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Cubosomes as Targeted Drug Delivery Systems - A Biopharmaceutical Approach

Fo nt N r P ha ot e m Fo rs S c o r D n ie a n is l U ce tri s P bu e u tio On bl n ly ish er s

Naga M. Lakshmi, Prasanna R. Yalavarthi*, Harini C. Vadlamudi, Jyotsna Thanniru, Gowri Yaga and Haritha K Pharmaceutics Division, Sree Vidyanikethan College of Pharmacy, Tirupati-517102, India

Abstract: Cubosomes are reversed bicontinuous cubic phases and possess unique physicochemical properties. These special systems are receiving much attention for the delivery of various hydrophilic, hydrophobic and amphiphilic drugs with enhanced bioavailability and high loading capacity. A wide variety of drugs are applicable for cubosome formulation for various routes of delivery. The lipids used in cubosome formulation are more stable and offer stability to the formulation during shelf-life. The article reviews about the back ground, techniques of cubosome preparation such as high pressure homogenization, probe ultrasonication and automated cubosome preparation; and also methods of cubosomes preparation such as top down, bottom up and other methods with pictorial presentation. This article emphasizes the phase transition and also targeted approaches of cubosomes. The characterization studies for cubosomes such as cryo transmission electron microscopy, differential scanning calorimetry and scanning electron microscopy followed by in-vitro and in-vivo evaluation studies of cubosomes were explained with appropriate examples. Recent applications of cubosomes were explained with reference to flurbiprofen, odorranalectin, diazepam and dexamethasone. The advantages, disadvantages and limitations of cubosomal technology were emphasized.

Keywords: Bottom up, glyceryl monooleate, high shear homogenization, phase transition, phytantriol, polaxomer, probe sonication, top down. 1. INTRODUCTION

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In late 1960s, the bicontinuous liquid crystal phase was first reported [1] and its geometric model was later provided in the mid-1970s [2]. Among these types of phase, cubosomes, especially made of binary systems and monoolein–water [3] are one of the most studied systems. The term cubosomes was first coined by Larsson, aimed to represent the cubic molecular crystallography [4]. Due to an emerging interest in pharmaceutical nanotechnology, there have been several investigations on the use of these systems as alternative drug delivery systems. So, they have been investigated for different pharmaceutical applications (peptides, enzymes, antimuscarinic drugs, antibiotics, analgesic delivery) and are extensively reviewed [5]. Cubosomes are the hydrated surfactant structures that can self- associate to form a bicontinuous liquid cubic crystal phase maintaining its thermodynamic stability. These are viscous isotropic binary systems, the highly twisted lipid bilayer and high surface area (~400 m2/g) of cubosomes results in higher adjuvant incorporation [6]. Improved stability of drug in formulation, desired particle size range, maximum drug loading capacity and well controlled drug release made cubosomal systems superior to other novel delivery systems like solid lipid nanoparticles, microemulsions, microspheres and liposomes. In addition, cubosomes can easily accommodate amphiphilic, lipid and water soluble molecules; their structure is different from liposomes and were easily prepared by *Address correspondence to this author at the Sree Vidyanikethan College of Pharmacy, Tirupati-517102, India; Mobile: +91- 98857 29290; E-mail: [email protected] 1875-6220/14 $58.00+.00

mechanical fragmentation [7, 8]. The cubosomes offer well controlled delivery to variety of drug candidates like antiinflammatory compounds, local anaethetics, antibiotics and anticancer drugs. The lipid entrapped cubosomal vaccines were administered with an adjuvant effect [9]. The special structural features of cubosome formulations are proposed on the basis of their nodal surfaces such as: (i) Diamond surface (Pn3m/ D-surface), (ii) Gyroid facade (Ia3d/ G-surface) and (iii) Primitive face (Im3m/ P-surface) [10]. The cubosome structure had been offered with enhanced efficacy and stability of vitamins [11] and proteins [12]. Addition of lipid polymers had synchronized the stability of the colloidal dispersions [13] and polymers are responsible for controlling the delivery of drugs where the active moiety will diffuse all the way through the channels present in the cubic phases. [14]. 2. SELECTION CRITERIA OF DRUGS AND EXCIPIENTMS Delivery of lipophilic drugs such as diazepam, rifampicin, propofol and griseofulvin to the target site was the main theme of the cubosome formulation, where the phospholipid coat causes increased permeability by minimizing the solubility problem of the lipophilic moiety. In case of hydrophilic glucose and insulin moieties, the permeability through the biomembrane is limited; this can be successfully overcome by incorporating them in lipid components. So, both types of drugs can be incorporated in the cubosomes, whereas till date the lipophilic drug encapsulation was reported to be higher. The effective delivery of peptides was

