Designing Selective and Non-Immunosuppressive Antifungal FK506 Analogs: Structures, Biophysics and Dynamics of Fungal and Human Calcineurin-Inhibitor Complexes

Calcineurin is an attractive antifungal target due to its central role in fungal pathogenesis. The development of specific antifungals targeting calcineurin is complex, as calcineurin inhibitors, such as FK506, are immunosuppressive. Using fungal calcineurin-inhibitor crystal structures we recently developed a less immunosuppressive FK506 analog, APX879, with broad-spectrum antifungal activity and efficacy in a murine model of invasive fungal infection. To better understand the interaction of the human and fungal FK506-binding proteins (FKBP12) required for calcineurin inhibition at a molecular level, and guide the design of fungal-selective and non-immunosuppressive FK506 analogs, here we report the high-resolution structures of the FKBP12 bound to FK506 and the human, and FKBP12 proteins bound to the FK506 analog, APX879. Combining structural, genetic and biophysical methodologies with molecular dynamics simulations, we identify critical variations in these structurally similar FKBP12:ligand complexes to enhance fungal-selectivity.