The major disadvantage of clinically established chemotherapeutic agents is their lack of selectivity for tumor cells. Therefore, for a pronounced antitumor effect, high doses of the chemotherapeutic drugs are needed, which often cause systemic toxicity and severe side effects. Some chemotherapeutic drugs do not exert their antitumor effects because they have hampered transport through the plasma membrane, but once inside the cell, they can be very potent. To overcome membrane restriction, much effort has been put into the development of drug delivery systems. These systems are aimed at facilitating the delivery of a chemotherapeutic drug into tumor cells at concentrations that are sufficient for effective cell killing and, at the same time, to minimize the drug concentration in normal tissue.