Project Summary Drugs targeting the liver stage offer many advantages over drugs that merely target the blood stage. First, drugs active against the liver stage represent true causally prophylactic agents that can prevent all disease symptoms, including death, associated with malaria. Secondly, it has been established that while wild-caught mosquitoes may harbor thousands of sporozoites, only ≈10 sporozoites are transferred in a single bite to the human host. Over the next 2-3 weeks the sporozoite reproduces in the liver to produce 10,000-30,000 descendants before the schizont ruptures and parasites flood into the bloodstream where the absolute parasite burden may increase to ten thousand billion (1013) circulating plasmodia. Clearly it is advantageous to strike at the liver stage where parasite numbers are low, to diminish the likelihood of selecting for a drug resistant mutant and before the infection has a chance to weaken the defenses of the human host. Our primary goal in this project is to develop a drug combination that is active against P. falciparum, causative agent of the most virulent, often fatal form of malaria. The drug combination will act synergistically while targeting the parasite in the liver, blood and also the vector stages that are critical for disease transmission. Studies (by us and others) have shown that ELQ-300 targets all 3 of these life cycle stages. The ultimate objective of our proposed work is the development of an inexpensive ELQ-300 prodrug that can be co-formulated with other antimalarials in a synergistic combination to prevent and treat malaria and support the worldwide effort to control and eradicate the disease. This study focuses on the combination of ELQ-300 prodrugs with biguanides including proguanil. We seek a better understanding of the biochemical mechanism that underlies the antimalarial synergism that exists between ELQ-300 and biguanides. A better understanding of this interaction will help to design a biguanide/ELQ-300 prodrug combination that is safe for use in humans for weekly prophylaxis and treatment of malaria.