Project Summary The control and eventual elimination of malaria in Africa is seriously challenged by drug resistance, in particular resistance to new artemisinin-based combination therapy (ACT) regimens. Worrisome signs suggest that high level resistance to components of ACT is on the way to Africa, a scary prospect, since the bulk of serious falciparum malaria occurs on this continent. Improved characterization of the extent of resistance and mechanisms of resistance is of critical importance. We hypothesize that antimalarial drug sensitivity is changing in Uganda, that resistance to each relevant drug is mediated by specific parasite mutations, that the rise of resistance will be dampened and obscured by parasite fitness costs, but that continued drug pressure will allow emergence of highly fit resistant parasites. Preemptive analysis of parasites will be critical to characterizing resistance mechanisms before the problem is widespread. To test our hypotheses we will measure the drug sensitivities of parasites freshly isolated from Ugandan patients, including individuals under varied levels of drug pressure; characterize the genetic profiles of these parasites; and assess the fitness costs of resistance in both clinical and laboratory settings. We believe that focused evaluations of fresh Ugandan isolates will best equip us to characterize drug sensitivity and the emergence of high level resistance in Africa, where the malaria problem is greatest, and where timely characterization of resistance mechanisms can be most valuable. Our studies will benefit from access to a wealth of clinical studies in Uganda, including trials studying the treatment and chemoprevention of malaria, surveillance programs collecting samples from sites around Uganda, and a cohort study following subjects at varied risks of malaria. Our program will also benefit from our established laboratories for the study of malaria parasites in Uganda. Our specific aims will be: (1) to longitudinally characterize the ex vivo sensitivity of malaria parasites to ACT components, (2) to characterize genetic mediators of varied antimalarial drug sensitivity, and (3) to characterize impacts of varied drug sensitivity on the fitness of malaria parasites. Our overall goal is to better characterize antimalarial drug sensitivity and resistance determinants before high level resistance becomes widespread, so that monitoring of these determinants can guide efforts to circumvent the spread of resistance.