Stabilization of a thermosetting emulsion system using ionic and nonionic surfactants

Combining branched copolymers with additives generates stable thermoresponsive emulsions with in situ gelation upon exposure to body temperature

Branched copolymer surfactants (BCS) containing thermoresponsive polymer components, hydrophilic components, and hydrophobic termini allow the formation of emulsions which switch from liquid at room temperature to a gel state upon heating. These materials have great potential as in situ gel-forming dosage forms for administration to external and internal body sites, where the emulsion system also allows effective solubilisation of a range of drugs with different chemistries. These systems have been reported previously, however there are many challenges to translation into pharmaceutical excipients. To transition towards this application, this manuscript describes the evaluation of a range of pharmaceutically-relevant oils in the BCS system as well as evaluation of surfactants and polymeric/oligomeric additives to enhance stability. Key endpoints for this study are macroscopic stability of the emulsions and rheological response to temperature. The effect of an optimal additive (methylcellulose) on the nanoscale processes occurring in the BCS-stabilised emulsions is probed by small-angle neutron scattering (SANS) to better comprehend the system. Overall, the study reports an optimal BCS/methylcellulose system exhibiting sol-gel transition at a physiologically-relevant temperature without macroscopic evidence of instability as an in situ gelling dosage form.

Pluronic F127-Folate Coated Super Paramagenic Iron Oxide Nanoparticles as Contrast Agent for Cancer Diagnosis in Magnetic Resonance Imaging

Contrast agents have been widely used in medicine to enhance contrast in magnetic resonance imaging (MRI). Among them, super paramagnetic iron oxide nanoparticles (SPION) have been reported to have low risk in clinical use. In our study, F127-Folate coated SPION was fabricated in order to efficiently target tumors and provide imaging contrast in MRI. SPION alone have an average core size of 15 nm. After stabilizing with Pluronic F127, the nanoparticles reached a hydrodynamic size of 180 nm and dispersed well in various kinds of media. The F127-Folate coated SPION were shown to specifically target folate receptor expressing cancer cells by flow cytometry analysis, confocal laser scanning microscope, as well as in vitro MRI. Furthermore, in vivo MRI images have shown the enhanced negative contrast from the F127-Folate coated SPION in tumor-bearing mice. In conclusion, our F127-Folate coated SPION have shown great potential as a contrast agent in MRI, as well as in the combination with drug delivery for cancer therapy.

Bare and Sterically Stabilized PLGA Nanoparticles for the Stabilization of Pickering Emulsions

Pickering emulsions were formulated using biodegradable and biocompatible poly(lactic- co-glycolic acid) (PLGA) nanoparticles (NPs) prepared without surfactants or any other polymer than PLGA. A pharmaceutical and cosmetic oil (Miglyol) was chosen as the oil phase at a ratio of 10% w/w. These emulsions were then compared with emulsions using the same oil but formulated with well-described PLGA-poly(vinyl alcohol) (PVA) NPs, i.e., with PVA as NP stabilizers. Strikingly, the emulsions demonstrated very different structures at macroscopic, microscopic, and interfacial scales, depending on the type of NPs used. Indeed, the emulsion layer was significantly thicker when using PLGA NPs rather than PLGA-PVA NPs. This was attributed to the formation and coexistence of multiple water-in-oil-in-water (W/O/W) and simple oil-in-water (O/W) droplets, using a single step of emulsification, whereas simple O/W emulsions were obtained with PLGA-PVA NPs. The latter NPs were more hydrophilic than bare PLGA NPs because of the presence of PVA at their surface. Moreover, PLGA NPs only slightly lowered the oil/water interfacial tension whereas the decrease was more pronounced with PLGA-PVA NPs. The PVA chains at the PLGA-PVA NP surface could probably partially desorb from the NPs and adsorb at the interface, inducing the interfacial tension decrease. Finally, independent of their composition, NPs were adsorbed at the oil/water interface without influencing its rheological behavior, possibly due to their mobility at their interface. This work has direct implications in the formulation of Pickering emulsions and stresses the paramount influence of the physicochemical nature of the NP surface into the stabilization of these systems.

