Release, uptake, and permeation behavior of salicylic acid in ointment bases

Skin models for dermal exposure assessment of phthalates

Phthalates are a class of compounds that have found widespread use in industrial applications, in particular in the polymer, cosmetics and pharmaceutical industries. While ingestion, and to a lesser degree inhalation, have been considered as the major exposure routes, especially for higher molecular weight phthalates, dermal exposure is an important route for lower weight phthalates such as diethyl phthalate (DEP). Assessing the dermal permeability of such compounds is of great importance for evaluating the impact and toxicity of such compounds in humans. While human skin is still the best model for studying dermal permeation, availability, cost and ethical concerns may preclude or restrict its use. A range of alternative models has been developed over time to substitute for human skin, especially in the early phases of research. These include ex vivo animal skin, human reconstructed skin and artificial skin models. While the results obtained using such alternative models correlate to a lesser or greater degree with those from in vivo human studies, the use of such models is nevertheless vital in dermal permeation research. This review discusses the alternative skin models that are available, their use in phthalate permeation studies and possible new avenues of phthalate research using skin models that have not been used so far.

Design and development of topical liposomal formulations in a regulatory perspective

The skin is the absorption site for drug substances intended to treat loco-regional diseases, although its barrier properties limit the permeation of drug molecules. The growing knowledge of the skin structure and its physiology have supported the design of innovative nanosystems (e.g. liposomal systems) to improve the absorption of poorly skin-permeable drugs. However, despite the dozens of clinical trials started, few topically applied liposomal systems have been authorized both in the EU and the USA. Indeed, the intrinsic complexity of the topically applied liposomal systems, the higher production costs, the lack of standardized methods and the more stringent guidelines for assessing their benefit/risk balance can be seen as causes of such inefficient translation. The present work aimed to provide an overview of the physicochemical and biopharmaceutical characterization methods that can be applied to topical liposomal systems intended to be marketed as medicinal products, and the current regulatory provisions. The discussion highlights how such methodologies can be relevant for defining the critical quality attributes of the final product, and they can be usefully applied based on the phase of the life cycle of a liposomal product: to guide the formulation studies in the early stages of development, to rationally design preclinical and clinical trials, to support the pharmaceutical quality control system and to sustain post-marketing variations. The provided information can help define harmonized quality standards able to overcome the case-by-case approach currently applied by regulatory agencies in assessing the benefit/risk of the topically applied liposomal systems.

Human skin models: From healthy to disease-mimetic systems; characteristics and applications

Skin drug delivery is an emerging route in drug development, leading to an urgent need to understand the behaviour of active pharmaceutical ingredients within the skin. Given, As one of the body's first natural defences, the barrier properties of skin provide an obstacle to the successful outcome of any skin drug therapy. To elucidate the mechanisms underlying this barrier, reductionist strategies have designed several models with different levels of complexity, using non-biological and biological components. Besides the detail of information and resemblance to human skin in vivo, offered by each in vitro model, the technical and economic efforts involved must also be considered when selecting the most suitable model. This review provides an outline of the commonly used skin models, including healthy and diseased conditions, in-house developed and commercialized models, their advantages and limitations, and an overview of the new trends in skin-engineered models.

In vitro skin models as a tool in optimization of drug formulation

(Trans)dermal drug therapy is gaining increasing importance in the modern drug development. To fully utilize the potential of this route, it is important to optimize the delivery of active ingredient/drug into/through the skin. The optimal carrier/vehicle can enhance the desired outcome of the therapy therefore the optimization of skin formulations is often included in the early stages of the product development. A rational approach in designing and optimizing skin formulations requires well-defined skin models, able to identify and evaluate the intrinsic properties of the formulation. Most of the current optimization relies on the use of suitable ex vivo animal/human models. However, increasing restrictions in use and handling of animals and human skin stimulated the search for suitable artificial skin models. This review attempts to provide an unbiased overview of the most commonly used models, with emphasis on their limitations and advantages. The choice of the most applicable in vitro model for the particular purpose should be based on the interplay between the availability, easiness of the use, cost and the respective limitations.