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182 Current Drug Discovery Technologies, 2014, Vol. 11, No. 3

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achieved by cubosomes with enhanced stability of peptides by reducing their exposure to different pH environmental conditions. Thus, the demerits of other site specific/targeted drug delivery systems could be minimized by proper selection of cubosome carrier which enables the delivery of drug moiety effectively at the specific tissue/organs [15].

• It requires a minimum large volume of sample (30 ml), and

The physicochemical factors to be considered for selection of the excipients are compatibility between drug and polymer and drug distribution in solid lipid matrix [16]. Among the physical characteristics, the selection priority goes to the melting point of polymers. The melting point of carrier should be greater than 45°C to minimize the stability problems [17]. The hydrophile-lipophile balance (HLB) value of core materials should be less than 2, so they are more lipophilic and have high chances to form solid matrices over the hydrophilic materials which form colloidal dispersions. The carrier should have the capability to solubilize the drug and to form particles of optimum size and strength enabling the drug release at desired site [18]. Glyceryl monooleate, monoolein and polaxomer-407 are commonly used in the cubosomal processing. Acyl glycerols (e.g. Glyceryl monooleate) are esters of glycerol and fatty acids. They are commonly known as amphiphilic lipids (polar lipid). In presence of water, these compounds are able to form various liquid crystals. Monoolein is a mixture of oleic acid glycerides and some other fatty acids mainly monooleate. Monoolein acts as a main precursor in the formulation of cubosomes [19-21].

3.2. Probe Ultrasonication

3. METHODS FOR PREPARATION OF CUBOSOMES

2) Each well has a capacity of 600 μl of solvent

3.1. High-Pressure Homogenization [22, 23]

3) Automated sonication is performed by the robot.

The most suitable method for processing of cubosomes with low polydispersity index that are highly stable and retain a long shelf-life is high pressure homogenization [24, 25]. It involves 3 main steps;

The main advantage of this technique is that the cubosomes are produced in large amounts. So their physicochemical properties can be easily assessed. The partitioning of the cubic phases tend to host the lipophilic, hydrophilic or amphiphilic molecules is shown in Fig. (1). Polar head of the surfactant molecules will localize the hydrophilic moieties whereas the lipophilic moieties will localize in the lipid bilayer and amphiphilic drugs at the interface [26].


• The shearing process is the determining step in the production of quality cubosomes before subjected to homogenization.

3.1.1. Gel Preparation

The process involves:

1) Preparation of gel involves addition of a stabilizer 2) Generation of cubic phase with equilibration of solvent 3) Ultrasonication

Amplitude and frequency are the main variables that are needed to be maintained so that to avoid the sample from overheat and phase transition due to pulsing frequency. 3.3. Automated Cubosome Preparation In this process robotic systems and probe sonicator are used to produce large number of cubosomes that are similar to probe sonication with small changes as. 1) 96 well plate is used for the preparation of gels

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Lipids and surfactants are dissolved in the suitable organic solvent and mixed well until all the content appears uniform followed by evaporation of the organic solvent by using rotary evaporator. This will form the gel phase of the preparation.

This method is mainly used for small volume samples. It can disperse the mixture into particles even with sample volume of 600 μl depending on probe size. This method is quick and time dependent process and involves the following steps [11].

