Lyotropic liquid crystal systems in drug delivery: a review

Abstract

Lamellar, cubic and hexagonal mesophases are some of the most common lyotropic liquid crystal systems, and have attracted much research attention because of their distinctive structures and physicochemical properties. Polar lipids and surfactants exhibit a range of phase behavior in an aqueous environment, depending on the composition of the lipids and surfactants. These characteristics have been investigated for a variety of applications in drug delivery, and lyotropic liquid crystal systems have potential as drug carriers for small molecules, peptides, and proteins. In this article we provide an overview of recent advances in the state of the art, including methods of preparation and applications in drug delivery. The scope and limitations of lyotropic liquid crystals for drug delivery are discussed, and future research perspectives are identified.

Full text

1 23
Journal of Pharmaceutical
Investigation
ISSN 2093-5552
Volume 45
Number 1
Journal of Pharmaceutical Investigation
(2015) 45:1-11
DOI 10.1007/s40005-014-0165-9
Lyotropic liquid crystal systems in drug
delivery: a review
Dong-Hwan Kim, Alexander Jahn, Sung-
Joon Cho, Jung Sun Kim, Min-Hyo Ki &
Dae-Duk Kim

1 23
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REVIEW
Lyotropic liquid crystal systems in drug delivery: a review
Dong-Hwan Kim •Alexander Jahn •Sung-Joon Cho •
Jung Sun Kim •Min-Hyo Ki •Dae-Duk Kim
Received: 4 August 2014 / Accepted: 30 November 2014 / Published online: 10 December 2014
ÓThe Korean Society of Pharmaceutical Sciences and Technology 2014
Abstract Lamellar, cubic and hexagonal mesophases are
some of the most common lyotropic liquid crystal systems,
and have attracted much research attention because of their
distinctive structures and physicochemical properties. Polar
lipids and surfactants exhibit a range of phase behavior in
an aqueous environment, depending on the composition of
the lipids and surfactants. These characteristics have been
investigated for a variety of applications in drug delivery,
and lyotropic liquid crystal systems have potential as drug
carriers for small molecules, peptides, and proteins. In this
article we provide an overview of recent advances in the
state of the art, including methods of preparation and
applications in drug delivery. The scope and limitations of
lyotropic liquid crystals for drug delivery are discussed,
and future research perspectives are identified.
Keywords Drug delivery Liquid crystals Cubosome 
Hexosome
Introduction
Controlled drug delivery systems are advanced methods for
the transport of pharmaceutical compounds within the
body, and can be used to overcome many of the limitations
of conventional drug formulations. The aim of this
approach is to create a higher concentration of the drug at a
specific site relative to that in the rest of the body, as well
as to develop controlled release formulations. Various
types of drug delivery system (DDS) have been developed,
including hydrogels, nanoparticulate delivery systems,
drug-loaded biodegradable microspheres, and drug poly-
mer conjugates.
One of the more recent advancements in DDS are liquid
crystals (LCs), which have emerged as injectable formula-
tions because of their sustained drug release properties (Guo
et al. 2010). Lyotropic mesophases form a long-range order
with the addition of a solvent, and have historically been
used to describe materials composed of amphiphilic mole-
cules. Lyotropic LC phases are formed when dissolving
amphiphilic molecules in a solvent, and are influenced by
the amphiphilic structure of the molecule, the presence of
additives and conditions of the solution. Lyotropic liquid
crystal systems (LLCSs) can be classified into lamellar (L
a
),
hexagonal and cubic phases, as shown in Fig. 1. The L
a
phase is a linear arrangement of lipid bilayers, in which the
hydrophilic head groups are in contact with water and the
hydrophobic tail groups are pointing toward the center of the
sheet. The hexagonal phase is the most common non-
lamellar phase formed by amphiphilic molecules mixed with
water (Rizwan et al. 2010). The normal hexagonal (H
1
)and
inverse hexagonal (H
2
) phases are cylindrical structures that
form a hexagonal lattice. A cubic phase (Q
2
) may exist
between the H
2
and lamellar L
a
phases, creating a dense
cubic lattice. The H
2
and Q
2
phases have been extensively
D.-H. Kim A. Jahn S.-J. Cho D.-D. Kim (&)
College of Pharmacy and Research Institute of Pharmaceutical
Science, Seoul National University, Seoul 151-742, Republic of
Korea
e-mail: ddkim@snu.ac.kr
J. S. Kim
Division of Health Sciences, Dongseo University,
Busan 617-716, Republic of Korea
M.-H. Ki
Chong Kun Dang Research Institute, CKD Pharmaceutics Inc.,
464-3 Jung-dong, Giheung-gu, Yongin-si, Gyeonggi-Do
446-916, Republic of Korea
123
Journal of Pharmaceutical Investigation (2015) 45:1–11
DOI 10.1007/s40005-014-0165-9
Author's personal copy

investigated, and have much potential for use as delivery
vehicles for a wide range of materials, from low-molecular-
weight drugs to proteins, peptides and nucleic acids (Garg
et al. 2007; Hirlekar et al. 2010). The H
2
phase consists of
rod-like water channels arranged in a two-dimensional lat-
tice, separated by lipid bilayers; and the Q
2
phase comprises
a curved water channel and a bicontinuous lipid bilayer that
extends in three dimensions. The inverse hexagonal and
cubic mesophases are spontaneously formed from the liquid
crystal-forming system (LCFS) in an aqueous fluid. The
resulting tortuous networks of aqueous nano-channels in
these mesophase particles can form passageways for sus-
tained release of drugs from liquid crystals (Ki et al. 2014).
