Chemotherapy and drug resistance in malaria

Interaction between Chitosan and Chloroquine against Plasmodium berghei and P. falciparum Using In-Vivo and In-Vitro Tests

Background:

The use of antimalarial drugs with number of compounds in combination form may potentiate each other’s activity.

Methods:

This study was conducted in the School of Public Health, Tehran University of Medical Sciences, Tehran, Iran in 2018. It was based on two methods including in vivo and in vitro tests with aim of considering interaction between chitosan and chloroquine against Plasmodium berghei and P. falciparum parasites using different ratios of the agents with ED50^s and IC50^s baselines.

Results:

Administrating 10 and 20 mg/kg (mouse body weight) of chitosan alone to the P. berghei –infected mice up to 4 successive days resulted in 37% and 45% inhibition of P. berghei respectively, while employing the compound with chloroquine in combination form with ratios of 90/10 and 70/30 (chloroquine/chitosan) had a considerable potentiation including 71.58% and 83.85% inhibition effectiveness against P. berghei. Moreover, 20 mg/L (CCM) concentration of chitosan alone could eliminate 69.55% of P. falciparum in culture medium while in combination with chloroquine in ratios of 90/10 (chloroquine/chitosan) had considerable potentiation including 79.14% inhibition effectiveness. Mean survival time of those mice received combination therapy in ratios of 90/10 and 70/30 (chloroquine/chitosan) was longer than those took up mono therapy of either chloroquine or chitosan based on their ED50^s doses.

Conclusion:

Interaction between chloroquine and chitosan showed considerable potentiation in combination form against either P. berghei or P. falciparum using in vivo and in vitro tests respectively. Meanwhile, interaction between the above mentioned agents resulted in a notable survival time for those P. berghei-infected mice treated with the combination.

Drug Repurposing: A Review of Old and New Antibiotics for the Treatment of Malaria: Identifying Antibiotics with a Fast Onset of Antiplasmodial Action

Malaria is one of the most life-threatening infectious diseases and constitutes a major health problem, especially in Africa. Although artemisinin combination therapies remain efficacious to treat malaria, the emergence of resistant parasites emphasizes the urgent need of new alternative chemotherapies. One strategy is the repurposing of existing drugs. Herein, we reviewed the antimalarial effects of marketed antibiotics, and described in detail the fast-acting antibiotics that showed activity in nanomolar concentrations. Antibiotics have been used for prophylaxis and treatment of malaria for many years and are of particular interest because they might exert a different mode of action than current antimalarials, and can be used simultaneously to treat concomitant bacterial infections.

New Anti SARS-Cov-2 Targets for Quinoline Derivatives Chloroquine and Hydroxychloroquine

The rapid spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has created a severe global health crisis. In this paper, we used docking and simulation methods to identify potential targets and the mechanism of action of chloroquine (CQ) and hydroxychloroquine (HCQ) against SARS-CoV-2. Our results showed that both CQ and HCQ influenced the functionality of the envelope (E) protein, necessary in the maturation processes of the virus, due to interactions that modify the flexibility of the protein structure. Furthermore, CQ and HCQ also influenced the proofreading and capping of viral RNA in SARS-CoV-2, performed by nsp10/nsp14 and nsp10/nsp16. In particular, HCQ demonstrated a better energy binding with the examined targets compared to CQ, probably due to the hydrogen bonding of the hydroxyl group of HCQ with polar amino acid residues.

Antiplasmodial Anthraquinones from Medicinal Plants: The Chemistry and Possible Mode of Actions

Malaria killed nearly half a million people in 2015, and 70% of this victims were young children. Malarial chemotherapy makes use of several drugs, each with its own pharmacological limitations, and with parasite resistance being the most challenging. People of low income nations often rely on traditional medicine as a treatment due to limited access to modern healthcare services. Despite uncertainties present in the outcome of traditional medicine, ethnomedicine approach has yielded important lead candidates. The investigation of medicinal plants utilized in the malaria endemic region yielded many antiplasmodial compounds with anthraquinone moiety. This paper describes natural anthraquinones extracted from medicinal plants utilized in traditional medicine for the treatment of malaria. In addition, the insight on structure-activity relationship and their mode of actions are also elaborated.

Novel applications for an established antimalarial drug: tumoricidal activity of quinacrine

Quinacrine (QC), a synthetic antimalarial drug, was consistently used worldwide to combat malaria during the last century. Interestingly, later studies revealed that it also displays various additional properties, specifically antitumor activity. QC's antitumor activity occurs via a variety of pathways, including DNA intercalation, angiogenesis inhibition, signal transduction regulation, cell cycle arrest and autophagy induction. In combination with traditional therapies such as chemotherapy and radiotherapy, QC has also displayed synergistic effects against tumors, which may open promising therapeutic avenues. However, the breadth and complexity of its antitumor mechanisms have not yet been fully elucidated. In this review, we have systematically categorized QC's reported antitumor mechanisms from recent studies, to enable a deeper understanding of its antitumor activity.

