Mechanism of action for MP. (Top left) Basic structure of MP. The tumor antigen-binding domain (Fv) targets CD22, which is overexpressed in HCL. Once MP binds to CD22, the complex is internalized into endosomes. In the endosomes, the disulfide bond reduction of catalytic domain III (C3) releases the active agent of PE38. This fragment eventually is processed through the endoplasmic reticulum and then translocates to the cytosol. This fragment then causes ADP ribosylation of elongation factor 2 (EL2), which inhibits protein synthesis leading to apoptosis of the cell.¹⁷  V_H, heavy-chain variable region; V_L, light-chain variable region.