Imidazole and polyene activity against chloroquine-resistant Plasmodium falciparum

Molecular Assessment of Domain I of Apical Membrane Antigen I Gene in Plasmodium falciparum: Implications in Plasmodium Invasion, Taxonomy, Vaccine Development, and Drug Discovery

Given its global morbidity and mortality rates, malaria continues to be a major public health concern. Despite significant progress in the fight against malaria, efforts to control and eradicate the disease globally are in jeopardy due to lack of a universal vaccine. The conserved short peptide sequences found in Domain I of Plasmodium falciparum apical membrane antigen 1 (PfAMA1), which are exposed on the parasite cell surface and in charge of Plasmodium falciparum invasion of host cells, make PfAMA1 a promising vaccine candidate antigen. The precise amino acids that make up these conserved short peptides are still unknown, and it is still difficult to pinpoint the molecular processes by which PfAMA1 interacts with the human host cell during invasion. The creation of a universal malaria vaccine based on the AMA1 antigen is challenging due to these knowledge limitations. This study used genome mining techniques to look for these particular short peptides in PfAMA1. Thirty individuals with Plasmodium falciparum malaria had blood samples taken using Whatman's filter papers. DNA from the parasite was taken out using the Chelex technique. Domain I of the Plasmodium falciparum AMA1 gene was amplified using nested polymerase chain reactions, and the amplified products were removed, purified, and sequenced. The DNA sequence generated was converted into the matching amino acid sequence using bioinformatic techniques. These amino acid sequences were utilized to search for antigenic epitopes, therapeutic targets, and conserved short peptides in Domain I of PfAMA1. The results of this investigation shed important light on the molecular mechanisms behind Plasmodium invasion of host cells, a potential PfAMA1 vaccine antigen sequence, and prospective malaria treatment options in the future. Our work offers fresh information on malaria medication and vaccine research that has not been previously discussed.

An epigrammatic status of the 'azole'-based antimalarial drugs

The development of multidrug resistance in the malarial parasite has sabotaged majority of the eradication efforts by restraining the inhibition profile of first line as well as second line antimalarial drugs, thus necessitating the development of novel pharmaceutics constructed on appropriate scaffolds with superior potency against the drug-resistant and drug-susceptible Plasmodium parasite. Over the past decades, the infectious malarial parasite has developed resistance against most of the contemporary therapeutics, thus necessitating the rational development of novel approaches principally focused on MDR malaria. This review presents an epigrammatic collation of the epidemiology and the contemporary antimalarial therapeutics based on the 'azole' motif.

Challenges in the chemotherapy of Chagas disease: Looking for possibilities related to the differences and similarities between the parasite and host

Almost 110 years after the first studies by Dr. Carlos Chagas describing an infectious disease that was named for him, Chagas disease remains a neglected illness and a death sentence for infected people in poor countries. This short review highlights the enormous need for new studies aimed at the development of novel and more specific drugs to treat chagasic patients. The primary tool for facing this challenge is deep knowledge about the similarities and differences between the parasite and its human host.

Exploiting large-scale drug-protein interaction information for computational drug repurposing

BACKGROUND

Despite increased investment in pharmaceutical research and development, fewer and fewer new drugs are entering the marketplace. This has prompted studies in repurposing existing drugs for use against diseases with unmet medical needs. A popular approach is to develop a classification model based on drugs with and without a desired therapeutic effect. For this approach to be statistically sound, it requires a large number of drugs in both classes. However, given few or no approved drugs for the diseases of highest medical urgency and interest, different strategies need to be investigated.

RESULTS

We developed a computational method termed "drug-protein interaction-based repurposing" (DPIR) that is potentially applicable to diseases with very few approved drugs. The method, based on genome-wide drug-protein interaction information and Bayesian statistics, first identifies drug-protein interactions associated with a desired therapeutic effect. Then, it uses key drug-protein interactions to score other drugs for their potential to have the same therapeutic effect.

CONCLUSIONS

Detailed cross-validation studies using United States Food and Drug Administration-approved drugs for hypertension, human immunodeficiency virus, and malaria indicated that DPIR provides robust predictions. It achieves high levels of enrichment of drugs approved for a disease even with models developed based on a single drug known to treat the disease. Analysis of our model predictions also indicated that the method is potentially useful for understanding molecular mechanisms of drug action and for identifying protein targets that may potentiate the desired therapeutic effects of other drugs (combination therapies).

