Retapamulin is a semi-synthetic pleuromutilin prepared by reacting pleuromutilin tosylate with tropine-3-thiol to give a more hydrophobic analogue with a tertiary amine. This enables formulation as a stable hydrochloride salt. Retapamulin is a broad spectrum antibiotic with no cross resistance to existing antibiotic classes, and is the first pleuromutilin approved for human use. Like all the pleuromutilins, retapamulin inhibits protein synthesis by binding to domain V of 23S rRNA.