Entamoeba histolytica and Giardia lamblia are Class B protozoa, which are major causes of water- and foodborne epidemics worldwide. Identification of new molecular targets for drug development is key for future therapy as resistance to the main class of drugs, imidazoles, has been detected in both parasites. This proposal brings together the expertise and interdisciplinary efforts of four major laboratories to build on basic research on parasite pathogenesis for a focus on drug development. We have shown that cysteine proteinases are critical for invasion of E. histolytica and excystation of G. lamblia. Cathepsins are an attractive drug target as they have been well characterized biochemically and structurally and are the focus of large inhibitor libraries. EhCP1, one of two cysteine proteinases unique to E. histolytica, is undergoing crystallization studies under Dr. Van Voorhis. The enzyme's unique requirement for arginine in the P2 position, led to the synthesis of a lead small molecule inhibitor by Dr. Roush's group with an IC50 for EhCP1 of <25 nM and >10-fold specificity for the amebic enzyme vs. human cathepsin B. We will test the proof of priniciple that we can develop an inhibitor that is sensitive, nontoxic, and specific for the protozoal cathepsins. Dr. Roush's group will synthesize additional compounds with improved potency and specificity for the parasitic enzymes based on SAR data we have generated. The lead compounds will then undergo basic toxicity testing and efficacy in standard models for in vitro and in vivo amebic infection. We will leverage the findings of the initial crystallization studies of EhCP1 to a structural based drug discovery to guide medicinal chemistry. The active, recombinant enzyme of the next drug target, GICP2, has been expressed by Dr. McKerrow's group and the effect of inhibitors on excystation will be validated with Dr. Gillin. We will also identify other potential classes of inhibitors by high throughput screening of Dr. McKerrow's focused 4000 compound library. These studies will provide the basis to identify novel cathepsin inhibitors against amebic and giardial cathepsins with the potential to target other protozoal enzymes in the future. Relevance. These studies will focus on new drug targets for amoebiasis and giardiasis, important causes of food- and waterborne outbreaks, and may lead to the development of a new class of antiparasitic drugs.