28-07-2012, 03:43 PM
Current Trends in Liposome Research
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Abstract
Among the several drug delivery systems, liposomes – phospholipid nanosized vesicles with a bilayered
membrane structure – have drawn a lot of interest as advanced and versatile pharmaceutical carriers for
both low and high molecular weight pharmaceuticals. At present, liposomal formulations span multiple
areas, from clinical application of the liposomal drugs to the development of various multifunctional
liposomal systems to be used in therapy and diagnostics. This chapter provides a brief overview of various
liposomal products currently under development at experimental and preclinical level.
Introduction
The clinical utility of most conventional therapies is limited either
by the inability to deliver therapeutic drug concentrations to the
target tissues or by severe and harmful toxic effects on normal
organs and tissues. Different approaches have been attempted to
overcome these problems by providing “selective” delivery of
drugs to the affected area using various pharmaceutical carriers.
Among the different types of particulate carriers, liposomes have
received the most attention. For more than three decades, liposomes –
artificial phospholipid vesicles – obtained by various methods
from lipid dispersions in water and capable of encapsulating the
active drug, have been recognized as the pharmaceutical carrier
of choice for numerous practical applications (1–3). From the
biomedical point of view, liposomes are biocompatible, cause very
little or no antigenic, pyrogenic, allergic and toxic reactions.
Long-circulating Liposomes
One of the drawbacks of the use of liposomes was the fast
elimination from the blood and capture of the liposomal preparations
by the cells of the RES, primarily, in the liver. To increase
liposomal drug accumulation in the desired areas, the use of
targeted liposomes with surface-attached ligands capable of
recognizing and binding to cells of interest, and potential
induction of the liposimal internalization has been suggested
(see Fig. 1). Targeted liposomes offer various advantages over
individual drugs targeted by means of polymers or antibodies.
One of the most compelling advantages is the dramatic increase
in drug payload that can be delivered to the target. Furthermore,
the number of ligand molecules exposed on the liposome surface
can be increased, improving ligand avidity and degree of the
uptake. Immunoliposomes also help provide a “bystander kill”
effect, because the drug molecules can diffuse into adjoining
tumor cells. Immunoglobulins of the IgG class, and their fragments
are the most widely used targeting moieties for liposomes
(termed “immunoliposomes” after the modification).