Room temperature ionic liquids and their mixtures: potential pharmaceutical solvents.

Room temperature ionic liquids (RTILs) are organic salts which are liquids at ambient temperature. Composed of relatively large asymmetric organic cations and inorganic or organic anions, they have generated interest as 'green' solvents. Here we report on the solvency of alkyl imidazolium salts (PF(6)(-)Br(-)Cl(-)) for poorly water-soluble model drugs, albendazole and danazol, indicating their potential application as pharmaceutical solvents/cosolvents. The solubility of albendazole, for example, is increased by more than 10,000 times by 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF(6)(-)). Ionic liquids can be water-miscible or water-immiscible. The aqueous miscibility of a poorly water-miscible RTIL such as of [bmim]PF(6)(-) can be improved by the inclusion of a second more miscible RTIL (e.g. 1-hexyl-3-methylimidazolium bromide ([hmim]Br(-))). The extent of improvement in water miscibility was found to correlate with the hydrophilicity of the second RTIL. This ability to modulate RTILs' aqueous miscibility increases their usefulness as pharmaceutical solvents.

Local controlled drug delivery to the brain: mathematical modeling of the underlying mass transport mechanisms.

The mass transport mechanisms involved in the controlled delivery of drugs to living brain tissue are complex and yet not fully understood. Often the drug is embedded within a polymeric or lipidic matrix, which is directly administered into the brain tissue, that is, intracranially. Different types of systems, including microparticles and disc- or rod-shaped implants are used to control the release rate and, thus, to optimize the drug concentrations at the site of action in the brain over prolonged periods of time. Most of these dosage forms are biodegradable to avoid the need for the removal of empty remnants after drug exhaustion. Various physical and chemical processes are involved in the control of drug release from these systems, including water penetration, drug dissolution, degradation of the matrix and drug diffusion. Once the drug has been released from the delivery system, it has to be transported through the living brain tissue to the target site(s). Again, a variety of phenomena, including diffusion, drug metabolism and degradation, passive or active uptake into CNS tissue and convection can be of importance for the fate of the drug. An overview is given of the current knowledge of the nature of barriers to free access of drug to tumour sites within the brain and the state of the art of: (i) mathematical modeling approaches describing the physical transport processes and chemical reactions which can occur in different types of intracranially administered drug delivery systems, and of (ii) theories quantifying the mass transport phenomena occurring after drug release in the living tissue. Both, simplified as well as complex mathematical models are presented and their major advantages and shortcomings discussed. Interestingly, there is a significant lack of mechanistically realistic, comprehensive theories describing both parts in detail, namely, drug transport in the dosage form and in the living brain tissue. High quality experimental data on drug concentrations in the brain tissue are difficult to obtain, hence this is itself an issue in testing mathematical approaches. As a future perspective, the potential benefits and limitations of these mathematical theories aiming to facilitate the design of advanced intracranial drug delivery systems and to improve the efficiency of the respective pharmacotherapies are discussed.

Adsorption of amphipathic dendrons on polystyrene nanoparticles.

Adsorption of dendrons onto nanoparticles may provide new model structures which may be useful in drug and gene delivery. Tritiated amphipathic dendrons having three lipidic (C(14)) chains coupled to branched (dendritic) lysine head groups with 8, 16 or 32 free terminal amino groups have been synthesised by solid phase peptide techniques. The interaction between these tritiated dendrons and 200 nm polystyrene latex nanoparticles was investigated in phosphate buffered saline. The amount of dendron adsorbed increased with increasing concentration of dendrons and then decreased. Maximum adsorption of dendrons per gram of nanoparticles was found to be between 8.2 and 84 x 10(-6)M, the amounts adsorbed being inversely proportional to the number of amino groups present in the molecule. The number of dendron molecules adsorbed per nanoparticle was found to be between 430 and 4421. The degree of adsorption was found to be slightly altered by the temperature.

Transcytosis of nanoparticle and dendrimer delivery systems: evolving vistas.

