5028
L.-M. Zhao et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5027–5029
H3COH2CO
OH
O
HO
OH
O
HO
OH
i
ii
CH3
CH3
OH
OH
iii
H3COCH2O
2
1
H3COH2CO
OH
HO
OH
R
R
iv
O
O
OH
H3COCH2O
4a~4m
3a~3m
Scheme 1. Reagents and conditions: (i) HCl/CH3CN/ZnCl2, 0 °C, 24 h and reflux, 2 h; (ii) ClCH2OCH3/K2CO3/acetone, rt, 2 h; (iii) aromatic
aldehyde/KOH/EtOH, rt, 24 h; (iv) 3 M HCl in MeOH, reflux, 0.5 h.
1
spectral data (IR, H NMR, and MS) obtained were
consistent with the structures proposed.18
These results suggested that substitution of the methoxy
group on chalcone B ring significantly increased the
antiplatelet activity. The O-phenylmethylation at C-4
of 4d (i.e., 4a), O-allyloxylation at C-4 of 4d (i.e., 4h),
and the demethoxylation at C-3 of 4e (i.e., 4i) enhanced
the inhibitory effects on arachidonic acid-induced plate-
let aggregation, although they were not as potent as that
of compound 4e. The replacement of the methoxyl at
C-4 of 4i with a methyl group (i.e., 4g) produced no
effect on the inhibition of platelet aggregation at a
concentration of 100 lM. These results indicated that
etherifying the phenolic hydroxyl of the B ring was re-
quired for the inhibition of platelet aggregation induced
by arachidonic acid. On the other hand, compound 4b
had no significant inhibitory effect, indicating that intro-
duction of a strong electron-donating group at the C-4
might attenuate its inhibitory effect. In addition, lipo-
philicity did not appear to be an important factor for
the antiplatelet activity, because a chloro group substi-
tuted at C-4 of 4l (i.e., 4j) or dichloro substituted at C-
2 and C-4 of 4l (i.e., 4k) was not more potent than that
of compound 4e. A similar inhibitory pattern was also
observed from the tested compounds in the collagen-
induced platelet aggregation, except for compound 4k
which did not show any inhibitory effect at a concentra-
tion of 100 lM. The IC50 values of compounds 4a, 4e,
4h, 4i, and 4j against collagen-induced platelet aggrega-
tion were 37.4 4.4, 27.4 4.4, 28.4 5.3, 33.3 4.6,
and 66.5 5.3 lM, respectively.
The antiplatelet activity was studied by measuring the
aggregation of washed rabbit platelet applying BornÕs
turbidimetric method.19 The washed platelet suspension
of rabbits was incubated at 37 °C for 4 min in an aggre-
gometer with stirring at 1000 rpm and the aggregation
was stimulated by adding arachidonic acid (100 lM)
or collagen (10 lg/ml) at concentrations giving maximal
aggregatory response, as Jin et al.20 described in detail.
The antiplatelet activity was expressed as percent inhibi-
tion with respect to control. Anti-aggregating potency of
the compounds was indicated by IC50 values that were
calculated by linear regression analysis of the concentra-
tion–response curves obtained for each compound. Data
are reported in Table 1.
As shown in Table 1, among the tested compounds, six
compounds showed potent inhibitory effects on arachi-
donic acid-induced washed rabbit platelet aggregation,
and the potencies of some compounds were better or
comparative to aspirin, a COX inhibitor which was used
as a positive control. Comparing with compound 4l,
compounds 4a, 4e, 4h, 4i, and 4j had potent antiplatelet
effects. It seemed that the substituent on chalcone B ring
might be important in the inhibition of platelet aggrega-
tion. However, compounds 4b, 4c, 4d, 4f, 4g, and 4m
that bore substituent(s) on the B ring did not show
any inhibitory effect at a concentration of 100 lM.
These results indicated that the character of substituent
on the B ring had a significant influence on the antiplate-
let activity. The compounds 4a, 4e, and 4h–4k concen-
tration-dependently inhibited washed rabbit platelet
aggregation induced by arachidonic acid, with the IC50
values of 28.5 2.2, 15.2 5.4, 19.8 3.5, 25.5 2.7,
45.2 6.7, and 70.6 3.5 lM, respectively. It seemed
that the increase of hydroxy group on chalcone A ring
could influence the inhibitory effect on platelet aggrega-
tion, but the potency depended on the variation of the
substituent of the B ring. Compound 4e was the most
potent in inhibiting arachidonic acid-induced platelet
aggregation: the demethylation at C-4 of 4e (i.e., 4d)
and the demethylation at C-3 and C-4 of 4e (i.e., 4c)
did not contribute so much to the antiplatelet effect.
In conclusion, some compounds showed good inhibito-
ry effects on platelet aggregation induced by arachidon-
ic acid and collagen. Some of them exhibited even
better potency than the reference drug, aspirin. The
antiplatelet effects of these compounds were probably
mediated through the suppression of cycloxygenase
activity and reduced thromboxane formation as it
had been reported that some chalcone derivatives
inhibited arachidonic acid-induced platelet aggregation
by inhibition of cycloxygenase activity and thrombox-
ane formation.16 Further experiments were needed to
elucidate their mechanism of action. Compound 4e
exhibited the most potent antiplatelet effect among
the tested compounds. Therefore, it merits further
investigation as the lead compound in continuing