Antimalarial Activity of Simalikalactone E, a New Quassinoid from Quassia amara L. (Simaroubaceae)

Abstract

We report the isolation and identification of a new quassinoid named simalikalactone E (SkE), extracted from a widely used
Amazonian antimalarial remedy made out of Quassia amara L. (Simaroubaceae) leaves. This new molecule inhibited the growth of Plasmodium falciparum cultured in vitro by 50%, in the concentration range from 24 to 68 nM, independently of the strain sensitivity to chloroquine.
We also showed that this compound was able to decrease gametocytemia with a 50% inhibitory concentration sevenfold lower than
that of primaquine. SkE was found to be less toxic than simalikalactone D (SkD), another antimalarial quassinoid from Q. amara, and its cytotoxicity on mammalian cells was dependent on the cell line, displaying a good selectivity index when tested
on nontumorogenic cells. In vivo, SkE inhibited murine malaria growth of Plasmodium vinckei petteri by 50% at 1 and 0.5 mg/kg of body weight/day, by the oral or intraperitoneal routes, respectively. The contribution of quassinoids
as a source of antimalarial molecules needs therefore to be reconsidered.

Full text

ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Oct. 2009, p. 4393–4398 Vol. 53, No. 10
0066-4804/09/$08.00⫹0 doi:10.1128/AAC.00951-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Antimalarial Activity of Simalikalactone E, a New Quassinoid from
Quassia amara L. (Simaroubaceae)
䌤
†
N. Cachet,
1,2
‡ F. Hoakwie,
1,2
§ S. Bertani,
3
¶ G. Bourdy,
1,2
E. Deharo,
1,4
D. Stien,
5
E. Houel,
5
H. Gornitzka,
6
J. Fillaux,
7
S. Chevalley,
1,2
A. Valentin,
1,2
储and V. Jullian
1,2
储*
Laboratoire de Pharmacochimie des Substances Naturelles et Pharmacophores Redox, UMR 152, UPS, Universite´ de Toulouse,
118 Route de Narbonne, F-31062 Toulouse Cedex 9, France
1
; Institut de Recherche pour le De´veloppement, UMR 152,
118 Route de Narbonne, F-31062 Toulouse Cedex 9, France
2
; USM 0307, Laboratoire de Parasitologie Compare´e et
Mode`les Expe´rimentaux, Muse´um National d’Histoire Naturelle, Paris, France
3
; Institut de Recherche pour le
De´veloppement, UMR 152, Mission IRD Casilla 18-1209 Lima, Peru
4
; CNRS, UMR Ecofog, Universite´ des
Antilles et de la Guyane, Cayenne, France
5
; CNRS, LCC, UPR 8241, 205 Route de Narbonne,
31077 Toulouse Cedex 4, France
6
; and Service de Parasitologie-Mycologie, CHU Rangueil,
1 Avenue de Pr. Jean Pouldhe`s, TSA 50032, 31095 Toulouse Cedex, France
7
Received 9 July 2009/Returned for modification 17 July 2009/Accepted 29 July 2009
We report the isolation and identification of a new quassinoid named simalikalactone E (SkE), extracted from a
widely used Amazonian antimalarial remedy made out of Quassia amara L. (Simaroubaceae) leaves. This new
molecule inhibited the growth of Plasmodium falciparum cultured in vitro by 50%, in the concentration range from
24 to 68 nM, independently of the strain sensitivity to chloroquine. We also showed that this compound was able to
decrease gametocytemia with a 50% inhibitory concentration sevenfold lower than that of primaquine. SkE was
found to be less toxic than simalikalactone D (SkD), another antimalarial quassinoid from Q.amara, and its
cytotoxicity on mammalian cells was dependent on the cell line, displaying a good selectivity index when tested on
nontumorogenic cells. In vivo, SkE inhibited murine malaria growth of Plasmodium vinckei petteri by 50% at 1 and
0.5 mg/kg of body weight/day, by the oral or intraperitoneal routes, respectively. The contribution of quassinoids as
a source of antimalarial molecules needs therefore to be reconsidered.
This study of the antiplasmodial properties of a new quassi-
noid from Quassia amara L. leaves results from our ongoing
work on traditional antimalarial remedies in French Guiana.
