Distribution of salicylic acid in human stratum corneum following topical application in vivo: a comparison of six different formulations

Structural and therapeutic properties of salicylic acid-solubilized Pluronic solutions and hydrogels

Salicylic acid (SA) finds extensive applications in the treatment of rheumatic and skin diseases because of its analgesic, anti-inflammatory and exfoliating properties. As it is lipophilic in nature, there is a need for appropriate delivery systems to harness these properties for different applications. Herein, we examined the suitability of Pluronic P123/F127 micellar systems as delivery media by investigating the structural, flow and antimicrobial properties of P123/F127-SA solutions and hydrogels using DLS, SANS, rheological and zone inhibition measurement techniques. SA modulates the aggregation characteristics of these surfactant systems and brings about spherical-to-worm-like micelle-to-vesicular structural transitions in the hydrophobic Pluronic P123 system, a spherical-to-worm-like micellar transition in the mixed P123/F127 system and an onset of inter-micellar attraction in the hydrophilic Pluronic F127 system. SA-solubilized systems of both hydrophobic and hydrophilic Pluronics inhibit the growth of Gram-positive and Gram-negative bacteria with comparable MIC values. This suggests that the interaction of SA molecules with the bacterial cell membrane remains unobstructed upon encapsulation in Pluronic micelles. F127 hydrogel-based SA formulations with rheological properties suitable for topical applications and up to 15% SA loading were prepared. These will be useful SA ointments as F127 is an FDA-approved excipient for topical drug delivery applications. The results indicate that Pluronics remain effective as delivery agents for SA and exhibit interesting structural polymorphism upon its solubilization.

Topical delivery of salicylates

Salicylates have a long history of use for pain relief. Salicylic acid and methyl salicylate are among the widely used topical salicylates namely for keratolytic and anti-inflammatory actions, respectively. The current review summarises both passive and active strategies, including emerging technologies employed to enhance skin permeation of these two salicylate compounds. The formulation design of topical salicylic acid targets the drug retention in and on the skin based on the different indications including keratolytic, antibacterial and photoprotective actions, while the investigations of topical delivery strategies for methyl salicylate are limited. The pharmacokinetics and metabolisms of both salicylate compounds are discussed. The current overview and future perspectives of the topical delivery strategies are also highlighted for translational considerations of formulation designs.

Topical ketoprofen nanogel: artificial neural network optimization, clustered bootstrap validation, and in vivo activity evaluation based on longitudinal dose response modeling

OBJECTIVES

This work aimed at investigating the potential of solid lipid nanoparticles (SLN) as carriers for topical delivery of Ketoprofen (KP); evaluating a novel technique incorporating Artificial Neural Network (ANN) and clustered bootstrap for optimization of KP-loaded SLN (KP-SLN); and demonstrating a longitudinal dose response (LDR) modeling-based approach to compare the activity of topical non-steroidal anti-inflammatory drug formulations.

METHODS

KP-SLN was fabricated by a modified emulsion/solvent evaporation method. Box-Behnken design was implemented to study the influence of glycerylpalmitostearate-to-KP ratio, Tween 80, and lecithin concentrations on particle size, entrapment efficiency, and amount of drug permeated through rat skin in 24 hours. Following clustered bootstrap ANN optimization, the optimized KP-SLN was incorporated into an aqueous gel and evaluated for rheology, in vitro release, permeability, skin irritation and in vivo activity using carrageenan-induced rat paw edema model and LDR mathematical model to analyze the time course of anti-inflammatory effect at various application durations.

RESULTS

Lipid-to-drug ratio of 7.85 [bootstrap 95%CI: 7.63-8.51], Tween 80 of 1.27% [bootstrap 95%CI: 0.601-2.40%], and Lecithin of 0.263% [bootstrap 95%CI: 0.263-0.328%] were predicted to produce optimal characteristics. Compared with profenid® gel, the optimized KP-SLN gel exhibited slower release, faster permeability, better texture properties, greater efficacy, and similar potency.

CONCLUSIONS

SLNs are safe and effective permeation enhancers. ANN coupled with clustered bootstrap is a useful method for finding optimal solutions and estimating uncertainty associated with them. LDR models allow mechanistic understanding of comparative in vivo performances of different topical formulations, and help design efficient dermatological bioequivalence assessment methods.

