In vitro antimalarial drug susceptibility in Thai border areas from 1998-2003

Emerging threat of artemisinin partial resistance markers (pfk13 mutations) in Plasmodium falciparum parasite populations in multiple geographical locations in high transmission regions of Uganda

BACKGROUND

Artemisinin-based combination therapy (ACT) is currently recommended for treatment of uncomplicated malaria. However, the emergence and spread of partial artemisinin resistance threatens their effectiveness for malaria treatment in sub-Saharan Africa where the burden of malaria is highest. Early detection and reporting of validated molecular markers (pfk13 mutations) in Plasmodium falciparum is useful for tracking the emergence and spread of partial artemisinin resistance to inform containment efforts.

METHODS

Genomic surveillance was conducted at 50 surveillance sites across four regions of Uganda in Karamoja, Lango, Acholi and West Nile from June 2021 to August 2023. Symptomatic malaria suspected patients were recruited and screened for presence of parasites. In addition, dried blood spots (DBS) were collected for parasite genomic analysis with PCR and sequencing. Out of 563 available dried blood spots (DBS), a random subset of 240 P. falciparum mono-infections, confirmed by a multiplex PCR were selected and used for detecting the pfk13 mutations by Sanger sequencing using Big Dye Terminator method. Regional variations in the proportions of pfk13 mutations were assessed using the chi square or Fisher's exact tests while Kruskal-Wallis test was used to compare absolute parasite DNA levels between wild type and mutant parasites.

RESULTS

Overall, 238/240 samples (99.2%) contained sufficient DNA and were successfully sequenced. Three mutations were identified within the sequenced samples; pfk13 C469Y in 32/238 (13.5%) samples, pfk13 A675V in 14/238 (5.9%) and pfk13 S522C in (1/238 (0.42%) samples across the four surveyed regions. The prevalence of pfk13 C469Y mutation was significantly higher in Karamoja region (23.3%) compared to other regions, P = 0.007. The majority of parasite isolates circulating in West Nile are of wild type (98.3), P = 0.002. Relative parasite DNA quantity did not differ in samples carrying the wild type, C469Y and A675V alleles (Kruskal-Wallis test, P = 0.6373).

CONCLUSION

Detection of validated molecular markers of artemisinin partial resistance in multiple geographical locations in this setting provides additional evidence of emerging threat of artemisinin partial resistance in Uganda. In view of these findings, periodic genomic surveillance is recommended to detect and monitor levels of pfk13 mutations in other regions in parallel with TES to assess potential implication on delayed parasite clearance and associated treatment failure in this setting. Future studies should consider identification of potential drivers of artemisinin partial resistance in the different malaria transmission settings in Uganda.

Progress towards malaria elimination in the Greater Mekong Subregion: perspectives from the World Health Organization

Malaria remains a global health challenge, disproportionately affecting vulnerable communities. Despite substantial progress, the emergence of anti-malarial drug resistance poses a constant threat. The Greater Mekong Subregion (GMS), which includes Cambodia, China's Yunnan province, Lao People's Democratic Republic, Myanmar, Thailand, and Viet Nam has been the epicentre for the emergence of resistance to successive generations of anti-malarial therapies. From the perspective of the World Health Organization (WHO), this article considers the collaborative efforts in the GMS, to contain Plasmodium falciparum artemisinin partial resistance and multi-drug resistance and to advance malaria elimination. The emergence of artemisinin partial resistance in the GMS necessitated urgent action and regional collaboration resulting in the Strategy for Malaria Elimination in the Greater Mekong Subregion (2015-2030), advocating for accelerated malaria elimination interventions tailored to country needs, co-ordinated and supported by the WHO Mekong malaria elimination programme. The strategy has delivered substantial reductions in malaria across all GMS countries, with a 77% reduction in malaria cases and a 97% reduction in malaria deaths across the GMS between 2012 and 2022. Notably, China was certified malaria-free by WHO in 2021. Countries' ownership and accountability have been pivotal, with each GMS country outlining its priorities in strategic and annual work plans. The development of strong networks for anti-malarial drug resistance surveillance and epidemiological surveillance was essential. Harmonization of policies and guidelines enhanced collaboration, ensuring that activities were driven by evidence. Challenges persist, particularly in Myanmar, where security concerns have limited recent progress, though an intensification and acceleration plan aims to regain momentum. Barriers to implementation can slow progress and continuing innovation is needed. Accessing mobile and migrant populations is key to addressing remaining transmission foci, requiring effective cross-border collaboration. In conclusion, the GMS has made significant progress towards malaria elimination, particularly in the east where several countries are close to P. falciparum elimination. New and persisting challenges require sustained efforts and continued close collaboration. The GMS countries have repeatedly risen to every obstacle presented, and now is the time to re-double efforts and achieve the 2030 goal of malaria elimination for the region.