Through a “knowledge, attitudes, and practices” survey fo-
cused on malaria and its treatments in this French overseas
department, we showed that Q.amara leaf tea was the most
frequently used antimalarial remedy (35). Q.amara belongs to
the Simaroubaceae, a family known to contain quassinoids,
secondary metabolites characteristic of the Sapindale order,
that display a wide range of biological activities, among them
antiparasitic activity and cytotoxicity (11, 17).
Simalikalactone D (SkD) was identified as one of the com-
pounds responsible for the activity of Quassia amara juvenile
leaf tea (6), but the small amount present in the traditional
preparation, made out of mature leaves (5), could not fully
explain the activity seen in vitro and in vivo (4). This is why we
looked for other active ingredients responsible for the antiplas-
modial activity and isolated a new quassinoid, named simalika-
lactone E (SkE). We report here our detailed studies of the
antimalarial and cytotoxic properties of this new compound.
MATERIALS AND METHODS
Plant material. Leaves of Quassia amara were collected in Re´mire-Montjoly,
French Guiana. A sample specimen (GB3012) was deposited in the Cayenne
Herbarium (CAY), and a specialist confirmed the botanical identification.
Chemistry. Detailed protocol for the isolation of SkE from the aqueous de-
coction or the methanolic extract can be found in the supplemental material.
Structural data for SkE. Structural data for simalikalactone E follow: APCIMS,
579 (MH
⫹
), 561 (MH
⫹
-H
2
O),
1
H nuclear magnetic resonance (
1
H NMR) (CDCl
3
,
500 MHz), ␦6.19 (m, 1H, H-15), 6.17 (s, 1H, H-3), 5.19 (dd, J⫽2.6 Hz, J⫽11.7 Hz,
1H, H-6), 4.75 (d, J⫽5.1 Hz, 1H, H-11), 4.70 (d, J⫽2.6 Hz, 1H, H-7), 4.65 (d, J⫽
7.4 Hz, 1H, H-17a), 4.19 (s, 1H, H-1), 3.83 (s, 1H, H-12), 3.70 (d, J⫽7.4 Hz, 1H,
H-17b), 3.37 (d, J⫽11.5 Hz, 1H, H-5), 2.51 (m, 1H, H-24), 2.48 (m, 1H, H-19), 2.45
(m, 1H, H-14), 2.43 (m, 1H, H-9), 2.08 (s, 3H, H-30), 1.79 (m, 2H, H-21a, H-26a),
1.61 (m, 1H, H-26b), 1.53 (m, 1H, H-21b), 1.45 (s, 3H, H-28), 1.35 (s, 3H, H-29), 1.21
(d, J⫽7.0 Hz, 3H, H-20), 1.19 (d, J⫽7.0 Hz, 3H, H-25), 1.01 (t, J⫽7.4 Hz, 3H,
H-22), 0.99 (t, J⫽7.4 Hz, 3H, H-27).
13
C NMR (CDCl
3
, 125 MHz), ␦196.5 (C-2),
176.2 (C-23), 175.2 (C-18), 166.5 (C-16), 163.0 (C-4), 126.5 (C-3), 82.8 (C-7), 81.8
(C-1), 80.0 (C-13), 79.8 (C-12), 74.2 (C-11), 70.9 (C-17), 69.1 (C-6), 67.3 (C-15), 52.7
(C-14), 50.4 (C-10), 46.1 (C-8), 45.9 (C-5), 41.3 (C-24), 41.3 (C-19), 41.1 (C-9), 27.2
(C-26), 26.7 (C-21), 26.1 (C-30), 22.8 (C-28), 16.7 (C-20), 15.6 (C-25), 12.5 (C-29),
11.7 (C-22), 11.5 (C-27). Infrared (KBr, cm
⫺1
): 2,965, 2,926, 2,855, 1,766, 1,738,
1,722, 1,667. [␣]
D26
⫽⫹94° (c ⫽0.35, CHCl
3
).
CCDC 739477 contain the supplementary crystallographic data for SkE. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Data Centre [CCDC], 12 Union Road,
Cambridge CB2 1EZ, United Kingdom. Fax: 44 1223 336033. E-mail:
deposit@ccdc.cam.ac.uk).
