Abstract
Background
The long-acting 8-aminoquinoline tafenoquine may be a good candidate for mass drug administration if it exhibits sufficient blood-stage antimalarial activity at doses low enough to be tolerated by glucose 6-phosphate dehydrogenase (G6PD)–deficient individuals.
Methods
Healthy adults with normal levels of G6PD were inoculated with Plasmodium falciparum 3D7-infected erythrocytes on day 0. Different single oral doses of tafenoquine were administered on day 8. Parasitemia and concentrations of tafenoquine and the 5,6-orthoquinone metabolite in plasma/whole blood/urine were measured and standard safety assessments performed. Curative artemether-lumefantrine therapy was administered if parasite regrowth occurred, or on day 48 ± 2. Outcomes were parasite clearance kinetics, pharmacokinetic and pharmacokinetic/pharmacodynamic (PK/PD) parameters from modelling, and dose simulations in a theoretical endemic population.
Results
Twelve participants were inoculated and administered 200 mg (n = 3), 300 mg (n = 4), 400 mg (n = 2), or 600 mg (n = 3) tafenoquine. The parasite clearance half-life with 400 mg or 600 mg (5.4 hours and 4.2 hours, respectively) was faster than with 200 mg or 300 mg (11.8 hours and 9.6 hours, respectively). Parasite regrowth occurred after dosing with 200 mg (3/3 participants) and 300 mg (3/4 participants) but not after 400 mg or 600 mg. Simulations using the PK/PD model predicted that 460 mg and 540 mg would clear parasitaemia by a factor of 106 and 109, respectively, in a 60-kg adult.
Conclusions
Although a single dose of tafenoquine exhibits potent P. falciparum blood-stage antimalarial activity, the estimated doses to effectively clear asexual parasitemia will require prior screening to exclude G6PD deficiency.
Clinical Trials Registration. Australian and New Zealand Clinical Trials Registry (ACTRN12620000995976).
Keywords: Plasmodium falciparum, tafenoquine, blood stage, antimalarial
A single oral dose of tafenoquine is effective against blood-stage Plasmodium falciparum infection. However, as the estimated dose to clear asexual parasitaemia is ≥460 mg (in adults), prior screening for glucose 6-phosphate dehydrogenase deficiency will be required.
(See the Editorial Commentary by Nicholas J. White on pages 1928–9.)
The global burden of malaria remains high, with an estimated 247 million cases and 619 000 deaths in 2021 [1]. The emergence of artemisinin (ACT)-resistant Plasmodium falciparum in Southeast Asia [2], and more recently in East Africa [3], threatens the utility of the current first-line ACT combination therapies and progress toward malaria eradication. There is growing recognition for the need to treat both asymptomatic and symptomatic malaria infections to reduce the parasite reservoir [4] and to slow the spread of drug-resistant parasites [5]. Mass drug administration (MDA) in endemic populations is 1 approach to rapidly reduce malaria disease burden and interrupt transmission. The World Health Organization defines malaria MDA as the administration of a full dose of antimalarial treatment to an entire population in a given area, except for those in whom the medicine is contraindicated [6]. Further, medicines used for MDA should have proven efficacy in the implementation area, a long half-life, and be different from those medicines used for first-line treatment [6].
The 8-aminoquinoline tafenoquine is a long-acting analogue of primaquine that is approved for malaria chemoprophylaxis and for radical cure of vivax malaria [7]. The very long half-life (∼16 days [8]) of tafenoquine suggests that it may be worth considering as a candidate for use in MDA, particularly given that it also has activity against transmissible gametocytes [9]. However, although the blood schizonticidal activity of tafenoquine has been established in humans [10, 11], the dose required to clear asexual P. falciparum parasites has not been determined. Furthermore, as with primaquine, the use of tafenoquine is limited by the risk of hemolysis in glucose-6-phosphate dehydrogenase (G6PD)–deficient individuals, and administration of tafenoquine at the doses currently recommended for chemoprophylaxis (200 mg) and for Plasmodium vivax radical cure (300 mg) requires prior G6PD testing, which can be costly and logistically difficult in endemic regions.
