Phase studies and particle size analysis of oil-in-water phospholipid microemulsions

Nanoemulsion: A Review on Mechanisms for the Transdermal Delivery of Hydrophobic and Hydrophilic Drugs

Nanoemulsions (NEs) are colloidal dispersions of two immiscible liquids, oil and water, in which one is dispersed in the other with the aid of a surfactant/co-surfactant mixture, either forming oil-in-water (o/w) or water-in-oil (w/o) nanodroplets systems, with droplets 20–200 nm in size. NEs are easy to prepare and upscale, and they show high variability in their components. They have proven to be very viable, non-invasive, and cost-effective nanocarriers for the enhanced transdermal delivery of a wide range of active compounds that tend to metabolize heavily or suffer from undesirable side effects when taken orally. In addition, the anti-microbial and anti-viral properties of NE components, leading to preservative-free formulations, make NE a very attractive approach for transdermal drug delivery. This review focuses on how NEs mechanistically deliver both lipophilic and hydrophilic drugs through skin layers to reach the blood stream, exerting the desired therapeutic effect. It highlights the mechanisms and strategies executed to effectively deliver drugs, both with o/w and w/o NE types, through the transdermal way. However, the mechanisms reported in the literature are highly diverse, to the extent that a definite mechanism is not conclusive.

Development of microemulsions for ocular delivery

Microemulsions (MEs) are thermodynamic stable dispersion of oily phase and aqueous phase stabilized by surfactants and co-surfactants, and are a small droplet size of less than 100 nm. MEs are appropriate systems for ocular drug delivery because they improve ocular drug retention, extended duration of action, high ocular absorption, permeation of loaded drugs and effortlessness of preparation and administration. This review is an effort to summarize the recent development in the area of MEs, self-emulsifying drug delivery systems, which are examined in relation to their uses in ocular drug delivery. The noteworthy patent, toxicity and stability issues related to these ME systems are also explored here.

Microemulsion: new insights into the ocular drug delivery

Delivery of drugs into eyes using conventional drug delivery systems, such as solutions, is a considerable challenge to the treatment of ocular diseases. Drug loss from the ocular surface by lachrymal fluid secretion, lachrymal fluid-eye barriers, and blood-ocular barriers are main obstacles. A number of ophthalmic drug delivery carriers have been made to improve the bioavailability and to prolong the residence time of drugs applied topically onto the eye. The potential use of microemulsions as an ocular drug delivery carrier offers several favorable pharmaceutical and biopharmaceutical properties such as their excellent thermodynamic stability, phase transition to liquid-crystal state, very low surface tension, and small droplet size, which may result in improved ocular drug retention, extended duration of action, high ocular absorption, and permeation of loaded drugs. Further, both lipophilic and hydrophilic characteristics are present in microemulsions, so that the loaded drugs can diffuse passively as well get significantly partitioned in the variable lipophilic-hydrophilic corneal barrier. This review will provide an insight into previous studies on microemulsions for ocular delivery of drugs using various nonionic surfactants, cosurfactants, and associated irritation potential on the ocular surface. The reported in vivo experiments have shown a delayed effect of drug incorporated in microemulsion and an increase in the corneal permeation of the drug.

Lecithin-based microemulsions for targeted delivery of ceramide AP into the stratum corneum: formulation, characterizations, and in vitro release and penetration studies

PURPOSE

To improve the solubility and penetration of Ceramide AP (CER [AP]) into the stratum corneum that potentially restores the barrier function of aged and affected skin.

METHODS

CER [AP] microemulsions (MEs) were formulated using lecithin, Miglyol® 812 (miglyol) and water-1,2 pentandiol (PeG) mixture as amphiphilic, oily and hydrophilic components, respectively. The nanostructure of the MEs was revealed using electrical conductivity, differential scanning calorimeter (DSC) and electron paramagnetic resonance (EPR) techniques. Photon correlation spectroscopy (PCS) was used to measure the sizes and shape of ME droplets. The release and penetration of the CER into the stratum corneum was investigated in vitro using a multi-layer membrane model.