The translocation of particulate matter across the gastrointestinal tract is now a well documented phenomenon offering new potential for the delivery of drugs with poor dissolution profiles and labile chemistries via encapsulation in biodegradable nanoparticles. The last few years have seen an acceleration in the number of publications describing the varying facets of this approach and the multidisciplinary nature of this field. This review delineates data from this rather fragmented area and from cognate fields to provide a physicochemical viewpoint of the importance of surface chemistries of oral drug delivery vehicles and their interactions in and with gut contents prior to uptake. The role of lymphoid and non-lymphoid tissues is examined, and the role of bioadhesion is discussed. The exciting potential of molecular encapsulation of drugs via dendrimers and star branched molecules is discussed in the context of nanotechnological applications for the oral route. Evolving vistas include a better understanding of the plasticity of the intestinal epithelium and M-cell induction as well as the influence of disease states on particulate uptake. In this review we address a number of issues deemed vital to an understanding of the subject including (i) some background knowledge on particulate uptake (the subject of several reviews), (ii) factors affecting uptake such as diameter and surface charge and character, (iii) the dynamic nature of particle interactions in the gut, (iv) the dynamic nature of the processes of capture, adhesion, uptake, transcytosis and translocation, and (v) the influence of surface ligands.

Non-aqueous emulsions: hydrocarbon-formamide systems.

There are few reports in the literature on formulation of non-aqueous emulsions. This study was designed to evaluate some design criteria for such systems. Formamide is the closest polar solvent that has the ability to replace water in emulsification when employing established non-ionic surfactants as stabilisers. For the majority of studies, linear alkanes (C6-C16) were dispersed in formamide as the continuous phase were stabilised with polysorbate 20. Initial studies involved gentle emulsification and observing mean globule size. The mean globule size varied in a non-linear fashion with alkyl chain length, the minimum being between C10 and C12. Sonication for 30 s led to smaller differences in the mean globule size. The effect of various parameters such as surfactant concentration and solvophilicity of the surfactant was observed. The surface activities of polysorbate 20, 40, 60 and 80 in formamide and critical micellar concentrations were determined. The latter were several orders of magnitude higher in formamide than in water, and the areas per molecule larger. The addition of water to the dodecane formamide systems did not destabilise the emulsion. Release of the model drug dehydroepiandrosterone from dodecane in formamide emulsions was studied in distilled water, the rate of release being dependent on the volume fraction of dodecane.

Interfacial behaviour and micelle formation of novel amphiphilic sequential lipid-lysine oligomers.

As part of work on the design and synthesis of new supramolecular carrier systems for drugs, a series of novel linear oligomers of alternating alpha-amino tetradecanoic acid and lysine having positively charged groups and lipid chains was synthesised. The smallest member of the series (n=2) is insoluble in water and diluted acid solutions, but the larger members are soluble in acid conditions and poorly soluble in alkaline conditions. Hence, in one series, one can conduct experiments both on the determination of micelle formation and spread monolayer behaviour. The surface pressure-area isotherms revealed limiting surface areas at the air/water interface ranged from 0.04 to 0.9 nm(2) according to the oligomer size, and a linear correlation between the observed area per molecule and that projected by computer-generated molecular models was demonstrated. The surface tension of the soluble members in dilute acid solution fell as the concentration of the oligomers was increased, indicating that all of these polymers were surface active with quite clearly defined critical micelle concentrations. The fluorescence intensity ratio of third to first band in the emission spectra of pyrene as a function of the polymer concentrations demonstrated that, even after normalising the data for the amount of lipid chains in the system, the (n=3) oligomer had fewer accessible hydrophobic sites for pyrene, and the forces of the repulsion between the charged head groups was crucial on the formation of micelles, especially in the case of the n=3 oligomer. Supramolecular fibre-like structures were observed in aqueous solution only when n=3 by transmission electron microscopy (TEM). Cryogenic TEM observation of the (n=3) solution also revealed that the micelles might elongate to form long cylindrical or fibrous structures. The diameter of these structures was estimated to be 6.0-13 nm, although their length varied.

