Olorofim susceptibility testing of 1423 Danish mould isolates 2018-2019 confirms uniform and broad-spectrum activity.

Background: Olorofim is a novel antifungal drug in Phase 2 trials. It has shown promising in vitro activity against various moulds except Mucorales. Initially, we observed a broad range of EUCAST MICs for Aspergillus fumigatus Here we explored the MIC variability in more detail and prospectively investigated the susceptibility of contemporary clinical mould isolates as population data is needed for future ECOFF setting.Methods: MIC variability: 15 A. fumigatus isolates previously found with low/medium/high MICs, respectively (≤0.002-0.25 mg/L), were tested repeatedly and EUCAST MICs read in a blinded fashion by three observers. pyrE, encoding the olorofim target enzyme dihydroorotate dehydrogenase (DHODH), was sequenced. Contemporary data: 1423 mould isolates (10 Aspergillus species complexes (including 1032 A. fumigatus) and 105 other mould/dermatophyte isolates). Olorofim susceptibility (modal MIC, MIC50, MIC90 and wild-type upper limits (WT-ULs) (species complexes with ≥15 isolates)) was determined and compared to that of four comparators.Results: MIC variability: MICs (mg/L) were within two two-fold dilutions (0.016-0.03) for 473/476 determinations. The MIC range spanned four dilutions (0.008-0.06). No significant pyrE mutations were found. Contemporary data: Modal MIC/WT-UL97.5% (mg/L) were 0.03/0.06 (A. terreus and A. flavus), 0.06/0.125 (A. fumigatus and Trichophyton rubrum), and 0.06/0.25 (A. niger and A. nidulans). The MIC range for Scedosporium spp. was 0.008-0.25. Olorofim susceptibility was similar for azole-resistant and -susceptible isolates of A. fumigatus but reduced for A. montevidensis and A. chevalieri (MICs>1).Conclusions: With experience, olorofim susceptibility testing is robust. Testing of isolates from our centre showed uniform and broad-spectrum activity. Single-centre WT-ULs are suggested.