RESULTS

The MEs exhibited excellent thermodynamic stability (>2 years) and loading capacity (0.5% CER [AP]). The pseudo-ternary phase diagrams of the MEs were obtained and PCS results showed that the droplets are spherical in shape and bigger in size. In vitro investigations showed that the MEs exhibited excellent rate and extent of release and penetration.

CONCLUSIONS

Stable lecithin-based CER [AP] MEs that significantly enhance the solubility and penetration of CER [AP] into the stratum corneum were developed. The MEs also have better properties than the previously reported polyglycerol fatty acid surfactant-based CER [AP] MEs.

Novel dithranol phospholipid microemulsion for topical application: development, characterization and percutaneous absorption studies

The objective of this study was to develop and characterize a novel dithranol-containing phospholipid microemulsion systems for enhanced skin permeation and retention. Based on the solubility of dithranol, the selected oils were isopropyl myristate (IPM) and tocopherol acetate (TA), and the surfactants were Tween 80 (T80) and Tween 20 (T20). The ratios of cosurfactants comprising of phospholipids and ethanol (1 : 10) and surfactant to co-surfactant (1 : 1 and 2.75 : 1) were fixed for the phase diagram construction. Selected microemulsions were evaluated for globule size, zeta potential, viscosity, refractive index, per cent transmittance, stability (freeze thaw and centrifugation), ex vivo skin permeation and retention. The microemulsion systems composed of IPM and T80 with mean particle diameter of 72.8 nm showed maximum skin permeation (82.23%), skin permeation flux (0.281 mg/cm²/h) along with skin retention (8.31%) vis-à-vis systems containing TA and T20. The results suggest that the developed novel lecithinized microemulsion systems have a promising potential for the improved topical delivery of dithranol.

Dermal targeting of tacrolimus using colloidal carrier systems

In the therapy of chronic inflammatory skin diseases, the epicutaneous application of anti-inflammatory drugs in combination with maintenance therapy leads to ideal therapeutic long term effects. In this work, the development of well-tolerated colloidal carrier systems (ME) containing tacrolimus is described. A comprehensive physico-chemical characterization of the novel systems was performed using different techniques. The potential of three ME compared to an ointment as suitable carrier for dermal delivery of tacrolimus was determined. The penetration studies demonstrated that in comparison to the standard vehicle ointment, all three ME resulted in higher concentrations of tacrolimus in the deeper skin layers independent of the time of incubation. Particularly, the percentage of the bioavailable amount of tacrolimus (sum of the amount found in the dermis and acceptor compartment) from the ME with concentrations up to 20.95 ± 12.03% after 1000 min incubation time differed significantly (p<0.01), when compared to the ointment which yielded a concentration of 6.41 ± 0.57%. As a result of these experiments, using colloidal carrier systems, the penetration profile of tacrolimus was enhanced significantly (p<0.01). High drug amounts penetrated the target site in a short period of time after applying the ME.

Microemulsions--modern colloidal carrier for dermal and transdermal drug delivery

Microemulsions are modern colloidal drug carrier systems. They form spontaneously combining appropriate amounts of a lipophilic and a hydrophilic ingredient, as well as a surfactant and a co-surfactant. Due to their special features, microemulsions offer several advantages for pharmaceutical use, such as ease of preparation, long-term stability, high solubilization capacity for hydrophilic and lipophilic drugs, and improved drug delivery. The article summarizes the level of research with respect to dermal and transdermal application. A large number of in vitro as well as some in vivo studies demonstrated that drugs incorporated into microemulsions penetrate efficiently into the skin. The enhancing activity seems to be attributable to a variety of factors depending on the composition and the resulting microstructure of the formulations. However, an extended use in practice depends on the choice of well-tolerated ingredients, mainly surfactants, and the restriction of their amounts in order to guarantee skin compatibility.

