Cubosomes – A liquid crystalline marvel

Dr Pradnya Palekar Shanbhag, Professor in Pharmaceutics, Saraswathi Vidya Bhavan’s College of Pharmacy, shares a study on cubosomes, its properties and its potential for use in diverse areas

Nanoparticulate systems characterised by different morphology and dimensions depending on production procedures have been obtained, namely cubosomes, nanovesicles, solid lipid nanoparticles and liposomes. The discovery of cubosomes is a unique story and spans the fields of food science, differential geometry, biological membranes, and digestive processes. Cubosomes are well-defined, three dimensional structures consisting of coexisting lipophilic and hydrophilic nano domains that can be either interconnected or isolated, depending on the phase structure. The hydrophobic effect drives amphiphilic molecules in polar solvents to spontaneously self-assemble into a rich array of thermodynamically stable lyotropic liquid crystalline phases with characteristic lengths on the nanometer scale. Cubosomes possess a sufficient average degree of molecular orientation order, characterised by their structural symmetry despite their liquid state, and often form in aqueous surfactant systems at relatively high amphiphile concentrations. Bicontinuous cubic phases consist of two separate, continuous but non-intersecting hydrophilic regions divided by a lipid bilayer that is contorted into a periodic minimal surface with zero average curvature. They have potential for controlled release through fictionalisation and they are an attractive choice for cosmetic applications as well as for drug delivery.

Properties

Size range: Large cubosomes (diameter >200nm) Smaller cubosomes and vesicles (diameter < 100nm)

Important properties are:

• Temperature stability
• Bicontinuous structure
• High internal surface area
• Solid-like viscosity
• Low cost of raw materials
• Optically isotropic
• Solid like liquid crystals with cubic crystallographic symmetry
• Biocompatible and bioadhesive in nature
• Cubosomes were found to get disintegrated by whole plasma as a result of the interaction with plasma components.

Cubic phase structure

Monoglycerides are polar lipids  having poor water solubility that exhibits aqueous phase behaviour, which are structurally mimicking to non-ionic surfactants. Lutton results the mono glycerides whose hydrocarbon chain lengths between c12 and c22 (Fig.1) of all the monoglycerides, particularly monoolein exhibits larger region of cubic phase. Monoolein is an unsaturated, c18 monoglyceride.

Cubic phases are often found sandwiched between lamellar and hexagonal liquid crystalline phases, especially in non-ionic surfactant systems. The monoolein-water system uniquely possess a cubic phase region and contains broad compositional and temperature range. Normally monoolein has continuous hydrophilic headed, hydrophobic tail end, producing reversed or inversed cubic phases, indicating the phases towards polar medium so that the cubic phase structures can be described using the concept of differential geometry and periodic minimal surfaces. Minimal surfaces are best described by analogy with soap films. Based on their curvatures, three types of minimal surfaces, studied in cubic phases was discovered mathematically by Schwarz.

The monoolein water system forms the D-surfaces at high water levels, and the G-surface at lower levels. The P-surface is formed in the monoolein water system, but only when a third component such caseins or amphiphilic block copolymer are added. Normally, using copolymers like poloxamer407-monoolein-water system because of the polymers utility it provides colloidal stability to cubosomes by recoalescence to bulk cubic phases. Poloxamer (peo99-ppo67-peo99), the ppo copolymer exists either at surface of cubic phase particle or with in bilateral structure, whereas the peo chain remains in the bulk water phase resulting in the formation of three-phased regions.

Drug release from cubosomes

Cubosomes have been proposed as a controlled release, intravenous drug delivery system. The pressure ultra-filtration method and equilibrium dialysis were used to elucidate the in vitro drug release mechanisms. On dilution of cubosomes, lipophilic compounds were released rapidly when studied by the pressure ultra-filtration method. In contrast, equilibrium dialysis incorrectly indicated sustained drug release from cubosomes. Research shows that cubosomes should be burst release delivery systems where drug is released by diffusion from the cubic phase matrix, and that pressure ultrafiltration may have benefits over dialysis methods for measurement of drug release from colloidal particle-based drug delivery systems.