A number of amphiphilic materials have been investi-
gated as LCFSs, including glycerol monooleate (GMO),
phytantriol (PT), glycerol dioleate (GDO), oleyl glycerate
(OG), and phytanyl glycerate (PG) (Guo et al. 2010). These
polar lipids are not water-soluble; however, they often form
Q
2
and H
2
mesophases in an aqueous environment, which
are reconstructed in such a way as to minimize the free
energy when in contact with water, so that they can be
dispersed in equilibrium with an excess of water to form
thermodynamically stable colloidal dispersions. Of these
systems, the phase behavior of GMO has been most
extensively studied, and the non-toxic, biodegradable and
bioadhesive properties of this material make it particularly
suitable for applications in drug delivery.
In this paper, we provide an overview of LLCSs for drug
delivery. We focus on the strategies used to prepare inverse
cubic and hexagonal mesophases, and describe approaches
for intravenous, gastrointestinal, subcutaneous, mucosal
and topical routes of administration. In addition, we discuss
the problems and limitations of these approaches, and
identify potential directions for future research.
Materials
Liquid crystals (LCs) are a class of soft matter, whereby
the constituent molecules exhibit positional and/or long-
range orientational order. Of greatest interest for pharma-
ceutical applications are lyotropic LCs, which consist of
two or more components whereby one acts as a solvent to
provide fluidity to the system, and the other provides an
anisometric shape. The arrangement of the LC molecules
in a particular solvent depends on various factors, including
temperature and concentration, as well as the shape of the
LC molecule. In 1976, Israelachvili et al. proposed the
Water in Oil CPP > 1 H2
Q2
Mirror plane CPP = 1 Lα
Oil in water CPP < 1/3 H1
Fig. 1 Schematic diagrams of
liquid crystal structures
2 D.-H. Kim et al.
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critical packing parameter (CPP) to predict how the mol-
ecules will be arranged in an LC phase (1976). This CPP is
given by:
p¼v=a0lc
ðÞ;
where vis the volume of the aliphatic chain, l
c
is the length
of the aliphatic chain and a
0
is the polar area of the surface
of the micelle, as shown in Fig. 1.
A diverse range of components in addition to LC mol-
ecules can be added to formulate the LCFS to stabilize the
LC mesophase, including phospholipids, ethanol and sur-
factants. Additives incorporated into the lipid bilayer may
vary the curvature of the bilayer, which can result in a
change of the thickness of the bilayer and/or the diameter
of the water channel. These changes may result in a change
to the lattice parameter or the structure of the phase (e.g.,
from hexagonal to cubic) (Fraser et al. 2013). It is known
that fatty materials, such as tocopherol and tricaprylin, can
induce a cubic or hexagonal phase by increasing the cur-
vature of the bicontinuous layer inside the liquid crystal
(Amar-Yuli et al. 2009).
