Basic principles of drug delivery systems - the case of paclitaxel.

Cancer is the second cause of death worldwide, exceeded only by cardiovascular diseases. The prevalent treatment currently used against metastatic cancer is chemotherapy. Among the most studied drugs that inhibit neoplastic cells from acquiring unlimited replicative ability (a hallmark of cancer) are the taxanes. They operate via a unique molecular mechanism affecting mitosis. In this review, we show this mechanism for one of them, paclitaxel, and for other (non-taxanes) anti-mitotic drugs. However, the use of paclitaxel is seriously limited (its bioavailability is <10%) due to several long-standing challenges: its poor water solubility (0.3 µg/mL), its being a substrate for the efflux multidrug transporter P-gp, and, in the case of oral delivery, its first-pass metabolism by certain enzymes. Adequate delivery methods are therefore required to enhance the anti-tumor activity of paclitaxel. Thus, we have also reviewed drug delivery strategies in light of the various physical, chemical, and enzymatic obstacles facing the (especially oral) delivery of drugs in general and paclitaxel in particular. Among the powerful and versatile platforms that have been developed and achieved unprecedented opportunities as drug carriers, microemulsions might have great potential for this aim. This is due to properties such as thermodynamic stability (leading to long shelf-life), increased drug solubilization, and ease of preparation and administration. In this review, we define microemulsions and nanoemulsions, analyze their pertinent properties, and review the results of several drug delivery carriers based on these systems.

The dielectric study of insulin-loaded reverse hexagonal (H(II)) liquid crystals.

The dielectric behavior of the insulin-loaded HII mesophase (containing GMO-TAG-water-glycerol-insulin) was studied using two empty reference systems (GMO-TAG-water and GMO-TAG-water-glycerol) at a frequency range of 10(-2)-10(6) Hz, and a temperature range of 290-333 K. Three clearly defined relaxation processes were observed and assigned to the reorientation of GMO polar heads, the tangential movement of counterions at the interface, and the movements of TAGs through the lipid tail. Upon addition of glycerol, a heterogeneous inner structure was formed within the HII cylinders: the water-glycerol core surrounded by a water rigid layer. Upon heating, two critical points were detected referring to the dehydration of the GMO heads (at 304 K, similar to the water-filled HII system) and to energetic modifications (at 316 K), resulting in breaking of the water layer allowing on-demand controlled release. Insulin incorporation combined the features of both reference HII systems. Yet, unlike the empty HII systems, insulin perturbed the GMO-water interface while decreasing the movement of the GMO headgroup, and reducing T0 (296 K). No interactions were formed between the dipole of each counterion at the interface region and the matrix (the GMO), fitting the Debye process. Dynamic behavior was observed, pointing to mobility between the hexagonal rods themselves, enabling controlled release from the HII carrier.

Inhibition of cholesterol transport into skin cells in cultures by phytosterol-loaded microemulsion.

Cholesterol and plant phytosterols are lipophilic compounds solubilized by intestinal micelles in a competitive manner. In this work, we used radioactive cholesterol- and phytosterol-loaded oil-in-water microemulsions to follow their incorporation and mutual competition in HaCaT keratinocytes, SZ95 sebocytes, and skin pieces in cultures. Dynamic light scattering showed homogenous nanostructures of 10.5+/-1.5 nm diameter and cryo-transmission electron microscopy confirmed the presence of uniform spherical droplets of 7.0+/-1.0 nm diameter. Up to 320 nmol/ml of cholesterol can be solubilized and transported into cells with minimal toxic effect by 0.5 wt% nanodroplets in a cell medium. Phytosterols inhibit incorporation of cholesterol into cells, in vitro, at molar ratios (phytosterols/cholesterol) of 4 and above. The loaded nanodroplets accumulate in intracellular vesicles (presumably endosomes). No metabolic conversion of cholesterol or phytosterols was found in these cells, in vitro, after 24 h, at 37 degrees C.