Structural Characterisation

X-Ray diffraction, Cryo-Transmission electron microscopy (cryo-TEM), Photon correlation spectroscopy (PCS), Atomic force microscopy and NMR investigations have been performed. X-ray diffraction measurements and C NMR are used to study the internal structure of liquid crystalline dispersion.

Using high resolution freeze-fracture electron microscopy direct visualisation of the internal structure of the 3-D cubosomes of double diamond type formed upon spontaneous lipid/protein assembly in excess of water.

Preparation

Cubosomes are usually produced by combining monoolein and water at 40°C for 24 hours. The resultant cubic liquid crystalline gel is dispersed into particles via the application of mechanical or ultrasonic energy.

Two main approaches can be used to prepare cubosomes:

Top-down approach: It applies high energy to fragment bulk cubic phase. It is the most widely used in research area, where by bulk cubic phase is first produced and then dispersed by high energy processing into cubosomes nanoparticles. Bulk cubic phase is resembles a clear rigid gel formed by water swollen crossed linked polymer chains. The bottom-up approach forms cubosomes from molecular solution by, for example, dilution of an ethanol-monoolein solution. They require formation of cubosomes prior to their use in a product.

Bottoms-up techniques: It avoids high-energy drawbacks and allow formation of cubosomes in use by a consumer or during product formulation. Both techniques require a colloidal stabiliser, like the tri-block copolymer Poloxamer 407, to prevent cubosome aggregation. For cubosomes in consumer products and pharma products, the most commonly envisioned uses are diffusive uptake/ release of materials and/ or deposition onto skin or tissue. In some cases, the bottom-up dilution process is favoured so that cubosomes form only during consumer use, such as by dilution via ingestion or sweating. Cubosomes are formed by sufficient dilution of liquid precursors prepared in the binary monoolein-ethanol system.

Liquid cubosome precursors

High-energy processes for formation of cubosomes can be expensive, difficult to scale up, and harmful to fragile temperature-sensitive active ingredients like proteins. The hydrotrope dilution process is found to consistently produce smaller, more stable cubosomes. In concept, particles are formed by nucleation and growth. This is achieved by dissolving the monoolein in a hydrotrope, such as ethanol, that prevents liquid crystalline formation. Subsequent dilution of this mixture spontaneously ‘crystallises’ or precipitates the cubosomes. The liquid precursor process allows for easier scale up of cubosome preparations and avoids bulk solids handling and potentially damaging high energy processes.

Powdered cubosome precursors

Powdered cubosome precursors are powders composed of dehydrated surfactant coated with polymer. Hydration of the precursor powders forms cubosomes with a mean particle size of 600 nm. Spray drying produces encapsulated particles from a dispersion of solid particles in a concentrated aqueous polymer solution. The continuous and dispersed phases are sprayed through a nozzle to create suspension droplets that are contacted with a heated, dry air stream flowing in the opposite direction. Excess water immediately evaporates, leaving dry powder particles composed of the dispersed phase encapsulated by the dissolved polymer. Finally, the polymer coating on the powder imparts surface properties to the hydrated cubosomes that can be tailored by proper selection of the encapsulating polymer. The production of starch-coated cubosome powder precursors requires high shear treatment of monoolein in aqueous starch solution to form a coarse cubosome dispersion that is then pumped through a nozzle and dried. The powder with the 3:1 ratio exhibits good encapsulation of the monoolein and small particle size.

