Lipid-Based Pharmaceutical Nanocarriers for Imaging Applications - Nanotechnology for Biomedical Imaging and Diagnostics - page 57

nanocarriers were prepared by ethanol evaporation method with encapsulation
efficiency up to 40% [55-57].
3.2.6 Key challenges in using Liposomes and micelles as carriers
for imaging agents
To facilitate the accumulation of contrast in the required zone, various nanovesi-
cles, such as liposomes and micelles, draw a special attention because of their
easily controlled properties and good pharmacological characteristics. pursuing
different in vivo delivery purposes, one can easily change the size, charge, and sur-
face properties of these nanocarriers simply by adding new ingredients to the lipid
or amphiphilic mixture before liposome or micelle preparation and/or by variation
of preparation methods.
The use of lipidic nanovesicles for the delivery of imaging agents has quite a long
history [14, 58]. The ability of these nanocarriers to entrap different substances into
both the aqueous phase and the nanocarrier membrane compartment made them suit-
able for carrying the diagnostic moieties used with all imaging modalities: gamma-
scintigraphy, mri, CT imaging, and even sonography. The different chemical nature
of reporter moieties used in different modalities requires different protocols to load
nanocarriers with the given contrast agent. All the imaging modalities, as listed in
Table 3.1, not only differ in their sensitivity and resolution but also require different
amounts of a diagnostic label to be delivered into the area of interest. These general
considerations, taken together, led to the development of the whole family of lipid-
based contrast agents for various purposes. since the micelle-based delivery of con-
trast agents is a relatively new approach [18, 43], most of the analysis of current
trends in carrier-mediated transport of imaging agents will, predominantly, involve
the data obtained with the use of liposomal nanocarriers.
phospholipid nanocarriers, if introduced into the circulation, are very rapidly
(usual half-clearance time is within 30 min) sequestered by the cells of rEs. Liver
cells are primarily responsible [59], and the sequestration is almost independent on
their size, charge, and composition of the nanovesicles. from the pharmacokinetic
point of view, it is important that circulating peripheral blood monocytes can also
endocytose nanovesicles and later infiltrate tissues and deliver endocytosed nanoves-
icles to certain pathological areas in the body [44]. The decrease in rEs uptake can
be achieved by decreasing nanovesicle size, increasing nanovesicle dose, presaturat-
ing rEs with “empty” nanolipidic vesicles or other particles, or modifying nanoves-
icle surface with certain “protective” polymers. To increase nanovesicle accumulation
in the “required” areas, the use of targeted nanocarriers has been suggested [60].
Nanocarriers with a specific affinity for an affected organ or tissue were believed to
increase the efficacy of liposomal pharmaceutical agents, including the imaging
ones. immunoglobulins, primarily of the igg class, are the most promising and
widely used targeting moieties for various drugs and drug carriers including nanoli-
pidic vesicles.
However, pharmacokinetic properties of immunoliposomes (as well as immu-
nomicelles, in a similar fusion) in many cases were far from desirable. despite