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Current Pharmaceutical Design, 2013, 19, 6290-6297
Nanostructed Cubosomes as Advanced Drug Delivery System Xin Pan1, Ke Han2, Xinsheng Peng3, Zhiwen Yang1, Lingzhen Qin1, Chune Zhu1, Xintian Huang1, Xuan Shi1, Linghui Dian1, Ming Lu1,* and Chuanbin Wu1,* 1
School of Pharmaceutical Sciences, Sun-Yat Sen University, Guangzhou, 510006, P.R. China; 2The Second Affiliated Hospital of Guangzhou Medical College, Guangzhou, 510260, P.R. China; 3School of Pharmaceutical Sciences, Guangdong Medical College, Dongguan, 523808, P.R. China Abstract: Some kinds of amphiphilic lipids can spontaneously self-assemble with a proper ratio of water to form liquid crystalline, also known as cubic phase. With a curved bi-continuous lipid bilayer and two congruent networks of water channels, cubic phases can enclose hydrophilic, amphiphilic and hydrophobic drugs for delivery. Nanostructured cubosomes, prepared by fragmentation of bulk cubic phase gels or lyotropic methods, retain the same inner structure of cubic phase and possess much larger specific surface area and lower viscosity. These unique properties make cubosomes excellent delivery systems applicable for oral, mucosal, transdermal and parenteral drug delivery. This article gave an overview of the accelerated development and current status of cubosomes research, with respect to their preparation, characteristics and applications in pharmaceutics.
L2+water H2 +water
ne n li
*Address correspondence to these authors at the School of Pharmaceutical Sciences, Sun-Yat Sen University, Guangzhou, 510006, P.R. China; Tel/Fax: 86-20-39943038; E-mail: [email protected]
School of Pharmaceutical Sciences, Sun-Yat Sen University, Guangzhou, 510006, P.R. China; Tel/Fax: 86-20-39943115; E-mail: [email protected]
; [email protected]
1. INTRODUCTION Amphiphilic lipids possess very low aqueous solubility and often self-assemble into lyotropic liquid crystalline phases in the presence of excess water [1-3]. The liquid crystalline structures may exhibit various mesophases including lamellar crystalline phase(L), which is sheet-like bilayers separated from each other by a water layer , and inverse hexagonal phase (H2) which is aggregates arranged as cylinders in a continuous matrix consisting of long rod-like micelles lying parallel to each other in a hexagonal array . Of particular interest is the inverse bi-continuous cubic phase, which is an isotropic, thermodynamically stable and highly viscous phase . And (Fig. 1) shows the general binary phase diagram of liquid crystalline phases. Cubic phase consists of a curved bi-continuous lipid bilayer extending in three dimensions and separating two congruent networks of water channels [1, 2, 8], so it can enclose hydrophilic, amphiphilic, and hydrophobic substances [2, 9]. As a promising delivery system for various actives ranging from low molecular weight drugs to proteins, peptides, amino acids, and nucleic acids, the cubic phase has received considerable attentions [2, 3]. Further studies of amphiphilic lipid-water systems have provided evidence for the existence of three cubic structures (Fig. 2), with the lipid bilayer following the gyroid (Ia3d), diamond (Pn3m), and schwarz primitive (Im3m) surface (with the minimal surface equivalent to the bilayer midsurface) . Typically, three forms of cubic phase have been used as drug delivery systems: cubic phase gel, cubic phase precursor, and cubosomes. Cubic phase gels have been commonly used in mucosal, vaginal, periodontal, and transdermal drug delivery, however, the stiffness and viscous nature of cubic phase gels limit their potential application as delivery system [2, 3]. Han et al. [12, 13] found when water content was less than 30% (w/w) in the ternary phytantriol-water-ethanol system, an injectable isotropic solution (cubic phase precursor) was formed and could transform into a bicontinuous cubic phase upon contacting with the dissolution media
Temperature [ o C]
Keywords: cubosomes, cubic phase, liquid crystalline, nanostructure, drug delivery, preparation, characteristic, application.