Functionalised cubic phase liquid crystals

Functionalisation is achieved by incorporating amphiphilic molecules into the liquid crystal; the hydrophobic portion of the amphiphile inserts into the bilayers of the cubic phase and the hydrophilic portions extend into the water channels. It is assumed that the loading and release properties from cubic phase liquid crystals are governed solely by the solubilised active. In a more simplified manner, higher affinity of the active for the liquid crystal leads to higher loading. At lower pH, the active is more hydrophobic and can be loaded to higher levels. This is observed for a wide range of actives including Lidocaine, Prilocaine and Clomethiazole (CMZ) and Phenol Butylamine.

Proposed advantages

• Patient/ clinical value
• Increased convenience and compliance (orally, topically and intravenously).
• Improved bioavailability due to size
• Improved efficacy
• Decreased side effects associated with high initial plasma levels from rapid drug release on injection (drug burst)
• Decreased health care costs due to simplified handling and less frequent administration
• Decreased risks of drug misuse and misdirection

Pharma value

• Means of solubilising, encapsulating, and transporting APIs.
• Broadly applicable (to small molecules, nucleotides, peptides and proteins).
• Enormous surface area of several hundred square meters per gram
• High drug loading
• Allows true controlled release
• Stability enhancement (improved shelf-life and/or in vivo stability)
• Allows targeting (e.g. of cancer drugs)
• Means of protecting sensitive molecules from degradation in vivo.
• Solubilisation of poorly water-soluble drugs could improve bioavailability
• Decrease of unwanted side effects,
• Improvement of intracellular penetration

Applications

• Monoglyceride-based cubosome dispersion can be proposed for topical use, such as for perctuneous or mucosal applications. Because of the microbicidal properties of monoglycerides, could be used to design intravaginal treatment of sexually transmitted diseases caused by viruses (e.g. HSV, HIV) or by bacteria (eg. Chlamydia trachomatis and neisseria genorrticae).
• Due to similarity between the cubic phase structure and the structure of the stratum corneum, it is reasonable to suppose the formation of mixture of cubosomal monolein with stratum corneum lipids. This kind of interaction might lead to the formation of a cubosome depot in this layer, from which drug can be released in a controlled fashion. The cubosome technology is used to develop a synthetic vernix.
• Indomethacin is an anti-inflammatory drug with potential side effects when administrated orally. The indomethacin-loaded cubosomes were suitable nanocarriers for efficiently prolonging anti-inflammatory activity and for controlling drug release.
• Cubosome particles are used as oil water emulsion stabilisers and pollutant absorbents in cosmetics.
• Cubosomes as an ophthalmic drug delivery system for Flurbiprofen (FB) to reduce ocular irritancy and improve bioavailability.
• As site-directed drug delivery or drug targeting, such formulation principles are of major interest in cancer chemotherapy, certain malignant diseases, for good prognosis which requires removal of the primary tumour by surgery as well the prevention of the development of lymphatic metastases. Current research and experimentation on the antineoplastic drugs e.g Darbazine loaded onto cubosomes has been carried out successfully.
• Cubic phase is attractive for controlled release because of its small pore size (ca. 5–10 nm); its ability to solubilise hydrophobic, hydrophilic, and amphiphilic molecules bio-degradability by simple enzyme action.
• Cubic phase is strongly bioadhesive and is thought to be a skin penetration enhancer.
• Cubosomes find applications in Parkinson’s disease using Bromocryptine and Levodopa.
• An oral lipid cubosomal formulation for delivery of insulin had been proposed by Chung et al (2002).

Clinical evaluation of skin conditioning by cubosomes

The first study contrasts the effects of two cubic phase formulations with two standard barrier creams containing petrolatum as the primary lipid base. The results indicate that the cubic phases are highly permeable to water and fail to exhibit a clear dose response as a function of film thickness. The formulations in the clinical study include:

1. 75 per cent monoolein, 25 per cent water,
2. 75 per cent monoolein, 20 per cent water, 5 per cent glycerin
3. petrolatum (long chain hydrocarbon mixture, 0 per cent water), and Eucerin cream (petrolatum, lanolin, 17 per cent water)

The effects of the various treatments on water handling properties are evaluated by means of the sorption-desorption test. In this standardised test, surface electrical capacitance readings are measured prior to and at measured intervals following topical application and removal of exogenous water.

