Relationship between chloroquine toxicity and iron acquisition in Saccharomyces cerevisiae

Antimalarial Quinacrine and Chloroquine Lose Their Activity by Decreasing Cationic Amphiphilic Structure with a Slight Decrease in pH

Quinacrine (QC) and chloroquine (CQ) have antimicrobial and antiviral activities as well as antimalarial activity, although the mechanisms remain unknown. QC increased the antimicrobial activity against yeast exponentially with a pH-dependent increase in the cationic amphiphilic drug (CAD) structure. CAD-QC localized in the yeast membranes and induced glucose starvation by noncompetitively inhibiting glucose uptake as antipsychotic chlorpromazine (CPZ) did. An exponential increase in antimicrobial activity with pH-dependent CAD formation was also observed for CQ, indicating that the CAD structure is crucial for its pharmacological activity. A decrease in CAD structure with a slight decrease in pH from 7.4 greatly reduced their effects; namely, these drugs would inefficiently act on falciparum malaria and COVID-19 pneumonia patients with acidosis, resulting in resistance. The decrease in CAD structure at physiological pH was not observed for quinine, primaquine, or mefloquine. Therefore, restoring the normal blood pH or using pH-insensitive quinoline drugs might be effective for these infectious diseases with acidosis.

The possible mechanisms of action of 4-aminoquinolines (chloroquine/hydroxychloroquine) against Sars-Cov-2 infection (COVID-19): A role for iron homeostasis?

The anti-malarial drugs chloroquine (CQ) and primarily the less toxic hydroxychloroquine (HCQ) are currently used to treat autoimmune diseases for their immunomodulatory and anti-thrombotic properties. They have also been proposed for the treatment of several viral infections, due to their anti-viral effects in cell cultures and animal models, and, currently, for the treatment of coronavirus disease 2019 (COVID-19), the pandemic severe acute respiratory syndrome caused by coronavirus 2 (Sars-Cov-2) infection that is spreading all over the world. Although in some recent studies a clinical improvement in COVID-19 patients has been observed, the clinical efficacy of CQ and HCQ in COVID-19 has yet to be proven with randomized controlled studies, many of which are currently ongoing, also considering pharmacokinetics, optimal dosing regimen, therapeutic level and duration of treatment and taking into account patients with different severity degrees of disease. Here we review what is currently known on the mechanisms of action of CQ and HCQ as anti-viral, anti-inflammatory and anti-thrombotic drugs and discuss the up-to-date experimental evidence on the potential mechanisms of action of CQ/HCQ in Sars-Cov2 infection and the current clinical knowledge on their efficacy in the treatment of COVID-19 patients. Given the role of iron in several human viral infections, we also propose a different insight into a number of CQ and HCQ pharmacological effects, suggesting a potential involvement of iron homeostasis in Sars-Cov-2 infection and COVID-19 clinical course.

Clioquinol induces G2/M cell cycle arrest through the up-regulation of TDH3 in Saccharomyces cerevisiae

Clioquinol (CQ) has been used as a classical antimicrobial agent for many years. However, its mode of action is still unclear. In our study, the growth of Candida albicans and Saccharomyces cerevisiae was inhibited by CQ. It did not kill yeast cells, but shortened G1 phase and arrested cell cycle at G2/M phase. By using two-dimensional electrophoresis based proteomic approach, six proteins were found to be significantly affected by CQ. Among them, four (PDC1, ADH1, TDH3, IPP1) were up-regulated and the other two (TDH1 and PGK1) were down-regulated. According to the Saccharomyces Genome Database (SGD), these proteins were involved in various biological processes including glycolytic fermentation, gluconeogenesis, glycolytic process, amino acid catabolism, redox reaction and reactive oxygen species metabolic process. It was noted that there was a link between TDH3 and cell cycle. The overexpression of TDH3 phenocopied CQ treatment and arrested the cell cycle at G2/M phase. RT-PCR analysis showed that the mRNA levels of CLN3 and CDC28, critical genes for passage through G1 phase, were up-regulated after the treatment of CQ as well as the overexpression of TDH3. It demonstrates that CQ inhibits the growth of yeast by up-regulating the expression of TDH3 to influence the cell cycle. It is expected to provide new insights for the antimicrobial mechanism of CQ.