A REVIEW ON THE CHEMISTRY AND PHARMACOLOGY OF Rennellia elliptica Korth

Rennellia elliptica, popularly dubbed as Malaysian Ginseng, is widely used in traditional medicine among the local Jakun community in Endau-Rompin State Park, Pahang, Malaysia. The decoction of the roots is traditionally taken for treatment of body aches, as postpartum tonic, as aphrodisiac and for the treatment of jaundice. In the effort of searching new botanical drugs and drug candidates from tropical rainforest, the team from this laboratory had conducted a sizeable phytochemical and biological screening program of tropical plant at Endau Rompin State Park, Pahang with the help from the indigenous people. R. elliptica showed strong antiplasmodial activity in vitro with the IC50 value of 4.04µg/mL. The comprehensive study on the root extract of R. elliptica in this laboratory yielded seventeen compounds from four different classes, including 2 new pyranoanthraquinones, one new anthraquinone, eleven known anthraquinones, one lactone triterpenoid, one coumarin and one phenolic acid. The chemical profile of the root extract was established using HPLC and the selected marker compounds were used as external standards and quantified using standard calibration curve. Nordamnacanthal 5, damnacanthal 7, 2-formyl-3-hydroxy-9,10-anthraquinone 6, 2-methyl-3-hydroxy-9,10-anthraquinone 11 and 1,2-dimethoxy-6-methyl-9,10-anthraquinone 3 were determined at 3.57, 10.32, 4.47, 12.18 and 4.09 µg/g, respectively. Owing to the toxicity of dichloromethane, the extraction of the desired marker compounds was attempted using accelerated solvent extraction and soxhlet extraction using ethanol and water at different compositions. R. elliptica root extract and the isolated anthraquinones showed potential antiplasmodial activity, and the active compounds were probed for their mode of action. In addition, the dichloromethane root extract of R. elliptica and the selected anthraquinones were screened for anticancer, antioxidant, and α-glucosidase inhibitory activities as well as toxicity study in vitro.  The review summarizes the findings on Rennellia elliptica which includes phytochemistry, toxicity and its biological activities. The chemotaxonomic significance of Rennellia elliptica is also discussed

Adaptive Landscape by Environment Interactions Dictate Evolutionary Dynamics in Models of Drug Resistance

The adaptive landscape analogy has found practical use in recent years, as many have explored how their understanding can inform therapeutic strategies that subvert the evolution of drug resistance. A major barrier to applications of these concepts is a lack of detail concerning how the environment affects adaptive landscape topography, and consequently, the outcome of drug treatment. Here we combine empirical data, evolutionary theory, and computer simulations towards dissecting adaptive landscape by environment interactions for the evolution of drug resistance in two dimensions-drug concentration and drug type. We do so by studying the resistance mediated by Plasmodium falciparum dihydrofolate reductase (DHFR) to two related inhibitors-pyrimethamine and cycloguanil-across a breadth of drug concentrations. We first examine whether the adaptive landscapes for the two drugs are consistent with common definitions of cross-resistance. We then reconstruct all accessible pathways across the landscape, observing how their structure changes with drug environment. We offer a mechanism for non-linearity in the topography of accessible pathways by calculating of the interaction between mutation effects and drug environment, which reveals rampant patterns of epistasis. We then simulate evolution in several different drug environments to observe how these individual mutation effects (and patterns of epistasis) influence paths taken at evolutionary "forks in the road" that dictate adaptive dynamics in silico. In doing so, we reveal how classic metrics like the IC50 and minimal inhibitory concentration (MIC) are dubious proxies for understanding how evolution will occur across drug environments. We also consider how the findings reveal ambiguities in the cross-resistance concept, as subtle differences in adaptive landscape topography between otherwise equivalent drugs can drive drastically different evolutionary outcomes. Summarizing, we discuss the results with regards to their basic contribution to the study of empirical adaptive landscapes, and in terms of how they inform new models for the evolution of drug resistance.

Adipose-derived stem cells retain their regenerative potential after methotrexate treatment

In musculoskeletal tissues like bone, chemotherapy can impair progenitor cell differentiation and proliferation, resulting in decreased bone growth and mineralization throughout a patient׳s lifetime. In the current study, we investigated the effects of chemotherapeutics on adipose-derived stem cell (ASC) function to determine whether this cell source could be a candidate for repairing, or even preventing, chemotherapy-induced tissue damage. Dose-dependent proliferation rates of ASCs and normal human fibroblasts (NHFs) were quantified after treatment with cytarabine (CY), etoposide (ETO), methotrexate (MTX), and vincristine (VIN) using a fluorescence-based assay. The influence of MTX on the multipotency of ASCs and freshly isolated stromal vascular fraction (SVF) cells was also evaluated using lineage-specific stains and spectrophotometry. ASC and NHF proliferation were equally inhibited by exposure to CY and ETO; however, when treated with MTX and VIN, ASCs exhibited greater resistance. This was especially apparent for MTX-treated samples, with ASC proliferation showing no inhibition for clinically relevant MTX doses ranging from 0.1 to 50 μM. Additional experiments revealed that the differentiation potential of ASCs was not affected by MTX treatment and that upregulation of dihydrofolate reductase possibly contributed to this response. Moreover, SVF cells, which include ASCs, exhibited similar resistance to MTX impairment, with respect to cellular proliferation, clonogenicity, and differentiation capability. Therefore, we have shown that the regenerative properties of ASCs resist the cytotoxicity of MTX, identifying these cells as a potential key for repairing musculoskeletal damage in patients undergoing chemotherapy.

Genetic diversity of Plasmodium falciparum and distribution of drug resistance haplotypes in Yemen

Background

Despite evident success of malaria control in many sites in the Arabian Peninsula, malaria remains endemic in a few spots, in Yemen and south-west of Saudi Arabia. In addition to local transmission, imported malaria sustains an extra source of parasites that can challenge the strengths of local control strategies. This study examined the genetic diversity of Plasmodium falciparum in Yemen and mutations of drug resistant genes, to elucidate parasite structure and distribution of drug resistance genotypes in the region.

