Investigating the Cytostatic and Cytocidal Mechanisms of Quinoline Drug Resistance in Plasmodium falciparum

Abstract

Efficacy of, mechanisms of action of, and resistance to chemotherapeutic agents targeted against the malarial parasite Plasmodium falciparum have historically been defined by quantifying cytostatic potency or growth-inhibition. However, recent evidence has suggested that, for some drugs, this is distinct from cytocidal potency or ability to promote cell death. From a practical standpoint, the major difference lies in level and duration of drug exposure. It becomes important, then, to define both new and established aspects of drug pharmacology from the perspective of cytocidal activity.

The cytostatic target of quinoline-based and related drugs is generally accepted to be crystallization of ferriprotoporphyrin IX heme, a consequence of parasitic hemoglobin digestion within the digestive vacuole. It is not known, however, whether this is the predominant target for cytocidal activity.

The cytostatic antiplasmodial effect is exerted via drug interacting with one or more pre-crystalline forms of heme that then prevents crystallization to hemozoin. This is a complex process, as heme/drug solubility and speciation and drug-heme interactions are highly dependent on pH, ionic strength, and lipid environment. Surprisingly, the molecular details of these interactions and how they influence antimalarial activity are largely unknown for many quinoline drugs, in particular, the Cinchona alkaloid quinine.

This thesis further defines cytostatic vs. cytocidal antimalarial activities for quinine and its stereoisomers as well as structural analogues of quinine and chloroquine. Using these compounds, I investigate the relationship between these two antiplasmodial measurements and a drug's ability to inhibit heme crystallization. I characterize quinine- and quinine stereoisomer-heme interactions and determine the activity-relevant complexes these drugs form with heme. Finally, in collaboration with several groups, I identify novel drug leads with potent activities against resistant P. falciparum. I also develop several interdisciplinary techniques for conducting such analyses, including high-throughput drug screening, organic synthesis, and spectroscopic methods.

Collectively, this work provides additional insight into cytostatic and cytocidal quinoline drug pharmacology and assists development of new, efficacious drugs in the fight against resistance.