Microemulsion-based media as novel drug delivery systems

Retention modelling in liquid chromatographic separation of simvastatin and six impurities using a microemulsion as eluent

A novel and unique approach was used for retention modelling in the separation of simvastatin and six impurities by liquid chromatographic using a microemulsion as mobile phase. A microemulsion is a modification of a micellar system where a lipophilic organic solvent is dissolved in the micelles; for that reason, microemulsions are usually treated as solvent-modified micellar solutions. When microemulsions are used as eluents in HPLC separations, solutes partition between the charged oil droplets and the aqueous buffer phase. The complexity of the composition of the microemulsion permits extensive manipulations to be made during method development in order to achieve acceptable resolution of such a complex mixture of substances. In order to avoid a laborious "trial and error" procedure, a 2(3) full factorial design was applied for choosing an optimal microemulsion composition to obtain good separation in a reasonable run time. Organic solvent, sodium dodecyl sulphate, and n-butanol content were varied within defined experimental domain. Optimal conditions for the separation of simvastatin and its six impurities were obtained using an X Terra 50 x 4.6 mm, 3.5 microm particle size column at 30 degrees C. The mobile phase consisted of 0.9% w/w of diisopropyl ether, 2.2% w/w of sodium dodecylsulphate (SDS), 7.0% w/w of co-surfactant such as n-butanol, and 89.9% w/w of aqueous 25 mM disodium phosphate pH 7.0.

Pluronic microemulsions as nanoreservoirs for extraction of bupivacaine from normal saline

We hypothesized that custom-designed microemulsions would effectively scavenge compounds from bulk media. Pluronic-based oil-in-water microemulsions were synthesized that efficiently reduced the free concentration of the local anesthetic bupivacaine in 0.9% NaCl. Both the molecular nature and concentration of the constituents in the microemulsions significantly affected extraction efficiencies. Pluronic F127-based microemulsions extracted bupivacaine more efficiently than microemulsions synthesized using other Pluronic surfactants (L44, L62, L64, F77, F87, F88, P104). Extraction was markedly increased by addition of fatty acid sodium salts due to greater oil/water interface area, increased columbic interaction between bupivacaine and fatty acids sodium salt, and greater surface activity. These data suggest that oil-in-water microemulsions may be an effective agent to treat cardiotoxicity caused by bupivacaine or other lipophilic drugs.

Characterization of caprylocaproyl macrogolglycerides based microemulsion drug delivery vehicles for an amphiphilic drug

Microemulsion systems composed of water, isopropyl myristate, PEG-8 caprylic/capric glycerides (Labrasol), and polyglyceryl-6 dioleate (Plurol Oleique), were investigated as potential drug delivery vehicles for an amphiphilic model drug (diclofenac diethylamine). Pseudo-ternary phase diagram of the investigated system, at constant surfactant/cosurfactant mass ratio (Km 4:1) was constructed at room temperature by titration, and the oil-to-surfactant/cosurfactant mass ratios (O/SC) that exhibit the maximum in the solubilization of water were found. This allowed the investigation of the continuous structural inversion from water-in-oil to oil-in-water microemulsions on dilution with water phase. Furthermore, electrical conductivity (sigma) of the system at Km 1:4, and O/SC 0.250 was studied, and the percolation phenomenon was observed. Conductivity and apparent viscosity (eta') measurement results well described colloidal microstructure of the selected formulations, including gradual changes during their formation. Moreover, sigma, eta', and pH values of six selected microemulsion vehicles which differ in water phase volume fraction (phi(w)) at the selected Km and O/SC values, were measured. In order to investigate the influence of the amphiphilic drug on the vehicle microstructures, each system was formulated with 1.16% (w/w) diclofenac diethylamine. Electrical conductivity, and eta' of the investigated systems were strongly affected by drug incorporation. The obtained results suggest that diclofenac diethylamine interacts with the specific microstructure of the investigated vehicles, and that the different drug release kinetics from these microemulsions may be expected. The investigated microemulsions should be very interesting as new drug carrier systems for dermal application of diclofenac diethylamine.

Influence of carrageenan on the rheology and skin permeation of microemulsion formulations

Three different microemulsions (A-C) and one semisolid preparation D were investigated in terms of viscoelastic properties and skin permeation of the model compound sodium fluorescein. The influence of the polysaccharide carrageenan on these parameters was investigated. Carrageenan is frequently used as food additive and has very interesting properties like good adhesiveness on skin which can be a benefit for topical application. The viscoelastic properties of the preparations (A-D) and of mixtures with carrageenan (A'-D') were characterized by oscillatory measurements. It was possible to adjust the rheologic properties of the formulations and to increase the sodium fluorescein permeation through porcine skin by mixing them with carrageenan gels. Therefore, the presented formulations as well as mixtures with carrageenan might be promising alternative drug carrier systems for topical pharmaceutical as well as cosmetics.