A wide variety of amphiphilic molecules form liquid
crystalline phases; however, only a few arrange into H
2
and
Q
2
phases over a wide range of concentrations. One of the
first to be discovered was GMO, as shown in Fig. 2, which
is thermodynamically stable in excess water, resulting in
particles that can maintain their three-dimensional structure
under a range of physiologically relevant conditions and at
low concentrations (Zhen et al. 2012; Jain et al. 2012). PT
(see Fig. 2) exhibits similar behavior in that it may form H
2
and Q
2
phases in excess water; however, it does not contain
an ester group, and exhibits improved stability in com-
parison with GMO (Fong et al. 2012; Nguyen et al. 2011).
It has recently been shown that both PT and GMO con-
stitute cubosomes, and may exhibit cytotoxicity.
A more recently discovered group of amphiphiles that
form a Q
2
phase are oleyl glycerate OG and PG (see
Fig. 2). These molecules were first patented in 2004 and
2005, and the phase behavior was described in a 2006
publication (Boyd et al. 2006a). OG, forms an inverse
hexagonal phase in water at concentrations in the range
7–100 % at 37 °C, with a threshold from pure H
2
to H
2
plus excess water at 29.9 ±1.1 %; PG also forms a Q
2
phase in water at concentrations in the range 7–100 % at
37 °C, but with a phase change to H
2
plus water at
39.0 ±2.7 % (Boyd et al. 2006a). Kaasgaard and Drum-
mond gave an in-depth review of the phase transition
characteristics of a wide range of amphiphiles, including
the transition from lamellar to cubic to hexagonal phases
(Kaasgaard and Drummond 2006).
Yamada et al. reported hexagonal and cubic phases of
erythritol phytanylacetic acid ester, together with favorable
skin-penetrating properties (2011), and a Q
2
phase of
monolinolein was described by Negrini and Mezzenga
(2012). A system of monolinolein and linoleic acid was
shown to exhibit a transition from cubic to hexagonal
phases as the pH was varied (Negrini and Mezzenga 2011).
It has recently been reported that 50-deoxy-5-fluoro-N4-
(phytanyloxycarbonyl) cytidine—a recently synthesized
fluorouracil prodrug-alkyl chain conjugate—forms a Q
2
phase (Gong et al. 2011). A mixture of poly(ethylene
glycol) monooleate and octylglucoside was also shown to
form Q
2
phase (Angelov et al. 2009).
Colloidal dispersions of inverse hexagonal or cubic
phases are generally preferred for DDS applications, which
are termed ‘hexosomes’ and ‘cubosomes’, respectively,
and form coarse colloids, which tend to aggregate rapidly
without the addition of a steric stabilizer (Guillot et al.
2010). The most frequently used system contains GMO and
the tri-block copolymer Pluronic F127 as a stabilizer, as
shown in Fig. 3(Boyd et al. 2009). Other commonly used
additives include various PEO stearates, phytosterol eth-
oxylates, Tween 85, Tween 80 and Cremophor EL (Chong
et al. 2012; Rossetti et al. 2011). The interaction between
GMO and F127 is not fully understood; however, it has
been reported that b-casein can be used as an alternative for
F127, which results in a decrease in the Q
2
–H
2
transition
temperature in GMO systems and promotes the formation
of H
2
phases in phytantriol formulations (Zhai et al. 2011).
Tricaprin has also been shown to stabilize the H
2
phase of
GMO (Garti et al. 2012).
D-a-tocopherol (see Fig. 3) is another commonly used
stabilizer, and has been shown to reduce the leakage of a
model dye from the LC bilayers (Bitan-Cherbakovsky et al.
2011; Quinn 2012). Rossetti et al. have shown that the
inclusion of the preservative Merguard 1200 (1,2-dibromo-
2,4-dicyanobutane) and the thickening agent Natrosol 250
(hydroxyethylcellulose) does not affect the formation of a
hexagonal phase (2011). Furthermore, Negrini and Mezz-
enga showed that the channel width in a cubic phase of
monolinolein could be controlled by varying the concen-
tration of sucrose stearate (Negrini and Mezzenga 2012).
Despite the numerous studies into LCFS, an LCFS DDS
has yet to be approved by the FDA for injection. Sorbitan
monooleate (SMO) (also known as Span 80) was recently
proposed as a material for LCFM. SMO has potential
advantages in subcutaneous injections because of favorable
safety and quality control, since its application as an
injectable emulsifier in pharmaceutical formulations is well
known (Ki et al. 2014).