New insights into eutectic cream skin penetration enhancement

The manner in which the eutectic cream EMLA enhances the percutaneous penetration of lidocaine and prilocaine into human skin is still not fully understood. The purpose of this study was to investigate if the modification of drug aggregation played a role in the way EMLA facilitates delivery. Light scattering analysis of lidocaine alone in water gave a critical aggregation concentration (CAC) of 572 μM and a mean aggregate size of 58.8 nm. The analysis of prilocaine in identical conditions gave a CAC of 1177 μM and a mean aggregate size of 105.7 ± 24.8 nm. When the two drugs were mixed at their eutectic 1:1 ratio in water the CAC reduced to 165.8 μM and the aggregate size was 43.82 nm. This lidocaine-prilocaine interaction in water was further modified upon addition of polyoxyethylene hydrogenated castor oil, the surfactant in the EMLA aqueous phase, to produce aggregates of <20 nm. The physical characterisation data suggested that it was the EMLA cream's surfactant that modified the drug molecular interactions in the aqueous continuous phase and caused a 6 fold higher drug penetration through human epidermal tissue compared to the oil formulations tested in this study.

Proteins, polysaccharides, and their complexes used as stabilizers for emulsions: alternatives to synthetic surfactants in the pharmaceutical field?

Emulsions are widely used in pharmaceutics for the encapsulation, solubilization, entrapment, and controlled delivery of active ingredients. In order to answer the increasing demand for clean label excipients, natural polymers can replace the potentially irritative synthetic surfactants used in emulsion formulation. Indeed, biopolymers are currently used in the food industry to stabilize emulsions, and they appear as promising candidates in the pharmaceutical field too. All proteins and some polysaccharides are able to adsorb at a globule surface, thus decreasing the interfacial tension and enhancing the interfacial elasticity. However, most polysaccharides stabilize emulsions simply by increasing the viscosity of the continuous phase. Proteins and polysaccharides may also be associated either through covalent bonding or electrostatic interactions. The combination of the properties of these biopolymers under appropriate conditions leads to increased emulsion stability. Alternative layers of oppositely charged biopolymers can also be formed around the globules to obtain multi-layered "membranes". These layers can provide electrostatic and steric stabilization thus improving thermal stability and resistance to external treatment. The novel biopolymer-stabilized emulsions have a great potential in the pharmaceutical field for encapsulation, controlled digestion, and targeted release although several challenging issues such as storage and bacteriological concerns still need to be addressed.

Growth, shrinking, and breaking of pluronic micelles in the presence of drugs and/or beta-cyclodextrin, a study by small-angle neutron scattering and fluorescence spectroscopy

The associative structures between F127 Pluronic micelles and four drugs, namely, lidocaine (LD), pentobarbital sodium salt (PB), sodium naproxen (NP), and sodium salicylate (SAL), were studied by small-angle neutron scattering (SANS). Different outcomes for the micellar aggregates are observed, which are dependent on the chemical nature of the drug and the presence of charge or otherwise: the micelles grow with LD, are hardly modified with PB, and decrease in size with both NP and SAL. The partition coefficient, determined by fluorescence spectroscopy, is directly correlated to the amount of charge, following NP approximately SAL < PB < LD. All drugs are found to lie at the interfacial layer, with a slightly deeper localization of LD and more superficial for PB. All drugs can form inclusion complexes with heptakis(2,6-di-O-methyl) beta-cyclodextrin (hep2,6 beta-CD). Hep2,6 beta-CD, as shown in previous studies (Joseph, J.; Dreiss, C. A.; Cosgrove, T. Langmuir, 2008, 24, 10005-10010; Dreiss, C. A.; Nwabunwanne, E.; Liu, R.; Brooks, N. J. Soft Matter, 2009, 5, 1888-1896), is also able to form a complex with F127, resulting in micellar breakup. In the ternary mixtures, a fine balance of forces is involved, which results in drastic micellar changes, as observed from the SANS patterns. Depending on the ratio of drug, polymer, and hep2,6 beta-CD and the nature of the interactions (which is directly linked to the drug chemical structure), the presence of drug either hinders micellar breakup by beta-CD (at high enough concentration of LD or PB) or leads to micellar growth (NP). These effects are mainly attributed to a preferential drug/beta-CD interaction (except for PB), which, at least in the conditions studied here, explains the higher beta-CD concentration needed for micellar breakup to occur.