Effect of colloidal carriers on ascorbyl palmitate stability

Active compounds can be protected against degradation by incorporation into colloidal carrier systems. The stabilizing effect of carrier systems for ascorbyl palmitate (AP) was investigated using microemulsions (ME), liposomes and solid lipid nanoparticles (SLN). Analysis of chemical stability by HPLC showed that AP is most resistant against oxidation in non-hydrogenated soybean lecithin liposomes, followed by SLN, w/o and o/w ME, and hydrogenated soybean lecithin liposomes. The molecular environment of the AP-like nitroxide probe (C(16)-Tempo) in colloidal carriers was characterized using electron paramagnetic resonance (EPR) spectroscopy. We have found that the nitroxide groups are located in environments with different polarity and mobility. The hydrophilic part of AP is the reactive moiety, and high stability is obtained in systems in which this part is exposed to a less polar environment. Additionally, the determined accessibility of nitroxide groups to reduction correlated well with the chemical stability of AP. It is more deeply immersed in the interface when entrapped in a liquid-state carrier than when applied in gel-state particles. Encapsulation of AP in SLN core leads to greater stability. We conclude that the location of the sensitive group of the drug-molecule in a carrier system is crucial for its stability.

Effect of added α-lactalbumin protein on the phase behavior of AOT–brine–isooctane systems

We have found that the presence of <1 wt% of the globular protein alpha-lactalbumin has a significant impact on the equilibrium phase behavior of dilute sodium bis(ethylhexyl) sulfosuccinate (AOT)/brine/isooctane systems. Nuclear magnetic resonance (NMR), Karl Fischer titration, and ultraviolet spectroscopy were used to determine the surfactant, oil, water, and protein content of the organic and aqueous phases as a function of the total surfactant and protein present. As a small amount of alpha-lactalbumin is added to the mixture, there is a substantial increase (up to 80%) in the maximum water solubility in the water-in-oil microemulsion phase. Dynamic light scattering measurements indicate that this increase is due to a decrease in the magnitude of the (negative) spontaneous curvature of the surfactant monolayer, as droplets swell in size. As the molar ratio of alpha-lactalbumin to AOT surpasses approximately 1:300, the partitioning of water, protein, and surfactant shifts to the excess aqueous phase, where soluble assemblies with positive curvature are detected by dynamic light scattering. Significant amounts of isooctane are solubilized in these aggregates, consistent with the formation of oil-in-water microemulsion droplets. Circular dichroism studies showed that the tertiary structure of the protein in the microemulsion is disrupted while the secondary structure is increased. In light of these findings, the protein most likely expands to a molten-globule type conformation in the AOT interfacial environment, but does not substantially unfold to become an extended chain.

Nanosizing: a formulation approach for poorly-water-soluble compounds

Poorly-water-soluble compounds are difficult to develop as drug products using conventional formulation techniques and are frequently abandoned early in discovery. The use of media milling technology to formulate poorly-water-soluble drugs as nanocrystalline particles offers the opportunity to address many of the deficiencies associated with this class of molecules. NanoCrystal Technology is an attrition process wherein large micron size drug crystals are media milled in a water-based stabilizer solution. The process generates physically stable dispersions consisting of nanometer-sized drug crystals. Nanocrystalline particles are a suitable delivery system for all commonly used routes of administration, i.e. oral, injectable (IV, SC, and IM) and topical applications. In addition, aqueous dispersions of nanoparticles can be post-processed into tablets, capsules, fast-melts and lyophilized for sterile product applications. The technology has been successfully incorporated into all phases of the drug development cycle from identification of new chemical entities to refurbishing marketed products for improving their performance and value.

Influence of ion pairing on topical delivery of retinoic acid from microemulsions

The purpose of the present study was to determinate the significance of ion pairing on the topical permeation of retinoic acid (R.A) using microemulsions as delivery vehicles. Phenylalanine methyl ester, phenylalanine ethylester, histidine methyl ester, tryptophan methyl ester and valine methyl ester were used as counter ions. Results of diffusion studies through polydimethylsiloxane membrane (PDMS) indicate that retinoic acid permeation from ethanol-pH 6.4 buffer mixture significantly increased in the presence of counter ions. A linear relationship was found between apparent partition coefficients and permeation coefficients. The highest values were with valine methyl ester and phenylalanine ethyl ester. In order to develop alternative formulations for topical administration of R.A, microemulsions were evaluated as delivery vehicles. Oil-in-water (O/W) and water-in-oil (W/O) microemulsion formulations were prepared using water, isopropyl myristate, lecithin, caprylyl-capryl glucoside and ethanol or 1,2 hexanediol. Experiments with PDMS membranes showed decreasing permeabilities of R.A from microemulsions in the presence of counter ions. This was related to the increased lipophilicity and different vehicle membrane affinity of the ion pairs The ability of the systems to deliver R.A through the skin was evaluated in vitro using pig-skin. R.A permeabilities were much lower with microemulsions than with solution, while a large increase in R.A skin deposition was observed only from O/W microemulsions in the presence of counter ions. The depth of skin accumulation was below 100 microm after 24 h application. The results suggest that O/W microemulsions containing a counter ion can be used to optimise drug targeting without a concomitant increase in systemic absorption.