The effect of antibacterial and non-antibacterial compounds alone or associated with antifugals upon fungi

During the last 30 years the incidence of fungal infections has increased dramatically. While the antifungal therapeutic options available are somewhat reduced, most pathogenic microorganisms have an incredible capacity to mutate and acquire resistance. In addition, multiple drugs are often required concomitantly to manage clinically complex disorders. The combination of antibiotics or other compounds with antifungal drugs, simultaneously or sequentially, is commonly adopted in clinical practice, although without a full knowledge of the consequences. Thus, the role of combined therapy and the effect of antibiotics upon fungal growth promotion need to be critically evaluated and understood in order to avoid undesirable drug interactions. With this review we intend discuss the studies that report about antibiotics inhibiting fungal growth, as well as studies describing the synergistic effect of the combined therapy, i.e., associations between antibiotics or other compounds with antifungal drugs. Alternative therapeutic protocols for fungal disease could be designed, taking advantage of such drug combinations. Critical revision of previously published data is crucial in order to define future research strategies.

Synergistic activity of chloroquine with fluconazole against fluconazole-resistant isolates of Candida species

The in vitro activity of chloroquine and the interactions of chloroquine combined with fluconazole against 37 Candida isolates were tested using the broth microdilution, disk diffusion, and Etest susceptibility tests. Synergistic effect was detected with 6 of 9 fluconazole-resistant Candida albicans isolates, with Candida krusei ATCC 6258, and with all 12 fluconazole-resistant Candida tropicalis isolates.

Signaling of chloroquine-induced stress in the yeast Saccharomyces cerevisiae requires the Hog1 and Slt2 mitogen-activated protein kinase pathways

Chloroquine (CQ) has been under clinical use for several decades, and yet little is known about CQ sensing and signaling mechanisms or about their impact on various biological pathways. We employed the budding yeast Saccharomyces cerevisiae as a model organism to study the pathways targeted by CQ. Our screening with yeast mutants revealed that it targets histone proteins and histone deacetylases (HDACs). Here, we also describe the novel role of mitogen-activated protein kinases Hog1 and Slt2, which aid in survival in the presence of CQ. Cells deficient in Hog1 or Slt2 are found to be CQ hypersensitive, and both proteins were phosphorylated in response to CQ exposure. CQ-activated Hog1p is translocated to the nucleus and facilitates the expression of GPD1 (glycerol-3-phosphate dehydrogenase), which is required for the synthesis of glycerol (one of the major osmolytes). Moreover, cells treated with CQ exhibited an increase in intracellular reactive oxygen species (ROS) levels and the effects were rescued by addition of reduced glutathione to the medium. The deletion of SOD1, the superoxide dismutase in yeast, resulted in hypersensitivity to CQ. We have also observed P38 as well as P42/44 phosphorylation in HEK293T human cells upon exposure to CQ, indicating that the kinds of responses generated in yeast and human cells are similar. In summary, our findings define the multiple biological pathways targeted by CQ that might be useful for understanding the toxicity modulated by this pharmacologically important molecule.

Essential functions of iron-requiring proteins in DNA replication, repair and cell cycle control

Eukaryotic cells contain numerous iron-requiring proteins such as iron-sulfur (Fe-S) cluster proteins, hemoproteins and ribonucleotide reductases (RNRs). These proteins utilize iron as a cofactor and perform key roles in DNA replication, DNA repair, metabolic catalysis, iron regulation and cell cycle progression. Disruption of iron homeostasis always impairs the functions of these iron-requiring proteins and is genetically associated with diseases characterized by DNA repair defects in mammals. Organisms have evolved multi-layered mechanisms to regulate iron balance to ensure genome stability and cell development. This review briefly provides current perspectives on iron homeostasis in yeast and mammals, and mainly summarizes the most recent understandings on iron-requiring protein functions involved in DNA stability maintenance and cell cycle control.

Repeat polymorphisms in the low-complexity regions of Plasmodium falciparum ABC transporters and associations with in vitro antimalarial responses