Interaction of cationic partial dendrimers with charged and neutral liposomes.

Amphipathic partial dendrimers having three lipidic (C(14)) chains coupled to dendritic lysine head groups with eight, 16 or 32 free terminal amino groups have been synthesised by solid-phase peptide synthesis. Liposomes were prepared with positive, negative and neutral charge using the dehydration-rehydration method and their interaction with the partial dendrimers studied. The interaction efficiency of the partial cationic dendrimers studied was greater than 88%. Interaction of the cationic partial dendrimer converted liposomes with very low or negative charge into positively charged species. Apparent vesicle size increased with the head size of the partial dendrimer but, in the case of negatively charged liposomes, large changes in properties were observed after ultracentrifugation due to the formation of myelin figures. To investigate the mode of interaction with the liposomes, adsorption studies were performed by adding the partial dendrimer after the preparation of dehydration-rehydration vesicles. The results indicated that adsorption is inversely proportional to the head size of the partial dendrimer molecules and the extent of adsorption was similar on both positively and negatively charged liposomes. Adsorption produced liposomes with greater or similar zeta potentials to liposomes that incorporated partial dendrimer through the dehydration-rehydration method. Taking account of the different interaction efficiencies, this suggests there is a degree of partial dendrimer entrappment inside the liposomes.

Increased vigabatrin entry into the brain by polysorbate 80 and sodium caprate.

The effects of a non-ionic surfactant, polysorbate 80, and the sodium salt of the saturated fatty acid, sodium caprate (C10), as potential brain absorption enhancers for vigabatrin were studied. Vigabatrin is an enzyme-activated irreversible inhibitor of gamma-aminobutyric acid (GABA) transaminase that increases brain and cerebrospinal GABA concentrations in animals and man. Before intravenous administration, a range of concentrations of the surfactants were tested using erythrocyte lysis or the red blood cell lysis test to establish the non-toxic concentration range. Vigabatrin was dissolved in 0.1% polysorbate 80 and 0.1% sodium caprate and administered intravenously in doses of 4 mL kg(-1) to male Wistar rats (230-250 g; n = 3). Rats were killed 2 h after drug and surfactant administration and the brains were immediately removed and homogenized in 0.4 M perchloric acid. Selected ion monitoring electrospray mass spectrometry was used to determine the concentration of vigabatrin and GABA directly from the perchloric acid extract of the rat brain. This method was developed to increase the speed and efficiency of the analysis by removing the need for complex extraction and derivatization procedures while retaining the specificity of the mass spectrometer as a detector. The stability of both vigabatrin and GABA in perchloric acid was established by monitoring their pseudo molecular ions in standard solutions at timed intervals over 24 h. Although the detection level for vigabatrin and GABA was at least 50 pg, only GABA was detected in rat brain. Vigabatrin caused a small increase in whole brain GABA. However, GABA levels were higher in the samples with vigabatrin + enhancer than in the samples where vigabatrin alone was administered. One-way analysis of variance indicated a significant effect of the surfactants on GABA levels (F (5,17) = 11.86, P < 0.01) and vigabatrin absorption was presumed. The rectal temperature of the rats is lowered by the presence of vigabatrin in the brain. Vigabatrin alone decreased rectal temperature by 6%. When given with either polysorbate 80 or sodium caprate, the extent of temperature lowering was significantly greater (P < 0.001). There was no significant difference after 2 h between polysorbate 80 + vigabatrin, and sodium caprate + vigabatrin.

DNA transfection and transfected cell viability using amphipathic asymmetric dendrimers.

