AbstractObjectivesTo evaluate the in vitro activity and in vivo efficacy of delafloxacin against Bacillus anthracis, the causative agent of anthrax.MethodsMICs were obtained according to CLSI guidelines for 30 virulent isolates and 14 attenuated antibiotic-resistant strains. For the in vivo efficacy study, mice were administered delafloxacin (30–62.5 mg/kg) subcutaneously, or ciprofloxacin (30 mg/kg) intraperitoneally beginning at either 24 or 48 ± 1 h post-challenge (post-exposure prophylaxis) and continued every 12 h for 14 days with study termination on day 30. The mean inhaled dose in the study was approximately 103 × LD50 equivalents, and the range was 87–120 × LD50.ResultsDelafloxacin (MIC90 = 0.004 mg/L) was 16-fold more potent than ciprofloxacin (MIC90 = 0.06 mg/L) against a 30-strain set of virulent B. anthracis. Against a panel of attenuated antibiotic-resistant strains, delafloxacin demonstrated potency ≥128-fold over that observed with ciprofloxacin. When evaluated in vivo, mice treated with all delafloxacin doses tested at 24 h post-challenge demonstrated equivalent survival compared with mice treated with the positive control ciprofloxacin. Because of the high challenge dose of spores, mice treated at 48 h showed rapid and high mortality in all groups including the positive control. Surviving animals in all delafloxacin- and ciprofloxacin-treated groups (24 and 48 h) showed complete splenic clearance of infection and <2.2 × 103 cfu/g lung tissue.ConclusionsGiven the high bar set by the 100 × LD50 challenge dose in this study, the results from delafloxacin treatment are promising for the treatment of inhaled anthrax.