Olorofim activity against multidrug-resistant <i>Fusarium</i> unveils intra-species and inter-species variability

Fungi in the genus <i>Fusarium</i> are plant pathogens but are also capable of causing a wide range of diseases in humans. The intrinsic multi-drug resistance of <i>Fusarium</i> often leads to a poor clinical outcome in patients with severe immune disorders. Olorofim, a member of the orotomide class, is a novel type of antifungal drug that interferes with pyrimidine biosynthesis by inhibiting dihydroorotate dehydrogenase and thereby prevents growth and cell division. In this study, the <i>in vitro</i> activity of olorofim was evaluated against 253 <i>Fusarium</i> isolates, of which 228 isolates belonging to the prevalent complexes involved in human infection, <i>F. solani</i> species complex (SC) and <i>F. fujikuroi</i> SC. All <i>Fusarium</i> isolates underwent species-level identification via multi-locus sequence typing (MLST) targeting <i>RPB1</i>, <i>RPB2</i>, and <i>TEF1</i> loci. Antifungal susceptibility testing was performed using a CLSI M38, 3rd ed. broth microdilution for olorofim. The geometric mean of the MICs of olorofim for all 253 isolates was 0.581 µg/mL, ranging from 0.015 µg/mL to >16 µg/mL. Olorofim demonstrated high MICs against <i>F. solani</i> SC, whereas greater potency (lower MICs) was observed against <i>F. fujikuroi</i> SC. Clinical isolates tended to have higher MIC values than environmental isolates, but this pattern was not consistent across all species complexes. Overall, olorofim demonstrated moderate <i>in vitro</i> activity against <i>Fusarium</i> isolates, suggesting it might be a potential candidate for treating fusarioses caused by multidrug-resistant strains.