The preliminary studies allow the following general conclusions:

1. Bulk cubic phases are difficult to handle and difficult to apply to human skin. In contrast, the relatively anhydrous lamellar phase of the monoolein-water admixture is relatively fluid and easy to apply.
2. The paradoxical addition of exogenous water to the lamellar phase of a topically applied monoolein-water admixture results in formation of the more viscous cubic architecture. Thus, the simple addition or removal of exogenous water provides a means of controlling the phase behaviour and, thus, the physical nature of the topical gel.
3. The cubic phase is highly vapour permeable when measured over human skin. Vapour permeability combined with a physical barrier is a desirable characteristic in wound healing applications. High viscosity and high vapour permeability are two physical properties distinguishing monoolein – water cubic phases from occlusive skin ointments such as petrolatum.
4. The cubic phase is hygroscopic on human skin as judged by instrumental tests such as the sorption – desorption test.
5. In the study presented, twice daily application of a cubic phase to the dry lower legs of normal adult females evoked increased erthyema, increased visual dryness, and increased trans-epidermal water loss compared to a standard glycerin-containing, petrolatum – based emollient.

Cubosomes as injectable formulations

The lipid liquid crystal gel technology is known as FluidCrystal and the nanoparticle systems featuring Cubosome as one of the nanoparticle carriers.

1. FluidCrystal is applied in the formulation of depot injection products for subcutaneous, intramuscular and intracavital administration.
2. FluidCrystal NP is applied in the formulation of intravenous (IV) products.

Be it in a prefilled syringe or vial, the product is presented as a simple non-aqueous liquid pre-concentrate. Only after injection, in situ on contact with minute quantities of aqueous fluid, does the inactive precursor transform into the active delivery system – a controlled release liquid crystal matrix.

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Products under development

Testosterone derivatives and opiates; multiple peptide agents including, Octreotide, Leuprolide, Somatostatin, Salmon Calcitonin, GLP-1 and analogues; and several therapeutic proteins have been used in the liquid crystalline formulations.

Two injectable products are currently in clinical development, including CAM2032, a long-acting LHRH agonist for the treatment of prostate cancer after a single subcutaneous administration of three different doses.

Another clinical-stage product is CAM2029, a long-acting formulation of Octreotide, for the treatment of acromegaly, carcinoid syndrome and vasoactive intestinal peptide (VIP) producing tumours.

Conclusion

Cubosomes are intriguing self-assembled materials with tremendous potential in areas as diverse as medicine, materials science, and consumer products. The discovery of cubosomes has urged a broad level of investigation that, as proposed applications become financially attractive, will continue to narrow and fill in many of the current gaps in our knowledge of cubosome formation and performance. Bicontinuous cubic liquid crystalline phases, either in bulk or cubosome form, offer unique properties of particular interest to the personal care industry for use in treatments of skin, hair, and other body tissue.

Interdisciplinary research in engineering, biology, medicine, and chemistry will be especially essential to bind together existing cubosome research and to provide a comprehensive understanding of these fascinating particles. The ability to form cubosomes either in use, during formulation, or during manufacture offers greatly enhanced flexibility for product development efforts. Cubic phase systems hold promise as drug delivery vehicles and platforms for adhesives, skin protectants, and biomonitoring devices. The utility of these binary nanostructured systems can be extended by the ability to control the physical phase of the system; e.g., the transition from lamellar to cubic phase and the use of cubosome powder precursor. Probably the most compelling direction of research into these systems seeks to understand the interactions between bicontinuous structures and biological systems in general.

(With inputs from Sucheta A D’sa, Priyank R Parikh, Rajvi J Wani from Saraswathi Vidya Bhavan’s College of Pharmacy, Dombivli and Vivekanand Education Society’s College Of Pharmacy, Mumbai)