Structural characteristics of oil-poor dilutable fish oil omega-3 microemulsions for ophthalmic applications

Docosahexaenoic acid (DHA) promotes synthesis of anti-inflammatory prostaglandins and relief of dry eye symptoms. However, topical ophthalmic application of DHA is difficult because of its lipophilic property. Therefore, it is important to develop aqueous-based formulation with enhanced capabilities. Novel, unique water-dilutable microemulsions (MEs) were constructed to allow loading of naturally occurring rigid long-chain triglyceride of DHA (TG-DHA). The TG-DHA serves as solubilizate and as the oil phase, therefore preparation is poor in oil. The structural transformations of MEs upon water dilution were studied by SAXS, viscosity, electrical conductivity, self-diffusion NMR, DSC, cryo-TEM, and DLS techniques. At low water content a new type of water-in-oil (W/O) structure is formed. The glycerol/water phase hydrates the headgroups of surfactants, and the oil solvates their tails, forming "ill-defined bicontinuous domains". Upon further water dilution more structured bicontinuous domains of high viscosity are formed. After additional dilution, the mesophases invert to oil-in-water (O/W) droplets of ∼8nm. In the structures composed of up to 25wt% water, the TG-DHA spaces and de-entangles the surfactant tails. Once the bicontinuous structures are formed, the surfactants and TG-DHA content decrease and their interfacial layer shrinks, leading to entanglement and buildup of viscous non-Newtonian mesophase. Above 70wt% water TG-DHA is embedded in the core of the O/W droplets, and its effect on the droplets' structure is minimal. This new dilutable ill-defined microemulsion can be a potential delivery vehicle for ophthalmic TG-DHA transport.

Water-dilutable microemulsions for transepithelial ocular delivery of riboflavin phosphate

Riboflavin phosphate (RFP) is an essential compound in the treatment of keratoconus - a degenerative, non-inflammatory disease of the cornea. Currently, the quantitative and efficient transport of riboflavin to the cornea is possible after mechanical removal of the epithelium. To avoid surgical intervention, it is therefore important to develop a method for quantitatively transporting riboflavin across the intact epithelium. In the present study, an RFP-loaded microemulsion was prepared, which could potentially function as an ocular drug delivery system crossing the eye epithelium. The specially designed water-dilutable microemulsion was based on a mixture of nonionic surfactants. Propylene glycol and glycerol acted as cosurfactant and cosolvent assisting in the solubilization of the RFP. The glycerol-rich water-free concentrate consisted of direct micelles for which glycerol served as the hydrophilic phase. In formulations with up to 40wt% water, the hydrophilic surfactant headgroups and glycerol strongly bind water molecules (DSC and SD-NMR). Above 60wt% water, globular, O/W nanodroplets, ∼14nm in diameter, are formed (SAXS, cryo-TEM, and SD-NMR). The structure of microemulsions loaded with 0.14-4.25wt% RFP (0.29-8.89mmol per 100g formulation) is not significantly influenced by the presence of the RFP. However, in the microemulsions containing 10-80wt% water, the mobility of RFP in the microemulsion is constrained by strong interactions with the surfactants and cosurfactant, and therefore free transport of the molecule can be achieved only upon higher (>80wt%) water dilutions.

Solubilization of simvastatin and phytosterols in a dilutable microemulsion system

The usual treatment of hypercholesterolemia includes a class of drugs known as statins (simvastatin among them), which inhibit the production of cholesterol. Another way of reducing cholesterol levels is with the use of phytosterols, which reduce the transport of exogenic cholesterol from the intestine into the blood stream. The two treatments can be combined, achieving an additive effect. However, both simvastatin and phytosterols are practically insoluble in water, and therefore their absorption and activity are low. Nanosized self-assembled structured liquid systems are modified microemulsions that present an alternative pathway for improving the bioavailability of poorly water-soluble drugs. The goal of this study was to solubilize the maximal quantity of both simvastatin and phytosterols in a single, fully dilutable microemulsion system. We constructed a water-dilutable liquid drug delivery system that includes sucrose monolaurate, propylene glycol, and oleyl lactate. This system exhibits high solubilization capacity for both simvastatin (7.0 wt%) and phytosterols (3.5 wt%) when each is solubilized separately in a water-free concentrate. When simvastatin and phytosterols were solubilized together at a wt ratio of 2.5:1, maximum solubilization was obtained with 4.7 wt% simvastatin and 1.9 wt% phytosterols. Structural and analytical methods were applied including rheology, DSC, SD-NMR, SAXS, and cryo-TEM. The water-free "concentrate" consisted of direct micelles for which propylene glycol served as the hydrophilic phase. Upon water dilution, the direct micelles appear to form "lipophilic compounds dispersed in hydrophilic continuous phase". The solubilizates are located in the droplet core and/or at the interface.

Dielectric analysis and Differential Scanning Calorimetry of water-in-oil microemulsions

The system investigated is a multicomponent thermodynamically stable liquid consisting of water spherical droplets, (approximately 10 nm diameter), stabilized by a surfactant shell and dispersed in a continuous oil phase (microemulsions). A comparative analysis is carried out among the results obtained with three different experimental approaches, namely, dielectric analysis, Thermally Stimulated Depolarization, and Differential Scanning Calorimetry. The different measurement procedures are briefly described. The attention was focused on the system water concentration. Two microemulsions, differing only in the oil used as continuous phase, were studied. For any concentration a comparison between the results obtained with the different experimental methods was undertaken. The evolution with water addition of the water/oil interphase region was analysed in some detail.

Freezing behaviour of microencapsulated water

The freezing behaviour of water in polyurea microcapsules was studied through DSC (differential scanning calorimetry) and ESR (electron spin resonance) measurements under a non-equilibrium condition to show that supercooling of water becomes more noticeable with decreasing droplet size of the liquid. Thermodynamics of small systems was found applicable to analyse the experimental findings, even though the process of water freezing in the microcapsules was not of an equilibrium nature.