3.1.2. Shearing

The formed gel is sheared with aqueous solvent to form the micro dispersion. 3.1.3. High-Pressure Homogenization

The dispersion is processed under high pressure homogenizer. This step is temperature sensitive and the temperature is set according to the thermal properties of selected lipid in the formulation. This method is best suitable for large volume sample systems having retention of cubic structure with good confidence (specified storage environment and amphiphile used). The methods assess the quality of the cubosome prior to use, but this is not a suitable preparative approach for low volumes sample systems. Even though it offers many advantages, some limitations follows: • A single sample can be prepared at a time,

Fig. (1). Reversed bicontinuous cubic structure.

4. SPECIAL TECHNIQUES FOR THE PREPARATION OF CUBOSOMES: [27-30] 4.1. Top-Down Technique Top down technique has been widely used to produce the bulk cubic phases with high energy. The bulk cubic phase would be formed by mixing the lipids with amphiphiles and then, mixture would be dispersed in an aqueous phase by

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requires less amount of energy for the production of submicron particles [39]. 5. STRUCTURAL CHARACTERISTICS OF CUBOSOMES 5.1. Structural Significance Either in bulk phase or in cubic phase, liquid bicontinuous cubic crystals possess specific characteristics that attract cosmetic industry. Cubosomal personal care products are prepared by mixing biocompatible lipids and aqueous phase which promotes their use in the production of skin care, hair care and other body care products. Cubosomal skin care products are gaining importance because of the possible interaction of stratum corneum and lipids used in cubosomal formulation promoting the permeation of drugs [40].


high pressure homogenization and followed by shearing or sonication to form the liquid crystal nanoparticles. Liquid crystalline phases will be observed at intermediate shear rate where as lamellar phases are formed at increased shear rate and defect phases are formed at high shear rate. Sonication and high pressure homogenization are used to produce the cubosomes based coarse dispersion. D-surface structure will be formed from the bulk cubic phase upon homogenization and it would be transformed to P-surface upon addition of polymers [31-33]. Initially lipid (glyceryl monooleate/ glyceryl mono stearate) is mixed with pluronics (acts as stabilizer) and then mixture is dispersed into the aqueous phase with the application of high shear energy (ultrasonication, high shear homogenization) to form lyotropic liquid crystals (LLC) as explained in Fig. (2) [34].

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Cubosomes being self-assembled cubic crystals are biocompatible and bioadhesive, thereby well suitable for oral administration which was proved with the oral administration of insulin loaded cubosomes for hypoglycemic effect [41-43].

Fig. (2). Top down technique.

4.2. Bottom-Up Technique

In bottom-up technique, the cubosomes would be formed by dispersing the droplets of inverse micellar phase into aqueous phase at 80°C and followed by slow cooling to crystallize out as cubosomes as shown in Fig. (3). This method is well suitable for vigorous production of cubosomes. The hydrotrope used in cubosome formulation would be responsible to prevent the formation of bulk cubic gel phase. The hydrotrope dissolves the cubic gel and further addition of water, with the application of sonication will decrease the solubility of the LLC particles thus leads to the formation of cubic particles [35].

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Fig. (3). Bottom up technique.

4.3. Other Methods

The emulsification was also employed to produce the cubosomes, by diluting monoolein-ethanol solution with poloxamer 407 aqueous solution [36]. Spray dried technique was also involved in the production of cubosomes in which monoolein is encrusted with polysaccharides such as either starch or dextran upon hydration and the polymers are added to stabilize the formulation [37]. The cubosomes usually follow high shear homogenization in their development. The high shear homogenization technique involves the addition of stabilizers to prevent the aggregation of the particles during shelf-life. Although it is the best method for the production of cubosomes, it has few limitations in the formulation of proteins and peptides due to application of high shear [38]. In order to circumvent the associated problems, liquid precursor or solvent dilution method was employed which

