Liposomes as targetable drug carriers.

The general problem of targeted drug transport is critically reviewed and three principle components of targeted systems are discussed: the target, the vector molecule, and the carrier. Different systems of drug targeting are briefly described: local drug application, chemical modification of the drug molecule, physical targeting under the action of pH, temperature, or magnetic field. The idea of a vector molecule is discussed and different methods of vector molecule coupling with the drug are reviewed (direct coupling, coupling via spacer group or polymer molecule, etc.). It is shown that the most promising approach seems to be the use of a drug-containing microcontainer with the vector molecule immobilized on its outer surface. Different types of microcontainers are briefly described: microcapsules, cell hosts, and liposomes. The advantages of liposomes as drug containers are shown and the main problems of their use for drug targeting in vitro and in vivo conditions are discussed. One of the most important problems is the problem of vector molecule immobilization on liposome surfaces. The principle four different immobilization methods: adsorbtion, incorporation, covalent binding, and hydrophobic binding. Targeted liposome transport is described in model systems, cell cultures, and experimental animals. It is shown that targeted liposomes may release a drug via diffusion, lysis, or endocytosis by appropriate cells. The problems of targeted liposome technology and clinical application are analyzed.