Abstract
Malaria remains a significant public health challenge in equatorial regions of the world, largely owing to the parasite's emerging resistance to the recently introduced drugs of the artemisinin (ART) family. In the human body, most ART-derivative drugs are metabolized to dihydroartemisinin (DHA), which, in the parasite, after activation by heme, can form a hematin-dihydroartemisinin adduct (H-DHA). Here we test whether and how H-DHA inhibits hematin crystallization, the main constituent of the heme detoxification pathway of malaria parasites. We find that H-DHA is a poor inhibitor of classical crystal growth-it weakly blocks the growth sites on crystal surfaces-and, counterproductively, a promoter of β-hematin nucleation, driven by a boost in the formation of precursors. We establish that at elevated hematin concentrations, H-DHA activates two nonclassical pathways that transform it into a potent β-hematin growth inhibitor. First, β-hematin crystallites, whose nucleation is promoted by H-DHA, incorporate into large β-hematin crystals and suppress their growth, likely by straining the crystal lattice. A second consequence of H-DHA is the generation of macrosteps on β-hematin crystal surfaces that hinder growth. Importantly, the induced growth suppression is irreversible and persists even in the absence of H-DHA. Our findings suggest that a partial resistance mechanism to artemisinin-class drugs in trophozoite-stage parasites may be due to the reduced concentrations of hematin and H-DHA, which deactivate the dual nonclassical mode of action of the adduct in the delayed-clearance parasite strains.