Although the dose–response relationship between tafenoquine and hemolysis has been incompletely characterized [12], it is conceivable that a low dose of tafenoquine may be safe in G6PD-deficient individuals, in a similar manner to low-dose primaquine [13]. We hypothesized that, if a sufficiently low schizonticidal dose of tafenoquine could be identified, this might allow tafenoquine to be used for MDA in endemic populations, without the need for G6PD testing. The purpose of the current study was to use the induced blood-stage malaria (IBSM) model to define the minimum single dose of tafenoquine required to clear asexual blood-stage P. falciparum.
METHODS
Study Design and Participants
This was an open-label, randomized, clinical trial using the IBSM model. Healthy malaria-naive males and females (nonpregnant, nonlactating) aged 18–55 years were eligible for inclusion (Supplementary Text 2). All participants were required to have normal G6PD activity at screening (quantitative test; normal range: 7.0–20.5 U/g hemoglobin). The study was conducted at the University of the Sunshine Coast Clinical Trials Unit (Morayfield, Australia) and was registered on the Australian and New Zealand Clinical Trials Registry (ACTRN12620000995976).
This study was approved by the QIMR Berghofer Human Research Ethics Committee (P3646) with mutual recognition by the Australian Departments of Defence and Veterans’ Affairs Human Research Ethics Committee (194–19). All participants gave written informed consent.
Procedures
Participants were inoculated intravenously with approximately 2800 viable P. falciparum 3D7–infected erythrocytes on day 0 [14]. Parasitemia was monitored throughout the study by quantitative polymerase chain reaction (qPCR) targeting the gene encoding P. falciparum 18S rRNA [15]. A single oral dose of tafenoquine (Kodatef; Biocelect) was administered on day 8. All participants in cohort 1 received the same tafenoquine dose (300 mg), while participants in cohorts 2 and 3 were randomized to a dose group on day 8. The randomization schedule was generated using STATA 15 (StataCorp Ltd). No blinding was performed.
Plasma, venous, and capillary whole blood and urine concentrations of tafenoquine (and the 5,6-orthoquinone metabolite) were measured by liquid chromatography–tandem mass spectrometry [16]. Participants received a curative course of artemether-lumefantrine (Riamet; Novartis Pharmaceuticals) if parasite regrowth was detected, or on day 48 ± 2. Safety assessments, including monitoring of adverse events (AEs), vital signs, hematology and biochemistry, physical examination, and electrocardiographs, were performed throughout the study.
Outcomes and Statistical Analysis
Analysis of parasite clearance kinetics following tafenoquine was performed in R 3.5.0 and RStudio 1.1.447 (R Foundation for Statistical Computing). The parasite reduction ratio over a 48-hour period (PRR48) and parasite clearance half-life (PCt1/2) were estimated using the slope of the optimal fit for the log-linear relationship of the parasitemia decay [17]. Noncompartmental pharmacokinetic analysis was performed using PKanalix 2019R1 (Lixoft).
Pharmacokinetic (PK) and PK/pharmacodynamic (PK/PD) modelling and simulations were conducted within R 3.6.3 combined to the IQRtools package 1.7.2 and MonolixSuite 2019R1 (Supplementary Text 1). PK/PD analyses were performed using nonlinear mixed-effects models. A population PK model was developed to obtain individual PK parameter estimates that adequately described the observed individual PK profiles. The PK/PD model was then built using the individual PK parameter estimates as regression parameters. Key efficacy parameters including the minimum inhibitory concentration (MIC) and the minimal parasiticidal concentration that achieved 90% of the maximum effect (MPC90) were derived from the final PK/PD model.
Simulations were performed in a theoretical endemic population to predict the dose of tafenoquine that would clear parasitemia by a factor of 106 (the proposed minimum level for a schizonticidal antimalarial [18]) and 109 in all patients, and to achieve adequate parasitological response on day 42 (APR42) with 90% probability (Supplementary Text 1). Population parameters were sampled from the uncertainty distribution, and individual parameters were sampled from the interindividual variability (variability of the maximum parasite killing rate [Emax] was assumed to be 20%). Single doses of 1 to 40 mg/kg tafenoquine (with an increment of 1 mg/kg) were simulated in 20 trials of 100 patients each with body weights of 5, 10, 15, 20, 30, 40, 50, and 60 kg. Baseline parasitemia was assumed to be log-normally distributed around a median of 107 parasites/mL, with an interindividual variability of 60% [19]. The PK and PK/PD relationships were assumed to be the same in patients as in the volunteers, and parasites were assumed to grow exponentially in patients at a growth rate of 0.048 hours−1 (equivalent to a multiplication rate of ∼10-fold per 48-hour asexual cycle) [20, 21].