Dermal Peptide Delivery Using Colloidal Carrier Systems

The advancement in synthetic and molecular biology techniques over the past years has resulted in the application of peptides or peptide-like drugs becoming a growing field in therapeutics. Because of the unfavorable chemical properties of peptides, it poses a challenge to find an optimized way of drug administration. The transdermal route has attracted interest as a promising way to advance the delivery of these drugs. The objective of this review is to summarize the level of research of microemulsions as colloidal carrier for dermal peptide drug delivery. The presented studies resulted in enhanced drug delivery or superior penetration profiles of peptides incorporated in microemulsions in comparison to conventional vehicles. Due to their benefits like high solubilization capacity, enhanced drug delivery, noninvasive administration or easy preparation, microemulsions offer a suitable vehicle for dermal and transdermal drug delivery.

Formulation of parenteral microemulsion containing itraconazole

The aim of this study was to develop an aqueous parenteral formulation containing itraconazole (ITZ) using an o/w microemulsion system. A mixture of benzyl alcohol and medium chain triglyceride (3/1) was chosen as the oil phase. Pseudoternary phase diagrams of the microemulsion formations were constructed in order to determine the optimum ratio of oils, the concentration range of surfactant and cosurfactant and the optimum ratio between them. Consequently, the suitability of the chosen microemulsion system as a parenteral formulation was evaluated using droplet size analysis and hemolysis tests. Among the surfactants and cosurfactants screened, a mixture of polyoxyethylene (50) hydrogenated castor oil and ethanol (3/1) showed the largest o/w microemulsion region in the phase diagram. The average droplet size of the microemulsions was < 150 nm, and the hemolysis test showed this formulation to be nontoxic to red blood cells. The pharmacokinetic profiles of the ITZ-microemulsion for itraconazole and its major metabolite, hydroxyitraconazole, were compared with those of a PEG 400 solution and cyclodextrin formulations in rats. Overall, these results highlight the potential of an ITZ-microemulsion formulation for the parenteral route.

Microemulsions: a potential delivery system for bioactives in food

Microemulsions are thermodynamically stable, transparent, low viscosity, and isotropic dispersions consisting of oil and water stabilized by an interfacial film of surfactant molecules, typically in conjunction with a cosurfactant. Microemulsions (so-called due to their small particle size; 5-100 nm) have found application in a wide variety of systems, such as pharmaceutical and oil recovery, but their application in food systems has been hindered by the types of surfactant permissible for use in food. The objective of this review is to provide an overview of the structures and phase behavior of microemulsions, methods of microemulsion formation, and techniques which may be used for characterization. A comprehensive review of previous work on both food-grade microemulsion systems, and non-food-grade systems of specific food interest is included. The application of microemulsions as reaction media, their ability to solubilize proteins and hence their use as a separation technique is also documented. In addition, attention is focused on the application of microemulsions as delivery systems for delivery of bioactive compounds, and the links between microemulsions and increased bioavailability. Future research, both applied and fundamental, should focus on surfactants which are not restricted for use in foods.

Lecithin-based microemulsion of a peptide for oral administration: preparation, characterization, and physical stability of the formulation

The objective of our study was to prepare and characterize a stable microemulsion formulation for oral administration of a peptide, e.g., rh-insulin. The microemulsions were prepared using Labrafil M 1944 CS, Phospholipon 90G (lecithin), absolute alcohol, and bidistilled water. Commercially available soybean lecithins (namely, Phospholipon 80, phosphatidylcholine purity 76 +/- 3%, and Phospholipon 90G, phosphatidylcholine purity 93 +/- 3%) were used in the study. The results showed that the phase diagram obtained using a low purity lecithin was not similar to that obtained with a high purity lecithin. We observed that the microemulsion area was wider at the phase diagram obtained with the higher purity lecithin. We found that the extent of the microemulsion region depended upon both the purity of the lecithin and the surfactant/co-surfactant (s/co-s) mixing ratios (K(m)). The rheological studies showed that microemulsions followed a Newtonian behavior. Such physical characteristics as viscosity, turbidity, density, conductivity, refractive index, droplet size, physical appearance, and phase separation of the microemulsion were measured at different temperatures (4 degrees C, 25 degrees C, and 40 degrees C) during 6 months. The results indicated that the physical characteristics of the developed microemulsions did not change under different storage temperatures (p > 0.05).

