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Recently we found that certain antibiotics which are markedly concentrated by human polymorphonuclear leukocytes (PMN) failed to kill susceptible, intraphagocytic Staphylococcus aureus, even though cellular drug levels were quite high. The possibility that specific antibiotics might adversely affect phagocyte antibacterial function was considered. Thus, we studied the effects of multiple antibiotics and adenosine, a known modulator of the PMN respiratory burst response, on neutrophil antibacterial function. At nontoxic concentrations, these drugs had no effect on degranulation in stimulated PMN. Adenosine was a potent inhibitor of formylmethionyl-leucyl-phenylalanine (FMLP)-stimulated superoxide and hydrogen peroxide generation in PMN but produced less inhibition of microbial particle-induced respiratory burst activity. Three of the tested antibiotics, all of which reach high concentrations in phagocytic cels, had a marked modulatory effect on the PMN respiratory burst. Clindamycin, which enters phagocytes by the ceUl membrane adenosine (nucleoside) transport system, had only a modest effect on FMLP-mediated superoxide production but inhibited the microbial particle-induced response by -50%. Roxithromycin and trimethoprim were efficient inhibitors of PMN superoxide generation stimulated by FMLP and concanavalin A (also inhibited by erythromycin) but had less effect on zymosan-mediated respiratory burst activity. Antibiotics which entered phagocytes less readily had no effect on the respiratory burst response in PMN. These results, as well as those of experiments with inhibitors of cell membrane nucleoside receptors, indicated that the antibiotic effect is mediated through intraphagocytic pathways. The possibility that antibiotic-associated inhibition of the PMN respiratory burst response might alter leukocyte antimicrobial and inflammatory function deserves further evaluation.The interaction of antimicrobial agents with leukocytes, and especially any influence on the fate of bacteria ingested by these phagocytic cells, may be of therapeutic importance. Entry of antibiotics into phagocytes is obviously a prerequisite for activity against intracellular organisms, but we recently demonstrated a discrepancy between uptake of certain antibiotics by human neutrophilic polymorphonuclear leukocytes (PMN) and the subsequent effect of these drugs on intraphagocytic bactericidal activity (18). Thus, clindamycin and erythromycin, which were markedly concentrated by PMN, exhibited poor activity against intraphagocytic Staphylococcus aureus, even though the intracellular drug levels exceeded the MBCs for this organism (18). The possibility that certain antibiotics might adversely influence phagocyte antibacterial function was considered. In the case of cindamycin, a potential modulatory role has been identified. We have shown that clindamycin enters PMN and macrophages by means of the cell membrane nucleoside (adenosine) transport system (16, 36). Since adenosine (by binding external cell membrane nucleoside receptors) regula...

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Dirithromycin, a new macrolide antibiotic, achieves prolonged, high levels in tissue. We previously demonstrated that certain macrolides are highly concentrated within phagocytic cells. This background information prompted us to evaluate the interactions of dirithromycin and human polymorphonuclear leukocytes (PMNs). After incubation with radiolabeled dirithromycin, antibiotic uptake by PMNs was determined by a velocity-gradient centrifugation technique and was expressed as the ratio of the cellular to the extracellular drug concentration (C/E). Dirithromycin was avidly accumulated by PMNs (C/E, 5 at 15 min, 10 at 30 min, 19 at 1 h, and 35 at 2 h). Uptake was dependent on cell viability, physiologic environmental temperature, and pH (optimum 8.6), but was not influenced by potential competitive inhibitors of membrane transport. Incubation with sodium cyanide caused an increase in dirithromycin accumulation by PMNs. Ingestion of microbial particles (mimicking in vivo infection) modestly inhibited the entry of dirithromycin into PMNs. After removal of extracellular drug, the efflux (release) of dirithromycin from PMNs was slow; only 10% was released within the first 30 min. This prolonged retention of dirithromycin within phagocytic cells might allow delivery and release of accumulated drug at sites of infection. The impact of intraphagocytic dirithromycin on cellular function was also evaluated. In a manner similar to that of other highly concentrated, weakly basic antibiotics, dirithromycin inhibited the respiratory burst response (superoxide production) in stimulated PMNs. The presence of dirithromycin slightly increased the intraphagocytic killing of Staphylococcus aureus in human PMNs. These interactions of dirithromycin with phagocytic cells may promote the extraphagocytic, and possibly the intraphagocytic, killing of infecting organisms.