Common dietary flavonoids inhibit the growth of the intraerythrocytic malaria parasite
© Lehane and Saliba; licensee BioMed Central Ltd. 2008
Received: 28 March 2008
Accepted: 18 June 2008
Published: 18 June 2008
Flavonoids are abundant plant phenolic compounds. More than 6000 have been identified to date, and some have been shown to possess antiparasitic activity. Here we investigate the effects of a range of common dietary flavonoids on the growth of two strains of the human malaria parasite Plasmodium falciparum.
A chloroquine-sensitive (3D7) and a chloroquine-resistant (7G8) strain of P. falciparum were tested for in vitro susceptibility to a range of individual dietary flavonoids and flavonoid combinations. Parasite susceptibility was measured in 96-well plates over 96 h using a previously described [3H]hypoxanthine incorporation assay. Of the eleven flavonoids tested, eight showed antiplasmodial activity against the 3D7 strain (with IC50 values between 11 and 66 μM), and all showed activity against the 7G8 strain (with IC50 values between 12 and 76 μM). The most active compound against both strains was luteolin, with IC50 values of 11 ± 1 μM and 12 ± 1 μM for 3D7 and 7G8, respectively. Luteolin was found to prevent the progression of parasite growth beyond the young trophozoite stage, and did not affect parasite susceptibility to the antimalarial drugs chloroquine or artemisinin. Combining low concentrations of flavonoids was found to produce an apparent additive antiplasmodial effect.
Certain common dietary flavonoids inhibit the intraerythrocytic growth of the 3D7 and 7G8 strains of P. falciparum. Flavonoid combinations warrant further investigation as antiplasmodial agents.
Flavonoid IC50 values for growth inhibition of P. falciparum
Major food sources 
In vitroantiplasmodial activity (IC50, μM)a
LogD (pH 7.0)b
Onions, kale, broccoli, apples, cherries, fennel, sorrel, berries, tea
3, 5, 7, 4' – OH
33 ± 7
25 ± 2
3, 5, 7, 3', 4', 5' – OH
40 ± 10
76 ± 23
3, 5, 7, 3', 4' – OH
15 ± 5
14 ± 1
3-gluc, 5, 7, 3', 4' – OH
66 ± 10
66 ± 18
Parsley, thyme, celery, sweet red pepper
5, 7 – OH 4' – OCH3
13 ± 2
5, 7, 4' – OH
20 ± 3
13 ± 2
5, 6, 7 – OH
32 ± 1
21 ± 6
5, 7 – OH
18 ± 3
22 ± 4
5, 7, 3', 4' – OH
11 ± 1
12 ± 1
Citrus fruits, prunes
5, 7, 4' – OH, single bond between 2 and 3
71 ± 10
5, 7, 4' – OH, B ring attached to 3
29 ± 5
0.006 ± 0.0003
0.084 ± 0.026
A number of reports have demonstrated growth inhibitory effects of flavonoids, particularly of the flavonol quercetin and of the flavone luteolin, on the protozoan parasite genera Toxoplasma , Trypanosoma  and Leishmania [4–6]. The majority of studies involving flavonoids and malaria describe the antiplasmodial activity-guided fractionation of plants (including species used in traditional medicine). Flavonoids (usually along with other compounds) have been identified in the antiplasmodial fractions of many plants, and in some cases have been shown to possess antiplasmodial activity when isolated [4, 7–12]. Despite this, until very recently only the structurally distinct chalcone subclass of flavonoids attracted much further investigation.
In this study, the flavanone naringenin, the isoflavone genistein, and a range of flavonols (kaempferol, myricetin, quercetin and isoquercitrin) and flavones (acacetin, apigenin, baicalein, chrysin and luteolin) were tested against a chloroquine-sensitive (3D7) and a chloroquine-resistant (7G8) strain of Plasmodium falciparum. The flavonoids were purchased from Sigma-Aldrich (Australia), and purities were ≥ 95% except for isoquercitrin which was ≥ 90%. A recent study demonstrated the in vitro antiplasmodial activities of a number of these compounds ; we confirm and extend these findings with different P. falciparum strains.
