Molecular genetics of drug resistance in Plasmodium faiciparum malaria

Association between chloroquine resistance phenotypes and point mutations in pfcrt and pfmdr1 in Plasmodium falciparum isolates from Thailand

The relationship between the in vitro susceptibility of Plasmodium falciparum isolates to the quinoline antimalarials chloroquine (CQ), mefloquine (MQ), and quinine (QN), and pfcrt and pfmdr1 gene polymorphisms were investigated. Field isolates (110 samples) were collected from various endemic areas of Thailand throughout 2002-2004. The pfcrt 76T allele was identified in 109 isolates (99.1%) while pfcrt 76K was found in a single (0.9%) isolate. The pfmdr 86N, 86Y, and the combination (86N+86Y) alleles were identified in 83 (75.5%), 22 (20%), and 5 (4.5%) isolates, respectively. The pfmdr1 1042N, 1042D alleles and a mixture (1042N+1042D) of the alleles were found in 94 (85.5%), 12 (10.9%) and 4 (3.6%) isolates, respectively. The pfmdr1 1246Y allele was detected in a single (0.9%) isolate. The pfmdr1 gene polymorphisms (86-1042-1246) was grouped into seven haplotypes as follows: N-N-D (68 isolates; 61.2%), Y-N-D (22 isolates; 19.8%), N-D-D (11 isolates; 9.9%), N-D-Y (1 isolate; 0.9%), N/Y-N-D (4 isolates; 3.6%), N-N/D-D (3 isolates; 2.7%), and N/Y-N/D-D (1 isolate; 0.9%). Eight different combinations of pfcrt-pfmdr1 genotypes were observed. Only one CQ-, MQ- and QN-sensitive isolate was found at the Thai-Laos border and no cases of QN resistance were found in this study.

Blood schizontocidal activity of selected 1,2,4-trioxanes (Fenozans) against the multidrug-resistant strain of Plasmodium yoelii nigeriensis (MDR) in vivo

Blood schizontocidal activity of 10 selected cis-fused cyclopenteno-1,2,4-trioxanes (namely Fenozan compound nos 6, 7, 11, 27, 32, 39, 44, 45, 48 and 51) have been re-investigated to establish their curative doses against the multidrug-resistant Plasmodium yoelii nigeriensis strain, which is lethal in Swiss mice. Freshly prepared formulations of these compounds prepared either in neutral groundnut (peanut) oil or in dimethyl sulfoxide (DMSO)-Tween-water, were compared for their antimalarial activity. Only 2 compounds, namely Fenozan derivatives 11 and 45, formulated in neutral groundnut oil for oral administration, showed highest activity with 100% cure rate in MDR P. yoelii nigeriensis-infected mice, while the DMSO-Tween-water formulations were inactive. Fenozan-48 produced 72.2% cure, when administered orally in groundnut oil (formulation) while its DMSO-Tween formulation was inactive. In the case of Fenozan 7, the oil and DMSO-Tween formulations produced 92.3 and 76.0% cures respectively. Fenozan derivatives nos 6, 27, 32, 39, 44 and 51 were not protective either in groundnut oil or DMSO-Tween oral formulations. The present study has applied more rigorous criteria for selection of active compounds, and has identified the 3,3-spirocyclopentane derivative Fenozan 11, and the 3,3-spirohydropyran derivative Fenozan 45, as potential blood schizontocides which can completely eliminate multidrug-resistant malaria infection in mice. Both these compounds are candidates for pre-clinical development. The present study advocates the preferred use of an oil vehicle for oral evaluation of potential antimalarial trioxanes/fenozans instead of the DMSO formulation, which gives inferior curative efficacy.

Antimalarial multi-drug resistance in Asia: mechanisms and assessment

The emergence and spread of drug-resistant parasites poses a major problem for management of Plasmodium falciparum malaria in endemic areas. Nowhere is this more apparent than in southeast Asia, where multi-drug resistance to chloroquine and sulfadoxine-pyrimethamine was exacerbated when mefloquine monotherapy began failing in the 1980s. A better understanding of mechanisms of (multi-) drug resistance is urgently warranted to monitor and guide antimalarial chemotherapy regimens more efficiently. Here we review recent advances on identification of molecular markers that can be employed in predicting in vitro and in vivo resistance in southeast Asia. Examples include amplification of PfMDR1 (P. falciparum multi-drug resistant gene 1) and mefloquine, K76T PfCRT and chloroquine, as well as mutations in the dihydroperoate synthase and dihydrofolate reductase genes and the antifolate class of drugs.