Amphipathic asymmetric dendrimers have been investigated for use in delivery of genes into cells, with the objective of optimising transfection efficiency and maintaining cell viability. We have synthesised amphipathic asymmetric dendrimers by solid phase methods. The ability of two of these to transfect BHK cells in culture with beta-galactosidase gene was determined by X-gal staining. Cell viability was measured by the MTT assay for BHK cells, and by spectroscopy for lysis of erythrocytes. Interactions between dendrimer and DNA were investigated by agarose gel electrophoresis. BHK cells were optimally transfected at 5:1 +/- charge ratio yielding 20% cells receiving at least one copy of the plasmid. Cell viability decreased when the dendrimer to DNA ratio exceeded 5:1. Raising the pH significantly affected the electrophoretic mobility of complexes of dendrimer and DNA. We conclude that amphipathic asymmetric dendrimers enable efficient plasmid DNA uptake into BHK cells. Cell viability is maintained at high concentrations of dendrimer when complexed with DNA at a 5:1 +/- charge ratio. Efficiency of transfection and cell viability suggest the system may be suitable for gene delivery in vivo.

The effect of processing variables on the physical characteristics of non-ionic surfactant vesicles (niosomes) formed from a hexadecyl diglycerol ether.

Niosomes are vesicles formed by self-assembly of non-ionic surfactants. In this investigation, the effects of processing variables, particularly temperature and sonication, on the physical characteristics and phase transitional behaviour of two niosomal systems based on a hexadecyl diglycerol ether (C(16)G(2)) have been studied. Systems containing C(16)G(2), cholesterol and poly-24-oxyethylene cholesteryl ether (Solulan C24) in the molar ratios 91:0:9 and 49:49:2 were prepared by aqueous dispersion of films, followed by examination of 5(6)-carboxyfluorescein entrapment, particle size and morphology. The thermal behaviour was examined using high sensitivity differential scanning calorimetry (HSDSC) and hot stage microscopy, while the effects of sonication were studied in terms of size and morphology, both immediately after preparation and on storing for 1 h at room temperature and 60 degrees C. Polyhedral niosomes were formed from systems containing C(16)G(2) and Solulan C24 alone, while cholesterol-containing systems formed spherical vesicles mixed with tubular structures; the polyhedral systems were found to have a larger particle size and higher CF entrapment efficiency. HSDSC studies showed the polyhedral systems to exhibit an endotherm at 45.4 degrees C and a corresponding exotherm at 39.1 degrees C on cooling which were ascribed to a membrane phase transition; no equivalent transition was observed for the cholesterol containing systems. Hot stage microscopy showed the polyhedral vesicles to convert to spherical structures at approximately 48 degrees C, while on cooling the spherical vesicles split into smaller structures and reverted to the polyhedral shape at approximately 49 degrees C. Sonication resulted in the polyhedral vesicles forming spherical structures which underwent a particle size increase on storage at room temperature but not at 60 degrees C. The study suggests that the polyhedral vesicles undergo a reversible transition to spherical vesicles on heating or sonication and that this morphological change may be associated with a membrane phase transition.

Oral uptake and translocation of a polylysine dendrimer with a lipid surface.

A series of lipidic peptide dendrimers based on lysine with 16 surface alkyl (C(12)) chains has been synthesised in our laboratories. One of the series, a fourth generation dendrimer with a diameter of 2.5 nm was chosen to study its absorption after oral administration to female Sprague-Dawley rats (180 g, 9 weeks old). It was synthesised as the tritiated derivative (all acetyl portions) and had a molecular weight of 6300 and log P (octanol/water) of 1.24. First a single oral dose 14 mg/kg was administered by gavage. Maximum levels of dendrimer observed were 15% in the small intestine, 5% in the large intestine and 3% in the blood at 6 h after administration, while 1.5% reached the liver, 0.1% the spleen and 0. 5% the kidneys. In a parallel study with a higher dose of 28 mg/kg, approximately 1% was absorbed via Peyer's patches of the small intestine at 3 h. The maximum uptake by small intestine enterocytes was 4% of the dose after 3 h. After 12 h, 0.3 and 4% dendrimer was measured respectively in Peyer's patches and enterocytes of the large intestine. When calculated on the basis of target tissue weight, the total percentage of the dose absorbed through Peyer's patches was greater than through normal enterocytes in the small intestine after 3 and 24 h, but the opposite was true in the large intestine. These levels of uptake and translocation are lower than those exhibited by polystyrene particles in the range from 50 to 3000 nm. This might suggest that there is an optimum size for nanoparticulate uptake by the gut.