Clinical pharmacokinetics of quinine and its relationship with treatment outcomes in children, pregnant women, and elderly patients, with uncomplicated and complicated malaria: a systematic review

Background

Standard dosage regimens of quinine formulated for adult patients with uncomplicated and complicated malaria have been applied for clinical uses in children, pregnant women, and elderly. Since these populations have anatomical and physiological differences from adults, dosage regimens formulated for adults may not be appropriate. The study aimed to (i) review existing information on the pharmacokinetics of quinine in children, pregnant women, and elderly populations, (ii) identify factors that influence quinine pharmacokinetics, and (iii) analyse the relationship between the pharmacokinetics and treatment outcomes (therapeutic and safety) of various dosage regimens of quinine.

Methods

Web of Sciences, Cochrane Library, Scopus, and PubMed were the databases applied in this systematic search for relevant research articles published up to October 2020 using the predefined search terms. The retrieved articles were initially screened by titles and abstracts to exclude any irrelevant articles and were further evaluated based on full-texts, applying the predefined eligibility criteria. Excel spreadsheet (Microsoft, WA, USA) was used for data collection and management. Qualitative data are presented as numbers and percentages, and where appropriate, mean + SD or median (range) or range values.

Results

Twenty-eight articles fulfilled the eligibility criteria, 19 in children, 7 in pregnant women, and 2 in elderly (14 and 7 articles in complicated and uncomplicated malaria, respectively). Severity of infection, routes of administration, and nutritional status were shown to be the key factors impacting quinine pharmacokinetics in these vulnerable groups.

Conclusions

The recommended dosages for both uncomplicated and complicated malaria are, in general, adequate for elderly and children with uncomplicated malaria. Dose adjustment may be required in pregnant women with both uncomplicated and complicated malaria, and in children with complicated malaria. Pharmacokinetics studies relevant to clinical efficacy in these vulnerable groups of patients with large sample size and reassessment of MIC (minimum inhibitory concentration) should be considered.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04065-1.

Hepatic safety of repeated treatment with pyronaridine-artesunate versus artemether-lumefantrine in patients with uncomplicated malaria: a secondary analysis of the WANECAM 1 data from Bobo-Dioulasso, Burkina Faso

Background

The use of pyronaridine-artesunate (PA) has been associated with scarce transaminitis in patients. This analysis aimed to evaluate the hepatic safety profile of repeated treatment with PA versus artemether–lumefantrine (AL) in patients with consecutive uncomplicated malaria episodes in Bobo-Dioulasso, Burkina Faso.

Methods

This study analysed data from a clinical trial conducted from 2012 to 2015, in which participants with uncomplicated malaria were assigned to either PA or AL arms and followed up to 42 days. Subsequent malaria episodes within a 2-years follow up period were also treated with the same ACT initially allocated. Transaminases (AST/ALT), alkaline phosphatase (ALP), total and direct bilirubin were measured at days 0 (baseline), 3, 7, 28 and on some unscheduled days if required. The proportions of non-clinical hepatic adverse events (AEs) following first and repeated treatments with PA and AL were compared within study arms. The association of these AEs with retreatment in each arm was also determined using a logistic regression model.

Results

A total of 1379 malaria episodes were included in the intention to treat analysis with 60% of all cases occurring in the AL arm. Overall, 179 non-clinical hepatic AEs were recorded in the AL arm versus 145 in the PA arm. Elevated ALT was noted in 3.05% of treated malaria episodes, elevated AST 3.34%, elevated ALP 1.81%, and elevated total and direct bilirubin in 7.90% and 7.40% respectively. Retreated participants were less likely to experience elevated ALT and AST than first episode treated participants in both arms. One case of Hy’s law condition was recorded in a first treated participant of the PA arm. Participants from the retreatment group were 76% and 84% less likely to have elevated ALT and AST, respectively, in the AL arm and 68% less likely to present elevated ALT in the PA arm. In contrast, they were almost 2 times more likely to experience elevated total bilirubin in both arms.

Conclusions

Pyronaridine-artesunate and artemether–lumefantrine showed similar hepatic safety when used repeatedly in participants with uncomplicated malaria. Pyronaridine-artesunate represents therefore a suitable alternative to the current first line anti-malarial drugs in use in endemic areas.

Trial registration Pan African Clinical Trials Registry. PACTR201105000286876

Molecular identification of two cestodes species parasitizing freshwater fishes in India

In the present study, molecular identification of two species of cestodes, Lytocestus indicus and Senga lucknowensis infecting freshwater fishes Clarias magur and Channa punctata, respectively in Manipur is carried out. To ascertain the taxonomic status of these helminth parasites, 18S gene marker was used. Phylogenetic analysis of 18S of Lytocestus sp. showed that it claded with L. indicus from Indian Isolate with a sequence similarity index of 99%. In case of Senga sp., the phylogenetic analysis revealed that it formed a separate clade with S. lucknowensis and Senga vishakapatnamensis, and the sequence similarity index showed maximum homogeneity with S. lucknowensis i.e., 99.8%. Thus, molecular characterization revealed that the two species of cestodes belong to L. indicus and S. lucknowensis.

