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The cellular uptake by human neutrophils and the intraphagocytic biological activity of the new macrolide antimicrobial agent dirithromycin (0.01-2 mg/L) compared with erythromycin was investigated in vitro. Staphylococcus aureus, Listeria monocytogenes and Legionella pneumophila were used as the test intracellular microbial pathogens. After coincubation (45 min at 37 degrees C) of neutrophils with a fixed concentration of 2 mg/L of each antibiotic the respective intracellular/extracellular ratios for erythromycin and dirithromycin were 6.1 +/- 2.5 and 10.6 +/- 2 respectively (P < 0.005). Using a combination of techniques (colony counting, radiometry and fluorescence microscopy) both erythromycin and dirithromycin at concentrations of 0.01 and 0.5 mg/L and higher, respectively, were found to possess dose-related intraphagocytic bacteristatic activity for each of the test microbial pathogens. The effects of dirithromycin and erythromycin (1-20 mg/L) on neutrophil chemotaxis and generation of reactive oxidants by these cells were also investigated in vitro. Both antimicrobial agents caused a dose-related stimulation of neutrophil migration which was associated with inhibition of leucoattractant-activated generation of superoxide and activity of the myeloperoxidase/H2O2/halide system. However, superoxide generation by neutrophils activited with opsonized zymosan or phorbol myristate acetate was unaffected by the macrolides. These findings demonstrate that dirithromycin accumulates in human neutrophils, is biologically active intracellularly and modulates leucoattractant-activated superoxide generation and chemotaxis.

The intraphagocytic bioactivities for Staphylococcus aureus of amoxicillin, clindamycin, and erythromycin (0.0075-20 micrograms/ml) were measured in human polymorphonuclear leukocytes (PMNLs) with the combination of a fluorochrome microassay and a radioassay. PMNLs with normal or depleted membrane-associated oxidative metabolism were used to investigate the interactions that may occur between the intrinsic O2-dependent antimicrobial systems of human phagocytes and antimicrobial agents in the elimination of intracellular microbial pathogens. Neutralization of O2-dependent antimicrobial systems with retention of phagocytic capacity was achieved with use of PMNLs from four children with chronic granulomatous disease (CGD) or NaF-pulsed normal PMNLs. None of the test antibiotics possessed intracellular bactericidal activity. Clindamycin and erythromycin possessed significant intracellular bacteriostatic activity relative to the modest activity of amoxicillin. Optimal intracellular bioactivity of all three antibiotics was obtained with normal PMNLs relative to NaF-pulsed or CGD PMNLs, a result indicating the existence of beneficial interactions between the antimicrobial agents and the O2-dependent antimicrobial systems of PMNLs.

The binding to human polymorphonuclear leucocytes and the intracellular bioactivity of the macrolide antibiotics erythromycin and roxithromycin on Legionella micdadei, Listeria monocytogenes and Staphylococcus aureus were investigated in vitro by the combination of a fluorochrome microassay and a radioassay. Polymorphs with intact or absent membrane-associated oxidative metabolism were used to investigate the interactions which may occur between the intrinsic oxygen-dependent antimicrobial systems of human polymorphs and the test antibiotics, in the elimination of intracellular microbial pathogens. Elimination of O2-dependent antimicrobial systems with retention of phagocytic activity was achieved by using polymorphs from children with chronic granulomatous disease NaF-pulsed normal polymorphs. Both antimicrobial agents were actively concentrated by polymorphs. Erythromycin was concentrated ten-fold and roxithromycin approximately thirty-fold above extra cellular levels. Both agents possessed intracellular bacteriostatic activity for all three test microbial pathogens. Depletion of polymorph O2-dependent intrinsic antimicrobial systems interfered with the intracellular bioactivity of both antibiotics. This emphasizes the importance of interactions between cell-associated antibiotics and phagocyte antimicrobial systems in the elimination of intracellular microbial pathogens. Like erythromycin, roxithromycin is concentrated by human phagocytes and is bioactive intracellularly.

Erythromycin base and its 6-0-methyl derivative clarithromycin were actively accumulated 7.3 +/- 1.2-fold and 9.2 +/- 2-fold respectively by human neutrophils in vitro. The intraphagocytic bioactivities of the antimicrobial agents were investigated using the combination of a radioassay, colony counting method and a fluorescence microassay which facilitates the distinction between intracellular bacteriostatic and bactericidal mechanisms. Staphylococcus aureus, Listeria monocytogenes and Legionella micdadei were used as the test intraphagocytic microbial pathogens. Both agents were found to possess intracellular bioactivity for all three species of bacteria with clarithromycin being consistently more active than erythromycin. Under the assay conditions used both agents were bacteriostatic (intracellularly) for S. aureus and Leg. micdadei and bactericidal for List. monocytogenes. Clarithromycin is clearly a potent intraphagocytic antibiotic and potentially superior in this respect to erythromycin.

The effects of erythromycin on the migration, phagocytosis and antimicrobial activity of human polymorphonuclear leucocytes (PMNLs) were investigated in vitro. Therapeutic concentrations of erythromycin potentiated PMNL staphylocidal activity without affecting phagocytosis. PMNL random motility and migration to leuco-attractant endotoxin-activated serum (EAS) were significantly enhanced by erythromycin at concentrations of greater than 5 x 10(-5)M (16,5 micrograms/ml). To assess the possible in vivo significance of these findings the same PMNL functions were investigated in healthy adult volunteers before and 90 minutes and 1 week after the ingestion of a single 500 mg tablet of erythromycin stearate. PMNL migration to EAS and antimicrobial activity were significantly increased after ingestion of erythromycin and eventually returned to normal levels. A single intraperitoneal injection of 1,5 mg erythromycin lactobionate significantly increased the mean survival time of mice experimentally infected with Candida albicans. These micro-organisms are resistant to the antimicrobial effects of erythromycin. These findings show that erythromycin possesses nonspecific immunopotentiating properties which may contribute to the antimicrobial activity of this antibiotic.