5.2. Phase Transition

Amphiphilic lipids in aqueous environments are characteristic to establish self-assembled nanostructures and facilitate cosmetic application [44, 45], drug delivery [46, 47] and diagnostics [48]. Reversible transitions occur among lamellar [L], hexagonal [H2] and cubic [V2 ] phases of liquid lipid crystalline structures have been of current concern [49, 50]. In the drug delivery field, the transition of phases from lamellar to cubic phase result in the transformation of drug release from slower pace in lamellar phase to rapid release from cubic phase. [47, 51, 52]. Inverse bicontinuous lipid cubic phase can be formed by the utilization of lipid such as phytantriol or glyceride lipids, for instance glyceryl monooleate (GMO) as well as monolinolein in combination with water or aqueous phase [52, 53]. Thus the cubic phase’ internal order is maintained in the cubosomal dispersions. Due to the chemical stability of phytantriol, its appilicability in the formation of cubosomes is escalating at a faster rate.

Addition of lipid additives to phytantriol based cubosomal formulation controls the self assembly of nanostructured phytantriol in water or aqueous phase. Combination of vitamin E acetate and phytantriol in water results in the formation of hexagonal phase at room temperature [11]. At low pH, a dispersion of dioleoylphosphatidylserine or monoolein multilamellar vesicles has undergone transition into the inverse bicontinuous double - diamond cubic phase [54]. Salt– sensitive cubosomal dispersion with phytantriol and didodecyldimethylammonium bromide (DDAB) was produced by addition of salt and resulted with the phase transition from lamellar [L] to cubic [V2]. This method is low energy process which is widely used in the formulation of cubosomal dispersions [55]. Ionic surfactant possess polar head group which is charged and non polar tail part with aliphatic side chains such as sodium bis(2-ethylhexyl) sulfosuccinate, anionic surfactant and the DDAB, cationic surfactant. They possess critical packing parameter nearly up to 1. Self assembly of these surfactants to form lamellar vesicles occurs


nificant difference in the well shaped monoolein cubosomal particles and arbitrary shaped monoelaidin dispersion [59]. The cryo-TEM analysis was similar with the dynamic light scattering measurements of flurbiprofen cubosomes [60]. The Cryo-TEM revealed the consistent particle arrangements in submicron size of faceted morphology of flurbiprofen cubosomes and had inner texture comparable from reversed bilayered cubosomes. Small angle X-ray scattering analysis was performed in order to assess the bioavailability of earthworm fibrinolytic enzyme in the inner eye. The results are evident that the cubosome formation is largely depending on the hydration level of the liquid precursor [61]. It was understood from the dynamic light scattering study on prepared particles, that the particle range was in between 50-80 nm for poorly water soluble drugs containing lipid crystalline nanoparticles [62]. The stability performance of photostabilizers was analyzed by dynamic light scattering from the liquid crystalline nanodispersions containing photostabilizers. The effect of drug loading and additives on the stability of LLC was quantified by small angle X-ray diffraction [63]. Increase in the soya phosphatidylcholine to glyceryl dioleate portion lead to transition of bulk lipid phase from reversed cubic phase to reversed hexagonal phase. Further cryo-TEM was evidated with addition of polysorbate 80 lead to more disorganized lamellar structures further increase in the ratio lead to extension of more lamellar layers surrounding the inner surface [64]. Ovalbumin cubosomes have small particle size when prepared freshly and upon long standing they formed aggregates. Dynamic light scattering is a well established and a non-invasive technique for measuring LLC particle dimension [65]. Differential scanning calorimetric analysis was performed to understand the absence of pure drug’ endothermic peak in 20(S)-protopanaxadiol cubosomes [66]. Magnesium trisilicate spray dried powder had irregular shape and diclofenac sodium shown large size crystals when were examined by Scanning electron microscope [67]. Internal tortuous layer with second external structure of phytantriol cubosomes was revealed with Cryo field emission scanning electron microscopy [68].


due to repulsions between polar head groups of similar charges. With an increase in the concentration of ionic surfactants, steric repulsions increases which consequences the negative curvature of mixed lipid system. Phase transition occurs from cubic to lamellar phase at this juncture in salt free conditions. On other instance, reverse phase transition occur from lamellar to cubic phase at high ionic strength [56]. So, the phase transition and self-assembly of ionic surfactant- phytantriol cubosomal dispersion in aqueous medium depends not only on the concentration of surfactantlipid mixture but also on the ionic strength.

