Comparison of azithromycin, roxithromycin, and cephalexin penetration kinetics in early and mature abscesses.

During the process of abscess formation, a myriad of changes are observed histologically that impede the penetration of antimicrobial agents into infection loci. A Staphylococcus aureus foreign body abscess, developed in rats, was employed to evaluate the penetration kinetics of azithromycin, roxithromycin and cephalexin at various stages of abscess development; the progressive patho-histological changes of abscess formation were also characterized in this model. In an early abscess (18 h post-challenge), azithromycin penetration into inflammatory fluid was enhanced (AUC of 351 vs 130 mg.h/kg) and residence prolonged relative to an inflammation control (half-life of 88 vs 27 h). In contrast, roxithromycin and cephalexin penetration into, and residence in, inflammatory fluid were unaltered in the early abscess. However, penetration into, and egress from, a mature abscess (ten days post-challenge) were impeded for all three antimicrobials (P < or = 0.03). The penetration kinetics of azithromycin into inflammatory fluid in an early abscess were independent of the dose regimen, but dependent on the total dose. The persistently high concentrations of azithromycin in inflammatory fluid within abscess were associated with the infiltration of phagocytic cells and encapsulation by fibrous tissue. These data are consistent with a phagocytic delivery mechanism for azithromycin, whereby the presence of high concentrations of azithromycin in inflammatory fluid are a consequence of augmented drug distribution via the release of accumulated intracellular drug from the infiltrating phagocytic cells and fibroblasts associated with abscess formation.