Ketoconazole, a cytochrome P(450) inhibitor can potentiate the antimalarial action of α/β arteether against MDR Plasmodium yoelii nigeriensis

The emergence of multidrug resistant (MDR) strains of Plasmodium falciparum in South East Asia and other tropical countries, is posing serious challenge for the international efforts to eradicate malaria. New drug target/ACT/non-ACT combinations need to be discovered to control the spread of MDR malaria. The present communication deals with antimalarial potential of a new combination comprising of ketoconazole (KTZ) (an antifungal/inhibitor of CYP3A4) and artemisinin derivative α/β arteether (ART). In vitro interactions of these drugs against chloroquine sensitive/resistant P. falciparum (Pf3D7/K1) have shown an overall additive interaction with mean sum fractional inhibitory concentrations (∑FICs) of 1.1±0.33 against 3D7 and 1.51±0.42 against K1 strains. Sub-curative doses of KTZ (150mg/kg×7 days) combined with ART (6.25-12.5mg/kg×5 days) both administered orally have shown 100% curative action against MDR P. yoelii nigeriensis in Swiss mice. Besides lower dose of KTZ (75mg/kg) which is non-curative itself, in combination with 12.5mg/kg×5 days of ART treatment, was also 100% curative. Further studies on mechanism of action of KTZ (150mg/kg single dose) have shown that significant inhibitory action of the antifungal drug is through very high level of suppression of CYP (nearly 90%) compared to that of healthy mice liver. The companion drug therapy comprising of KTZ together with ART (25mg/kg×1 dose) also produced more than 50% inhibitory effect on the CYP enzyme level. Since the ART is known to be rapidly metabolized by the liver cytochrome P450 (CYP) 3A4 to Dihydroquinghasu, the combined therapy with KTZ (a strong CYP 3A4 inhibitor) may influence the pharmacokinetics of ART and consequently slow down the drug metabolism and prolong the plasma life of the active drug, which would contribute to enhanced antimalarial action of ART against MDR P. yoelii nigeriensis.

In vitro activity of antifungal drugs against Plasmodium falciparum field isolates

The increasing resistance of the malaria parasite Plasmodium falciparum to currently available drugs necessitates a continuous effort to develop new antimalarial agents. We therefore aimed to assess the in vitro activity of the antifungal drugs clotrimazole, fluconazole, ketoconazole, itraconazole, voriconazole, flucytosine, amphotericin B, and caspofungin against field isolates of P. falciparum from Lambaréné, Gabon. Using the histidin-rich protein 2 (HRP-2) assay we determined the drug susceptibility (EC(50), EC(90)) of 16 field isolates obtained from outpatients attending the Albert Schweitzer Hospital in Lambaréné, Gabon. For fluconazole, itraconazole and caspofungin the in vitro growth inhibition of these drugs is reported for the first time. Our data indicate that clotrimazole, fluconazole, itraconazole and caspofungin show median EC(50) values of 3.1 µg/mL, 1.9 µg/mL, 1.1 µg/mL and 1.1 µg/mL respectively. Ketoconazole, voriconazole, flucytosine and amphotercin B showed no relevant growth inhibition within the range of drug concentrations used in this study.

Antimalarial drugs inhibiting hemozoin (β-hematin) formation: A mechanistic update

Digestion of hemoglobin in the food vacuole of the malaria parasite produces very high quantities of redox active toxic free heme. Hemozoin (beta-hematin) formation is a unique process adopted by Plasmodium sp. to detoxify free heme. Hemozoin formation is a validated target for most of the well-known existing antimalarial drugs and considered to be a suitable target to develop new antimalarials. Here we discuss the possible mechanisms of free heme detoxification in the malaria parasite and the mechanistic details of compounds, which offer antimalarial activity by inhibiting hemozoin formation. The chemical nature of new antimalarial compounds showing antimalarial activity through the inhibition of hemozoin formation has also been incorporated, which may help to design future antimalarials with therapeutic potential against multi-drug resistant malaria.

MOLECULAR MECHANISMS OF RESISTANCE IN ANTIMALARIAL CHEMOTHERAPY: The Unmet Challenge

▪ Abstract 

The enormous public health problem posed by malaria has been substantially worsened in recent years by the emergence and worldwide spread of drug-resistant parasites. The utility of two major therapies, chloroquine and the synergistic combination of pyrimethamine/sulfadoxine, is now seriously compromised. Although several genetic mechanisms have been described, the major source of drug resistance appears to be point mutations in protein target genes. Clinically significant resistance to these agents requires the accumulation of multiple mutations, which genetic studies of parasite populations suggest arise focally and sweep through the population. Efforts to circumvent resistance range from the use of combination therapy with existing agents to laboratory studies directed toward discovering novel targets and therapies.

