We present a new antibody-directed enzyme prodrug therapy strategy (ADEPT) based on a post-proline cleaving endopeptidase and prodrugs, in which cytotoxic moieties are linked to a proline-contg. peptide. Human prolyl endopeptidase was expressed in Escherichia coli and purified to homogeneity. The enzyme was active in buffer and in human serum but was rapidly thermally inactivated by incubation at 37 DegC, thus preventing applications in vivo. While prolyl endopeptidase display on filamentous phage abolished viral infectivity and prevented directed evolution strategies based on phage display, we robotically screened 10752 individual colonies of mutant enzymes using a fluorogenic assay to improve enzyme stability. A single amino acid mutation (Glu289 -> Gly) improved protein stability, resulting in a half-life of 16 h at 37 DegC in phosphate buffer. Two prodrugs were synthesized, in which an N-protected glycine-proline dipeptide was covalently coupled to doxorubicin and melphalan. (Benzyloxycarbonyl)glycylprolylmelphalan, but not the more sterically hindered doxorubicin prodrug, could be efficiently activated by prolyl endopeptidase [specific activity = 813.3 nmol min-1 (mg of enzyme)-1 at 25 DegC]. The melphalan prodrug was essentially nontoxic to CHO, F9 teratocarcinoma, MCF7 breast adenocarcinoma, and p3U1 mouse myeloma cells up to millimolar concns., while prodrug incubation with the engineered prolyl endopeptidase mutant led to a cell killing profile superimposable to the one of melphalan. The prolyl endopeptidase mutant was then chem. coupled to the human antibody L19, specific to the EDB domain of fibronectin, a marker of angiogenesis. The resulting immunoconjugate retains antigen binding and enzymic activity, thus opening the way to anticancer ADEPT applications. [on SciFinder (R)]