Reduced resources applied to antimalarial drug development

This statement was made in 1984 (Ref. I): 'The Agency for International Development (AID) announced a major breakthrough in the development of a vaccine against the most deadly form of malaria in human beings. The vaccine should be ready for use around the world, especially in developing countries, within five years.' Since then, the spending on development of drugs against malaria has been on the decline. Brian Schuster and Wilbur Milhous wonder. did we declare victory too soon? Wouldn't the prudent approach have been to look at vaccines and drugs as complementary techniques rather than alternative approaches?

Mining the Plasmodium genome database to define organellar function: what does the apicoplast do?

Apicomplexan species constitute a diverse group of parasitic protozoa, which are responsible for a wide range of diseases in many organisms. Despite differences in the diseases they cause, these parasites share an underlying biology, from the genetic controls used to differentiate through the complex parasite life cycle, to the basic biochemical pathways employed for intracellular survival, to the distinctive cell biology necessary for host cell attachment and invasion. Different parasites lend themselves to the study of different aspects of parasite biology: Eimeria for biochemical studies, Toxoplasma for molecular genetic and cell biological investigation, etc. The Plasmodium falciparum Genome Project contributes the first large-scale genomic sequence for an apicomplexan parasite. The Plasmodium Genome Database (http://PlasmoDB.org) has been designed to permit individual investigators to ask their own questions, even prior to formal release of the reference P. falciparum genome sequence. As a case in point, PlasmoDB has been exploited to identify metabolic pathways associated with the apicomplexan plastid, or 'apicoplast' - an essential organelle derived by secondary endosymbiosis of an alga, and retention of the algal plastid.

Hemoglobin metabolism in the malaria parasite Plasmodium falciparum

Hemoglobin degradation in intraerythrocytic malaria parasites is a vast process that occurs in an acidic digestive vacuole. Proteases that participate in this catabolic pathway have been defined. Studies of protease biosynthesis have revealed unusual targeting and activation mechanisms. Oxygen radicals and heme are released during proteolysis and must be detoxified by dismutation and polymerization, respectively. The quinoline antimalarials appear to act by preventing sequestration of this toxic heme. Understanding the disposition of hemoglobin has allowed identification of essential processes and metabolic weakpoints that can be exploited to combat this scourge of mankind.

A Plasmodium falciparum aminopeptidase gene belonging to the M1 family of zinc-metallopeptidases is expressed in erythrocytic stages

A new single copy gene has been isolated from Plasmodium falciparum, by immunoscreening a genomic DNA expression library. The gene appears devoid of introns, displays the classical A + T richness and codon usage of P. falciparum genes, and is transcribed into a 4 kb mRNA in erythrocytic stages. The deduced amino acid sequence corresponds to a 1056 residue protein (122 kDa) containing the canonical HExxHx18E signature of zinc-metallopeptidase active sites of the M1 family at position 467-490, a downstream conserved tyrosine residue involved in catalysis in position 551, and the GAMEN conserved motif characteristic of aminopeptidases in the M1 family, at position 431-435. The greatest similarities were found with aminopeptidases N of Escherichia coli and Haemophilius influenza (more than 80% identical residues in the canonical signature of the active site) but significant similarities centred on the active site region exist with all other members of the M1 family such as other prokaryotic aminopeptidases, eukaryotic aminopeptidases A and N and leukotriene A4 hydrolases (40-50% identical residues in the canonical signature of the active site). A polyclonal serum raised to a synthetic peptide deduced from the gene labelled schizont proteins of 96 and 68 kDa purified to homogeneity and both displaying aminopeptidase activity, as well as cytoplasmic structures in schizont stages.

