Olorofim is a novel antifungal agent with in vitro activity against Aspergillus and some other moulds. Here we addressed technical aspects for EUCAST olorofim testing and generated contemporary MIC data.
EUCAST E.Def 9.3.1 testing was performed comparing two plate preparation methods (serial-dilution in medium (serial-plates) versus pre-dilution in DMSO (ISO-plates)), two lots of olorofim, visual (visual-MIC) versus spectrophotometer (spec-MIC) reading and four polystyrene plates using 34-53 Aspergillus isolates from five genera. Subsequently, olorofim MICs were compared to itraconazole, voriconazole, posaconazole and amphotericin B MICs for 298 clinical mould isolates (2016-2017). Wild-type upper limits (WT-UL) were determined following EUCAST principles for ECOFF setting.
Olorofim median MICs comparing serial-plates and ISO-plates were identical (25/36, 69%) or one dilution apart (11/36, 31%). Inter-person agreement for visual-MICs was 92-94%/100% for ≤1/≤2 dilutions, respectively. Visual-MICs across tested microtitre plates and olorofim lots revealed only discrete differences (≤1 dilution lower for treated plates). No single spec-MIC criteria was applicable to all species.
Olorofim MICs were low against 275 Aspergillus spp. isolates (modal-MIC=0.06 mg/L, MIC range <0.004-0.25 mg/L) and three dermatophytes (MICs 0.03-0.06 mg/L). MICs against Fusarium were diverse with full inhibition of F. proliferatum (MIC=0.016), 50% growth inhibition of Fusarium solani at 1-2 mg/L and no inhibition of F. dimerum.
Olorofim displayed potent in vitro activity against most mould isolates and was associated with limited variation in EUCAST susceptibility testing.
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