Soft drug delivery systems

This brief review aims at providing some illustrative examples on the use of soft drug delivery systems formed by surfactants, polymers, and lipids. Such delivery systems are discussed and exemplified regarding both more traditional soft drug delivery systems such as micelles, liquid crystalline phases, liposomes and polymer gels, as well as more novel structures, , carbon nanotubes, polyelectrolyte multilayer capsules, and liquid crystalline particles.

Polymer stabilisers for temperature-induced dispersion gelation: Versatility and control

In this study the temperature-induced gelation of butadiene-acrylonitrile latex containing the added temperature-responsive polymer surfactant, poly(NIPAM-co-PEGMa) is investigated for the first time. (NIPAM and PEGMa are N-isopropylacrylamide and poly(ethylene glycol)methacrylate, respectively.) The results are compared with temperature-induced gelation of oil-in-water emulsions containing 1-bromohexadecane. The effect of added anionic surfactant, NaDBS (sodium dodecylbenzene sulfonate) on the temperature-induced gelation process and mechanism is considered. It was found that the gelation temperature (T(gel)) for the latex occurs at the cloud point temperature (T(cpt)) of the polymer and that T(gel) is much less affected by added NaDBS than is the case for emulsion gelation. The mathematical predictive theory recently derived for temperature-induced emulsion gelation was applied to the latex data and gave a good fit (i.e., T(gel) approximately 1/C(p), where C(p) is the concentration of added poly(NIPAM-co-PEGMa)). However, the causes for the variation of T(gel) with C(p) for temperature-induced latex and emulsion gelation are different. The variation of T(gel) for latex gelation in the presence of added NaDBS originates from surfactant association with poly(NIPAM-co-PEGMa) which increased T(cpt). In the case of emulsion gelation there are electrostatic interactions above T(cpt) which control T(gel). The subtle difference in the temperature-induced latex gelation mechanism is a consequence of the very high latex surface area (cf. emulsion), small inter-particle separation and the presence of electrolyte. The reason that T(gel) follows 1/C(p) for the latex is due to a fortuitous T(cpt) approximately 1/C(p) relationship that applies for poly(NIPAM-co-PEGMa) solution in the presence of NaDBS. The work presented here shows that addition of poly(NIPAM-co-PEGMa) to dispersions gives a versatile method for temperature-triggered gelation. Furthermore, the theory presented provides a framework for predicting their gelation temperatures.

Sustained release of lidocaine from Poloxamer 407 gels

In this work, we show that alteration of P407 gel content can affect drug release rates. The inorganic salts and PEG 400 commonly included in the formulation of P407 gels can also change the rate at which a drug is released. Lidocaine was selected as a model drug because, although widely used in the treatment of pain, its use is limited by short duration of its effects. The use of P407 gels prolongs the residence time of the lidocaine at the injection site, sustains drug release and increases therapeutic efficacy. Release studies were performed in a diffusion system. During release, data followed the Higuchi square root law time kinetic (r>0.98). Increased polymer concentration in the gel increases viscosity and reduces lidocaine release rates and diffusion coefficients via extended gel dissolution time and prolonged drug diffusion through the gel matrix. Lidocaine release rates and diffusion coefficients increased in gels composed of NaCl or PEG 400 aqueous solution. Because these additives are hydrophilic, they reduce gel dissolution time, thereby accelerating drug diffusion. Poloxamer is biocompatible and the results support the possibility of using Poloxamer gel as a sustained release injectable formulation.

