Deposition of salicylic acid into hamster sebaceous.

In an earlier paper, we identified vehicles that are miscible with sebum, using differential scanning calorimetry (DSC). In this paper, the potential of these vehicles to deliver salicylic acid (SA) into the sebum-filled follicles of hamster ears is examined. The main objective of this study is to correlate the melting transitions of a model sebum with the follicular delivery of SA, using two different types of vehicles (fatty and polar). Generally, the fatty vehicles show higher deposition than the polar vehicles. Follicular delivery of salicylic acid correlates well with its solubility in the respective vehicles. This extent of deposition also shows a relationship with the effect of the vehicle on thermal behavior of the model sebum. The nature of the relationship depends on the vehicle (polar or fatty) tested. We conclude that DSC could be used to identify appropriate vehicles for drugs whose follicular delivery depends on solubility. The results also suggest that delivery into the sebaceous glands occurs by two different mechanisms, depending upon the polarity of the vehicle and the physicochemical properties of the drug. The results of these experiments are further extended to investigate follicular delivery of SA from two different types of oil-in-water emulsion formulations. From these studies we conclude that either increasing the volume of the oil phase or changing the emulsion to a water-in-oil emulsion would increase follicular deposition. Our research highlights the role of sebum, its compatibility with drug molecules, and vehicle selection in the transport of drugs into the follicles. The overall results of these experiments provide a reasonable understanding of the mechanisms underlying the transport of drugs to, and subsequently through, the sebaceous follicle.

Effect of formulation on the delivery and metabolism of alpha-tocopheryl acetate.

The effect of delivery system on the permeation and metabolism of alpha-tocopheryl acetate (alpha-TAc) was studied in micro-Yucatan pig skin, which closely resembles human skin. Various alpha-tocopheryl acetate formulations, including a simple isopropyl myristate (IPM) solution, an o/w emulsion, microemulsions, which differed in their oily phase content, and alcoholic and hydroalcoholic gels were made. A suitable HPLC method was developed and validated to separate and quantify alpha-TAc and alpha-tocopherol (alpha-T). Dulbecco's modified phosphate-buffered saline with 3% bovine serum albumin (DMPBS-BSA 3%) served as the receptor media to ensure tissue viability and to maintain skin conditions. Finite doses (5 microl) of the formulations were applied to viable pig skin using a statistically approved randomized complete block design. Data were analyzed using Tukey's studentized range test, and interday variability was estimated using an F-test. About 70% of the active was recovered from the wash, representing the amount adhering to the surface of the skin. alpha-TAc underwent metabolism in pig skin to the active antioxidant, alpha-T. The identity of the HPLC peaks were confirmed by spiking studies using known standards. The extent of metabolism was found to be formulation-dependent. No alpha-T was, however, detected in the stratum corneum. A higher extent of metabolism was obtained for the IPM solution, a microemulsion containing IPM as the oily phase, and the hydroalcoholic gel, when calculated based on the percent of total alpha-TAc permeated in the viable skin. Metabolism occurred in pig skin to the extent of 15-20% in terms of the total amount of alpha-TAc permeated in the viable skin and stratum corneum. Thus the topical delivery and metabolism of alpha-TAc were found to be dependent on formulation.

Differential scanning calorimetry studies of sebum models.

