Interaction of liposomes with human skin in vitro--the influence of lipid composition and structure.

Liposomes have been suggested as a vehicle for dermal and transdermal drug delivery, but the knowledge about the interaction between lipid vesicles and human skin is poor. Therefore, we visualized liposome penetration into the human skin by confocal laser scanning microscopy (CLSM) in vitro. Liposomes were prepared from phospholipids in different compositions and labeled with a fluorescent lipid bilayer marker, N-Rh-PE (L-alpha-phosphatidylethanolamine-N-lissamine rhodamine B sulfonyl). Fluorescently labelled liposomes were not able to penetrate into the granular layers of epidermis. However, the fluorescence from liposome compositions containing DOPE (dioleylphosphatidyl ethanolamine) was able to penetrate deeper into the stratum corneum than that from liposomes without DOPE. Pretreatment of skin with unlabeled liposomes containing DOPE or lyso-phosphatidyl choline (lyso-PC) enhanced the subsequent penetration of the fluorescent markers, N-Rh-PE and sulforhodamine B into the skin, suggesting possible enhancer activity, while most liposomes did not show such enhancement. Resonance energy transfer (RET) and calcein release assay between stratum corneum lipid liposomes (SCLLs) and the phospholipid vesicles suggested that the liposomes containing DOPE may fuse or mix with skin lipids in vitro and loosen the SCLL bilayers, respectively. Among the factors not affecting stratum corneum penetration were: negative charge, cholesterol inclusion and acyl chain length of the phospholipids. In conclusion, fusogenicity of the liposome composition appears to be a prerequisite for the skin penetration.

Silica xerogel as an implantable carrier for controlled drug delivery--evaluation of drug distribution and tissue effects after implantation.

The purpose of the present study was to examine controlled delivery of toremifene citrate from subcutaneously implanted silica xerogel carrier and to evaluate silica xerogel related tissue effects after implantation. Toremifene citrate was incorporated into hydrolyzed silica sol in a room temperature process. Toremifene citrate treated silica xerogel implants were tested both in vitro and in vivo using healthy mice. Silica xerogel with tritium-labelled toremifene was implanted subcutaneously in mice for 42 d. To determine the amount of tritiated toremifene remaining in the silica discs at the implantation site, the discs were excised periodically and radioactivity measured. The amount of tritiated toremifene in the implant after 42 d was still about 16% and the amount of silica xerogel about 25%. In a histopathological study silica xerogel did not show any tissue irritation at the site of the implantation. A fibrotic capsule was formed around the implant. No silica xerogel related histological changes in liver, kidney, lymph nodes and uterus were observed during the implantation period. The silica xerogel discs showed a sustained release of toremifene citrate over 42 d. Histologically, toremifene-related changes in the uterus were also detectable at all studied time points. These findings suggest that silica xerogel is a promising carrier material for implantable controlled drug delivery system.

Phospholipids affect stratum corneum lipid bilayer fluidity and drug partitioning into the bilayers.

Phospholipids, e.g. fluid-state EPC (l-alpha-phosphatidylcholine from egg yolk), may diffuse into the stratum corneum and enhance dermal and transdermal drug penetration, while many other phospholipids, e.g. gel-state DSPC (distearoylphosphatidyl choline), are not able to do this. These effects are suggested to be due to the interactions between the phospholipids and the skin lipid bilayers, and so an in vitro method was developed to evaluate the influence of phospholipids on the distribution of drugs to stratum corneum lipids. The distribution coefficients of estradiol, progesterone and propranolol between stratum corneum lipid liposomes (SCLLs) without phospholipids or with EPC, DSPC, SPC (l-alpha-phosphatidylcholine from soybean) or DOPE (dioleylphosphatidyl ethanolamine), and pH 7.4 buffer were determined. Fluid-state phospholipids in SCLLs increased the partitioning of drugs into SCLLs, while gel-state lipid, DSPC, did not. The increased distribution of drugs into the SCLLs was at least partially due to the increased fluidity of SCLL bilayers by phospholipids, which was shown using steady-state fluorescence anisotropy. This in vitro method enables screening of the effects of phospholipids and other permeation enhancers on stratum corneum bilayer fluidity and drug partitioning.

Alkyl-substituted silica gel as a carrier in the controlled release of dexmedetomidine.