Lecithin-based oil-in-water microemulsions for parenteral use: pseudoternary phase diagrams, characterization and toxicity studies

Pseudoternary phase diagrams have been constructed to evaluate the phase behavior of systems containing water/lecithin/polysorbate 80/isopropyl myristate at different polysorbate 80:lecithin weight ratios (K(m)). Oil-in-water microemulsion regions were accurately determined and the influence of the K(m) on the area of existence of such disperse systems was also examined. Viscosity studies as well as particle size analysis by dynamic light scattering were carried out on oil-water microemulsions, and the influence of the oil phase content, the total amount of surfactants and K(m) on the rheological behavior, viscosity, and droplet size of such disperse systems was evaluated. All systems studied showed a water-rich isotrope region (oil-in-water microemulsion area), that was seen to be highly dependent upon the surfactant/cosurfactant weight ratio. Most of the microemulsions analyzed showed a non-Newtonian rheological behavior and both, droplet size, and viscosity of the disperse systems, were found to be much more influenced by the total content of oil phase and surfactants present in the microemulsion than by the K(m). The selected system underwent both stability and in vivo acute toxicity studies, and seemed to be highly stable, even at extreme conditions, and very low toxic according to the results obtained.

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.

Microemulsions as ocular drug delivery systems: recent developments and future challenges

Eye drops are the most used dosage form by ocular route, in spite of low bioavailability and the pulsed release of the drug. However, due to their intrinsic properties and specific structures, the microemulsions are a promising dosage form for the natural defence of the eye. Indeed, because they are prepared by inexpensive processes through autoemulsification or supply of energy, and can be easily sterilized, they are stable and have a high capacity of dissolving the drugs. The in vivo results and preliminary studies on healthy volunteers have shown a delayed effect and an increase in the bioavailability of the drug. The proposed mechanism is based on the adsorption of the nanodroplets representing the internal phase of the microemulsion, which constitutes a reservoir of the drug on the cornea and should then limit their drainage.

Microemulsion-based media as novel drug delivery systems

Microemulsions are clear, stable, isotropic mixtures of oil, water and surfactant, frequently in combination with a cosurfactant. These systems are currently of interest to the pharmaceutical scientist because of their considerable potential to act as drug delivery vehicles by incorporating a wide range of drug molecules. In order to appreciate the potential of microemulsions as delivery vehicles, this review gives an overview of the formation and phase behaviour and characterization of microemulsions. The use of microemulsions and closely related microemulsion-based systems as drug delivery vehicles is reviewed, with particular emphasis being placed on recent developments and future directions.

Phospholipid-based microemulsions of flurbiprofen by the spontaneous emulsification process

The purpose of this study was to investigate the possibility for parenteral delivery of flurbiprofen without chemical modification using a phospholipid-based microemulsion system. Microemulsions composed of ethyl oleate, lecithin and distearoylphosphatidyl-ethanolamine-N-poly(ethyleneglycol) 2000 (DSPE-PEG) were prepared using ethanol as a cosolvent. The effect of formulation variables on the particle size of the microemulsion was investigated. Flurbiprofen concentrations in plasma and various organs after the intravenous administration of flurbiprofen-loaded microemulsion were measured and compared with those after the intravenous administration of flurbiprofen axetil-entrapped emulsion (Lipfen(R), 50 mg/5 ml as flurbiprofen axetil) and flurbiprofen solution. Phospholipid-based microemulsions could solubilize more than 10 mg ml-1 of flurbiprofen at the ratio of vehicle to drug at least 10:1, if the oil contents (10 or 20%) of common parenteral emulsions were used. The half-life, AUC and MRT of flurbiprofen loaded in microemulsion (ethyl oleate:lecithin:DSPE-PEG:flurbiprofen=8:3:1:1.2) increased significantly. The biodistribution of flurbiprofen loaded in this microemulsion was quite different from others. Reticuloendothelial uptake of flurbiprofen loaded in microemulsion decreased compared with that in solution or Lipfen(R). It is concluded that the current microemulsion system might be applicable to formulate the parenteral dosage form of poorly water-soluble flurbiprofen without chemical modification.