Block copolymer stabilized nonaqueous biocompatible sub-micron emulsions for topical applications

Polyethylene glycol (PEG) 400/Miglyol 812 non-aqueous sub-micron emulsions were developed due to the fact that they are of interest for the design of drug-loaded biocompatible topical formulations. These types of emulsions were favourably stabilized by poly (2-vinylpyridine)-b-poly (butadiene) (P2VP-b-PBut) copolymer with DP_But > DP_2VP, each of these sequences being well-adapted to the solubility parameters of PEG 400 and Miglyol 812, respectively. This type of block copolymers, which might limit the Ostwald ripening, appeared to be more efficient stabilizers than low molecular weight non-ionic surfactants. The emulsion characteristics, such as particle size, stability and viscosity at different shear rates were determined as a function of the phase ratio, the copolymer concentration and storage time. It was further shown that Acyclovir, as a model drug of low water solubility, could be incorporated into the PEG 400 dispersed phase, with no significant modification of the initial emulsion characteristics.     

Amphiphilic association systems for Amphotericin B delivery

The present study describes the production and characterization of amphiphilic association systems for Amphotericin B (AMB). In particular, three different classes of microemulsions and different monoglyceride–water systems were produced. Formulations were characterized for macroscopic aspect, pH, rheology, mean size and size distribution, both in the absence and in the presence of AMB.

AMB solubility was investigated in the different formulations by HPLC studies. The formulations increased AMB solubility up to 20-fold with respect to the single oil and aqueous phases employed for microemulsion production.

AMB diffusion studies from two microemulsions taken as models were performed in a Franz cell system using a nylon membrane.

The physical and chemical stability of AMB-containing amphiphilic association systems were investigated for three months after production. For physical stability studies both the macroscopic aspect, droplet mean size and dimensional distribution were analysed. For chemical stability studies, the AMB content of the formulations was quantified by HPLC analysis. Microemulsions and monoglyceride–water systems were free from phase separation for up to three months and in some cases the AMB content was unchanged even after three months.     

Crystal growth studies involving phase transitions in aqueous drug suspensions

The growth of oxyclozanide crystals in quiescent suspensions has been monitored using the Coulter Counter. Increase in particle size is the result of an isothermal, solvent-mediated phase transition between two unsolvated polymorphs, one having a lower solubility than the other. Effects due to Ostwald ripening and temperature cycling are absent. Theophylline, which shows crystal growth in suspension by hydration, has been studied by photomicrography. Preliminary results indicate that the initial rate of growth in such systems may be qualitatively described in terms of steady-state diffusion theory.     

A study on the influence of emulsion droplet size on the skin penetration of tetracaine

OBJECTIVES/AIMS: The influence of emulsion droplet size on the skin penetration of a model drug, tetracaine, was studied. For this purpose, in vitro dermal and transdermal delivery of tetracaine from 6 emulsions (3 macro-emulsions with droplet sizes >1 microm and 3 nano-emulsions with droplet sizes <100 nm) were tested. METHODS: Two approaches were used: in the first one, the composition of the emulsions was kept constant, while in the second one, the surfactant concentration in the aqueous phase was kept constant by varying the overall surfactant concentration. RESULTS: The results from emulsions differing only in droplet size did not provide statistically significant evidence for the anticipated increase in transdermal or dermal delivery (after 24 h) when reducing emulsion droplet size. The same results were obtained when the surfactant concentration in the aqueous phase was kept constant, indicating that there is no influence of emulsion droplet size on the skin penetration of tetracaine within the droplet size range studied. CONCLUSION: This is in contrast to what has been reported in various publications that claim penetration to increase with reducing droplet size. It should be noted that the results reported so far are based on emulsions that apart from droplet size also differed in composition and/or system components.     