Methods

Five polymorphic loci (MSP-2, Pfg377 and three microsatellites on chromosome 8) not involved in anti-malarial drug resistance, and four drug resistant genes (pfcrt, pfmdr1, dhfr and dhps) were genotyped in 108 P. falciparum isolates collected in three sites in Yemen: Dhamar, Hodeidah and Taiz.

Results

High diversity was seen in non-drug genes, pfg377 (He = 0.66), msp-2 (He = 0.80) and three microsatellites on chr 8, 7.7 kb (He = 0.88), 4.3 kb (He = 0.77) and 0.8 kb (He = 0.71). There was a high level of mixed-genotype infections (57%), with an average 1.8 genotypes per patient. No linkage disequilibrium was seen between drug resistant genes and the non-drug markers (p < 0.05). Genetic differentiation between populations was low (most pair-wise F_ST values <0.03), indicating extensive gene flow between the parasites in the three sites.

There was a high prevalence of mutations in pfmdr1, pfcrt and dhfr; with four mutant pfmdr1 genotypes (NFCDD[57%], NFSND[21%], YFCDD[13%] and YFSND[8% ]), two mutant pfcrt genotypes (CVIET[89%] and SVMNT[4%]) and one mutant dhfr genotype (ICNI[53.7%]). However, no dhps mutations were detected.

Conclusion

The high diversity of P. falciparum in Yemen is indicative of a large parasite reservoir, which represents a challenge to control efforts. The presence of two distinct pfcrt genotype, CVIET and SVMNT, suggests that chloroquine resistance can possibly be related to a migratory path from Africa and Asia. The absence of the triple mutant dhfr genotype (IRN) and dhps mutations supports the use of artesunate + sulphadoxine-pyrimethamine as first-line therapy. However, the prevalent pfmdr1 genotype NFSND [21%] has previously been associated with tolerance/resistance response to artemisinin combination therapy (ACT). Regular surveys are, therefore, important to monitor spread of pfmdr1 and dhfr mutations and response to ACT.

Distribution of drug resistance genotypes in Plasmodium falciparum in an area of limited parasite diversity in Saudi Arabia

Two hundred and three Plasmodium falciparum isolates from Jazan area, southwest Saudi Arabia, were typed for Pfcrt, Pfmdr1, dhps, and dhfr mutations associated with resistance to chloroquine, mefloquine, halofantrine, artemisinin, sulfadoxine-pyrimethamine, and the neutral polymorphic gene Pfg377. A large proportion (33%) of isolates harbored double mutant dhfr genotype (51I,59C,108N). However, only one isolate contained mutation dhps-437G. For Pfcrt, almost all examined isolates (163; 99%) harbored the mutant genotype (72C,73V,74I,75E,76T), whereas only 49 (31%) contained the mutant Pfmdr1 genotype (86Y,184F,1034S,1042N), 109 (66%) harbored the single mutant genotype (86N,184F,1034S,1042N), and no mutations were seen in codons 1034, 1042, and 1246. Nonetheless, three new single-nucleotide polymorphisms were detected at codons 182, 192, and 102. No differences were seen in distribution of drug resistance genes among Saudis and expatriates. There was a limited multiplicity (5%), mean number of clones (1.05), and two dominant multilocus genotypes among infected individuals in Jazan. A pattern consistent with limited cross-mating and recombination among local parasite was apparent.

Does the drug sensitivity of malaria parasites depend on their virulence?

Background

Chemotherapy can prompt the evolution of classical drug resistance, but selection can also favour other parasite traits that confer a survival advantage in the presence of drugs. The experiments reported here test the hypothesis that sub-optimal drug treatment of malaria parasites might generate survival and transmission advantages for virulent parasites.

Methods

Two Plasmodium chabaudi lines, one derived from the other by serial passage, were used to establish avirulent and virulent infections in mice. After five days, infections were treated with various doses of pyrimethamine administered over 1 or 4 days. Virulence measures (weight and anaemia), parasite and gametocyte dynamics were followed until day 21.

Results

All treatment regimes reduced parasite and gametocyte densities, but infections with the virulent line always produced more parasites and more gametocytes than infections with the avirulent line. Consistent with our hypothesis, drug treatment was disproportionately effective against the less virulent parasites. Treatment did not affect the relative transmission advantage of the virulent line. Neither of the lines contained known mutations conferring classical drug resistance.

Conclusion

Drug-sensitivity of malaria parasites can be virulence-dependent, with virulent parasites more likely to survive sub-optimal treatment. If this proves to be general for a variety of drugs and parasite species, selection imposed by sub-optimal drug treatment could result in the evolution of more aggressive malaria parasites.

Cloning, expression, and characterization of Babesia gibsoni dihydrofolate reductase-thymidylate synthase: inhibitory effect of antifolates on its catalytic activity and parasite proliferation