The Plasmodium falciparum genome is rich in regions of low amino acid complexity which evolve with few constraints on size. To explore the extent of diversity in these loci, we sequenced repeat regions in pfmdr1, pfmdr5, pfmdr6, pfmrp2, and the antigenic locus pfmsp8 in laboratory and cultured-adapted clinical isolates. We further assessed associations between the repeats and parasite in vitro responses to 7 antimalarials to determine possible adaptive roles of these repeats in drug tolerance. Our results show extensive repeat variations in the reference and clinical isolates in all loci. We also observed a modest increase in dihydroartemisinin activity in parasites harboring the pfmdr1 sequence profile 7-2-10 (reflecting the number of asparagine repeats, number of aspartate repeats, and number of asparagine repeats in the final series of the gene product) (P = 0.0321) and reduced sensitivity to chloroquine, mefloquine, quinine, and dihydroartemisinin in those with the 7-2-11 profile (P = 0.0051, 0.0068, 0.0011, and 0.0052, respectively). Interestingly, we noted an inverse association between two drugs whereby isolates with 6 asparagine repeats encoded by pfmdr6 were significantly more susceptible to piperaquine than those with 8 (P = 0.0057). Against lumefantrine, those with 8 repeats were, however, more sensitive (P = 0.0144). In pfmrp2, the 7-DNNNTS/NNNNTS (number of DNNNTS or NNNNTS motifs; underlining indicates dimorphism) repeat group was significantly associated with a higher lumefantrine 50% inhibitory concentration (IC50) (P = 0.008) than in those without. No associations were observed with pfmsp8. These results hint at the probable utility of some repeat conformations as markers of in vitro antimalarial response; hence, biochemical functional studies to ascertain their role in P. falciparum are required.

Iron economy in Chlamydomonas reinhardtii

While research on iron nutrition in plants has largely focused on iron-uptake pathways, photosynthetic microbes such as the unicellular green alga Chlamydomonas reinhardtii provide excellent experimental systems for understanding iron metabolism at the subcellular level. Several paradigms in iron homeostasis have been established in this alga, including photosystem remodeling in the chloroplast and preferential retention of some pathways and key iron-dependent proteins in response to suboptimal iron supply. This review presents our current understanding of iron homeostasis in Chlamydomonas, with specific attention on characterized responses to changes in iron supply, like iron-deficiency. An overview of frequently used methods for the investigation of iron-responsive gene expression, physiology and metabolism is also provided, including preparation of media, the effect of cell size, cell density and strain choice on quantitative measurements and methods for the determination of metal content and assessing the effect of iron supply on photosynthetic performance.

Chloroquine sensitizes biofilms of Candida albicans to antifungal azoles

Biofilms formed by Candida albicans, a human pathogen, are known to be resistant to different antifungal agents. Novel strategies to combat the biofilm associated Candida infections like multiple drug therapy are being explored. In this study, potential of chloroquine to be a partner drug in combination with four antifungal agents, namely fluconazole, voriconazole, amphotericin B, and caspofungin, was explored against biofilms of C. albicans. Activity of various concentrations of chloroquine in combination with a particular antifungal drug was analyzed in a checkerboard format. Growth of biofilm in presence of drugs was analyzed by XTT-assay, in terms of relative metabolic activity compared to that of drug free control. Results obtained by XTT-metabolic assay were confirmed by scanning electron microscopy. The interactions between chloroquine and four antifungal drugs were determined by calculating fractional inhibitory concentration indices. Azole resistance in biofilms was reverted significantly (p < 0.05) in presence of 250 μg/mL of chloroquine, which resulted in inhibition of biofilms at very low concentrations of antifungal drugs. No significant alteration in the sensitivity of biofilms to caspofungin and amphotericin B was evident in combination with chloroquine. This study for the first time indicates that chloroquine potentiates anti-biofilm activity of fluconazole and voriconazole.

Discovering thiamine transporters as targets of chloroquine using a novel functional genomics strategy

Chloroquine (CQ) and other quinoline-containing antimalarials are important drugs with many therapeutic benefits as well as adverse effects. However, the molecular targets underlying most such effects are largely unknown. By taking a novel functional genomics strategy, which employs a unique combination of genome-wide drug-gene synthetic lethality (DGSL), gene-gene synthetic lethality (GGSL), and dosage suppression (DS) screens in the model organism Saccharomyces cerevisiae and is thus termed SL/DS for simplicity, we found that CQ inhibits the thiamine transporters Thi7, Nrt1, and Thi72 in yeast. We first discovered a thi3Δ mutant as hypersensitive to CQ using a genome-wide DGSL analysis. Using genome-wide GGSL and DS screens, we then found that a thi7Δ mutation confers severe growth defect in the thi3Δ mutant and that THI7 overexpression suppresses CQ-hypersensitivity of this mutant. We subsequently showed that CQ inhibits the functions of Thi7 and its homologues Nrt1 and Thi72. In particular, the transporter activity of wild-type Thi7 but not a CQ-resistant mutant (Thi7^T287N) was completely inhibited by the drug. Similar effects were also observed with other quinoline-containing antimalarials. In addition, CQ completely inhibited a human thiamine transporter (SLC19A3) expressed in yeast and significantly inhibited thiamine uptake in cultured human cell lines. Therefore, inhibition of thiamine uptake is a conserved mechanism of action of CQ. This study also demonstrated SL/DS as a uniquely effective methodology for discovering drug targets.

