Determination of Disk Diffusion and MIC Quality Control Ranges for Nafithromycin (WCK 4873), a New Lactone-Ketolide

Disk diffusion and MIC quality control (QC) ranges were determined for nafithromycin, a new lactone-ketolide, following the completion of a nine-laboratory, Clinical and Laboratory Standards Institute (CLSI) document M23-defined tier 2 study. Five QC strains consistent with the spectrum of activity of nafithromycin were tested: Staphylococcus aureus ATCC 25923 (disk only), S. aureus ATCC 29213 (broth only), Enterococcus faecalis ATCC 29212 (broth only), Streptococcus pneumoniae ATCC 49619 (disk and broth), and Haemophilus influenzae ATCC 49247 (disk and broth). Nafithromycin disk diffusion QC ranges were determined to be 25 to 31 mm for S. aureus ATCC 25923, 25 to 31 mm for S. pneumoniae ATCC 49619, and 16 to 20 mm for H. influenzae ATCC 49247. Nafithromycin MIC QC ranges were determined to be 0.06 to 0.25 μg/ml for S. aureus ATCC 29213, 0.016 to 0.12 μg/ml for E. faecalis ATCC 29212, 0.008 to 0.03 μg/ml for S. pneumoniae ATCC 49619, and 2 to 8 μg/ml for H. influenzae ATCC 49247. All disk diffusion and MIC QC ranges established in this study were approved by the CLSI Subcommittee on Antimicrobial Susceptibility Testing at their June 2015 meeting and were initially reported in the 2017 M100S document. The QC ranges established in this study should be used for determining the in vitro activity of nafithromycin in phase 2 and phase 3 human clinical trials and subsequently for testing patient isolates and isolates in phase 4 surveillance studies.