On structural transitions in a discontinuous micellar cubic phase loaded with sodium diclofenac.

An intermediate mesophase of lyotropic liquid crystalline structure from the ternary mixtures of glycerol monooleate, water, and ethanol was recently characterized in our lab. This mesophase, termed Q(L), consists of discrete discontinuous micelles arranged in a cubic array. The Q(L) phase can solubilize very significant loads of water-insoluble anti-inflamatory drug sodium diclofenac (Na-DFC). Close examination of the internal structures of the lyotropic liquid structure upon increasing the solubilization loads reveals the existence of three structural transitions controlled by the Na-DFC levels. Up to 0.4 wt% Na-DFC, the Q(L) structure remains intact with some influence on the hydration of the headgroups and on the intermicellar forces. However, at 0.8 to 1.2 wt% Na-DFC, the discontinuous micellar cubic phase is transformed into a more condensed mesophase of a bicontinuous cubic phase. At > or =1.2 wt% Na-DFC, the cubic phase is converted into a lamellar phase (L(alpha)). Within 5.5 to 7.3 wt% Na-DFC the mesophase is progressively transformed into a less ordered lamellar structure. At 12 wt% Na-DFC crystals tend to precipitate out. At low Na-DFC concentrations the drug behaves like a lyotropic or kosmotropic salt and can salt-out the surfactant from its water layer, but at higher levels it behaves like a hydrotropic, chaotropic salt and can salt-in the surfactant. The Na-DFC location and position within the interface as well as its polarization and partial ionization are strongly affected by its solubilization contents and the structure that it is inducing. In the cubic phase the drug is located less close to the hydration layer while once transition occurs it is exposed more to the water layer and the surfactant headgroups.

A novel dispersion method comprising a nucleating agent solubilized in a microemulsion, in polymeric matrix II. Microemulsion characterization.

This second part of our paper focuses on structural characterization of the microemulsion for increasing the crystallization rate of polypropylene (PP) through the entrapment of nucleating agent (nucleator) HPN-68, serving as a transport vehicle. Our concept is based on creating an advantage in dispersion capability of the nucleator that is dissolved in a nanoreactor vehicle, compared with its conventional loading as a crystalline powder. The advantage was achieved by solubilizing the HPN-68 in a microemulsion to decrease its size from micro- to nanoscale. The microemulsions were introduced to the target PP using a mixer. By the end of the mixing, when the water phase had evaporated, only the nucleator and the surfactant remained in the matrix. DSC results showed a 24% improvement in nucleation efficiency of PP by this novel method. It was shown that solubilization of the nucleator depends on the water activity in the microemulsion, and the presence of the nucleator opposes formation of the W/O phase. Light scattering, SD-NMR, and SAXS results showed that HPN-68 is accommodated in the water phase and at the interface, and significantly reduces the level of order in the microemulsion. In intermediate water content, a worm-like structure was proposed instead of the classical bicontinuous one. The structure was confirmed by SAXS and SD-NMR analysis. Viscosity measurements revealed structural transitions in the microemulsions.

A novel dispersion method comprising a nucleating agent solubilized in a microemulsion, in polymeric matrix I. Dispersion method and polymer characterization.

This is the first of a two-part study focusing on a novel dispersion method which enables increasing the crystallization rate of polypropylene (PP) through the incorporation of nucleating agent HPN-68 into the molten polymer using a microemulsion as a nanovehicle. The cycle time for processing the PP is significantly reduced and thus the effectiveness of its production is increased. Our concept is based on creating an advantage in dispersion capability of the nucleator that is dissolved in a nanoreactor vehicle in comparison with its conventional introduction as a crystalline powder. The microemulsions were introduced to the target PP using a mixer. By the end of the mixing, when the water phase had evaporated, only the nucleator and the surfactant remained in the matrix. The microemulsion components that solubilized the HPN-68 were mineral oil, alcohol, surfactant, and water. DSC results showed a 24% improvement in nucleation efficiency of PP by this method. WAXS results showed that HPN-68 is a gamma-nucleator. It causes polymorphism by significantly raising the gamma-phase concentration in the PP. SEM results showed a four-fold decrease in the PP spherulite size due to the improved dispersion of HPN-68 within the matrix via microemulsion compared to conventional nucleator incorporation.