RESULTS
Participants
The study was conducted from 27 October 2020 to 9 April 2021. Twelve participants were enrolled across 3 cohorts (Figure 1). Participants were healthy, malaria-naive males (n = 6) or females (n = 6) aged 19 to 53 years (Table 1). All 3 participants in cohort 1 received 300 mg tafenoquine, whereas participants in cohorts 2 and 3 were randomized to a dose group (200 mg, n = 3; 300 mg, n = 1; 400 mg, n = 2; or 600 mg, n = 3). Doses were selected following review of interim data to optimally characterize the PK/PD relationship. All participants received the allocated treatment, completed the study, and were included in the analysis of study endpoints.
Figure 1.
Trial profile. Eligible participants were enrolled in 1 of 3 cohorts. All participants in cohort 1 were allocated the same dose of tafenoquine (300 mg). Participants in cohorts 2 and 3 were randomized within each cohort to a dose group on the day of dosing with tafenoquine (8 days following challenge with blood-stage P. falciparum). All participants completed the study and were included in the analysis of study endpoints.
Table 1.
Demographic and Baseline Characteristics of Participants
| Tafenoquine | |||||
|---|---|---|---|---|---|
| 200 mg (n = 3) |
300 mg (n = 4) |
400 mg (n = 2) |
600 mg (n = 3) |
Total (N = 12) |
|
| Sex, n (%) | |||||
| Female | 1 (33) | 3 (75) | 2 (100) | 0 (0) | 6 (50) |
| Male | 2 (67) | 1 (25) | 0 (0) | 3 (100) | 6 (50) |
| Race, n (%) | |||||
| White | 2 (67) | 4 (100) | 2 (100) | 3 (100) | 11 (92) |
| Asian | 1 (33) | 0 (0) | 0 (0) | 0 (0) | 1 (8) |
| Age, median (range), years | 40 (28–53) | 34 (20–45) | 31 (20–41) | 23 (19–23) | 30 (19–53) |
| Body weight, median (range), kg | 87 (61–95) | 81 (71–108) | 52 (51–53) | 67 (65–100) | 72 (51–108) |
| Body mass index, median (range), kg/m2 | 29 (26–30) | 27 (27–31) | 20 (20–21) | 21 (20–30) | 27 (20–31) |
| G6PD activity, median (range), units/g hemoglobin | 12.4 (11.8–13.7) | 13.3 (12.4–14.1) | 12.5 (11.8–13.2) | 12.6 (12.4–13.3) | 12.9 (11.8–14.1) |
| CYP2D6 phenotype, n (%) | |||||
| Normal metabolizer | 2 (67) | 3 (75) | 0 (0) | 1 (33) | 6 (50) |
| Intermediate metabolizer | 1 (33) | 1 (25) | 2 (100) | 2 (67) | 6 (50) |
Abbreviations: CYP2D6, cytochrome P450 2D6; G6PD, glucose-6-phosphate dehydrogenase.
Parasite Clearance Kinetics and Regrowth
The progression of parasitemia following intravenous inoculation was consistent between participants up to day 8, when a single dose of tafenoquine was administered (Figure 2). An initial reduction in parasitemia occurred in all participants following tafenoquine, with a trend of an increased rate of parasite clearance with increasing dose of tafenoquine (Table 2). Parasite regrowth occurred after dosing with 200 mg (3/3 participants; 8–22 days postdose) and 300 mg (3/4 participants; 18–27 days postdose) tafenoquine but not after dosing with 400 mg or 600 mg tafenoquine up to day 48 when definitive antimalarial treatment with artemether-lumefantrine was initiated (Figure 2).
Figure 2.
Individual participant parasitemia-time profiles. Participants were inoculated intravenously with Plasmodium falciparum–infected erythrocytes on day 0 and were administered a single oral dose of 200 mg (A), 300 mg (B), 400 mg (C), or 600 mg (D) TQ on day 8 (indicated by the vertical dashed line). Definitive antimalarial treatment with a standard course of A/L was initiated in response to parasite regrowth or on day 48 ± 2 (indicated by the vertical dashed line). Parasitemia was measured using qPCR targeting the gene encoding P. falciparum 18S rRNA. The horizontal dotted line indicates the lower limit of quantitation of the assay (32 parasites/mL) [22]. Abbreviations: A/L, artemether-lumefantrine; qPCR, quantitative polymerase chain reaction; TQ, tafenoquine.
