The well-known antiparasitic compound licochalcone A is a potent membrane-active agent that transforms normal erythrocytes into echinocytes in parallel with the inhibition of growth of Plasmodium falciparum cultures, the in vitro antiplasmodial effect apparently being an indirect effect on the host cell. In vitro experiments with synchronous cultures demonstrate that inhibition of invasion is the principal mechanism of growth inhibition. The erythrocyte membrane-modifying effect was also transiently observed in vivo in mice after intravenous administration.Licochalcone A (Fig. 1) was originally isolated (39, 44) as a constituent of roots and rhizomes of various species of Glycyrrhiza L. (licorice root), and its structure was confirmed by synthesis (20,22,44). Subsequent studies resulted in reports of a variety of biological effects of licochalcone A, notably antibacterial (13-15, 17, 18, 36, 43), antileishmanial (3, 4, 8, 34, 45, 46), and antiplasmodial (6, 7, 23) ones. Thus, the compound inhibited growth of Plasmodium falciparum strain 3D7 parasites in vitro with 50% inhibitory concentrations (IC 50 s) of 5.6 Ϯ 0.6 M (35) and reduced parasitemia in mice infected with Plasmodium yoelii (7,23). Compounds related to licochalcone A exhibited comparable antiplasmodial activity in vitro (5, 35).The licochalcone A used in the present work was synthesized as previously described (22,33,44). The purity of the compound was assessed by high-performance liquid chromatography (C 18 column, acetonitrile gradient in water) and 600-MHz 1 H nuclear magnetic resonance and was higher than 99.9%. Determination of the in vitro IC 50 (Fig. 2) for P. falciparum strain 3D7 (initial parasitemia, 1.5%) was performed as previously described (49) using 12 concentrations (each used in triplicate) in the range from 0.098 to 25.0 g/ml (0.29 to 73.9 M); an average IC 50 calculated from three independent determinations was 2.10 Ϯ 0.56 g/ml (6.21 Ϯ 1.65 M), which was practically identical to the reported value (35). Microscopic examination of the cultures showed the presence of dead merozoites and ruptured schizonts at concentrations of 1.56 g/ml and above, and no internal parasites beyond the ring stage were observed.However, simultaneously with the growth inhibition, the exposure of the parasite cultures to licochalcone A caused pronounced membrane perturbations of the erythrocytes in which the parasites grow. The effect of licochalcone A on erythrocytes was the same regardless of whether parasitized or nonparasitized erythrocytes were used and was observed in the same concentration range as that of the in vitro antiplasmodial activity. In order to assess these membrane effects systematically, nonparasitized erythrocytes were incubated with licochalcone A at five different concentrations (0.098, 1.56, 3.13, 5.0, and 25.0 g/ml) for 48 h as in the growth inhibition assay. The deformations of the cell shape were investigated by light microscopy after fixation to glass slides with methanol followed by staining with Giemsa as well as by the hangi...