The spiramycin paradox

Spiramycin has been found to be effective in a variety of clinical and experimental infections despite modest in-vitro activity. In animal models of infection, spiramycin has been found to be as effective as or more effective than erythromycin despite inferior in-vitro activity. These paradoxical results are explained in part by spiramycin's ability to achieve intra-cellular and tissue concentrations that exceed serum concentrations by a factor of ten or more. Furthermore, spiramycin clearance from these sites is much lower resulting in sustained tissue and intracellular concentrations. Finally, spiramycin appears to produce a substantial post-antibiotic effect and, possibly, subinhibitory effects that may further enhance its in-vivo activity.