By using a novel SL/DS methodology in the model organism yeast, we discovered that the antimalarial drug CQ inhibits thiamine transporters and consequently causes thiamine (vitamin B1) deficiency and growth defects. This mechanism of action (MOA) is conserved in human cells and possibly also in other organisms. Given that both thiamine deficiency and treatment with CQ cause retinal, neurological, and cardiovascular disorders in humans, our results suggest that thiamine deficiency might be a root cause of some of CQ's adverse effects, which might be preventable with concomitant dietary thiamine supplementation. Such a MOA by CQ could also be responsible for its therapeutic effects against malarial parasites, which need exogenous thiamine for survival. Such a possibility needs to be investigated before dietary thiamine supplementation can be used to prevent CQ's adverse effects.

Yeast toxicogenomics: genome-wide responses to chemical stresses with impact in environmental health, pharmacology, and biotechnology

The emerging transdisciplinary field of Toxicogenomics aims to study the cell response to a given toxicant at the genome, transcriptome, proteome, and metabolome levels. This approach is expected to provide earlier and more sensitive biomarkers of toxicological responses and help in the delineation of regulatory risk assessment. The use of model organisms to gather such genomic information, through the exploitation of Omics and Bioinformatics approaches and tools, together with more focused molecular and cellular biology studies are rapidly increasing our understanding and providing an integrative view on how cells interact with their environment. The use of the model eukaryote Saccharomyces cerevisiae in the field of Toxicogenomics is discussed in this review. Despite the limitations intrinsic to the use of such a simple single cell experimental model, S. cerevisiae appears to be very useful as a first screening tool, limiting the use of animal models. Moreover, it is also one of the most interesting systems to obtain a truly global understanding of the toxicological response and resistance mechanisms, being in the frontline of systems biology research and developments. The impact of the knowledge gathered in the yeast model, through the use of Toxicogenomics approaches, is highlighted here by its use in prediction of toxicological outcomes of exposure to pesticides and pharmaceutical drugs, but also by its impact in biotechnology, namely in the development of more robust crops and in the improvement of yeast strains as cell factories.

A genome-wide screen identifies yeast genes required for protection against or enhanced cytotoxicity of the antimalarial drug quinine

Quinine is used in the treatment of Plasmodium falciparum severe malaria. However, both the drug's mode of action and mechanisms of resistance are still poorly understood and subject to debate. In an effort to clarify these questions, we used the yeast Saccharomyces cerevisiae as a model for pharmacological studies with quinine. Following on a previous work that examined the yeast genomic expression program in response to quinine, we now explore a genome-wide screen for altered susceptibility to quinine using the EUROSCARF collection of yeast deletion strains. We identified 279 quinine-susceptible strains, among which 112 conferred a hyper-susceptibility phenotype. The expression of these genes, mainly involved in carbohydrate metabolism, iron uptake and ion homeostasis functions, is required for quinine resistance in yeast. Sixty-two genes whose deletion leads to increased quinine resistance were also identified in this screen, including several genes encoding ribosome protein subunits. These well-known potential drug targets in Plasmodium are associated with quinine action for the first time in this study. The suggested involvement of phosphate signaling and transport in quinine tolerance was also studied, and activation of phosphate starvation-responsive genes was observed under a mild-induced quinine stress. Finally, P. falciparum homology searches were performed for a selected group of 41 genes. Thirty-two encoded proteins possess homologs in the parasite, including subunits of a parasitic vacuolar H(+)-ATPase complex, ion and phosphate importers, and several ribosome protein subunits, suggesting that the results obtained in yeast are good candidates to be transposed and explored in a P. falciparum context.

