Olorofim Susceptibility Testing of 1,423 Danish Mold Isolates Obtained in 2018-2019 Confirms Uniform and Broad-Spectrum Activity

Abstract

Olorofim is a novel antifungal drug in phase 2 trials. It has shown promising in vitro activity against various molds, except for 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 mold isolates, as population data are needed for future epidemiological cutoff (ECOFF) settings. Fifteen A. fumigatus isolates previously found with low/medium/high MICs (≤0.002 to 0.25 mg/liter) 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. A total of 1,423 mold isolates (10 Aspergillus species complexes [including 1,032 A. fumigatus isolates] and 105 other mold/dermatophyte isolates) were examined. 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. MICs (mg/liter) were within two 2-fold dilutions (0.016 to 0.03) for 473/476 determinations. The MIC range spanned four dilutions (0.008 to 0.06). No significant pyrE mutations were found. Modal MIC/WT-UL_97.5 (mg/liter) values 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 to 0.25. Olorofim susceptibility was similar for azole-resistant and -susceptible isolates of A. fumigatus but reduced for A. montevidensis and A. chevalieri (MICs of >1). With experience, olorofim susceptibility testing is robust. The testing of isolates from our center showed uniform and broad-spectrum activity. Single-center WT-ULs are suggested.

Keywords: Aspergillus; Cyp51A; DHODH; EUCAST; F901318; Scedosporium; antifungal susceptibility; azole resistance; olorofim; pyrE.

FIG 1

Repetitive olorofim MICs against the 15 A. fumigatus isolates from the previous study (2016 to 2017) compared to the original A. fumigatus MICs of all isolates from that study (a), MICs against contemporary isolates in this study (2018 to 2019) (b), and QCs (A. fumigatus ATCC 204305) from the two study periods (c). The original MIC values for each isolate included in the variability study are marked in blue circles in the 2016–2017 data set. Yellow lines indicate the median and 25% interquartile range.

FIG 2

Comparison of olorofim MICs for isolates of A. fumigatus (left) and A. terreus SC (right) in relation to azole susceptibility (percentage of the total number of isolates with a given susceptibility classification). Azole-susceptible (S) isolates are shown below the x axis, whereas resistant (R) isolates are shown above the x axis. For A. fumigatus, only azole-resistant cyp51A sequenced isolates were included (79 of 112 azole-resistant isolates). For A. terreus, nonsequenced isolates with azole susceptibility profiles similar to those of other sequenced isolates from the same patient harboring Cyp51A alterations were included to show the MIC variability.