Biological tests. (i) Activity against erythrocytic stages of cultured Plasmodium
falciparum.SkE antiplasmodial activity was determined against three strains of P.
falciparum: chloroquine (CQ)-sensitive F32 Tanzania (CQ 50% inhibitory con-
centration [IC
50
], 36 ⫾3 nM) and chloroquine-resistant FcB1 Colombia and W2
Indochina (CQ IC
50
s, 167 ⫾32 nM and 196 ⫾16 nM, respectively).
* Corresponding author. Mailing address: Institut de Recherche
pour le De´veloppement, UMR 152, Faculte´ de Pharmacie, Universite´
de Toulouse 3, 35 Chemin des Maraichers, Toulouse 31062, France.
Phone: 33 5 62 25 68 23. E-mail: jullian@cict.fr.
† Supplemental material for this article may be found at http://aac
.asm.org/.
‡ Present address: LCMBA, UMR 6001, CNRS-Universite´ Nice
Sophia Antipolis, 06108 Nice Cedex 02, France.
§ Present address: LSPCMIB, UMR 5068, CNRS-Universite´ Paul
Sabatier, 31062 Toulouse Cedex 09, France.
¶ Present address: Department of Biochemistry; University of Cal-
ifornia, Riverside, CA 92521.
储These two senior investigators made equal contributions to this
study.
䌤
Published ahead of print on 10 August 2009.
4393
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Parasites were cultured by the method of Trager and Jensen (32) with mod-
ifications (34). The cultures were synchronized by a combination of magnetic
enrichment and 5% D-sorbitol lysis (Merck, Darmstadt, Germany) (22, 27). In
vitro antimalarial activity testing was performed by the method of Desjardins et
al. (12) with modifications (34).
The sensitivity of the different asexual erythrocytic stages of P.falciparum to SkE
was determined on the FcB1 strain, synchronized on a 4-hour period as previously
described (19). Cultures in 24-well plates (1% parasitemia, 2% hematocrit) were
subjected to 8-hour pulses of SkE at 8.6 and 86.5 nM. After the pulses, the cultures
were washed three times with culture medium and returned to normal culture
conditions. At the end of the experiment (70 hours, young trophozoite stage of the
next erythrocytic cycle), parasitemia was evaluated in each well by visual examination
by counting Giemsa-stained smears (5,000 erythrocytes/smear). Results are ex-
pressed as the percentage of inhibition of parasitic growth.
(ii) In vitro activity against cultured P.falciparum gametocytes. P.falciparum
gametocytogenesis was induced on day 0 with the W2 strain by the method of
Ifediba and Vanderberg (18) with modifications (3, 31). At day 13 (3 days after
the addition of N-acetylglucosamine), increasing dilutions of SkE were added to
the culture medium. At day 15, a thin smear corresponding to each concentration
was made and Giemsa stained. Visual estimation of the gametocytemia was
carried out by counting at least 10,000 erythrocytes. Negative and positive con-
trols were performed without SkE and with primaquine, respectively.
(iii) Cytotoxicity. Cytotoxicity was evaluated with three cell lines: (i) Vero, a
monkey kidney cell line; (ii) MCF7, a human breast cancer cell line; and (iii) THP1,
a human leukemia cell line. All the cell lines were cultured under the same condi-
tions as used for P.falciparum, except for the 5% human serum, which was replaced
by 10% fetal calf serum (Boehringer). Cell growth was measured by [
3
H]hypoxan-
thine (Amersham-France) incorporation after a 48-hour incubation with serial drug
dilutions. The amount of [
3
H]hypoxanthine incorporated in the presence of drugs
was compared with that of control cultures without the test compounds (30).