Dihydrofolate reductase-thymidylate synthase (DHFR-TS) is a well-validated antifolate drug target in certain pathogenic apicomplexans, but not in the genus Babesia, including Babesia gibsoni. Therefore, we isolated, cloned, and expressed the wild-type B. gibsoni dhfr-ts gene in Escherichia coli and evaluated the inhibitory effect of antifolates on its enzyme activity, as well as on in vitro parasite growth. The full-length gene consists of a 1,548-bp open reading frame encoding a 58.8-kDa translated peptide containing DHFR and TS domains linked together in a single polypeptide chain. Each domain contained active-site amino acid residues responsible for the enzymatic activity. The expressed soluble recombinant DHFR-TS protein was approximately 57 kDa after glutathione S-transferase (GST) cleavage, similar to an approximately 58-kDa native enzyme identified from the parasite merozoite. The non-GST fusion recombinant DHFR enzyme revealed K(m) values of 4.70 +/- 0.059 (mean +/- standard error of the mean) and 9.75 +/- 1.64 microM for dihydrofolic acid (DHF) and NADPH, respectively. Methotrexate was a more-potent inhibitor of the enzymatic activity (50% inhibition concentration [IC(50)] = 68.6 +/- 5.20 nM) than pyrimethamine (IC(50) = 55.0 +/- 2.08 microM) and trimethoprim (IC(50) = 50 +/- 12.5 microM). Moreover, the antifolates' inhibitory effects on DHFR enzyme activity paralleled their inhibition of the parasite growth in vitro, indicating that the B. gibsoni DHFR could be a model for studying antifolate compounds as potential drug candidates. Therefore, the B. gibsoni DHFR-TS is a molecular antifolate drug target.

Folate metabolism as a source of molecular targets for antimalarials

Folate metabolism of the malaria parasites provides two targets for current antimalarials: dihydrofolate reductase and dihydropteroate synthase. Dihydrofolate reductase inhibitors have been used as antimalarials over the past few decades, often in combination with dihydropteroate synthase inhibitors. Resistance to these antifolate drugs developed through mutations in both target enzymes. However, limited mutation possibilities gave opportunities for the development of new drugs. Furthermore, other enzymes in the folate and related pathways are potential new targets that remain to be exploited. These include thymidylate synthase, an enzyme fused with dihydrofolate reductase in the same protein chain, serine hydroxymethyltransferase, methylene tetrahydrofolate dehydrogenase, methionine synthase and enzymes in the glycine cleavage pathway.

Plasmodium berghei: the application of cultivation and purification techniques to molecular studies of malaria parasites

Species of malaria parasites that infect rodents provide models for the study of the biology of malaria parasites that infect humans. In this article, Chris Janse and Andy Waters describe some of the recent advances in the cultivation and purification methodology of one of these species, Plasmodium berghei. The improvement of these techniques, and the increasing knowledge about the molecular biology of P. berghei enhance the value of this particular rodent model for the investigation of many aspects of the biology of Plasmodium.

Synthesis of 5′-methylenearisteromycin and its 2-fluoro derivative with potent antimalarial activity due to inhibition of the parasite S-adenosylhomocysteine hydrolase1

5'-methylenearisteromycin 5 and its 2-fluoro derivative 6, which were designed as antimalarial agents because of their AdoHcy hydrolase inhibition, were synthesized from D-ribose, using a stereoselective intramolecular radical cyclization as the key step to construct the carbocyclic structure. These compounds were evaluated as AdoHcy hydrolase inhibitors with the recombinant human and malarial parasite enzymes. Although 5 and 6 were both potent inhibitors of the malarial parasite AdoHcy hydrolase, the 2-fluoro derivative 6 proved to be superior due to its lower inhibitory effect on the human enzyme. In addition, 6 was identified as a potent antimalarial agent using an in vitro assay system with Plasmodium falciparum.

Proteomics in Malaria

The recent completion of human, Anopheles gambiae, and Plasmodium falciparum genomes relevant to the study of human malaria allows the application of modern proteomic technologies to complement previously implemented conventional approaches. Proteomic analysis has been employed to elucidate global protein expression profiles, subcellular localization of gene products, and host-pathogen interactions that are central to disease pathogenesis and treatment. The high-throughput nature of these techniques is in accord with the pace of drug and vaccine development that have the potential to directly reduce the morbidity and mortality of disease.

Efficacy of proton pump inhibitor drugs against Plasmodium falciparum in vitro and their probable pharmacophores

The substituted benzimidazoles omeprazole, lansoprazole, rabeprazole, and pantoprazole were found to have in vitro activity against three different isolates of Plasmodium falciparum: D6 (which is chloroquine and pyrimethamine sensitive), W2 (chloroquine and pyrimethamine resistant), and TM91C235 (multidrug resistant). Lansoprazole and rabeprazole were the most effective against all three isolates, with a 50% inhibitory concentration (IC(50)) range of 7 to 11 microM. Omeprazole showed intermediate activity against D6 and W2 isolates, with IC(50)s of 27 to 28 microM, but had poor activity against TM91C235, with an IC(50) of 76 microM. Pantoprazole was the least effective, with IC(50)s of 73 microM against D6, 53 microM against W2, and 39 microM against TM91C235. A pharmacophore model describing the important features responsible for potent activity of the drugs was developed using computational techniques of semiempirical quantum chemical methods and the three-dimensional QSAR procedure of the CATALYST software. The important features of the pharmacophore, according to the findings based on the CATALYST procedures, are the hydrogen bond acceptor and donor sites at the benzimidine nitrogen atoms and the two aromatic hydrophobic sites in the molecules. AM1 quantum chemical calculations identified the electrostatic potential surface surrounding the sulfoxide atom as crucial for potent activity.

New neplanocin analogues. 12. Alternative synthesis and antimalarial effect of (6'R)-6'-C-methylneplanocin A, a potent AdoHcy hydrolase inhibitor

An improved method for the synthesis of (6'R)-6'-C-methylneplanocin A (RMNPA, 2), a potent S-adenosyl-L-homocysteine (AdoHcy) hydrolase inhibitor, was developed via a chelation-controlled stereoselective addition of MeTiCl(3) to the neplanocin A 6'-aldehyde derivative 6. Compound 2 effectively inhibited the growth of malaria parasites both in vitro and in vivo. The antimalarial EC(50) value of 2 against Plasmodium berghei in mice was 1.0 mg/kg/day, which was superior to that of chloroquine (EC(50) = 1.8 mg/kg/day).

