Antimalarial activity of prodrugs of N-branched acyclic nucleoside phosphonate inhibitors of 6-oxopurine phosphoribosyltransferases

Acyclic nucleoside phosphonates (ANPs) that contain a 6-oxopurine base are good inhibitors of the human and Plasmodium falciparum 6-oxopurine phosphoribosyltransferases (PRTs), key enzymes of the purine salvage pathway. Chemical modifications, based on the crystal structures of several inhibitors in complex with the human PRTase, led to the design of a new class of inhibitors - the aza-ANPs. Because of the negative charges of the phosphonic acid moiety, their ability to cross cell membranes is, however, limited. Thus, phosphoramidate prodrugs of the aza-ANPs were prepared to improve permeability. These prodrugs arrest parasitemia with IC values in the micromolar range against Plasmodium falciparum-infected erythrocyte cultures (both chloroquine-sensitive and chloroquine-resistant Pf strains). The prodrugs exhibit low cytotoxicity in several human cell lines. Thus, they fulfill two essential criteria to qualify them as promising antimalarial drug leads

The Novel bis-1,2,4-Triazine MIPS-0004373 Demonstrates Rapid and Potent Activity against All Blood Stages of the Malaria Parasite

Novel bis-1,2,4-triazine compounds with potent in vitro activity against Plasmodium falciparum parasites were recently identified. The bis-1,2,4-triazines represent a unique antimalarial pharmacophore and are proposed to act by a novel but as-yet-unknown mechanism of action. This study investigated the activity of the bis-1,2,4-triazine MIPS-0004373 across the mammalian life cycle stages of the parasite and profiled the kinetics of activity against blood and transmission stage parasites in vitro and in vivo. MIPS-0004373 demonstrated rapid and potent activity against P. falciparum, with excellent in vitro activity against all asexual blood stages. Prolonged in vitro drug exposure failed to generate stable resistance de novo, suggesting a low propensity for the emergence of resistance. Excellent activity was observed against sexually committed ring stage parasites, but activity against mature gametocytes was limited to inhibiting male gametogenesis. Assessment of liver stage activity demonstrated good activity in an in vitro P. berghei model but no activity against Plasmodium cynomolgi hypnozoites or liver schizonts. The bis-1,2,4-triazine MIPS-0004373 efficiently cleared an established P. berghei infection in vivo, with efficacy similar to that of artesunate and chloroquine and a recrudescence profile comparable to that of chloroquine. This study demonstrates the suitability of bis-1,2,4-triazines for further development toward a novel treatment for acute malaria