Structure
Both the inverse cubic and inverse hexagonal phases are
binary amphiphilic systems, whereby amphiphilic
Lyotropic liquid crystal systems in drug delivery 3
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molecules (e.g., lipids) are arranged in such a way as to
form bulk penetrating polar solvent (e.g., water) channels,
surrounded by a layer of amphiphilic molecules (Fraser
et al. 2013; Amar-Yuli et al. 2007). These phases can be
differentiated by the polarization-dependent optical
refraction; i.e., birefringence (Aeinleng et al. 2012).
Fig. 2 Chemical structures of glyceryl monooleate (GMO), phytantriol (PT), oleyl glycerate (OG) and phytanyl glycerate (PG)
Fig. 3 Chemical structures of
Pluronic F127 and DL-a-
tocoperol acetate as stabilizers
4 D.-H. Kim et al.
123
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In a Q
2
phase, the amphiphilic molecules form curved
bilayers, which can be described as minimal surfaces with
two non-contacting penetrating water channels. The lipid
domains are arranged in a cubic pattern, but are con-
nected, forming a continuous layer throughout the bulk of
the phase, resulting in a relatively large specific interface
area of approximately 500 m
2
/g (Drummond and Fong
1999). The water channels are also continuous and con-
nected, and so the phase is bicontinuous (Lapteva and
Kalia 2013). The different spatial arrangements of the
inverse cubic phases are not straightforward to analyze;
they are, however, of considerable interest to soft matter
physicists. A result of the minimal surface arrangement
in an inverse cubic phase is an average surface curvature
of zero, resulting in optical isotropy. Thus, inverse cubic
phases can be identified by the lack of bifringence. In
2011, Nguyen et al. demonstrated that cinnarizine
encapsulated in the cubosome exhibits sustained release
following oral administration. Cryogenic transmission
electron microscopy (Cryo-TEM) images of GMO-cubo-
somes exhibited a non-spherical appearance, resulting
from a bicontinuous cubic (i.e., Q
2
) phase, as shown in
Fig. 4(Nguyen et al. 2011).
In an H
2
phase, the amphiphilic molecules form inverse
micellar columns of indefinite length, with a negative mean
curvature of the enclosed water channels. These columns
are arranged in a two-dimensional hexagonal pattern, and
the curvature of the interface is more pronounced than in
inverse cubic phases. Thus, in a system of similar com-
position, a higher temperature is required to produce a
hexagonal phase (Libster et al. 2011). A Cryo-TEM image
of dispersed hexagonal particles was published in 2011, as
shown in Fig. 5(Kuntsche et al. 2011).
Predictions of both open and closed water channels in
Q
2
phases have been made; however, recent studies have
shown that the aqueous channels are open (Rizwan et al.
2007). H
2
phases do not exhibit contact between the water
channels and the surrounding medium (Larsson 1999). In
addition, an understanding the absorption of drugs from
LC phases into biological tissue is essential for predicting
biological uptake and skin permeation, and an under-
standing of the oral bioavailability or absorption following
subcutaneous injection is important. It has been reported
that cubic phases (GMO and PT) generally exhibit greater
absorption than hexagonal phases onto hydrophobic sur-
faces (Dong et al. 2011,2012). Drug localization within
the Q
2
or H
2
phase is typically influenced by the lipo-
philicity of the drug. Hydrophilic drugs are typically
located in the water channels, whereas lipophilic drugs
tend to localize to the lipid layers. More complex inter-
actions at the interface tend to occur between amphiphilic
drugs or additives (Fraser et al. 2013; Cohen-Avrahami
et al. 2012).
Method of preparation
Gel-like mesophases can be prepared simply by injecting
the LCFS into an aqueous solution. LC gels can be pre-
pared by mixing the LCFS with aqueous phase using a
vortex or using ultrasonication (Boyd et al. 2006b); how-
ever, preparation of lyotropic LC nanoparticles, cubosomes
Fig. 4 Cryo-TEM micrographs of inverse cubic mesophases particles
dispersed in water. Figures was modified from Nguyen et al. (2011)
Fig. 5 Cryo-TEM micrographs of inverse hexagonal mesophases
particles dispersed in water. This figure was modified from Kuntsche
et al. (2011)
Lyotropic liquid crystal systems in drug delivery 5
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or hexosomes is more complicated, and thus will be dis-
cussed in greater detail in the following sections.