Genome comparison of progressively drug resistant Plasmodium falciparum lines derived from drug sensitive clone

Chloroquine has been the mainstay of malaria chemotherapy for the past five decades, but resistance is now widespread. Pyrimethamine or proguanil form an important component of some alternate drug combinations being used for treatment of uncomplicated Plasmodium falciparum infections in areas of chloroquine resistance. Both pyrimethamine and proguanil are dihydrofolate reductase (DHFR) inhibitors, the proguanil acting primarily through its major metabolite cycloguanil. Resistance to these drugs arises due to specific point mutations in the dhfr gene. Cross resistance between cycloguanil and pyrimethamine is not absolute. It is, therefore, important to investigate mutation rates in P. falciparum for pyrimethamine and proguanil so that DHFR inhibitor with less mutation rate is favored in drug combinations. Hence, we have compared mutation rates in P. falciparum genome for pyrimethamine and cycloguanil. Using erythrocytic stages of P. falciparum cultures, progressively drug resistant lines were selected in vitro and comparing their RFLP profile with a repeat sequence. Our finding suggests that pyrimethamine has higher mutation rate compared to cycloguanil. It enhances the degree of genomic polymorphism leading to diversity of natural parasite population which in turn is predisposes the parasites for faster selection of resistance to some other antimalarial drugs.

The ABC transporter genes of Plasmodium falciparum and drug resistance

The seminal observations that (a) chloroquine-resistant Plasmodium falciparum strains accumulate less drug than more sensitive parasites, and (b) chloroquine resistance could be modulated in vitro by the classic multidrug-resistance (MDR) modulator verapamil, suggested not only that parasite resistance to multiple drugs may be similar to the MDR phenotype described in mammalian cancer cells, but that homologous proteins may be involved. These findings prompted search for MDR-like genes in the parasite. To date, three full-length ABC transporter genes have been isolated from P. falciparum: two P-glycoprotein-like homologues, pfmdr1 and pfmdr2, and a homologue of the yeast GCN20 gene, pfgcn20.

Positive and negative control of multidrug resistance by the Sit4 protein phosphatase in Kluyveromyces lactis

The nuclear gene encoding the Sit4 protein phosphatase was identified in the budding yeast Kluyveromyces lactis. K. lactis cells carrying a disrupted sit4 allele are resistant to oligomycin, antimycin, ketoconazole, and econazole but hypersensitive to paromomycin, sorbic acid, and 4-nitroquinoline-N-oxide (4-NQO). Overexpression of SIT4 leads to an elevation in resistance to paromomycin and to lesser extent tolerance to sorbic acid, but it has no detectable effect on resistance to 4-NQO. These observations suggest that the Sit4 protein phosphatase has a broad role in modulating multidrug resistance in K. lactis. Expression or activity of a membrane transporter specific for paromomycin and the ABC pumps responsible for 4-NQO and sorbic acid would be positively regulated by Sit4p. In contrast, the function of a Pdr5-type transporter responsible for ketoconazole and econazole extrusion, and probably also for efflux of oligomycin and antimycin, is likely to be negatively regulated by the phosphatase. Drug resistance of sit4 mutants was shown to be mediated by ABC transporters as efflux of the anionic fluorescent dye rhodamine 6G, a substrate for the Pdr5-type pump, is markedly increased in sit4 mutants in an energy-dependent and FK506-sensitive manner.

Potent in vivo antimalarial activity of 3,15-di-O-acetylbruceolide against Plasmodium berghei infection in mice

The antimalarial activity of the O-acylated bruceolide derivative, 3,15-di-O-acetylbruceolide, was evaluated against Plasmodium berghei in vivo. The concentration of 3,15-di-O-acetylbruceolide required for 50% suppression (ED50) of P. berghei in mice was 0.46 +/- 0.06 mg/kg/day, whereas bruceolide was only half as effective as 3,15-di-O-acetylbruceolide. Two antimalarial drugs used clinically, chloroquine and artemisinin, demonstrated only low activity corresponding to 1/4 and 1/12 of the ED50 value of 3,15-di-O-acetylbruceolide, respectively. These results may be helpful in the design of better chemotherapeutic bruceolides against falciparum malaria.

Antimalarial activity of 20 crude extracts from nine African medicinal plants used in Kinshasa, Congo

Twenty extracts including ten EtOH and ten CH2Cl2 from different parts of nine African medicinal plants used in Congolese traditional medicine for the treatment of malaria, were submitted to a pharmacological test in order to evaluate their effect on P. falciparum growth in vitro. Of these plant species, 14 (70%) extracts including EtOH and CH2Cl2 from Cassia occidentalis leaves, Cryptolepis sanguinolenta root bark, Euphorbia hirta whole plant, Garcinia kola stem bark and seeds, Morinda lucida leaves and Phyllanthus niruri whole plant produced more than 60% inhibition of the parasite growth in vitro at a test concentration of 6 microg/ml. Extracts from E. hirta, C. sanguinolenta and M. morindoides showed a significant chemosuppression of parasitaemia in mice infected with P. berghei berghei at orally given doses of 100-400 mg/kg per day.