Water content [%wt] Fig. (1). Identity and location of liquid crystalline phases in the temperaturewater composition phase diagram of amphiphilic lipids . L: lamellar phase; L2: microemulsion; H2: reverse hexagonal; Pn3m and Ia3d: liquid crystalline phases.
Fig. (2). Structure of bi-continuous cubic phases based on periodic minimal surfaces . (A): diamond (Ia3d); (B): gyroid (Pn3m); and (C): schwarz primitive (Im3m). The top panels show one unit cell representation whilst the bottom panels show these surfaces extending in space to form infinite periodic minimal surfaces.
© 2013 Bentham Science Publishers
Nanostructed Cubosomes as Advanced Drug Delivery System
or body fluids. The cubic phase precursor has also been successfully used for arterial transcatheter chemoembolization on hepatocellular carcinoma [12, 13]. Cubosomes are discrete, sub-micron or nanostructured particles formed from fragmentation and steric stabilization of inverse bi-continuous cubic phases of lipids [14, 15]. Therefore, cubosomes have much larger specific surface area and still remain the inner structures and the sustained release property and mechanism of cubic phase as delivery system. (Fig. 3) illustrates the typical square profiles of cubosomes which could be easily discriminated from hoxosomes, which are hexagon and formed from inverse hexagonal phase (Figs. 3 and 4). Drug release rate of cubosomes as delivery system could be increased at the initial time as compared to cubic gel, and this is benefit for the escape of insoluble/hydrophobic drug . As relatively insoluble cubic phaseforming lipids, cubosomes could exist at almost any dilution levels in water and possess much lower viscosity than the bulk cubic phase. Compared to liposomes, cubosomes showed better storing stability at room temperature [19-21]. All these properties are favorable for drug delivery, so, in this review the recent progress in the studies of cubosomes as a universal vehicle for drug delivery was briefly summarized. 2. PREPARATION OF LIQUID CRYSTALLINE PHASES 2.1. Materials used for Preparing Liquid Crystalline Phases Amphiphilic lipids such as monoglycerides, phospholipids, urea-based lipids, and glycolipids could self-assemble spontaneously in water to form various well-ordered liquid crystalline phases . However, the most commonly studied material is the unsaturated monoglycerides, in particular glycerylmonooleate (GMO, Fig. 5A) and mixtures of GMO with other lipids or structural derivatives based on GMO . GMO is biodegradable due to its ester moiety, and it was considered innoxious in the human body . Barauskas  reported that GMO was safe for use in, at least, oral drug delivery system, while a few literatures showed the hemolysis induced by GMO in vivo for parenteral administration [25-27]. Oleyl glycerate (OG), which has a structure closely related to that of GMO (Fig. 5B), is another interesting lipid at present. Nevertheless, OG can form a reverse hexagonal phase in excess
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water at physiological temperatures, providing an interesting point of differentiation with GMO, which instead forms a bi-continuous cubic phase. The structural difference between reverse hexagonal phase and bi-continuous cubic phase resulted in distinct release rates for the same material from a OG-based reverse hexagonal matrix and a GMO-based cubic phase . However, the esterbased structure of these lipids may limit their practical applications, as ester hydrolysis may lead to chemical instability and disruption of the liquid crystalline structure. Pathologic investigation of duodenum, jejunum or ileum indicated that OG was safe for oral administration . Phytantriol (PYT, Fig. 5C), a lipid of more recent interest forming liquid crystalline phase, does not possess an ester bond structurally, and hence would likely retain the cubic phase structure under digestive conditions [6, 28], which is benefit for long-time controlled and sustained release. Also, PYT could act as a penetration enhancer to improve the moisture retention of skin and hair, and it is commonly utilized for hair and skin care in the cosmetics industry . Therefore, PYT offers several advantages such as structural stability and higher purity over GMO or GMObased derivatives [23, 30]. As cubosomes are initially prepared by mechanical fragmentation of cubic phase in a three-phase region containing a liposomal dispersion, the above mentioned materials used in preparation of liquid crystalline phases can also be used for cubosomes preparation. 2.2. Preparation Methods for Liquid Crystalline Phases Amphiphilic lipids can arrange themselves into different ordered arrays upon the addition of water, and the formation of liquid crystalline phases depends on the water content, temperature and the presence of solutes. Usually, a multistep vortex-centrifugation method including heating the lipid into molten state and the application of equilibration for several weeks is used. For preparing lipid-based colloidal nanostructured dispersions (cubosomes and hexosomes), several methods which can basically be classified as fragmentation techniques have been introduced in literatures:
Fig. (3). Representative cryo-TEM and cryo-FESEM micrographs of cubosomes having square profiles with an internal cubic Pn3m phase [10, 16, 17]. (A, B): cryo-TEM photographs; (C, D): cryo-FESEM photographs; and (E, F): mathematically characterized cubosome structures using nodal surface approximations.
