Mechanisms of the Macrolide-Induced Inhibition of Superoxide Generation by Neutrophils

The effect of macrolides on the superoxide (O₂ ⁻) production by neutrophils was studied. Resting neutrophils become primed by lipopolysaccharide (LPS) or N-formyl-methionyl-leucyl-phenylalanine (fMLP), and primed neutrophils generate O₂ ⁻ in response to fMLP or adhesion, respectively. Both LPS-primed fMLP-stimulated O₂ ⁻ generation by macrolide-treated neutrophils and adhesion-stimulated O₂ ⁻ generation by macrolide-treated fMLP-primed neutrophils were inhibited. Macrolide inhibition of O₂ ⁻ generation was dependent on serum or pH. Serum could be substituted by NaHCO₃. The intensity of inhibition was azithromycin = roxithromycin > clarithromycin > erythromycin, in that order. Non-antimicrobial derivatives of erythromycin, that is, EM703 and EM900, inhibited O₂ ⁻ generation at pH 7.4. NH₄Cl abolished the activity of azithromycin (AZ) only when added to neutrophils with AZ but not after incubation with AZ, suggesting that NH₄Cl prevented the influx of AZ. AZ did not affect the expression of alkaline phosphatase, CD11b, and cytochrome b₅₅₈ in both resting and LPS-primed neutrophils. These results suggested that macrolides did not affect granule mobilization but inhibited O₂ ⁻ generation selectively.