Human sebum is a mixture of triglycerides, fatty acids, wax esters, squalene, cholesterol, and cholesterol esters. P. acnes, a bacterium that is normally found on the skin, hydrolyzes certain triglycerides to fatty acids, thereby changing the sebum composition. The objective of this study was to examine the physical state of a model sebum and the effect of variations in its composition on its physical properties including (a) the carbon chain length of the components, (b) the ratio of unsaturated to saturated components, and (c) the ratio of triglycerides to fatty acids. A model sebum mixture was prepared based on a composition reported in the literature and evaluated by differential scanning calorimetry (DSC). Since cholesterol and cholesterol esters contribute insignificantly to sebum composition, they were not included. Squalene was kept constant (13%), while the concentration of the rest of the components was varied. Variations of sebum were prepared by dissolving all components in a 3:1 chloroform-methanol mixture for uniformity. Subsequently the solvent was evaporated at room temperature. The samples were then analyzed using DSC. Four distinct endotherms (namely, Mp-1, Mp-2, Mp-3, and Mp-4) were observed between -50 degrees C and 100 degrees C. Mp-1 and Mp-2 occurred below 0 degrees C and were contributed by unsaturated components. Mp-3 and Mp-4, which represent the saturated components, occurred above 30 degrees C. Thus, at normal skin temperature (skin surface temperature is 32 degrees C), sebum contains both a solid and a liquid phase. All the transition temperatures increased with an increase in carbon chain length for the same ratio of unsaturation to saturation. A replacement of unsaturated components with corresponding saturated components led to a decrease in the transition temperatures for the former (Mp-1 and Mp-2) and an increase in the transition temperatures for the latter (Mp-3 and Mp-4). Replacement of triglycerides with corresponding fatty acids (mimicking the action of anaerobic bacteria) caused an increase in Mp-2 and a decrease in Mp-4. In all cases, the final melting temperature (Mp-4) was greater than the temperature of the human skin surface (32 degrees C); thus components contributing to these endotherms are still solids at skin temperature. All variations in the sebum model led to mixtures of solids and liquids at skin temperature. Considering a reduction in Mp-3 and/or Mp-4 to represent sebum "fluidization," it was achieved by a decrease in carbon chain length, an increase in unsaturation, or a substitution of triglycerides by corresponding fatty acids. Preferential enrichment with the saturated species will lead to enrichment of solids versus liquids in the sebum, presumably making it difficult for the liquid phase to dissolve the solids. It seems plausible that perturbation of the balance of solid and liquid components of sebum, such as by P. acnes action, may lead to blockage of the follicle. Future research will investigate strategies to dissolve and/or liquify the solid phase of sebum.

Kinetics of permeation and metabolism of alpha-tocopherol and alpha-tocopheryl acetate in micro-Yucatan pig sin.

The objective of this research was to investigate the permeation and metabolism of alpha-tocopheryl acetate (alpha-TAc) and alpha-tocopherol (alpha-T) from solution and emulsion formulations and to delineate the kinetics of such metabolism. Simple formulations containing alpha-TAc and alpha-T were applied to fresh, viable micro-Yucatan skin dermatomed to a thickness of 250-300 microns, as a finite dose in a flow-through diffusion system. The experiments were stopped at time intervals of 2, 6, 12, and 24 hours. At the end of each time interval, the amounts removed by washing, retained in the stratum corneum (SC), and penetrated into the viable skin and receptor were determined by a validated HPLC method. Receptor concentrations were below the limit of detection. alpha-TAc underwent metabolism in pig skin to the active antioxidant alpha-T. The metabolite appeared as early as two hours after application. The extent of metabolism was highest at 6-12 hours after application. No metabolism was detected in the stratum corneum. Delivery of alpha-T from isopropyl myristate (IPM) solution was more efficient than utilization of alpha-TAc from the same solution. Approximately 1.5% of alpha-T yielded the same viable skin concentration as 5% alpha-TAc. Topical application of alpha-tocopherol or its prodrug acetate was capable of enhancing the overall antioxidant capacity of pig skin. The hydrolytic pathway of alpha-TAc leading to the active antioxidant alpha-T could possibly be saturable.

The quality of skin care products and their ingredients.

Several ingredients used in skin products have been criticized as being excessively harsh, allergenic, or otherwise unsuitable for use, especially in the elderly population. Preservatives, in particular, have been condemned, leading to a proliferation of "preservative-free" products. Other descriptive/promotional phrases with negative connotations are "fragrance-free" and "emulsifier-free." Inferences regarding these designations might suggest that preservatives, fragrances, emulsifiers, and a number of other ingredients serve no important function, are superfluous in terms of product quality, and, therefore, should be left out of all skin products. While this is obviously not the case, neither is the obverse. Ingredients used in skin care products should be carefully chosen to support or maintain the overall effectiveness and utility of the product, and the concentration of such ingredients should be given careful consideration. After briefly reviewing skin structure and changes that occur during aging, this article examines the concept of product quality. Major nondrug ingredient categories will be addressed, including the reasons for using such ingredients in skin care products, the products in which they are required, the limitations and choices available within each category, and guidelines for product selection.

