© Copyright 2002 by the American Chemical Society and the American Society of Pharmacognosy
Volume 65, Number 10
October 2002
Full Papers
Antiplasmodial Activity of Alkaloids from Various Strychnos Species
Michel Frédérich,*,†,‡ Marie-José Jacquier,§ Philippe Thépenier,§ Patrick De Mol,‡ Monique Tits,†
Geneviève Philippe,† Clément Delaude,† Luc Angenot,† and Monique Zèches-Hanrot§
University of Liège, Natural and Synthetic Drugs Research Center, Laboratory of Pharmacognosy, Avenue de l’Hôpital 1,
B36, B-4000 Liège, Belgium, University of Liège, Laboratory of Medical Microbiology, Liège, Belgium, Laboratoire de
Pharmacognosie, UMR 6013 CNRS, Bâtiment 18, Moulin de la Housse, 51687 Reims Cedex 2, France
Received February 28, 2002
The in vitro antiplasmodial activities of 69 alkaloids from various Strychnos species were evaluated against
chloroquine-resistant and chloroquine-sensitive lines of Plasmodium falciparum. The compounds,
comprising mainly indolomonoterpenoid alkaloids, exhibited a wide range of biological potencies in the
antiplasmodial assays. The most active alkaloids were also tested for cytotoxicity against HCT-116 colon
cancer cells to determine their antiplasmodial selectivity. As a result of these studies, structure-activity
relationships for these alkaloids have begun to emerge. Alkaloids presenting four types of bisindole skeleton
exhibited potent and selective activities against Plasmodium. They were sungucine-type (IC50 values
ranging from 80 nM to 10 µM), longicaudatine-type (IC50 values ranging from 0.5 to 10 µM), matopensinetype (IC50 values ranging from 150 nM to 10 µM), and usambarine-type alkaloids. Within the last structural
type, isostrychnopentamine (49) and ochrolifuanine A (46) were found to be active against chloroquinesensitive and -resistant strains (IC50 values of 100-150 and 100-500 nM, respectively), and dihydrousambarensine (51) exhibited a 30-fold higher activity against the chloroquine-resistant strain (IC50 ) 32 nM)
than against the chloroquine-sensitive one.
Malaria is the major parasitic infection in many tropical
and subtropical regions, leading to more than one million
deaths (principally among young African children) out of
400 million cases each year.1 More than half of the world’s
population lives in areas where they remain at risk of
malaria infection. During recent years, the situation has
worsened in many ways, mainly due to malarial parasites
becoming increasingly resistant to several antimalarial
drugs.2 This resistance concerns numerous drugs, but is
thought to be most serious with chloroquine, the cheapest
and most widely used drug to treat malaria. Furthermore,
the control of malaria is becoming more complicated by the
parallel spread of resistance of the mosquito vector to
* To whom correspondence should be addressed. Tel: + 32 4 366 43 38.
Fax: + 32 4 366 43 32. E-mail: M.Frederich@ulg.ac.be.
†
Laboratory of Pharmacognosy, University of Liège.
‡
Laboratory of Medical Microbiology, University of Liège.
§ Laboratoire de Pharmacognosie, Reims.
10.1021/np020070e CCC: $22.00
currently available insecticides. Urgent efforts are therefore
necessary to identify new classes of antimalarial drugs.3
The in vitro antiplasmodial, antiamebic, and cytotoxic
activities of several indole alkaloids, particularly those
isolated from various Strychnos species, have been previously investigated.4-9 To complement these studies it was
considered to be of interest to test the antiplasmodial
activity of other Strychnos alkaloids available in our
laboratories. In total, 69 alkaloids with various structures
have been tested against three lines of P. falciparum, thus
allowing us to determine preliminary structure-activity
relationships.
Results and Discussion
Among a total of 69 alkaloids examined, compounds 1-4
were of the monoterpenoid type, compounds 5-45 were
monoindole alkaloids, and compounds 46-69 were bisindole alkaloids. Compounds exhibiting IC50 values higher
than 10 µM, between 1 and 10 µM, and lower than 1µM
© 2002 American Chemical Society and American Society of Pharmacognosy
Published on Web 08/16/2002
1382 Journal of Natural Products, 2002, Vol. 65, No. 10
Frédérich et al.