Effect of added surfactant on temperature-induced gelation of emulsions

This paper involves an investigation of the effect of added ionic surfactant on the temperature-induced gelation of oil-in-water (O/W) emulsions stabilized by a responsive copolymer. The oil phase used in this study is 1-bromohexadecane. The copolymer is poly(NIPAM-co-PEGMa) (NIPAM and PEGMa are N-isopropylacrylamide and poly(ethylene glycol) methacrylate, respectively). The lower critical solution temperature for the copolymer was 39.5 degrees C. The ionic surfactant used in this work was sodium dodecylbenzenesulfonate (NaDBS). The critical association concentration for NaDBS and poly(NIPAM-co-PEGMa) was measured at 0.30 mM using fluorescence measurements (pyrene was the probe molecule). Gelation temperatures were measured for the O/W emulsions to establish the effect of added NaDBS and copolymer concentration (Cp) on the gelation temperature (Tgel). The strength of the gels was measured using dynamic oscillatory measurements. These measurements allowed the shear modulus of the gel at Tgel to be estimated as 100 Pa. A theoretical model based on transient network theory was developed that predicts the dependence of Tgel on Cp. The study revealed that NaDBS has two effects on the overall cross-link density of the emulsion gels: it contributes a source of cross-linking via micellar cross-links and also decreases the proportion of transient cross-links due to electrostatic repulsion.

Development of a size exclusion chromatography method for the determination of molar mass for poloxamers

An aqueous size exclusion chromatography (SEC) method for determination of the molar mass of poloxamers 188 and 407 has been developed as an alternative to the pharmacopoeia methods. During the development work two different columns and several different eluent compositions were investigated. With a PL-aquagel-OH column, non-exclusion behaviour was obtained. A TSKgel column gave good separation of both poloxamers. The best separation was obtained with an eluent consisting of sodium chloride (0.01 M)-methanol (90:10, v/v) on the TSKgel column. The method was shown to be linear within the elution time of the two poloxamers and to have acceptable precision. The results from the SEC method was compared to results obtained using SEC with online multi angle light scattering detection (MALS) and to results obtained with matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS).

Improvement of physicochemical properties of N-4472 part I formulation design by using self-microemulsifying system

The optimization of oral dosage form formulation has been developed for N-4472, N-[2-(3,5-di-tert-butyl-4-hydroxyphenethyl)-4,6-difluorophenyl]-N'-[4-(N-benzylpiperidyl)] urea, which was a poorly water-soluble drug having a lipid-lowering effect. Formulations that contained various surfactants and water-soluble polymers were prepared and the solubility of N-4472 was evaluated in JP XIV first fluid (pH 1.2), JP XIV second fluid (pH 6.8), and distilled water. The highest solubility of N-4472 was achieved when L-ascorbic acid (VC), Gelucire 44/14, and HCO-60 were used as additives. It was confirmed that this formulation could create microemulsion droplets with a mean droplet size of approximately 20 nm and a sharp droplet distribution pattern in JP XIV first fluid, JP XIV second fluid, and distilled water. When JP XIV second fluid was used as a dissolution medium, however, an eventual decrease of solubility was observed, that is, the fluid became white and cloudy as time passed. It was found that the addition of sodium dodecyl sulfate (SDS) was effective to prevent the lowering of solubility, and that a weight ratio of 1.0/1.5/11.4/4.9/3.8 for N-4472/VC/Gelucire 44/14/HCO-60/SDS was optimum for the self-microemulsifying formulation. It was assumed that electrostatic repulsion of microemulsion droplets and an increase of the cloud point by the addition of SDS were responsible for the successful formation of a stable microemulsion.

Local anaesthetic block copolymer system undergoing phase transition on dilution with water

The possibility of formulating a local anaesthetic system displaying in situ gelation on dilution with water, as well as its dependence on concentration of active ingredients and pH was investigated. For this purpose Lutrol F68, water, a eutectic mixture of lidocaine and prilocaine and Akoline MCM were mixed in different ratios and investigated using crossed polarisers, small-angle X-ray diffraction, rheology, conductivity and NMR self-diffusion measurements. In particular, an isotropic phase of low viscosity turning into a high viscous hexagonal phase upon dilution with water was found. The increase in viscosity is only weakly dependent on temperature in the temperature range of 20-37 degrees C. The rheology and in vitro drug release of these systems were studied and the elastic modulus was found to be fairly independent of concentration of active ingredients and pH in the investigated region. The in vitro release of lidocaine and prilocaine was found to increase with increasing concentration of the active ingredients and with decreasing pH, the latter as a consequence of the pH-dependent ionisation of these substances. The behaviour of the system is promising from a pharmaceutical point of view, since the isotropic low-viscous phase can be injected into, e.g. a periodontal pocket where the presence of saliva will cause a temporal transition into a rigid hexagonal phase thus making the formulation stay at the application site. At even higher water content, either as a result of longer application time or rinsing with water, the hexagonal phase is effectively dissolved through transformation to a water-rich micellar phase.