Percutaneous absorption of sunscreens through micro-yucatan pig skin in vitro.

PURPOSE: The objectives of this study were to develop an in vitro model for studying sunscreen permeation in skin, and evaluate the influence of formulation differences. METHODS: The sunscreens studied were two of the most widely used agents, octyl methoxycinnamate (OMC) and benzophenone-3. Preparations containing radiolabeled actives were applied to micro-Yucatan pig skin dermatomed to a thickness of 250-300 microm as a finite dose in a flow-through diffusion system. At the end of each experiment the amounts removed by washing, retained inside stratum corneum (SC) and penetrated into receptor and viable skin were determined. RESULTS: The two sunscreens reached a peak level in SC within an hour. Benzophenone-3 penetrated skin to a greater extent than OMC. The opposite was true when comparisons of SC retention were made. The ratio of retained to penetrated amount of sunscreens from a hydroalcoholic formulation at the end of 10 hours was higher when the sunscreens were present together than alone. CONCLUSIONS: Despite the highly lipophilic nature of sunscreens, particularly OMC, SC is the rate limiting skin layer for penetration. Penetration and SC retention were formulation dependent. The ratio of SC content to the amount penetrated is a useful tool for evaluating sunscreen permeation.

Assessment of value and applications of in vitro testing of topical dermatological drug products.

The FDA recently issued a guidance covering practices of scaleup and post approval changes with semisolids (SUPAC-SS). This guidance outlines the steps that must be taken by a company to maintain certification of its semisolid dermatological products after quantitative changes have been made in their compositions and/or after changes have been made in the sourcing of their key ingredients, in their processing, in their batch sizes, and/or after their site of manufacture has been relocated. A key element within the guidance is a release test to be used to determine if the diffusional release of a drug found in a formulation is the same after changes have been made to the formulation as it was prior to implementing the changes. The AAPS-FDA sponsored workshop was set up to explore this qualifying test. The stated aims of the workshop were: a) to illustrate the methodology and techniques of in vitro release testing, b) to show the sensitivity of in vitro release with respect to manufacturing variables and to variations in components and composition (of specific formulations), c) to recognize in vitro release testing as a useful procedure for SUPAC documentation, d) to highlight and evaluate other applications of in vitro release testing, e) to explore the degree to which in vitro release testing and bioavailability may be related, and f) to evaluate the role of in vitro release testing of topical dosage forms as a tool to improve product quality.

In vitro release of betamethasone dipropionate from petrolatum-based ointments.

The purpose of this research was to develop new in vitro methodology for measuring release from petrolatum-based semisolids and to determine whether two ointments, both of which contained betamethasone dipropionate, 0.05%, but with different formulations, could be distinguished by release measurements. Several receptor media were explored to optimize the procedure utilizing Franz-type cells. Analysis was by HPLC. The release slope was 1.5 to 6 times greater from the ointment than the "augmented" ointment (which had greater clinical potency). Release was highest with a receptor consisting of a 5% solution of hexane in acetonitrile. Even so, it was necessary to subject samples of receptor from the augmented ointment to evaporation followed by reconstitution with a smaller volume of mobile phase to bring corticosteroid concentrations up to quantifiable levels. In another series of experiments, the HPLC mobile phase was used as the receptor and a relatively large volume (100 microliters) was injected onto the column. With the second approach, measured concentrations were lower but more reproducible. Quantifiable levels of betamethasone dipropionate were obtained for both formulations beginning from the first data point (at 1 hr), with satisfactory linearity of plots of amount released per unit area of membrane versus the square root of time. Using this methodology, it was possible to distinguish the effect of formulation differences in two ointments containing the same drug in the same concentration.

Noninvasive assessment of anesthetic activity of topical lidocaine formulations.

The effectiveness of a series of lidocaine formulations in producing anesthesia after topical application was evaluated in human volunteers. The formulations, five suspensions in 20% propylene glycol and one cream, were applied to the forearms for 3 h with occlusion with Hilltop chambers. Testing for anesthesia was performed electrometrically. All lidocaine-containing formulations produced significantly greater anesthesia than the blanks. The formulation containing tetradecyltrimethylammonium bromide produced greater anesthesia than that containing octadecyltrimethylammonium chloride. Changing the pH of the formulation from 7.9 to 10.0 had no significant effect. Other formulations (sodium lauryl sulfate and the cream) were no more effective than the plain formulation without surfactants. The rank order for the suspension formulations was the same as for steady-state permeation in in vitro experiments. However, application of the cream formulation produced greater effect in vivo than was anticipated from in vitro flux values.

