Structure-guided design and synthesis of C22- and C32-modified FK520 analogs with enhanced activity against human pathogenic fungi

Invasive fungal infections are a leading cause of death worldwide. Translating molecular insights into clinical benefits is challenging because fungal pathogens and their hosts share similar eukaryotic physiology. Consequently, current antifungal treatments have limited efficacy, may be poorly fungicidal in the host, can exhibit toxicity, and are increasingly compromised by emerging resistance. We have established that the phosphatase calcineurin (CaN) is required for invasive fungal disease and an attractive target for antifungal drug development. CaN is a druggable target, and there is vast clinical experience with the CaN inhibitors FK506 and cyclosporin A (CsA). However, while FK506 and its natural analog FK520 exhibit antifungal activity, they are also immunosuppressive in the host and thus not fungal-selective. We leverage our pathogenic fungal CaN-FK506-FKBP12 complex X-ray structures and biophysical data to support structure-based ligand design as well as structure-activity relationship analyses of broad-spectrum FK506/FK520 derivatives with potent antifungal activity and reduced immunosuppressive activity. Here, we apply molecular docking studies to develop antifungal C22- or C32-modified FK520 derivatives with improved therapeutic index scores. Among them, the C32-modified FK520 derivative JH-FK-44 (