Figure 1. Structures of alkaloids 29-31 and 41-56.
were respectively considered as weak, moderately active,
and potent antiplasmodial agents. Compounds 1-40 were
inactive or showed less than 30% growth inhibition at a
concentration of 50 µM (Supporting Information). The
results obtained for compounds 5-40 were largely confirmatory of previous studies where other monoindole alkaloids (akagerine, retuline, strychnine, icajine, vomicine,
novacine derivatives, diaboline, holstiine) were shown to
be devoid of any antiplasmodial activity.7-9 However, five
monoindole alkaloids evaluated in the present study showed
a weak activity, with antiplasmodial IC50 values of 10-30
µM: ngouniensine (41), epingouniensine (42), akagerine
lactone (43), demethoxycarbonyl 3,14-dihydrogambirtanine
(44), and 9-methoxy 16(R)-E-isositsirikine (45) (Table 1).
Since the compounds possess highly varied structures, no
structure-activity relationships could be deduced from
these data. The most active of the monomeric compounds
was 45, with an IC50 value of approximately 10 µM against
all P. falciparum strains tested. This compound contains
an additional methoxy substituent on the aromatic ring,
compared to its inactive analogues 29-31.
All the other active alkaloids were bisindole derivatives
(Table 1). Twelve compounds (48, 53, 54, 57, 59, 63, 64,
65, 66, 67, 68, 69) exhibited moderate to weak activity, with
antiplasmodial IC50 values of 2-20 µM in at least one of
the Plasmodium lines. The remaining 12 compounds (46,
47, 49, 50, 51, 52, 55, 56, 58, 60, 61, 62) showed IC50 values
of <2 µM against all Plasmodium lines tested. Of these,
the two compounds isostrychnopentamine A (49) and
ochrolifuanine A (46) demonstrated very potent activity of
Alkaloids from Strychnos Species
Journal of Natural Products, 2002, Vol. 65, No. 10 1383
Figure 2. Structures of alkaloids 57-69.
<500 nM against all lines tested, and one compound,
dihydrousambarensine (51), was selectively highly active
(39 nM) against the chloroquine-resistant strain W2.
Among these 24 bisindole compounds it was possible to
distinguish four principal structural classes: usambarinetype or quasi-dimeric-type (i.e., made of two tryptamine
units with a single iridoid unit) (46-51), matopensine-type
(52-55), longicaudatine-type (56-58), and sungucine-type
(59-62) alkaloids.
Among alkaloids of the usambarine-type (46-51), most
of the compounds evaluated exhibited potent antiplasmodial activity (IC50 below 1 µM). The most interesting
compound was isostrychnopentamine (49), which possessed
an IC50 value of about 100 nM against all plasmodial lines
tested. The absence of the hydroxyl substituent at C-11 and
of the pyrrolidine ring at C-12 in its analogue usambarine
(50) led to a 20-fold reduction in activity. As previously
reported,7 the 11-hydroxy derivative of usambarine was
also 3 times more active than usambarine. Ochrolifuanine
A (46) also exhibited potent activity against the three
strains tested (100-500 nM). This compound was also of
interest, as its total synthesis has been recently described.10
Ochrolifuanine E (47), exhibiting the H-3β and H-3’R
configuration, was clearly less potent than ochrolifuanine
A (46) (H-3’β, H-3R and an ethyl side chain). Furthermore,
within this group, two compounds (50, 51), presenting a
fully or partially aromatized lower portion of the molecule,
were clearly more potent against the chloroquine-resistant
strain, particularly dihydrousambarensine (51), which
showed a 30-fold lower IC50 value for the chloroquine-
1384 Journal of Natural Products, 2002, Vol. 65, No. 10
Frédérich et al.
