Eosin B as a Novel Antimalarial Agent for Drug-Resistant Plasmodium falciparum

Abstract

4,5-Dibromo-2,7-dinitrofluorescein, a red dye commonly referred to as eosin B, inhibits Toxoplasma gondii in both enzymatic and cell culture studies with a 50% inhibitory concentration (IC 50 ) of 180 M. As a non-active-site inhibitor of the bifunctional T. gondii dihydrofolate reductase-thymidylate synthase (DHFR-TS), eosin B offers a novel mechanism for inhibition of the parasitic folate biosynthesis pathway. In the present study, eosin B was further evaluated as a potential antiparasitic compound through in vitro and cell culture testing of its effects on Plasmodium falciparum. Our data revealed that eosin B is a highly selective, potent inhibitor of a variety of drug-resistant malarial strains, with an average IC 50 of 124 nM. Furthermore, there is no indication of cross-resistance with other clinically utilized compounds, suggesting that eosin B is acting via a novel mechanism. The antimalarial mode of action appears to be multifaceted and includes extensive damage to membranes, the alteration of intracellular organelles, and enzymatic inhibition not only of DHFR-TS but also of glutathione reductase and thioredoxin reductase. In addition, preliminary studies suggest that eosin B is also acting as a redox cycling compound. Overall, our data suggest that eosin B is an effective lead compound for the development of new, more effective antimalarial drugs.Plasmodium falciparum, the causative agent of malaria, is a protozoan parasite that has a significant impact on health worldwide. Due to the emergence and spread of resistance to antimalarial drugs, novel chemotherapeutics for the treatment of malaria are urgently needed. To achieve this goal, antiparasitic research needs not only to identify new, chemically diverse molecules for the circumvention of cross-resistance but also to discover novel targets (44). Ideal putative targets will have a variety of characteristics, including involvement in an essential feature of the parasite life cycle, parasitic selectivity over the host, and low potential for resistance development (44).Along these lines, we recently identified a compound that acts as an inhibitor for a novel chemotherapeutic target in the related apicomplexan organism Toxoplasma gondii. This compound, eosin B (4Ј,5Ј-dibromo-2Ј,7Ј-dinitrofluorescein), targets a unique non-active-site region on the bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) enzyme (2). Inhibitors specific to this area have the potential to be powerful antiparasitics since they target folate metabolism, an essential feature of the parasitic life cycle, through a different mode of action than the currently utilized antifolate therapies. Furthermore, the nonactive target site is unique to the bifunctional parasitic enzyme-it is not contained within mammalian TS and DHFR counterparts-and therefore provides for a high level of selectivity.Cell culture and biochemical assays agree that the average 50% inhibitory concentration (IC 50 ) of eosin B for T. gondii is 180 M (2). Importantly, the relative safety of eosin has b...

Redox-Active Antiparasitic Drugs

“…Inhibition of PfTrxR may affect the parasite at several vulnerable points, resulting in enhanced oxidative stress, ineffective DNA synthesis, hindrance in cell division, and disturbed redox regulatory processes (16). Eosin B (3d) and chalcone derivatives (3e) exhibit antiplasmodial activity and also inhibit PfTrxR (100,105). Mannich bases (3f) and nitro compounds (3g-i), which are PfTrxR inhibitors, show antimalarial activity against CQ-R (K1) strain and no toxicity against mammalian cells (8,41,97).…”

Inhibition of Disulfide Reductases as a Therapeutic Strategy

“…Fig. (3)) inhibits in vitro the growth of both Toxoplasma gondii and P. falciparum The antimalarial mode of action appears to be multifaceted and includes extensive damage to membranes, the alteration of intracellular organelles, and enzymatic inhibition not only of dihydrofolate reductase-thymidylate synthase as in T. gondii, but also of glutathione reductase and thioredoxin reductase [63]. The evolution of thioredoxin reductases has made the human enzymes to diverge more from the bacterial enzymes than from those of parasites, justifying the study of the antibacterial properties of drugs that inhibit TrxB.…”

Development of binding assays to screen ligands for Plasmodium falciparum thioredoxin and glutathione reductases by ultrafiltration and liquid chromatography/mass spectrometry

“…Even though a number of in vitro spectrophotometric studies to screen thioredoxin reductase and glutathione reductase inhibitors by monitoring the oxidation of NADPH have been reported [12][13][14][15], detection of small molecules as ligands with selective binding affinity to a target protein is critical in the drug discovery. With the advent of new ionization techniques, MS-based screening methods have emerged as an important analytical tool in identification and characterization of biologically active compounds for protein targets of interest [16][17][18][19].…”