Of the eleven flavonoids tested, eight showed antiplasmodial activity against the 3D7 strain, with mean IC50 values in the range 11–66 μM (Table 1). The remaining three (acacetin, naringenin and genistein) did not achieve 50% parasite killing at the highest concentrations tested (which varied according to solubility in aqueous media; DMSO concentrations were kept ≤ 0.1% to prevent any effect on parasite growth). All the flavonoids tested showed measurable activity against the chloroquine-resistant 7G8 strain, with IC50 values between 12 and 76 μM. The most active compound in both strains was luteolin, with IC50 values of 11 ± 1 μM and 12 ± 1 μM for 3D7 and 7G8, respectively (Table 1).
Marked differences in the susceptibilities of the two strains to some flavonoids (most notably to acacetin) were observed in this study and in that of Tasdemir et al. . In both studies, the chloroquine-resistant strain was generally more susceptible to growth inhibition. Whether enhanced sensitivity to some flavonoids relates to the chloroquine resistance phenotype or other difference(s) in genetic background is not known. These strain differences make structure-activity determinations difficult. Furthermore, potential differences in cell permeability should also be considered. The higher antiplasmodial activity of quercetin (logD 2.09) compared to the quercetin glucoside isoquercitrin (logD 1.59) (P = 0.002, paired t-test) could be a result of better cell permeability. Similarly, the flavanone naringenin (logD 2.17) differs from apigenin (logD 2.77) only in the absence of a double bond in the C ring but is much less active (P = 0.02, paired t-test for 7G8). Also, the isoflavone genistein (logD 1.53) differs from apigenin (logD 2.77) only in that the B ring is attached to carbon 3 of the C ring rather than carbon 2, and is less active against both strains (P = 0.03, paired t-test for 7G8). However, the testing of a number of additional flavonoids would be required to conclude that differences in activities are a result of different cell permeabilities.
The effect of a number of the flavonoids tested here on the viability of two normal (non-cancerous) human cell lines has been reported . In the cases of apigenin, kaempferol, luteolin and quercetin, the IC50 values against both strains of P. falciparum are at least 4-fold lower than the IC50 values reported against both human cell lines. Naringenin has relatively low activity against P. falciparum (Table 1) and human cells . In addition, Mamani-Matsuda et al.  investigated quercetin toxicity towards haemopoietic cells and found that the IC50 value was greater than 100 μM (> 6-fold higher than the IC50 against P. falciparum reported here; Table 1).
There have been several reports of flavonoids showing synergistic or antagonistic activities with various antiparasitic drugs [10, 15, 16]. We therefore tested the effect of luteolin (at concentrations of 2.5 μM, 5 μM, 10 μM and 20 μM) on the susceptibility of 7G8 to two important antimalarial drugs, chloroquine and artemisinin. Luteolin did not significantly affect the IC50 values of either drug (not shown). Therefore, luteolin should not antagonise the activities of chloroquine or artemisinin if used in combination therapy.
The concentrations of diet-derived individual flavonoids in plasma are unlikely to reach those required for complete parasite killing. The extent to which flavonoids are absorbed from the gastrointestinal tract depends on factors such as the flavonoid itself and the dietary source [1, 17, 18]. Another important issue is the metabolism of flavonoids in vivo to form potentially less active conjugates . Thus, it will be important to investigate the activities of the main conjugates of promising antiplasmodial flavonoids. Nevertheless, our study raises the possibility of using clinically relevant, non-toxic concentrations of flavonoid combinations as antiplasmodial agents.
The authors are grateful to AP Wasson for helpful discussions and to the Canberra branch of the Australian Red Cross Blood Service for the provision of human blood.
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