ABSTRACT
The main objective of using liposomes as drug carriers is to achieve selective, and sufficiently high, localization of ‘‘active’’ drug at disease sites such as tumors and inflamed tissues. In addition, in order to achieve therapeutic efficacy, the liposomal encapsulated/associated drug should become available to the target cells. In this respect, the liposome differs from other controlled release systems, in which drug release occurs either in plasma or at the site of administration. Selective localization can be obtained using either passive or active targeting. Passive targeting is a process by which the physical properties of the liposomes combined with the microanatomy of the vasculature at the target tissue determine drug selective localization. Active targeting requires, in addition to the ability to reach the disease site by passive targeting, a homing device (antibody, receptor ligand, etc.) as part of the liposome surface so that the liposomes can recognize the ‘‘sick’’ cells, bind to them
selectively, and either be internalized by these cells or be broken down by either enzymatic hydrolysis or processes such as ultrasonic irradiation to release the drug near the cell surface so it will be taken up by the target cells (1).