Alcohol enhanced permeation in model membranes. Part I. Thermodynamic and kinetic analyses of membrane permeation

While it is well recognised that formulation components influence drug permeation, few studies have addressed the influence of vehicles on drug transport in artificial or biological membranes Previously we have investigated the effects of temperature on the uptake of model vehicles (i.e. alcohols) into silicone membrane. The present study evaluates the permeation of the model drug methyl paraben in the presence of butanol or heptanol. Drug permeation through silicone membranes was studied at different temperatures for each vehicle. Thermodynamic and kinetic analyses of the permeation data were conducted to elucidate the possible mechanisms of drug transport. Independent examination of the partition and diffusion coefficients estimated for the permeation studies at different temperatures showed a break point occurring near 20 degrees C for butanol, but not heptanol. This transition temperature separated two different mechanisms of solute diffusion and partitioning, which may be associated with a change in the properties of the solvent. This was not observed from an analysis of flux data, owing to compensatory influences on the diffusion and partition behaviour of the drug. The study underlines the importance of appropriate temperature control when studying drug permeation.

Effects of various vehicles on the penetration of flufenamic acid into human skin

The effect of various vehicles (polyacrylate gels and wool alcohol ointments) on the penetration of flufenamic acid into excised human skin was investigated. Physico-chemical properties of the formulations were examined and discussed. Penetration data was gathered using two different in vitro test systems: the Saarbruecken penetration model (SB-M) and the Franz diffusion cell (FD-C). With wool alcohol ointments, drug concentration in the formulation was the decisive parameter for drug liberation and penetration. The incorporation of water into wool alcohol ointment led to increased drug amounts within the deeper skin layers (DSL), especially after longer incubation times. The drug concentration within the stratum corneum (SC) was not influenced by the bleeding effect of lipophilic, liquid components of the various wool alcohol ointments. With polyacrylate gels different results for liberation and penetration were observed. These results could be related to the effects of the drug concentration within the formulation and the penetration enhancers incorporated into the gels. Especially the effects of penetration enhancers clearly illustrated that liberation experiments do not predict the situation in the skin, but make experiments with a biological barrier essential. The high water content of the gels led to hydration of the skin specimen for the SB-M and the FD-C and therefore, in contrast to previous findings, comparable data were obtained in the penetration studies with both models. Furthermore, the quasi steady-state drug amount in the SC could be calculated for all formulations using an equation derived from a Michaelis-Menten kinetics. The data from both test systems were linearly correlated to each other. In addition, a direct linear relationship between the SC drug amount and the drug amount in the DSL was found as long as the quasi steady-state drug amount in the SC was not reached. A combination of all results might offer the chance to reduce the costs and to simplify the development of a new drug formulation.

Chemical peeling with salicylic acid in polyethylene glycol vehicle suppresses skin tumour development in hairless mice

BACKGROUND

Chemical peeling with salicylic acid in polyethylene glycol (PEG) vehicle is used clinically to improve the cosmetic appearance of skin that has been damaged by exposure to the sun. It is well known that cancers of the skin such as basal cell carcinoma and squamous cell carcinoma may be induced by the sun. However, the carcinogenic potential of chemical peeling agents has not been studied.

OBJECTIVES

To evaluate the effects of chemical peeling with 30% salicylic acid in PEG on skin tumour formation in treated vs. control mice.

METHODS

To serve as a model of sun-damaged skin, hairless SKH/hr1 mice were irradiated with ultraviolet (UV) B for 14 weeks, with or without treatment every 2 weeks with 30% salicylic acid in PEG for a total of 18 weeks.

RESULTS

Not only was the total number of tumours greatly reduced in the treated vs. the control mice, but skin tumour development was also slower in the treated vs. the control mice. At the final treatment, the fractions of T and B lymphocytes and natural killer cells from spleens of both groups of mice were comparable, and interferon-gamma production did not differ.

CONCLUSIONS

Our findings suggest that chemical peeling with salicylic acid in PEG may help to prevent as well as to reduce the number of UVB-induced skin tumours.

New formulation of chemical peeling agent: histological evaluation in sun-damaged skin model in hairless mice

BACKGROUND

chemical peelings injure the superficial skin, which is then restored by healing of the wound.

OBJECTIVES

to document the acute and chronic histological changes produced by applying chemical peeling agents used clinically to the UVB-irradiated skin of hairless mice, which served as a model of sun-damaged skin.

