The present invention provides methods of specifically targeting compounds to cells overexpressing matrix metalloproteinases, plasminogen activators, or plasminogen activator receptors, by administering a compound and a mutant protective antigen protein comprising a matrix metalloproteinase or a plasminogen activator-recognized cleavage site in place of the native protective antigen furin-recognized cleavage site, wherein the mutant protective antigen is cleaved by a matrix metalloproteinase or a plasminogen activator overexpressed by the cell, thereby translocating into the cell a compound comprising a lethal factor polypeptide comprising a protective antigen binding site.