G Biagini1; G Camarda1; S March1; R Priestley1; A Saif1; A Miller1; P Jirawatcharadech1; M H Wong1; S Leung1; D Baker1; P Alano1; M J Paine1; S Bhatia1; P M O’Neill1; S A Ward1;
1 Liverpool School of Tropical Medicine, UK
DiscussionPrimaquine (PQ) is an 8-aminoquinline class FDA-approved drug on the World Health Organization's (WHO) List of Essential Medicines, and is currently the only registered drug available for radical cure of relapse malaria showing activity against Plasmodium vivax and P. ovale dormant liver stages. PQ is also active against P. falciparum liver stages and against the sexual gametocyte stages of Plasmodium species, making this drug available for prophylaxis and in transmission blocking strategies such as in elimination programmes. However, its widespread use in mass drug administration intervention is limited due to severe side effects occurring in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency.
Despite being in use for some 70 years, PQ mechanism of action is still poorly understood. Metabolic activation has long been known to be required for PQ to gain activity, and recently CYP2D6 enzyme has been shown to be required to achieve P. vivax radical cure in humans. It is hypothesised CYP2D6 mediates the generation of hydroxylated metabolites, which in turn could exert the anti-parasitic effect via a redox cycling mechanism.
Here, by using a chemical biology approach we provide the first direct definitive evidence of the role of hydroxylated and quinoneimine primaquine metabolites in anti-gametocyte and liver stage inhibitory activity. Furthermore, we present biochemical evidence consistent with a mechanism of action whereby redox cycling of catalytic quantities of PQ metabolites can generate pharmacologically-relevant levels of hydrogen peroxide (H2O2) which lead to parasite killing. Finally, we demonstrate how redox cycling of PQ metabolites by host enzymes leads to the selective killing of malaria parasite gametocytes and liver stages.
The identification of the biochemical events responsible for the anti-parasite activity of primaquine not only answers to a long-asked question, but also opens the way to the possibility of designing new 8AQs with improved therapeutic profiles.