(iv) In vivo antimalarial activity. In vivo antimalarial activity of SkE was tested
with the 4-day suppressive test performed on Plasmodium vinckei petteri-infected CD
female mice (8). Mice (mean body weight, 20 ⫾2 g) were infected with 10
6
infected
red blood cells in RPMI on day 0. Groups of five mice were treated intraperitoneally
(i.p.) or orally (p.o.) from days 0 to 3 with increasing doses (0.5 to 20 mg/kg of body
weight) of the drugs. On day 4, Giemsa-stained blood smears (tail blood) were made
for each mouse, and parasitemia was estimated by visual counting of at least 5,000
erythrocytes. The survival time was also monitored until day 21. Chloroquine was
used as the reference drug. The percentage of inhibition was calculated with the
following formula: (control parasitemia ⫺parasitemia with test drug)/(control par-
asitemia) ⫻100. All the procedures involving animals conformed to European
regulations (European Economic Community [EEC] directive 86/609).
Statistical analysis. A survival analysis was performed. Each treatment was
tested by the log rank test for survivor functions, in which the analysis time was
the survival time. Failure was defined as death. The mean survival time reported
was calculated as the area under the Kaplan-Meier survivor function. As some
mice were still alive at the end of the study, the longest analysis time was not
available. If the observation with the longest analysis time was not available, the
survivor function did not go to zero. Consequently, the area under the curve
underestimated the mean survival time. If the longest observed analysis time was
not available, the extended mean reported was calculated as the area under the
extension of the survivor function from the last observed time to zero using an
exponential function. All tests were performed using Stata 9.2 (Intercooled Stata
9.2 for Windows; StataCorp, College Station, TX).
RESULTS
The chloroform extract of a tea made with defatted dry and
mature leaves, which retained biological activity, was depig-
mented and further fractionated by countercurrent chromatogra-
phy (see Scheme S1 in the supplemental material). Seven frac-
tions were obtained (fraction 1 [F1] to F7). Previous purification
with the same protocol but on a smaller quantity allowed us to
identify two active fractions (IC
50
⬍1␮g/ml) with a similar
composition to F1, F2, and F6 (according to their thin-layer chro-
matography profiles). F1, F2, and F6 were therefore further pu-
rified. From F6, we were not able to isolate any pure compounds.
SkD was isolated from F2 (0.0002% yield [i.e., 2 mg from 1 kg
{dry weight} of plant]). More interestingly, a new quassinoid
structurally related to SkD (that we called SkE) was isolated from
F1 and showed a very good activity in vitro on the different P.
falciparum strains (Table 1). However, the yield obtained for this
active compound was very low (0.00035% [i.e., 3.5 mg from 1 kg
{dry weight} of plant]) and precluded any further investigation of
its antiplasmodial properties.
We then set up an improved extraction procedure from the
methanol extract of the mature dry leaves of Q.amara and
increased the yield to 0.004% (40 mg from 1 kg of plant).
Careful analysis of infrared, optical rotation, and mass and
NMR spectra enabled us to establish the structure of SkE.
Crystals were also obtained from deuterated methanol, and
X-ray diffraction confirmed the established structure and its
relative stereochemistry (Fig. 1 and 2).
Other already known quassinoids could be also identified:
quassin (from fraction F3), picrasin H (from F4), picrasin B
(from F5), and picrasin J (from F7). These compounds showed
no significant antiplasmodial activity (D. Stien, S. Bertani, G.
Bourdy, E. Deharo, E. Houe¨l, V. Jullian, A. Valentin, and S.
Chevalley, presented at the ZingConference on Natural Prod-
ucts Chemistry, Antigua, 10 to 13 January 2008).
The antiplasmodial activity of SkE was determined on three
strains of P.falciparum and gave IC
50
s ranging from 24 to 68
nM (Table 1). When tested on highly synchronized parasite
cultures, SkE had a maximal activity beginning at the second
half of the erythrocytic cycle (Fig. 3). This peak decreased at
the 40- to 48-h pulse for the lowest concentration tested (about
1/10 of the IC
50
) (2).
