Traditional chemotherapeutic drugs are often restricted by severe side effects and lack of tumor specificity. Peptide prodrugs cleavable by peptidases present in the tumor environment have been explored to improve the therapeutic index of cytotoxic drugs. One such prodrug of doxorubicin (Dox), CPI-0004Na [N-succinyl-beta-alanyl-L-leucyl-L-alanyl-L-leucyl-Dox (sALAL-Dox)] has been shown to have an improved antitumor efficacy profile with reduced toxicity compared with Dox in tumor xenograft models (V. Dubois et al., Cancer Res., 62: 2327-2331, 2002). In this study, we demonstrate that CD10, a cell surface metalloprotease expressed on a variety of tumor cell types, is capable of cleaving CPI-0004Na and related peptide prodrugs such as N-succinyl-beta-alanyl-L-isoleucyl-L-alanyl-L-leucyl-Dox (sAIAL-Dox). This proteolytic cleavage generates leucyl-Dox, which is capable of entering cells and generating intracellular Dox. In a [(3)H]thymidine proliferation assay, analogues of CPI-0004Na showed a 100-300-fold increase in potency on CD10(+) cells compared with CD10(-) cells. Cytotoxicity of CPI-0004Na was inhibited by phosphoramidon, a known inhibitor of CD10 enzymatic activity. Furthermore, Chinese hamster ovary CHO-S cells, which are resistant to CPI-0004Na, could be sensitized to the cytotoxic effect of the prodrug by transfection of a CD10 cDNA. Tumor xenograft studies using LNCaP prostate tumor cells support the important role of CD10 in the antitumor efficacy of these prodrugs against tumors expressing CD10. CPI-0004Na and sAIAL-Dox achieved statistically significant 70% tumor growth inhibition at day 22. CD10 is expressed on many types of human tumors including B-cell lymphoma, leukemia, and prostate, breast, colorectal, and lung carcinomas; therefore, CD10-cleavable prodrugs may be effective in a range of different tumor types.