Table 1. In Vitro Activities of Some Strychnos Alkaloids Against Three Lines of P. falciparum
FCA 20/ Ghana
(chloroquine-sensitive line)
compound
monoindole alkaloids
ngouniensine (41)
epingouniensine (42)
akagerine lactone (43)
demethoxycarbonyl 3,14dihydrogambirtannine (44)
9-methoxy 16(R) E-isositsirikine (45)
usambarine-type bisindole
alkaloids
ochrolifuanine A (46)
ochrolifuanine E (47)
usambarine (48)
isostrychnopentamine (49)
usambarensine (50)
dihydrousambarensine
(51)
matopensine-type
bisindole alkaloids
matopensine (52)
matopensine-N-oxide (53)
18-hydroxymatopensine
(54)
16-methoxyisomatopensine
(55)
longicaudatine-type
bisindole alkaloids
longicaudatine (56)
longicaudatine F (57)
tetradehydrolongicaudatine
Y (58)
sungucine-type bisindole
alkaloids
sungucine (59)
isosungucine (60)
18-hydroxyisosungucine
(61)
strychnogucine B (62)
various bisindole alkaloids
strychnofuranine (63)
janussine A (64)
janussine B (65)
S-panganensine (66)
panganensine Y (67)
panganensine X (68)
16,17-dehydroisostrychnobiline (69)
reference compounds
chloroquine (70)
quinine (71)
artemisinin (72)
IC50 µM (SDa
IC90 µM
FCB1-R/Colombia
(moderately chloroquine-resistant line)
nb
IC50 µM (SDa
IC90 µM
W2/Indochina
(chloroquine-resistant line)
nb
IC50 µM (SDa
IC90 µM nb
26.8
7.8
17.0
11.3
71.0
26.9
60.7
42.2
1
1
1
1
17.8
16.3
27.5
12.9
62.8
49.7
55.2
47.4
2
1
1
2
20.2
15.8
25.9
18.3
70.1
52.8
53.3
89.5
1
1
1
1
13.6 ( 5.9
37.8
3
12.5
36.3
2
9.9
34.6
1
3
1
1
2
6
5
0.266 ( 0.185
0.702 ( 0.431
ND
0.104 ( 0.036
ND
ND
1.02
2.42
3
6
0.386
3
0.492 ( 0.145
1.90
2.36c
0.152c
0.594 ( 0.052c
0.032 ( 0.002c
4.11
24.8
11.8
3
2
4
0.243
8.66
0.121
9.52
3
3.64
33.4
2
2
0.118 ( 0.024
0.276
2.50c
0.120c
1.52 ( 0.031c
0.857 ( 0.061c
0.495
1.49
9.21
0.450
4.41
2.49
1.25
13.2
5.11
6.63
73.3
12.3
1
1
2
0.362 ( 0.035
4.67
2.43 ( 0.37
0.54
2.53
2
1.54 ( 0.21
0.986
7.70
1.236c
3.87
26.7
7.970
2
1
1
0.560
11.7
ND
7.82 ( 1.14d
1.32 ( 0.25d
0.847 ( 0.141d
26.3
7.06
4.35
3
3
4
ND
ND
0.207 ( 0.126
1.47
3
10.1 d
0.265d
0.140d
3.79
2
0.529 ( 0.038
2.24
3
0.085d
33.5
28.3
33.1
12.7
43.7
26.1
7.13
2
2
2
2
2
2
2
0.617d
4.11
9.55
12.6
9.77
10.4
14.9
4.58
0.011 ( 0.005
0.269 ( 0.006
ND
15.0
30.9
34.9
17.7
68.2
34.8
18.7
0.071
1.91
2
2
1
1
1
1
6
3
10.8
15.9
17.0
5.27
13.5
12.9
2.12
0.032 ( 0.019
0.200 ( 0.033
0.005
0.084
2.74
0.013
3
4
2
1.38
6.76
7.29
0.628
4.83
4.68
3
1
2
2
3
4
7.74
86.7
2.26
1
1
2
0.122 ( 0.076
2.84
4
0.569 ( 0.228
9.58
0.958c
4.83
28.1
12.087
3
2
2
33.2
1.72
1.35
2
2
2
2.80
12.7
4.75
> 20
17.4
17.0
3.31
0.284 ( 0.017
0.413 ( 0.011
0.002
0.358
16.0
27.6
27.8
78.7
85.8
18.5
1.75
1.72
0.017
2
2
2
2
1
1
1
1
5
3
1
a Values are expressed as mean ( standard deviation (for n > 2). ND ) not determined. b n ) number of independent experiments.