High-temperature dispersion
The procedure described by Gustafsson et al., whereby
homogenization at high temperatures produces a coarse
dispersion, resulting in well-ordered particles of cubo-
somes (1997). A homogeneous melt of lipids, surfactants
and the drug were added dropwise to water to form the
coarse dispersion. Further reductions in size via homoge-
nization were achieved at higher temperatures. The ele-
vated temperature promotes transformation of the non-
cubic vesicles to cubic vesicles.
Wo
¨rle et al. reported a method of preparing homoge-
neous cubic nanoparticles by autoclaving an aqueous
coarse dispersion (2006). The heat treatment contributes to
a reduction in the particle size, and a narrow size distri-
bution and good colloidal stability were reported. Although
temperature is important, the heat treatment duration did
not appear to affect the size of the particles, nor the dis-
tribution or transformation of the dispersions. Small
changes in temperature may alter the visual appearance and
structure of the particles; however, temperature-sensitive
drugs, such as proteins and peptides, may not be suitable
for this method.
Mechanical agitation
To prepare lyotropic liquid crystal (LLC) nanoparticles,
high-pressure homogenization, sonication or shearing is
typically necessary. The viscous bulk phase can be pre-
pared form lipids, surfactants and stabilizers; the resulting
mixture is then injected into an aqueous solution, together
with mechanical agitation (Spicer 2005). Mechanical stir-
ring or homogenization is commonly used, and Wo
¨rle et al.
have reported GMO-based LLC nanoparticles via stirring
and homogenization (2007). They investigated GMO-based
cubosomes formed at different temperatures and with
varying concentrations of F127 as a stabilizer. Heat treat-
ment is frequently used to prepare LLC nanoparticles
combined with mechanical agitation. These methods are
well established and easy to use in a laboratory setting;
however, they can result in mixed structures, including
cubosomes, hexosomes and lamellar liquid crystalline
phase. Salentinig et al. used shearing to fabricate LLC
nanoparticles (2008); shearing can be used to prepare more
stable and homogeneous cubosomes and hexosomes with a
large hydrophobic phase content compared with ultrason-
ication. The temperature and viscosity of the sample during
emulsification are also important parameters.
Hydrotropic solvents
Hydrotropic solvents can be used to stabilize LLC phases.
LLC nanoparticles prepared using a hydrotrope exhibit
long-term stability due to the homo-dispersed stabilizers in
the particles (Spicer 2005). LLC nanoparticles can be
prepared simply, and have favorable properties compared
with other methods of preparation.
Swarnakar et al. used ethanol to prepare isotropic LCs,
whereby lipids and oil were solubilized (2007). Ethanol
was used as a hydrotropic solvent to create a liquid pre-
cursor, and diluted to form a colloidal dispersion of
hexosomes.
Although hydrotropes have both hydrophilic and
hydrophobic parts, they generally do not have the critical
micelle concentration, because the hydrophobic part is too
small to form self-assembled particles. Hydrotropes also
exhibit salting-in, thereby increasing the solubility of lipids
and surfactants in aqueous solution; however, the use of
hydrotropic solvents requires an understanding of the phase
behavior, and thus lipid-water-hydrotrope charting trajec-
tories on the ternary phase diagram should be investigated
to determine the optimal point for the formation of stable
LLC nanoparticles.
Applications in drug delivery
LLCSs have been considered as a new method of drug
delivery, because of the unique physicochemical proper-
ties. LLCSs have the following advantages as drug delivery
systems (Garg et al. 2007): (i) effective solubilization
compared with traditional carriers; (ii) a high carrying
capacity for a range of water-insoluble drugs; (iii) 20 to
[100-fold improved bioavailability of water-soluble pep-
tides; and (iv) they are a promising vehicle that can protect
the sensitive drugs from enzymatic degradation.
Cubic LLCSs generally exhibit great flexibility in terms
of their composition. It is possible to upload drugs with a
wide range of polarities and sizes. They are also biode-
gradable and the viscosity facilitates slow and sustained
release of the incorporated drug.
However, LLCSs have a variety of limitations. One
practical limitation is due to their viscosity, especially
when the cubic and hexagonal phases are injected.
Administration of viscous LLCSs by means of traditional
delivery routes is impractical because the viscous sub-
stance may induce irritation or other adverse effects when
injected into the human body. However, alternative strat-
egies for utilizing viscous cubic or hexagonal phase have
been reported over the past two decades. One is to for-
mulate the active component within a less viscous lamellar
6 D.-H. Kim et al.
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phase gel, which is easier to administer (Lynch et al. 2003;
Engstro
¨m et al. 1992).