Fabrication of highly selective molecularly imprinted membranes for the selective adsorption of methyl salicylate from salicylic acid

Highly selective molecularly imprinted membranes for methyl salicylate were synthesized with 4-vinylpyridine, acrylamide or methacrylic acid as the functional monomer based on the Al₂O₃ microporous ceramic membrane.

Relative bioavailability of salicylic acid following dermal application of a 30% salicylic acid skin peel preparation

A single-center, single-sequence, two-period crossover study was performed to compare the systemic exposure to salicylic acid (SA) following facial application of a 30% SA cosmetic skin peel formulation applied for 5 min and an oral dose of 650 mg aspirin in nine healthy male and female subjects. The mean (SD) maximum SA concentration (Cmax) was 0.81 (0.32) microg/mL and 56.4 (14.2) microg/mL. The AUC-based safety margin ratio was 50:1. A depot effect was observed during topical application of the skin peel solution as the absorption of SA continued beyond the 5-min application period. Plasma SA Cmax values were achieved from 1.4 to 3.5 h after topical application and from 0.5 to 1.5 h after oral aspirin. The plasma concentrations in the present study (30%; 5 min) were similar to that of a low concentration (2%) applied in a leave-on product to the same body surface area. In conclusion, our results suggest that the use of this SA facial peel should not pose any significant systemic health risks.

Stable isotope dilution analysis of salicylic acid and hydroquinone in human skin samples by gas chromatography with mass spectrometric detection

A sensitive and accurate gas chromatographic-mass spectrometric (GC-MS) method has been developed for the quantitative determination of salicylic acid (SA) and hydroquinone (HQ) from human skin samples and cosmetic emulsions. Deuterium labeled SA-d(6) and HQ-d(6) were used as internal standards (IS). The samples were extracted with methanol, dried under nitrogen and derivatized with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA)+1% trimethylchlorosilane (TMCS). Quantification was performed in SIM mode with a limit of quantification (LOQ) of 50 ng ml(-1) for SA and 10 ng ml(-1) for HQ. The inter-day variation (R.S.D.) was less than 5% and the accuracy was better than 13.3% for both compounds. The recoveries from the different matrices ranged between 93.1 and 103.3% for SA, and 97.3 and 100.8% for HQ.

Evaluation of skin penetration of topically applied drugs in humans by cutaneous microdialysis: acyclovir vs. salicylic acid

BACKGROUND AND OBJECTIVE

Cutaneous drug application is used for both local drug therapy and systemic treatment. For both types of treatment, the drug concentration profile in, and transport across, the skin is important. To evaluate skin penetration of topically-applied drugs we recently used cutaneous microdialysis. The aim of this study was the use of this method for studying acyclovir and salicylic acid.

METHOD

Five per cent acyclovir cream was applied on intact and tape-stripped skin of healthy volunteers and 5% salicylic acid ointment-onto intact skin of other volunteers. Microdialysis probes with 2 kDa molecular weight cut-off were inserted intradermally and were perfused with Ringer solution. Drug concentrations were measured by high-performance liquid chromatography.

RESULTS

Following topical application of 5% acyclovir cream onto intact skin of eight healthy volunteers, no drug was determinable in the skin (cutaneous microdialysate) in any of the subjects studied. After partial removal of the stratum corneum the penetration of this drug into skin increased markedly. The mean maximum skin concentration was about 2 x 5 micromol/L after 2 x 4 +/- 0 x 7 h. Topically applied salicylic acid penetrated intact skin with a maximum concentration in the cutaneous microdialysate of 7 x 57 +/- 3 x 90 micromol/L after 5 x 3 +/- 0 x 4 h.

CONCLUSION

Cutaneous microdialysis is a valuable method for estimating skin concentration of topically-applied drug. It allows evaluation after application to a small skin area, of about 2 cm(2), thereby reducing the risk of systemic toxicity. The method may be helpful for evaluating the influence of skin condition on the transport process.

‘Keratolytic’ Properties of Benzoyl Peroxide and Retinoic Acid Resemble Salicylic Acid in Man

OBJECTIVES

Retinoic acid (RA) and benzoyl peroxide (BP) were studied, comparing their keratolytic efficacy and water barrier disruption to that of salicylic acid (SA), a well-established keratolytic, under similar conditions.