Table 2.
Parasite Clearance Parameters Following Single-Dose Tafenoquine Administration to Healthy Volunteers With Induced Plasmodium falciparum Parasitemia
| Tafenoquine | ||||
|---|---|---|---|---|
| 200 mg (n = 3a) | 300 mg (n = 4) | 400 mg (n = 2) | 600 mg (n = 3) | |
| Log10PRR48 (95% CI) | 1.23 (1.12–1.33) | 1.51 (1.41–1.61) | 2.68 (2.46–2.89) | 3.42 (3.20–3.64) |
| PCt1/2, hours (95% CI) | 11.77 (10.84–12.87) | 9.57 (8.96–10.27) | 5.40 (5.00–5.87) | 4.23 (3.97–4.51) |
| Parasite regrowth incidence, n (%) | 3 (100) | 3 (75) | 0 (0) | 0 (0) |
Abbreviations: Log10PRR48, logarithm to base 10 of the parasite reduction ratio over a 48-hour period; PCt1/2, parasite clearance half-life.
One participant dosed with 200 mg tafenoquine had a nonsignificant regression fit and was therefore omitted from the PRR48 and PCt1/2 estimates.
Tafenoquine Exposure
Dose-related increases in tafenoquine exposure (maximum concentration [Cmax] and area under the concentration-time curve from time 0 to the last measured time point [AUC0-last]) were observed across the dose range (Table 3). Absorption was relatively slow, with time to maximum concentration (tmax) of approximately 12 hours, while a long plasma elimination half-life (t1/2) of approximately 13 days was observed. Compared with plasma, venous whole-blood tafenoquine exposure (Cmax and AUC0-last) was higher across the dose range, whereas tmax and t1/2 were comparable (Table 3). Concentrations of the 5,6-orthoquinone metabolite were considerably lower than the parent compound in plasma (Supplementary Figure 1), venous whole blood (Supplementary Figure 2), and capillary whole blood (Supplementary Figure 3), but substantially higher in urine (Supplementary Figure 4).
Table 3.
Tafenoquine Plasma and Venous Whole-Blood Noncompartmental Pharmacokinetic Parameters Associated With Single-Dose Administration to Healthy Volunteers With Induced Plasmodium falciparum Parasitemia
| Tafenoquine 200 mg (n = 3) |
Tafenoquine 300 mg (n = 4) |
Tafenoquine 400 mg (n = 2) |
Tafenoquine 600 mg (n = 3) |
|||||
|---|---|---|---|---|---|---|---|---|
| Plasma | Whole Blood | Plasma | Whole Blood | Plasma | Whole Blood | Plasma | Whole Blood | |
| Cmax, ng/mL | 138.2 (24.7) | 146.5 (40.9) | 205.8 (25.6) | 242.5 (30.2) | 324.8 (21.7) | 489.6 (24.1) | 340.2 (24.6) | 476.8 (35.3) |
| tmax, hours | 12.2 (12.1–24.0) | 12.2 (12.1–48.1) | 12.0 (8.1–24.0) | 12.0 (8.0–12.0) | 10.0 (8.1–12.0) | 8.0 (8.0–8.1) | 12.1 (12.0–24.2) | 12.0 (8.0–24.2) |
| AUC0-last, hours × ng/mL | 47 091 (9.2) | 53 552 (17.0) | 67 303 (33.4) | 83 531 (28.4) | 117 107 (25.0) | 158 573 (22.5) | 128 140 (28.2) | 157 541 (33.7) |
| t1/2, hours | 309.4 (17.7) | 333.3 (5.0) | 357.3 (10.3) | 302.2 (14.2) | 313.9 (13.5) | 344.5 (14.6) | 314.4 (14.0) | 327.0 (4.7) |
| CL/F, L/hour | 3.8 (7.4) | 3.3 (13.6) | 3.9 (33.7) | 3.2 (26.9) | 3.1 (29.2) | 2.2 (26.5) | 4.1 (29.9) | 3.4 (33.4) |
| Vz/F, L | 1708 (22.0) | 1585 (18.2) | 2003 (33.5) | 1378 (34.0) | 1379 (15.2) | 1109 (11.6) | 1863 (24.6) | 1585 (30.8) |
Data are geometric means (coefficient of variation [%]), except for tmax, which is median (range). Abbreviations: AUC0-last, area under the concentration-time curve from time 0 (dosing) to the last sampling time at which the concentration is at or above the lower limit of quantification; CL/F, apparent total clearance; Cmax, maximum observed concentration; t1/2, apparent terminal half-life; tmax, time to reach the maximum observed concentration; Vz/F, apparent volume of distribution.
