Antagonism Rather Than Synergy: Ibuprofen-Antifungal Interactions Depend on Strain Genetics and Nutrient Environment

Drug interaction outcomes-synergism, additivity, or antagonism-represent complex phenotypes. While drug repurposing aims to identify compounds that potentiate conventional antifungals, when given in combination, the modulators of these interactions in fungi remain largely unexplored. We hypothesize that the response to repurposed-conventional antifungal pairs is a complex trait modulated by genetic and environmental factors. To study the impact of genotype on the outcome, we screened six diverse Saccharomyces cerevisiae isolates, including clinical, wild, and fermentation strains, for their responses to combinations of ibuprofen with either clotrimazole or caspofungin. We evaluated the role of the environment using rich and minimal media and assessed the influence of assay type by comparing solid- and liquid-rich media assays. Our results reveal that ibuprofen-clotrimazole interactions are highly dynamic, predominantly antagonistic, with limited synergy observed. These outcomes are significantly modulated by genetic background, media composition, assay type, and, in specific genotypes, even by the drug dosage, reflecting a complex, multi-parametric phenotype. However, the ibuprofen-caspofungin combination is more predictable, exhibiting only synergy or additivity. Interaction outcomes correlate with baseline sensitivity to caspofungin: caspofungin-resistant isolates consistently demonstrate synergy, while sensitive strains exhibit additivity. These findings shift the paradigm of drug discovery by demonstrating that synergism and antagonism are not static properties of drug pairs but are dynamic, context-dependent outcomes. This study highlights the need to use clinically relevant models and patient-specific isolates before clinical application, as drug interactions cannot be generalized from a single dosage, strain, or environmental condition.