Correlation of water and lidocaine flux enhancement by cationic surfactants in vitro.

The penetration of radiolabeled water and lidocaine through human epidermis was studied simultaneously using a flow-through apparatus under infinite-dose conditions. The donors were suspensions of lidocaine in propylene glycol:water mixtures containing cationic surfactants of varying alkyl chain length from three classes: alkyl dimethylbenzyl ammonium halides, alkyl trimethyl ammonium halides, and alkyl pyridinium halides. Each skin sample served as its own control; each was subjected to sequential treatments of control formulation with no surfactant, test formulation, and then a repeat control. Higher surfactant concentration resulted in greater enhancement ratios for both permeants. Peak surfactant enhancement effects were seen at alkyl chain lengths of 12 or 14 carbons. Strong correlation was noted between the enhancement ratios of water and lidocaine. Water permeation can serve as a predictor of the effects of surface-active compounds on the permeation of drugs. The data suggest that water and lidocaine utilize the same pathway through the horny layer.

Evaluation of a noninvasive method for monitoring percutaneous absorption of lidocaine in vivo.

The pharmacodynamic measurement of in vivo skin penetration of lidocaine was explored with an instrument used in dentistry to determine tooth pulp vitality. The instrument delivers a low-current, pulsatile electrical waveform of increasing intensity with time. The readings, which are reproducible, are in arbitrary units on a scale of 0-80. Testing of naive sites showed variation as a function of location, even over relatively small distances. The response at a marked site over a 12-hr period generally was consistent in five subjects. Following intradermal administration of 1 or 2% lidocaine hydrochloride injection in one subject, the instrument reached its maximum value within 2 min. This was followed by a sustained plateau and then a gradual falloff of the effect. Topical formulations containing 5% lidocaine base and corresponding blank formulations were applied under occlusion within Hilltop chambers to intact skin on the forearms of human volunteers for 3 hr. While the response to a 40% propylene glycol formulation was not significantly different from the corresponding control, a cream exhibited slow development of profound anesthesia that lasted for several hours following chamber removal.

A model for alcohol-enhanced permeation through polydimethylsiloxane membranes.

In this study the influence of paraben concentration on flux from solution in 1-propanol through polydimethylsiloxane membranes was investigated. Alcohol was sorbed by the polymer membrane leading to changes in membrane dimensions and increased membrane capacity to contain paraben (partition coefficient). Diffusion coefficients were not significantly influenced by alcohol sorption. Flux was increased 5-30 fold over nonimbibed donors such as water and polyols. An increase in paraben concentration reduced alcohol activity, its uptake by the membrane, and consequently the partition coefficient of paraben. As a result, flux increased with paraben concentration, reached a peak, and then declined. Maximal membrane concentration involves a trade-off between alcohol-membrane interaction (solvent activity) and paraben concentration, and occurred at paraben concentrations in the range of 1.3 to 1.4 mmol/g. At equimolar concentrations, flux was highest for methylparaben and declined as the series was ascended. Differences in flux between parabens at a specified molar concentration were due only to differences in diffusivity.

Membrane-solvent-solute interaction in a model permeation system.

In this study, methyl- and propylparaben flux from various alcohol donors through polydimethylsiloxane membranes was investigated. Flux from saturated alcohol vehicles was markedly increased relative to water and glycol systems. The uptake of neat alcohol, a measure of solvent membrane interaction, gave a good rank order correlation to the flux data for a particular paraben. The major influence of the alcohols was an increase in membrane solubility of paraben, with a smaller effect on the diffusion coefficient. High paraben donor solubility indirectly reduced the solvent-membrane interaction leading to attenuated flux. Paraben membrane solubility was influenced by the amount of alcohol sorbed from saturated systems and the affinity of the paraben for the alcohol. This conforms to the concept of imbibed alcohol molecules being organized into clusters. The alteration in barrier properties of the membrane was found to require the presence of sorbed alcohol and was reversible upon removal of the solvent.