All tests were realized in duplicate. c Data from ref 7. d Data from ref 5.
resistant line W2 (32 nM). Its IC90 value, however, was
similar to that obtained with the chloroquine-sensitive line,
FCA20.
For alkaloids of the matopensine-type (52-55), the
N-oxide derivative (53) was clearly less potent than the
other compounds in this class. This may be attributed to
the ionic nature of the N-oxide, since quaternary alkaloids
have consistently been inactive in our assays (data not
published). The three alkaloids 52, 54, and 55 were slightly
more potent against the chloroquine-resistant strain, but
no clear structure-activity relationships could be deduced
for these three compounds.
For alkaloids of the longicaudatine-type, only three
compounds were available (56-58). The most interesting
was longicaudatine itself (56), which exhibited an IC50 of
0.5-1 µM. Longicaudatine F (57), whose structure differs
from that of 56 only by the opening of ring G in the
strychnine part of the molecule, was less active. Compound
58, which possessed a β-carboline structure in the lower
part of the dimer (as in 50), demonstrated enhanced
activity against the chloroquine-resistant strain. It would
be interesting to test additional analogues of longicaudatine.
Among alkaloids of the sungucine-type, compounds 6062 possessed potent activities against the different lines
of Plasmodium falciparum. Isosungucine (60), a compound
possessing a C-16′/C-17′ double bond in the lower part of
the dimer, was more potent than its isomer sungucine (59).
The presence, in 18-hydroxyisosungucine (61), of an additional hydroxyl substituent on the 18-19 ethylidene side
chain resulted in a small increase in activity. Finally,
cyclization of ring G in the lower portion of the molecule
Alkaloids from Strychnos Species
Journal of Natural Products, 2002, Vol. 65, No. 10 1385
Table 2. Cytotoxic Activity Against the Human Colon Cancer
Cell Line HCT-116 and Antiprotozoal Selectivity Indexa of the
More Potent Antiplasmodial Alkaloids
compound
ochrolifuanine A (46)
ochrolifuanine E (47)
isostrychnopentamine (49)
dihydrousambarensinec
(51)
matopensine (52)
18-hydroxymatopensine
(54)
16-methoxyisomatopensine
(55)
longicaudatine (56)
sungucinec (59)
18-hydroxyisosungucinec
(61)
strychnogucine Bc (62)
chloroquine (70)
IC50,
µMb
FCA
SI
FCB1
SI
16.1
5.70
7.47
12.0
136
20.7
62.3
14
60.7
32.8
8.12
3.0
71.8
49.1
ND
375
12.0
42.5
9.59
8.31
24.5
45.3
4.93
6.2
16.2
4.59
0.79
19.1
15
33.7
24.3
3063
W2
SI
33.1
17.5
49.5
351
15.9
200
8.81
7.96
ND
0.611
78.3
115
28.3
1053
176.4
118.6
a Selectivity index (SI) is defined as the ratio of 50% cytotoxicity
over 50% antiplasmodial activity. b Number of independent experiments ) 2. Each experiment was realized in triplicate. c Data on
KB cells from refs 5 and 7.
and the presence of a hydroxyl substituent on the C-18 in
the upper portion led to strychnogucine B (62), the most
potent derivative in this group of alkaloids. In contrast,
strychnogucine A, presenting a C-16′/C-17′ double bond,
with a G cyclization on the upper part of the dimer, was
more active than sungucine but markedly less active than
60-62 (IC50 near 3 µM for all strains).4 Thus, strychnogucine B (62) and 18-hydroxyisosungucine (61) were found
to be the most active compounds in this series, with 12120 times higher activity than sungucine (59) against the
three plasmodial lines. Furthermore, the activities were
several-fold higher against the chloroquine-resistant lines
W2 and FCB1-R than against the chloroquine-sensitive
strain FCA of Plasmodium falciparum.
To further assess the clinical potential of these antiplasmodial bisindolomonoterpene alkaloids, the most active
compounds were evaluated for cytotoxicity with a human
colon cancer cell line (HCT-116), to distinguish general
cellular toxicity from specific antiplasmodial activity (Table
2). Many cytotoxic compounds also possess antiplasmodial
properties under conditions of in vitro testing. Hence, the
selectivity index (SI) of a compound was used, defined as
IC50(in HCT-116 cells)/IC50(in P. falciparum). This selectivity index is an attempt to estimate the potential of tested
compounds to inhibit the growth of the intracellular
malaria parasite without cellular toxicity. The HCT-116
cell line was selected because it demonstrated no resistance
to current anticancer drugs.11 In addition, this cell line was
easily cultured and exhibited comparable sensitivity to
noncancer cell lines (such as WI38 fibroblasts).