Pharmacokinetic/Pharmacodynamic Analysis
The PK of tafenoquine and 5,6-orthoquinone tafenoquine was described by a mammillary physiological model comprising 7 disposition compartments (plasma, red blood cell, and urine for both tafenoquine and 5,6-orthoquinone, and an additional tissue compartment for tafenoquine) with first-order absorption and linear elimination (Supplementary Figure 5 and Supplementary Table 1). Modelling identified time-dependent parasitemia as a significant covariate on tafenoquine clearance, resulting in decreasing clearance with increasing parasitemia. Visual predictive checks demonstrated that the observed profiles of tafenoquine fit well within the 5th and 95th percentiles of the simulated predictions, indicating the model adequacy in characterizing the observed data (Supplementary Figure 6).
The relationship between tafenoquine red blood cell concentrations and parasite killing was described by an Emax PK/PD model. Modelling was unable to identify a significant relationship between 5,6-orthoquinone concentrations and parasite killing. Population PD parameters (Supplementary Table 2) were estimated with satisfactory precision (relative standard error <30%). The interindividual variability in the parasite growth rate constant was estimated with low precision, and thus fixed to 0.07 based on retrospective P. falciparum parasite growth modelling in IBSM studies. Visual predictive checks demonstrated that the 5th, 50th, and 95th percentiles of the observed data were within the 95% confidence interval of the simulated data (Supplementary Figure 7). PK/PD modelling using population estimates predicted an MIC of 74 ng/mL (0.16 nmol/mL) and an MPC90 of 561 ng/mL (1.21 nmol/mL) (Supplementary Tables 3 and 4).
Tafenoquine Dose Predictions
The single doses of tafenoquine required to clear parasitemia by a factor of 106 and 109, and to achieve APR42 with 90% probability, in all patients within a hypothetical endemic population were predicted using the PK/PD model with the associated interindividual variability, covariate effect, and model parameter uncertainty. Analyses were performed using body weights ranging from 5 to 60 kg, and baseline parasitemia log-normally distributed around a median of 107 parasites/mL (interindividual variability: 60%), to simulate dosing an entire endemic population in the context of MDA. The predicted dose required to clear parasitemia by a factor of 106 ranged from 50 mg (10 mg/kg) for a 5-kg infant to 460 mg (7.7 mg/kg) for a 60-kg adult (Table 4 and Supplementary Figure 8). Higher doses were predicted to be required to clear parasitemia by a factor of 109 (65 mg for a 5-kg infant and 540 mg for a 60-kg adult) and to achieve APR42 (Table 4, Supplementary Figures 8 and 9).
Table 4.
Predicted Single Doses of Tafenoquine Required to Clear Asexual Plasmodium falciparum Parasitemia in All Patients Within a Hypothetical Endemic Population
| Body Weight (kg) | Dose to Clear Parasitemia by a Factor of | Dose to Achieve APR42 With 90% Probability (mg) [mg/kg] | |
|---|---|---|---|
| 106 (mg) [mg/kg] | 109 (mg) [mg/kg] | ||
| 5 | 50 [10] | 65 [13] | 90 [18] |
| 10 | 95 [9.5] | 120 [12] | 170 [17] |
| 15 | 140 [9.3] | 170 [11.3] | 260 [17.3] |
| 20 | 170 [8.5] | 210 [10.5] | 340 [17] |
| 30 | 250 [8.3] | 300 [10] | 480 [16] |
| 40 | 320 [8] | 380 [9.5] | 640 [16] |
| 50 | 390 [7.8] | 460 [9.2] | 780 [15.6] |
| 60 | 460 [7.7] | 540 [9] | 960 [16] |
Single doses of 1 to 40 mg/kg tafenoquine, with an increment of 1 mg/kg, were simulated in 20 trials of 100 patients with each body weight. Simulations were performed using the pharmacokinetic/pharmacodynamic model developed from the data obtained in healthy volunteers with induced blood-stage Plasmodium falciparum. Abbreviation: APR42, adequate parasitological response on day 42, defined as a decrease in parasitemia below 1 parasite in the body (or below the lower limit of quantification [LLOQ]) within 42 days, without early treatment failure (parasitemia above baseline at day 2 or above 25% of baseline at day 3) or late clinical failure (parasitemia above the LLOQ after day 4).