Effects of some non-ionic surfactants on transepithelial permeability in Caco-2 cells.

The effects of the non-ionic surfactants polysorbate 20, polysorbate 60, polysorbate 85, cholesteryl poly (24) oxyethylene ether (Solulan C24) and the lanolin-based poly (16) oxyethylene ether (Solulan 16) on the epithelial integrity of monolayers of human intestinal epithelial (Caco-2) cells has been studied using metformin as a model drug. The aim was to identify the surfactants and their optimal concentrations capable of enhancing drug transport while causing no, or only minor, cellular damage. Effects on cell permeability were assessed by measurements of the transport of metformin, a hydrophilic drug, by monitoring transepithelial electrical resistance. Cell viability was determined by the diphenyltetrazolium bromide test (the MTT test). All the surfactants studied demonstrated concentration-dependent effects on cell permeability and cell viability. The effects on transepithelial electrical resistance correlated with cell viability, i.e. increased transepithelial electrical resistance and increased cell-monolayer permeability for metformin corresponded to decreased cell viability. The results indicate that the Solulan and polysorbate surfactants were active as absorption enhancers, Solulan C24 and 16 being more effective than polysorbates 20, 60 or 85, causing an increase in metformin transport at lower concentrations than the polysorbates. Polysorbate 20 exerted its greatest effect at a concentration of 5%-increasing the flux of metformin after 3 h by a factor of around 20 over the control. Large increases in the transport of metformin, especially at surfactant levels of 0.05%, 0.1% and 0.5%, were related to the effect of Solulan C24 and Solulan 16 on the cell permeability. The Caco-2 cell monolayer experiments confirmed the ability, especially of polysorbate 20, Solulan C24 and Solulan 16, to increase the absorption of metformin. The polysorbates increased permeability as a result of solubilisation of membrane components, while Solulans did so by penetrating and solubilising the membrane. Correlation between increase in membrane permeability and the toxicity of the surfactants towards the cell membrane has been established.

Osmotic behaviour of polyhedral non-ionic surfactant vesicles (niosomes).

In addition to common spherical non-ionic surfactant vesicles (niosomes), disc-like, tubular, and polyhedral niosomes have also been reported. The permeability and osmotic activity of niosomes are important in determining their use as controlled-release drug-delivery systems. These properties have been compared for polyhedral niosomes prepared by hydrating a mixture of a hexadecyl diglycerol ether (C16G2), a poly(24)oxyethylene cholesteryl ether (Solulan C24), 91:9 or 98:2, and conventional spherical niosomes prepared from the same surfactants but with cholesterol. When subjected to osmotic gradients, polyhedral niosomes, the membranes of which are in the gel phase, swell and shrink less than their spherical counterparts and they are more permeable to the hydrophilic solute 5(6)-carboxyfluorescein. In 2 M NaCl the rate of release of carboxyfluorescein from polyhedral niosomes (both containing 9% Solulan C24) into either a hypotonic (water) or an isotonic medium (2 M NaCl) was low. This contrasted with similarly loaded spherical niosomes and polyhedral niosomes containing 2% Solulan C24, from which release was high in hypotonic media (e.g. water) but less in an isotonic medium (2 M NaCl). For both polyhedral and spherical niosomes encapsulating carboxyfluorescein (pKa = 6.4), release rates were higher at pH 8 than at pH 5. Polyhedral niosomes are thus, in general, less osmotically active than spherical niosomes because of their rigid but highly permeable membranes. The unusual polyhedral membrane impermeability to carboxyfluorescein co-entrapped with salt in hypotonic media is a function of Solulan C24 content, and is possibly a result of salting out of the polyoxyethylene chains; this is, therefore, a property that might be manipulated in the design of a drug-delivery system.

