Background Artemisinin derivatives are the leading class of antimalarial drugs due to their rapid onset of action and rapid clearance of circulating parasites. The parasite clearance (PC) half-life measures the rate of loss of parasites from blood after treatment, and this is currently used to assess antimalarial activity of novel agents and to monitor resistance. However, a number of recent studies have challenged the use of PC to measure drug activity, arguing that many circulating parasites may be non-viable. Methods Plasmodium falciparum infected subjects (n=10) in a malaria volunteer infection study were administered a single dose of artesunate (2 mg/kg). Circulating parasite concentration was assessed by quantitative PCR (qPCR). Parasite viability after artesunate administration was estimated by mathematical modelling of the ex vivo growth of parasites collected from subjects. Results We showed that in artemisinin-sensitive infection, viable parasites declined to <0.1% of baseline within 8 h after artesunate administration, while the total number of circulating parasites measured by qPCR remained unchanged. In artemisinin-resistant infections over the same interval, viable parasites declined to 51.4% (SEM: 4.6%) of baseline. Conclusions These results demonstrate that in vivo drug activity of artesunate is faster than is indicated by the PC half-life.

The rate at which parasitemia declines in a host after treatment with an antimalarial drug is a major metric for assessment of antimalarial drug activity in preclinical models and in early clinical trials. However, this metric does not distinguish between viable and nonviable parasites. Thus, enumeration of parasites may result in underestimation of drug activity for some compounds, potentially confounding its use as a metric for assessing antimalarial activity in vivo . Here, we report a study of the effect of artesunate on Plasmodium falciparum viability in humans and in mice.

Background In the absence of a method to culture Plasmodium vivax, the only way to source parasites is ex vivo. This hampers many aspects of P. vivax research. This study aimed to assess the safety of apheresis, a method for selective removal of specific components of blood as a means of extracting and concentrating P. vivax parasites. Methods An iterative approach was employed across four non-immune healthy human subjects in single subject cohorts. All four subjects were inoculated with ~ 564 blood stage P. vivax (HMP013-Pv) and subjected to apheresis 10 to 11 days later. Blood samples collected during apheresis (haematocrit layers 0.5% to 11%) were tested for the presence and concentration of P. vivax by microscopy, flow cytometry, 18S rDNA qPCR for total parasites, and pvs25 qRT-PCR for female gametocyte transcripts. Safety was determined by monitoring adverse events. Malaria transmission to mosquitoes was assessed by membrane feeding assays. Results There were no serious adverse events and no significant safety concerns. Apheresis concentrated asexual parasites by up to 4.9-fold (range: 0.9–4.9-fold) and gametocytes by up to 1.45-fold (range: 0.38–1.45-fold) compared to pre-apheresis densities. No single haematocrit layer contained > 40% of all the recovered P. vivax asexual parasites. Ex vivo concentration of parasites by Percoll gradient centrifugation of whole blood achieved greater concentration of gametocytes than apheresis. Mosquito transmission was enhanced by up to fivefold in a single apheresis sample compared to pre-apheresis. Conclusion The modest level of parasite concentration suggests that the use of apheresis may not be an ideal method for harvesting P. vivax. Trial Registration Australia New Zealand Clinical Trials Registry (ANZCTR) Trial ID: ACTRN12617001502325 registered on 19th October 2017. https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=373812.

Background Blocking the transmission of parasites from humans to mosquitoes is a key component of malaria control. Tafenoquine exhibits activity against all stages of the malaria parasite and may have utility as a transmission blocking agent. We aimed to characterize the transmission blocking activity of low dose tafenoquine. Methods Healthy adults were inoculated with P. falciparum 3D7-infected erythrocytes on day 0. Piperaquine was administered on days 9 and 11 to clear asexual parasitemia while allowing gametocyte development. A single 50 mg oral dose of tafenoquine was administered on day 25. Transmission was determined by enriched membrane feeding assays pre-dose and at 1, 4 and 7 days post-dose. Artemether-lumefantrine was administered following the final assay. Outcomes were the reduction in mosquito infection and gametocytemia post-tafenoquine, and safety parameters. Results Six participants were enrolled, and all were infective to mosquitoes pre-tafenoquine, with a median 86% (range: 22–98) of mosquitoes positive for oocysts and 57% (range: 4–92) positive for sporozoites. By day 4 post-tafenoquine, the oocyst and sporozoite positivity rate had reduced by a median 35% (IQR: 16–46) and 52% (IQR: 40–62), respectively, and by day 7, 81% (IQR 36–92) and 77% (IQR 52–98), respectively. The decline in gametocyte density post-tafenoquine was not significant. No significant participant safety concerns were identified. Conclusion Low dose tafenoquine (50 mg) reduces P. falciparum transmission to mosquitoes, with a delay in effect.

Background In the absence of a method to culture Plasmodium vivax, the only way to source parasites is ex vivo. This hampers many aspects of P. vivax research. This study aimed to assess the safety of apheresis, a method for selective removal of specific components of blood as a means of extracting and concentrating P. vivax parasites. MethodsAn iterative approach was employed across four non-immune healthy human subjects in single subject cohorts. All four subjects were inoculated with ~564 blood stage P. vivax (HMP013-Pv) and subjected to apheresis 10 to 11 days later. Blood samples collected during apheresis (haematocrit layers 0.5% to 11%) were tested for the presence and concentration of P. vivax by microscopy, flow cytometry, 18S rDNA qPCR for total parasites, and pvs25 qRT-PCR for female gametocyte transcripts. Safety was determined by monitoring adverse events. Malaria transmission to mosquitoes was assessed by membrane feeding assays. ResultsThere were no serious adverse events and no significant safety concerns. Apheresis concentrated asexual parasites by up to 4.9-fold (range: 0.9 - 4.9-fold) and gametocytes by up to 1.45-fold (range: 0.38 - 1.45-fold) compared to pre-apheresis densities. No single haematocrit layer contained >40% of all the recovered P. vivax asexual parasites. Ex vivo concentration of parasites by Percoll gradient centrifugation of whole blood achieved greater concentration of gametocytes than apheresis. Mosquito transmission was enhanced by up to 5-fold in a single apheresis sample compared to pre-apheresis. ConclusionThe modest level of parasite concentration suggests that the use of apheresis may not be an ideal method for harvesting P. vivax.