In Papua, Indonesia, the antimalarial susceptibility of Plasmodium vivax (n ‫؍‬ 216) and P. falciparum (n ‫؍‬ 277) was assessed using a modified schizont maturation assay for chloroquine, amodiaquine, artesunate, lumefantrine, mefloquine, and piperaquine. The most effective antimalarial against P. vivax and P. falciparum was artesunate, with geometric mean 50% inhibitory concentrations (IC 50 In vitro drug susceptibility assays assess antimicrobial activity in the absence of the confounding effects of the host. Although such assays have become useful for monitoring the antimalarial resistance of Plasmodium falciparum, the assay has been of limited use with P. vivax. This is in part a consequence of a perception of the importance of antimalarial drug resistance with P. vivax, compounded by difficulties in standardizing a field-based assay. Over the last decade, a number of clinical studies have demonstrated the emergence of highgrade chloroquine resistance in Papua, Indonesia, and Papua, New Guinea (1, 18, 21), and its spread to other regions of Asia (6) and South America (20). However, assessment of the clinical efficacy of antimalarial drugs against P. vivax infection is confounded by the occurrence of both reinfections and relapses, making the attributable fraction of recurrent infections due to intrinsic parasite resistance difficult to gauge (2, 3, 10). To confirm the emergence of the spread of antimalarial drug resistance of P. vivax and to investigate alternative antimalarial drugs, it is critical that a standardized in vitro assay be developed and validated. The aim of this study was to define the in vitro susceptibility profiles of a range of antimalarial drugs and to investigate the confounding factors that modulate the derived estimate of drug efficacy. MATERIALS AND METHODS Field location and sample collection. Between March 2004 and May 2007,Plasmodium isolates were collected from patients attending malaria clinics in Timika, located in the southern part of Papua province, Indonesia. Timika is a region of endemicity for multidrug-resistant strains of P. vivax and P. falciparum, with a risk of treatment failure of 65% within 28 days after chloroquine monotherapy for P. vivax malaria and 48% failure after multidrug therapy with chloroquine-sulfadoxine-pyrimethamine for P. falciparum malaria (16). In 2004, treatment guidelines were changed accordingly to recommend an artemisinin combination therapy for both P. falciparum and P. vivax infection, precluding further clinical studies of the use of chloroquine monotherapy in this region (15). Patients with symptomatic malaria who presented to an outpatient facility were recruited into the study if they were singly infected with P. falciparum or with P. vivax, with a parasitemia of between 2,000 l Ϫ1 and 80,000 l Ϫ1 . Patients treated with antimalarials in the previous 3 weeks were excluded from this study. Venous blood (5 ml) was collected by venipuncture and, after the host white blood cells were removed using a CF11 column, 2 ml of packed infected red bl...

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Background A very large biomass of intact asexual-stage malaria parasites accumulates in the spleen of asymptomatic human individuals infected with Plasmodium vivax. The mechanisms underlying this intense tropism are not clear. We hypothesised that immature reticulocytes, in which P. vivax develops, may display high densities in the spleen, thereby providing a niche for parasite survival. Methods and findings We examined spleen tissue in 22 mostly untreated individuals naturally exposed to P. vivax and Plasmodium falciparum undergoing splenectomy for any clinical indication in malaria-endemic Papua, Indonesia (2015 to 2017). Infection, parasite and immature reticulocyte density, and splenic distribution were analysed by optical microscopy, flow cytometry, and molecular assays. Nine non-endemic control spleens from individuals undergoing spleno-pancreatectomy in France (2017 to 2020) were also examined for reticulocyte densities. There were no exclusion criteria or sample size considerations in both patient cohorts for this demanding approach. In Indonesia, 95.5% (21/22) of splenectomy patients had asymptomatic splenic Plasmodium infection (7 P. vivax, 13 P. falciparum, and 1 mixed infection). Significant splenic accumulation of immature CD71 intermediate- and high-expressing reticulocytes was seen, with concentrations 11 times greater than in peripheral blood. Accordingly, in France, reticulocyte concentrations in the splenic effluent were higher than in peripheral blood. Greater rigidity of reticulocytes in splenic than in peripheral blood, and their higher densities in splenic cords both suggest a mechanical retention process. Asexual-stage P. vivax-infected erythrocytes of all developmental stages accumulated in the spleen, with non-phagocytosed parasite densities 3,590 times (IQR: 2,600 to 4,130) higher than in circulating blood, and median total splenic parasite loads 81 (IQR: 14 to 205) times greater, accounting for 98.7% (IQR: 95.1% to 98.9%) of the estimated total-body P. vivax biomass. More reticulocytes were in contact with sinus lumen endothelial cells in P. vivax- than in P. falciparum-infected spleens. Histological analyses revealed 96% of P. vivax rings/trophozoites and 46% of schizonts colocalised with 92% of immature reticulocytes in the cords and sinus lumens of the red pulp. Larger splenic cohort studies and similar investigations in untreated symptomatic malaria are warranted. Conclusions Immature CD71+ reticulocytes and splenic P. vivax-infected erythrocytes of all asexual stages accumulate in the same splenic compartments, suggesting the existence of a cryptic endosplenic lifecycle in chronic P. vivax infection. Findings provide insight into P. vivax-specific adaptions that have evolved to maximise survival and replication in the spleen.

