INTRACELLULAR ACCUMULATION, SUBCELLULAR-DISTRIBUTION, AND EFFLUX OF TILMICOSIN IN CHICKEN PHAGOCYTES

Tilmicosin is a semi-synthetic macrolide antibiotic, currently approve d for veterinary use in cattle and swine respiratory disease, and is i n development for use in poultry mycoplasma air sacculitis. In order t o provide an understanding of clinical efficacy, the in vitro interact ion of tilmicosin with three types of chicken phagocytes (MQ-NCSU macr ophages, monocytemacrophages, and heterophils) was evaluated. After in cubation with radiolabeled tilmicosin,uptake was determined and expres sed as the ratio of the-cellular (Cc) to the extracellular (Ce) drug c oncentration (Cc:Ce). Tilmicosin was avidly accumulated by heterophils (Cc: Ce 138 at 4 h incubation vs 32 and 66, respectively, in MQ-NCSU and monocyte-macrophages) with 61 to 88% localized in the lysosomes. U ptake was dependent on cell viability, temperature, and pH, but was no t influenced by metabolic inhibitors. However,phagocytosis of Pasteure lla multocida and, lipopolysaccharide exposure increased tilmicosin up take by the chicken phagocytes. Upon removal of extracellular tilmicos in, 50% of the intracellular tilmicosin was effluxed within the first 30 min,but after 4 h of incubation in antibiotic-free medium, 30% rema ined cell-associated Opsonized P. multocida significantly enhanced the release of tilmicosin from all three types of chicken phagocytes. Til micosin uptake was observed to increase lysosomal enzyme (acid phospha tase, lysozyme, avidin, and. beta-glucuronidase) production. Finally,n eutrophils were shown to transport and efflux bioactive tilmicosin in a test system measuring both neutrophil chemotaxis under agarose and a bioassay measuring inhibition of bacterial growth in the presence of antibiotic in agar. These in vitro observations of cellular pharmacolo gy suggest a complex interaction between phagocytes and tilmicosin tha t contribute to clinical efficacy.