Novel monomethylauristatin E (MMAE) prodrug 8 was designed and prepared that bound cell surface glycoprotein integrin αvβ3, and was activated using legumain protease as a catalyst. Upon activation, prodrug 8 strongly induced the death of MDA-MB-435 cells that express integrin αvβ3 on cell surface. Efficacies of prodrug 8 were also determined in vivo using animal models of 4T1 murine breast cancer, D121 Lewis lung carcinoma, and MDA-MB-435 human breast cancer. The results demonstrated that prodrug 8 decreased tumor growth and metastasis effectively. In comparison to the parent cytotoxin, MMAE, and prodrug 3, prodrug 8 was less toxic to mouse white blood cells. The latter caused no loss in weight gain of mice at a dose 3 mg/kg, which is over 30 times in excess to MMAE (0.1 mg/kg). We hypothesize that overexpression and colocalization of integrin αvβ3 and legumain protease on tumor cells, tumor vasculature, and/or tumor microenvironments can be exploited to enhance the efficacy and selectivity of potent cytotoxins, such as MMAE, which is otherwise too toxic to use for therapy.