LOCALIZATION OF AZITHROMYCIN IN TOXOPLASMA GONDII-INFECTED CELLS

Agents effective against intracellular pathogens must enter infected c ells, crossing vacuolar membranes surrounding the organisms and then p enetrating into the microbe and localizing to the microbial target sit e. We have characterized these parameters for azithromycin entry into Toxoplasma gondii-infected Chinese hamster ovary cells and murine macr ophage-like J774 cells. Azithromycin uptake into infected host cells w as concentrative and was dependent upon proton gradients. Subcellular fractionation of azithromycin-loaded infected CHO cells demonstrated > 95% intracellular drug in host cell lysosomes and cytosol, with <5% as sociated with the parasite. Uptake of azithromycin into the T. gondii vacuole increased if parasites were coated with antibody prior to inte rnalization by murine J774 cells, conditions which result in the forma tion of acidified phagolysosomes. No redistribution or retention of az ithromycin in the parasite was observed when drug efflux from antibiot ic-loaded infected CHO cells was monitored. Azithromycin entry into ex tracellular T. gondii was concentrative, was temperature and pH depend ent, and was not different when azithromycin-sensitive and -resistant parasites were compared. These results demonstrate that azithromycin c oncentrates primarily in acidified compartments in parasites and host cells. The high concentration of azithromycin within these compartment s may not be biologically relevant to inhibition of intracellular para site growth by this agent.