Formulation and Physicochemical Characterization of Imwitor 308 Based Self Microemulsifying Drug Delivery Systems

Abstract

Microemulsions are thermodynamically stable, transparent, isotropic systems, consisting of oil, water and surfactant with or without a cosurfactant. They are considered ideal for drug delivery due to their low viscosity, ease of preparation, improved drug stability and solubility, small droplet size, surfactant provoked permeability and protection against enzymatic degradation.1) The later characteristics are responsible for improved permeability of the drug across biological membranes including the skin. Although the oil and surfactant constituents are known to enhance the solubility of lipophilic drugs, at higher weight fraction of water (j), their solubility may be greatly compromised. To overcome this effect microemulsions containing cyclodexterine have been investigated. Incorporation of cyclodexterines has been shown to improve the solubility of the model steroid drug progesterone in selected microemulsions by 3300 folds.2) Self Microemulsifying Drug Delivery Systems (SMEDDS) can be considered a novel alternative to conventional transdermal delivery systems.3) SMEDDS are preconcentrated (waterfree) microemulsions, made up of oils and surfactants. Upon dilution with an aqueous medium and gentle agitation, these systems can form microemulsions. 4) Enhanced solubility and improved bioavailability are amongst the main advantages of SMEDDS.Before SMEDDS can be used as drug delivery systems it is necessary to characterize the internal structure of the resulting microemulsion. This is highly critical as multiple colloidal and coarse dispersions may coexist. Amongst these are microemulsions, coarse emulsions, various liquid crystalline systems (hexagonal, reverse hexagonal, lamellar, cubic) and gels. These can form upon mixing oils, surfactants and water. Moreover microemulsions are known to have different microstructures namely oil in water (o/w), water in oil (w/o) and bicontinuous. 5) Elucidating the internal structure of microemulsions is of a great interest as the phase behaviour of these systems can influence drug solubility, stability and in vitro release. Phase behaviour of microemulsions can be monitored through visual inspection, measuring electrical conductivity, viscosity, droplets size, density determination, surface tension measurements, and diffusion coefficient measurements. Visual observation is one of the first and foremost techniques used to establish the phase boundaries and distinguish microemulsions from coarse emulsions, liquid crystals and gels. To further differentiate microemulsions form lamellar liquid crystals one can take advantage of other experimental techniques such as polarized light microscopy and viscosity measurements. In general microemulsions display Newtonian flow with low viscosity whilst liquid crystals exhibit non-Newtonian flow and are of relatively high viscosity.5) Furthermore viscosity can also be used to study the microstructure of microemulsions. [6][7][8] Normally o/w microemulsions have a lower viscosity compared to w/o microemulsions in which the more viscous oil ...

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