PATIENTS/METHODS

Six volunteers were included in this blinded study. Eleven randomized test sites were marked on the volar forearms, containing sites for untreated skin at time zero, unoccluded, occlusion, and vehicle controls for 3 and 6 h, and each of BP, RA, and SA solutions for 3 and 6 h. At each time point, occlusion at 5 of the test sites was removed, and chromameter measurements were performed over 30 min. Each site then underwent 25 stratum corneum (SC) tape strippings. At 1, 5, and 30 min after the last stripping at each site, TEWL measurements were performed. Quantitative protein analysis of the SC from the tapes was then performed.

RESULTS AND CONCLUSION

after 3 h, bp was significantly more effective in disrupting sc cohesion than sa and ra, indicating bp is a moderate keratolytic agent in addition to its antimicrobial properties. After 6 h, all three agents were similarly effective in keratolysis. Barrier disruption, as measured by TEWL, paralleled depth of SC removal. SA tended to exhibit the greatest keratolytic efficacy superficially, hence its clinical effectiveness in superficial conditions such as comedonal acne, whereas BP was more effective at deeper levels, complimenting its antimicrobial effects and enabling it to treat deeper, more inflammatory lesions. None of the agents significantly affected skin erythema. These techniques provide a robust and rapid assay for in vivo keratolytic demonstration.

Polyvinyl Alcohol—Gelatin Patches of Salicylic Acid: Preparation, Characterization and Drug Release Studies

Polyvinyl alcohol—gelatin patches were developed and salicylic acid was incorporated at different stages of preparation of the patches. The patches were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry, tensile strength test, and scanning electron microscopy. The release patterns of the drug from the patches were also studied. The FTIR spectra of the blank patch indicated complete esterification of the free carboxylic groups of gelatin. The XRD studies indicated a crystalline form of the drug entrapped in the patches. Release of the drug from the patches followed Higuchian/Fickian kinetics indicating a diffusion-controlled release process.

Difference in percutaneous absorption and intracutaneous distribution in guinea pigs among topical antifungal drugs (tioconazole solution, tioconazole cream, miconazole nitrate solution and bifonazole solution)

Tioconazole (TCZ) is an imidazole antifungal agent with broad spectrum activity. Percutaneous absorption and intracutaneous distribution of TCZ solution have been compared with TCZ cream, miconazole nitrate (MCZ) solution and bifonazole (BFZ) solution following a single topical application to abdominal skin of guinea pigs. Following application of TCZ solution, TCZ concentrations in the stratum corneum, epidermis-cutis and subcutaneous tissue were higher than those after TCZ cream application suggesting superior percutaneous penetration after TCZ solution application. The percutaneous penetration after applications of MCZ solution and BFZ solution was comparable to that of TCZ cream, but inferior to that of TCZ solution. TCZ concentrations in the stratum corneum were much higher than those in epidermis-cutis and subcutaneous tissue after applications of both TCZ formulations. The majority of applied TCZ remained in the stratum corneum at high levels for a long duration. TCZ concentrations in the stratum corneum within 24 h after applications of both TCZ formulations were more than several hundred times higher than the minimum inhibitory concentrations against most of the dermatophytes and yeasts. The effectiveness of both TCZ formulations against dermatophytoses may be due to this favorable pharmacokinetic property in the skin tissues, together with its potent antifungal activity. Percutaneous absorption of TCZ after applications of both formulations was negligible suggesting that these treatments are unlikely to produce systemic side effects.

Cutaneous bioassay of salicylic acid as a keratolytic

Keratolytic efficacy of topical preparations containing salicylic acid was studied in humans utilizing adhesive tape stripping and quantifying SC removal by protein analysis. In combination with tape stripping, squamometry was used to evaluate the influence of salicylic acid on skin surface scaliness and desquamation. Furthermore, skin barrier perturbation and skin irritancy was recorded and related to the dermatopharmacological effect of the preparations. In contrast to squamometry, tape stripping combined with protein analysis was sensitive in detecting keratolytic effect of salicylic acid within hours of application. Importantly, whereas the pH of the preparations only minimally influenced efficacy, local dermatotoxicity was significantly increased at acidic pH. This indicates that the quest to increase the amount of free, non-dissociated SA is, in fact, counterproductive as the more acidic preparations resulted in skin irritation and barrier disruption.