The effect of structural variation of alcohols on water solubilization in nonionic microemulsions 2. Branched alcohols as solubilization modifiers: results and interpretation.

In this second part of a paper dealing with the effect of branched alcohols on solubilization, an attempt has been made to provide explanations of experimental data related mostly to the system Brij 97/branched alcohol + dodecane = 1:1 (by weight)/water at 27+/-0.2 degrees C. Applying the Hou-Shah mechanism it was shown that for many C4-C6 branched alcohol isomers having one methyl branch, solubilization behavior is readily interpreted by assuming control of the critical radius, R(c). Two parameters, both included in the definition of the branching factor, F(b) (which was treated in the first part of the paper), were also used to analyze solubilization data. The first, l(i), is defined as the distance from the free end of the alcohol molecule to the methyl branch. The second, d, is virtually N(A), the chain length of the alcohol. When l(i)>3, the solubilization becomes dominated by the natural radius of curvature, R0. Also, we have suggested that for R(c)-control, solubilization will be enhanced in direct proportion to the distance d-l(i), whereas for R0-control, solubilization will increase with decreasing d-l(i). The validity of our assumptions was demonstrated in many cases. Some examples of the more complicated case of double branching (two methyl groups along the alcohol chain) were also analyzed.

The effect of structural variation of alcohols on water solubilization in nonionic microemulsions 1. From linear to branched amphiphiles--general considerations.

This article is the first part of a two-part study that exemplifies how to treat the solubilization of water in multicomponent surfactant-based systems. In particular, it aims at clarifying the role of cosurfactants in water solubilization in these systems. The judicious selection of the components in such systems to maximize water solubilization is occasionally thought to be dictated by the chain length compatibility principle, which may be expressed quantitatively by the BSO (Bansal, Shah, O'Connell) equation. Here we demonstrate some limitations of the equation. For example, in our best model system, C12(EO)8/dodecane+pentanol=1:1 (by weight)/water at 27+/-0.2 degrees C, the BSO equation predicts that no alcohol is needed for maximum water solubilization, contrary to our experimental findings. We discuss how to optimize the alcohol/oil weight ratio needed for stabilizing four-component microemulsions. In our model systems C12(EO)8 or C(18:1)(EO)10/pentanol/dodecane/water, this optimal weight ratio is 1:1. We also highlight the difference between the effect of normal alcohols on water solubilization-which passes via a maximum-and their effect on percolation processes and structured changes of proteins, which depends solely upon the alcohol hydrophobicity. For the investigation of the effect of branching on phase behavior the utilization of an extended form of the geometrical branching factor F(b) is suggested. The meaning of this factor is elucidated by comparing it with topological indices.

Double emulsions stabilized with hybrids of natural polymers for entrapment and slow release of active matters.

The main focus and efforts for the next few years in the area of emulsion technology will be to improve stability and control the release of active matter in double emulsions (3rd World Congress on Emulsions, Lyon, France, September 2002). Almost any possible blends of low-molecular weight emulsifiers, oils, cosolvents and coemulsifiers have been already tested. Biopolymers, synthetic graft and comb co-polymers and polymerizable emulsifiers that impart steric or mechanical stabilization with improved stability and better controlled release were explored. Amphiphilic macromolecules, natural occurring or synthetic, that increase the viscosity of each of the phases, complex with the oil or the emulsifiers and form systems that will behave much like microcapsules, microspheres and/or mesophasic liquid crystals have been mentioned as possible new technologies for improved stability. This review will concentrate only on the most recent findings that can enhance stability of the double emulsions and/or will reduce droplets sizes for potential food applications. The attempts and achievements include: selection of food-grade blends of emulsifiers to enhance emulsion stability at both inner and outer interfaces and use of new polymeric amphiphiles (carriers, complexing agents, natural polymeric emulsifiers) to control and reduce the reverse micellar transport phenomena and to control the addenda transport.