Safety and Tolerability
A total of 160 AEs were reported, the majority (102/160) being mild to moderate signs and symptoms of malaria (Table 5). There were no serious AEs. There were 39 AEs considered related to tafenoquine (Supplementary Table 5), with an increase in blood methemoglobin above the upper limit of normal (1.2%) being the most commonly reported (10/12 participants). The peak methemoglobin concentration was related to the tafenoquine dose administered, with 400- or 600-mg doses resulting in a greater elevation (2–5% at peak) compared with 200- or 300-mg doses (<2% at peak) (Supplementary Figure 11). Blood methemoglobin AUC0-last and Cmax correlated with the AUC0-last and Cmax of tafenoquine and 5,6-orthoquinone in whole blood and plasma (Supplementary Figures 12 and 13). The methemoglobin elevations were not accompanied by symptoms or signs of methemoglobinemia. No direct correlation was detected between methemoglobin levels and the rate of parasite clearance or time to parasite regrowth (Supplementary Figures 14 and 15). A mild decrease in hemoglobin from baseline (pre–malaria inoculation) was recorded for 3 participants (largest decrease: 24 g/L). Overall, hemoglobin concentrations did not appear to be dose related (Supplementary Figure 10).
Table 5.
Summary of Adverse Events
| Number of Participants With at Least 1 AE (%); Total Number of AEs | |||||
|---|---|---|---|---|---|
| AE Categorya | Tafenoquine 200 mg (n = 3) |
Tafenoquine 300 mg (n = 4) |
Tafenoquine 400 mg (n = 2) |
Tafenoquine 600 mg (n = 3) |
Total (N = 12) |
| Any AE | 3 (100); 39 | 4 (100); 74 | 2 (100); 22 | 3 (100); 25 | 12 (100); 160 |
| AE related to malaria | 3 (100); 24 | 4 (100); 61 | 2 (100); 6 | 3 (100); 11 | 12 (100); 102 |
| AE related to tafenoquineb | 3 (100); 9 | 4 (100); 15 | 2 (100); 5 | 3 (100); 10 | 12 (100); 39 |
| Grade 2c AE | 3 (100); 11 | 4 (100); 18 | 2 (100); 5 | 3 (100); 12 | 12 (100); 46 |
| Grade 2c AE related to tafenoquine | 3 (100); 6 | 3 (75.0); 5 | 2 (100); 3 | 3 (100); 8 | 11 (91.7); 22 |
| Grade 3c AE | 2 (66.7); 2d | 1 (25.0); 1d | 0 (0); 0 | 0 (0); 0 | 3 (25.0); 3d |
Adverse events were recorded from administration of the malaria challenge agent until the end of the study. The investigator assessed the relatedness of all AEs to tafenoquine, malaria, and artemether-lumefantrine (related or unrelated). Adverse events may have been considered related to more than 1 study intervention. Abbreviation: AE, adverse event.
None of the adverse events met the predefined criteria for a serious adverse event or resulted in a participant discontinuing the study.
Adverse events related to tafenoquine are specified in Supplementary Table 5.
The medical assessment of AE severity was recorded in accordance with the Common Terminology Criteria for Adverse Events version 5.0, published 27 November 2017 (mild = grade 1; moderate = grade 2; severe = grade 3; potentially life-threatening = grade 4; death related to AE = grade 5). No grade 4 or grade 5 AEs were recorded.
The grade 3 AEs were a transient decrease in neutrophil count (0.7 × 109/L [normal range: 2.0–7.5 × 109/L]) in 1 participant (300-mg group) and a transient decrease in lymphocyte count (0.4 × 109/L [normal range: 1.1–4 × 109/L]) in 2 participants (200-mg group); all 3 events normalized within 5 days and were considered related to malaria.