The effect of alkyl substitution of the silica xerogel matrix on the release rate of dexmedetomidine was evaluated. Silica sol was processed by either casting or spray drying. When the reaction precursor tetraethylorthosilicate (TEOS) was partially substituted with tri- or dialkoxysilane, the release of dexmedetomidine and degradation of the matrix were decreased compared with 100% TEOS-based gel. Increasing the number or length of the organic groups attached to silicon, modified the silica gel structure and reduced the release rate of dexmedetomidine from monoliths. The release of dexmedetomidine from alkyl-substituted silica gel microparticles, however, showed a burst in drug release. Subcutaneously administered silica xerogel matrices (manufactured by casting, containing 25 mol% dimethyldiethoxysilane at two different doses of dexmedetomidine) were studied in dogs. Sustained delivery of dexmedetomidine was obtained for at least 48 h.

Dependence between dissolution rate and porosity of compressed erythromycin acistrate tablets.

The correlation between dissolution rate and porosity of compressed erythromycin acistrate tablets was studied. The total porosity of the tablets, the pore size distribution and the specific surface area of the pores were determined using high-pressure mercury porosimetry. The particle size and specific surface area of the raw material and of the dry granulated mass of the tablets were also determined. The results show that the pore size distribution, showing the differences in pore structure, is more informative than total intruded volume of mercury. However, it is very difficult to explain the dissolution behaviour of erythromycin acistrate tablets only by porosity results of the tablets, and more work is still needed in this field.

Liposome-skin interactions and their effects on the skin permeation of drugs.

The aim of the study was to evaluate the interaction of phospholipid liposomes with skin and stratum corneum lipid liposomes (SCLLs). The influence of phospholipid liposomes on the skin permeability of model drugs was also studied. The transdermal flux of the drugs applied in various phospholipid containing formulations through human epidermis was studied in diffusion chambers. Liposomes in water solutions did not enhance the skin permeability of the drugs, but when ethanol (32% w/v) was present in the donor with EPC (egg yolk lecithin), permeabilities of some model drugs were substantially increased. Confocal microscopy studies revealed that EPC do not penetrate into the skin from water solutions, while from ethanol solutions, EPC penetrates deeply into the stratum corneum. Also, resonance energy transfer between different liposome compositions and the release of calcein from SCLLs showed that interactions between phospholipid liposomes and SCLLs increased with increasing ethanol concentration in the liposome solutions.

Transdermal delivery of levosimendan.

The aim of this study was to determine if transdermal penetration of levosimendan, a novel positive inotropic drug, could be enhanced and controlled by formulation modifications. Penetration of levosimendan across human epidermis in vitro was determined using abdominal excised skin and diffusion cells. Predicted steady-state plasma concentrations of levosimendan were estimated using permeabilities and pharmacokinetic parameters of levosimendan. For penetration enhancement we used different pH values, co-solvents, cyclodextrins, surfactants, penetration enhancers, liposomes, and iontophoresis. Sodium lauryl sulfate, ethanol, oleic acid, and soya phosphatidylcholine or their combinations clearly increased levosimendan permeation across the skin in vitro. Iontophoresis was also an efficient method to increase transdermal permeation of levosimendan. A hydrophilic co-solvent/penetration enhancer is needed to achieve better permeability of levosimendan across the skin. In conclusion, transdermal delivery of levosimendan can be significantly increased by formulation modification. Based on kinetic calculations, therapeutic plasma concentrations may be achievable transdermally.

Determination of optimal combination of surfactants in creams using rheology measurements.

The effect of surfactant on the rheological properties of some cream formulations was studied. Two surfactants from two different series were combined to determine the combination which yielded the most viscoelastic structure for creams. The surfactants were the soybean derivatives soya sterol, polyethylene glycol 10 soya sterol and polyethylene glycol 25 soya sterol and the sorbitol derivatives sorbitan monooleate and sorbitan trioleate. The rheological properties of the creams were studied using oscillation stress sweep, oscillation frequency sweep and viscosity tests. Droplet size distribution and conductivity of the creams were also determined. The combination polyethylene glycol 10 soya sterol and sorbitan trioleate yielded the most viscoelastic structure with linearly viscoelastic behaviour.

Silica xerogel carrier material for controlled release of toremifene citrate.