Most compounds evaluated (Table 2) exhibited a 5- to
400-fold higher activity against Plasmodium than against
the human cancer cells used, thus indicating some antiplasmodial selectivity (except 59). The most generally
selective compounds were isostrychnopentamine (49) and
ochrolifuanine A (46), which showed selectivity indices of
50-70 and 30-140 times, respectively. Dihydrousambarensine (51), 18-hydroxymatopensine (54), and 16-methoxyisomatopensine (55) yielded a favorable SI only for the
W2 strain (SI ) 400, 350, and 200, respectively). Even
though sungucine (59) exhibited no selectivity against
Plasmodium, 18-hydroxysungucine (61) and strychnogucine B (62) exhibited 20-180-fold higher activity against
the P. falciparum lines than against the human cancer cell
line, thus indicating a satisfactorily high selectivity.
The results of the present study confirmed the previously
reported antiplasmodial activities of Strychnos bisindole
alkaloids8 and more particularly of four alkaloid subtypes.
All active compounds were tertiary dimers. A certain
degree of basicity seems to be necessary for the antiplasmodial activity of this family of compounds. For example,
usambarine (48) (possessing a methyl substituent at N4′)
was less active than the ochrolifuanines (46 and 47) (N4′-H
being more basic) or isostrychnopentamine (49) (which
possesses a third basic nitrogen in the pyrrolidine ring).
Also, N-oxide or quaternary derivatives were also less
active than the respective tertiary compounds. These
observations were consistent with the ability of basic
compounds to accumulate to higher levels in the acidic food
vacuole of the parasite, as has been hypothesized for
chloroquine. Chloroquine is thought to be selectively accumulated (to at least a 1000-fold level) in the parasite food
vacuole, where digestion of hemoglobin takes place. The
weakly basic properties of chloroquine explain its accumulation in the food vacuole: at neutral pH, chloroquine
has the ability to diffuse freely through membranes, but
at the acidic pH of the food vacuole, the compound is
protonated and is, therefore, sequestered.12,13
In the case of the sungucine-type alkaloids, it was
apparent that the antiplasmodial selectivity was clearly
associated with the presence of a 16′-17′ double bond or a
17′-18′ ether link. Isostrychnopentamine (49) exhibited
potent and selective activities against all three lines of
Plasmodium used in this study. The activity of this
compound seemed to be directly linked to the presence of
the basic methylpyrrolidine substituent. It would be useful
to test semisynthetic and closely related compounds possessing these kinds of structures.
Dihydrousambarensine (51), has been previously tested
in vivo against Plasmodium berghei but was inactive at a
dose of 30 mg/kg/day.8 Nevertheless, this compound was
essentially active against chloroquine-resistant strains of
P. falciparum and the P. berghei strain used in this study
was chloroquine-sensitive. This could explain its inactivity
in vivo, in addition to the differences between the biology
of the two species. It is now desirable to examine if other
lead alkaloids are able to inhibit Plasmodium growth in
animal models and if they possess original modes of action
(in particular, if they act or not on the heme polymerization
process).
Experimental Section
Chemicals. All alkaloids tested (1-69) were isolated and
purified in our laboratories from various Strychnos species as
previously described. Compounds 1, 3, and 29 were isolated
from Strychnos pungens Solered.;14,15 2, 26, and 31 from S.