The prevention and management of drug resistance are among the most important practical problems of tropical medicine and public health.

Leonard J. Bruce-Chwatt, 1972

Inhibition of heme crystal growth by antimalarials and other compounds: implications for drug discovery

During intraerythrocytic infection, Plasmodium falciparum parasites crystallize toxic heme released during hemoglobin catabolism. The proposed mechanism of quinoline inhibition of crystal growth is either by a surface binding or a substrate sequestration mechanism. The kinetics of heme crystal growth was examined in this work using a new high-throughput crystal growth determination assay based on the differential solubility of free vs. crystalline FP in basic solutions. Chloroquine (IC(50)=4.3 microM) and quinidine (IC(50)=1.5 microM) showed a previously not recognized reversible inhibition of FP crystal growth. This inhibition decreased by increasing amounts of heme crystal seed, but not by greater amounts of FP substrate. Crystal growth decreases as pH rises from 4.0 to 6.0, except for a partial local maxima reversal from pH 5.0 to 5.5 that coincides with increased FP solubility. The new crystal growth determination assay enabled a partial screen of existing clinical drugs. Nitrogen heterocycle cytochrome P450 inhibitors also reversibly blocked FP crystal growth, including the azole antifungal drugs clotrimazole (IC(50)=12.9 microM), econazole (IC(50)=19.7 microM), ketoconazole (IC(50)=6.5 microM), and miconazole (IC(50)=21.4 microM). Fluconazole did not inhibit. Both subcellular fractionation of parasites treated with subinhibitory concentrations of ketoconazole and in vitro hemozoin growth assays demonstrated copurification of hemozoin and ketoconazole. The chemical diversity of existing CYP inhibitor libraries that bind FP presents new opportunities for the discovery of antimalarial drugs that block FP crystal growth by a surface binding mechanism and possibly interfere with other FP-sensitive Plasmodium pathways.

Potent antimalarial activity of clotrimazole in in vitro cultures of Plasmodium falciparum

The increasing resistance of the malaria parasite Plasmodium falciparum to currently available drugs demands a continuous effort to develop new antimalarial agents. In this quest, the identification of antimalarial effects of drugs already in use for other therapies represents an attractive approach with potentially rapid clinical application. We have found that the extensively used antimycotic drug clotrimazole (CLT) effectively and rapidly inhibited parasite growth in five different strains of P. falciparum, in vitro, irrespective of their chloroquine sensitivity. The concentrations for 50% inhibition (IC(50)), assessed by parasite incorporation of [(3)H]hypoxanthine, were between 0.2 and 1.1 microM. CLT concentrations of 2 microM and above caused a sharp decline in parasitemia, complete inhibition of parasite replication, and destruction of parasites and host cells within a single intraerythrocytic asexual cycle (approximately 48 hr). These concentrations are within the plasma levels known to be attained in humans after oral administration of the drug. The effects were associated with distinct morphological changes. Transient exposure of ring-stage parasites to 2.5 microM CLT for a period of 12 hr caused a delay in development in a fraction of parasites that reverted to normal after drug removal; 24-hr exposure to the same concentration caused total destruction of parasites and parasitized cells. Chloroquine antagonized the effects of CLT whereas mefloquine was synergistic. The present study suggests that CLT holds much promise as an antimalarial agent and that it is suitable for a clinical study in P. falciparum malaria.

Energy dependence of chloroquine accumulation and chloroquine efflux in Plasmodium falciparum

Chloroquine inhibits the growth of susceptible malaria parasites at low (nanomolar) concentrations because of an energy-requiring drug-concentrating mechanism in the parasite secondary lysosome (food vacuole) which is dependent on the acidification of that vesicle. Chloroquine resistance results from another energy-requiring process: efflux of chloroquine from the resistant parasite with a half-time of 2 min. Chloroquine efflux is inhibited reversibly by the removal of metabolizable substrate (glucose); it is also reduced by the ATPase inhibitor vanadate. These results suggest that chloroquine efflux is an energy-requiring process dependent on the generation and hydrolysis of ATP. Chloroquine efflux cannot be explained by differences in drug accumulation between chloroquine-susceptible and -resistant parasites because the 40-50-fold difference in initial efflux rates between -susceptible and -resistant parasites is unchanged when both parasites contain the same amount of chloroquine. Although chloroquine efflux is phenotypically similar to the efflux of anticancer drugs from multidrug-resistant (mdr) mammalian cells, it is not linked to either of the mdr-like genes of the parasite.