Sol-gel processed silica xerogel was used as a carrier material for toremifene citrate in order to develop an implantable controlled release formulation which could be localised to a desired site providing targeted and long-lasting disease control and resulting in a reduced amount of drug needed. Toremifene citrate, an anti-estrogenic compound, was incorporated into silica xerogel matrixes during polycondensation of organic silicate, tetraethyl ortho silicate (TEOS). The effects of drug amount, drying temperature and polyethylene glycol (PEG) on the release rate of toremifene citrate and degradation of the silica xerogel matrixes were investigated. Addition of PEG (M(w) 4600/10000) decreased the specific surface area of the matrix and lowered the release rate of the drug. Reducing the amount of drug in the matrix also decreased the release rate of toremifene citrate. However, drying temperature did not affect the release rate of silica or toremifene citrate. The release profiles of toremifene citrate were according to zero order kinetics, suggesting that drug release was controlled by erosion of the silica xerogel matrix. These results suggest that the toremifene citrate release rate can be controlled to some extent by adding (PEG) or by varying the amount of drug in the silica xerogel matrix.

Microemulsions for topical delivery of estradiol.

Estradiol has been widely used for the treatment of hormonal insufficiencies. Due to its extensive first pass metabolism after oral administration, transdermal administration of estradiol in gels and emulsions has been used to improve its bioavailability, prolong activity and to optimize metabolic profile. The purpose of this study was to investigate microemulsions as delivery systems for estradiol. Various o/w microemulsions were used to deliver estradiol across human abdominal skin in vitro. Trasdermal flux of estradiol was determined using Franz-type diffusion cells and the samples were analyzed by high-performance liquid chromatography (HPLC). The permeation data showed that microemulsion formulations increased estradiol flux 200-700-fold over the control, but permeability coefficients were decreased by 5-18 times. The superior transdermal flux of estradiol was due to 1500-fold improvement in solubilization of estradiol by microemulsions. The results suggest that microemulsions are potential vehicles for improved topical delivery of estradiol.

In vitro evaluation of biodegradable epsilon-caprolactone-co-D, L-lactide/silica xerogel composites containing toremifene citrate.

Poly(epsilon-caprolactone-co-D,L-lactide) polymers were blended with toremifene citrate or with toremifene citrate impregnated silica xerogel in order to develop a controlled release formulation. The copolymers were synthesized by bulk polymerization and characterized by nuclear magnetic resonance, size exclusion chromatography and differential scanning calorimetry analyses. The in vitro release of toremifene citrate, an antiestrogenic compound, and silica was carried out in simulated body fluid (pH 7.4) containing 0.5 wt% sodium dodecylsulphate at 34 degrees C. The in vitro release studies indicate that the release flux of toremifene citrate increases with increasing weight fraction of caprolactone in the copolymer. Silica xerogel had a minor enhancing effect on the release rate of toremifene citrate. Copolymers containing larger amounts of D,L-lactide (PLA-CL20 and PLA-CL40 copolymers) were not suitable matrices for the delivery of toremifene citrate in a controlled manner because of the burst effect. The fraction of toremifene citrate released from PLA-CL80 matrix increased with the increasing loading of toremifene citrate. The results of the study indicate that the in vitro release of toremifene citrate can be adjusted by varying the polymer composition and also the initial drug loading.

Effect of the molecular weight of poly(epsilon-caprolactone-co-DL-lactide) on toremifene citrate release from copolymer/silica xerogel composites.

The purpose of this study was to develop a biodegradable polymeric carrier system for toremifene citrate based on epsilon-caprolactone/DL-lactide copolymers and silica xerogel. The effect of the molecular weight of poly(epsilon-caprolactone-co-DL-lactide) affecting the release rate of toremifene citrate from copolymer/silica xerogel composites was evaluated by in vitro dissolution study. Lower and higher molecular weight copolymers (LMW 60000 g/mol and HMW 300000 g/mol) were used in the devices. Drug release was compared from the (copolymer/drug) matrix device and the (copolymer/drug impregnated silica xerogel) composite device. Hydrolysis of the copolymer devices was evaluated by water absorption, weight loss and change of molecular weight by size exclusion measurements (SEC). Controlled release of toremifene citrate was obtained from both matrix and composite devices and the release rate was most affected by the initial molecular weight of the copolymer. Throughout the study better results were obtained with LMW devices, since drug release was steady for nearly 1 year and no changes in the release rate were observed. The drug release was diffusion controlled from both LMW matrix and composite devices. Incorporation of toremifene citrate into the silica xerogel was found to enhance the drug release rate. The copolymer matrices degraded by random hydrolytic chain scission and, unexpectedly, HMW P(CL/LA) degraded faster than LMW P(CL/LA). The release of toremifene citrate from HMW devices was not complete before the second stage of polymer degradation began.