Changes in enzyme activity and expression of DHFR of Toxoplasma gondii by antifolates

The responses to antifolates of Toxoplasma gondii were investigated by measuring the dihydrofolate reductase (DHFR) activity, quantity of DHFR mRNA, and single-strand conformational polymorphism (SSCP) pattern. Pyrimethamine (PYM) and methotrexate (MTX) were tested as antifolates. When T. gondii was treated with PYM, the viability was decreased by the increasing concentration of PYM, DHFR activity tended to increase as the passage proceeded, and the quantity of mRNA expressed was also increased according to passages. The viability of T. gondii was decreased by the increasing concentration of MTX, but it was maintained over 40% up to 100 microM MTX. DHFR activity was 77.4% in the 1st passage (1 microM). 82.2% in the 4th passage (10 microM), and 141.3% in the 7th passage (100 microM). But no changes were detected in SSCP pattern of T. gondii exposed to PYM and MTX, both. These results suggested that the response of T. gondii to PYM was regulated by transcriptional level and that, in MTX, the viability of T. gondii was derived from increasing DHFR activity.

4-Aminoquinolines--past, present, and future: a chemical perspective

The 4-aminoquinoline chloroquine (1) can be considered to be one of the most important synthetic chemotherapeutic agents in history. Since its discovery, chloroquine has proved to be a highly effective, safe, and well-tolerated drug for the treatment and prophylaxis of malaria. However, the emergence of chloroquine-resistant strains of the malarial parasite has underlined the requirement for a synthetic alternative to chloroquine. This review describes structure-activity relationships for the 4-aminoquinolines, along with views on the mechanism of action and parasite resistance. A description of drug metabolism and toxicity also is included, with a brief description of potential approaches to the design of new synthetic derivatives.

Transformation with human dihydrofolate reductase renders malaria parasites insensitive to WR99210 but does not affect the intrinsic activity of proguanil

Increasing resistance of Plasmodium falciparum malaria parasites to chloroquine and the dihydrofolate reductase (DHFR) inhibitors pyrimethamine and cycloguanil have sparked renewed interest in the antimalarial drugs WR99210 and proguanil, the cycloguanil precursor. To investigate suggestions that WR99210 and proguanil act against a target other than the reductase moiety of the P. falciparum bifunctional DHFR-thymidylate synthase enzyme, we have transformed P. falciparum with a variant form of human DHFR selectable by methotrexate. Human DHFR was found to fully negate the antiparasitic effect of WR99210, thus demonstrating that the only significant action of WR99210 is against parasite DHFR. Although the human enzyme also resulted in greater resistance to cycloguanil, no decrease was found in the level of susceptibility of transformed parasites to proguanil, thus providing evidence of intrinsic activity of this parent compound against a target other than DHFR. The transformation system described here has the advantage that P. falciparum drug-resistant lines are uniformly sensitive to methotrexate and will complement transformation with existing pyrimethamine-resistance markers in functional studies of P. falciparum genes. This system also provides an approach for screening and identifying novel DHFR inhibitors that will be important in combined chemotherapeutic formulations against malaria.

Variations in frequencies of drug resistance in Plasmodium falciparum

Continual exposure of malarial parasite populations to different drugs may have selected not only for resistance to individual drugs but also for genetic traits that favor initiation of resistance to novel unrelated antimalarials. To test this hypothesis, different Plasmodium falciparum clones having varying numbers of preexisting resistance mechanisms were treated with two new antimalarial agents: 5-fluoroorotate and atovaquone. All parasite populations were equally susceptible in small numbers. However, when large populations of these clones were challenged with either of the two compounds, significant variations in frequencies of resistance became apparent. On one extreme, clone D6 from West Africa, which was sensitive to all traditional antimalarial agents, failed to develop resistance under simple nonmutagenic conditions in vitro. In sharp contrast, the Indochina clone W2, which was known to be resistant to all traditional antimalarial drugs, independently acquired resistance to both new compounds as much as a 1,000 times more frequently than D6. Additional clones that were resistant to some (but not all) traditional antimalarial agents acquired resistance to atovaquone at high frequency, but not to 5-fluoroorotate. These findings were unexpected and surprising based on current views of the evolution of drug resistance in P. falciparum populations. Such new phenotypes, named accelerated resistance to multiple drugs (ARMD), raise important questions about the genetic and biochemical mechanisms related to the initiation of drug resistance in malarial parasites. Some potential mechanisms underlying ARMD phenotypes have public health implications that are ominous.