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6. CUBOSOMAL PRECURSOR FORMS

Development of cubosomal precursors results in the production of in-situ cubosomes, which not only avoids high energy procedure which is an expensive and difficult to scale-up but also to protect the thermosensitive moieties like proteins. 6.1. Liquid Precursors

A strong driving force is required to produce cubosomes from liquid phase precursors. Upon dilution of liquid precursors, more stable cubosomes of desired size can be produced. In hydrotrope dilution method, cubosomal particles are produced by nucleation and growth mechanism which is similar to crystallization and precipitation procedure. Liquid phase precursors are widely used in mouth washes, hand washes, where cubosomes can be formed during rinsing, washing respectively. Since high shear is not required in this method and thereby degradation of active moiety can be minimized in the cubic crystals [57]. Thus, liquid precursors offer an easy scale-up technique for production of cubosomes reducing the handling of bulk solids and avoidance of high energy process techniques that degrade the drug. 6.2. Powdered Precursors

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Powdered cubosomal precursors are made up of dehydrated surfactants coated with suitable polymer. Upon reconstitution of powdered precursors with water, cubosomes with an average particle size of 600 nm are formed as conformed by characterization studies such as light scattering technique and cryo- transmission electron microscopy (cryo-TEM) [36, 58]. Spray drying is an excellent technique to formulate cubosomal powdered precursors. It involves the production of encapsulated particles from liquid droplets in emulsion as well as from dispersed solid particles in concentrated aqueous polymer solution. Spray drying technique is well suitable to scale-up the production of consumer goods like foods and detergents. This process also helps to provide an easy way to preload potent drug in cubosomes before drying. These powdered precursors propose process and performance oriented advantages than the liquid phase cubosomal precursors. 7. CHARACTERIZATION OF UNIQUENESS OF CUBOSOMES Transformation of lipid vesicles to cubic phase (cubosomes) results in the self assembly of lipid as evidenced by the Cryo-TEM technique. Self assembly in monoelaidin cubosomal dispersions represents the formation of closely arranged onion like vesicles. There exists a sig-

8. EVALUATION OF CUBOSOMES Permeation is the key determinant for the success of cubosomes in which most of the hydrophobic drug particles are to be absorbed through Peyer’s patch [69, 70]. The cubosomes with good entrapment efficiency (EE) can entrap lipid based particles with loading capacity (LC). Permeation studies of the prepared cubosomes are used to conduct with the vertical Franz diffusion cell. The major limitation for Amphotericin B (AmB) oral administration is poor bioavailability. To overcome above limitation and to improve oral antifungal efficacy of AmB, GMO based AmB lipidic cubosomes were prepared. Their transport efficacy across intestinal barrier was evaluated using human colon adenocarcinoma cell line (caco-2). Transportation of AmB into caco-2 cells from AmB loaded GMO cubosomes occurs by clathrin as well as caveolae mediated endocytosis mechanism not by means of macropinocytosis. AmB cubosomal dispersion has shown profound enhancement in the oral bioavilability and was effective against renal tissues on 2nd day dose as well. Thus AmB–GMO cubosomes serves as lipidic nanovectors facilitating the delivery of AmB orally [71].