Another limitation of LLCSs is that the active sub-
stances may influence the membrane properties of the
LLCS, which can in turn affect the release pattern and
delivery properties. Several studies have reported that the
addition of drugs or solvents to GMO/water systems leads
to a modification of the liquid crystalline structure, and
alteration of the release kinetics of the uploaded drugs
(Lynch et al. 2003; Engstro
¨m et al. 1992; Engstro
¨m and
Engstro
¨m1992; Nylander et al. 1996).
In addition, a lack of suitable, scalable manufacturing
methods to prepare structurally well-defined and stable
dispersions is a considerable barrier to the commerciali-
zation of cubic LCs for sustained drug delivery systems.
Furthermore, the range of lipids available with suitable
phase behavior for the preparation of these systems is
limited.
Despite these limitations, however, various potential
routes of drug delivery using LLCS have been investigated.
Many studies of drug delivery using LLCS have been
reported and can be categorized according to the route of
administration used, as described in the following
subsections.
Intravenous administration of LLCS drug delivery
systems
The high viscosity of inverse bicontinuous cubic and
hexagonal phases, together with the mechanical stiffness, is
problematic for intravenous injection (Drummond and
Fong 1999; Malmsten 2007). Several methods have been
proposed to overcome this difficulty, however, including
the application of flowable forms (e.g., lamellar phases)
and the use of lyotropic liquid crystal nanoparticles (Boyd
et al. 2006b; Leesajakul et al. 2004; Cervin et al. 2009).
Irinotecan is an anticancer drug that exhibits a pH-
dependent equilibrium between the active lactone and
inactive carboxylate forms, with rapid conversion to the
carboxylate form occurring at pH 7, and concurrent
cleavage of the side chain to the highly cytotoxic SN-38
(Hirlekar et al. 2010). Boyd et al. developed glycerate-
based hexosomes, which can improve the retention of iri-
notecan in the non-toxic lactone form at neutral pH. Fur-
thermore, the dimensions of the particle appear to be
suitable for intravenous administration (2006a).
Somatostatin is a peptide hormone that is active in the
regulation of the endocrine system, and has much potential
in the treatment of diseases including acromegaly, acute
pancreatitis and gastroenteropathic endocrine tumors
(Dalm et al. 2008). However, the practical use of this drug
is limited because of its short half-life of only a few min-
utes. Cervin et al. showed that the combination of
somatostatin with lipid-based liquid crystalline nanoparti-
cle carriers significantly increased the half-life of the
peptide when injected intravenously into rats (2009).
Subcutaneous administration of LLCS drug delivery
systems
Sustained drug release via subcutaneous (SC) injection has
potential to maintain an effective plasma concentration for
up to several months, with minimal side effects in com-
parison with intravenous injection or multiple dosing. A
number of studies have shown that LLCS may be applied
to achieve improved drug delivery via SC injections.
Somatostatin and desmopressin uploaded in a GMO–
water system showed a prolonged in vivo release profile,
following SC injection (Engstro
¨m et al. 1992). More
recently, Fong et al. formulated phytantriol- and glyceryl
monooleate-based bicontinuous cubic and inverse hexag-
onal nanostructures, which were converted in response to
changes in temperature. When injected subcutaneously at
40 °C, in vivo absorption studies have shown slow release
of the hexagonal phase; however, when the temperature
decreased to 30 °C (at which temperature a transition from
the hexagonal phase to the cubic phase occurred), a sta-
tistically significant increase in the plasma concentration of
drugs was observed. In addition, this system exhibited
more sustained release profiles compared with the other
control formulations (Fong et al. 2009). It has recently been
reported that an LLCS prepared by using SMO for SC
injection of leuprolide acetate showed a sustained release
for up to 1 month in rats and in dogs (Ki et al. 2014).
