The future of liposomal drug delivery

Liposomes are microvesicles composed of continuous bilayers of phospholipid surrounding an aqueous phase. They have been extensively used to deliver antibiotics, antifungal agents, and antiviral compounds for the treatment of animal model infections. Three rationales have been invoked to justify their use as carriers of anti-infectious agents: (i) passive targeting of compounds to the mononuclear phagocytic system for enhanced effects at this site; (ii) targeted drug delivery; and (iii) site-avoidance delivery, wherein the rate of transfer of drug to the toxic site(s) is reduced but not the drug transfer rate to the active site. All strategies result in an increase in the therapeutic index of the drug; however, to obtain optimal drug therapy the characteristics for a particular drug for site-specific delivery differ from those of site-avoidance delivery. Future developments that will permit a larger variety of agents to be delivered in liposomes include the development of fusogenic liposomes and of liposomes that avoid the reticuloendothelial system.