Evaluation of in vivo bioequivalence methodology for topical clobetasol 17-propionate based on pharmacodynamic modeling using Chinese skin

The United States Food and Drug Administration recommends pilot dose duration-response and pivotal bioequivalence studies to be conducted using reflectance colorimetry for assessment of the in vivo bioequivalence of topical dermatologic corticosteroids. The major objectives of the present studies were to examine the applicability of the standardized pharmacodynamic modeling-based methodology to super-potent clobetasol 17-propionate (CP) in the Chinese population and to evaluate the bioequivalence of two generic ointments and four generic creams containing 0.05% (w/w) CP with respect to Dermovate formulations using such methodology. In the pilot dose duration-response study, although the E(max) model (where E(max) is the maximum fitted value of AUEC, which is the area under the baseline-corrected, untreated control-site-corrected a* scale data from 0 to 24 h after drug removal) did not provide acceptable model fits, E(max) parameter estimates of -38.97 +/- 3.62 and -41.89 +/- 11.28 a*-scale. h, and ED(50) (dose duration required to achieve 50% of the fitted E(max) value) estimates of 0.40 +/- 0.37 and 0.42 +/- 0.16 h were obtained for Dermovate ointment and cream, respectively, by population analyses. The estimates for the two formulations were not statistically different, so in vivo bioequivalence studies were conducted at an ED(50)dose duration of approximately 0.5 h for both Dermovate formulations. The results demonstrated that one generic ointment was bioequivalent to Dermovate, whereas the other was not. None of the generic creams were shown to be bioequivalent to Dermovate cream. The in vivo bioequivalence data from the vasoconstriction assay were linearly correlated with stratum corneum uptake of the drug at the same dose duration until the maximal vasoconstriction response was achieved. The studies illustrated the applicability of the standardized pharmacodynamic modeling-based methodology in detecting the product differences between a variety of generic 0.05% CP formulations and reference Dermovate formulations in Chinese skin.

Non-invasive method for quantitative evaluation of exogenous compound deposition on skin

Topical application of active compounds on skin is common to both pharmaceutical and cosmetic industries. Quantification of the concentration of a compound deposited on the skin is important in determining the optimum formulation to deliver the pharmaceutical or cosmetic benefit. The most commonly used techniques to date are either invasive or not easily reproducible. In this study, we have developed a noninvasive alternative to these techniques based on spectrofluorimetry. A mathematical model based on diffusion approximation theory is utilized to correct fluorescence measurements for the attenuation caused by endogenous skin chromophore absorption. The limitation is that the compound of interest has to be either fluorescent itself or fluorescently labeled. We used the method to detect topically applied salicylic acid. Based on the mathematical model a calibration curve was constructed that is independent of endogenous chromophore concentration. We utilized the method to localize salicylic acid in epidermis and to follow its dynamics over a period of 3 d.

Topical application of 5-aminolevulinic acid hexyl ester and 5-aminolevulinic acid to normal nude mouse skin: differences in protoporphyrin IX fluorescence kinetics and the role of the stratum corneum

An important limitation of topical 5-aminolevulinic acid (ALA)-based photodetection and photodynamic therapy is that the amount of the fluorescing and photosensitizing product protoporphyrin IX (PpIX) formed is limited. The reason for this is probably the limited diffusion of ALA through the stratum corneum. A solution to this problem might be found in the use of ALA derivatives, as these compounds are more lipophilic and therefore might have better penetration properties than ALA itself. Previous studies have shown that ALA hexyl ester (ALAHE) is more successful than ALA for photodetection of early (pre)malignant lesions in the bladder. However, ALA pentyl ester slightly increased the in vivo PpIX fluorescence in early (pre)malignant lesions in hairless mouse skin compared to ALA. The increased PpIX fluorescence is located in the stratum corneum and not in the dysplastic epidermal layer. In the present study, ALA- and ALAHE-induced PpIX fluorescence kinetics are compared in the normal nude mouse skin, of which the permeability properties differ from the bladder. Application times and ALA(HE) concentrations were varied, the effect of a penetration enhancer and the effect of tape stripping the skin before or after application were investigated. Only during application for 24 h, did ALAHE induce slightly more PpIX fluorescence than ALA. After application times ranging from 1 to 60 min, ALA-induced PpIX fluorescence was higher than ALAHE-induced PpIX fluorescence. ALA also induced higher PpIX production than ALAHE after 10 min of application with concentrations ranging from 0.5 to 40%. The results of experiments with the penetration enhancer and tape stripping indicated that the stratum corneum acts a barrier against ALA and ALAHE. Use of penetration enhancer or tape stripping enhanced the PpIX production more in the case of ALAHE application than in the case of ALA application. This, together with the results from the different application times and concentrations indicates that ALAHE diffuses more slowly across the stratum corneum than ALA.