DESIGN

three chemical peeling agents, 30% salicylic acid, dissolved in macrogol (a new formulation), 35% trichloroacetic acid (TCA) dissolved in distilled water and 20% glycolic acid dissolved in glycerin were applied to the backs of UVB-irradiated hairless mice. Untreated, irradiated areas of skin served as controls. Specimens were evaluated histologically at 3, 14, 28, and 70 days.

RESULTS

chronic UVB irradiation produced an irregular hypertrophy of the epidermis. The treated areas of irradiated skin recovered by day 70. At 28 days, all skin specimens treated with chemical peeling agents exhibited a unique connective tissue layer composed of fine collagen fibers beneath the epidermis. While 35% TCA produced severe tissue damage marked by inflammation up to day 14, no inflammatory infiltrates were seen with 30% salicylic acid in macrogol at 70 days.

CONCLUSIONS

chemical peeling with 30% salicylic acid dissolved in macrogol led to reorganization of the epidermis and a rebuilding of the superficial dermal connective tissue important in reducing wrinkles, and without evidence of inflammatory infiltrates in an animal model of sun-damaged skin. Findings suggest a possible clinical benefit.

Improvement in the percutaneous absorption of beclomethasone dipropionate by gamma-cyclodextrin complexation

Inclusion complex formation of beclomethasone dipropionate (BMDP) with gamma-cyclodextrin (gamma-CyD) in water and in solid state was assessed by solubility analysis and X-ray diffractometry. The solid complex of BMDP with gamma-CyD in a molar ratio of 1:2 was prepared and its in-vitro release behaviour was investigated using an ointment release simulator. The release of BMDP from hydrophilic ointment was significantly improved by the gamma-CyD complexation. Permeation and uptake studies indicated that the enhanced release of BMDP from the ointment may be due to the faster dissolution and the lower binding affinity of the complex in the ointment base. The vasoconstrictor activity of BMDP in man was found to be increased by gamma-CyD complexation, suggesting an improvement in the percutaneous absorption of BMDP.

Effect of complex formation between 4-hexylresorcinol and ethyl myristate on release rate of 4-hexylresorcinol from petrolatum base

The effect of ethyl myristate on the release rate of 4-hexylresorcinol from petrolatum base was studied at pH 7.4 and 37 degrees. 4-Hexylresorcinol was analyzed spectrophotometrically at 278 nm. The release rate of hexylresorcinol from the ointments was directly proportional to the square root of time and depended on the percentage of ethyl myristate in the ointment base. For 0, 0.5, 1, 3, and 5% of ethyl myristate, the release rates were 29.6, 35.4, 38.3, 55.7, and 70.0 micrograms/hr1/2/cm2, respectively. The solubility of hexylresorcinol in the petrolatum base was determined as a function of ethyl myristate using partitioning techniques. The enhancement in hexylresorcinol solubility was rationalized on the basis of 1:1 and 1:2 complexes between hexylresorcinol and ethyl myristate. The complexation constants of these complexes were estimated to be 10 M-1 and 206.1 M-2, respectively. The diffusion coefficient of hexylresorcinol in the petrolatum base was estimated to be 1.31 x 10(-8) cm2/sec.

Systemic absorption of topical salicylic acid

The systemic absorption of salicylic acid in humans following topical application in either hydrophilic ointment or polyethylene glycol 400 vehicle was studied. Drug absorption was minimal following application to intact skin; however, measurable levels (8 mg/dl) were attained when the stratum corneum was removed prior to application of the drug in hydrophilic ointment. A one-compartment open model with first order absorption and elimination processes was fitted to the plasma salicylate concentrations as a function of time. Computer simulations predict that plasma salicylate levels associated with toxicity in some patients may be present after repetitive application of the drug in hydrophilic ointment.

Release of corticoids from oleaginous ointment bases containing drug in suspension

Simplified methods for studying the release of drugs suspended in oleaginous ointment bases were developed. These procedures were used in studying the release rates of two corticoids, fluocinonide and flucloronide, from white petrolatum and petrolatum containing various adjuvants. A practical method for measuring drug solubilities was developed and used in determining solubilities of these corticoids in ointment bases. When using physical data obtained from model ointments, the release rates of drugs from modified ointment bases were predicted. Comparisons of the observed and predicted rates from ointments containing hydrophobic adjuvants indicated the usefulness of the physical model approach in predicting the release rates. For ointments containing emulsifying agents, the simple model used did not provide useful predictions.