Biosynthesis and maturation of the malaria aspartic hemoglobinases plasmepsins I and II

During the intraerythrocytic stage of infection, the malaria parasite Plasmodium falciparum digests most of the host cell hemoglobin. Hemoglobin degradation occurs in the acidic digestive vacuole and is essential for the survival of the parasite. Two aspartic proteases, plasmepsins I and II, have been isolated from the vacuole and shown to make the initial cleavages in the hemoglobin molecule. We have studied the biosynthesis of these two enzymes. Plasmepsin I is synthesized and processed to the mature form soon after the parasite invades the red blood cell, while plasmepsin II synthesis is delayed until later in development. Otherwise, biosynthesis of the plasmepsins is identical. The proplasmepsins are type II integral membrane proteins that are transported through the secretory pathway before cleavage to the soluble form. They are not glycosylated in vivo, despite the presence of several potential glycosylation sites. Proplasmepsin maturation appears to require acidic conditions and is reversibly inhibited by the tripeptide aldehydes N-acetyl-L-leucyl-L-leucyl-norleucinal and N-acetyl-L-leucyl-L-leucyl-methional. These compounds are known to inhibit cysteine proteases and the chymotryptic activity of proteasomes but not aspartic proteases. However, proplasmepsin processing is not blocked by other cysteine protease inhibitors, nor by the proteasome inhibitor lactacystin. Processing is also not blocked by aspartic protease inhibitors. This inhibitor profile suggests that unlike most other aspartic proteases, proplasmepsin maturation may not be autocatalytic in vivo, but instead could require the action of an unusual processing enzyme. Compounds that block processing are expected to be potent antimalarials.

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.

Cell surface receptor directed targeting of toxin to human malaria parasite, Plasmodium falciparum

Gelonin (a toxin and type II ribosome inactivating protein) when linked to human transferrin can be targeted to Plasmodium falciparum. The transferrin toxin conjugate is significantly toxic to parasite growth and is 25 times more potent than toxin alone in inhibiting parasite protein synthesis. The mechanism of its entry into the intraerythrocytic parasite is discussed.

Current status of the Plasmodium falciparum genome project

The Plasmodium falciparum Genome Project is a collaborative effort by many laboratories that will provide detailed molecular information about the parasite, which may be used for developing practical control measures. Initial goals are to prepare an electronically indexed clone bank containing partially sequenced clones representing up to 80% of the parasite's genes and to prepare an ordered set of overlapping clones spanning each of the parasite's 14 chromosomes. Currently, clones of genomic DNA, prepared as yeast artificial chromosomes, are arranged into contigs covering approximately 70% of the genome of parasite clone 3D7, gene sequence tags are available from more than contigs covering approximately 70% of the genome of parasite clone 3D7, gene sequence tags are available from more than 20% of the parasite's genes, and approximately 5% of the parasite's genes are tentatively identified from similarity searches of entries in the international sequence databases. A total of > 0.5 Mb of P. falciparum sequence tag data is available. The gene sequence tags are presently being used to complete YAC contig assembly and localize the cloned genes to positions on the physical map in preparation for sequencing the genome. Routes of access to project information and services are described.

Kinetic analysis of plasmepsins I and II aspartic proteases of the Plasmodium falciparum digestive vacuole

Plasmepsins I and II are Plasmodium falciparum aspartic proteases implicated in hemoglobin degradation. Using a synthetic fluorogenic peptide substrate based on the initial hemoglobin cleavage site, we have analyzed kinetic parameters of the two enzymes in native and recombinant forms. Both native plasmepsins cleave the model substrate well. Recombinant plasmepsin II behaves similarly to native enzyme, substantiating its usefulness for inhibition and structural studies. In contrast, recombinant plasmepsin I does not resemble its native homolog kinetically. A hybrid molecule, in which the polyproline loop of plasmepsin I has been replaced by the homologous sequence from plasmepsin II, still maintains the specificity/kinetics of plasmepsin II. This suggests that the polyproline loop, important for substrate recognition in the mammalian aspartic protease renin, does not play a similar role in the plasmepsins.

DNA diagnosis of pulmonary infections: particular emphasis on Mycobacterium tuberculosis

The past two decades have brought significant changes to the clinical laboratory. Microbiologists now have highly sensitive, rapid and specific molecular methods of identifying infectious agents by the direct detection of DNA or RNA sequences unique to a particular organism. Advanced DNA technology such as nucleic-acid hybridization, PCR and DNA fingerprinting have been used in the direct detection of causative organisms in clinical specimens, with resultant benefits such as increased sensitivity and specificity of the diagnostic approach and reduction of turnaround time. This review outlines a brief description of the various DNA diagnostic tools used in the detection of pulmonary infections with emphasis on their applications in the diagnosis of Mycobacterium tuberculosis.

