This document provides an overview of ocular liposomes. It discusses the structural components of liposomes including phospholipids, sterols, and sphingolipids. The advantages of liposomes for ophthalmic drug delivery include enhancing permeation and residence time on the corneal surface. Various types of liposomes are described based on structural parameters, composition, and preparation method. Common preparation techniques include conventional method, sonication, extrusion, solubilization, and reverse phase evaporation. Mechanisms of permeation through the ocular surface include adsorption, endocytosis, fusion, and lipid exchange. Liposomes show potential for improving ophthalmic drug pharmacokinetics and reducing toxicity.
22. Reverse phase evaporation technique
It consists of a rapid injection of aqueous solution of the drug into an organic solvent,
which contains the lipid dissolved with simultaneous bath sonication of the mixture
leading to the formation of water droplets in the organic solvent (i.e., a “water-in-oil”
emulsion). The resulting emulsion is dried down to a semi solid gel in a rotary evaporator.
The next step is to subject the gel to vigorous mechanical agitation to induce a phase
reversal from water-in-oil to oil-in-water dispersion (i.e., an aqueous suspension of the
vesicles). During the agitation, some of the water droplets collapse to form the external
phase while the remaining portion forms the entrapped aqueous volume. Large unilamellar
vesicles (diameter 0.1–1μm) are formed in the process. This method has been used to
encapsulate both small and macromolecules such as RNA and various enzymes without
loss of activity. The expected limitation of this method is the exposure of the material to
be encapsulated to organic solvents and mechanical agitation, which can lead to the
denaturation of some proteins or breakage of DNA strands.
29. EVALUATION OF LIPOSOMES
Vesicle shape and lamellarity:
Vesicle shape can be assessed using Electron Microscopic
Techniques. Lamellarity of vesicles i.e. number of bilayers presents in liposomes is
determined using Freeze-Fracture Electron Microscopy and Nuclear Magnetic Resonance
Analysis
Optical Microscopy:
The microscopic method includes use of Bright-Field, Phase Contrast
Microscope and Fluorescent Microscope and is useful in evaluating vesicle size of large
vesicle.
Negative Stain TEM :
Electron Microscopic Techniques used to assess liposome shape and size are mainly
negative-stain TEM and Scanning Electron Microscopy. The latter technique is less
preferred. Negative Stain Electron Microscopy visualizes bright areas against dark
background (hence termed as negative stain) The negative stains used in TEM analysis are
ammonium molybdate or Phosphotungstic acid (PTA) or uranyl acetate. Both PTA and
ammonium molybdate are anionic in nature while uranyl acetate are cationic in nature