Tools for surveillance of anti-malarial drug resistance: an assessment of the current landscape

To limit the spread and impact of anti-malarial drug resistance and react accordingly, surveillance systems able to detect and track in real-time its emergence and spread need to be strengthened or in some places established. Currently, surveillance of anti-malarial drug resistance is done by any of three approaches: (1) in vivo studies to assess the efficacy of drugs in patients; (2) in vitro/ex vivo studies to evaluate parasite susceptibility to the drugs; and/or (3) molecular assays to detect validated gene mutations and/or gene copy number changes that are associated with drug resistance. These methods are complementary, as they evaluate different aspects of resistance; however, standardization of methods, especially for in vitro/ex vivo and molecular techniques, is lacking. The World Health Organization has developed a standard protocol for evaluating the efficacy of anti-malarial drugs, which is used by National Malaria Control Programmes to conduct their therapeutic efficacy studies. Regional networks, such as the East African Network for Monitoring Antimalarial Treatment and the Amazon Network for the Surveillance of Antimalarial Drug Resistance, have been set up to strengthen regional capacities for monitoring anti-malarial drug resistance. The Worldwide Antimalarial Resistance Network has been established to collate and provide global spatial and temporal trends information on the efficacy of anti-malarial drugs and resistance. While exchange of information across endemic countries is essential for monitoring anti-malarial resistance, sustainable funding for the surveillance and networking activities remains challenging. The technology landscape for molecular assays is progressing quite rapidly, and easy-to-use and affordable new techniques are becoming available. They also offer the advantage of high throughput analysis from a simple blood spots obtained from a finger prick. New technologies combined with the strengthening of national reference laboratories in malaria-endemic countries through standardized protocols and training plus the availability of a proficiency testing programme, would contribute to the improvement and sustainability of anti-malarial resistance surveillance networks worldwide.

Population Pharmacokinetic and Pharmacodynamic Modeling of Artemisinin Resistance in Southeast Asia

Orally administered artemisinin-based combination therapy is the first-line treatment against uncomplicated P. falciparum malaria worldwide. However, the increasing prevalence of artemisinin resistance is threatening efforts to treat and eliminate malaria in Southeast Asia. This study aimed to characterize the exposure-response relationship of artesunate in patients with artemisinin sensitive and resistant malaria infections. Patients were recruited in Pailin, Cambodia (n = 39), and Wang Pha, Thailand (n = 40), and received either 2 mg/kg/day of artesunate mono-therapy for 7 consecutive days or 4 mg/kg/day of artesunate monotherapy for 3 consecutive days followed by mefloquine 15 and 10 mg/kg for 2 consecutive days. Plasma concentrations of artesunate and its active metabolite, dihydroartemisinin, and microscopy-based parasite densities were measured and evaluated using nonlinear mixed-effects modeling. All treatments were well tolerated with minor and transient adverse reactions. Patients in Cambodia had substantially slower parasite clearance compared to patients in Thailand. The pharmacokinetic properties of artesunate and dihydroartemisinin were well described by transit-compartment absorption followed by one-compartment disposition models. Parasite density was a significant covariate, and higher parasite densities were associated with increased absorption. Dihydroartemisinin-dependent parasite killing was described by a delayed sigmoidal Emax model, and a mixture function was implemented to differentiate between sensitive and resistant infections. This predicted that 84% and 16% of infections in Cambodia and Thailand, respectively, were artemisinin resistant. The final model was used to develop a simple diagnostic nomogram to identify patients with artemisinin-resistant infections. The nomogram showed > 80% specificity and sensitivity, and outperformed the current practice of day 3 positivity testing.

In vitro sensitivity of antimalarial drugs and correlation with clinico-parasitological response following treatment with a 3-day artesunate-mefloquine combination in patients with falciparum malaria along the Thai-Myanmar border

A 3-day artesunate-mefloquine combination therapy has been using as first-line treatment for acute uncomplicated Plasmodium falciparum malaria in Thailand since 1995 on the background of mefloquine resistance. The aim of the present study was to assess sensitivity of P. falciparum isolates (n=44) in an area along the Thai-Myanmar border (year 2009) to artesunate, mefloquine, chloroquine and quinine, including their correlation with clinico-parasitological response. Twenty, 19, and 5 isolates were collected from patients with 'Adequate Clinical and Parasitological Response (ACPR)', 'Late Parasitological Failure (LPF)' and 're-infection', respectively. The IC₅₀ of artesunate and mefloquine were significantly higher in patients with LPF compared with ACPR and re-infection. The proportion of isolates with declined artesunate or mefloquine sensitivity in the LPF group (47.4%) was significantly higher than the ACPR group (5.0%). A weak but statistical significant correlation (r=0.384, p=0.01) was observed between IC₅₀ values of artesunate and parasite clearance time (PCT). There was no significant relationship between in vitro sensitivity of parasite isolates to chloroquine or quinine and clinical response. In vitro susceptibility of P. falciparum isolates to artesunate and mefloquine may be used as a useful reliable tool to predict clinico-pathological response following a 3-day artesunate-mefloquine combination therapy.

