In vitro activity:Retapamulin is a potent inhibitor of protein synthesis with an IC50 of 0.33 μM in lysates prepared from erythromycin-susceptible E. coli cells. Retapamulin (100 μM) is ineffective in inhibiting eukaryotic translation when tested in a rabbit reticulocyte lysate system with the cellular components necessary for mammalian protein synthesis. Retapamulin binds to Erys ribosomes and fully displaces the labeled ligand with an IC50 of 26.1 nM. Retapamulin partially inhibits the ability of charged, N-blocked tRNA to bind to the P-site of E. coli ribosomes, with an IC50 of 17.4 nM (maximum inhibition of 80%). Retapamulin inhibits Staphylococcus aureus and Streptococcus pyogenes with MIC90 of 0.12 μg/mL and ≤0.03 μg/mL, respectively. Retapamulin inhibits S. aureus subset with MIC50/90 values of 0.06/0.12 μg/mL. Retapamulin shows excellent activity against these isolates, with only two requiring a MIC of 0.06 μg/mL. Retapamulin is very active against the S. pyogenes isolates tested with MIC90 of 0.016 μg/mL, and based on MIC90s, is 32- and>1,024-fold more active than mupirocin and fusidic acid, respectively. Retapamulin binds to a unique site on the bacterial ribosome, and by virtue of its novel mode of action. Retapamulin (<2 mg/L) inhibits 37/52 (71%) strains of the B. fragilis group and 85/87 (98%) of the other Gram-negative bacilli. Retapamulin is more active than clindamycin, metronidazole and ceftriaxone against Propionibacterium acnes and anaerobic Gram-positive cocci. Retapamulin inhibits total viable cells (TVC), Protein synthesis and 50S subunit synthesis in both wild-type (wt) Staphylococcus aureus strain RN1786 with IC50 of 12 ng/mL, 5 ng/mL and 27 ng/mL, respectively.