Accumulation of chloroquine by membrane preparations from Plasmodium falciparum

Chloroquine susceptibility and resistance have been associated respectively with the uptake and efflux of chloroquine by Plasmodium falciparum. We made membrane preparations from parasitized and unparasitized red cells in order to study chloroquine accumulation in a cell-free system. The accumulation of [3H]chloroquine by these preparations is inhibited by unlabeled chloroquine and thus is specific. Only membranes from parasitized red cells demonstrate time-dependent chloroquine accumulation; membranes from unparasitized red cells do not. Chloroquine accumulation is eliminated by detergent (0.05% Triton X-100) and reduced by a hypertonic medium, consistent with accumulation inside membrane vesicles rather than binding to membranes. Accumulation is energy dependent; it has a specific requirement for ATP, which cannot be replaced with GTP, CTP, UTP, TTP or ADP, an apparent Km of 21 microM and an apparent Vmax of 4.6 pmol (mg protein)-1 h-1. Vesicle acidification is MgATP dependent, and is reversed by NH4Cl. Chloroquine accumulation is inhibited by reduced medium pH, N-ethylmaleimide or oligomycin, but not by vanadate or ouabain. These studies demonstrate that membrane vesicles prepared from parasitized red cells provide a model system for the study of chloroquine accumulation by P. falciparum.

The use of 5-fluorocytosine and ketoconazole in the culture of the erythrocytic stages of Plasmodium falciparum and some tumor cell lines

In vitro culture systems are often contaminated by bacteria and fungi. It is therefore often necessary to supplement culture media with agents such as penicillin/streptomycin, gentamycin or amphotericin B. The latter cannot be used in the in vitro culture of erythrocytic stages of P. falciparum, and thus anti-fungal agents have not been regularly used in this system. We describe the prophylactic use of 5-fluorocytosine (5-FC) and ketoconazole (KTZ) in tissue cultures at concentrations up to 300 and 10 micrograms/ml respectively which have no effect on the growth of P. falciparum (FCR-3 strain). A melanoma cell line (C32) and a line of uterine carcinoma (C41) were also unaffected by similar concentrations of 5-FC and KTZ. When dissolved in complete culture medium (RPMI 1640) with 10% human plasma, the minimum inhibitory concentration of 5-FC for a susceptible strain of Candida remained below 2 micrograms/ml. These experiments suggest that 5-FC (at 50 micrograms/ml) alone or in combination with KTZ (at 1 microgram/ml) is a useful addition to the armamentarium of antimicrobials available to the tissue culture biologist for a variety of cell culture systems.

The M13 repeat probe detects RFLPs between two strains of the protozoan malaria parasite Plasmodium falciparum

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Efflux of chloroquine from Plasmodium falciparum: mechanism of chloroquine resistance

Chloroquine-resistant Plasmodium falciparum accumulate significantly less chloroquine than susceptible parasites, and this is thought to be the basis of their resistance. However, the reason for the lower accumulation of chloroquine was unknown. The resistant parasite has now been found to release chloroquine 40 to 50 times more rapidly than the susceptible parasite, although their initial rates of chloroquine accumulation are the same. Verapamil and two other calcium channel blockers, as well as vinblastine and daunomycin, each slowed the release and increased the accumulation of chloroquine by resistant (but not susceptible) Plasmodium falciparum. These results suggest that a higher rate of chloroquine release explains the lower chloroquine accumulation, and thus the resistance observed in resistant Plasmodium falciparum.

Inoculum effect with chloroquine and Plasmodium falciparum

In the studies reported here, we examined the inoculum effect observed with chloroquine and Plasmodium falciparum. The 50% effective doses observed with both chloroquine-susceptible and -resistant parasites increased five- to sevenfold from their baseline values as the inoculum was increased from 2 X 10(5) to 2 X 10(7) parasitized erythrocytes per ml (parasitemias of 0.1 to 10% with a hematocrit of 2%). Increasing the inoculum also decreased the chloroquine uptake per parasitized erythrocyte 15- to 20-fold with both chloroquine-susceptible and -resistant parasites. However, because of the 100-fold increase in the inoculum, the total amount of chloroquine taken up actually increased sufficiently to reduce the extracellular chloroquine concentration in vitro by 60 to 90%. These studies suggest that a chloroquine uptake of greater than or equal to 2.0 pmol/10(6) parasitized erythrocytes is necessary for chloroquine to inhibit parasite growth. More marked reductions in the amount of chloroquine uptake per parasitized erythrocyte were observed with a hematocrit of 40% using similar parasitemias of 0.1 to 10% (inocula of 4 X 10(6) to 4 X 10(8) parasitized erythrocytes per ml). Thin-layer chromatography of [3H]chloroquine taken up by chloroquine-resistant P. falciparum revealed no evidence of drug alteration by the parasite. These studies define the mechanism responsible for the inoculum effect observed with chloroquine and P. falciparum in vitro.

