Transmission Blocking Activity of Low-dose Tafenoquine in Healthy Volunteers Experimentally Infected With Plasmodium falciparum.

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 Plasmodium 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 predose and at 1, 4, and 7 days postdose. Artemether-lumefantrine was administered following the final assay. Outcomes were the reduction in mosquito infection and gametocytemia after tafenoquine and safety parameters. RESULTS: Six participants were enrolled, and all were infective to mosquitoes before tafenoquine, with a median 86% (range, 22-98) of mosquitoes positive for oocysts and 57% (range, 4-92) positive for sporozoites. By day 4 after tafenoquine, the oocyst and sporozoite positivity rate had reduced by a median 35% (interquartile range [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 after tafenoquine was not significant. No significant participant safety concerns were identified. CONCLUSIONS: Low-dose tafenoquine (50 mg) reduces P. falciparum transmission to mosquitoes, with a delay in effect.

Parasite Viability as a Measure of In Vivo Drug Activity in Preclinical and Early Clinical Antimalarial Drug Assessment.

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. We first measured the drug effect in mice by estimating the decrease in parasite viability after treatment using two independent approaches to estimate viability. We demonstrate that, as previously reported in humans, parasite viability declines much faster after artesunate treatment than does the decline in parasitemia (termed parasite clearance). We also observed that artesunate kills parasites faster at higher concentrations, which is not discernible from the traditional parasite clearance curve and that each subsequent dose of artesunate maintains its killing effect. Furthermore, based on measures of parasite viability, we could accurately predict the in vivo recrudescence of infection. Finally, using pharmacometrics modeling, we show that the apparent differences in the antimalarial activity of artesunate in mice and humans are partly explained by differences in host removal of dead parasites in the two hosts. However, these differences, along with different pharmacokinetic profiles, do not fully account for the differences in activity. (This study has been registered with the Australian New Zealand Clinical Trials Registry under identifier ACTRN12617001394336.).

Parasite Viability as a Superior Measure of Antimalarial Drug Activity in Humans.

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 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 parasite clearance to measure drug activity, arguing that many circulating parasites may be nonviable. 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 means of quantitative polymerase chain reaction (qPCR). Parasite viability after artesunate administration was estimated by mathematical modeling 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 hours after artesunate administration, while the total number of circulating parasites measured with quantitative polymerase chain reaction remained unchanged. In artemisinin-resistant infections over the same interval, viable parasites declined to 51.4% (standard error of the mean, 4.6%) of baseline. CONCLUSIONS: These results demonstrate that in vivo drug activity of artesunate is faster than is indicated by the parasite clearance half-life.

Safety and feasibility of apheresis to harvest and concentrate parasites from subjects with induced blood stage Plasmodium vivax infection.

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.

Longitudinal changes in iron homeostasis in human experimental and clinical malaria.

BACKGROUND: The interaction between iron status and malaria is incompletely understood. We evaluated longitudinal changes in iron homeostasis in volunteers enrolled in malaria volunteer infection studies (VIS) and in Malaysian patients with falciparum and vivax malaria. METHODS: We retrieved data and samples from 55 participants (19 female) enrolled in malaria VIS, and 171 patients (45 female) with malaria and 30 healthy controls (13 female) enrolled in clinical studies in Malaysia. Ferritin, hepcidin, erythropoietin, and soluble transferrin receptor (sTfR) were measured by ELISA. FINDINGS: In the VIS, participants' parasitaemia was correlated with baseline mean corpuscular volume (MCV), but not iron status (ferritin, hepcidin or sTfR). Ferritin, hepcidin and sTfR all increased during the VIS. Ferritin and hepcidin normalised by day 28, while sTfR remained elevated. In VIS participants, baseline ferritin was associated with post-treatment increases in liver transaminase levels. In Malaysian patients with malaria, hepcidin and ferritin were elevated on admission compared to healthy controls, while sTfR increased following admission. By day 28, hepcidin had normalised; however, ferritin and sTfR both remained elevated. INTERPRETATION: Our findings demonstrate that parasitaemia is associated with an individual's MCV rather than iron status. The persistent elevation in sTfR 4 weeks post-infection in both malaria VIS and clinical malaria may reflect a causal link between malaria and iron deficiency. FUNDING: National Health and Medical Research Council (Program Grant 1037304, Project Grants 1045156 and 1156809; Investigator Grants 2016792 to BEB, 2016396 to JCM, 2017436 to MJG); US National Institute of Health (R01-AI116472-03); Malaysian Ministry of Health (BP00500420).

Longitudinal changes in iron homeostasis in human experimental and clinical malaria.

Background: The interaction between iron deficiency and malaria is incompletely understood. We evaluated longitudinal changes in iron homeostasis in volunteers enrolled in malaria volunteer infection studies (VIS) and in Malaysian patients with falciparum and vivax malaria. Methods: We retrieved samples and associated data from 55 participants enrolled in malaria VIS, and 171 malaria patients and 30 healthy controls enrolled in clinical studies in Malaysia. Ferritin, hepcidin, erythropoietin, and soluble transferrin receptor (sTfR) were measured by ELISA. Results: In the VIS, participants' parasitaemia was correlated with baseline mean corpuscular volume (MCV), but not iron status (ferritin, hepcidin or sTfR). Ferritin, hepcidin and sTfR all increased during the VIS. Ferritin and hepcidin normalised by day 28, while sTfR remained elevated. In VIS participants, baseline iron status (ferritin) was associated with post-treatment increases in liver transaminase levels. In Malaysian malaria patients, hepcidin and ferritin were elevated on admission compared to healthy controls, while sTfR increased following admission. Hepcidin normalised by day 28; however, ferritin and sTfR both remained elevated 4 weeks following admission. Conclusion: Our findings demonstrate that parasitaemia is associated with an individual's MCV rather than iron status. The persistent elevation in sTfR 4 weeks post-infection in both malaria VIS and clinical malaria may reflect a causal link between malaria and iron deficiency.