Functional genomics of drug-induced ion homeostasis identifies a novel regulatory crosstalk of iron and zinc regulons in yeast

Pyrrolidine dithiocarbamate (PDTC), a known inhibitor of NFκB activation, has antioxidative as well as antiviral activities. PDTC is effective against several virus families, indicating that its antiviral mechanism targets host rather than viral functions. To investigate its mode of action, we used baker's yeast as a simple eukaryotic model system and two types of genome-wide analysis. First, expression profiling using whole-genome DNA microarrays identifies more than 200 genes differentially regulated upon PDTC exposure. Interestingly, the Aft1-dependent iron regulon is a main target of PDTC, indicating a lack of iron availability. Moreover, the PDTC-caused zinc influx triggers a strong regulatory effect on zinc transporters due to the cytoplasmic zinc excess. Second, phenotypic screening the EUROSCARF collection for PDTC hypersensitivity identifies numerous mutants implicated in vacuolar maintenance, acidification as well as in transport, mitochondrial organization, and translation. Notably, the screening data indicate significant overlaps of PDTC-sensitive genes and those mediating zinc tolerance. Hence, we show that PDTC induces cytoplasmic zinc excess, eliciting vacuolar detoxification, which in turn, disturbs iron homeostasis and activates the iron-dependent regulator Aft1. Our work reveals a complex crosstalk in yeast ion homeostasis and the underlying regulatory networks.

Transcriptomic profiling of the Saccharomyces cerevisiae response to quinine reveals a glucose limitation response attributable to drug-induced inhibition of glucose uptake

Quinine has been employed in the treatment of malaria for centuries and is still used against severe Plasmodium falciparum malaria. However, its interactions with the parasite remain poorly understood and subject to debate. In this study, we used the Saccharomyces cerevisiae eukaryotic model to better understand quinine's mode of action and the mechanisms underlying the cell response to the drug. We obtained a transcriptomic profile of the yeast's early response to quinine, evidencing a marked activation of genes involved in the low-glucose response (e.g., CAT8, ADR1, MAL33, MTH1, and SNF3). We used a low inhibitory quinine concentration with no detectable effect on plasma membrane function, consistent with the absence of a general nutrient starvation response and suggesting that quinine-induced glucose limitation is a specific response. We have further shown that transport of [(14)C]glucose is inhibited by quinine, with kinetic data indicating competitive inhibition. Also, tested mutant strains deleted for genes encoding high- and low-affinity hexose transporters (HXT1 to HXT5, HXT8, and HXT10) exhibit resistance phenotypes, correlating with reduced levels of quinine accumulation in the mutants examined. These results suggest that the hexose transporters are facilitators of quinine uptake in S. cerevisiae, possibly through a competitive inhibition mechanism. Interestingly, P. falciparum is highly dependent on glucose uptake, which is mediated by the single-copy transporter PfHT1, a protein with high homology to yeast's hexose transporters. We propose that PfHT1 is an interesting candidate quinine target possibly involved in quinine import in P. falciparum, an uptake mechanism postulated in recent studies to occur through a still-unidentified importer(s).

Sitamaquine sensitivity in Leishmania species is not mediated by drug accumulation in acidocalcisomes

Sitamaquine (WR6026), an 8-aminoquinoline derivative, is a new antileishmanial oral drug. As a lipophilic weak base, it rapidly accumulates in acidic compartments, represented mainly by acidocalcisomes. In this work, we show that the antileishmanial action of sitamaquine is unrelated to its level of accumulation in these acidic vesicles. We have observed significant differences in sitamaquine sensitivity and accumulation between Leishmania species and strains, and interestingly, there is no correlation between them. However, there is a relationship between the levels of accumulation of sitamaquine and acidotropic probes, acidocalcisomes size, and polyphosphate levels. The Leishmania major AP3delta-null mutant line, in which acidocalcisomes are devoid of their usual polyphosphate and proton content, is unable to accumulate sitamaquine; however, both the parental strain and the AP3delta-null mutants showed similar sensitivities to sitamaquine. Our findings provide clear evidence that the antileishmanial action of sitamaquine is unrelated to its accumulation in acidocalcisomes.