DISCUSSION
The objective of this study was to characterize the activity of tafenoquine against blood-stage P. falciparum, including estimation of the dose required to effectively clear asexual parasitemia, with a view to the potential use of tafenoquine as a component of MDA. We considered that tafenoquine may be suitable for MDA given that it has a very long elimination half-life and exhibits activity against asexual blood-stage parasites [10, 11] and transmissible gametocytes [23–25]. Although the potential use of tafenoquine for MDA is limited by the risk of hemolysis in G6PD-deficient individuals, we hypothesized that a low dose of tafenoquine may be safe in such individuals, and thus we considered it important to determine whether a low dose of tafenoquine could effectively clear parasitemia.
Results of this study showed that single low doses of tafenoquine were not effective at clearing asexual parasitemia. Doses of 200 or 300 mg resulted in slow parasite clearance (PCt1/2: 11.8 hours and 9.6 hours, respectively) and led to recrudescence in 6 of 7 participants. In contrast, doses of 400 or 600 mg tafenoquine resulted in rapid parasite clearance (PCt1/2: 5.4 hours and 4.2 hours, respectively), faster than that previously characterized for 10 mg/kg mefloquine (PCt1/2: 6.2 hours) using the IBSM model [26], and were not associated with recrudescence in any participant. Dose simulations in an endemic population took into account the higher parasite burdens and greater variability in parasitemia, compared with the volunteers in the current study. Adult doses of 460 mg (7.7 mg/kg) and 540 mg (9 mg/kg) were predicted to clear parasitemia by a factor of 106 and 109, respectively, whereas 960 mg (16 mg/kg) would achieve APR42 with 90% probability. The estimation of APR42 was considered informative albeit not entirely relevant for MDA, given that, in practice, all curative antimalarial treatments consist of combinations of 2 or more drugs and/or require multiple doses.
The adult dose of tafenoquine predicted to clear asexual P. falciparum parasitemia is higher than the 300-mg dose currently recommended for P. vivax radical cure, but similar to the 450-mg dose that has recently been proposed as more effective for preventing P. vivax relapses [27]. Thus, if a 450-mg dose of tafenoquine were to be adopted for P. vivax radical cure, then this dose would have the added benefit of clearing circulating P. falciparum parasites. Furthermore, there is evidence to suggest that, in co-endemic areas, P. vivax radical cure should be given alongside treatment of falciparum malaria, even in the absence of detectable P. vivax infection [28, 29]. Tafenoquine may be particularly suitable for this purpose, given the predicted effectiveness of a 450-mg dose for clearing asexual P. falciparum parasitemia and achieving radical cure of P. vivax.
Although the results of this study indicate that tafenoquine exhibits potent blood-stage antimalarial activity, the estimated doses required to clear asexual parasitemia will require screening for G6PD deficiency. A safe dose of tafenoquine that could be safely administered to G6PD-deficient individuals has not been established. However, a dose of 300 mg was found to result in a hemoglobin decrease of more than 2.5 g/dL in a small group of G6PD heterozygous females with enzyme activity 40–60% of normal [12]. Thus, it seems unlikely that the doses required to clear asexual parasitemia could be safely administered in MDA without G6PD screening. However, we and others have demonstrated that single low doses of tafenoquine (<100-mg adult dose) can reduce the transmission of P. falciparum to mosquitoes [24, 25]. Thus, the use of tafenoquine for this purpose, in combination with partner drugs to clear asexual parasitemia, may be worthy of investigation. Furthermore, tafenoquine has been shown in vitro to be mostly synergistic with ACT-partner drugs against asexual P. falciparum parasites [30]. Investigation for an optimal partner drug for tafenoquine is warranted, particularly if it enables use of tafenoquine at lower doses, thus reducing hemolytic risk.