Chitosan nanoparticles as a dual growth factor delivery system for tissue engineering applications

The effect of stabilizer type (small molecule vs. polymeric) and the amount of micellar solubilized drug on Ostwald ripening of nanosuspensions was investigated. Indomethacin nanosuspensions were prepared with small molecule stabilizers (sodium lauryl sulfate (SLS) and Dowfax 2A1 (DF)) and a polymeric stabilizer (hydroxypropyl methyl cellulose (HPMC)). Two different drug:stabilizer ratios were used to evaluate the effect of micellar solubilized drug. The Ostwald ripening potential of nanosuspensions was evaluated by subjecting them to various stress conditions (temperature (15, 25, 35 and 45°C), thermal cycling, and mechanical shaking) for three months. The mean particle size increased in all SLS and DF formulations stored under different stress conditions. No effect of micellar solubilized drug on the Ostwald ripening rate was observed. In the case of HPMC formulations only those stored at higher temperatures (35 or 45°C) exhibited an increase in mean particle size. The increase in size in the HPMC formulation stored at 45°C was attributed to dehydration of the HPMC chains and subsequent loss of protection of the nanoparticles. The cube of the mean particle diameter versus time plot was determined to be non-linear for all formulations exhibiting Ostwald ripening. Therefore, according to the Lifshitz, Slyozov and Wagner theory the process was not diffusion controlled. The most probable mechanism for Ostwald ripening was surface nucleation controlled.     

A comparative study of top-down and bottom-up approaches for the preparation of micro/nanosuspensions

Nano-sizing offers a promising method for the formulation of poorly aqueous soluble compounds. Nanosuspensions can be prepared by top-down or bottom-up approaches. The different conditions encountered in these two approaches can greatly affect nanosuspension characteristics. In this study, milling via microfluidization and precipitation via sonication were compared to study their effects on the formation and stability of ibuprofen nanosuspensions. Various stabilizers (SLS, PVP K-30, Pluronic F-68 and F-127, Tween 80 and different hydroxypropyl methylcelluloses (HPMCs)) were evaluated. Both processes resulted in a similar trend in the initial particle size and comparable short-term physical stability of suspensions. Of all the stabilizers investigated, the HPMCs were the most effective both in terms of particle size reduction and short-term physical stability. Differences in stabilizer efficacy were observed between the two processing methods. The initial particle size of the suspensions prepared using microfluidization correlated with the solubility of ibuprofen in the respective stabilizer solutions. Whereas, the initial particle size of suspensions prepared using precipitation under sonication correlated with the HLB values of the stabilizers. The solubility of ibuprofen in the stabilizer solution also played a significant role in the increase in particle size on storage, indicating Ostwald ripening.     

Oxidation of linoleic acid encapsulated with gum arabic or maltodextrin by spray-drying

Linoleic acid was emulsified with gum arabic or maltodextrin at various weight ratios of the acid to the polysaccharide in the presence or absence of a small-molecule emulsifier. The emulsions were spray-dried to produce microcapsules. Emulsions prepared with gum arabic were smaller in droplet size and more stable than those prepared with maltodextrin, and linoleic acid in a gum arabic-based microcapsule was also most resistant to oxidation than that in a maltodextrin-based microcapsule. Although the oil droplet size in the emulsion with maltodextrin decreased and the emulsion stability was improved by addition of a small-molecule emulsifier to linoleic acid, the oxidative stability of the encapsulated linoleic acid was not significantly improved. Encapsulated linoleic acid of small droplet size oxidized more slowly than that of large droplet size.     

Correlation of physical parameters of an oil in water emulsion with manufacturing procedures and stability

The effect of nine process variables for the manufacture of a semi solid paraffin-in-water emulsion prepared with two nonionic surfactants, steareth 21 and sorbitan oleate were examined initially and following ageing for 2 and 4 years. The techniques employed included differential scanning calorimetry (DSC), small angle X-ray diffraction (SAXD) and microscope (differential interference contrast (DIC) and crossed polars). A previous paper (Lashmar, Int. J. Pharm., (1993) 59–67) used particle size analysis and theological techniques to evaluate the emulsions. It was concluded that the total DSC enthalpy change was an important indicator of the stability of an oil-in-water emulsion and that the appearance of a sample under crossed polars could be used as a guide. SAXD was not found to be suitable for evaluation of process parameters. The thermal properties of the emulsions and their appearance under crossed polars suggested that the homogenisation speed of the manufacturing vessel probably was the single most important factor for the production of a stable emulsion and that extending the homogenisation time would improve the stability of the emulsion. Cooling the emulsion slowly appeared to benefit its stability, whereas the speed at which the water phase was added to the oily phase during the emulsification process, the emulsification temperature and the speed of the agitator during cooling seemed to have little effect on the stability of the emulsion. Adding the surfactants to the aqueous phase before emulsification and adding the oil phase to the water phase during the emulsification produced inferior emulsions. On cooling, the optimum stability was achieved when the agitator was stopped around the temperature corresponding to the lower cooling exotherm for the sample. Results from this and the previous paper (Lashmar, 1993) indicated that the droplet size and the rheological properties could be manipulated by process parameters without affecting the stability of the product. A later paper will investigate other techniques to evaluate emulsions and manufacturing parameters.     