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Fig. (4). Representative cryo-FESEM micrographs of hexosome particles showing the clear hexagonal profile .
A OH HO
B OH HO
Fig. (5). Chemical structures of glycerylmonooleate (GMO), oleyl glycerate (OG), and phytantriol (PYT) [23, 24].
(1) Time-consuming methods involving high-energy input, e.g., microuidization, ultrasonication, and homogenization [14, 31-33]. (2) Reconstitution of dispersions from dried lipid/stabilizer lm [34, 35]. (3) Precipitation upon diluting lipids in the presence of solution, e.g., ethanol, with an aqueous phase [20, 36] or upon dialysing a mixed micellar solution  to form nanostructured cubosome. (4) Heat treatment, e.g., autoclaving, a novel method proposed for the formation of homogeneous dispersions (cubosomes) with regard to particle structure and size which are desirable in pharmaceutical applications .
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2.3. Other Materials Used in Preparing Liquid Crystalline Phases As mentioned above, liquid-crystalline phases with different internal structure, size, and morphology have been prepared using a wide range of lipids and lipid mixtures to explore their applications as drug-delivery carriers . However, it is important to point out that the utilization of these phases in pharmaceutical area is still very limited. Their high viscosities may cause handling difficulty and furthermore, these bulk phases might cause irritation in contact with the biological epithelia [22, 40]. Nanometer scaled cubosome with low viscosity may overcome these shortcomings, however, cubosome in water-dispersion form tends to aggregate, and the dispersing agents are thus vital for the cubosome applications. The first aqueous dispersions of cubic phases were prepared with bile salts, amphiphilic proteins, and amphiphilic block copolymers as dispersing agents [41, 42]. The cubic phase formed from a monoglyceride-water mixture was dispersed in the presence of micellar solutions of bile salts [41, 43], and stabilized by the formation of a lamellar envelope composed of bile salt and monoglycerides shielding the inner cubic structure. Casein, an amphiphilic protein and a very effective emulsifier, was also used to disperse the monoglyceride-water cubic phase. In this strategy, the protein was believed to partition into the outer layer of the lipid, making it more hydrophilic and therefore easy to disperse . In the search of dispersing agents, Landh  found that amphiphilic nonionic triblock copolymers F-127, also called Poloxamer 403, composed of a middle block of 67 units of poly(propylene oxide) (PPO) and both ends capped with 99 units of poly(ethylene oxide) (PEO), was suitable for the dispersion of cubic phases. F-127 is by far the most widely used dispersing and stabilizing agent [14, 46-48], as the hydrophobic PPO block is adsorbed onto or incorporated at the surface of the stabilized particle, and the hydrophilic PEO blocks extend to cover the particle surface, providing steric shielding and stabilization for the colloid particles [49, 50]. PEG-lipids [51, 52], Tween-80 [53, 54], D-alpha-tocopheryl PEO1000 , Polyvinyl alcohol (PVA) , and Laponite XLG [55-57] have also been used as the stabilizing agents for cubosome dispersions. Cubosome may also be formed without a stabilizer. For example, Rosa et.al  reported when a binary catanionic amino acid-based surfactant system was mixed with water, cubosome particles with cubic internal structures were spontaneously formed. The cubosome may be stable within several months, but its longer term stability should be carefully studied. 2.4. Drugs Embedded in Liquid Crystalline Phases Self-assembled structures consist of nanoscaled hydrophilic and hydrophobic domains, consequently such structures can be used to incorporate guest molecules of hydrophilic, lipophilic and/or amphiphilic nature , surely providing new opportunities for liquid crystalline in food and pharmaceutical applications . It is believed that hydrophilic molecules will locate close to the polar head-group of the amphiphilic lipids or in the water-rich area, lipophilic molecules will be intercalated within the apolar tail (domains) of the amphiphilic lipids, and amphiphilic solutes will be accommodated close to the interface. In recent years, the liquid crystalline