Improved oil solubilization in oil/water food grade microemulsions in the presence of polyols and ethanol.

Microemulsions based on five-component mixtures for food applications and improved oil solubilization have been studied. The compositions included water, oil phase [such as R(+)-limonene and medium-chain triglycerides (MCT)], short-chain alcohols (such as ethanol), polyols (propylene glycol and glycerol), and several surfactants and their corresponding mixtures (nonionic, such as ethoxylated sorbitan esters, polyglycerol esters, sugar ester, and anionic, such as phosphatidylcholine). The phase behavior of these systems is discussed with respect to the influence of polyols and short-chain alcohols on the degree of solubilization of oils in the aqueous phase. The alcohol and polyols modify the interfacial spontaneous curvature and the flexibility of the surfactant film, enhancing the oil solubilization capacity of the microemulsions. The solubilization of R(+)-limonene was dramatically improved in the presence of the alcohol and polyols, whereas the improvement of solubilization for triglycerides containing MCT was less pronounced. In some systems high oil solubilization was achieved, and some of them can be easily diluted to infinity both with the aqueous phase and with the oil phase. Viscosity measurements along selected dilution lines [characterized by a single continuous microemulsion region starting from a pseudo binary solution (surfactant/oil phase) to the microemulsion (water/polyol corner)] indicate that at a certain composition the system inverts from a W/O to an O/W microemulsion.

Water Solubilization in Nonionic Microemulsions Stabilized by Grafted Siliconic Emulsifiers.

Microemulsions containing octanol, decanol, or dodecanol as the oil phase and oligomeric, grafted nonionic amphiphiles based on ethoxylated polymethylsiloxanes (Silwets) have been studied. It was demonstrated that significant amounts of water can be solubilized only when the hydrophobic siliconic backbone is very short (trimers). The water solubilization was evaluated using SAXS, DSC, and conductivity measurements. It was found that up to 40 wt% of water can be solubilized in dodecanol and Silwet L-7607 (MW 1000 and 75 wt% ethylene oxide (EO)). Surprisingly, no free water was detected in the aggregate core. All the solubilized water was confined in the vicinity of the interphasal region and froze at -10 degrees C and below. Up to three molecules of water can be associated with each EO headgroup. Based on SAXS measurements, the structural units of the microemulsions were interpreted to be lamellar-like, a form previously found for the related monomeric microemulsions. Copyright 2001 Academic Press.

Structured fluids as microreactors for flavor formation by the Maillard reaction.

Thermal reactions of cysteine/furfural and cysteine/ribose mixtures were studied in model systems to gain more insight into the influence of structured fluids such as L(2) microemulsions and cubic phases on the generation of aroma compounds. Formation of 2-furfurylthiol from cysteine/furfural was particularly efficient in L(2) microemulsions and cubic phases compared to aqueous systems. The reaction led to the formation of two new sulfur compounds, which were identified as 2-(2-furyl)thiazolidine and, tentatively, N-(2-mercaptovinyl)-2-(2-furyl)thiazolidine. Similarly, generation of 2-furfurylthiol and 2-methyl-3-furanthiol from cysteine/ribose mixtures was strongly enhanced in structured fluids. The cubic phase was shown to be even more efficient in flavor generation than the L(2) microemulsion. It was denoted "cubic catalyst" or "cubic selective microreactor". The obtained results are interpreted in terms of a surface and curvature control of the reactions defined by the structural properties of the formed surfactant associates.

Some characteristics of sugar ester nonionic microemulsions in view of possible food applications.