Oxidation of linoleic acid encapsulated with gum arabic or maltodextrin by spray-drying

Linoleic acid was emulsified with gum arabic or maltodextrin at various weight ratios of the acid to the polysaccharide in the presence or absence of a small-molecule emulsifier. The emulsions were spray-dried to produce microcapsules. Emulsions prepared with gum arabic were smaller in droplet size and more stable than those prepared with maltodextrin, and linoleic acid in a gum arabic-based microcapsule was also most resistant to oxidation than that in a maltodextrin-based microcapsule. Although the oil droplet size in the emulsion with maltodextrin decreased and the emulsion stability was improved by addition of a small-molecule emulsifier to linoleic acid, the oxidative stability of the encapsulated linoleic acid was not significantly improved. Encapsulated linoleic acid of small droplet size oxidized more slowly than that of large droplet size.     

Effect of oil loading on microspheres produced by spray drying

Oil-loaded microspheres were produced by spray drying emulsions consisting of fish oil and modified starch suspensions with different oil loadings. The emulsion stability was assessed by oil droplet size analysis. Microspheres were characterized in terms of size, morphology, yield and microencapsulation efficiency. It was found that an increase in oil loading resulted in emulsions containing larger oil droplets. This corresponded with larger mean microsphere diameters and rounder microspheres. However, high oil loadings produced lower yields and affected microencapsulation efficiencies.     

Effect of oil loading on microspheres produced by spray drying

Oil-loaded microspheres were produced by spray drying emulsions consisting of fish oil and modified starch suspensions with different oil loadings. The emulsion stability was assessed by oil droplet size analysis. Microspheres were characterized in terms of size, morphology, yield and microencapsulation efficiency. It was found that an increase in oil loading resulted in emulsions containing larger oil droplets. This corresponded with larger mean microsphere diameters and rounder microspheres. However, high oil loadings produced lower yields and affected microencapsulation efficiencies.     

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.     

Effect of methylcellulose on the stability of oil-in-water emulsions: influence of the disperse phase

The influence of nature of the disperse phase on the stability of oil-in-water emulsions containing nonionic emulsifiers and methylcellulose 4000 as an auxiliary emulsifier was investigated. One stable and three unstable base emulsions each of olive oil and of mineral oil were formulated with an emulsifier blend of Tween® and Span®. The stable emulsion (SE) contained 2% emulsifier blend optimized for maximum stability. Three unstable emulsions were formulated from the SE formulation: one with 0.5% emulsifier blend as of the SE formulation (UE1), one with excessive hydrophilic emulsifier (UE2) and one with excessive lipophilic emulsifier (UE3). A series of emulsions was prepared containing increasing amounts of methylcellulose 4000 for each base emulsion. The particle size of all olive oil emulsions was reduced (UE2>SE>UE3) and the viscosity was increased (UE2>SE>UE3) on addition of methylcellulose. The stability of these emulsions improved in the presence of methylcellulose. However, the addition of the polymer caused instability in mineral oil emulsions containing lower concentrations of the hydrophilic emulsifier (Tween®). These results suggest that: (i) methylcellulose and the hydrophilic emulsifier associate to form a complex; (ii) this complex when present at the mineral oil-water interface would be dislodged from the interface due to less interaction between the non-polar oil and the polyoxyethylene (POE) chain of the hydrophilic emulsifier; and (iii) this complex when present at the olive oil-water interface would stabilize emulsions due to higher interaction between the polar oil and the POE chain of the hydrophilic emulsifier.     

A novel stable supersaturated submicron lipid emulsion of tirilazad.

