(662g) Interactions Between Antimalarials and Hematin Crystal Surface Sites Determine the Mode of Growth Inhibition

Dynamic events at solid-liquid interfaces mediate the kinetics of crystallization wherein the molecular interactions between the crystal surface and solvent or adsorbates can alter anisotropic growth rates. These fundamental processes mediate natural and synthetic crystallization pathways, giving rise to materials with optical, mechanical, and electrical properties for a variety of applications. Our studies employ a model crystal system, hematin, which has a wide range of potential applications. In malaria pathophysiology, hematin crystallization ensures the survival of the parasite Plasmodium falciparum during its asexual life cycle.^1-3 Survival within its human host involves the parasite entering erythrocytes (red blood cells), catabolizing hemoglobin and releasing free heme (Fe²⁺), which undergoes rapid oxidation to form toxic hematin (Fe³⁺) within its digestive vacuole (DV). The parasite utilizes the process of heme detoxification by sequestering the toxin into innocuous crystalline hemozoin within the complex DV environment. Current antimalarial compounds are thought to prevent the growth of hematin crystals, referred to as hemozoin (in vivo) or beta-hematin (in vitro). Identification of antimalarial drug action on this phase transformation could provide a foundation for drug design to overcome parasite drug resistance. Here, we will discuss our effort to understand the molecular mechanism(s) of hematin crystallization and the modes of antimalarial drug action in physiologically-relevant growth media.

In situ atomic force microscopy (AFM) reveals that hematin crystallization occurs by a classical mechanism of layer-by-layer growth involving the addition of individual molecules to specific sites presented on the crystal surface. We show that hematin crystals form in a two-phase medium, which we designed to mimic the parasiteâ??s DV comprising a complex system with a primarily acidic aqueous phase and lipid subphase.² We quantified the rate of layer generation and the velocities of anisotropic step advancement as a function of supersaturation in the absence and presence of antimalarials.¹ We identified that current antimalarials act in unique modes of inhibition. Our current studies aim to elucidate the relationship between the functional moieties present on current antimalarials and free hematin as well as the solid-liquid interface presented on predominant {100} crystal surfaces. In parallel, we studied the prominent functional groups of antimalarial compounds using a combination of in situ AFM studies and chemical force microscopy (CFM) measurements. Identifying key factors governing molecular recognition of inhibitors to specific crystal surface sites is crucial to the development of rational design platforms for screening new drug compounds.

References:

[1] Olafson, K.N., Ketchum, M.A., Rimer, J.D., Vekilov, P.G., Mechanisms of Hematin Crystallization and Inhibition by the Antimalarial Drug Chloroquine, Proc. Natl. Acad. Sci. 16 (2015) 4946-4951

[2] Olafson, K.N., Rimer, J.D., Vekilov, P.G., Growth of Large Hematin Crystals in Biomimetic Solutions, Cryst. Growth Des. 14 (2014) 2123-2127

[3] Ketchum, M.A., Olafson, K.N., Rimer, J.D., Vekilov, P.G., Hematin Crystallization from Aqueous and Organic Solvents, J. Chem. Phys. 139 (2013) 121911(1-9)