SkE reduced gametocytemia by 50% at a dose sevenfold
TABLE 1. Antiplasmodial activity against asexual (F32, FcB1, and W2 strains) and sexual (W2 Indochina strain) stages and
cytotoxicity of SkE
Drug
a
Antiplasmodial activity (IC
50
)
b
against P.falciparum strain: Cytotoxicity
b
against cell line:
F32
Tanzania
FcB1
Colombia
W2
Indochina
W2 Indochina
(gametocytes) Vero MCF7 THP1
SkE 68 ⫾12 45 ⫾32 24 ⫾10 1,120 ⫾400 6,574 ⫾264 47 ⫾233⫾5
SkD NT 1 NT NT 58 ⫾11 ⬍20
c
⬍2
c
CQ 36 ⫾3 167 ⫾32 196 ⫾16 NT ⬎500
d
⬎500
d
⬎500
d
PMQ 6.17 ⫾1.2 8.9 ⫾1.1 7.14 ⫾0.9 8.9 ⫾2.3 340 ⫾29 NT NT
a
SkE, simalikalactone E; SkD, simalikalactone D; CQ, chloroquine; PMQ, primaquine.
b
Values are expressed in nanomolar (except for PMQ, which is shown in micromolar). Each value corresponds to the mean ⫾standard error of the mean from at
least three independent experiments. NT, not tested.
c
Lower concentration tested.
d
Higher concentration tested.
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lower than primaquine, a reference molecule for the elimina-
tion of gametocytes (Table 1) (33).
The cytotoxicity of SkE varied, with an IC
50
from 6 ␮M
(Vero cells) to 33 nM (THP1 cells) (Table 1).
SkE was then orally and intraperitoneally administered to P.
vinckei petteri-infected mice. The i.p. route (50% effective dose
[ED
50
], 0.5 mg/kg/day) was almost twice as effective as the p.o.
route (ED
50
, 1 mg/kg/day), the control being chloroquine given
i.p. (ED
50
, 3 mg/kg/day) (Table 2). The survival of mice was
monitored for 3 weeks, and the mean survival time of the mice
was evaluated (Table 3). The highest values were obtained with
mice treated with SkE i.p. at 1 mg/kg/day (18.6 days; extended
mean, 93.89 days) and with CQ at 10 mg/kg/day (19.20 days;
extended mean, 43.87 days). Lower survival means were ob-
tained with SkE given i.p. at a higher dose or by oral route.
However, all the treated mice had a significantly higher sur-
vival time than that of control mice (P⬍0.05 for CQ given i.p.
[1 mg/kg/day] and P⬍0.01 for the other treatments). The
Kaplan-Meier curves (Fig. 4) showed similar aspects for SkE
given 0.5 and 1 mg/kg/day i.p. and for CQ given 10 mg/kg/day i.p.
DISCUSSION
Biodiversity is clearly a source of new drugs (26). When
biodiversity analysis combines with traditional treatments,
there is the hope of finding some promising candidate mole-
cules for pharmaceutical development. Here, we describe a
new quassinoid, obtained after bioguided fractionation of a
widely used Amazonian traditional remedy for malaria (35).
This quassinoid, named simalikalactone E, exhibited very good
antiplasmodial activity against the three P.falciparum strains
tested, whatever their geographic origin or chloroquine sensitiv-
ity. The IC
50
s obtained were in the range of most commercially
available antimalarial drugs tested under similar conditions (16)
FIG. 1. Chemical structures of simalikalactone E (compound 1)
and simalikalactone D (compound 2).
FIG. 2. Structure of SkE obtained from X-ray diffraction experiment.
FIG. 3. Impact of SkE on the eythrocytic cycle of P.falciparum.A
parasite culture synchronized on a 6-h period was subjected to 8-h pulses
of SkE at the IC
50
(gray bars) and 1/10 the value (white bars). After the
pulse, the culture was washed and returned to normal culture conditions
until the beginning of the second erythrocytic cycle, and then parasitemia
was determined. The scale bar at the top of the figure shows time (in
hours). Major events along the eythrocytic cycle are shown. The dotted
line shows protein synthesis, and the solid black line shows DNA synthe-
sis. This figure was adapted from the Annals of Tropical Medicine and
Parasitology (2) with the permission of the publisher.
TABLE 2. Inhibition of parasitemia at day 5
a
Treatment
and route
b
% Inhibition of
parasitemia
DMSO........................................................................................... 0
CMC.............................................................................................. 0
CQ i.p.
1................................................................................................. 0
5.................................................................................................100
10...............................................................................................100
SkE
i.p.
0.5.......................................................................................... 59
1............................................................................................. 98
5.............................................................................................100
p.o.