angolensis Gilg;16 4, 11, 20, 56, and 57 from S. longicaudata
Gilg;17-19 5, 6, 12, 18, 19, 41, and 42 from S. ngouniensis
Pellegr.;18,20,21 7, 8, 27, and 28 from S. potatorum L.;22,23 9, 10,
53, and 69 from S. kasengaensis De Wild;24 13 from S. xantha
Leeuwenberg;25 14, 15, 16, 17, 21, 22, 43, 44, 64, and 65 from
S. johnsonii Hutch. et M.B. Moss;26,27 23, 24, 25, 32, 33, 34,
and 37 from S. henningsii Gilg;28 30, 52, 54, 55, and 63 from
S. matopensis S. Moore;29,30 39 from S. staudtii Gilg;31 45 from
S. lucens Bak.;32 46 and 47 from S. potatorum L.;22 48, 49, 50,
51, and 58 from S. usambarensis Gilg;33-35 59, 60, 61, and 62
from Strychnos icaja Baill.; 4,5 and 66, 67, and 68 from S.
panganensis Gilg.36 The purity (>95%) of the compounds has
been determined by TLC and spectroscopic (UV, IR, MS, NMR)
comparison with authentic samples. Chloroquine diphosphate
(Sigma, Bornem, Belgium), artemisinin (Sigma, Bornem,
1386 Journal of Natural Products, 2002, Vol. 65, No. 10
Belgium), and quinine base (Aldrich 14590-4) were used as
antimalarial references. [3H]Hypoxanthine was from NEN Life
Science Products (Zaventem, Belgium).
Plasmodium falciparum Lines. Three P. falciparum
lines were used in this study: the W-2 chloroquine-resistant
line from Indochina, the FCB1-R line from Columbia, and the
chloroquine-sensitive FCA 20 from Ghana. These lines were
provided by Prof. P. Grellier (Laboratoire de Biologie Parasitaire et Chimiothérapie, Muséum d’Histoire Naturelle, Paris),
Prof. J. Le Bras (Hôpital Bichat-Claude Bernard, Laboratoire
de Parasitologie, Centre National de Référence de la Chimiosensibilité du Paludisme, Paris), and Prof M. Wéry (Tropical
Medicine Institute, Antwerp, Belgium), respectively. The
FCB1-R line was described as moderately chloroquineresistant, but IC50 values near 30 nM were obtained (the
chloroquine resistance threshold was set at 100 nM).
In Vitro Antiplasmodial Testing. Continuous in vitro
cultures of asexual erythrocytic stages of the three P. falciparum strains were maintained following the procedure of
Trager and Jensen37 and as described previously.5 Each test
sample was applied in a series of eight 4-fold dilutions (final
concentrations ranging from 20 to 0.0012 µg/mL) and was
tested in duplicate. Parasite growth was estimated by the
determination of [3H]hypoxanthine incorporation as described
by Desjardins et al.38 and modified by Mirovsky et al.39 The
Student t-test was used to test the significance of differences
between results obtained for different samples. Statistical
significance was set at p e 0.05.
Evaluation of Cytotoxic Potential. The HCT-116 colon
cancer cell line was cultured as described previously.40 Compounds were tested in 96-well microplates using the tetrazolium salt WST-1 (Boehringer) colorimetric assay based on the
cleavage of the reagent by mitochondrial succinate-tetrazolium
reductase in living cells. Altogether, 5000 cells were seeded
per well in 200 µL of medium supplemented with adequate
concentrations of tested drugs. Corresponding controls with
analogous concentrations of DMSO were included in parallel.
At least two experiments were performed per cell line. After
a 72 h incubation, 20 µL of WST-1 was added to the well. After
30 min at 37 °C, plates were shaken and absorbance values
were recorded as described in the commercial assay, against
a background control as blank (medium plus WST-1 without
cells). Relative absorbance values were expressed as percent
of the respective controls (100% in ordinate). Means ( SEM
were calculated. IC50 values were calculated from graphs.
Acknowledgment. The authors wish to thank Prof. J.
Boniver (Anatomie et Cytologie Pathologique, Université de
Liège) for liquid scintillation measurements, V. Bours and M.
P. Merville (CTCM, ULg) for HCT-116 cell lines and access to
their laboratory, and M. Bentires (CTCM, ULg) for his
technical assistance. This research was supported by the
Belgian National Fund for Scientific Research (FNRS) (Grant
No. 3453201 and fellowship for M.F.).
Supporting Information Available: Table of alkaloids exhibiting
less than 30% growth inhibition at a concentration of 50 µM against
P. falciparum FCA-20 Ghana line. This material is available free of
charge via the Internet at http://pubs.acs.org.
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