Cloning and partial characterization of the proteasome S4 ATPase from Plasmodium falciparum

Certad, G., Abrahem, A., and Georges, E. 1999. Cloning and Partial characterization of the proteasome S4 ATPase from Plasmodium falciparum. Experimental Parasitology 93, 123-131. The ATP-ubiquitin-proteasome pathway mediates the nonlysosomal degradation of cytosolic proteins in eukaryotic cells. The activities of this pathway have been shown to regulate cell growth and differentiation through modulation of regulatory proteins. The proteasome is a large complex consisting of two multisubunit structures, the 20S and 19S(PA700) or P28 complexes, that combine to form the 26S particles. In this study, we describe the cloning of a cDNA encoding the proteasome subunit 4 ATPase homologue from Plasmodium falciparum (PFS4). Analysis of the PFS4 cDNA sequence shows an open reading frame encoding a deduced protein of 455 amino acids. Moreover, comparison of PFS4 cDNA sequence to that of genomic fragments encoding PFS4 showed identical sequences with no detectable introns. Database searches revealed a high sequence identity to those of rice, yeast, mouse, Drosophila, and human S4 ATPases. However, PFS4 contains two unique inserts of nine and seven amino acid residues in the N-terminal domain. Interestingly, only the rice S4 contains the latter (seven amino acids) insert with four identical amino acids. In vitro expression of the full-length cDNA encoding the PFS4, using a transcription-translation-coupled reticulocyte lysate, shows a 50-kDa [(35)S]methionine-labeled protein which was immunoprecipitated with PFS4 anti-peptide antiserum. Southern blot analysis of genomic DNA digests shows a single gene copy of PFS4 in P. falciparum. Of interest was the effect of the proteasome-specific natural product, lactacystin, on the growth of the parasite, with IC(50) values of 0.6-0.92 microM. The latter IC(50) values of lactacystin for different clones of P. falciparum are comparable to those obtained for mammalian cell lines (0.65 microM), suggesting the presence of a conserved proteasome complex. Moreover, lactacystin was equally toxic to drug-sensitive and resistant parasites.

A chloroquine resistance locus in the rodent malaria parasite Plasmodium chabaudi

We have located a possible chloroquine resistance locus in the genome of the rodent malaria parasite Plasmodium chabaudi. Two genetically distinct clones of the parasite were grown in vivo and allowed to undergo genetic crossing. The clones differed from each other in their susceptibility to chloroquine; AS(3CQ) had been selected for a low level of resistance to the drug whereas AJ is chloroquine-sensitive. Independent recombinant progeny (20) were cloned from the products of two crosses, phenotyped for their susceptibility to chloroquine, and genotyped for their inheritance of 46 chromosome-specific markers. No association was found between chloroquine susceptibility and the inheritance of pcmdr1, the P. chabaudi homologue of the pfmdr1 multi-drug resistance gene of P. falciparum. Also, there was no association between chloroquine susceptibility and the inheritance of a marker linked to a putative chloroquine resistance locus in a P. falciparum cross. However, 16 of the progeny clones showed co-segregation of four linked markers on chromosome 11 with their resistance phenotype. This result suggests that a locus for chloroquine resistance exists on this chromosome in P. chabaudi.

Antimalarial and cytotoxic potential of four quassinoids from Hannoa chlorantha and Hannoa klaineana, and their structure-activity relationships

Hannoa chlorantha and Hannoa klaineana (Simaroubaceae) are used in traditional medicine of Central African countries against fevers and malaria. Four stem bark extracts from H. klaineana and four quassinoids from H. chlorantha were examined in vitro against Plasmodium falciparum NF 54. The extracts displayed good activities, while the quassinoids were highly active, with IC50 values well below 1 microgram ml-1, those of chaparrinone and 15-desacetylundulatone being much lower than 0.1 microgram ml-1 (0.037 and 0.047 microgram ml-1, respectively). Chaparrinone is five times more active than 14-hydroxychaparrinone against P. falciparum, indicating that the hydroxyl function at C-14 is unfavourable for antiplasmodial activity. As 14-hydroxychaparrinone has a seven-times higher cytotoxic activity against P-388 cells than chaparrinone, the latter compound has the better antiplasmodial therapeutic index. All four quassinoids were evaluated in vivo in a standard 4-day test as well. 15-Desacetylundulatone was proven to be the most active compound, almost totally suppressing the parasitaemias of OF1 mice for at least 7 days, while both chaparrinone and 14-hydroxychaparrinone were active for at least 4 days. Quassinoids have ED50 values much lower than 50 mg kg-1 body weight day-1 and none of them caused obvious side effects. The keto function at C-2 in 15-desacetylundulatone is apparently of crucial importance for its high activity. 6-alpha-Tigloyloxyglaucarubol was not active at all. Chaparrinone is considered the most interesting of the investigated quassinoids and its in-vivo antimalarial potential will be examined further.

The efficacy of benzimidazole drugs against Plasmodium falciparum in vitro

The sensitivities in vitro of Plasmodium falciparum to the benzimidazoles, albendazole, thiabendazole, mebendazole, omeprazole and 2 albendazole metabolites, albendazole sulphone and albendazole sulphoxide, were investigated and compared to those of the commonly used antimalarial drugs chloroquine and quinine. Quinine and chloroquine were the most potent drugs tested (EC50 values of 8 x 10(-9)-6 x 10(-8) mol/L and 5-7 x 10(-9) mol/L, respectively). Thiabendazole, mebendazole, albendazole sulphone and albendazole sulphoxide reached maximum growth inhibitions of 13-36% at the highest concentration tested (1 x 10(-4) mol/L). Albendazole (EC50 range: not achieved-2 x 10(-6) mol/L) and omeprazole (EC50 range: 2-4 x 10(-5) mol/L) were the most effective benzimidazoles. The activity of albendazole was pH dependent, as was that of chloroquine, and variable. Albendazole has its primary mode of action on trophozoites, suggesting that the drug may target parasite tubulin polymerization. Omeprazole, although also primarily effective against trophozoites, had additional activity against schizonts and ring forms, suggesting a distinct or additional parasitic target. Given the variable activity of albendazole and its rapid metabolism in vivo into compounds with even less antimalarial activity, it appears unlikely that this benzimidazole will be useful in the treatment of malaria. The rapid activity and different stage-specific profile of the more soluble benzimidazole omeprazole warrants further investigation.

