Dxr is essential in <Emphasis Type="Italic">Mycobacterium tuberculosis</Emphasis>and fosmidomycin resistance is due to a lack of uptake

   

Fosmidomycin is a phosphonic antibiotic which inhibits 1-deoxy-D-xylulose 5-phosphate reductoisomerase (Dxr), the first committed step of the non-mevalonate pathway of isoprenoid biosynthesis. In Mycobacterium tuberculosis Dxr is encoded by Rv2870c, and although the antibiotic has been shown to inhibit the recombinant enzyme [1], mycobacteria are intrinsically resistant to fosmidomycin at the whole cell level. Fosmidomycin is a hydrophilic molecule and in many bacteria its uptake is an active process involving a cAMP dependent glycerol-3-phosphate transporter (GlpT). The fact that there is no glpT homologue in the M. tuberculosis genome and the highly impervious nature of the hydrophobic mycobacterial cell wall suggests that resistance may be due to a lack of cellular penetration.     

Modifications around the hydroxamic acid chelating group of fosmidomycin,an inhibitor of the metalloenzyme 1-deoxyxylulose 5-phosphate reductoisomerase (DXR)

Fosmidomycin derivatives in which the hydroxamic acid group has been replaced by several bidentate chelators as potential hydroxamic alternatives were prepared and tested against the DXR from Escherichia coli. These results illustrate the predominant role of the hydroxamate functional group as the most effective metal binding group in DXR inhibitors.     

Antimalarial dosing regimens and drug resistance

The contribution of underdosing to antimalarial treatment failure has been underappreciated. Most recommended dosage regimens are based on studies in non-pregnant adult patients. Young children and pregnant women, who bear the heaviest malaria burden, have the highest treatment failure rates. This has been attributed previously to lower immunity, although blood concentrations of many antimalarial drugs are significantly lower in pregnant women and young children than in non-pregnant adults. Nevertheless, there have been no studies of higher dosages. Sub-therapeutic concentrations will certainly contribute to poorer responses to treatment and will fuel the emergence and spread of antimalarial drug resistance. There is an urgent need for studies to optimise antimalarial dosage regimens in infants, young children and pregnant women, both to improve cure rates and to prolong the useful therapeutic lives of antimalarial drugs.     

Drug discovery for malaria: a very challenging and timely endeavor

The prevalence of resistance to known antimalarial drugs has resulted in the expansion of antimalarial drug discovery efforts. Academic and nonprofit institutions are partnering with the pharmaceutical industry to develop new antimalarial drugs. Several new antimalarial agents are undergoing clinical trials, mainly those resurrected from previous antimalarial drug discovery programs. Novel antimalarials are being advanced through the drug development process, of course, with the anticipated high failure rate typical of drug discovery. Many of these are summarized in this review. Mechanisms for funding antimalarial drug discovery and genomic information to aid drug target selection have never been better. It remains to be seen whether ongoing efforts will be sufficient for reducing malaria burden in the developing world.     

Synthesis and evaluation of alpha,beta-unsaturated alpha-aryl-substituted fosmidomycin analogues as DXR inhibitors

Fosmidomycin, which acts through inhibition of 1-deoxy-D-xylulose phosphate reductoisomerase (DXR) in the non-mevalonate pathway, represents a valuable recent addition to the armamentarium against uncomplicated malaria. In this paper, we describe the synthesis and biological evaluation of E- and Z-alpha,beta-unsaturated alpha-aryl-substituted analogues of FR900098, a fosmidomycin congener, utilizing a Stille or a Suzuki coupling to introduce the aryl group. In contrast with our expectations based on the promising activity earlier observed for several alpha-substituted fosmidomycin analogues, all synthesized analogues exhibited much lower binding affinity for DXR than fosmidomycin.     

Structure-activity relationships of antileishmanial and antimalarial chalcones

A series of oxygenated chalcones which have been evaluated earlier for antimalarial activity (Plasmodium falciparum K1) were tested for antileishmanial activity against Leishmania donovani amastigotes. A comparison of structure-activity relationships reveal that different physicochemical and structural requirements exist for these two activities. Antileishmanial activity is associated with less lipophilic chalcones, in particular those with 4'-hydroxyl-substituted B rings and hetero/polyaromatic A rings. In contrast, chalcones with good antimalarial activity have alkoxylated B rings and electron-deficient A rings. Visualization of the steric and electrostatic fields generated from comparative molecular field analysis (CoMFA) indicate that the ring A of chalcones make a more significant contribution to antileishmanial activity while both rings A and B are important for antimalarial activity. Despite different requirements, two alkoxylated chalcones (8, 19) were identified which combined good antimalarial and antileishmanial activities.     