In vitro evaluation of sol-gel processed spray dried silica gel microspheres as carrier in controlled drug delivery.

The objective of this study was to evaluate sol-gel-derived spray dried silica gel microspheres as carrier material for dexmedetomidine HCl and toremifene citrate. The drug was dissolved in sol-gel processed silica sol before spray drying with Büchi laboratory scale equipment. Microspheres with a low specific surface area were spherical by shape with a smooth surface without pores on the external surface. The particle size distribution was quite narrow. The in vitro release of toremifene citrate and dexmedetomidine HCl showed a dose-dependent burst followed by a slower release phase, that was proportional to the drug concentration in the concentration range between 3.9 and 15.4 wt.%. The release period for toremifene citrate was approximately 10 days and for dexmedetomidine HCl between 7 and 50 days depending on drug concentration. Spray drying is a promising way to produce spherical silica gel particles with a narrow particle size range for controlled delivery of toremifene citrate and dexmedetomidine HCl.

In vitro release of dexmedetomidine from silica xerogel monoliths: effect of sol-gel synthesis parameters.

Dexmedetomidine, an alpha 2-agonist, was incorporated as a hydrochloride salt into silica xerogel in order to evaluate the effect of sol-gel synthesis parameters: pH of the sol, water/alkoxide molar ratio, drug concentration and size of the device on the drug release rate and degradation rate of the matrix. This study showed that diffusion controlled the release of dexmedetomidine from silica xerogel prepared between pH 1 and pH 5. The drug release was, however, slowest near the zero charge of silica xerogel (pH 2-3). The burst of dexmedetomidine, a lipophilic, but in the form of hydrochloride salt water-soluble drug, was increased from the matrix prepared either below or above the isoelectric point. It follows that the optimum pH for preparing a drug delivery device for dexmedetomidine, is near the zero charge of silica xerogel, where the degradation of the matrix was also slowest. In addition to processing pH, the release rate of drugs can be controlled by changing the water/alkoxide molar ratio of the sol.

Sol-gel-processed sintered silica xerogel as a carrier in controlled drug delivery.

Sol-gel-processed sintered silica xerogel was studied as a controllable, dissolvable, implantable material. The erosion of the matrix and the release of the preadsorbed drug toremifene citrate was investigated both in vitro and in vivo using mice. In an in vitro dissolution study, 50 to 60% of the drug was released after 24 h, according to the square root of time kinetics, and the weight loss of the silica was 24 wt %. Silica xerogel with tritium-labeled toremifene was implanted subcutaneously in mice for 56 days. To determine the amount of tritiated drug remaining in the silica disks at the implantation site, the disks were excised periodically and the radioactivity measured. About 40% of the radioactivity was released during the first 4 days and all of it within 28 days. Radioactivity also was measured in the liver, lungs, kidneys, uterus, and blood. The radioactivity reached a maximum level after 4 days in the liver, kidneys, and lungs and slowly decreased until all of the drug had been released from the matrix after 28 days. After release of the drug (28 days) the amount of remaining silica xerogel implant was 45 wt %, and at the end of the study (56 days) it was 24 wt %. In the histopathological study, sintered silica xerogel did not show any tissue toxicity at the site of the implantation, in the liver, or in the kidneys. It was concluded that sintered silica xerogel is a biocompatible and controllably resorbable material and therefore is a promising matrix for use in the sustained delivery of drugs.

In vitro release behavior of toremifene citrate from sol-gel processed sintered silica xerogels.

Factors affecting the adsorption and desorption of toremifene citrate (TC) on sintered silica xerogels were investigated in vitro. TC was attached onto sol-gel processed sintered silica xerogel grains or disks by adsorption. The adsorption of TC on the surface of silica was pH dependent. The results support the conclusion that large pore size results in highest drug adsorption. Adsorption of TC was most effective in xerogels sintered at 700 degrees C and containing the largest pores and lowest specific surface area of the silica xerogels studied in the adsorption tests. The release of TC from the xerogel matrix was linear with respect to the square root of time. The release of TC from the grains was very rapid for the first 5 hr, followed by a slower release. All drug was released from the grains, and 60% to 80% was released from the disks in 24 hr. All drug-silica xerogel formulations showed sustained in vitro release profiles.