Pharmaceutical Manufacturing Handbook: Production and

448 LIPOSOMES AND DRUG DELIVERY
and dicetyl phosphate (DCP), and dioctadecyldimethylammonium bromide
(DODAB).
The last group of amphiphiles contains sterols that are present in the membranes
of cells. The most popular among them is cholesterol (Chol), which can be easily
incorporated in lipid bilayers, increasing their rigidity and making them less permeable,
due to the interactions taking place with phospholipids in lipid membranes
which result in modifi cation of the lipid acyl - chain conformation.
Recently polyethyleneglycol (PEG, of varying molecular weight) – lipid conjugates
have become commercially available and are frequently used in liposome
applications. Aditionally, functionalized phospholipids exist for the covalent or noncovalent
attachment of proteins, peptides, or drugs to the liposome surface. Most of
these lipids fall into three major classes of functionality: Conjugation through amide
bond formation, disulfi de or thioether formation, or biotin/streptavidin binding.
Active lipids — mostly with anticancer activity — have also been added in liposome
membrane for production of active liposomes. Examples of such lipids are
ether lipids [18] and arsonolipids [19] .
Several techniques are employed for the physicochemical characterization of
lipid membranes, as summarized in Table 2 . Thermal analysis, mainly differential
scanning calorimetry (DSC), has been used extensively, offering information on the
thermodynamics of various types of liposomes. The phase behavior of lipid components
of membranes determines membrane fl uidity. Each lipid has a characteristic
lipid chain transition temperature, T m . Changes in the structure of lipids occur below
and above this temperature [20] . The temperature at which these changes occur
depends on the head group, the chain length, and the degree and type of unsaturation
of each lipid [21] . Using DSC studies in has been demonstrated that heat capacity
curves are affected by the size of vesicles [20, 22] , and can be modifi ed by
introduction of drugs [23, 24] or peptides [25, 26] in the lipid membranes (due to
interactions between incorporated molecules and lipids).
TABLE 2 Methods for Physicochemical Characterization of Lipid Membranes
Method
Information
References
Thermal analysis, mainly
Membrane fl uidity
20 – 26
Differential scanning calorimetry
(DSC)
Lipid chain transition temperature, T m
Fluorescence spectroscopy
Phase transitions
27, 28
Membrane dynamics
29, 30
Nuclear magnetic resonance (NMR) Polymorphism
31, 32
Lamellarity
33
Membrane dynamics
34, 35
Electron paramagnetic resonance Fluidity of membranes 36
(EPR)
Liposomal internal volume
37
Membrane dynamics 38
Membrane – drug interactions
39
Fluorescence quenching
Fusion processes
40, 41
X - ray diffraction Structural information; thickness of
the membrane and water layers
42 – 45