Extrusion of niosomes from capillaries: approaches to a pulsed delivery device.

We describe an early prototype of a pulsatile delivery system for drug containing vesicles. Nonionic surfactant vesicles (niosomes) of average diameter 4-30 microm are extruded from glass capillaries (exit diameter, 5-10 microm), using air pressures of 0.5-5 p.s.i. The formulation of the vesicles is vital. Extrusions were affected by the size, shape, and membrane composition of the niosomes used. Spherical or polyhedral niosomes, formed by polyoxyethylene alkyl ethers with and without cholesterol, respectively, with diameters larger than the exit diameter of the capillary do not retain their membrane integrity on extrusion and were sheared to form new ultrastructures. The expulsion of single or groups of intact polystyrene microspheres or tetradecyl-beta-D-maltoside niosomes with sizes smaller than the exit diameter can be achieved readily. The stepwise release profile of luteinizing hormone releasing hormone (LHRH) obtained after pulsatile expulsion of groups of niosomes entrapping LHRH indicates the feasibility of this system for pulsatile delivery of vesicles, although it requires miniaturization.

Sorbitan monostearate/polysorbate 20 organogels containing niosomes: a delivery vehicle for antigens?

Multi-component organogels formed using the non-ionic surfactant sorbitan monostearate as gelator have been formulated to contain niosomes. The purpose of this study was to evaluate the potential of these vesicle-in-water-in-oil (v/w/o) gels as delivery vehicles for vaccines. Bovine serum albumin (BSA) and haemagglutinin (HA) were used as model antigens in depot and immunogenicity studies respectively. The complex gels were prepared by the addition of a hot (60 degrees C) aqueous niosome suspension (v/w) to the sol phase (o, an organic solution of the gelator); a vesicle-in-water-in-oil (v/w/o) emulsion was produced which cools to an opaque, semi-solid, thermoreversible v/w/o gel. Light microscopy of the organogel revealed that the microstructure consists of a tubular network of surfactant aggregates in the organic medium, the niosome suspension being dispersed in these surfactant tubules. Therefore, in such gels, the vaccine is thought to be entrapped in the niosomes, themselves located within the sorbitan monostearate tubular network in the organic medium. In vivo, a depot effect was observed following intramuscular administration of the gel containing the entrapped bovine serum albumin, cleared from the injection site over a period of days. The relatively short-lived nature of the depot was thought to arise due to interactions between the gel and the local interstitial fluid which results in gel disintegration in situ. Thus, the niosomes containing antigens are believed to be released from the organic gel. Immunogenicity studies showed that the v/w/o gel as well as one of the controls, the water-in-oil (w/o) gel, possess immunoadjuvant properties and enhance the primary and secondary antibody titres (of total IgG, IgG1, IgG2a and IgG2b) to haemagglutinin antigen. As far as humoral immunity is concerned, the w/o gel showed stronger immunoadjuvant properties compared to the v/w/o gel, being effective at a lower antigen dose i.e 0.1 microg HA.

Interaction of a nonionic surfactant-based organogel with aqueous media.

In an attempt to explain the rather short half-life of molecules at the injection site after their intra-muscular administration in a sorbitan monostearate organogel, in vitro studies were carried out to study the effects of an aqueous medium (simulating interstitial fluid at injection site) on the physical form of the organogel. When the gel mass comes in contact with an aqueous phase, the latter penetrates into the organic gel via the sorbitan monostearate tubular network, resulting in gel breakdown into smaller fragments. The surfactant tubular network act as a conduit for water penetration into the gel. Meanwhile, emulsification, aided by the surfactants present in the gel, also occurs at the gel surface between the organogel and the aqueous phase. This leads to a gradual erosion of the gel as oil droplets bud off from the gel mass. From these in vitro observations, we speculate that after gel administration in vivo, dynamic interactions occur between the local interstitial fluid and the gel mass: fluid penetration into the gel and emulsification at the gel surface is thus responsible for gel breakdown and so a relatively short duration of drug at the injection site.