phases formed by amphiphilic lipids have been studied with emphasis on accommodating biologically active molecules such as vitamins, enzymes, and other proteins, as well as crystallizing membrane proteins [11, 23]. Owing to the high surface area of the internal mesophase structure (up to 400 m2/g) and the large pore size (about 5 nm as fully swollen) , the cubic phase can be used to incorporate a typical globular protein which has the similar size as the dimensions of the water channels in the bi-continuous cubic phases [60, 61]. Chung et. al  developed a new precursor-type formula of nanocubosome for oral insulin delivery where insulin was shielded from
Nanostructed Cubosomes as Advanced Drug Delivery System
proteolytic enzymes by an easy preparation procedure and was stable during storage. Additionally, Boyd  found that incorporation of irinotecan into hexosomes at neutral pH did not result in conversion from the active lactone to the inactive carboxylate form during storage, hence this provides a promising alternative to the current low pH formulation of irinotecan which requires to inhibit the adverse conversion. Moreover, cationic amphiphilic bilayer is an efficient vehicle to integrate foreign DNA and introduce it into cells to achieve gene transfer . However, it was noted that the incorporation of guest component, either hydrophilic, hydrophobic, or co-surfactant, to some extent might influence the phase behavior and transformations of the self-assembled structure . For instance, protein entrapment in the cubic phase depends on the type of protein, interactions between protein and the lipid bilayer, and dimensions of the water channels. Recently, Kraineva et. al  investigated the impact of entrapping insulin in the structure of cubic phase, and found that the addition of insulin even at concentration as low as 0.1 wt % significantly altered the phase behavior of monoolein (MO). Therefore, it should be very cautious of incorporating macromolecular enzymes in the cubic phase, since this may modify the structures of protein and cubic phase [22, 65]. 3. CHARACTERISTICS OF CUBOSOMES The characteristics of cubosomes can be categorized into the structure of liquid crystalline and the particle size. The general analysis technology for nanoparticles can be utilized for particle size analysis of cubosomes. Therefore, this article more specifically introduced the technology to inspect the structure of liquid crystalline in cubosomes. 3.1. Polarizing Microscopy Polarizing microscopy can offer the morphology of liquid crystalline based on the optical birefringence phenomena of liquid crystalline. Because of the anisotropic molecular arrangement of layered and hexagonal liquid crystalline, in polarizing photographs the layered one will show striated or cross pattern , or coexistence of both (Fig. 6A~B), and the hexagonal one will present fan-shaped or cone-shaped hexagonal mosaic (Fig. 6C). However, the molecular arrangement of cubic liquid crystalline is non-birefringence (Fig. 6D), showing dark field in the polarizing photograph . As the polarizing microscopic photographs only indicate the phase of liquid crystalline, other methods should be utilized to confirm the crystallographic structure of cubic liquid crystalline. 3.2. Differential Scanning Calorimetry (DSC) Liquid crystalline is a thermodynamic equilibrium system, and the phase transition is usually accompanied by endothermic or exothermic energy changes. Through the thermal effects, differential scanning calorimetry (DSC) can determine whether the phase transition occurs. Chang  used DSC to measure the phase-transition temperature of binary liquid crystalline system to investigate its stability. The system showed good stability under room temperature, and the phase-transition temperature decreased with the water content. 3.3. Small Angle X-ray Scattering (SAXs) As a compensation for the deficiency of polarizing microscopy and DSC, SAXs can determine the crystallographic structure of liquid crystalline accurately. Due to the low long-rang order of liquid crystalline, the 1~10 ° small angle X-ray scattering is used based on the Bragg formula [5, 68]:
2sin q = 2
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Fig. (6). Polorizing photographs of the lemellar phase (A, B), hexagonal phase (C), and cubic phase (D) .