This study explores some characteristics of microemulsions composed of sucrose monostearate (SMS), medium-chain triglycerides (MCT), or R-(+)-limonene, alcohols, and water. The systems are homogeneous, soft, and waxy solids at room temperature but liquefy and structure into homogeneous microemulsions when heated to >40 degrees C. The amount of solubilized water is enhanced as a function of the alcohol/oil ratio and is inversely proportional to the alcohol chain length. Over 60 wt % water can be solubilized in systems consisting of propanol/MCT/SMS at a weight ratio of 3:1:4 (initial weight ratio). These microemulsions are unique and differ from nonionic ethoxylated-based microemulsions in that their viscosity is very low and is reduced with increasing amounts of solubilized water. The electrical conductivity increases only slightly as a function of the water content and does not show typical bicontinuous or percolated behavior. The water in the core of the microemulsion strongly binds to the headgroups of the surfactant. Only at >15 wt % solubilization of water was free or bulk water detected in the core of the microemulsions. Such unique behavior of the core water might have a possible application in systems requiring monitoring of enzymatic (lipase) reactions carried out in the microemulsions as microreactors.

Solubilization of Hydrophilic Compounds in Copolymer Aggregates.

The solubilization of five hydrophilic water-soluble aroma compounds in self-aggregating triblock amphiphilic copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), with similar percentages of PEO and different molecular weights, was studied. The five hydrophilic compounds (diacetyl, 2-methylpyrazine, pyrrole, furfural, guaiacol) were carefully selected to represent hydrophilic molecules with a similar molecular weight and molecular volume, but with different abilities to interact with the micellar core of PPO moieties and with the PEO palisade side chains. It was found that the solubilized solute mole fraction increased and the aggregate-water partition coefficients of the solutes decreased with increasing free solute concentration in the aqueous phase. The partition coefficients were smaller than those obtained for hydrophobic compounds and equilibrium was reached at lower solubilization values. Guaiacol was the least hydrophilic molecule and had the highest partition coefficient. Diacetyl was the most water-soluble compound and exhibited the smallest partition coefficient. The data reveal that the higher molecular weight polymers solubilized more solute than the low-molecular-weight polymers. Moreover it is supposed that at low solute concentrations, guaiacol (containing a hydroxyl electron acceptor group) penetrates the core of the micelle and displaces water while at more elevated concentrations it seems to be solubilized in the micelle corona. Diacetyl, the most hydrophilic solute investigated (consisting of electron donor groups), prefers mainly the corona since its affinity for the polymeric core is very weak. The solubilization occurs in the palisade layer and the partition coefficient is independent of the free solute concentration. Selective site (palisade vs core) solubilization of hydrophilic compounds in polymeric micelles can be a powerful tool to protect sensitive materials from reactants present in the continuous water phase and to conduct surface-sensitive organic reactions. Furthermore, selective release properties of reactants and products can be designed. Copyright 2000 Academic Press.

Structural Polymorphism in a Four-Component Nonionic Microemulsion

The Winsor IV microemulsion system composed of octaethylene glycol mono n-dodecylether [C12(EO)8]/1-dodecane + n-pentanol (1:1 by weight)/water has been investigated at constant temperature using small angle X-ray scattering and electrical conductivity measurements. The results obtained are interpreted in terms of structural evolution of the molecular aggregates as a function of the stepwise addition of water or (oil + alcohol). The size and shape of a variety of microstructures are described: small, spherical micelles near the water corner and hexagonal and lamellar mesophases, which are oil- and alcohol-poor. Simple multishell models of these provide some insight into how dilution with water or swelling with (oil + alcohol) influences the overall symmetry of the aggregates, pentanol and dodecane partitioning, surfactant headgroup conformation, and the contribution of pentanol to oil solubilization. The previously identified "local" lamellar structure [O. Regev et al., Langmuir 12, 668 (1996)], which is a surfactant-rich and (oil + alcohol)-rich intermediate state between the W/O and O/W regions, is characterized here as a type of ordered, but highly obstructed, bicontinuous microemulsion.