The mechanisms of action of antiprotozoal and anthelmintic drugs in man

The mechanisms of action of antiprotozoal and anthelmintic drugs are reviewed according to: (1) drugs interfering with metabolic processes; (2) drugs interfering with reproduction and larval physiology; and (3) drugs interfering with neuromuscular physiology of parasites.

Multigenic drug resistance among inbred malaria parasites

Recent population genetic studies on the malaria parasite Plasmodium falciparum have confirmed that selfing is more frequent where the transmission rate is lower, with inbreeding coefficients estimated to be 0.33 and 0.92 for sites in Tanzania and Papua New Guinea (PNG), respectively. These geographical differences in Plasmodium mating patterns have been linked to the rate of spread of chloroquine resistance CQR, which, according to some measures, has been slower in Tanzania than in PNG. It has been proposed that the former observation explains the latter, although the theoretical argument linking the two is based on limited simulation studies. Taking a more analytical approach here, we first establish the relevant relationship between the coefficient of inbreeding (F, within loci) and the recombination rate r, between loci, defining an 'effective recombination rate', r = r (1-F. We then show that the emergence of multigenic drug resistance can indeed be slowed (or even quickened) by more outcrossing, but only when resistance is determined by two or more genes, none of which independently confers significant protection. The resistance genes should both be initially rare, and subject to low selection pressure. The analysis does not completely discount the hypothesis that inbreeding significantly influences the spread of CQR, but we show that it can only do so under a restrictive set of conditions, and that these conditions are not satisfied by some laboratory and field data. We discuss some of the wider implications of these results for the evolution of multigenic resistance.

4-aminoquinoline analogs of chloroquine with shortened side chains retain activity against chloroquine-resistant Plasmodium falciparum

We have synthesized several 4-aminoquinolines with shortened side chains that retain activity against chloroquine-resistant isolates of Plasmodium falciparum malaria (W. Hofheinz, C. Jaquet, and S. Jolidon, European patent 94116281.0, June 1995). We report here an assessment of the activities of four selected compounds containing ethyl, propyl, and isopropyl side chains. Reasonable in vitro activity (50% inhibitory concentration, < 100 nM) against chloroquine-resistant P. falciparum strains was consistently observed, and the compounds performed well in a variety of plasmodium berghei animal models. However, some potential drawbacks of these compounds became evident upon in-depth testing. In vitro analysis of more than 70 isolates of P. falciparum and studies with a mouse in vivo model suggested a degree of cross-resistance with chloroquine. In addition, pharmacokinetic analysis demonstrated the formation of N-dealkylated metabolites of these compounds. These metabolites are similarly active against chloroquine-susceptible strains but are much less active against chloroquine-resistant strains. Thus, the clinical dosing required for these compounds would probably be greater for chloroquine-resistant strains than for chloroquine-susceptible strains. The clinical potential of these compounds is discussed within the context of chloroquine's low therapeutic ratio and toxicity.

In vitro stage-specific sensitivity of Plasmodium falciparum to quinine and artemisinin drugs

The inhibitory effects of quinine, chloroquine and 4 qinghaosu drugs, artemisinin, artemether, artesunate and dihydroartemisinin, on 4 culture-adapted isolates and 2 standard clones of Plasmodium falciparum were determined in vitro. All isolates were sensitive to the widely used antimalarial drugs quinine (EC50 range 3 x 10(-8)-1 x 10(-7) mol/L) and chloroquine (EC50 range 1 x 10(-9)-7 x 10(-9) mol/L), irrespective of the geographical origin or treatment history of the patients from which they were taken. In general, the qinghaosu drugs were more potent than the conventional antimalarials, having EC50 values of 3 x 10(-11)-3 x 10(-8) mol/L. Stage-specific data indicated that quinine has a primary mode of action on mature parasite forms, achieving 80-100% growth inhibition within 2-4 h of drug exposure. The stage-specific activity of the 3 qinghaosu drugs artemisinin, artemether and dihydroartemisinin differed from that of quinine, and each derivative displayed a unique stage-specific profile. Artemisinin was rapidly effective against both rings and schizonts, achieving 100% growth inhibition within 6-8 h. The inhibitory effects of artemether were less rapid, requiring 10 h to achieve 70-80% ring stage growth inhibition. Dihydroartemisinin was highly effective against all parasite stages in most cases achieving 100% growth inhibition within 2-4 h of exposure. The results confirm that the qinghaosu drugs are potent antimalarials, and suggest different stage-specific profiles compared to conventional antimalarial drugs.

