Azithromycin May Antagonize Inhaled Tobramycin When Targeting Pseudomonas aeruginosa in Cystic Fibrosis

Abstract Rationale Recent studies of inhaled tobramycin in subjects with cystic fibrosis (CF) find less clinical improvement than previously observed. Nonhuman data suggest that in some strains of Pseudomonas aeruginosa, azithromycin can antagonize tobramycin. Objectives We tested the hypothesis that concomitant azithromycin use correlates with less improvement in key outcome measures in subjects receiving inhaled tobramycin while not affecting those receiving a comparative, nonaminoglycoside inhaled antibiotic. Methods We studied a cohort of 263 subjects with CF enrolled in a recent clinical trial comparing inhaled tobramycin with aztreonam lysine. We performed a secondary analysis to examine key clinical and microbiologic outcomes based on concomitant, chronic azithromycin use at enrollment. Measurements and Main Results The cohort randomized to inhaled tobramycin and reporting azithromycin use showed a significant decrease in the percent predicted FEV1 after one and three courses of inhaled tobramycin when compared with those not reporting azithromycin use (28 d: −0.51 vs. 3.43%, P Conclusions Oral azithromycin may antagonize the therapeutic benefits of inhaled tobramycin in subjects with CF with P. aeruginosa airway infection.