Genomewide expression profiling of cryptolepine-induced toxicity in Saccharomyces cerevisiae

We have used the budding yeast Saccharomyces cerevisiae to identify genes that may confer sensitivity in vivo to the antimalarial and cytotoxic agent cryptolepine. Five S. cerevisiae strains, with different genetic backgrounds in cell permeability and DNA damage repair mechanisms, were exposed to several concentrations of cryptolepine. Cryptolepine showed a relatively mild toxicity for wild-type strains, which was augmented by either increasing cell permeability (Deltaerg6 or ISE2 strains) or disrupting DNA damage repair (Deltarad52 strains). These results are compatible with the ability of cryptolepine to intercalate into DNA and thus promote DNA lesions. The effects of low concentrations of cryptolepine (20% and 40% inhibitory concentrations [IC(20) and IC(40)]) were analyzed by comparing the gene expression profiles of treated and untreated Deltaerg6 yeast cells. Significant changes in expression levels were observed for 349 genes (117 upregulated and 232 downregulated). General stress-related genes constituted the only recognizable functional cluster whose expression was increased upon cryptolepine treatment, making up about 20% of upregulated genes. In contrast, analysis of the characteristics of downregulated genes revealed a specific effect of cryptolepine on genes related to iron transport or acid phosphatases, as well as a significant proportion of genes related to cell wall components. The effects of cryptolepine on the transcription of iron transport-related genes were consistent with a loss of function of the iron sensor Aft1p, indicating a possible disruption of iron metabolism in S. cerevisiae. Since the interference of cryptolepine with iron metabolism is considered one of its putative antimalarial targets, this finding supports the utility of S. cerevisiae in drug-developing schemes.

Plasmodium falciparum: Genome wide perturbations in transcript profiles among mixed stage cultures after chloroquine treatment

A genomic approach was taken to study the effect of chloroquine (CQ) on Plasmodium falciparum cultures in multiple cell states, following short and long exposures to drug at varying concentrations. Six hundred genes from numerous functional groups were responsive to CQ amongst all cell states assayed in a micro-array analysis; however, the amplitude of fold-change was low in the majority of cases. Moreover, alterations in specific, functionally related cascades could not be discerned, leading us to believe there is no single signature response to CQ at the transcript level in P. falciparum. Instead, cell cycle changes appear to have a more pronounced effect on gene expression; only a fraction of the drug responsive loci (approximately 5%) were shared between two separate starting cultures that varied in staging profile in the current study, as well as a previous published analysis using SAGE technology [Gunasekera, A.M., Patankar, S., Schug, J., Eisen, G.,Wirth, D.F., 2003. Drug-induced alterations in gene expression of the asexual blood forms of Plasmodium falciparum. Molecular Microbiology 50, 1229-1239]. These findings are important to report, given the striking contrast to similar studies in other model eukaryotic organisms.

What is the relevance of bioinformatics to pharmacology?

Although bioinformatics achieved prominence because of its central role in genome data storage, management and analysis, its focus has shifted as the life sciences exploit these data. In pharmacology, genomic, transcriptomic and proteomic data are being used in the quest for drugs that fulfill unmet medical needs, are disease modifying or curative and are more effective and safer than current drugs. Bioinformatics is used in drug target identification and validation and in the development of biomarkers and toxicogenomic and pharmacogenomic tools to maximize the therapeutic benefit of drugs. Now that the 'parts list' of cellular signalling pathways is available, integrated computational and experimental programmes are being developed, with the goal of enabling in silico pharmacology by linking the genome, transcriptome and proteome to cellular pathophysiology.

Cellular uptake of a catechol iron chelator and chloroquine into Plasmodium falciparum infected erythrocytes

Our study demonstrates the capacity of FR160, a catechol iron chelator, to reach and accumulate into infected Plasmodium falciparum erythrocytes and parasites (cellular accumulation ratio between 12 and 43). Steady-state FR160 accumulation is obtained after 2 hr of exposure. After 2 hr exposure, it reaches intracellular levels that are 4- to 10-fold higher in infected red blood cells than those attained in normal erythrocytes. There is quite a good correlation between the accumulation of chloroquine and FR160 in the different strains (r=0.939) and in the IC(50) values (r=0.719). In contrast, the accumulation of FR160 and its activity is poorly correlated (r=0.500), suggesting that activity of FR160 may be independent of its penetration into infected erythrocytes. The mechanism of accumulation is yet unknown but based on inhibitor studies, the uptake of FR160 seems to be not associated with the calcium pump or channel, the potassium channel or the Na(+)/H(+) exchanger. Combinations of FR160 with verapamil, diltiazem, clotrimazole, amiloride, diazoxide, 4-aminopyridine, and picrotoxin should be avoided (antagonistic effects). The potent in vitro activity of FR160 on chloroquine-resistant strains or isolates, its lower toxicity against Vero cells, its mechanisms of action, its capacity to reach rapidly and accumulate into infected erythrocytes suggest that FR160 holds much promise as a new structural lead and effective antimalarial agent or at least a promising adjuvant in treatment of malaria.