Intracellular accumulation, subcellular distribution, and efflux of tilmicosin in bovine mammary, blood, and lung cells.

Tilmicosin is a semisynthetic macrolide antibiotic currently approved for veterinary use in cattle and swine to combat respiratory disease. Because the concentrations of tilmicosin are generally low in bovine serum, the interaction of tilmicosin with three types of bovine phagocytes (monocyte-macrophages, macrophages, and neutrophils from blood, lungs, and mammary gland, respectively) and mammary gland epithelial cells was evaluated to provide an understanding of potential clinical efficacy. After incubation with radiolabeled tilmicosin, uptake was determined and expressed as the ratio of the intracellular to the extracellular drug concentration. Accumulation of tilmicosin at 4 h of incubation by the alveolar macrophages (Cc/Ce 193) was 4 to 13 times more than that observed in monocyte-macrophages (Cc/Ce 43), neutrophils, (Cc/Ce 13), or mammary epithelial cells (Cc/Ce 20). Subcellular distribution showed that 70 to 80% of tilmicosin was localized in the lysosomes. Uptake in mammary gland cells was dependent on cell viability, temperature, and pH, but was not influenced by metabolic inhibitors or anaerobiosis. However, lipopolysaccharide exposure increased tilmicosin uptake by the bovine mammary macrophages and epithelial cells. When neutrophils and epithelial cells were incubated in the presence of tilmicosin and extracellular tilmicosin was then removed, 40% of the intracellular tilmicosin remained cell associated after 4 h of incubation (i.e., 60% effluxed), but only 25% remained in macrophages. These in vitro interactions of tilmicosin with bovine phagocytes and epithelial cells suggest an integral role in effecting clinical efficacy.