Chemistry Reference
In-Depth Information
to catalyze the enantioselective Diels-Alder reaction of cyclopentadiene and
crotonaldehyde (Fig. 17), with ee and de values identical to those of the parent
chiral imiazolinone catalyst in solution [120]. In this system, the initial chiral
imidazolinone monomer did not display any LLC properties; but upon pro-
tonation to the active catalyst salt form, different LLC phases were obtained
depending on the nature and amount of acid used. Transient salt forma-
tion via protonation with Brønsted acids may be another general method for
inducing highly functionalized monomers with weak or no mesogenic prop-
erties in their pristine state, to form well-defined LLC phases [119].
5
LLC-based Drug Delivery Systems
The use of delivery systems for the sustained release of drugs at optimum
therapeutic ranges is extremely important not only for maintaining drug ef-
ficacy, but also for reducing the number of applications or injections needed
for therapy [121]. Matrices for drug delivery for intravenous, oral, or der-
mal application must meet certain criteria, such as non-toxicity, compatibility
with the drug to be delivered, good chemical stability under physiologi-
cal conditions, and eventual biodegradability/excretion from the body [122].
A number of hydrophilic polymer systems and surfactant systems that form
vesicles and micelles meet these above criteria, and have been employed as
drug delivery vehicles [123, 124].
Over the last two decades, it has been recognized in the drug delivery com-
munity that water-based LLC phases can also be used as viable matrices for
the sustained and controlled release of drugs. The fact that many LLC phases
have periodic nanometer-scale pores allows for a high and uniform loading
of drugs, as well as good controlled diffusion out of the pores [121]. In add-
ition, the ability of unpolymerized LLC assemblies to be dispersed by eventual
or purposely triggered changes in solvent concentration or temperature, al-
lows for easy removal of the delivery matrix at a desired point in time after
its job has been accomplished. For the most part, only certain LLC phases
containing open water channels or pores (i.e., the L and inverted (type II)
LLC phases) have been explored for drug delivery because most drugs are hy-
drophilic or water-soluble molecules [121]. As such, LLC phases containing
water compartments are required for initially encapsulating these drugs and
then performing controlled and sustained release.
Only a few surfactant systems and their derivatives have been explored as
LLC-based drug delivery systems because of the chemical and physiological
compatibility requirements listed above, and because of cost and availability
issues. These LLCs include commercially available non-ionic surfactants such
as esters of oleic acid and related fatty acids [e.g., glyceryl monooleate (GMO)
(aka, monoolein)], and oligo(ethylene oxide)-alkyl ether surfactants (e.g.,
Search WWH ::
Custom Search