Melphalan flufenamide ethyl ester (melflufen, previously denoted J1) is a peptidase-potentiated dipeptide prodrug of alkylating agent melphalan. Melflufen readily penetrates membranes and an equilibrium is rapidly achieved, followed by enzymatic cleavage in aminopeptidase positive cells, which results in trapping of less lipophilic metabolites. This targeting effect results in very high intracellular concentrations of its metabolite melphalan and subsequent apoptotic cell death. This results in a potency increase (melflufen vs melphalan) ranging from 10- to several 100-fold in different in vitro models. Melflufen triggers a rapid, robust, and an irreversible DNA damage which may account for its ability to overcome melphalan-resistance in multiple myeloma cells. Melflufen has been granted orphan drug designation for the treatment of multiple myeloma.