Background Neutrophil activation results in Plasmodium parasite killing in vitro, but neutrophil products including neutrophil extracellular traps (NETs) mediate host organ damage and may contribute to severe malaria. The role of NETs in the pathogenesis of severe malaria has not been examined. Methods In Papua, Indonesia, we enrolled adults with symptomatic Plasmodium falciparum (n = 47 uncomplicated, n = 8 severe), Plasmodium vivax (n = 37), or Plasmodium malariae (n = 14) malaria; asymptomatic P falciparum (n = 19) or P vivax (n = 21) parasitemia; and healthy adults (n = 23) without parasitemia. Neutrophil activation and NETs were quantified by immunoassays and microscopy and correlated with parasite biomass and disease severity. Results In patients with symptomatic malaria, neutrophil activation and NET counts were increased in all 3 Plasmodium species. In falciparum malaria, neutrophil activation and NET counts positively correlated with parasite biomass (Spearman rho = 0.41, P = .005 and r2 = 0.26, P = .002, respectively) and were significantly increased in severe disease. In contrast, NETs were inversely associated with parasitemia in adults with asymptomatic P falciparum infection (r2 = 0.24, P = .031) but not asymptomatic P vivax infection. Conclusions Although NETs may inhibit parasite growth in asymptomatic P falciparum infection, neutrophil activation and NET release may contribute to pathogenesis in severe falciparum malaria. Agents with potential to attenuate these processes should be evaluated.

Histone acetylation plays an important role in regulating gene transcription and silencing in Plasmodium falciparum. Histone deacetylase (HDAC) inhibitors, particularly those of the hydroxamate class, have been shown to have potent in vitro activity against drug-resistant and -sensitive laboratory strains of P. falciparum, raising their potential as a new class of antimalarial compounds. In the current study, stage-specific ex vivo susceptibility profiles of representative hydroxamate-based HDAC inhibitors suberoylanilide hydroxamic acid (SAHA), 2-ASA-9, and 2-ASA-14 (2-ASA-9 and 2-ASA-14 are 2-aminosuberic acid-based HDAC inhibitors) were assessed in multidrug-resistant clinical isolates of P. falciparum (n ‫؍‬ 24) and P. vivax (n ‫؍‬ 25) from Papua, Indonesia, using a modified schizont maturation assay. Submicromolar concentrations of SAHA, 2-ASA-9, and 2-ASA-14 inhibited the growth of both P. falciparum (median 50% inhibitory concentrations [IC 50 s] of 310, 533, and 266 nM) and P. vivax (median IC 50 s of 170, 503, and 278 nM). Inverse correlation patterns between HDAC inhibitors and chloroquine for P. falciparum and mefloquine for P. vivax indicate species-specific susceptibility profiles for HDAC inhibitors. These HDAC inhibitors were also found to be potent ex vivo against P. vivax schizont maturation, comparable to that in P. falciparum, suggesting that HDAC inhibitors may be promising candidates for antimalarial therapy in geographical locations where both species are endemic. Further studies optimizing the selectivity and in vivo efficacy of HDAC inhibitors in Plasmodium spp. and defining drug interaction with common antimalarial compounds are warranted to investigate the role of HDAC inhibitors in antimalarial therapy.