1............................................................................................. 55
10........................................................................................... 95
20...........................................................................................100
a
The control mice treated with dimethyl sulfoxide alone showed 100% para-
sitemia.
b
Treatment and routes were as follows: chloroquine (CQ) (1, 5, or 10 mg/kg/day for
4 days); simalikalactone E (SkE) (0.5, 1, 5, 10 or 20 mg/kg/day); p.o., oral route; i.p.,
intraperitoneal route. DMSO, dimethyl sulfoxide, control for i.p. route; CMC, carboxy-
methylcellulose, control for p.o. route.
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and varied from 24 to 68 nM. These data clearly indicate that SkE
does not interfere with CQ resistance pathways.
Against P.falciparum gametocytes, a stage which is funda-
mental for transmission to mosquitoes, SkE was more active
(IC
50
, 1.2 ␮M) than the reference compound primaquine
(IC
50
, 8.9 ␮M). In a previous study, Benoit-Vical et al. (3)
showed that an artemisinin derivative, artesunate, inhibited
gametocyte growth at a 10-fold-lower dose (around 0.1 ␮M).
Regarding cytotoxicity, the SkE IC
50
s were similar to those
obtained against P.falciparum when using cancer-derived cell
lines (MCF7 and THP1). However, when tested on Vero cells
(which are of primate origin and are not cancer-derived cells)
SkE displayed an IC
50
around 100 times higher than that ob-
served on the asexual stages of the parasite, and is less toxic
than SkD (but more toxic than CQ). This particularly good
selectivity index prompted us to perform in vivo experiments.
In vivo, at day 5, the lowest tested doses (0.5 mg/kg/day i.p.
and 1 mg/kg/day p.o.) were close to the ED
50
. Higher doses led
to a complete cure of the mice at day 5 (Table 2). However,
when the survival of mice was evaluated over a longer period,
clear differences appeared between the i.p. and p.o. routes.
The analysis of survival times showed that the i.p. route, at
doses in the same range, seemed to be more efficient than the
p.o. route as illustrated by the Kaplan-Meier curves (Fig. 4). It
is to be noted that SkE administered by the i.p. route was
considerably more active than CQ (about 10-fold more effi-
cient when looking at SkE [1 mg/kg/day i.p.] and CQ [10
mg/kg/day i.p.], Table 3 and Fig. 4). On the other hand, at the
higher i.p. dose (5 mg/kg/day), the survival time was shorter
than for the other drugs, which could be explained by toxicity.
The statistical analysis of the survival times (Table 3) showed
better activity of SkE compared with the control, and at the
doses tested, a better activity than that of CQ at 1 mg/kg/day,
except for the highest dose (5 mg/kg/day, i.p. route) where a
toxic effect was probably emerging. It is also to be noted that
there were no significant differences between the SkE-treated
mice given 1 and 0.5 mg/kg/day (i.p.) and the CQ-treated mice
given 10 mg/kg/day (i.p.). Taken together, these data demon-
strated that SkE showed good antimalarial activity in mice, with
the best dose for a complete cure being around 1 mg/kg/day.
This activity was in the same range as in vivo activities
previously reported for other quassinoids. When administered
by the i.p. route, sergeolide (13), glaucarubinone (23), cedro-
nine (24), and bruceolide and its carbonate derivatives (25)
had ED
50
s between 0.2 mg/kg/day and 1.8 mg/kg/day. We also
showed that SkD inhibits 50% of Plasmodium yoelii yoelii ro-
dent malaria parasite growth at 3.7 mg/kg/day in vivo by the
oral route (6).
Toxicity was also monitored, and at the higher dose of SkE
(5 mg/kg/day given i.p., 10 times the IC
50
), an immediate tox-
icity close to the LD
50
was observed (three deaths out of five
mice in 3 days), while at lower doses, the antimalarial activity
was clear.
The study of the activity of SkE on the different stages of the
P.falciparum life cycle showed that SkE had a better inhibitory
effect on stages where DNA synthesis occurred, but our results
do not enable a distinction to be made between an SkE-DNA
interaction and an inhibition of proteins implicated in DNA
synthesis. DNA and protein synthesis inhibition in P.falcipa-
rum has been reported for several quassinoids. In general, the
inhibition of DNA synthesis was less pronounced and seemed
to be a consequence of protein synthesis inhibition (15, 21, 28).