Flocculation of suspensions containing nonionic surfactants by sorbitol.

Aqueous suspensions of sulfamerazine, salicylamide, and butamben, containing either polysorbate 20 or polysorbate 80 as a wetting agent, were deflocculated. The presence of relatively high concentrations of sorbitol in the suspensions resulted in flocculation due to dehydration of surfactant polyoxyethylene groups (reflected by the cloud point). The critical flocculation concentration of sorbitol was reduced by raising the storage temperature, lowering the surfactant concentration, switching from polysorbate 80 to polysorbate 20, or adding sodium sulfate to the suspension formulation. Both the cloud point and the sorbitol critical flocculation concentration depended on the particular drug that was suspended. Methylparaben lowered the cloud point, suggesting that the choice of preservative could influence the suspension characteristics in certain cases. The various dehydrating influences were additive. By assuming that interparticle repulsion becomes negligible at the cloud point, it was possible to use cloud point curves to estimate the critical flocculation concentration of sorbitol.

Evaluation of penetration enhancement of lidocaine by nonionic surfactants through hairless mouse skin in vitro.

The effect of two nonionic surfactants (polyoxyethylene sorbitan monoesters) on percutaneous absorption of lidocaine in the presence of various concentrations of propylene glycol is reported. Comparisons were made in vitro using excised hairless mouse skin as the barrier membrane. Under infinite dose conditions, steady-state flux was enhanced by surfactants at high propylene glycol concentrations. The same trend was observed following application of a thin layer of formulation to the skin (finite-dose conditions). However, penetration behavior was complex due to: (a) changes in vehicle composition following application, (b) temperature changes resulting from evaporation or moisture uptake, and (c) depletion of lidocaine as a result of penetration with compositions that lost water by evaporation. Two peaks in the flux versus time curve were observed. Surfactant monomer concentration in the vehicles was increased in the presence of propylene glycol.

Stabilization of sulfamerazine suspensions by xanthan gum.

Suspensions of sulfamerazine (10%) containing 0.2% docusate sodium were deflocculated because of repulsion between the negatively charged particles. Flocculation was induced by salts or by xanthan gum, which is anionic, in the presence of salts at concentrations below those at which salt flocculation resulted. The amount of gum necessary to produce a flocculated system was lower the higher the concentration of salt present. Calcium chloride and magnesium chloride were considerably more effective in this regard than sodium chloride. Gum flocculation produced aggregates with fewer particles and weaker bonding forces than did salt flocculation. The sedimentation rate of the suspensions decreased 5 to 10 times for each 0.1% increase in the gum concentration.

Viscosity of xanthan gum solutions at low shear rates.

The viscosity of xanthan gum solutions in the low shear region was investigated with the aid of a Couette instrument. All solutions were highly pseudoplastic . Solutions containing 0.3-0.5% of the gum exhibited a highly ordered phase at very low shear. Viscosity, the degree of pseudoplasticity , and the value of the transition from soft gel to pseudoplastic behavior were directly related to gum concentration. The effect of the addition of a salt on viscosity depended on the xanthan gum concentration. The viscosity of a 0.3% xanthan gum solution was practically unaffected by the salts. Higher gum concentrations exhibited a viscosity increase when salt was present. Concentrations less than 0.3% exhibited a viscosity decrease in the presence of a salt. All viscosity effects seemed to reach limiting values at approximately 10(-3) to 3.3 X 10(-3) N salt. No major differences were observed between sodium chloride, calcium chloride, and sodium citrate in their influence on xanthan gum viscosity.

Interaction of xanthan gum with suspended solids.

Xanthan gum was adsorbed significantly by magnesium carbonate, aluminum hydroxide, zinc oxide, and calcium carbonate, giving Langmuir-type isotherms. Saturation adsorption was higher from 0.9% NaCl than from water due to reduced mutual repulsion of polymer segments in the presence of the salt. Adsorption resulted from electrostatic attraction between positively charged particles and the negatively charged polymer. zeta-Potential measurements correlated with the adsorption data but were not predictive of the flocculation state. The result indicate that flocculation of magnesium carbonate and aluminum hydroxide by xanthan gum is consistent with a bridging mechanism.