Light-scattering studies of testosterone enanthate containing soybean oil/C18:1E10/water oil-in-water microemulsions

Total-intensity light scattering (TILS) and photon correlation spectroscopy (PCS) techniques have been used to determine the droplet size of concentrated, oil-in-water microemulsions formed from soybean oil, polyoxyethylene-10-oleyl ether (C(18:1)E(10)), and water, both in the presence and absence of the lipophilic drug, testosterone enanthate. The TILS data were analyzed using the hard-sphere model of Percus-Yevick to account for interparticulate interactions experienced in the concentrated systems studied and the volume fraction of the hard-sphere droplet obtained from these analyses used to correct the PCS data. Correction of the light-scattering data in this manner yielded a satisfactory agreement between the size of the microemulsion droplets calculated using both techniques. Both the TILS and PCS data showed that, for a constant surfactant concentration, the size of the microemulsion droplets increased with increasing oil content. For example, droplets of radius 52.5 and 65.1 A (as determined by TILS) were obtained at 10.0% w/w C(18:1)E(10) and 0.5 and 2.0% w/w soybean oil, respectively. In contrast, for a constant oil concentration, microemulsion droplet size decreased with increasing surfactant concentration. For example, droplet sizes of 65.1, 59.3, 56.6, 54.5, and 53.3 A were seen with 2.0% w/w soybean oil and 10, 14.0, 18, 22, or 26% w/w C(18:1)E(10), respectively. Furthermore, in the presence of 1.0% w/w of the lipophilic drug, testosterone enanthate, the size of the microemulsion droplet increased by about 6-10 A depending on the concentration of the surfactant; the higher the concentration of the surfactant, the smaller the increase in size. The slight increase in size of the drug-containing microemulsion droplets suggests that some of the drug has penetrated into the core of the droplet.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Time-dependent solubilization of IgG in AOT-brine-isooctane microemulsions: role of cluster formation

The stability and structure of protein-containing water-in-oil (w/o) microemulsions were investigated by using the large protein immunoglobulin G (IgG, MW 155,000) in a mixture comprised of brine, sulfosuccinic acid bis [2-ethylhexyl]ester (sodium salt), and isooctane. We explored factors affecting the initial uptake of IgG into the w/o microemulsion and its subsequent release to a solid (precipitate) phase, and the kinetics of the latter process. Influences of such parameters as pH, ionic strength, and protein concentration on the solubilization and precipitation of bovine IgG in the organic phase are described. The structure and dynamics in microemulsions containing bovine IgG were probed by using dynamic light scattering, and it was found that the presence of IgG in the microemulsion induced strong attractive forces between the droplets. Based on results obtained by using these various experimental approaches, a model for protein solubilization and release is proposed. In this model, we propose the formation of clusters within which bovine IgG resides and which substantially slow the kinetics of protein release from the droplets to the precipitate phase.

Gel-microemulsions as vaginal spermicides and intravaginal drug delivery vehicles

There is a need for novel formulations to improve the bioavailability through the vaginal/rectal mucosa of microbicidal drug substances against sexually transmitted diseases. In addition, there is a need for more effective and less toxic vaginal spermicides. Here we review our recent discovery of novel gel-microemulsions (GM) as nontoxic, dual-function intravaginal spermicides, which can be used as delivery vehicles for lipophilic drug substances targeting sexually transmitted pathogens. We describe the formulation and biologic properties of 2 novel, submicron-particle-size GMs, GM-4 and GM-144, which were prepared from commonly available pharmaceutical excipients. These GMs comprising oil-in-water microemulsion and polymeric hydrogels were designed to solubilize lipophilic antiviral/antimicrobial agents and exhibited rapid spermicidal activity in human semen. Preclinical studies comparing the in vivo contraceptive efficacy of GM-4 and GM-144 versus nonoxynol-9-based detergent spermicide (Gynol II) in the rigorous rabbit model confirmed the potent contraceptive activity of these GMs. Unlike nonoxynol-9, repeated intravaginal applications of GM-4 and GM-144 in the rabbit vaginal irritation test were not associated with local inflammation or damage of the vaginal mucosa or epithelium. Furthermore, in short-term toxicity studies performed in mice, repetitive intravaginal application of spermicidal GM-4 and GM-144 for up to 13 weeks was not associated with any local, systemic, or reproductive toxicity. Spermicidal GMs have unprecedented potential as dual function microbicidal contraceptives to improve vaginal bioavailability of poorly soluble antimicrobial agents without causing significant vaginal damage.