Attenuation of Plasmodium falciparum in vitro drug resistance phenotype following culture adaptation compared to fresh clinical isolates in Cambodia

Background

There is currently no standardized approach for assessing in vitro anti-malarial drug susceptibility. Potential alterations in drug susceptibility results between fresh immediate ex vivo (IEV) and cryopreserved culture-adapted (CCA) Plasmodium falciparum isolates, as well as changes in parasite genotype during culture adaptation were investigated.

Methods

The 50 % inhibitory concentration (IC₅₀) of 12 P. falciparum isolates from Cambodia against a panel of commonly used drugs were compared using both IEV and CCA. Results were compared using both histidine-rich protein-2 ELISA (HRP-2) and SYBR-Green I fluorescence methods. Molecular genotyping and amplicon deep sequencing were also used to compare multiplicity of infection and genetic polymophisms in fresh versus culture-adapted isolates.

Results

IC₅₀ for culture-adapted specimens were significantly lower compared to the original fresh isolates for both HRP-2 and SYBR-Green I assays, with greater than a 50 % decline for the majority of drug-assay combinations. There were correlations between IC₅₀s from IEV and CCA for most drugs assays. Infections were nearly all monoclonal, with little or no change in merozoite surface protein 1 (MSP1), MSP2, glutamate-rich protein (GLURP) or apical membrane antigen 1 (AMA1) polymorphisms, nor differences in P. falciparum multidrug resistance 1 gene (PfMDR1) copy number or single nucleotide polymorphisms following culture adaptation.

Conclusions

The overall IC₅₀ reduction combined with the correlation between fresh isolates and culture-adapted drug susceptibility assays suggests the utility of both approaches, as long as there is consistency of method, and remaining mindful of possible attenuation of resistance phenotype occurring in culture. Further study should be done in higher transmission settings where polyclonal infections are prevalent.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-1021-8) contains supplementary material, which is available to authorized users.

A cysteine protease inhibitor of plasmodium berghei is essential for exo-erythrocytic development

Plasmodium parasites express a potent inhibitor of cysteine proteases (ICP) throughout their life cycle. To analyze the role of ICP in different life cycle stages, we generated a stage-specific knockout of the Plasmodium berghei ICP (PbICP). Excision of the pbicb gene occurred in infective sporozoites and resulted in impaired sporozoite invasion of hepatocytes, despite residual PbICP protein being detectable in sporozoites. The vast majority of these parasites invading a cultured hepatocyte cell line did not develop to mature liver stages, but the few that successfully developed hepatic merozoites were able to initiate a blood stage infection in mice. These blood stage parasites, now completely lacking PbICP, exhibited an attenuated phenotype but were able to infect mosquitoes and develop to the oocyst stage. However, PbICP-negative sporozoites liberated from oocysts exhibited defective motility and invaded mosquito salivary glands in low numbers. They were also unable to invade hepatocytes, confirming that control of cysteine protease activity is of critical importance for sporozoites. Importantly, transfection of PbICP-knockout parasites with a pbicp-gfp construct fully reversed these defects. Taken together, in P. berghei this inhibitor of the ICP family is essential for sporozoite motility but also appears to play a role during parasite development in hepatocytes and erythrocytes.

Four years' monitoring of in vitro sensitivity and candidate molecular markers of resistance of Plasmodium falciparum to artesunate-mefloquine combination in the Thai-Myanmar border

Background

The decline in efficacy of artesunate (AS) and mefloquine (MQ) is now the major concern in areas along the Thai-Cambodian and Thai-Myanmar borders.

Methods

The correlation between polymorphisms of pfatp6, pfcrt, pfmdr1 and pfmrp1 genes and in vitro sensitivity of Plasmodium falciparum isolates to the artemisinin-based combination therapy (ACT) components AS and MQ, including the previously used first-line anti-malarial drugs chloroquine (CQ) and quinine (QN) were investigated in a total of 119 P. falciparum isolates collected from patients with uncomplicated P. falciparum infection during 2006–2009.

Results

Reduced in vitro parasite sensitivity to AS [median (95% CI) IC₅₀ 3.4 (3.1-3.7) nM] was found in 42% of the isolates, whereas resistance to MQ [median (95% CI) IC₅₀ 54.1 (46.8-61.4) nM] accounted for 58% of the isolates. Amplification of pfmdr1 gene was strongly associated with a decline in susceptibility of P. falciparum isolates to AS, MQ and QN. Significant difference in IC₅₀ values of AS, MQ and QN was observed among isolates carrying one, two, three, and ≥ four gene copies [median (95% CI) AS IC₅₀: 1.6 (1.3-1.9), 1.8 (1.1-2.5), 2.9 (2.1-3.7) and 3.1 (2.5-3.7) nM, respectively; MQ IC₅₀: 19.2 (15.8-22.6), 37.8 (10.7-64.8), 55.3 (47.7-62.9) and 63.6 (49.2-78.0) nM, respectively; and QN IC₅₀: 183.0 (139.9-226.4), 256.4 (83.7-249.1), 329.5 (206.6-425.5) and 420.0 (475.2-475.6) nM, respectively]. The prevalence of isolates which were resistant to QN was reduced from 21.4% during the period 2006–2007 to 6.3% during the period 2008–2009. Pfmdr1 86Y was found to be associated with increased susceptibility of the parasite to MQ and QN. Pfmdr1 1034C was associated with decreased susceptibility to QN. Pfmrp1 191Y and 1390I were associated with increased susceptibility to CQ and QN, respectively.

