Mechanism of azithromycin uptake in human polymorphonuclear leucocytes

The new antibiotic azithromycin (CP-62.993) is enriched in human polymorphonuclear leucocytes by up to 300-fold the extra-cellular concentrations. To approach an understanding of the underlying mechanism of this unique behavior of azithromycin, some characteristics of the uptake process were investigated in vitro. The speed of the uptake in human polymorphonuclear leucocytes was found to be independent of the extracellular starting concentrations of azithromycin. There was no indication of saturation up to extracellular concentrations of 100 micrograms/ml. The uptake was largely determined by incubation temperatures. At 4 degrees C no penetration into the cells could be observed. The activation energy of azithromycin uptake came to 144 kJ mol-1, twice the value of erythromycin uptake. The presence of various inhibitors of cell metabolism did not change the intracellular accumulation compared to control, nor did the presence of some agents which interfere with certain transport channels. These findings suggest that passive diffusion is an essential mechanism of azithromycin transport through the phagocyte membrane, while active transport is less important. The high cellular enrichment of azithromycin and the relatively high activation energy of the uptake process could be explained by accumulation of the drug in phagocytic lysosomes and this would also be in keeping with pH partition considerations.