Induction of crystallization of calcium oxalate dihydrate in micellar solutions of anionic surfactants.

Calcium oxalate dihydrate (CaC2O4 center dot (2+x)H2O; COD; x < or = 0.5) does not readily crystallize from electrolytic solutions but appears as a component in crystalluria. In this paper, we review in vitro studies on the factors responsible for its nucleation and growth with special attention given to the role of surfactants. The following surfactants were tested: dodecyl ammonium chloride (cationic), octaethylene mono-hexadecylether (non-ionic), sodium dodecyl sulfate (SDS, anionic), dioctyl sulphosuccinate (AOT, anionic), and sodium cholate (NaC, anionic). The cationic and some of the anionic surfactants (SDS, AOT) induced different habit modifications of growing calcium oxalate crystals by preferential adsorption at different crystal faces. In addition, the anionic surfactants effectively induced crystallization of COD at the expense of COM, the proportion of COD in the precipitates abruptly increasing above a critical surfactant concentration, close to, but not necessarily identical with the respective CMC. A mechanism is proposed, whereby crystallization of COD in the presence of surfactants is a consequence of the inhibition of COM by preferential adsorption of surfactant hemimicelles (two-dimensional surface aggregates) at the surfaces of growing crystals.

Effect of electrolytes, stirring and surfactants in the coacervation and microencapsulation processes in presence of gelatin.

The objectives of the present study were to investigate the parameters affecting simple coacervation and the ability to encapsulate oleic acid using this technique. Coacervation has been achieved using different types of gelatin (bloom number, charge) and various electrolytes. The electrolytes used for the coacervation can be divided into three groups: (1) inert salts; (2) phase separation inducers, (a) precipitation inducing agents (PIA), and (b) coacervation inducing agents (CIA); (3) coacervation inhibiting agents. The encapsulation of oleic acid was evaluated with two types of gelatin and various emulsifiers (anionic, cationic and nonionic). For positively charged gelatin, it was found that the encapsulation is incomplete in presence of cationic emulsifiers. For negatively charged gelatin no general trend was observed. The stirring rate for each step of the preparation of the microcapsules was evaluated. It was found that high stirring is essential only in the cooling stage. The study was carried out in view of encapsulation of particular bacteria dispersed in the oil phase.

The amino acid factor in stone formers' and normal urines.

The composition of amino-acids in kidney stone matrices and inhibitory materials from normal urines, reported in different independent studies has been reviewed. No obvious difference was found between the composition of amino acids from healthy and pathological sources. Studies carried out in this laboratory showed a specific marked effect of glutamic acid on the crystallization of calcium oxalate while aspartic acid and alanine affected the process very slightly. It is known that aspartic acid and alanine are transformed into glutamic acid by enzyme activity. A short survey performed in this laboratory showed that the relevant enzyme activity was relatively high in healthy urine and low in stone formers' urine. The AA factor (FAA) proposed is FAA = [Glutamic acid]/[( Aspartic acid] + [Alanine]) its value in fractions of non-potent inhibitory material and in kidney stone matrices is below 0.6. In potent portions of the inhibitory material separated from healthy people and young animals urine the value is 0.8 and above.

Transparent macroemulsions for cosmetic applications.

Synopsis This study proposes a method for preparation of transparent emulsions for cosmetic applications. The continuous phase in oil-in-water emulsions consisted of both water and several polyols in order to equalize the refractive indices of the two phases. Sorbitol, glycerol and mannitol were the main polyols used in the adequate concentration and isopropyl myristate was the oil phase. Stabilities of a few days in 60 degrees C incubation and a few months on the shelf were obtained. Combinations of triethanolamine oleate together with sorbitan esters and ethoxylated sorbitan esters were found to improve the emulsion stability. The optimal water concentration was checked by spectrophotometer measurements at 660 nm in order to achieve best transparency. In conclusion, it was demonstrated that relatively stable transparent macroemulsions can be prepared by equalizing the refractive indices of the two phases composing the emulsion.