iThe declining efficacy of artemisinin derivatives against Plasmodium falciparum highlights the urgent need to identify alternative highly potent compounds for the treatment of malaria. In Papua Indonesia, where multidrug resistance has been documented against both P. falciparum and P. vivax malaria, comparative ex vivo antimalarial activity against Plasmodium isolates was assessed for the artemisinin derivatives artesunate (AS) and dihydroartemisinin (DHA), the synthetic peroxides OZ277 and OZ439, the semisynthetic 10-alkylaminoartemisinin derivatives artemisone and artemiside, and the conventional antimalarial drugs chloroquine (CQ), amodiaquine (AQ), and piperaquine (PIP). Ex vivo drug susceptibility was assessed in 46 field isolates (25 P. falciparum and 21 P. vivax). The novel endoperoxide compounds exhibited potent ex vivo activity against both species, but significant differences in intrinsic activity were observed. Compared to AS and its active metabolite DHA, all the novel compounds showed lower or equal 50% inhibitory concentrations (IC 50 s) in both species (median IC 50 s between 1.9 and 3.6 nM in P. falciparum and 0.7 and 4.6 nM in P. vivax). The antiplasmodial activity of novel endoperoxides showed different cross-susceptibility patterns in the two Plasmodium species: whereas their ex vivo activity correlated positively with CQ, PIP, AS, and DHA in P. falciparum, the same was not apparent in P. vivax. The current study demonstrates for the first time potent activity of novel endoperoxides against drug-resistant P. vivax. The high activity against drug-resistant strains of both Plasmodium species confirms these compounds to be promising candidates for future artemisinin-based combination therapy (ACT) regimens in regions of coendemicity.A pproximately 3.3 billion people (i.e., almost 50% of the world's population) are at risk of malaria with two Plasmodium species responsible for the majority of infections: P. falciparum and P. vivax (6,7,43). Traditionally, malaria control and research efforts have focused on P. falciparum, the dominant species in Africa. However, outside Africa, P. falciparum almost invariably coexists with P. vivax (7), with both species inflicting significant morbidity, particularly in infants and pregnant women (18,27).Chloroquine (CQ)-resistant P. falciparum is already well established, with emerging evidence that susceptibility to CQ in P. vivax is also declining across much of the world in which vivax is endemic. This combined threat is driving the investigation of alternative schizonticidal treatment regimens, such as artemisininbased combination therapy (ACT), for deployment against both P. falciparum and P. vivax (29). ACTs have become the mainstay of antimalarial chemotherapy, adopted in more than 100 countries worldwide (42). This huge demand for artemisinin and its derivatives relies on isolation from the plant source Artemisia annua and is vulnerable to harvest and production costs and intermittent supply (2, 11). Of particular concern are recent reports of prolonged ...

Ferroquine (FQ; SSR97193), a ferrocene-containing 4-aminoquinoline derivate, has potent in vitro efficacy against chloroquine (CQ)-resistant Plasmodium falciparum and CQ-sensitive P. vivax. In the current study, ex vivo FQ activity was tested in multidrug-resistant P. falciparum and P. vivax field isolates using a schizont maturation assay. Although FQ showed excellent activity against CQ-sensitive and -resistant P. falciparum and P. vivax (median 50% inhibitory concentrations [IC 50 s], 9.6 nM and 18.8 nM, respectively), there was significant cross-susceptibility with the quinoline-based drugs chloroquine, amodiaquine, and piperaquine (for P. falciparum, r ‫؍‬ 0.546 to 0.700, P < 0.001; for P. vivax, r ‫؍‬ 0.677 to 0.821, P < 0.001). The observed ex vivo cross-susceptibility is likely to reflect similar mechanisms of drug uptake/efflux and modes of drug action of this drug class. However, the potent activity of FQ against resistant isolates of both P. falciparum and P. vivax highlights a promising role for FQ as a lead antimalarial against CQ-resistant Plasmodium and a useful partner drug for artemisinin-based combination therapy.

BackgroundThe emergence and spread of multidrug-resistant Plasmodium falciparum and Plasmodium vivax highlights the need for objective measures of ex vivo drug susceptibility. Flow cytometry (FC) has potential to provide a robust and rapid quantification of ex vivo parasite growth.MethodsField isolates from Papua, Indonesia, underwent ex vivo drug susceptibility testing against chloroquine, amodiaquine, piperaquine, mefloquine, and artesunate. A single nucleic acid stain (i.e., hydroethidine (HE) for P. falciparum and SYBR Green I (SG) for P. vivax) was used to quantify infected red blood cells by FC-based signal detection. Data derived by FC were compared to standard quantification by light microscopy (LM). A subset of isolates was used to compare single and double staining techniques.ResultsIn total, 57 P. falciparum and 23 P. vivax field isolates were collected for ex vivo drug susceptibility testing. Reliable paired data between LM and FC was obtained for 88 % (295/334) of these assays. The median difference of derived IC50 values varied from −5.4 to 6.1 nM, associated with 0.83–1.23 fold change in IC50 values between LM and FC. In 15 assays (5.1 %), the derived difference of IC50 estimates was beyond the 95 % limits of agreement; in eleven assays (3.7 %), this was attributable to low parasite growth (final schizont count < 40 %), and in four assays (1.4 %) due to low initial parasitaemia at the start of assay (<2000 µl−1). In a subset of seven samples, LM, single and double staining FC techniques generated similar IC50 values.ConclusionsA single staining FC-based assay using a portable cytometer provides a simple, fast and versatile platform for field surveillance of ex vivo drug susceptibility in clinical P. falciparum and P. vivax isolates.