LLCSs have also been applied in SC injection for
immunization. Cubosomes with the Toll-like receptor
agonists monophosphoryl lipid A and imiquimod, exhib-
ited sustained release kinetics and induced a more robust
immune response in mice compared with liposome and
alum formulations (Rizwan et al. 2013). Moreover, LLCSs
may also be combined with microneedle (MN) technology,
which could provide improved methods of transcutaneous
immunization (TCI). Rattanapak et al. utilized MN and
cubosomes as a synergistic approach with TCI. Whereas
MN increased the permeation of an aqueous mixture
through the skin, a cubosome formulation containing the
peptide was retained in the skin. It was thus proposed that
this approach using MN and cubosome has potential for
TCI applications (Rattanapak et al. 2013).
Oral administration of LLCS drug delivery systems
To achieve effective oral delivery, three important factors
must be considered. First, the formulation must possess the
inherent property of sustained release; second, it should
exist in a stable form in the gastrointestinal (GI) fluids to
Lyotropic liquid crystal systems in drug delivery 7
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provide an enduring matrix from which drugs can be
released; and third, it should exhibit bioadhesive properties
to extend the retention time of the formulations in the GI
tract (Boyd et al. 2007). It has recently been reported that
glycerated monooleate-based mesophase formulation
exhibited enhanced bioavailability of co-administered
poorly water-soluble drugs, which exhibited the first and
third features discussed above, but does not exhibit sus-
tained release due to the sensitivity to the digestive process
(Larsson 1999; Charman et al. 1993; Lee et al. 2009). It has
been reported that LLCSs prepared using phytantriol and
oleyl glycerate also exhibit enhanced bioavailability and
sustained release of orally administered drugs (Guo et al.
2010).
Cinnarizine is a poorly water-soluble drug that is used as
an antihistamine. The poor water solubility and resulting
poor absorption were significantly improved when cinnar-
izine was entrapped in the oleyl-glycerate-based inverse
hexagonal phases. This formulation exhibited a sustained
release pattern over several days, accompanied by extended
absorption (Boyd et al. 2007).
Chung et al. reported GMO-based cubosomes contain-
ing insulin, and described a hypoglycemic effect of the
formulation when administered orally. Their results
showed that the insulin–cubosome formulation provided a
hypoglycemic effect that was comparable to insulin
injected intravenously over 6 h (Chung et al. 2002). Sim-
vastatin uploaded in GMO-based cubosomes administered
orally showed an increase in bioavailability of 241 %
compared with a control comprising a crystal powder for-
mulation of the drug. In addition to the enhanced bio-
availability, the cubosome formulation exhibited sustained
release of simvastatin over a period of 12 h in beagles (Lai
et al. 2009). Cubic nanoparticles loaded with 20(S)-proto-
panaxadiol (PPD), exhibited a 169 % increase in the rela-
tive bioavailability compared with raw PPD (Jin et al.
2013).
Transdermal and topical administration of LLCS drug
delivery systems
Transdermal administration is an interesting alternative to
oral administration because it avoids a number of the
limitations of oral delivery, including the hepatic first-pass
effect and low oral bioavailability, as well as several dose-
dependent side effects and compliance problems. Nielsen
et al. reported that the high viscosity of the cubic phase
formulation was able to adhere to jejunum and vaginal
cavity in an in situ study using rabbits (Nielsen et al. 1998).
The bioadhesive nature of the cubic phase may be useful
for enhancing the transdermal penetration of drugs;
however, transdermal delivery typically exhibits limited
absorption of drugs. The stratum corneum is believed to be
the major rate-limiting barrier for transdermal delivery
(Nemanic and Elias 1980). LLCSs can form a thin surface
film consisting of a liquid crystal matrix, which can be
controlled to achieve an optimal delivery profile, and can
provide temporary protection for sore and sensitive skin
(Garg et al. 2007). A number of studies have shown that
LLCSs, including cubic and hexagonal mesophase formu-
lations, can penetrate through the stratum corneum, and are
promising candidates as transdermal drug delivery systems.
Cyclosporin A is an immunosuppressive undecapeptide.
When it was solubilized into inverse hexagonal crystalline
structures with three dermal penetration enhancer, the
formulation showed enhanced penetration through the
stratum corneum with less skin irritation compared with a
control oil formulation (Lopes et al. 2006; Dima et al.
2007).
LLCSs also have potential applications as topical
delivery systems for prophylaxis and in the treatment of
post-surgical wound infections, as well as for the treatment
of post-surgical pain. Several studies have shown that local
anesthetics, including bupivacaine and lidocaine, formu-
lated as a cubic phase gel, exhibited sustained release of the
drug when it was applied at the site of the wound (Eng-
stro
¨m and Engstro
¨m1992; Park et al. 1998). Local anti-
biotic delivery is also considered to be an effective method
of preventing post-surgical wound infections. Sadhale and
Shah reported improved stability of cefazolin and ce-
furoxime in GMO cubic-phase gels (Sadhale and Shah
1998).
