The invention relates to improvements in the field of drug delivery. More particularly, the invention relates to polypeptides having a hydrolyzable covalent bond to a therapeutic agent that includes, etoposide, etoposide 4&prime；-dimethylglycine or doxorubicin. These polypeptide conjugates can be used as vectors to transport the podophyllotoxin derivative across the blood brain barrier (BBB) or into particular cell types such as ovary, liver, lung, or kidney. The invention also relates to pharmaceutical compositions that include the compounds of the invention and to uses thereof in methods of treatment.