Micellization and gelation in block copolymer systems containing local anesthetics

A formulation consisting of a eutectic mixture of lidocaine and prilocaine, Lutrol((R)) F68 and Lutrol((R)) F127, suitable for anesthetizing the periodontal pocket has previously been developed. This consists of discrete micelles with a diameter of 20-30 nm and has a suitable gelation temperature, a good release profile and excellent long-term stability. In this study, the unimer/micelle transition and gel formation of the formulation, in its concentrated state, are investigated using differential scanning calorimetry (DSC), dye solubilization, rheology, and nuclear magnetic resonance (NMR) self-diffusion. The critical micellization temperature (cmt) and gelation temperature are found to be interconnected and influenced by cosolutes, such as electrolytes and hydrophobic substances, the latter as found particularly for the eutectic mixture of the local anesthetic agents lidocaine and prilocaine. Both cmt and the gelation temperature decrease with increasing pH of the system, i.e. at reduced solubility of the active ingredients. Moreover, both cmt and the gelation temperature increase upon diluting the system with water. The ratio between the two block copolymers present in the system also has an impact on both cmt and the gelation temperature, resulting in a decrease in onset temperature of both processes with an increase of Lutrol((R)) F127. The amount of the active ingredients present in the micelle phase depends on the pH of the system being approximately 0% w/w at pH 5, 50-60% w/w at pH 7.8 and 80% w/w at pH 9.

Nonionic Cellulose Ethers as Potential Drug Delivery Systems for Periodontal Anesthesia

Nonionic cellulose ethers displaying a lower consolute temperature, or cloud-point, close to body temperature were investigated as potential carrier systems for the delivery of local anesthetic agents to the periodontal pocket. The interaction between the polymers, i.e., ethyl(hydroxyethyl)cellulose (EHEC) and hydrophobically modified EHEC (HM-EHEC), and ionic surfactants was determined in the absence and in the presence of the local anesthetic agents lidocaine and prilocaine. The cloud-point and rheology data indicate interactions between the polymer and both anionic and cationic surfactants. More precisely, a number of ionic surfactants were found to result in an increase in cloud-point at higher surfactant concentrations, a surfactant-concentration-dependent thickening, and a temperature-induced gelation upon heating. Upon addition of the local anesthetic agents lidocaine and prilocaine in their uncharged form to EHEC and HM-EHEC, in the absence of surfactants, only minor interaction with the polymer could be inferred. However, these substances were found to affect the polymer-surfactant interaction. In particular, the drug release rate in vitro as well as the stability and temperature-dependent viscosity were followed for an EHEC/SDS system and EHEC/myristoylcholine bromide system upon addition of lidocaine and prilocaine. The data indicate a possibility of formulating a local anesthetic drug delivery system suitable for administration into the periodontal pocket where at least small amounts of active ingredients can be incorporated into the system without severely affecting the gelation behavior. The results found for the cationic myristoylcholine bromide system are particularly interesting for the application in focus here since this surfactant is antibacterial and readily biodegradable. Copyright 2000 Academic Press.

Thermosetting microemulsions and mixed micellar solutions as drug delivery systems for periodontal anesthesia

In the present study, thermosetting microemulsions and mixed micellar solutions were investigated as drug delivery systems for anesthetizing the periodontal pocket. The structure of the systems, consisting of the active ingredients lidocaine and prilocaine, as well as two block copolymers (Lutrol F127 and Lutrol F68), was investigated by NMR spectroscopy and photon correlation spectroscopy (PCS). The results obtained for dilute (1-3% w/w) solutions show discrete micelles with a diameter of 20-30 nm and a critical micellization temperature of 25-35 degrees C. Gel permeation chromatography (GPC) was used to study the distribution of the active ingredients, and indicates a preferential solubilization of the active components in micelles over unimers. Analogous to the Lutrol F127 single component system these formulations display an abrupt gelation on increasing temperature. The gelation temperature was found to depend on both the drug ionization and concentration. These systems have several advantages over emulsion-based formulations including good stability, ease of preparation, increased drug release rate, and improved handling due to the transparency of the formulations.