Comparative efficacy of ketoconazole and mebendazole in experimental trichinosis

The therapeutic efficacy of ketoconazole and mebendazole was studied in ICR/CD-1 mice infected with Trichinella spiralis for 17 to 20 weeks. Efficacy of both drugs was over 70% when compared with results in control mice. This study indicates that both ketoconazole and mebendazole should be considered in the treatment of trichinosis in humans.

In vitro activity of ketoconazole against herpes simplex virus

The effects of ketoconazole alone and in combination with acyclovir and adenine arabinoside upon the replication of herpes simplex virus types 1 and 2 (HSV-1 and -2) were investigated by using a yield reduction assay. Ketoconazole demonstrated antiviral activity against HSV-1 and -2 and synergistic antiviral activity when it was combined with acyclovir. Combinations of ketoconazole with adenine arabinoside resulted in either interference or indifference. The effects of ketoconazole upon the protein synthesis of HSV-2-infected cells were also determined in an effort to define the mechanism of action for the antiviral activity of ketoconazole. There was no reduction of HSV proteins when compared with acyclovir. These findings suggest that further investigations of the use of ketoconazole for the treatment of HSV infections are warranted.

Antimalarials increase vesicle pH in Plasmodium falciparum

The asexual erythrocytic stage of the malarial parasite ingests and degrades the hemoglobin of its host red cell. To study this process, we labeled the cytoplasm of uninfected red cells with fluorescein-dextran, infected those cells with trophozoite- and schizont-rich cultures of Plasmodium falciparum, and harvested them 110-120 h later in the trophozoite stage. After lysis of the red cell cytoplasm with digitonin, the only fluorescence remaining was in small (0.5-0.9 micron) vesicles similar to the parasite's food vacuole. As measured by spectrofluorimetry, the pH of these vesicles was acid (initial pH 5.2-5.4), and they responded to MgATP with acidification and to weak bases such as NH4Cl with alkalinization. These three properties are similar to those obtained with human fibroblasts and suggest that the endocytic vesicles of plasmodia are similar to those of mammalian cells. Each of the antimalarials tested (chloroquine, quinine, and mefloquine) as well as NH4Cl inhibited parasite growth at concentrations virtually identical to those that increased parasite vesicle pH. These results suggest two conclusions: (a) The increases in vesicle pH that we have observed in our digitonin-treated parasite preparation occur at similar concentrations of weak bases and antimalarials in cultures of parasitized erythrocytes, and (b) P. falciparum parasites are exquisitely dependent on vesicle pH during their asexual erythrocytic cycle, perhaps for processes analogous to endocytosis and proteolysis in mammalian cells, and that antimalarials and NH4Cl may act by interfering with these events.

Activity of ketoconazole and its deacyl derivative against Plasmodium falciparum and Candida isolates

Deacylketoconazole was 15- to 50-fold more active against Plasmodium falciparum than was ketoconazole, based on [(3)H]hypoxanthine uptake and quantitative parasite counts. In contrast, there were no significant differences between these drugs in their activity against clinical isolates of Candida spp.

Lysis of Plasmodium falciparum by ferriprotoporphyrin IX and a chloroquine-ferriprotoporphyrin IX complex

Ferriprotoporphyrin IX (FP) and a chloroquine-FP complex lysed isolated Plasmodium falciparum parasites as judged by decreases in the turbidity of parasite suspensions and by ultrastructural changes. Exposure of parasite suspensions to 50 microM FP or to a complex formed from 50 microM FP and 20 MicroM chloroquine reduced the number of identifiable parasites and caused swelling and loss of internal detail in those that were identifiable. The amount of lysis was dose-dependent over the range of 10 to 50 microM FP. Formation of a chloroquine-FP complex reduced, but did not eliminate, the toxicity of FP. Since there is evidence indicating that a chloroquine-FP complex forms when chloroquine-susceptible parasites are exposed to chloroquine, we suggest that accumulation of this complex may account for the chemotherapeutic effect of chloroquine against P. falciparum.