where S is the scattering vector, q is the scattering factor, is the scattering angle, and is the wavelength of X-ray. With the scattering vector ratio of each corresponding peak in SAXs patterns, the type and the crystallographic structure of liquid crystalline can be determined. The peak scattering vector ratios of layered, hexagonal and cubic liquid crystalline are shown as follows [38, 69]: layered liquid crystalline： S1：S2：S3：S4=1：2：3：4 hexagonal liquid crystalline： S1：S2：S3：S4=1： 3：2： 7 cubic liquid crystalline： Fd3m S1：S2：S3：S4= 3： 4： 11： 16 Pn3m S1：S2：S3：S4= 2： 3： 4： 6 Im3m S1：S2：S3：S4= 2： 4： 6： 8 Ia3d S1：S2：S3：S4= 6： 8： 14： 16 3.4. Cryo-transmission Electron Microscopy (Cryo-TEM) The microstructure of lyotropic liquid crystalline could not be directly observed under electron microscopy since: (1) electron microscopy requires vacuum environment, while the lyotropic liquid crystalline system holds a high vapor pressure for existence and evaporation of solvent; (2) liquid crystalline with light density leaves poor image contrast under electron microscopy; (3) highenergy electron beam may induce structure changes in liquid crystalline phase. Cryo-TEM, which was firstly introduced by Gustafsonand's group [10, 33] to detect the cubosome characteristics, can overcome all the above defects of regular electron microscopy. It is an ideal and powerful tool to observe the shape of single particle, photograph the morphology of particle surface, detect the insight information for analyzing the mechanism of phase transition or particle stabilization . The core technology of Cryo-TEM is to freeze the sample rapidly, and then fracture the sample. The following volatilization of the solvent may leave the cross-section structure of the sample clearly presented. After that the fracture surface is moulded on a platinum-carbon, and the carbon film is detected by electron microscopy to illustrate the inner structure of liquid crystalline . The Cryo-TEM photographs showed that cubosomes always coexist with vesicles, and the previous studies considered the vesicles as the stabilizers for cubosomes.
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t (hr) 7a
B: Model drug is allure red
Allura Red released (%)
t (hr) 7b Fig. (7). Release profiles for hydrophilic model drugs from the cubic (GMO: and PYT: ), and reversed hexagonal (PYT: ) liquid crystalline matrices in phosphate-buffered saline (n=3). The model drug for A and B were glucose and allura red respectively .
3.5. Cryo-field Emission Scanning Electron Microscopy (CryoFESEM) Equipped with a high performance electronic lighting source, Cryo-FESEM holds a higher resolution than Cryo-SEM. CryoFESEM was employed to observe the three-dimensional informations about surface and structure of cubosomes  (Fig. 3C~D). Even though Cryo-FESEM could only detect the morphology of particles and the nodes of cubosome surface, not revealing the inner structure of particles, it is still irreplaceable as compared to other techniques. Usually, Cryo-FESEM is used in combination with Cry-SEM and SAXs to explore the virtual structure of particles. 4. DRUG RELEASE MECHANISM OF CUBIC LIQUID CRYSTALLINE It is generally considered that the drug release mechanism from cubosomes is based on the principle of drug diffusion, and the drug concentration gradient across the cubosomes is the driving force of the diffusion equation [9, 54, 72, 73]. Therefore, the drug release rate from cubosomes is generally coincidence with the Higuchi or Fick diffusion equation. Certainly, there are many factors influencing the drug release rate, such as (1) drug solubility, diffusion coefficient, partition coefficient, etc.; (2) cubic liquid crystalline geome-
try, pore size and distribution, and the interface curvature; (3) temperature, pH, and ionic strength of the release medium [72, 74]. Chang  reported that the release rate of model drug chlorpheniramine maleate from lamellar liquid crystals was faster than that from cubic phase. Also, the release mechanism of several hydrophilic model drugs from the cubic and reversed hexagonal liquid crystalline was investigated . These studies indicated that diffusion is the predominant mechanism of drug release, and the drug release rate from hexagonal liquid crystalline was slower than that from the cubic ones (Fig. 7). Furthermore, the in vivo drug release profiles of 14C-glucose from cubosomes and hexagonal phase (Fig. 8), were consistent with the in vitro release profiles, which indicated the nanostructure of cubosomes and the nature of lipid itself could be utilized to control the release rate of hydrophilic drugs with varying molecular weight . Interestingly, it is difficult for hydrophobic drug to escape from the cubosomes in vitro due to the affinity of drug with the hydrophobic domain in the cubic phase. That’s why Lai  pointed out that in vitro evaluation of apparent release rate was not suitable for cubosomes. Lian  investigated the release profiles of hydrophobic drug loading cubosomes in distilled water and digestion medium respectively (Fig. 9), and found the drug release rate in digestion medium was dramatically improved. Silymarin plasma concentration in vivo also indicated an increased drug release rate from cubosome formulation as compared to the commercial capsule (Fig. 10).