Conclusion

High prevalence of CQ and MQ-resistant P. falciparum isolates was observed during the four-year observation period (2006–2009). AS sensitivity was declined, while QN sensitivity was improved. Pfmdr1 and pfmrp1 appear to be the key genes that modulate multidrug resistance in P. falciparum.

Direct comparison of the histidine-rich protein-2 enzyme-linked immunosorbent assay (HRP-2 ELISA) and malaria SYBR green I fluorescence (MSF) drug sensitivity tests in Plasmodium falciparum reference clones and fresh ex vivo field isolates from Cambodia

Background

Performance of the histidine-rich protein-2 enzyme-linked immunosorbent assay (HRP-2 ELISA) and malaria SYBR Green I fluorescence (MSF) drug sensitivity tests were directly compared using Plasmodium falciparum reference strains and fresh ex vivo isolates from Cambodia against a panel of standard anti-malarials. The objective was to determine which of these two common assays is more appropriate for studying drug susceptibility of “immediate ex vivo” (IEV) isolates, analysed without culture adaption, in a region of relatively low malaria transmission.

Methods

Using the HRP-2 and MSF methods, the 50% inhibitory concentration (IC₅₀) values against a panel of malaria drugs were determined for P. falciparum reference clones (W2, D6, 3D7 and K1) and 41 IEV clinical isolates from an area of multidrug resistance in Cambodia. Comparison of the IC₅₀ values from the two methods was made using Wilcoxon matched pair tests and Pearson’s correlation. The lower limit of parasitaemia detection for both methods was determined for reference clones and IEV isolates. Since human white blood cell (WBC) DNA in clinical samples is known to reduce MSF assay sensitivity, SYBR Green I fluorescence linearity of P. falciparum samples spiked with WBCs was evaluated to assess the relative degree to which MSF sensitivity is reduced in clinical samples.

Results

IC₅₀ values correlated well between the HRP-2 and MSF methods when testing either P. falciparum reference clones or IEV isolates against 4-aminoquinolines (chloroquine, piperaquine and quinine) and the quinoline methanol mefloquine (Pearson r = 0.85-0.99 for reference clones and 0.56-0.84 for IEV isolates), whereas a weaker IC₅₀ value correlation between methods was noted when testing artemisinins against reference clones and lack of correlation when testing IEV isolates. The HRP-2 ELISA produced a higher overall success rate (90% for producing IC₅₀ best-fit sigmoidal curves), relative to only a 40% success rate for the MSF assay, when evaluating ex vivo Cambodian isolates. Reduced sensitivity of the MSF assay is likely due to an interference of WBCs in clinical samples.

Conclusions

For clinical samples not depleted of WBCs, HRP-2 ELISA is superior to the MSF assay at evaluating fresh P. falciparum field isolates with low parasitaemia (<0.2%) generally observed in Southeast Asia.

Optimizing the HRP-2 in vitro malaria drug susceptibility assay using a reference clone to improve comparisons of Plasmodium falciparum field isolates

Background

Apparent emerging artemisinin-resistant Plasmodium falciparum malaria in Southeast Asia requires development of practical tools to monitor for resistant parasites. Although in vitro anti-malarial susceptibility tests are widely used, uncertainties remain regarding interpretation of P. falciparum field isolate values.

Methods

Performance parameters of the W2 P. falciparum clone (considered artemisinin “sensitive”) were evaluated as a reference for the HRP-2 immediate ex vivo assay. Variability in W2 IC₅₀s was assessed, including intra- and inter-assay variability among and between technicians in multiple experiments, over five freeze-thaw cycles, over five months of continuous culture, and before and after transport of drug-coated plates to remote field sites. Nominal drug plate concentrations of artesunate (AS) and dihydroartemisinin (DHA) were verified by LC-MS analysis. Plasmodium falciparum field isolate IC₅₀s for DHA from subjects in an artemisinin-resistant area in Cambodia were compared with W2 susceptibility.

Results

Plate drug concentrations and day-to-day technical assay performance among technicians were important sources of variability for W2 IC₅₀s within and between assays. Freeze-thaw cycles, long-term continuous culture, and transport to and from remote sites had less influence. Despite variability in W2 susceptibility, the median IC₅₀s for DHA for Cambodian field isolates were higher (p <0.0001) than the W2 clone (3.9 nM), both for subjects with expected (less than 72 hours; 6.3 nM) and prolonged (greater or equal to 72 hours; 9.6 nM) parasite clearance times during treatment with artesunate monotherapy.

Conclusion

The W2 reference clone improved the interpretability of field isolate susceptibility from the immediate ex vivo HRP-2 assay from areas of artemisinin resistance. Methods to increase the reproducibility of plate coating may improve overall assay interpretability and utility.

