Failure of artesunate-mefloquine combination therapy for uncomplicated Plasmodium falciparum malaria in southern Cambodia

Mapping the origins and spread of antifolate-resistant malaria parasites

Evaluation of: Pearce RJ, Pota H, Evehe MSB et al.: Multiple origins and regional dispersal of resistant dhps in African Plasmodium falciparum malaria. PLoS Med. 6(4), e1000055 (2009). Widespread resistance to current antimalarial drugs is a major factor in the extremely high levels of mortality and disabling illness that still prevail in many developing countries. It is important to understand how frequently resistant malaria parasite strains arise and their patterns of propagation and dispersal across borders and continents. By studying the DNA sequences of both the gene encoding the drug target and its flanking regions, it is possible to collect and map such data, providing a considerable asset in devising and evaluating future strategies of drug use and deployment. In this article, Pearce et al. analyze a large number of parasite samples collected over a decade from countries across Africa, allowing them to present for the first time a detailed picture of the origins and relatively recent spread of resistance to sulfa-drugs, key components of antifolate drug combinations that have been used extensively as part of the antimalarial armory.

Discovery, mechanisms of action and combination therapy of artemisinin

Despite great international efforts, malaria still inflicts an enormous toll on human lives, especially in Africa. Throughout history, antimalarial medicines have been one of the most powerful tools in malaria control. However, the acquisition and spread of parasite strains that are resistant to multiple antimalarial drugs have become one of the greatest challenges to malaria treatment, and are associated with the increase in morbidity and mortality in many malaria-endemic countries. To deal with this grave situation, artemisinin-based combinatory therapies (ACTs) have been introduced and widely deployed in malarious regions. Artemisinin is a new class of antimalarial compounds discovered by Chinese scientists from the sweet wormwood Artemisia annua. The potential development of resistance to artemisinins by Plasmodium falciparum threatens the usable lifespan of ACTs, and therefore is a subject of close surveillance and extensive research. Studies at the Thai-Cambodian border, a historical epicenter of multidrug resistance, have detected reduced susceptibility to artemisinins as manifested by prolonged parasite-clearance times, raising considerable concerns on resistance development. Despite this significance, there is still controversy on the mode of action of artemisinins. Although a number of potential cellular targets of artemisinins have been proposed, they remain to be verified experimentally. Here, we review the history of artemisinin discovery, discuss the mode of action and potential drug targets, and present strategies to elucidate resistance mechanisms.

Plasmodium falciparum neutral aminopeptidases: new targets for anti-malarials

The neutral aminopeptidases M1 alanyl aminopeptidase (PfM1AAP) and M17 leucine aminopeptidase (PfM17LAP) of the human malaria parasite Plasmodium falciparum are targets for the development of novel anti-malarial drugs. Although the functions of these enzymes remain unknown, they are believed to act in the terminal stages of haemoglobin degradation, generating amino acids essential for parasite growth and development. Inhibitors of both enzymes are lethal to P. falciparum in culture and kill the murine malaria P. chabaudi in vivo. Recent biochemical, structural and functional studies provide the substrate specificity and mechanistic binding data needed to guide the development of more potent anti-malarial drugs. Together with biological studies, these data form the rationale for choosing PfM1AAP and PfM17LAP as targets for anti-malarial development.

Seasonal distribution of anti-malarial drug resistance alleles on the island of Sumba, Indonesia

BACKGROUND

Drug resistant malaria poses an increasing public health problem in Indonesia, especially eastern Indonesia, where malaria is highly endemic. Widespread chloroquine (CQ) resistance and increasing sulphadoxine-pyrimethamine (SP) resistance prompted Indonesia to adopt artemisinin-based combination therapy (ACT) as first-line therapy in 2004. To help develop a suitable malaria control programme in the district of West Sumba, the seasonal distribution of alleles known to be associated with resistance to CQ and SP among Plasmodium falciparum isolates from the region was investigated.

METHODS

Plasmodium falciparum isolates were collected during malariometric surveys in the wet and dry seasons in 2007 using two-stage cluster sampling. Analysis of pfcrt, pfmdr1, pfmdr1 gene copy number, dhfr, and dhps genes were done using protocols described previously.

RESULTS AND DISCUSSION

The 76T allele of the pfcrt gene is nearing fixation in this population. Pfmdr1 mutant alleles occurred in 72.8% and 53.3%, predominantly as 1042D and 86Y alleles that are mutually exclusive. The prevalence of amplified pfmdr1 was found 41.9% and 42.8% of isolates in the wet and dry seasons, respectively. The frequency of dhfr mutant alleles was much lower, either as a single 108N mutation or paired with 59R. The 437G allele was the only mutant dhps allele detected and it was only found during dry season.

CONCLUSION

The findings demonstrate a slighly higher distribution of drug-resistant alleles during the wet season and support the policy of replacing CQ with ACT in this area, but suggest that SP might still be effective either alone or in combination with other anti-malarials.

Malaria vaccine research: lessons from 2008/9

Vaccination remains a crucial component of any initiative to control or eradicate malaria. With increasing reports of insecticide resistance in mosquitoes, and malaria parasite resistance to first-line drugs, it is clear that the development of an effective malaria vaccine is an urgent requirement for the improvement of global human health. This article highlights malaria vaccine research-related discoveries from 2008/9 to suggest that the time is now ripe for researchers to develop malaria vaccines that target many antigens from multiple stages of the parasite’s lifecycle. We also call for greater bidirectional information transfer between preclinical and clinical trials, to facilitate more efficient improvement of malaria vaccine candidates.