Cubosomes as Targeted Drug Delivery Systems

9. STABILITY ASPECTS OF CUBOSOMES

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bar/5 cycles) homogenization to produce cubic nanoparticle containing bulk gel and proved with increased bioavailability for 20(S)-protopanaxadiol [66]. The GMO/phytantriol based cubosomes were formulated with minute addition of vitamin E to phytantriol using hydrophilic drugs (glucose, Allura Red and FITC-dextrans) with increasing molecular weights. With oral administration of GMO/phytantriol based cubosomes to rats, the absorption kinetics of 14C-glucose precursor was monitored by scintillation counting and found to have increased therapeutic effect [80]. In a study with hydrophilic polymers such as methyl cellulose (MC), dextran, carrageenan and pluronic-R, the MC and dextran had no effect, whereas carrageenan had no transition, phytantriol did lower transition and pluronic-R enhanced the sol-gel transition with increased stability and imparted better sustained release for antigen from poloxamer 407 cubosomes as thermo responsive depot systems [81]. In another application of high pressure homogenization, the optically isotropic cubic gels of AmB deoxycholate with phytantriol/poloxamer 407 were fragmented by probe sonication to get milky coarse dispersion with unique morphology [41].


The cubosomes formation region is observed in the binary and ternary phase diagrams with a wide gap in the miscibility region of cubic phase and solvent phase. Steric stabilization which prevents coalescence and aggregation can be attained by polymers such as poloxamer 407. Fragmentation of lamellar and cubic phases in aqueous medium results in the formation of vesicles and cubosomes respectively. The cubosomes produced in this manner have bilayered openings exposes the hydrocarbon chains to aqueous phase and limits the cubosomal stability. This can be overcome by coating the openings with solid bilayered crystalline coating as well as liquid lamellar crystalline coating. Solid bilayered coating offers superior stability to liquid lamellar coatings. Though the diameter of cubosomes is 50 nm, solid rigid coating results in the cubosomal diameter of 100 nm due to restrictions in curvature. Besides this, sponge phase coatings can also be used as stabilizing coatings for cubosomes [72]. The lipids like GMO and monoolein, used in the formulation are also responsible for the stability of dosage form and controlled release of drug [1]. Majority of cubosomal formulations are usually included with cationic, nonionic surfactants, polymer chains and also some natural lipids. GMO is one among them which is the most widely used lipid which have the ability to form spontaneous colloidal dispersions and thus offer thermal stability to cubosomal formulations [73, 74].


10. APPLICATIONS OF CUBOSOMES WITH REGARDING TARGETED DRUG DELIVERY SYSTEMS

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The cubic delivery systems were proved as meritorious to formulate sensitive membrane proteins. It was explianed that cubic delivery systems contain 1-o-phytanyl--d-xyloside, a lipid glycoside has ‘Pn3m’nodal cubic phase in association with triblock copolymer as dispersion stabilizer and was able to process cubic dexamethasone (DM) particles and which is an alternative to GMO [75]. The DM cubosomes had exhibited internal and exterior cubic structure and possessed high permeability, retention and area under curve (AUC) than eye drops when applied on rabbit corneal tissue cross sections [65]. The cubic structure has no effect on certain complexation techniques, which are meant to enhance the solubility of drug candidates. This was observed with molten monoolein, when it was hydrated by sonication in presence of Pluronic F127 with hydroxyl propyl -cyclodextrin (HP -CD) / minoxidil (MXD) complexes to entrap cubic nanoparticles. The structure of cubic phases has no effect with complex formation and in addition, the skin permeation of MXD loaded cubosomes was highest over MXD solution [76]. Employing high pressure on cubosomal systems had profound effect on pharmacological actions which was evidenced with albumin bound cubosomes. They had gone for prolonged circulation in system and offered sustained delivery which was possible by high-pressure emulsification of monoolein–water mixture in presence of dispersing agent pluronic F127, a cubic component. Adsorption onto monoolein and subsequent binding with albumin leads destabilization and resulted remnant particles remain in plasma [77]. The flurbiprofen cubosomes were developed with high-pressure (350 bar/8 cycles, 11000 rpm) homogenization and has enhanced transcorneal permeation without ocular irritation [78, 79]. The similar approach was extended with hot melt and high pressure (670

