New insights on the structure-function principles & design of quinoline antimalarial drugs

Plasmodium falciparum degrades hemoglobin releasing toxic ferriprotoporphyrin IX (FPIX) within the digestive vacuole (DV). Unlike the host, it lacks FPIX degrading enzymes. Consequently, it has adapted an alternative detoxification pathway, FPIX crystallization into non-toxic hemozoin (Hz). The antiplasmodial activity of quinolines is partially due to inhibition of this process. However, molecular-level understanding of this mechanism and how it is presumably altered in drug resistant parasites remains elusive.;Herein, I present evidence to further aid in elucidating the mechanism of action of quinoline antimalarials. First, using rationally designed quinoline analogs, I illustrate that predicted accumulation at their site of action, the DV, is essential for their antiplasmodial activity and can be enhanced through specific structural modifications.;Secondly, owing to the ability of quinolines to inhibit Hz crystallization in vivo, I methodically explore different factors affecting this crystallization process in vitro. I develop a novel high throughput beta-hematin growth assay that mimics physiologic conditions, which is an improvement over current conventional assays. My findings show that lipids and pH are crucial for crystallization efficiency and crystal growth inhibitory activity for quinolines. There are currently contradicting views regarding possible correlation between antiplasmodial activity and Hz growth inhibition for these drugs. Using a large set of common and candidate quinoline antimalarial drugs and this assay, I provide evidence supporting a weak correlation between these two parameters for both chloroquine sensitive and resistant strains.;Finally, limited reports are currently available on drug-substrate interaction and structural modification for quinoline methanols. I perform a comprehensive structure-function study of quinine demonstrating that the hydroxyl functionality and a rigidly positioned nitrogen are vital to its activity. Based on extensive analyses of an isolated quinine-FPIX adduct formed under aqueous conditions, I propose a novel quinine-heme binding model. This model envisions that the hydroxyl oxygen and the iron center of heme associate via a non-covalent electrostatic interaction aided by hydrogen bonding between the hydroxyl proton and the quinuclidyl nitrogen.;Collectively, this work provides additional insight into the mechanism of action of quinoline antimalarial drugs, and should therefore aid in the future design of efficacious new therapies for drug resistant malaria.