A nearly complete reversal of chloroquine (CQ) resistance in the CQ-resistant Plasmodium falciparum K-1 strain, with a significant decrease in the mean ؎ standard deviation (SD) 50% inhibitory concentration (IC 50 ) from 1,050 ؎ 95 nM to 14 ؎ 2 nM, was achieved in vitro by the simultaneous administration of 2-aminoethyl diphenylborinate (2-APB). The CQ resistance-reversing activity of 2-APB, which showed the same efficacy as verapamil, was also observed in an in vivo mouse infection model with the CQ-resistant Plasmodium chabaudi AS(30CQ) strain.
Malaria continues to be a worldwide public health problem, causing significant morbidity and mortality. In addition, the resistance of the causative parasite to existing antimalarial drugs has been a growing problem in countries where the disease is endemic (1). Thus, there is an urgent need to develop new antimalarial drugs (2-4) and find novel agents to reverse antimalarial resistance. Understanding the signaling pathways governing the blood-stage growth of the causative parasite may aid in the discovery of new therapeutic targets for antimalarial drugs. As such, the calcium ion (Ca 2ϩ ) homeostasis and signaling pathways in the malaria parasite Plasmodium might be promising targets. Ca 2ϩ is a ubiquitous intracellular signal responsible for controlling a wide range of cellular activities in eukaryotic cells (5). In protozoan parasites, Ca 2ϩ -mediated signaling controls various vital functions, such as protein secretion, motility, cell invasion, and differentiation (6-9). We recently, for the first time, demonstrated spontaneous Ca 2ϩ oscillation in Plasmodium falciparum. Furthermore, we showed that the blockage of this oscillation in the trophozoite stage by 2-aminoethyl diphenylborinate (2-APB) inhibited 1,4,5-trisphosphate (IP 3 )-induced Ca 2ϩ release (10-12), resulting in the death of the parasite (13). Chloroquine (CQ) is thought to exert its toxic effect in the intraerythrocytic parasite at the digestive vacuole (14). The compound was also found to induce Ca 2ϩ release and disrupt Ca 2ϩ and H ϩ homeostasis in the cytoplasm of Plasmodium chabaudi cells (15, 16). Therefore, with regard to the disruption of intracellular Ca 2ϩ homeostasis, it was hypothesized that the potentiation of CQ activity could be achieved in the malaria parasite with the simultaneous administration of 2-APB.To evaluate the potential of 2-APB for reversing CQ resistance, a CQ-resistant K-1 strain of P. falciparum was cultured with the modified method of Trager and Jensen (17) in RPMI 1640 medium (Life Technologies Japan Co., Tokyo, Japan) supplemented with 0.5% AlbuMAX (Life Technologies Japan), 25 mM HEPES, 24 mM sodium bicarbonate, 0.5 g/liter L-glutamine, 50 mg/liter hypoxanthine, 25 g/liter gentamicin, and human erythrocytes (from healthy Japanese volunteers) at a hematocrit level of 2%. Growth synchronization was achieved with 5% D-sorbitol (18). The outcome of the in vitro drug susceptibility test was assessed using the SYBR green I method (19). In brief, P. falciparum-infected eryth...