The morphology is usually confirmed with equilibrium dialysis method for cubosomes containing lipophilic compounds such as diazepam, griseofulvin, rifampicin and propofol [82]. A new approach of surface engineering technique was applied by linking between odorranalectin and streptavidin through thiol group of maleimide cubosomal delivery to assess the neuroprotective effect. The intranasally administered dose of odorranalectin cubosomes to amyloid- 25–35-treated rats was navigated with coumarin6, a lipophilic fluorescent probe for its distribution in brain [83]. Currently, marketed cosmetic product, L’Oreal and Nivea possesses cubosomal moieties as stabilizers for oil-inwater emulsion as well as pollutant absorbents. Annexin-V decorated phosphatidyl serine (PS) containing phytantriol cubosomes acts as probe for the enhanced detection of apoptotic membranes in both model and in-vitro systems. In healthy cells, PS is articulated on the cytosolic part of plasma membrane in healthy cells. At the early stages of apoptosis, translocation of PS from phospholipid bilayer to extracllular membrane occurs which can be detected by using Annexin-V. They have demonstrated that lipid bound protein, Annexin- V that can be used to target the cubosome systems to biological surfaces [84]. Uptake of insulin nanocubicles by caco-2 cells occurs at a higher level which is supported by the controlled serum glucose level for more than 6 h. [85]. Monoolein based cubosomes were formulated which are doped with two fluorescent probes namely flourescein and dansyl. Later it was modified with a hydrocarbon chain to improve the monoolein palisade and hydrophobic quercetin was loaded. Cubosomes doped with the modified flourescein probe and quercitin were successfully exploited for cancer therapy [86, 87]. 11. ADVANTAGES The cubosomes offer controlled and targeted release profiles for many drug candidates. Most of the lipids used in the formulation are biocompatible [88, 89], nontoxic, non-allergic, nonirritating and offer high drug encapsulation efficiency. Lipids used in the formulation are thermally stable. Because of the presence of bilayered lipid surfaces


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they can easily accommodate both lipophilic drugs like cinnarizine, and hydrophilic dugs like cyclosporines [90, 91]. GMO has been used most commonly in the formulations since it has rigid structure and maintains the integrity of the structure even at altered temperatures and imparts stability to drug delivery systems. The methods of cubosome preparation mainly involve with the shear and homogenization techniques which don’t require usage of organic solvents [92, 93].

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12. DISADVANTAGES

As these cubosomes are nanosize particles with 50 m, upon long standing they lead to the growth of the particles which is a major drawback for parenteral formulations. Irregular gelation of the particles may occur during shelf-life. Due to changes in the external environment they have the capability to change their dynamics i.e., phase transition [94]. Lipids offer mostly crystalline nature for the particles and allow them to incorporate the active moieties in low doses which may not lead to desired therapeutic effect. 13. LIMITATIONS OF CUBOSOMES

Despite of the potential, the scale-up of cubosomal production is difficult due to their complex phase behaviour and viscous properties. High pressure induced drug degradation and irregular gelation of the particles were found as draw backs associated with cubosome processing [15]. To quantify cubosome formation mechanism by cryo-TEM is difficult. Upon dilution with polaxomer-407 solution; interfacial turbulence is created promoting the spontaneous emulsification which results in the formation of sub micron sized particles [95]. CONCLUSION

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As cubosomes are bicontinuous lipid cubic phases, they have capability to incorporate many hydrophilic and lipophilic drugs that get delivered to the targeted tissues such as central nervous system and brain efficiently. Stability can be imparted to potent and highly sensitive moieties by these cubic systems which contain selective and stable lipids. Two reproducible methods such as top down and bottom up approaches could be easily employed to produce cubosomes either by high pressure homogenization or ultrasonication techniques. Cubosomes are applicable to wide range of drug candidates, proteins, immunogenic substances and also to cosmetics. Due to the potential site specificity, the cubosomal preparations may be widely employed as targeted drug delivery systems for ophthalmic, diabetic and also for anticancer therapy. The cubosome technology is relatively new with high through output and would have wide scope of research in developing new formulations with commercial and industrial viability.

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CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest.

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ACKNOWLEDGEMENTS

[24]

Declared none.


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Received: March 26, 2014

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Revised: April 26, 2014

Accepted: April 28, 2014