Dreamtime, devastation and deviation: Australia's contribution to the chemotherapy of human parasitic infections

There are three areas in which Australian scientists have made outstanding contributions to the study of the chemotherapy of human parasitic infections. Naturally occurring products of plants have great potential as antiparasitic agents and although several native species have been shown to have antimalarial and anthelmintic activity, their potential as chemotherapeutic agents has not been fully realised; secondly, the demands of war ensured that the Army Malaria Unit at Cairns carried out meticulous and exceptional studies to evaluate new antimalarial compounds. Not only were they able to prove the effectiveness of atebrin, Proguanil and chloroquine as prophylactics, they also obtained much new information on the pharmacokinetics of antimalarials and about the infection itself. Full recognition of these pioneering studies involving over 1000 volunteers infected with malaria, which can never be repeated, has not been appreciated. The third significant contribution is the molecular studies on the mechanisms of drug resistance Plasmodium falciparum of both the antifolate- and quinoline-containing drugs and the identification and subsequent biochemical and molecular analysis of drug resistance in Giardia intestinalis infections.

Drug-resistant malaria in children and in travelers

Malaria remains a significant cause of childhood morbidity and mortality worldwide. Drug resistance in Plasmodium falciparum has become widespread in the past 30 years, and in some parts of the world multidrug resistance is common. Chloroquine resistance in Plasmodium vivax has recently been recognized in Indonesia. The mechanisms of drug resistance have been defined for the antifolate antimalarial agents but remain incompletely understood for the quinolines. Judicious use of antimalarial compounds will be essential to prevent the emergence and spread of further drug resistance. The history, geographic distribution, and mechanisms of drug resistance are reviewed, together with current recommendations regarding prophylaxis and therapy.

Conserved location of genes on polymorphic chromosomes of four species of malaria parasites

The number of chromosomes and the chromosomal location and linkage of more than 50 probes, mainly of genes, have been established in four species of Plasmodium which infect African murine rodents. We expected that the location and linkage of genes would not be conserved between these species of malaria parasites since extensive inter- and intraspecific size differences of the chromosomes existed and large scale internal rearrangements and chromosome translocations in parasites from laboratory lines had been reported. Our study showed that all four species contained 14 chromosomes, ranging in size between 0.5 and 3.5 Mb, which showed extensive size polymorphisms. The location and linkage of the genes on the polymorphic chromosomes, however, was conserved and nearly identical between these species. These results indicate that size polymorphisms of the chromosomes are more likely due to variation in non-coding (subtelomeric, repeat) sequences and show that a high plasticity of internal regions of chromosomes that may exist does not frequently affect chromosomal location and linkage of genes.

In vitro drug interaction between amantadine and classical antimalarial drugs in Plasmodium falciparum infections

The interactions of amantadine with classical antimalarial drugs were evaluated against a chloriquine-resistant and a chloroquine-sensitive strain of Plasmodium falciparum in vitro. Amantadine potentiated the effect of chloroquine and quinine in both strains; it also potentiated the effect of mefloquine, halofantrine and primaquine in the chloroquine-resistant strain but had no effect in the chloroquine-sensitive strain. Amantadine had no effect on the response to pyrimethamine of either strain. Amantadine does not interfere with the activity of these compounds and may possibly enhance it.

The susceptibility of the malarial parasite Plasmodium falciparum to quinoline-containing drugs is correlated to the lipid composition of the infected erythrocyte membranes

The anti-malarial action of quinoline-containing compounds depends on various membrane-related processes, and drug resistance could depend, among other factors, on the membrane lipid composition. To verify this hypothesis, the constitution of phospholipid classes and the content of cholesterol of various strains of Plasmodium falciparum-infected human erythrocytes grown in in vitro cultures have been assessed in conjunction with drug susceptibility. It was found that uninfected erythrocytes in the culture serve as a major source for the increased lipid content of malaria-infected cells. Alterations of the phospholipid composition of infected cells that result from parasite lipid metabolism are also reflected in the constitution of uninfected red cells, implying lipid exchange between infected and uninfected cells. An inverse relationship between the content of acidic phospholipids and cholesterol has been observed. Some strains resistant to chloroquine and quinine were sensitive to mefloquine, and vice versa. Resistance to chloroquine or quinine was found to be directly related to the content of acidic phospholipids, while that of mafloquine displayed an inverse correlation. Concomitantly, the resistance to chloroquine was inversely related to the content of cholesterol, while the sensitivity to mefloquine decreased with cholesterol concentration. The possible mechanisms that could account for these observations are briefly discussed.

Quinoline-containing antimalarials--mode of action, drug resistance and its reversal. An update with unresolved puzzles

Malaria constitutes one of the major health threats in the tropical and sub-tropical areas of the world. Yet, few advances were made in recent years in revealing the mode of action of the common and most economically affordable antimalarial drugs, the schizontocidal 4-aminoquinolines. Data presented indubitably repudiate the previous notions that these drugs act by either halting the feeding of the parasite on its host erythrocyte cytosol or repressing nucleic acid synthesis due to intercalation into the parasite's DNA. A novel target for drugs is outlined, i.e. they are shown to inhibit in vitro the release of iron from acidified host cell cytosol, consisting mostly of hemoglobin, a process that could provide this trace element to the parasite. Resistance to quinoline-containing drugs is the principal reason for the present resurgence of malaria. Drug-resistant parasites accumulate less of these weak base-like drugs in the acidic digestive vacuoles. A kinetic model is presented, indicating that diminishing drug accumulation is due to decreased vacuolar proton pump activity and is not a result of a putative multidrug resistance (MDR) efflux pump. Findings to date on the molecular biology of parasite mdr genes are reviewed. These indicate no correlation between gene expression or mutations and phenotypic drug resistance. Reversal of parasite drug resistance by relevant compounds in MDR cancer cells seems to involve mechanism(s) different from the inhibition of the MDR pump in cancer cells.