In Vitro Activity of ASP2397 against Aspergillus Isolates with or without Acquired Azole Resistance Mechanisms

Abstract

ASP2397 is a new compound with a novel and as-yet-unknown target different from that of licensed antifungal agents. It has activity against Aspergillus and Candida glabrata. We compared its in vitro activity against wild-type and azole-resistant A. fumigatus and A. terreus isolates with that of amphotericin B, itraconazole, posaconazole, and voriconazole. Thirty-four isolates, including 4 wild-type A. fumigatus isolates, 24 A. fumigatus isolates with alterations in CYP51A TR/L98H (5 isolates), M220 (9 isolates), G54 (9 isolates), and HapE (1 isolate), and A. terreus isolates (2 wild-type isolates and 1 isolate with an M217I CYP51A alteration), were analyzed. EUCAST E.Def 9.2 and CLSI M38-A2 MIC susceptibility testing was performed. ASP2397 MIC50 values (in milligrams per liter, with MIC ranges in parentheses) determined by EUCAST and CLSI were 0.5 (0.25 to 1) and 0.25 (0.06 to 0.25) against A. fumigatus CYP51A wild-type isolates and were similarly 0.5 (0.125 to >4) and 0.125 (0.06 to >4) against azole-resistant A. fumigatus isolates, respectively. These values were comparable to those for amphotericin B, which were 0.25 (0.125 to 0.5) and 0.25 (0.125 to 0.25) against wild-type isolates and 0.25 (0.125 to 1) and 0.25 (0.125 to 1) against isolates with azole resistance mechanisms, respectively. In contrast, MICs for the azole compounds were elevated and highest for itraconazole: >4 (1 to >4) and 4 (0.5 to >4) against isolates with azole resistance mechanisms compared to 0.125 (0.125 to 0.25) and 0.125 (0.06 to 0.25) against wild-type isolates, respectively. ASP2397 was active against A. terreus CYP51A wild-type isolates (MIC 0.5 to 1), whereas MICs of both azole and ASP2397 were elevated for the mutant isolate. ASP2397 displayed in vitro activity against A. fumigatus and A. terreus isolates which was independent of the presence or absence of azole target gene resistance mutations in A. fumigatus. The findings are promising at a time when azole-resistant A. fumigatus is emerging globally.

FIG 1

Histogram illustrating the uniform activity of ASP2397 (EUCAST MICs are shown above the x axis) across wild-type and azole-resistant A. fumigatus isolates compared to that of itraconazole (shown below the x axis). The strains are named according to the genotype, with “wt” indicating the wild-type strain versus the strains harboring different CYP51A alterations at position G54, M220, or TR/L98H. Reference strains are indicated with official strain numbers preceded by “EUCAST” or “CLSI.”

FIG 2

ASP2397 growth inhibition curves (light blue) for the atypical G54E and M220K mutant A. fumigatus isolates compared to those for itraconazole (dark blue) and posaconazole (red). (a and b) Growth curves for the G54E isolate determined by the EUCAST method (a) and a modified EUCAST method using a lower inoculum (b) show the fully resistant phenotype of this specific isolate. (c) In contrast, the phenotype of partial growth inhibition of the M220K isolate tested using the EUCAST method is shown. Results of growth in 8 growth control wells are shown by the upper broken black line. The horizontal dotted lines indicate 50% growth inhibition. Background absorbance in the medium corresponds to an optical density (OD at 490 nm) of approximately 0.1.