Synthesis and antimalarial evaluation of novel isocryptolepine derivatives

A series of mono- and di-substituted analogues of isocryptolepine have been synthesized and evaluated for in vitro antimalarial activity against chloroquine sensitive (3D7) and resistant (W2mef) Plasmodium falciparum and for cytotoxicity (3T3 cells). Di-halogenated compounds were the most potent derivatives and 8-bromo-2-chloroisocryptolepine displayed the highest selectivity index (106; the ratio of cytotoxicity (IC(50)=9005 nM) to antimalarial activity (IC(50)=85 nM)). Our evaluation of novel isocryptolepine compounds has demonstrated that di-halogenated derivatives are promising antimalarial lead compounds.     

Interactions of curcumin with the PfATP6 model and the implications for its antimalarial mechanism

Despite curcumin has been proved to possess antimalarial effects, the underlying mechanism remains to be elucidated. In this letter, the active site binding modes of curcumin in PfATP6, an important antimalarial target, were investigated using computational docking. It was revealed that curcumin interacts with PfATP6 mainly through hydrophobic interactions and hydrogen bonds. Moreover, the theoretically predicted binding affinity implies that curcumin can efficiently inhibit PfATP6, which gains some deeper insights into the antimalarial mechanism of curcumin.     

Antimalarial activity of compounds comprising a primary benzene sulfonamide fragment

Despite the urgent need for effective antimalarial drugs with novel modes of action no new chemical class of antimalarial drug has been approved for use since 1996. To address this, we have used a rational approach to investigate compounds comprising the primary benzene sulfonamide fragment as a potential new antimalarial chemotype. We report the in vitro activity against Plasmodium falciparum drug sensitive (3D7) and resistant (Dd2) parasites for a panel of fourteen primary benzene sulfonamide compounds. Our findings provide a platform to support the further evaluation of primary benzene sulfonamides as a new antimalarial chemotype, including the identification of the target of these compounds in the parasite.     

Second-generation antimalarial endoperoxides

Artemisinin, derived from a Chinese herbal remedy, is a potent peroxide-containing antimalarial. New types of peroxides, derived from this structure, as well as other naturally occurring antimalarial peroxides, have been synthesized and found to have potent antimalarial activities. Studies on the activities, modes of action, and toxicities of these compounds are discussed here by Steven Meshnick and colleagues.     

Synthetic medicinal chemistry of selected antimalarial natural products

Natural products remain a rich source of novel molecular scaffolds for novel antimalarial agents in the fight against malaria. This has been well demonstrated in the case of quinine and artemisinin both of which have served as templates for the development of structurally simpler analogues that either served or continue to serve as effective antimalarials. This review will expound on these two natural products as well as other selected natural products that have served either as antimalarial agents or as potential lead compounds in the development of antimalarial drugs.     

Apicoplast translation, transcription and genome replication: targets for antimalarial antibiotics

Several antibiotics possess antimalarial properties, although the mechanisms by which they kill malaria parasites have been poorly understood. Recent data suggest that the target for multiple antimalarial antibiotics is the apicoplast, a chloroplast-like organelle of uncertain function. Translation inhibitors (such as tetracyclines, clindamycin and macrolides) and gyrase inhibitors (such as ciprofloxacin) cause modest antimalarial effects initially but are much more potent against the progeny of treated parasites. These progeny inherit nonfunctional apicoplasts, suggesting that blocking production of apicoplast proteins causes the 'delayed-death effect'. Interestingly, the antibiotics thiostrepton and rifampin are fast acting and might target additional processes outside the apicoplast.     

2-N-Methyl modifications and SAR studies of manzamine A

Quaternary carbolinium salts have been reported to show improved antimalarial activity and reduced cytotoxicity as compared to electronically neutral beta-carbolines. In this study, mono- and di-methylated quaternary carbolinium cations of manzamine A were synthesized and evaluated for their in vitro antimalarial and antimicrobial activity, cytotoxicity, and also their potential for glycogen synthase kinase (GSK-3beta) inhibition using molecular docking studies. Among the analogs, 2-N-methylmanzamine A (2) exhibited antimalarial activity (IC(50) 0.7-1.0microM) but was less potent than manzamine A. However the compound was significantly less cytotoxic to mammalian kidney fibroblasts and the selectivity index was in the same range as manzamine A.     

Antiplasmodial activity of synthetic ellipticine derivatives and an isolated analog

Ellipticine has been shown previously to exhibit excellent in vitro antiplasmodial activity and in vivo antimalarial properties that are comparable to those of the control drug chloroquine in a mouse malaria model. Ellipticine derivatives and analogs exhibit antimalarial potential however only a few have been studied to date. Herein, ellipticine and a structural analog were isolated from Aspidosperma vargasii bark. A-ring brominated and nitrated ellipticine derivatives exhibit good in vitro inhibition of Plasmodium falciparum K1 and 3D7 strains. Several of the compounds were found not to be toxic to human fetal lung fibroblasts. 9-Nitroellipticine (IC₅₀ = 0.55 μM) exhibits greater antiplasmodial activity than ellipticine. These results are further evidence of the antimalarial potential of ellipticine derivatives.     