Solution GMO+Cubic PHY+Cubic PYT+Hexagonal
C-Glucose released (%)
Dose normalised [ C-glucose] in plasma (mg/ml)
A: Model drug is glucose
Pan et al.
Time (hr) Fig. (8). Normalized plasma concentration profiles of 14C-glucose after administration to rats by oral gavage (n=4) . Aqueous solution (), GMO cubosomes (), PYT cubosomes (), and PYT hexagonal ().
5. CUBOSOMES AS DRUG DELIVERY SYSTEM 5.1. Cubosomes for Oral Delivery Cubosome has been proposed as an excellent candidate for oral drug delivery system. In most cases, cubosomes as drug delivery system were explored for poorly water-soluble drugs to enhance the oral bioavailability. Sustained release of drug under a controlled manner can also be achieved by using the nanostructure lipid-based liquid crystalline system. An in vitro study reported the sustained levels of rifampicin lipophilic drug release from cubosomes . Another study reported the sustained release of amphiphilic drug amphotericin B from cubosome system . Recently, cubosomes containing a model protein ovalbumin were successfully developed with a high entrapment ratio and slow release behavior of ovalbumin in vitro, which highlighted the potential of cubosomes as a novel vaccine delivery system . Currently, in the majority of reported studies on liquid crystalline dispersion systems, glyceryl monoolein (GMO) was used for
Cumulative percent released (%)
Nanostructed Cubosomes as Advanced Drug Delivery System
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their release rates, are susceptible to digestive processes. And the manner, in which this effect impacts on the application of cubosomes in vivo, is not fully understood. Cubic nanoparticles and their mechanisms of enhancing drug absorption need to be elucidated in future research.
Cubosomes precursor (digestion media) Silymarim pure drug (distilled water) Cubosomes precursor (distilled water)
100 80 60 40 20 0
Plasma silybin concentration (mg/ml)
Fig. (9). In vitro release profile of cubosomes precursor and silymarin pure drug in distilled water media (pH 6.5) and cubosomes precursor in digestion media (n=3). Adapted and modified with kind permission from .
Cubosomes precursor Liquid crystalline gel Legalon
3 2 1 0
Fig. (10). The mean plasma Silymarin concentration vs. time plot in beagle dogs (n=6) after oral administration of cubosomes precursor, liquid crystalline gel, and Legalon® (a commercial capsule formulation) .
