42
L. Fang et al. / Bioorg. Med. Chem. Lett. 24 (2014) 40–43
to that of 3f which possess an electron-withdrawing F substituent.
The FRSA of the target compounds towards DPPH radicals is gener-
ally better than that towards galvinoxyl radicals. This is probably
because they are two different types of radicals: DPPH is a kind
of typical nitrogen radicals while galvinoxyl radicals belong to
reactive oxygen species. From a chemical point of view, DPPH rad-
icals are more reactive than galvinoxyl radicals, and are more eas-
ily captured by phenolic hydroxy groups. In comparison with
curcumin, the FRSA of the target compounds is significantly im-
proved. This is probably due to the introduction of the dimethyl-
aminomethyl substituent which is an electron donating group
and may subsequently enhance the free radical-capturing ability
of the phenolic hydroxy group.
1.2
1
(A)
0.8
0.6
0.4
0.2
0
Curcumin
3a
2a
Ab, especially Ab42, and its aggregates have been considered as
the key etiological factors triggering a neurotoxic cascade and fi-
nally causing neurodegeneration in AD brains.1 In vivo monomeric
Ab42 peptides come from the amyloid precursor protein (APP)
1.2
1
(B)
0.8
0.6
0.4
0.2
0
cleaved by b- and
c-secretase. The monomeric Ab42 peptides,
Curcumin
spontaneously or bioactivated by various factors, prone to aggre-
gate, forming more toxic oligomeric intermediates and plaque-
associated amyloid fibrils. Thus, the inhibition of Ab-aggregation
has been presented as a promising approach for the development
of anti-AD agents. In order to investigate the inhibitory effect of
the derivatives on Ab self-aggregation, compound 3a (5, 50,
3a
2a
100
min was used as positive control. The results are presented in Fig-
ure 3. At concentration of 5 M, neither curcumin nor 3a showed
significant inhibitory effect on the self-aggregation of Ab42. As
the concentration increased, the inhibition rate of curcumin and
lM) was selected to perform Thioflavin T (ThT) assay. Curcu-
Figure 4. Stability measured by visible absorption in the presence (A) or absence
(B) of 0.1% FBS. A0 means absorption of the solution measured at 410 nm at 0 min; A
means absorption of the solution measured at 410 nm. The data of 2a and curcumin
were cited from Ref. 10
l
3a was enhanced. At concentration of 50
of curcumin and 3a reached 24% and 28%, respectively. When the
concentration was raised to 100 M, the inhibition rate of curcu-
min and 3a was also increased to 29% and 32%, respectively, which
is indicative of a dosage-dependent manner of the inhibition.
Clearly, in addition to the antioxidant activity compound 3a also
possesses positive anti-Ab42 self-aggregation activity, which may
alleviate the Ab-induced toxicity and eventually benefit the treat-
ment of AD.
The poor stability of curcumin is a major problem limiting its
clinical application. An investigation showed that 50% of curcumin
has decomposed after 8 h culture in cell culture medium contain-
ing 0.1% fetal bovine serum (FBS) or in human blood.6 In vivo cur-
cumin can be rapidly metabolized into curcumin glucuronides and
sulfates conjugating at the phenolic hydroxy groups. Thus, the
introduction of dimethylaminomethyl groups, which have a large
lM, the inhibition rate
steric hindrance, to the ortho position of the hydroxy groups may
slow down the formation of the glucuronides and sulfates, and pro-
long the half time of the target compounds. To investigate the sta-
bility of the target compounds in a physiological media, the
absorption variation of curcumin and 3a was measured in phos-
phate-buffered solutions (pH = 7.4) in the presence or absence of
0.1% FBS under daylight condition using a previous reported meth-
od (Fig. 4).6 It was found that curcumin degraded very rapidly. In
the absence of 0.1% FBS more than 80% curcumin degraded within
120 min, which is consistent with the previous report.6 In the pres-
ence of 0.1% FBS, curcumin was a little stable, but still 60% curcu-
min degraded during the measurement. The stability of 2a was also
poor, but better than curcumin. About half of 2a were decomposed
within 120 min under both measurement conditions. In contrast,
the analogue 3a was much more stable. The absorption of 3a only
slight decreased during the measurement, no matter 0.1% FBS was
present or absent, indicating a good stability of 3a. Analyzing the
structures of 2a and 3a, it can be found that the structural differ-
ence between these two compounds is that the methoxy group
of 2a was replaced with the dimethylaminomethyl group of 3a.
The improvement of the stability of 3a, as compared with 2a, indi-
cated that the introduction of the dimethylaminomethyl group can
indeed prevent the degradation of the compound.
l
1.5
1.0
0.5
0.0
Curcumin
Aβ alone
3a
**
**
**
**
Table 2
The logP values of the target compounds and curcumin
Compound
LogP
Compound
LogP
b
3a
3b
3c
3d
2a
3.70a (3.48
3.84a
)
3e
3f
3g
3h
4.32a
3.84a
4.70a
4.83a
3.85a
5 µM
50 µM
100 µM
4.06a
3.92a
3.72a (3.65b)
Curcumin
Figure 3. Inhibition of amyloid formation by compound 3a and curcumin moni-
tored by ThT fluorescence. Each result is the mean value of 3 measurements
(n = 3) SEM. Statistical analysis was performed by the one-way ANOVA followed
by Tukey’s multiple comparison test. ⁄⁄p < 0.01 versus Ab alone.
a
Calculated by software Marvin sketch.
Experimental values determined in the n-octanol–water system.
b
Fang, Lei