Comparison of a chromosomal segment of P. vivax with that of P. falciparum

P. falciparum and P. vivax are two human malaria pathogens. Although these parasites share the same hosts, they are evolutionary distant species, differing in genome composition and organisation, parasite life cycle and clinical manifestations. Genome comparison of these two species is fundamental to increase our understanding of common biological processes, as well as unique properties of each pathogen, which is necessary for the development of species specific or pan-species diagnostic tools, antimalarial drugs and vaccines. To investigate the level of conservation between P. falciparum and P. vivax a continuous region of a P. vivax chromosome cloned in the YAC 1H14 was sequenced. Within this 199,866 base pair portion of P. vivax chromosome, a conserved linkage group was identified consisting of at least 41 genes homologous to P. falciparum genes located on chromosome 3. Within this conserved linkage group, the gene order and structure are identical to that of P. falciparum chromosome 3. Furthermore, this conserved linkage group may contain as many as 190 genes. Although there are large regions of similarity between the P. falciparum and P. vivax chromosomes, there were two regions where no homology with P. falciparum chromosome 3 was observed. The first region contained the P. falciparum cytoadherence-linked asexual genes clag 3.2, clag 3.1 and a var C pseudogene for which no homologues could be found on the section of P. vivax chromosome analysed. Secondly, there was no homology observed between the subtelomeric regions of the P. vivax chromosome and P. falciparum chromosome 3. In addition, the subtelomeric region of the analysed P. vivax chromosome is 90 kb longer than that of P. falciparum chromosome 3. The overall size difference of at least 900 kb between the entire homologous P. vivax chromosome and P. falciparum chromosome 3 is presumably due to a translocation from another chromosome. The region of the P. vivax chromosome homologous to P. falciparum has a much higher DNA GC-content compared to that of P. falciparum. There is a preference for amino acids using GC-rich codons in the deduced proteins of P. vivax compared to P. falciparum orthologues. Nevertheless, there is a high degree of functional conservation between the orthologous proteins identified within this locus. The degree of sequence similarity between the orthologous proteins varied from 23% to 97%. Thus, both highly conserved and diverged proteins could be readily identified by the comparative analysis. This approach is of great value for initial functional characterisation of the genes in both species. Pairwise alignments were used to search for phylogenetic footprints within noncoding sequences. In contrast to the coding sequences, the intergenic regions and introns of P. vivax have diverged substantially from those of P. falciparum. The number of footprints found within the non-coding sequences of P. vivax and P. falciparum is considerably less than that between P. falciparum and the rodent parasite P. yoelii. Only 4% of nucleotides appear to be conserved between the P. vivax and P. falciparum intergenic sequences. This is much lower than the fraction of conserved nucleotides in the coding sequences and is indicative of a different type of selection pressure acting on non-coding sequences. The effect of sequence divergence on the cross-species functionality of promoter regions was tested. The ability of several homologous P. vivax, P. falciparum and P. yoelii promoter regions to drive expression of a reporter gene was compared. The expression of the reporter gene under control of the homologous P. falciparum and P. yoelii promoter regions was detected at comparable levels, whereas expression under control of the P. vivax promoter regions was not detectable. The decrease in activity of the P. vivax promoters appears to be consistent with the lack of conservation between the non-coding sequences of P. vivax and P. falciparum. In conclusion, this thesis combines molecular biology techniques and bioinformatic analyses to characterise a P. vivax chromosomal segment. This study revealed for the first time a high degree of conservation between the genomes of the two evolutionary distant parasites P. falciparum and P. vivax, and demonstrated the power of comparative genomics for studying Plasmodium species. This work also contributed to the P. vivax genome sequencing initiative leading to the whole genome sequencing of P. vivax

Leveraging cell cycle analysis in anticancer drug discovery to identify novel plasmodial drug targets

Cancer and malaria are life threatening diseases killing millions of people each year. In spite of our best efforts, both continue to resist full control and eradication. If untreated, both malaria and cancer can lead to death. Only a few antimalarial drugs have been developed over the last decades and new drugs are urgently needed to combat drug-resistant parasites. Significant progress has been made in understanding the molecular mechanisms of cancer and designing new anticancer therapies. However, similar to malaria, majority of cancers quickly develop resistance to single target-based therapy. Novel cancer therapeutics are being developed with the aim of targeting multiple signalling pathways in tumour cells, an approach that may be applicable to antimalarial therapy. In this review we compare cell signalling pathways targeted by cancer drugs with similar pathways in the malaria parasite. We placed particular emphasis on cell cycle regulation and cell cycle checkpoints since the associated molecular machinery controlling these processes are conserved in Plasmodium. Furthermore, a large number of cancer drugs target cell cycle control mechanisms and, therefore, these compounds may possess antimalarial activity. We tried to demonstrate that promising areas of anticancer drug development can be incorporated in the existing antimalarial drug discovery program as well as deepen our understanding of parasite biology. © 2010 Bentham Science Publishers Ltd

Targeting protein kinases in the malaria parasite: update of an antimalarial drug target

The emergence of drug-resistant malaria highlights the need for new agents. A desired characteristic of candidate antimalarials is rapid killing of parasites. This is typically measured by the rate of exponential clearance of parasitemia following treatment. However, this clearance rate excludes the highly variable lag phase, when the parasitemia level may increase, remain constant, or decrease. Understanding factors determining this lag phase is important for drug development