Ex vivo drug sensitivity profiles of Plasmodium falciparum field isolates from Cambodia and Thailand, 2005 to 2010, determined by a histidine-rich protein-2 assay

Background

In vitro drug susceptibility assay of Plasmodium falciparum field isolates processed “immediate ex vivo” (IEV), without culture adaption, and tested using histidine-rich protein-2 (HRP-2) detection as an assay, is an expedient way to track drug resistance.

Methods

From 2005 to 2010, a HRP-2 in vitro assay assessed 451 P. falciparum field isolates obtained from subjects with malaria in western and northern Cambodia, and eastern Thailand, processed IEV, for 50% inhibitory concentrations (IC₅₀) against seven anti-malarial drugs, including artesunate (AS), dihydroartemisinin (DHA), and piperaquine.

Results

In western Cambodia, from 2006 to 2010, geometric mean (GM) IC₅₀ values for chloroquine, mefloquine, quinine, AS, DHA, and lumefantrine increased. In northern Cambodia, from 2009–2010, GM IC₅₀ values for most drugs approximated the highest western Cambodia GM IC₅₀ values in 2009 or 2010.

Conclusions

Western Cambodia is associated with sustained reductions in anti-malarial drug susceptibility, including the artemisinins, with possible emergence, or spread, to northern Cambodia. This potential public health crisis supports continued in vitro drug IC₅₀ monitoring of P. falciparum isolates at key locations in the region.

Molecular analysis of pfatp6 and pfmdr1 polymorphisms and their association with in vitro sensitivity in Plasmodium falciparum isolates from the Thai-Myanmar border

The association between pfatp6, pfmdr1 polymorphisms (gene mutation and amplification) and in vitro susceptibility to mefloquine (MQ), artesunate (AS), quinine (QN), and chloroquine (CQ) was investigated in a total of sixty-three Plasmodium falciparum isolates collected from the Thai-Myanmar border. The mutations of pfatp6 at codons R37K, G639D, S769N, and I898I and of pfmdr1 at codons N86Y, Y184F, N1042D, and D1246Y were analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Pfatp6 and pfmdr1 gene copy numbers were analyzed by quantitative real time-polymerase chain reaction (qRT-PCR). In vitro susceptibility test was successful in 58 culture-adapted isolates. Median (range) IC(50) values for MQ, AS, QN, and CQ were 28.96 (3.4-100.5), 1.74 (0.8-5.57), 223.9 (14.99-845.47), and 69.93 (9.6-183.18) nM, respectively. There was a significant positive correlation (R(2) = 0.58) of parasite susceptibility to MQ, AS, and QN. Twelve isolates showed marked decline in susceptibility to AS [median (range) IC(50) = 3.78 (3.07-5.57) nM]. Almost all isolates carried wild-type pfatp6 and pfmdr1 alleles at the investigated codons, while only three isolates (5%) carried pfmdr1 mutation alleles at codon 86. Mutation at codon 86 was associated with a significant increase in the susceptibility of parasite isolates to MQ and QN. All of the sixty-three isolates carried only one pfatp6 copy number. Thirty-three out of the 58 isolates showed increase in pfmdr1 gene copies, which was associated with reduced in vitro susceptibility to MQ, AS, and QN. No association between mutation or amplification of pfatp6 gene and in vitro susceptibility of P. falciparum isolates was found.

How artemisinin-containing combination therapies slow the spread of antimalarial drug resistance

Antimalarial drug therapies containing artemisinins, 'ACTs', have become the mainstay for treating uncomplicated malaria in endemic countries. This is a major public health achievement requiring substantial political, financial and scientific input. The most compelling scientific argument for ACT deployment employed a very simple basic rationale that emphasised their role in slowing the origin of drug resistance while largely neglecting the additional role(s) of ACTs in slowing or preventing the spread of resistance once it has arisen. Recent reports suggest that early stages of resistance to artemisinins and/or its partner drugs could be occurring, thus it is timely to briefly review exactly how ACTs slow the origin and spread of resistance and to interpret the threat of resistance within this context.

Functional studies of Plasmodium falciparum dipeptidyl aminopeptidase I using small molecule inhibitors and active site probes

The widespread resistance of malaria parasites to all affordable drugs has made the identification of new targets urgent. Dipeptidyl aminopeptidases (DPAPs) represent potentially valuable new targets that are involved in hemoglobin degradation (DPAP1) and parasite egress (DPAP3). Here we use activity-based probes to demonstrate that specific inhibition of DPAP1 by a small molecule results in the formation of an immature trophozoite that leads to parasite death. Using computational methods, we designed stable, nonpeptidic covalent inhibitors that kill Plasmodium falciparum at low nanomolar concentrations. These compounds show signs of slowing parasite growth in a murine model of malaria, which suggests that DPAP1 might be a viable antimalarial target. Interestingly, we found that resynthesis and activation of DPAP1 after inhibition is rapid, suggesting that effective drugs would need to sustain DPAP1 inhibition for a period of 2-3 hr.

