Using chemical probes to investigate the sub-inhibitory effects of azithromycin

The antibacterial drug azithromycin has clinically beneficial effects at sub-inhibitory concentrations for the treatment of conditions characterized by chronic Pseudomonas aeruginosa infection, such as cystic fibrosis. These effects are, in part, the result of inhibition of bacterial biofilm formation. Herein, the efficient synthesis of azithromycin in 4 steps from erythromycin and validation of the drug's ability to inhibit biofilm formation at sub-MIC (minimum inhibitory concentration) values are reported. Furthermore, the synthesis of immobilized and biotin-tagged azithromycin analogues is described. These chemical probes were used in pull-down assays in an effort to identify azithromycin's binding partners in vivo. Results from these assays revealed, as expected, mainly ribosomal-related protein binding partners, suggesting that this is the primary target of the drug. This was further confirmed by studies using a P. aeruginosa strain containing plasmid-encoded ermC, which expresses a protein that modifies 23S rRNA and so blocks macrolide entry to the ribosome. In this strain, no biofilm inhibition was observed. This work supports the hypothesis that the sub-inhibitory effects of azithromycin are mediated through the ribosome. Moreover, the synthesis of these chemical probes, and proof of their utility, is of value in global target identification in P. aeruginosa and other species.