As expected, no significant safety signals were associated with tafenoquine in volunteers with normal levels of G6PD in the current study. A mild transient decrease in hemoglobin occurred in a few participants, although this did not appear to be dose related and has been observed previously with blood-stage malaria challenge studies [31]. A dose-related transient increase in blood methemoglobin was observed, as has been reported following tafenoquine dosing of patients with vivax malaria [32–34]. The production of dose-dependent methemoglobinemia by 8-aminoquinoline antimalarials is associated with efficacy against latent P. vivax and Plasmodium ovale [27, 35]. The results of our study suggest that a similar characteristic may exist for tafenoquine against asexual blood-stage P. falciparum. Doses of 400 or 600 mg resulted in a notably greater elevation in methemoglobin than did doses of 200 or 300 mg, while also resulting in faster parasite clearance without subsequent parasite regrowth. However, in this small study, we did not identify a correlation between methemoglobin levels and the rate of parasite clearance or time to recrudescence. The specific mechanisms of tafenoquine-induced parasite killing, hemolytic toxicity, and methemoglobinemia remain unclear. It is hypothesized that tafenoquine exhibits parasite stage–specific activity associated with different metabolites and modes of action [36]. Activity against blood-stage parasites is thought to be independent of CYP-2D6 metabolism, potentially involving interference with heme polymerization, or oxidative mechanisms [36]. The 5,6-orthoquinone metabolite of primaquine is considered a surrogate marker for active metabolites of primaquine [37]. Although we measured the 5,6-orthoquinone metabolite of tafenoquine, a relationship between its concentration and parasite killing was not observed. Further work to better understand the role of the metabolite and the mode of action of tafenoquine is warranted.
The primary limitation of this study is that the dose predictions were based on the assumption that the PK/PD relationship characterized in the small number of volunteers would be the same in the target population. We cannot discount the possibility that the PK/PD relationship would be influenced by other factors such as host immunity, G6PD activity, and other bioactive metabolites. However, estimates of therapeutic doses of other antimalarial drugs in volunteers has translated well to patient populations [26, 38].
In conclusion, this study has demonstrated that single-dose tafenoquine exhibits potent P. falciparum blood-stage antimalarial activity. However, the predicted doses required to clear asexual parasitemia will require prior G6PD screening. This would limit the use of tafenoquine for the purpose of clearing asexual parasitemia in MDA where widespread G6PD screening would prove costly and logistically difficult.
Supplementary Data
Supplementary materials are available at Clinical Infectious Diseases online. Consisting of data provided by the authors to benefit the reader, the posted materials are not copyedited and are the sole responsibility of the authors, so questions or comments should be addressed to the corresponding author.
Supplementary Material
Contributor Information
Bridget E Barber, QIMR Berghofer Medical Research Institute, Brisbane, Australia; University of the Sunshine Coast, Morayfield, Australia; Royal Brisbane and Women's Hospital, Brisbane, Australia.
Azrin N Abd-Rahman, QIMR Berghofer Medical Research Institute, Brisbane, Australia.
Rebecca Webster, QIMR Berghofer Medical Research Institute, Brisbane, Australia.
Adam J Potter, QIMR Berghofer Medical Research Institute, Brisbane, Australia.
Stacey Llewellyn, QIMR Berghofer Medical Research Institute, Brisbane, Australia.
Louise Marquart, QIMR Berghofer Medical Research Institute, Brisbane, Australia; The University of Queensland, Brisbane, Australia.
Nischal Sahai, University of the Sunshine Coast, Morayfield, Australia.
Indika Leelasena, University of the Sunshine Coast, Morayfield, Australia.
Geoffrey W Birrell, Australian Defence Force Malaria and Infectious Disease Institute, Brisbane, Australia.
Michael D Edstein, Australian Defence Force Malaria and Infectious Disease Institute, Brisbane, Australia.
G Dennis Shanks, Australian Defence Force Malaria and Infectious Disease Institute, Brisbane, Australia.
David Wesche, Certara, Ann Arbor, Michigan, USA.
Joerg J Moehrle, Medicines for Malaria Venture, Geneva, Switzerland.
James S McCarthy, QIMR Berghofer Medical Research Institute, Brisbane, Australia; The Peter Doherty Institute for Infection and Immunity, Melbourne, Australia.
Notes
Acknowledgments. The authors thank all of the volunteers who participated in the study, staff at the University of the Sunshine Coast Clinical Trials Unit who conducted the trial, staff at the Queensland Paediatric Infectious Diseases laboratory for qPCR analysis, Dr. Scott Miller from the Bill and Melinda Gates Foundation for valuable discussions as part of the Safety and Data Review Team, and Karin Van Breda from the Australian Defence Force Malaria and Infectious Disease Institute for the measurement of tafenoquine concentrations in biospecimens.
Disclaimer. The views expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Australian Defence Force, Joint Health Command, or any extant Australian Defence Force policy.
Financial support. This work was supported by the Bill and Melinda Gates Foundation (grant number INV-001965). Additional funding support was provided to the Clinical Malaria Group within the QIMR Berghofer Medical Research Institute by Medicines for Malaria Venture.
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