This paper describes a novel supersaturated submicron lipid emulsion for parenteral drug delivery. Tirilazad was used as an amphiphilic model drug for the preparation and characterization of the supersaturated emulsions with concentrations more than two times higher than the drug solubility in the emulsion. Analysis of particle size distribution using photon correlation spectroscopy indicated that the mean particle diameters of the supersaturated 10 and 20% lipid emulsions of tirilazad were approximately 210 and 240 nm, respectively, and that the mean particle size and size distribution of the supersaturated tirilazad emulsions were not different from those of the non-supersaturated tirilazad emulsions. The apparent viscosity of the supersaturated tirilazad emulsions, as measured by continuous rheometry, was similar to that of the non-supersaturated tirilazad emulsions. However, the zeta potential of the supersaturated tirilazad emulsions, as determined by the electrophoretic light-scattering technique, was found to be dependent on drug load and was lower than that of the non-supersaturated tirilazad emulsion. Fractionation by ultracentrifugation showed that at low drug loads, the amount of drug associated with the emulsion particles and the infranatant fraction appeared to increase linearly with increasing concentration of the drug. By contrast, the amount of drug associated with the emulsion particles increased rapidly, whereas the amount of drug associated with the infranatant fraction remained constant for emulsions with a high drug load. These results suggest that the excess drug in the supersaturated emulsions is largely associated with the lipid phase. Accelerated stability studies under stress conditions (i.e., autoclaving and shaking) and long-term storage stability tests demonstrated that the supersaturated tirilazad emulsions had excellent physical stability over the study period of 16 months.     

The influence of alkali fatty acids on the properties and the stability of parenteral O/W emulsions modified with solutol HS 15.

Arachis oil based parenteral O/W emulsions were prepared using soya bean phosphatidylcholine (SPC) and different combinations of co-emulsifiers containing polyethylene glycol fatty acid esters (Solutol HS 15) and alkali fatty acids (sodium laurate, sodium stearate). The parameters measured were droplet size (both by photon correlation spectroscopy and laser diffractometry), pH and zeta potential. All emulsions were subjected to autoclaving. The addition of polyethylene glycol 12-hydroxy stearate (Solutol HS 15) led to a significant decrease of mean oil droplet size. For long-term stability the amount added turned out to be the most important factor. With increased amounts of Solutol HS 15 the packing density of the emulsifier layer and the zeta potential decreased leading to instability. The optimum load of Solutol HS 15 was found to be 15 micromol/ml. Alkali fatty acids markedly improved the physical stability of the emulsions. Improved stability properties conferred to emulsions by alkali fatty acids could be attributed to the zeta potential increase even in the presence of Solutol HS 15. Consequently a mixed emulsifier film was established in which the ionized fatty acids determined the interface charge. In addition to this a strengthening of the molecular interactions occurring between phospholipid and Solutol HS 15 emulsifier in the presence of ionized fatty acids at the O/W interface can be assumed (L. Rydhag, The importance of the phase behaviour of phospholipids for emulsion stability, Fette Seifen Anstrichm. 81 (1979) 168-173). Different co-emulsifier mixtures were shown to have a pronounced impact on the plasma protein adsorption onto emulsion droplets.     

Comparison of oil-in-water emulsions manufactured by microfluidization and homogenization

The purpose of this study was to compare drug-free model submicron oil-in-water (o/w) emulsions manufactured by high-speed homogenization and microfluidization. The study was aimed at evaluating the influence of these two manufacturing processes on the stability of the emulsions with respect to emulsifier concentration. Stability was defined in terms of dispersed droplet diameter growth over time. The study was also directed towards identifying the minimum emulsifier concentrations required by either processing method within the same model o/w systems to produce emulsions viable throughout the study period of three months. The Microfluidizer 110L was found to be more effective than the homogenizer in producing stable o/w submicron emulsions using triglycerides of caprylic/capric acid as the oil phase and combinations of emulsifiers (polyoxyethylene sorbitan oleate with high HLB and sorbitan monooleate with low HLB) at low emulsifier concentrations. Submicron emulsions prepared by the microfluidization process had smaller droplet diameters and exhibited less droplet diameter growth over time compared to high-speed homogenization. At emulsifier concentrations below 20% w/w, the droplet diameter or stability of the dispersed phase of the sub-micron emulsions prepared by either process was found to be dependent on the emulsifier content.     

