Design and Development of Theranostic Nanomedicines - Nanotechnology for Biomedical Imaging and Diagnostics - page 430

15.3 colloiDal systems
15.3.1 liposomes
liposomes for drug delivery have more than 50-year-long history of preclinical and
clinical use. They represent, from a design point of view, the groundwork behind
many nanomedicine systems we know today [71]. one of the first reports of targeted
anticancer drug delivery was using liposomes [72]. Therefore, it is also easy to see
them as the pioneering nanosystems for imaging and theranostic. By definition,
liposomes are colloidal, thermodynamically stabilized systems comprised of a
phospholipid bilayer. Based on their structure, liposomes are classified into several
categories. Based on the size and number of bilayers, liposomes can be grouped into
two categories: multilamellar vesicles (Mlv), with onion-like structure of several
phospholipid bilayers, and unilamellar vesicles, where single phospholipid bilayer
surrounds a water-filled core. Unilamellar vesicles are then divided to large unila-
mellar vesicles (lUv) and small unilamellar vesicles (sUv) [71]. They are unique
in their ability to carry both lipophilic and hydrophilic compounds (diagnostic or
therapeutic), biologics, and macromolecules. Their surface can be modified to facil-
itate both passive and active drug targeting [73]. liposomes are also well-explored
platform for carrying imaging agents with magnetoliposomes the most well known
[74, 75]. here, we will highlight several examples where liposomes carrying both
drugs and imaging agents for image-guided drug delivery.
liposomes were explored as nanocarriers for a variety of imaging agents, such
as radioisotopes, Mri, speCT, peT, and optical imaging [76-81]. in these formula-
tions, the imaging agents were conjugated to the liposomal surface, incorporated into
the lipid bilayer, or added to the inner core of the liposome. liposomes also showed
great versatility for multimodal imaging. For example, gd-decorated liposomes were
used for target-selective imaging of tumor angiogenesis in rats [82]. in theranostic
liposomes, radioisotopes, Mri, and optical imaging agents were used [83, 84]. The
choice of imaging modality and how it is incorporated influences what role the imaging
agent plays in the final theranostic formulation. if the agent is conjugated to the sur-
face of the liposome while the drug is in its inner core, the imaging agent can provide
insight into overall liposomal biodistribution and indicate whether the liposome
reaches the target. Depending on the drug release mechanism involved, surface
labeling of the liposome with imaging agent could support imaging of drug release. it
is critical that drug and imaging agent should be chemically or physically compatible
in the theranostic formulation. To facilitate chemical and physical compatibility, the
drug and imaging agents reside in separate compartments of the liposome structure.
imaging liposomes offer insight into drug delivery effectiveness and potentially allow
us to see if the drug is released at target site. These insights can in turn help us modify
dose and treatment plan for a particular drug and therefore may decrease systemic
side effects.
in recent years, Mls emerged as attractive formulations for not only imaging
drug delivery but also controlling drug release using magnetic field. Mls is a versa-
tile theranostic platform that offers treatment of cancers with molecular imaging