Resistance to the antifungal activity of Aprotinin occurs through mutations in genes that function in cation homeostasis

An increase in the prevalence of fungal infections is coinciding with an increase of resistance to current clinical antifungals, placing pressure on the discovery of new antifungal candidates. One option is to investigate drugs that have been approved for use for other medical conditions that have secondary antifungal activity. Aprotinin, also known as Bovine Pancreatic Trypsin inhibitor (BPTI), is an antifibrinolytic that has been approved for systemic use in patients in some countries. Bleackley and coworkers (2014) revealed that BPTI also has antifungal activity against S. cerevisiae and C. albicans and does this by targeting the magnesium transporter ALR1. Here we have further investigated the potential for aprotinin to be used as an antifungal by assessing the development of resistance. We used an in vitro model to assess the evolution of BPTI resistance/tolerance whereby BPTI was serial passaged with the model organism S. cerevisiae. Resistance to BPTI developed more quickly than resistance to the plant defensin NaD1 and the clinical antifungal, caspofungin. Full genome sequencing of resistant lines revealed that resistance to BPTI developed as the result of a deleterious mutation in either the ptk2 or sky1 genes. This revealed that cation homeostasis and transport functions were particularly affected in S. cerevisiae after exposure to BPTI. Therefore, the mutations in these genes probably decreases release of magnesium and other cations from the cell, protecting the yeast from the limiting intracellular magnesium levels that arise when BPTI blocks the magnesium transporter Alr1p.