Stability of parenteral nanoemulsions loaded with paclitaxel: the influence of lipid phase composition,drug concentration and storage temperature

Abstract

Paclitaxel was loaded into licensed parenteral nutrition nanoemulsions (Clinoleic® and Intralipid®) using bath sonication, and the stability of the formulations was investigated following storage for two weeks at room temperature or at 4?°C. In general, Clinoleic droplets were smaller than Intralipid droplets, being around 255 and 285?nm, respectively, for blank and freshly loaded emulsions. Regardless of storage temperature, the Clinoleic exhibited a very slight or no increase in droplet size upon storage, whilst the droplet size of the Intralipid emulsion increased significantly. The droplet size of both emulsions was minimally affected by paclitaxel concentration within the range of 0, 1, 3 and 6?mg/ml. The pH of both emulsions markedly decreased upon storage at room temperature, which was possibly attributed to the production of fatty acids resulting from phospholipid hydrolysis. However, at 4?°C, the pH of Clinoleic emulsion was unaffected by storage or paclitaxel concentration while the Intralipid emulsion demonstrated a trend for pH reduction. Both nanoemulsions had a negative zeta potential, with the Clinoleic formulations having the highest charge, possibly explaining the better size stability of this emulsion. Overall, this study has shown that paclitaxel was successfully loaded into clinically licensed parenteral emulsions and that Clinoleic showed greater stability than the Intralipid.     

Characterization of Oil-in-Water Emulsions Prepared from Solid-State Emulsions: Effect of Matrix and Oil Phase

Emulsions (o/w) were prepared from solid-state emulsions comprised of various matrix materials and oils and the resultant particle size properties determined. Results suggest that for those matrices that can form solid-state emulsions, the droplet size decreased as a function of time, as previously observed. The final droplet size was dependent on the oil utilized but was independent of the matrix material. The use of mineral oil resulted in the smallest droplet diameter (1.5 µm) while isopropyl myristate resulted in the largest droplet diameter (3 µm). With the exception of mineral oil, the oil/water interfacial tension was found to be directly proportional to the droplet diameter. The rate of emulsification appeared to be bi-phasic. The initial emulsification phase appeared to be independent of the matrix material while the terminal phase was a function of the matrix material. Most importantly, it was found that solid state emulsions could be prepared from a diverse, yet specific, list of matrices.     

The influence of physicochemical properties of preservative compounds on their distribution into various phases of oil in water submicron emulsion

Phospholipids--stabilized submicron emulsions require the addition of preservatives to destroy or inhibit the growth of microorganisms when these preparations are non-sterile products or when packed in multi-dose containers. This study examined the distribution of four paraben esters--methylparaben, ethylparaben, propylparaben, and butylparaben--that were added into submicron emulsions by de novo emulsification. The distribution of preservative compounds among different phases was determined after separation of submicron emulsions by ultracentrifugation. The compounds with higher lipophilicity were concentrated in the oil phase and phospholipids-rich phase, leading to an increase in oil droplet size. However, the effect of the paraben distribution on zeta potential and pH of emulsion fluctuated depending on the type of phospholipid used. The lower lipophilic compounds were mostly found in the aqueous phase and mesophase. These results signify the possibility that the chemical structures and lipophilicity of preservative compounds affected their distribution in phospholipid-stabilized submicron emulsions. We conclude that the higher concentration of preservatives or their combination may be required for efficient preservation of submicron emulsion products.     

Multiple emulsions containing rifampicin

A stable multiple emulsion containing rifampicin in the internal aqueous phase was prepared by the incorporation of additives in both aqueous and oily phases. The formulation and process variables were optimized for primary and secondary emulsification. Drug release studies were performed on selected multiple emulsions to observe the effect of dilution. The release data were fitted to the Higuchi equation for slab and spherical geometry and effective diffusion coefficients were calculated. Stability studies for three months revealed good stability of the multiple emulsions with respect to creaming, phase separation, viscosity change, drug leakage, change in droplet size upon storage and exposure to osmotic and shear stress. The in vivo studies performed with stable multiple emulsions administered orally in human volunteers showed prolonged plasma drug levels. The multiple emulsions were found suitable for an improved tuberculosis therapy.