Anti-proliferative activity: GI50 (lM)
ACHN
NCI-H23
MDA-MB231
HCI-15
NUGC-3
PC-3
1a
1b
1c
1d
1e
1f
1g
1h
1i
8.98
2.98
3.08
11.32
6.70
9.81
12.35
3.74
3.67
>30
7.08
2.72
2.28
12.45
7.08
11.61
13.51
3.36
3.65
>30
5.88
2.31
2.48
13.14
5.90
6.38
14.33
3.20
2.69
>30
8.09
2.07
1.15
12.23
3.53
3.79
12.21
2.39
2.99
>30
5.46
1.89
1.22
11.50
2.71
2.94
15.89
1.08
0.67
>30
7.40
2.63
2.19
12.67
5.16
4.26
16.45
2.45
2.76
>30
1j
1k
1l
>30
>30
>30
>30
>30
>30
>30
>30
>30
>30
>30
>30
1m
1n
ADRb
>30
>30
1.63
>30
>30
1.66
>30
>30
1.79
>30
>30
1.34
>30
>30
1.00
>30
>30
1.65
a
GI50 values are taken as a mean from three experiments.
ADR : adriamycin.
b
emphasize that the structural modification with hydrophobic
groups at this position do not have much effect on the activity of
1a.
In the next set of experiment, our focus was shifted to find the
role of phenylimino group of 1a and thus the planar phenyl group
at 2-position was replaced with the bulky cyclohexyl group. The
For finding the correlation between inhibitory activity of NF-jB
and anti-proliferative activity, all the compounds 1a–n were eval-
uated for their anti-proliferative against various cancer cell lines
namely, ACHN (renal), NCI-H23 (lung), MDA-MB-231 (breast),
HCT-15 (colon), NUGC-3 (stomach) and PC-3 (prostate) cancer cell
lines as shown in Table 2 (GI50 values are listed in parenthesis be-
low along with this sequence of these cell lines written here).
resulted compounds 1d (IC50 = 12.5
(IC50 = 13.1 M, ClogP = 6.491) showed less inhibition as com-
pared to the corresponding phenylimino analogs 1a–c toward
NF- B. Therefore, the planarity of phenyl group seems to be impor-
lM, ClogP = 6.082) and 1e
l
Compounds 1a (GI50 = 8.98, 7.08, 5.88, 8.09, 5.46, 7.40
respectively), 1b (2.98, 2.72, 2.31, 2.07, 1.89, 2.63 M, respectively)
and 1c (3.08, 2.28, 2.48, 1.15, 1.22, 2.19 M, respectively) showed
good anti-proliferative activity against all cancer cell lines.
Although 1a showed similar level of activity against NF- B com-
lM,
l
j
l
tant for good inhibition at this position.
Along with the importance of the planarity of phenyl ring at
j
2-position of 1a toward NF-
jB inhibition, substituent effect on
pared to those of 1b and 1c, its anti-proliferative activity decreased
phenyl moiety was examined. Accordingly, analogs 1f–l associated
with p-chloro, p-fluoro or 2,4,6-trichloro substituted phenyl groups
have been synthesized and tested for their inhibition against
as compared to 1b and 1c. Compounds 1d (11.32, 12.45, 13.14,
12.23, 11.50, 12.67
3.53, 2.71, 5.16 M, respectively) showed moderate anti-prolifera-
tive activity as well as NF- B inhibition. The chloro analogs 1f
(9.81, 11.61, 6.38, 3.79, 2.94, 4.26 M, respectively) and 1g
(12.35, 13.51, 14.33, 12.21, 15.89, 16.45 M, respectively) also
showed moderate anti-proliferative activity. Interestingly, the flu-
oro analogs, 1h (3.74, 3.36, 3.20, 2.39, 1.08, 2.45 M) and 1i (3.67,
3.65, 2.69, 2.99, 0.67, 2.76 M) showed potent anti-proliferative
activity against all cancer cell lines. However, compounds 1j
(>30, >30, >30, >30, >30, >30 M), 1k (>30, >30, >30, (>30, >30,
>30 M), 1l (>30, >30, >30, >30, >30, >30 M r), 1m ((>30, >30,
>30, >30, >30, >30 M) and 1n (>30, >30, >30, (>30, >30, >30 M)
did not show any anti-proliferative activity against any cancer cell
lines and their NF- B inhibition were almost negligible. The above
results indicate relatively good correlation between NF- B inhibi-
lM, respectively) and 1e (6.70, 7.08, 5.90,
l
NF-
ring decreases the inhibition as shown in 1f (IC50 = 7.5
ClogP = 6.199), 1g (IC50 = 19.5 M, ClogP = 5.680) compared to
the corresponding un-substituted analogs. The fluoro-substituted
analogs also reduced the activity as shown in 1h (IC50 = 10.6 M,
ClogP = 5.629) and 1i (IC50 = 7.9 M, ClogP = 6.038). These chloro
jB, respectively. The para chloro substitution on the phenyl
j
lM,
l
l
l
l
l
l
l
or fluoro substitutions slightly affect the activity. However, 2,4,6-
trichloro substituted analogs resulted in the total loss of activity
l
as shown in 1j (IC50 = >30
lM, ClogP = 7.625), 1k (IC50 = >30
lM,
l
l
ClogP = 7.106) and 1l (IC50 = >30
lM, ClogP = 8.034). The hydro-
l
l
phobicity of trichloro substituted analogs 1j–l appears very large
compared to those of mono-or un-substituted analogs. Thus this
might be the reason for the total loss of activity in 1j–k.
j
j
To explore the importance of benzoxathiole ring of 1a, benzodioxole
analog 1m (IC50 = >30 lM, ClogP = 4.781) was synthesized and evalu-
ated. Although it was expected that the isosteric substitution of sulfur
with oxygen would maintain the activity toward the inhibition of NF-
tory activity and anti-proliferative activity of compounds 1a–n.
Structure–activity relationship of 1a analogs could be summa-
rized as shown in Figure 2. Dihydrobenzo[d][1,3]oxathiol-4(5H)-
one motif in the structure of 1a appears to be essential for both its
jB, surprisingly this replacement completely abolished the activity.
NF-jB inhibitory activity and anti-proliferative activity. Slight vari-
This might indicate that the sulfur atom in 6,7-dihydro-
benzo[d][1,3]dioxol-4(5H)-ones has specific role in binding to the
receptor. Thus benzoxathiole group of 1a is essential for the inhibition
ation of hydrophobicity by replacement of dimethyl group of 1a at
6-position with bulky isopropyl group and introduction of para-flu-
oro substitution on 2-phenyl group showed good NF-jB inhibitory
of NF-jB activity.
activity as well as anti-proliferative activity. However, excessive in-
To explore the importance of cyclohex-2-enone moiety of 1a, a
crease in hydrophobicity with 2,4,6-trichloro groups on phenyl
ring truncated derivative, 1-(2-(phenylimino)-5-propyl-1,3-oxath-
group of 1a resulted in the decrease or loss of both NF-
activity and anti-proliferative activity. Taken together, all the deriv-
atives (1a–n) maintained a good correlation between their NF-jB
jB inhibitory
iol-4-yl)ethanone (1n, IC50 = >30
lM, ClogP = 5.461), was prepared
and tested for its inhibitory activity of NF-
jB. It showed complete
loss of the activity. This confirmed that the 6,7-dihydro-5H-benzo-
[1,3]oxathiol-4-one of 1a is important for the activity.
inhibition and anti-proliferative activity though the extent is not
completely proportional to each other.23 This might indicate that