Retapamulin is a semisynthetic pleuromutilin derivative being developed as a 
topical antibiotic for treating bacterial infections of the skin. It is potent 
in vitro against susceptible and multidrug-resistant organisms commonly 
associated with bacterial skin infections. We report detailed mode of action 
studies demonstrating that retapamulin binds to the bacterial ribosome with high 
affinity, inhibits ribosomal peptidyl transferase activity, and partially 
inhibits the binding of the initiator tRNA substrate to the ribosomal P-site. 
Taken together, these data distinguish the mode of action of retapamulin from 
that of other classes of antibiotics. This unique mode of action may explain the 
lack of clinically relevant, target-specific cross-resistance of retapamulin 
with antibacterials in current use.