Summary The treatment and control of malaria are seriously challenged by drug resistance. With widespread resistance to older agents, artemisinin-based combination therapies (ACTs) are the mainstay for antimalarial treatment, but ACT efficacy is threatened by resistance to artemisinins and partner drugs. New antimalarial drugs are needed. Spearheaded by the Medicines for Malaria Venture (MMV), a robust pipeline of new lead antimalarial compounds is under development. However, resistance to new antimalarial agents can be anticipated. In a number of cases drug targets and resistance mechanisms have been identified, but studies have focused on small numbers of P. falciparum laboratory strains. It is critical also to consider sensitivity to lead antimalarials of fresh P. falciparum field isolates, especially isolates from Africa. In pilot studies we have identified important variability in ex vivo sensitivity of fresh Ugandan P. falciparum isolates to lead antimalarial compounds. In some cases this variability appears to be linked to mechanisms of resistance identified in the laboratory, but not previously seen in field isolates. A better understanding of the extent of and mechanisms of decreased sensitivity in African field isolates will be of great value in informing optimal development of next-generation combination antimalarial therapies. This project will build on an ongoing collaboration between our group and MMV, which has provided pilot data from Uganda, and another longstanding collaboration in Burkina Faso. Our program will offer state-of-the-art assessment of ex vivo P. falciparum sensitivities linked to genotypic and phenotypic characterization of field samples. We hypothesize that P. falciparum isolates from Uganda and Burkina Faso will demonstrate varied sensitivity to lead antimalarial compounds, and that characterization of genotypes and phenotypes of field isolates will identify shared resistance mechanisms and optimal combination therapies. Our specific aims will be: (1) to characterize ex vivo sensitivity to lead antimalarial compounds of P. falciparum field isolates, (2) to characterize genotypes of drug sensitivity outliers to identify mediators of decreased sensitivity in field isolates, and (3) to characterize phenotypes of drug sensitivity outliers to elucidate mechanisms of resistance of lead antimalarial compounds. Our studies will define resistance mechanisms for the most important new compounds under development as antimalarials and inform choices of optimal antimalarial drug combinations and the direction of continued drug discovery efforts.