Identification and characterization of novel strategies for the chemotherapeutic inhibition of Toxoplasma gondii and Plasmodium falciparum.

摘要

Plasmodium falciparum, the causative agent of malaria, and Toxoplasma gondii, a pathogen that causes toxoplasmosis, are two apicomplexan parasites responsible for considerable worldwide human morbidity and mortality. Due to the lack of vaccines to prevent contraction of the aforementioned diseases, chemotherapy is utilized as a means for parasitic control and disease mitigation. One pathway largely targeted by antiparasitic drugs is de novo folate biosynthesis. Unfortunately, poor patient tolerance and emerging parasitic resistance to the current antifolate therapies are growing and new drugs are urgently required. Due to its central role in parasite survival, folate biosynthesis still represents a viable target for novel therapeutics. Along this line, this dissertation examines apicomplexan folate acquisition pathways in order to identify and characterize new strategies and/or lead compounds that can be utilized in the design of novel therapies for both T. gondii and P. falciparum infection. Several potential tactics emerged from this work. First, we provide evidence that non-active site inhibition of bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) offers a novel mode for the inhibition of parasitic folate biosynthetic enzymes. Second, through characterization studies of a DHFR-TS non-active site inhibitor, eosin B, we identified that T. gondii is capable of exogenous folate salvage; the results from these studies provide the first direct evidence for the existence of a folate transport mechanism in T. gondii, demonstrate the importance of folate salvage as a factor in the effectiveness of antifolate therapy, and identify another novel pathway that could be exploited in the development of new drugs. Third, the characterization of eosin B in Plasmodium exemplified that the dual non-active site inhibition of multiple enzymes by one compound is possible which, in turn, represents a powerful strategy for the development of new antiparasitic inhibitors. Finally, this research also identified eosin B as a respectable candidate for antiparasitic drug development. All together, this dissertation provides information that will aid in the design of newer, more effective therapeutic treatments for toxoplasmosis and malaria.