Malaria chemotherapy: resistance to quinoline containing drugs in Plasmodium falciparum

Resistance to quinoline containing drugs, particularly chloroquine (CQ), is a major impediment to the successful chemotherapy and prophylaxis of malaria. CQ-resistant parasites fail to accumulate as much drug as their sensitive counterparts and two major hypotheses have been proposed to account for this phenomenon. CQ-resistant parasites are thought to maintain lower intracellular drug levels by means of an active efflux system, similar to that found in multi-drug resistant cancer cells, despite major differences in both the genetic and biochemical manifestations of drug resistance in the two cell types. Alternatively, CQ-resistance could be linked to a defective CQ uptake mechanism, possibly an impaired acidification process in the food vacuole of the resistant parasite. These two theories are discussed in detail in the following review. The potential of pharmacological intervention to override these resistance mechanisms is also discussed.

The chemotherapy of rodent malaria. XLVIII. The activities of some synthetic 1,2,4-trioxanes against chloroquine-sensitive and chloroquine-resistant parasites. Part 1: Studies leading to the development of novel cis-fused cyclopenteno derivatives

The new Chinese antimalarial blood schizontocide, artemisinin, derived from the plant Artemisia annua, displays a high level of activity against polyresistant Plasmodium falciparum. Several synthetic 1,2,4-trioxanes were examined in a search for compounds that exhibit a similar type of action against drug-resistant parasites. This paper, the first of a series, describes the examination of these trioxanes against drug-sensitive and drug-resistant malaria parasites in a rodent model, using artemisinin and arteether as comparison standards. Cis-fused cyclohexeno-1,2,4-trioxanes (10-17) substituted with various side-chains revealed for the most part variable but weak antimalarial activity. On the other hand, cis-fused cyclopenteno-1,2,4-trioxanes (18-19) showed greater activity, 19 showing about 1/30th of the activity of arteether against drug-sensitive Plasmodium berghei in vivo, thereby providing a clue to the structure-activity relationship.

The chemotherapy of rodent malaria. XLVII. Studies on pyronaridine and other Mannich base antimalarials

The activities of Mannich base antimalarials, including pyronaridine, have been explored against drug-sensitive (Plasmodium berghei N) and chloroquine-resistant (Plasmodium yoelii NS) rodent malaria parasites in vivo. Lines of these parasites have been developed with resistance to pyronaridine, amodiaquine, or WR 228,258. The responses and patterns of cross-resistance of these lines to Mannich bases and other blood schizontocides are inconsistent. It is concluded that some Mannich bases may prove still to be of value inthe treatment of chloroquine-resistant Plasmodium falciparum infection.

Pyrimethamine resistant mutations in Plasmodium falciparum

Three mutations in Plasmodium falciparum yielding increased resistance to pyrimethamine were obtained following treatment with chemical mutagens and selection in presence of pyrimethamine. From parasite clone TM4/8.2 a mutant, TM4/8.2/4.1, was produced which raised pyrimethamine resistance about 500 times and was found to involve an amino acid change in the DHFR-TS enzyme molecule from Ser108 to Asn108. A clone of another isolate, T9/94, yielded a mutant, T9/94/300.300, raising pyrimethamine resistance about 10 times and involving an amino acid change from Ile164 to Met164. However, another mutant from T9/94, T9/94/M1-1(b3), although it raised the pyrimethamine resistance 100 times, did not involve any changes in the coding sequence of the DHFR-TS gene, but resulted in the production of about twice as much DHFR-TS enzyme as the original clone T9/94. No amplification of the DHFR-TS gene was detected. It is concluded that changes in pyrimethamine resistance of malaria parasites may arise in at least 2 ways: (1) by structural changes in the DHFR domain of the DHFR-TS gene (as previously found by other workers); (2) by other changes, possibly affecting the expression of the DHFR-TS gene. The relative importance of these 2 mechanisms in causing resistance in wild populations of P. falciparum is discussed.