Recent reinvestigation of the antineoplastic activity of various
quassinoids showed that NF-␬B activation (9) and downregu-
lation of c-myc (10) were implicated in the cell differentiation
and apoptosis induced by quassinoids. Mitochondrial mem-
brane depolarization and caspase 3 activation also played a
role in this process (29). It has also been shown that 6␣-
tigloyloxychaparrinone was an inhibitor of hypoxia-inducible
factor 1 (20). Ailanthinone, glaucarubinone, and 6␣-senecio-
TABLE 3. Mean survival time of the treated mice and statistical significance
Treatment
and route
a
No. of
mice
Mean survival time (days)
关95% CI兴
b
Extended mean survival
time (days)
Statistical significance (Pvalue)
compared to the following group:
Control CQ
(i.p., 1 mg/kg/day)
Control 15 4.33 关3.76–4.91兴
CQ 5 7.40 关6.70–8.10兴0.013
i.p.
1
5 5 12.80 关11.51–14.09兴⬍0.001 0.026
10 5 19.20 关16.82–21.58兴
c
43.87 ⬍0.001 0.002
SkE
i.p.
0.5 5 16.40 关11.45–21.35兴
c
41.07 ⬍0.001 0.005
1 5 18.60 关14.39–22.81兴
c
93.89 ⬍0.001 0.003
5 5 11.40 关4.51–18.29兴
c
20.57 ⬍0.001 0.047
p.o.
1 5 11.00 关6.46–15.54兴
c
13.61 ⬍0.001 NS
d
10 5 12.40 关8.42–16.38兴
c
15.01 ⬍0.001 0.041
20 5 13.80 关9.32–18.28兴
c
16.41 ⬍0.001 0.014
a
Treatment and routes were as follows: chloroquine (CQ) (1, 5, or 10 mg/kg/day for 4 days); simalikalactone E (SkE) (0.5, 1, 5, 10 or 20 mg/kg/day); p.o., oral route;
i.p., intraperitoneal route.
b
95% CI, 95% confidence interval.
c
As some mice were still alive at the end of the study, the largest observed analysis time was not available, and therefore, the mean is underestimated.
d
NS, not significant.
4396 CACHET ET AL. ANTIMICROB.AGENTS CHEMOTHER.
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nylchaparrin were also identified as inhibitors of the transcrip-
tion factor AP-1, but this function did not correlate with cyto-
toxicity or protein synthesis inhibition (7). Those new findings
suggest that the antiplasmodial mechanism of action of quassi-
noids merits further detailed investigation.
The structural requirements for the antimalarial activity of
quassinoids are well-documented, and both SkD and SkE meet
them: they have an ␣,␤-unsaturated lactone on ring A and an
oxymethylene bridge between C-8 and C-11 or between C-8
and C-13. The isolation of SkE allowed us to compare the
antiplasmodial potencies of SkE and SkD and thus evaluate
the effect of the carboxylate group in the C-6 position. C-6
substitution occurs sometimes in quassinoids. In a review de-
scribing 230 quassinoids, 10% were shown to be substituted on
C-6, and hydroxy or carboxy groups were the only substituents
reported (11). For one of these quassinoids, 15-desacetylun-
dulatone, isolated from Quassia undulata (1) and Hannoa chlo-
rantha (14), a good antiplasmodial activity was reported. How-
ever, there is no clear evidence in the literature of the influence
of a substituent on the C-6 position on antiplasmodial activity.
We have shown here that this activity in vitro is lower for SkE
than for SkD, but the selectivity index when using Vero cells is
better for SkE (the selectivity index of SkE is 111, while the
selectivity index for SkD is 58). The C-6 carboxylation could
contribute to lowering the cytotoxicity of the quassinoid.
The present report demonstrated that despite their reputa-
tion as toxic molecules, quassinoids remain potentially inter-
esting as antimalarials, and further research should be done on
rationalizing the effect of the C-6 substitution to improve their
efficacy as drugs and lower their toxicity. Because quassinoids
are the active ingredients of many traditional antimalarial
preparations all over the world, this type of research would be
of great interest for people living in places where malaria is
endemic and relying on these preparations.
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