Plasmodium vivax malaria is a major cause of morbidity and is a significant contributor to mortality in tropical regions (Battle et al. 2019 The Lancet). Relapses are the major cause of illness and in higher transmission settings contribute to severe anaemia and death in young children as well as pregnancy loss.

SE Asia has the highest proportion of the estimated global burden of P. vivax malaria burden 51.2% (7.2 million of 14.3 million) in 2017.

In SE Asia over half of the patients treated for an acute P. vivax blood stage infection will develop at least one relapse (Betuela et al. 2012 J. Infect. Dis.; Chu et al. 2018 Clin. Infect. Dis.; Commons et al. 2019 BMC Med; Luxemburger et al. 1999 Trans R Soc Trop Med Hyg; Poespoprodjo et al. 2009 Clin. Infect. Dis.; Sutanto et al. 2013 AAC) if an anti-relapse drug is not given. The only widely available drug to prevent relapses (radical cure) currently is the 8-aminoquinoline primaquine. The doses of primaquine required to prevent relapses in Southeast Asia and the Western Pacific region are higher than elsewhere. P. vivax relapses more in these regions where with large populations over 80% of the global P. vivax burden occurs so it is likely that most of the world's relapses occur in East Asia and thus the benefits of effective radical cure are greatest in this area.

Adherence to the currently recommended 14-day primaquine regimen is variable and this compromises widespread primaquine use and efficacy (Cheoymang et al. 2015 Acta tropica; Grietens et al. 2010 ASTMH; Leslie et al. 2004 Trans R Soc Trop Med Hyg; Maneeboonyang et al. 2011 Southeast Asian J. Trop. Med. Public Health) . Recently a slowly eliminated 8-aminoquinoline, tafenoquine, has become available and has been registered in several countries. Tafenoquine can be given in a single dose (Lacerda et al. 2019 NEJM; Llanos-Cuentas et al. 2014a The Lancet; Llanos-Cuentas et al. 2019 NEJM) which allows supervised dosing. However, tafenoquine is contraindicated in persons with G6PD activity below 70% as it may cause significant haemolysis in G6PD deficiency (including in female heterozygotes who may test as normal with current qualitative screens). Thus, quantitative G6PD deficiency testing is required to identify individuals with intermediate G6PD activity to ensure that female G6PD deficiency heterozygotes are not enrolled. Near patient quantitative G6PD tests (G6PD Biosensor) (Bancone et al. 2018 PLoS One; Pal et al. 2019 ASTMH; Zobrist et al. 2021 PLOS Negl. Trop. Dis.) which provide a quantitative result will be used before tafenoquine is prescribed.

The currently recommended tafenoquine dose is sub-optimal. The 300 mg dose proved significantly inferior to low dose primaquine in a meta-analysis of the phase 3 studies when restricted to the Southeast Asian region (Llanos-Cuentas et al. 2019 NEJM; Watson et al. 2022a Elife).

These data and an individual patient data meta-analysis of all (\>1000) patients in the pre-registration studies (J. A.Watson et al. 2022a Elife) suggest that a 450mg adult dose of tafenoquine is needed in the Southeast Asian and Western Pacific regions. Doses up to 1200mg have proved safe and well tolerated in G6PD normal individuals.

However a recent study suggests that ACTs may antagonize the curative efficacy of tafenoquine. If true this would complicate current treatment of both falciparum and vivax malaria with a single blood stage drug. The results from the INSPECTOR trial in Indonesian soldiers returning to non-endemic areas showed very low radical curative efficacy with tafenoquine in combination with DHA-PPQ (21%) - an efficacy which was similar to DHA-PPQ alone (11%) (Baird et al. 2020 ASTMH Annual meeting).

This has prompted a product label update for Krintafel® (the branded form of tafenoquine in the USA) that tafenoquine should be combined only with chloroquine and not with other antimalarials such as artemisinin-based combination therapies (ACT)(CDC 2020). This now prevents the use of tafenoquine in areas with chloroquine resistant P. vivax parasites where national malaria programmes recommend ACTs for vivax malaria. As chloroquine resistance in P. vivax is increasing this poses a serious challenge to the potential use of this new radical cure antimalarial. However, this result is at variance with our earlier studies with the structurally and mechanistically similar primaquine, where radical curative efficacies were similar with ACTs (either DHA-PPQ or AL) and chloroquine combinations (Chu et al. 2019 Clin. Infect. Dis.). Whether DHA-PPQ or AL can be combined effectively with tafenoquine needs to be resolved quickly to guide deployment.

Tafenoquine could be particularly valuable in conflict-torn Myanmar where vivax malaria is now out of control and health services have broken down. This clinical trial will be performed in the clinics and health centres in Myawaddy district supervised by Shoklo Malaria Research Unit.

Scientific Rationale

Tafenoquine, as a single dose regimen, has a tremendous advantage over the longer courses of primaquine needed for radical cure. Its use will be progressively restricted if it cannot be used in combination with ACTs. This trial will assess whether or not ACTs (either AL or DHA-PPQ) significantly reduces tafenoquine radical curative efficacy in comparison with chloroquine.