An open-label, randomised study of dihydroartemisinin-piperaquine versus artesunate-mefloquine for falciparum malaria in Asia

BACKGROUND

The artemisinin-based combination treatment (ACT) of dihydroartemisinin (DHA) and piperaquine (PQP) is a promising novel anti-malarial drug effective against multi-drug resistant falciparum malaria. The aim of this study was to show non-inferiority of DHA/PQP vs. artesunate-mefloquine (AS+MQ) in Asia.

METHODS AND FINDINGS

This was an open-label, randomised, non-inferiority, 63-day follow-up study conducted in Thailand, Laos and India. Patients aged 3 months to 65 years with Plasmodium falciparum mono-infection or mixed infection were randomised with an allocation ratio of 2:1 to a fixed-dose DHA/PQP combination tablet (adults: 40 mg/320 mg; children: 20 mg/160 [DOSAGE ERROR CORRECTED] mg; n = 769) or loose combination of AS+MQ (AS: 50 mg, MQ: 250 mg; n = 381). The cumulative doses of study treatment over the 3 days were of about 6.75 mg/kg of DHA and 54 mg/kg of PQP and about 12 mg/kg of AS and 25 mg/kg of MQ. Doses were rounded up to the nearest half tablet. The primary endpoint was day-63 polymerase chain reaction (PCR) genotype-corrected cure rate. Results were 87.9% for DHA/PQP and 86.6% for AS+MQ in the intention-to-treat (ITT; 97.5% one-sided confidence interval, CI: >-2.87%), and 98.7% and 97.0%, respectively, in the per protocol population (97.5% CI: >-0.39%). No country effect was observed. Kaplan-Meier estimates of proportions of patients with new infections on day 63 (secondary endpoint) were significantly lower for DHA/PQP than AS+MQ: 22.7% versus 30.3% (p = 0.0042; ITT). Overall gametocyte prevalence (days 7 to 63; secondary endpoint), measured as person-gametocyte-weeks, was significantly higher for DHA/PQP than AS+MQ (10.15% versus 4.88%; p = 0.003; ITT). Fifteen serious adverse events were reported, 12 (1.6%) in DHA/PQP and three (0.8%) in AS+MQ, among which six (0.8%) were considered related to DHA/PQP and three (0.8%) to AS+MQ.

CONCLUSIONS

DHA/PQP was a highly efficacious drug for P. falciparum malaria in areas where multidrug parasites are prevalent. The DHA/PQP combination can play an important role in the first-line treatment of uncomplicated falciparum malaria.

TRIAL REGISTRATION

Controlled-Trials.com ISRCTN81306618.

Monitoring of in vitro susceptibilities and molecular markers of resistance of Plasmodium falciparum isolates from Thai-Myanmar border to chloroquine, quinine, mefloquine and artesunate

Malaria is one of the major causes of morbidity and mortality worldwide. The major factor which has aggravated the situation is the emergence of multidrug resistant Plasmodium falciparum malaria. To successfully deal with the problem, thorough understanding of the molecular bases for reduced parasite sensitivity to existing antimalarial drugs is of considerable importance. The objective of this work was to broaden the insight into the molecular mechanisms of resistance of P. falciparum to quinoline-containing antimalarials and artemisinin derivatives. Polymorphisms of the candidate genes pfmdr1 and pfcrt were investigated in relation to the susceptibility (in vitro sensitivity) of P. falciparum isolates to chloroquine (CQ), mefloquine (MQ), quinine (QN) and the artemisinin derivative - artesunate (AS). A total of 26 P. falciparum isolates were successful cultured. In vitro sensitivity results indicate the increase in susceptibility of P. falciparum strains in Thailand to CQ, while the susceptibility to MQ and QN was markedly declined. The pattern of cross-resistance was observed between MQ vs QN vs AS. Only one point mutation in the pfmdr1 gene, i.e., N86Y was observed with low prevalence of 7.7% (2/26). In contrast, the mutations at positions 76T, 220S, 271E, 326S, 356T and 371I in the pfcrt gene were identified in almost all isolates (25 isolates, 96.2%). The association between polymorphisms of the pfmdr1 and susceptibility of the parasite to MQ and QN was observed (increased susceptibilities to MQ and QN in isolates with mutations). Moreover, the correlation between pfmdr1 gene amplification and susceptibility of the parasite to MQ, QN and AS was observed (decreased susceptibilities to MQ, QN and AS in isolates with increased pfmdr1 copy number).

Effectiveness of quinine versus artemether-lumefantrine for treating uncomplicated falciparum malaria in Ugandan children: randomised trial

OBJECTIVE

To compare the effectiveness of oral quinine with that of artemether-lumefantrine in treating uncomplicated malaria in children.

DESIGN

Randomised, open label effectiveness study.

SETTING

Outpatient clinic of Uganda's national referral hospital in Kampala.

PARTICIPANTS

175 children aged 6 to 59 months with uncomplicated malaria.

INTERVENTIONS

Participants were randomised to receive oral quinine or artemether-lumefantrine administered by care givers at home.

