Stimuli-Responsive Lipid -Based Self-Assembled Systems - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 256

et al., 2003). Harnessing this information is key to utilizing the transformation
between lipid systems.

9.2 APPLICATION OF SELF-ASSEMBLED SYSTEMS IN DRUG
DELIVERY AND THE CASE FOR STIMULI-RESPONSIVE SYSTEMS

9.2.1 Benefi ts of Self-Assembled Systems in Drug Delivery

The use of self-assembled systems based on lipids and surfactants in drug
delivery applications is predicated primarily on three main features.

1 . Enhance Solubilization The self-assembly provides an internal hydro-
phobic environment for enhanced solubilization of drug molecules that are
otherwise poorly water soluble and limits the dose able to be administered.
Emulsions, micelles, and liposomes have all been utilized for this purpose in
a range of pharmaceutical products already on the market. While micelles
have a disadvantage of potential loss of solubilization on dilution beyond the
critical micelle concentration of the surfactant, emulsions and liposomes
do not.
2 . Manipulate Pharmacokinetic Behavior Side effects due to drug admin-
istration are most often associated with high blood concentrations immedi-
ately after administration. Incorporation of drug into a carrier system can slow
down the availability of drug to induce side effects without reducing the total
dose of drug administered. One example of this is the use of liposomes to
encapsulate amphotericin B, an antifungal drug.
3 . Target Drug to Particular Tissues The functionalization of micelles and
liposomes has been utilized to direct the carrier to the desired tissue site, either
by passive or active targeting. Passive targeting involves surface coverage with
polyethylene glycol (PEG) chains to prevent nonspecifi c removal from the
circulatory system, leading to preferential deposition in tumor tissues and sites
of infl ammation. Active targeting involves functionalizing the surface of the
particle with a tissue-specifi c moiety such as an antibody or antigen, to encour-
age specifi c interaction with diseased tissues that differentially express the
complementary entity to the functional group compared to healthy tissues.

While the three drivers for using a self-assembled system in drug delivery are
important in providing improved therapeutic outcomes using self-assembled
systems, there are still a number of unmet therapeutic needs. In particular,
external control over the release of incorporated agents is currently not
achievable. There are a number of therapeutic scenarios in which external
control over drug release may be benefi cial including:

• Frequently injected compounds with short half - lives: Reducing the fre-
quency of administration of drugs that require multiple injections per day
to, for example, once daily with external activation when required, such