Ophthalmic and nasal administration of LLCS drug
delivery systems
Proper ophthalmic drug delivery is becoming increasingly
important because even a small overdose can induce irri-
tation to the eyes, particularly with flubiprofen (FB) solu-
tion (Ahuja et al. 2008). LLCS drug delivery may
contribute to improved ophthalmic applications. Han et al.
developed cubosomes containing FB and reported reduced
ocular irritancy and improved bioavailability compared
with FB solution (Han et al. 2010). Gan et al. also dem-
onstrated LC nanoparticles (cubosomes) containing dexa-
methasone (DEX) as a novel ophthalmic delivery system,
with an enhance bioavailability (eightfold increase of
AUC) in the in vivo compared with a DEX suspension.
Nasal administration using LLCS has also been investi-
gated, and LC vehicles containing zidovudine (AZT)
exhibited increased absorption of AZT in rats. These
results demonstrate that the LC formulation administered
via the nasal route may represent a promising tool for
systemic delivery of drugs (Gan et al. 2010).
8 D.-H. Kim et al.
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Other administration routes of LLCS drug delivery
systems
Vaginal delivery of the antimuscarinic drugs propantheline
bromide and oxybutynin hydrochloride for treatment of
urinary incontinence using a GMO–water LC system has
been demonstrated (Geraghty et al. 1996). The incorpo-
rated drug induced the formation of a lamellar phase, and
formed a cubic phase when it was retained in the vaginal
cavity, due to its bioadhesive characteristics. Both drugs
were released via diffusion following square-root time
kinetics over a period of 18 h.
Periodontal delivery of antibiotics using LLCS has been
considered for the effective prevention and treatment of
infection. Viscous cubic and inverse hexagonal phases have
been shown to transform from lamellar phases containing
metronidazole when injected into the periodontal pocket.
However, cubic phases did not exhibit the desired release
characteristics (Norling et al. 1992). In another study, Es-
posito et al. characterized both poloxamer-based and GMO-
based cubic phase formulations when injected into the
periodontal pocket (Esposito et al. 2005). The results of
these studies support the application of LLCSs as a cubic
phase for local and periodontal delivery. Swarnakar et al.
prepared a hexosomal dispersion loaded with progesterone,
which was tested for oromucosal delivery systems. In vitro
release tests showed an increased transmucosal flux and a
decreased lag time across the mucosa of albino rabbit
(Swarnakar et al. 2007).
Conclusions and outlook
The unique structure and physicochemical properties of
LLCSs make them suitable for use as a drug delivery
carrier, with numerous potential applications in pharma-
ceuticals. LLCS formulations have exhibited significant
entrapment efficiency, sustained drug release, and
improved stability.
There have been many promising results of these
LLCSs; however, a number of limitations need to be
overcome for their clinical application. For example, with
the direct administration of liquid crystal gels, problems
associated with the viscosity and the frequent occurrence of
burst-release must be resolved. Although monoolein has
been regarded as being generally safe, little is known about
its adverse effects upon parenteral administration. In
addition, the number of available lipids that exhibit suitable
phase behavior for application in humans is limited.
Although some new materials, such as PT, OG, PG and
SMO have been shown to exhibit promising properties,
questions remain regarding their safety and biological
stability in vivo.
The considerable potential advantages of LLCSs are
expected to provide the motivation for these challenges to
be addressed. Further basic research—including in vivo
studies and validations of methodology—is anticipated, as
well as methods of evaluating their applicability in human
subjects, which is expected to bring LLCS formulations
closer to market as pharmaceuticals.
Acknowledgments This article does not contain any studies with
human and animal subjects performed by any of the authors. All
authors (D.-H. Kim, A. Jahn, S.-J. Cho, J. S. Kim, M.-H Ki, and D.-D.
Kim) declare that they have no conflict of interest. This work was
supported by the National Research Foundation of Korea (NRF)
Grant funded by the Korean government (MSIP) (No. 2009-0083533)
and the MarineBio Research Program (NRF-C1ABA001-2011-
0018561).
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