MAIN OUTCOME MEASURES

Primary outcomes were parasitological cure rates after 28 days of follow-up unadjusted and adjusted by genotyping to distinguish recrudescence from new infections. Secondary outcomes were adherence to study drug, presence of gametocytes, recovery of haemoglobin concentration from baseline at day 28, and safety profiles.

RESULTS

Using survival analysis the cure rate unadjusted by genotyping was 96% for the artemether-lumefantrine group compared with 64% for the quinine group (hazard ratio 10.7, 95% confidence interval 3.3 to 35.5, P=0.001). In the quinine group 69% (18/26) of parasitological failures were due to recrudescence compared with none in the artemether-lumefantrine group. The mean adherence to artemether-lumefantrine was 94.5% compared with 85.4% to quinine (P=0.0008). Having adherence levels of 80% or more was associated with a decreased risk of treatment failure (0.44, 0.19 to 1.02, P=0.06). Adverse events did not differ between the two groups.

CONCLUSIONS

The effectiveness of a seven day course of quinine for the treatment of uncomplicated malaria in Ugandan children was significantly lower than that of artemether-lumefantrine. These findings question the advisability of the recommendation for quinine therapy for uncomplicated malaria in Africa.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00540202.

Drug resistance and in vitro susceptibility of Plasmodium falciparum in Thailand during 1988-2003

The aim of the present study was to investigate antimalarial drug pressure resulting from the clinical use of different antimalarials in Thailand. The phenotypic diversity of the susceptibility profiles of antimalarials, i.e., chloroquine (CQ), quinine (QN), mefloquine (MQ), and artesunate (ARS) in Plasmodium falciparum isolates collected during the period from 1988 to 2003 were studied. P. falciparum isolates from infected patients were collected from the Thai-Cambodian border area at different time periods (1988-1989, 1991-1992, and 2003), during which 3 different patterns of drug use had been implemented: MQ + sulphadoxine (S) + pyrimethamine (P), MQ alone and MQ + ARS, respectively. The in vitro drug susceptibilities were investigated using a method based on the incorporation of [(3)H] hypoxanthine. A total of 50 isolates were tested for susceptibilities to CQ, QN, MQ, and ARS. Of these isolates, 19, 16, and 15 were adapted during the periods 1988-1989, 1991-1993, and 2003, respectively. P. falciparum isolates collected during the 3 periods were resistant to CQ. Sensitivities to MQ declined from 1988 to 2003. In contrast, the parasite was sensitive to QN, and similar sensitivity profile patterns were observed during the 3 time periods. There was a significantly positive but weak correlation between the IC(50) values of CQ and QN, as well as between the IC(50) values of QN and MQ. Drug pressure has impact on sensitivity of P. falciparum to MQ. A combination therapy of MQ and ARS is being applied to reduce the parasite resistance, and also increasing the efficacy of the drug.

Drug resistance and genetic mapping in Plasmodium falciparum

Drug resistance in malaria parasites is a serious public health burden, and resistance to most of the antimalarial drugs currently in use has been reported. A better understanding of the molecular mechanisms of drug resistance is urgently needed to slow or circumvent the spread of resistance, to allow local treatments to be deployed more effectively to prolong the life span of the current drugs, and to develop new drugs. Although mutations in genes determining resistance to drugs such as chloroquine and the antifolates have been identified, we still do not have a full understanding of the resistance mechanisms, and genes that contribute to resistance to many other drugs remain to be discovered. Genetic mapping is a powerful tool for the identification of mutations conferring drug resistance in malaria parasites because most drug-resistant phenotypes were selected within the past 60 years. High-throughput methods for genotyping large numbers of single nucleotide polymorphisms (SNPs) and microsatellites (MSs) are now available or are being developed, and genome-wide association studies for malaria traits will soon become a reality. Here we discuss strategies and issues related to mapping genes contributing to drug resistance in the human malaria parasite Plasmodium falciparum.

Malaria Chemotherapeutics Part I: History of Antimalarial Drug Development, Currently Used Therapeutics, and Drugs in Clinical Development

Since ancient times, humankind has had to struggle against the persistent onslaught of pathogenic microorganisms. Nowadays, malaria is still the most important infectious disease worldwide. Considerable success in gaining control over malaria was achieved in the 1950s and 60s through landscaping measures, vector control with the insecticide DDT, and the widespread administration of chloroquine, the most important antimalarial agent ever. In the late 1960s, the final victory over malaria was believed to be within reach. However, the parasites could not be eradicated because they developed resistance against the most widely used and affordable drugs of that time. Today, cases of malaria infections are on the rise and have reached record numbers. This review gives a short description of the malaria disease, briefly addresses the history of antimalarial drug development, and focuses on drugs currently available for malaria therapy. The present knowledge regarding their mode of action and the mechanisms of resistance are explained, as are the attempts made by numerous research groups to overcome the resistance problem within classes of existing drugs and in some novel classes. Finally, this review covers all classes of antimalarials for which at least one drug candidate is in clinical development. Antimalarial agents that are solely in early development stages will be addressed in a separate review.