Antimalarial Drug Discovery and Target Identification from Phenotypic High-throughput Screening Hits

The drive to propagate a species genes is among the strongest biological forces, shared by humans and pathogens alike. For pathogens like Plasmodium parasites, the etiological agent of human malaria, self-preservation comes at the cost of hundreds of thousands of human lives annually. Between 2000 and 2016, worldwide cases of malaria were progressively declining. Although there are many causes for the recent increase in global malaria cases, parasite drug resistance is a likely contributor. Thus, research is desperately needed to identify druggable targets and develop novel therapeutics capable of more than symptom alleviation. This dissertation highlights the use of key strategies that have resulted in new preclinical drug candidates, namely the systematic investigation of vast small molecule libraries. In Chapter 1, the investigation of more than 100 marine derived natural products identified six compounds with promising antiparasitic activity and selectivity relative to the host cell. Additionally, this chapter describes the successful target identification of choice screening hits, such as hectochlorin and its newly validated target, actin. In Chapter 2 high throughput screening methods are embraced to explore the activity of nearly 70,000 small molecules, testing them, with collaborators, against all malaria parasite stages that dwell in the human host. Hundreds of these molecules are discovered to have activity against one or more of these stages. Studies in collaboration with Manu Vanaerschot show the target of one such molecule acting within the mitochondrial electron transport chain, against cytochrome bc1. Despite affecting a well-characterized drug target in Plasmodium, this target rediscovery legitimizes our strategy for antimalarial hit selection from untested chemical libraries. Because drug target discovery is vital to the development of novel therapeutics, and can guide drug design to minimize the likelihood of off target effects, Chapter 3 describes the search for the target of a potent asexual blood stage (ABS) inhibitor. Here, the protein cytoplasmic isoleucyl-tRNA synthetase (PF3D7_1332900) is shown as the target of a drug-like scaffold, TCMDC-124553. This protein was previously shown to be critical in P. falciparum ABS, and our data also suggest it is essential in the liver stage of infection as well