preparing oral formulations. However, GMO-based cubosomes consisting of lipid composition have the same digestion process in the gastrointestinal tract as micelles, liposomes, and niosomes. The ester-based structure may be rapidly cleaved by pancreatic lipase, and the ester hydrolysis is likely to induce chemical instability and disruption of the liquid crystalline structure during the digestion. To avoid this problem, one possible alternative to the glyceride lipids is phytantriol, because phytantriol does not possess a susceptible ester bond in its molecular structure, and the cubic phase structure can be likely retained under the digestive condition . Cubosome delivery system was assumed to produce sustained release formulation in addition to being retained in the stomach through its bioadhesive nature, so that increasing its gastric residence time was used to improve the oral bioavailability of drug and at the same time obtain a sustained release . Additionally, cubosomes are liquid crystalline phase with cubic crystallographic symmetry formed by self-assembly of amphiphilic or surfactantlike molecules. This unique liquid crystalline structure of cubosomes may provide protection for entrapped drug from degradation in the gastrointestinal tract . Moreover, it is assumed that cubic nanoparticles will break down in the intestinal uid by esterase, and smaller cubic nanoparticles and other secondary carriers will form and transport the loaded drug across the unstirred water layer resulting in enhanced absorption. However, for cubosomes as oral drug carrier, the interactions between cubosomes intrinsic and
5.2. Cubomes for Mucosal and Transdermal Drug Delivery The properties of cubosomes with well-defined morphology, particle size and compatibility with human tissues and cells suggest a great potential of cubosomes as mucosal and transdermal drug delivery system. Due to the similarity between cubosomes inner structure and the epithelium cells, as well as the good permeability of cubosomes, drug contained in cubosomes can easily penetrate the epidermis of mucosal and skin, resulting in enhanced drug bioavailability. Gan  and Han  used cubosomes as carriers of dexamethasone and flurbiprofen for the ocular treatment respectively, and the in vitro/vivo studies showed the apparent permeability and the bioavailability of dexamethasone and flurbiprofen containing cubosomes were greatly increased as compared to the corresponding phosphate eye drops. The ocular tolerance and histological examination also revealed the good biocompatibility of these two cubosome formulations. Additionally, a novel mucosal drug delivery system, named small odorranalectin-bearing cubosomes, was prepared for brain delivery through intranasal path to enhance the therapeutic effects of S14G-HN-loaded cubosomes on Alzheimer's disease . Although excellent points have been presented for cubosomes application, little is known about cubosomes application for other topical mucosal administrations, such as buccal mucosa and vaginal mucosa. Furthermore, the mechanism of cubosomes enhancing the permeation efficiency leaves a new area for the further research of cubosomes. 5.3. Cubosomes for Parenteral Drug Delivery Cubosomes have been developed as attractive vehicles for drug release system due to their unique solubilization, effective encapsulation, sustained release behavior, and in vivo stabilization . Besides, cubosomes have lower viscosity than liquid crystalline phase and still keep the property of controlled release. Cervin  demonstrated the terminal half-life of somatostatin cubosomes injected intravenously in rats was signicantly extended more than six-fold as compared to the corresponding somatostatin solution. Moreover, cubosome is a desirable substitution of ordinary microsphere and implant, due to its good syringeability and low consumption of solvent during preparing . However, some literatures reported the self-assembled materials of monoglyceride and GMO could induce hemolysis in vivo for intravenous administration, thus parenteral administration of cubosomes based drug delivery system was restricted [25, 54]. 6. SUMMARY AND CONCLUSION This review focused on the recent advances in characterization and potential applications of cubosomes. Being kinetically stable in the presence of certain dispersing agents, cubosome dispersions possess a nanometer scaled inner structure identical to the bulk cubic phase but have much lower viscosity as well as higher bilayer area-to-particle volume ratio. Cubosome dispersions are convenient for using as oral, transdermal and parenteral drug delivery systems. Even though there is no well-known commercialized cubosome product yet, a significant number of publications have presented cubosomes as attractive pharmaceutical delivery system. Thanks to the self-assembled structures of lipids and the similarity between the cubosome inner structure and human skin [86, 87], the applications of cubosomes in gene transfection , biosensor [89, 90] and transdermal drug delivery system will attract more researchers. It is believed with the required safety testing and the development of industrial nanoparticle technology for cubosome preparation, cubosomes have a promising future.
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CONFLICT OF INTEREST The authors confirm that this article content has no conflicts of interest. ACKNOWLEDGEMENTS The authors acknowledge National Science (81173002＆81001643) for research foundation.
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Received: January 16, 2013
Accepted: March 1, 2013
Current Pharmaceutical Design, 2013, Vol. 19, No. 35  
  
  
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