Investigation of Dual Stage Acridones as a Potent Malaria Treatment

Author

Stephanie Huezo, Dominican University of CaliforniaFollow

Graduation Date

4-2015

Document Type

Master's Thesis

Degree Name

Master of Science

Department or Program

Biological Sciences

Department or Program Chair

Maggie Louie, PhD

First Reader

Roland A. Cooper, PhD

Second Reader

Philip J. Rosenthal, MD

Abstract

The need for potent antimalarials to prevent the emergence of drug resistant Plasmodium falciparum is urgent. Discovery of novel acridone chemotypes have shown promise for a new antimalarial drug treatment. Presently, two acridone chemotypes have intrinsic antimalarial potency against chloroquine sensitive and multidrug resistant parasites. Acridones lacking an N10 side chain are known as chemotype I acridones, whereas, chemotype II acridones are defined as having an alkyl side chain at the same position. The N10 substitution of chemotype II acridones is thought to target heme and inhibit hemozoin formation within the parasite’s digestive vacuole, and is known to provide synergistic effects with other antimalarials such as quinine. Currently, the molecular target(s) of chemotype I acridones are unknown; therefore, we sought to identify the target of these acridones. Using the chloroquine resistant P. falciparum Dd2 line and the chemotype I compound, T13, we selected stable chemotype I acridone resistance by using multiple rounds of 24 hour intermittent pressure and incremental continuous pressure. Control parasites exhibited an IC₅₀ value of ~40 nM T13, while cloned resistant parasite lines showed IC₅₀ values of ~1300-1700 nM T13. Cross-resistance was seen with other chemotype I acridones, but not chemotype II acridones, indicating the importance of the N10 substitution in avoiding the resistance mechanism. Slight cross-resistance to atovaquone and ELQ-400 was seen in T13-resistant parasites, indicating that T13 may target the component of the electron transport chain pathway that inhibits the binding of ubiquinone to cytochrome b. Two previously reported mutations have been identified in the Q_o site of cytochrome b, M133I and V259L, which confer low level (IC₅₀ = 242 ± 18 nM T13) and high level (IC₅₀ = 1439 ± 49) T13 resistance, respectively, in vitro. A third, novel mutation has been identified in the Q_o site, A138T, which accompanies the M133I mutation, and confers high-level T13 resistance (1678 ± 78). Selection of acridone resistance has led to identification of cytochrome b as the main target of chemotype I acridones and the mechanism of action within P. falciparum.