Antisense and chemical suppression of the nonmevalonate pathway affects ent-kaurene biosynthesis in Arabidopsis

Transgenic plants of Arabidopsis thaliana (L.) Heynh. (ecotype Columbia) expressing the antisense AtMECT gene, encoding 2- C-methyl- D-erythritol 4-phosphate cytidylyltransferase, were generated to elucidate the physiological role of the nonmevalonate pathway for production of ent-kaurene, the latter being the plastidic precursor of gibberellins. In transformed plants pigmentation and accumulation of ent-kaurene were reduced compared to wild-type plants. Fosmidomycin, an inhibitor of 1-deoxy- D-xylulose 5-phosphate reductoisomerase (DXR), caused a similar depletion of these compounds in transgenic plants. These observations suggest that both AtMECT and DXR are important in the synthesis of isopentenyl diphosphate and dimethylallyl diphosphate and that ent-kaurene is mainly produced through the nonmevalonate pathway in the plastid.     

Investigation of some medicinal plants traditionally used for treatment of malaria in Kenya as potential sources of antimalarial drugs

Malaria is a major public health problem in many tropical and subtropical countries and the burden of this disease is getting worse, mainly due to the increasing resistance of Plasmodium falciparum against the widely available antimalarial drugs. There is an urgent need for discovery of new antimalarial agents. Herbal medicines for the treatment of various diseases including malaria are an important part of the cultural diversity and traditions of which Kenya′s biodiversity has been an integral part. Two major antimalarial drugs widely used today came originally from indigenous medical systems, that is quinine and artemisinin, from Peruvian and Chinese ancestral treatments, respectively. Thus ethnopharmacology is a very important resource in which new therapies may be discovered. The present review is an analysis of ethnopharmacological publications on antimalarial therapies from some Kenyan medicinal plants.     

Synthesis and antimalarial activity in vitro of potential metabolites of ferrochloroquine and related compounds

In man, the two major metabolites of the antimalarial drug chloroquine (CQ) are monodesethylchloroquine (DECQ) and didesethylchloroquine (di-DECQ). By analogy with CQ, the synthesis and the in vitro tests of some amino derivatives of ferrochloroquine (FQ), a ferrocenic analogue of CQ which are presumed to be the oxidative metabolites of FQ, are reported. Desmethylferrochloroquine 1a and didesmethylferrochloroquine 2 would be more potent against schizontocides than CQ in vitro against two strains (HB3 and Dd2) of Plasmodium falciparum. Other secondary amino derivatives have been prepared and proved to be active as antimalarial agents in vitro, too.     

Discovery and biochemical characterization of Plasmodium thioredoxin reductase inhibitors from an antimalarial set

Plasmodium falciparum is the most prevalent and deadly species of the human malaria parasites, and thioredoxin reductase (TrxR) is an enzyme involved in the redox response to oxidative stress. Essential for P. falciparum survival, the enzyme has been highlighted as a promising target for novel antimalarial drugs. Here we report the discovery and characterization of seven molecules from an antimalarial set of 13533 compounds through single-target TrxR biochemical screens. We have produced high-purity, full-length, recombinant native enzyme from four Plasmodium species, and thioredoxin substrates from P. falciparum and Rattus norvegicus. The enzymes were screened using a unique, high-throughput, in vitro native substrate assay, and we have observed selectivity between the Plasmodium species and the mammalian form of the enzyme. This has indicated differences in their biomolecular profiles and has provided valuable insights into the biochemical mechanisms of action of compounds with proven antimalarial activity.     

Antimalarial drug synergism and antagonism: mechanistic and clinical significance

Interactions between antimicrobial agents provide clues as to their mechanisms of action and influence the combinations chosen for therapy of infectious diseases. In the treatment of malaria, combinations of drugs, in many cases acting synergistically, are increasingly important in view of the frequency of resistance to single agents. The study of antimalarial drug interactions is therefore of great significance to both treatment and research. It is therefore worrying that the analysis of drug-interaction data is often inadequate, leading in some cases to dubious conclusions about synergism or antagonism. Furthermore, making mechanistic deductions from drug-interaction data is not straightforward and of the many reported instances of antimalarial synergism or antagonism, few have been fully explained biochemically. This review discusses recent findings on antimalarial drug interactions and some pitfalls in their analysis and interpretation. The conclusions are likely to have relevance to other antimicrobial agents.