Pharmaceutical Manufacturing Handbook: Production and

458 LIPOSOMES AND DRUG DELIVERY
connected with a water bath. This is repeated many times until the size of the liposomes
is signifi cantly reduced. They can operate with volumes from 0.1 up to 10 L
of liposomes and with concentrations up to 300 m M , which is by far the highest
capacity from all other methods. Small unilamellar vesicles less than 100 nm can be
produced, but the population is not completely homogeneous.
Reverse - Phase Evaporation The REV method was developed by Szoka and Papahadjopoulos
[132] . Lipids are dissolved in organic solvent and the solvent is removed
with evaporation. The thin fi lm is resuspended in diethyl ether (1 mL solvent/mL
liposomes) followed by the addition of one - third of water and sonication in a bath
sonicator for 1 min. This water - in - oil (w/o) - emulsion is evaporated until a dry gel is
formed, and fi naly the gel is broken by agitation and water addition. Sometimes this
step is quite diffi cult. The remnants of the organic solvent are removed by evacuation
and the resulting dispersion is REV liposomes.
Dehydrated – Rehydrated Vesicles DRV liposomes were developed by Kirby and
Gregoriadis in 1984 [133] and are capable of encapsulating high amounts of aqueous
soluble molecules under mild conditions (conditions that do not cause decomposition
or loss of activity). The high entrapment ability of this type of liposomes is due
to the fact that preformed, “ empty ” SUVs are disrupted during a freeze - drying step
in the presence of the solute destined for entrapment. Subsequently, during controlled
rehydration that is carried out in the presence of concentrated solution of
the solute (to be encapsulated), the vesicles fuse into large oligolamellar vesicles
entrapping high amounts of solute. The produced liposomes are multilamellar and
their size is between 200 and 400 nm up to a few micrometers. Recently, with the
addition of certain amounts of sucrose, DRV liposomes with diameter between 90
and 200 nm were obtained entrapping considerable proportions (up to 87) of the
solute [134] .
Giant Vesicles Large or giant liposomes have been developed by Reeves and
Dowben [135] . Briefl y, lipids are dissolved in chloroform/methanol 2 : 1 and dispersed
on a piece of glass. Water is added for their rehydration; however, their
population is quite heterogeneous. Other types of particle - encapsulating giant liposomes
[136] can be prepared by applying a double - emulsion technique followed by
a freeze – drying step.
Detergent Depletion With this method phospholipid – detergent mixed micelles are
initially produced, and during controlled - rate detergent removal, liposomes are
formed. The rate and method of detergent removal determine the size and size
homogeneity of the liposomes produced. Gel fi ltration and dialysis are the most
popular approaches [137, 138] . Although liposomes are produced under mild conditions
(low temperature and low shear mechanical forces applied), this method
suffers from low encapsulation effi ciency of hydrophilic drugs.
Large Unilamellar Vesicles from Cochleates Large unilamellar vesicles can be produced
with the “ cochleate ” approach [139] . Small unilamellar vesicles consisted
from phosphatidylserine adopt a cochleate shape after addition of calcium. Addition
of EDTA creates complexes with calcium, turning the cochleates to LUVs.