Stimuli-Responsive Lipid -Based Self-Assembled Systems - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 269

employed to induce drug release from vesicles such as liposomes. The free
radicals cause membrane destabilization through lipid hydrolysis. Thompson
and co-workers have manipulated the effects of plasmalogen photooxidation
on membrane permeability in order to release the drug from liposomes. The
photooxidation of light-sensitive components promotes bilayer fusion and
contents release (Anderson and Thompson, 1992; Collier et al., 2001; Gera-
simov et al., 1999).
Systems have also been synthesized that undergo photofragmentation or
polymerization of the stabilizing element on exposure to light. Photocleavable
lipid derivatives in conjunction with a lipid, for example, DOPE, have been
used to form liposomes. On exposure to UV irradiation, the fragmentation of
the modifi ed lipid destabilizes the bilayer and contents release ensues. Zhang
and Smith (1999) achieved 50% calcein release through the fragmentation
of NVOC-DOPE, a photocleavable nitroveratryloxycarbonyl derivative of
dioleoylphosphatidylethanolamine. Activation of photopolymerizing compo-
nents in self-assembled systems has also been used to cause drug release.
Polymerization of these components condenses some of the interface, result-
ing in the formation of pores. For instance, Spratt et al. (2003) achieved a
28,000-fold increase in liposome bilayer permeability on activation of their
synthesized photoreactive lipid bis-SorbPC 17,17 .
Photothermal Systems Light can be used to impart heat into systems in two
ways. First, NIR lasers can penetrate tissues into the posterior segment of the
eye and as deep as 10 cm under the skin (Weissleder, 2001). A major focus
of the application of photothermal systems concentrates on drug delivery to
the posterior section of the eye, especially the retina and choroid, in order to
treat degenerative conditions such as choroidal neovascularisation (CNV), a
cause of age-related macular degeneration (AMD). Current methods of drug
delivery are based on topical, periorbital, intravitreal, and systemic adminis-
tration. These are problematic as low penetration is gained through topical
and periorbital administration, periorbital and intravitreal administration is
invasive, and systemic administration of the drugs presents toxicity issues
as the whole body is exposed to large concentrations of drug in order to get
a therapeutic amount into the target tissue. An interesting method developed
by Zeimer et al., called light-targeted drug delivery (LTD), uses a noninvasive
laser source to gently heat liposomes and so trigger drug release (Zeimer
and Goldberg, 2001; Zeimer et al., 1988), the intended target of this system
being the posterior section of the eye. The method involves the intravenous
administration of drug encapsulated in a heat-sensitive liposome, and release
of its contents at the target tissue by gently warming up the target tissue to
41°C with a directed laser light pulse. LTD, like PTD, is limited by the need
to keep doses of the photosensitizer and light low enough to avoid collateral
damage.
Second, light can be used to activate metallic nanoparticles (NPs), which
are able to act as “nanoheaters” on a highly localized scale. The evolved heat