Mitochondrial membrane potential in a small subset of Artemisinin-induced dormant Plasmodium falciparum parasites in vitro

Artemisinin induced dormancy is a proposed mechanism for failures of mono-therapy and is linked with artemisinin resistance in Plasmodium falciparum. The biological characterization and dynamics of dormant parasites are not well understood. Here we report that following dihydroartemisinin (DHA) treatment in vitro, a small subset of morphologically dormant parasites was stained with rhodamine 123 (RH), a mitochondrial membrane potential (MMP) marker, and persisted to recovery. FACS sorted RH-positive parasites resumed growth at 10,000/well while RH-negative parasites failed to recover at 5 million/well. Furthermore, transcriptional activity for mitochondrial enzymes was only detected in RH-positive dormant parasites. Importantly, after treating dormant parasites with different concentrations of atovaquone, a mitochondrial inhibitor, the recovery of dormant parasites was delayed or stopped. This demonstrates that mitochondrial activity is critical for survival and regrowth of dormant parasites and that RH staining provides a means of identifying these parasites. These findings provide novel paths for studying and eradicating this dormant stage

Development of pyridyl thiosemicarbazones as highly potent agents for the treatment of malaria after oral administration

Objectives Drug resistance exists to all current and investigational antimalarial drug classes. Consequently, we have set out to develop chemically and mechanistically discrete antimalarials. Here we report on the development of thiosemicarbazone (TSC) antimalarials, with TSC3 as the most advanced lead. Methods Thiosemicarbazones were generated through simple condensation reactions of thiosemicarbazides and ketones. TSC3 was selected and tested for in vitro antimalarial activities against MDR Plasmodium falciparum lines using the [3H]hypoxanthine growth assay, in vitro cytotoxicity against mammalian cell lines using the alamarBlue fluorescence cell viability assay, in vivo potency in the mouse–Plasmodium berghei model and blood exposure in mice measured by LC-MS for pharmacokinetic analysis. Results TSC3 showed potent in vitro activity against atovaquone-, dihydroartemisinin-, chloroquine- and mefloquine-resistant P. falciparum lines (EC50 500 in two of three mammalian cell lines. In P. berghei-infected mice, TSC3 showed potent activity in the Peters 4 day suppression test (ED50 1.2 mg/kg/day) and was as potent as artesunate and chloroquine in the curative modified Thompson test. A single oral dose of TSC3 at 16 mg/kg in healthy mice achieved a mean maximum blood concentration of 1883 ng/mL at 1 h after dosing and an elimination half-life of 48.7 h in groups of five mice. Conclusions TSC3 shows promise as a persistent, potent and orally effective antimalarial. This, coupled with the extremely low cost of synthesis, suggests that the further development of antimalarial thiosemicarbazones is clearly warrante

The spiroindolone KAE609 does not induce dormant ring stages in Plasmodium falciparum parasites

In vitro drug treatment with artemisinin derivatives, such as dihydroartemisinin (DHA), results in a temporary growth arrest (i.e., dormancy) at an early ring stage in Plasmodium falciparum. This response has been proposed to play a role in the recrudescence of P. falciparum infections following monotherapy with artesunate and may contribute to the development of artemisinin resistance in P. falciparum malaria. We demonstrate here that artemether does induce dormant rings, a finding which further supports the class effect of artemisinin derivatives in inducing the temporary growth arrest of P. falciparum parasites. In contrast and similarly to lumefantrine, the novel and fast-acting spiroindolone compound KAE609 does not induce growth arrest at the early ring stage of P. falciparum and prevents the recrudescence of DHA-arrested rings at a low concentration (50 nM). Our findings, together with previous clinical data showing that KAE609 is active against artemisinin-resistant K13 mutant parasites, suggest that KAE609 could be an effective partner drug with a broad range of antimalarials, including artemisinin derivatives, in the treatment of multidrug-resistant P. falciparum malaria