Azithromycin was shown to achieve high concentrations in human skin fibroblasts. IntraceUular penetration occurred rapidly (10 ig/mg of celular protein after 3 h) and then increased progressively over a 3-day period; azithromycin accumulated up to 21 times more than erythromycin (61.1 versus 2.9 ,ig/mg of protein). Uptake was dependent on the extracellular concentration, was inhibited at 4°C, did not occur in nonviable cells, and was reduced by a low pH. Intracellular accumulation was not affected by the metabolic inhibitor 2,4-dinitrophenol or sodium fluoride or by the nucleoside transport inhibitor 2-chloroadenosine. Once concentrated in cells, azithromycin remained intracellular and was released slowly in the absence of extracelular drug, compared with erythromycin (17 versus 78% released after 1 h). After 48 h of incubation in drug-free medium, 27% of the initial amount of azithromycin remained cell associated. The release of azithromycin was not affected by various monokines reported to stimulate fibroblasts (interleukin-1 or tumor necrosis factor) or by exposure to bacteria. Incubation of azithromycin-loaded fibroblasts with human polymorphonuclear leukocytes resulted in a higher intraceUlular accumulation of azithromycin in polymorphonuclear leukocytes than in cells incubated with free nonintracelular azithromycin for the same time (8.3 versus 2.2 gug/ml after 2 h), suggesting a more efficient or rapid uptake through ceUl-to-cell interaction. The widespread distribution of fibroblasts in tissues suggests a potential for these ceUls, and possibly other lysosome-containing tissue cells, to serve as a reservoir for azithromycin, slowly releasing it for activity against extracellular organisms at sites of infection and passing it to phagocytes for activity against intracellular pathogens and potential transport to sites of infection.

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