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Structural Biological Development of Fungal-Specific Calcineurin Inhibitors

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Invasive fungal infections are a leading cause of death in immunocompromised patients. Current antifungals have limited clinical efficacy, are poorly fungicidal, are in some cases toxic, and are increasingly ineffective due to emerging resistance. We have established that the conserved phosphatase calcineurin is broadly required for invasive fungal disease. The FDA-approved calcineurin inhibitor FK506 is active in vitro against major invasive fungal pathogens, but also suppresses host immunity. Our approach seeks to overcome the fungal versus human specificity barrier to significantly advance antifungal treatment. The objective of this study is to utilize a structural biology-based strategy to define fungal-mammalian calcineurin structural differences, validate fungal-specific targets, and generate and optimize novel FK506 analogs to treat invasive fungal diseases. Our central hypothesis is that by employing a structural biological approach using both crystallography and NMR spectroscopy that we will define novel targetable fungal-specific areas in the calcineurin complex critical for fungal pathogenesis. For maximum clinical breadth, we will focus on the two major clinical pathogens: the yeast Candida albicans and the mold Aspergillus fumigatus. Our preliminary studies document proof of principle non-immunosuppressive FK506 analogs with robust antifungal activity. We hypothesize that structures of the calcineurin A and B complex, coupled with calmodulin and the immunophilin complex (FKBP12-FK506), will reveal novel fungal-specific targets for inhibition. We have recently solved the structure for C. albicans, and will now solve structures for the calcineurin heterodimer alone and complexed with FKBP12-FK506/analogs from A. fumigatus. The multiple molecular views will allow identification of sites that are distinct between human and fungal calcineurin complexes that can be exploited for targeted inhibitor development. Protein regions that are dynamic or resist crystallization will be structurally characterized by NMR. Putative inhibitory domains will be validated via genetic and biochemical assays, utilizing site-directed mutagenesis of key contact and surface residues to examine structural stability, fungal phenotype, and drug action/resistance. Non-immunosuppressive fungal-specific FK506 analogs will be generated by Amplyx Pharmaceuticals based on the SAR results of our first iteration. This will guide the production of second generation analogs optimized for retention of antifungal activity and abrogation of immunosuppression by capitalizing on the unique structural differences between the host and fungal enzymes. Medicinal chemistry and inhibitor docking experiments will be conducted to alter analogs based on screening results. Lead compounds will be tested using an iterative approach both in vitro and in murine models of invasive candidiasis and invasive aspergillosis. We will capitalize on structural biology as a new approach to targeting calcineurin by defining fungal-specific features with no mammalian counterpart to generate novel antifungal therapeutics.

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