Antimicrobial Activity of Solithromycin against Clinical Isolates of Legionella pneumophila Serogroup 1

The activity of solithromycin was evaluated against clinical Legionella pneumophila serogroup 1 (Lp1) isolates (n = 196) collected in Ontario, Canada, from 1980 to 2011. Its in vitro activity was compared to that of azithromycin (AZM) using the broth microdilution method. Solithromycin had a MIC50 of <= 0.015 mu g/ml and a MIC90 of 0.031 mu g/ml, making its activity at least 8-fold to 32-fold higher than that of AZM (MIC50 and MIC90, 0.125 mu g/ml and 1 mu g/ml, respectively). Ninety-nine percent of the isolates had MICs for solithromycin ranging from <= 0.015 mu g/ml to 0.031 mu g/ml, whereas 83.6% of the isolates showed MICs for AZM ranging from 0.062 mu g/ml to 0.25 mu g/ml. Interestingly, 96.7% (30 out of 31 clinical isolates) identified with higher AZM MICs (0.5 mu g/ml to 2 mu g/ml) belonged to the clinically prevalent sequence type 1. To investigate the intracellular activity of solithromycin, in vitro invasion assays were also performed against a subset of representative Lp1 isolates internalized within human lung epithelial cells. Solithromycin and AZM both inhibited growth of all intracellular Lp1 isolates at 1 x or 8 x MICs, displaying bacteriostatic effects, as would be expected with protein synthesis inhibitor rather than bactericidal activity. Solithromycin demonstrated the highest in vitro and intracellular potency against all Lp1 isolates compared to AZM. Given the rapid spread of resistance mechanisms among respiratory pathogens and the reported treatment failures in legionellosis, the development of this new fluoroketolide, already in phase 3 oral clinical studies, constitutes a promising alternative option for the treatment of legionellosis.