Predicting skin penetration of actives from complex cosmetic formulations: an evaluation of inter formulation and inter active effects during formulation optimization for transdermal delivery

Twenty products, containing a radiolabelled form of each active in typical cosmetic formulations, were made and applied to female human epidermal membranes mounted in Franz diffusion cells for 48 h under ‘in use’ conditions. The products consisted of combinations of five formulations (a hydro‐alcoholic gel, an oil in water emulsion, a water in oil emulsion, a microemulsion and an oil) with four model drug actives (testosterone, hydrocortisone, 5‐fluorouracil and ketoconazole). Steady‐state flux appeared to be reached by 8 h and maintained for all products, other than for the microemulsions, consistent with the actives being present in the residual formulation on the skin at saturation. The recovery for each active at the end of the 48‐h study (from a series of stratum corneum tape strips, the remaining skin, cumulative amount penetrating into the receptor solution, product washed from the skin and on the donor chamber cap) ranged from 86.5% to 100.6%. The rank order of the fluxes for the actives from the hydro‐alcoholic gel is consistent with the known active molecular size and polarity determinants for maximum epidermal flux. Actives with similar steady‐state (maximum) fluxes from a range of formulations had retention in the stratum corneum and similar transport rate constants through the stratum corneum. The microemulsion formulation significantly enhanced both the stratum corneum steady‐state flux and transport rate constant for 5‐fluorouracil, hydrocortisone and testosterone. The penetration flux of each active could be related to its size and polarity and appeared maximal when the actives in the different cosmetic formulations applied to the skin under ‘in use’ conditions were likely to remain in the residual product on the skin as a saturated solution after solvent evaporation. Enhanced penetration fluxes can be achieved by formulation selection and an appropriate choice/mix of emollients/adjuvants. The principles described here provide a framework for understanding the delivery of cosmetic ingredients from various formulations.     

Formulating for efficacy

Active ingredients have been around in cosmetics for a long time but have they really resulted in active cosmetic products? In order to achieve this, the right active needs to be delivered to the right location at the right concentration for the correct period of time. And the extent (and therefore the concentration) of this delivery depends on the formulation. From a rather theoretical approach based on the polarity of the active ingredient, the stratum corneum and the oil phase, the Relative Polarity Index was established that enables the selection of a suitable emollient for ensuring skin penetration of the active ingredient. Practical examples subsequently show the validity of this approach that demonstrates that one can regulate the delivery of an active molecule (and therefore the efficacy of a cosmetic formulation) by selection and control of the emollient system. Cosmetic formulations are generally quite complex mixtures and subsequent experiments using different emulsifier systems indicated that this component of a cosmetic formulation could also have an impact on steering the active ingredient to the right layer of the skin, although it is too early to be able to derive general rules from this.     

Stability study of lipoic acid in the presence of vitamins A and E in o/w emulsions for cosmetic application

The effectiveness of any cosmetic product containing a functional ingredient is determined by the skin delivery of the active molecule, which is influenced by the type of carrier and the molecule itself. Furthermore, the functional ingredient should be stable in the formulation. The purpose of this paper is to study the stability of lipoic acid in the presence of vitamins A (as palmitate) and E (as acetate) in semisolids for cosmetic use. The systems formulated were studied in regard to their aspect, pH, stability under centrifugation and rheological behavior. The chemical analyses of lipoic acid and vitamins A and E were carried out by HPLC after studying the specificity of the method employed in each case. The quantitation of the active principles was performed by HPLC with C18 (5 μm) columns. The mobile phase was methanol for the vitamins, with spectrophotometric detection at 325 nm for vitamin A and 230 nm for vitamin E. The mobile phase for lipoic acid was methanol : water (80 : 20) and phosphoric acid at pH 3.0, with spectrophotometric detection at 332 nm. All systems were stable to centrifugation, and no significant modification of rheological behavior was observed in relation to the base emulsion used as control. The chemical studies performed indicated that although lipoic acid is not very stable in these formulations, the presence of vitamin A favors its chemical stability.     

Formulation effects of topical emulsions on transdermal and dermal delivery

It has been recognized that the vehicle in which a permeant is applied to the skin has a distinctive effect on the dermal and transdermal delivery of active ingredients. The cutaneous and percutaneous absorptions can be enhanced, e.g. by an increase in thermodynamic activity, supersaturation and penetration modifiers. Furthermore, dermal and transdermal delivery can be influenced by the interactions that may occur between the vehicle and the skin on the one hand, and interactions between the active ingredient and the skin on the other hand. Emulsions are widely used as cosmetic and pharmaceutical formulations because of their excellent solubilizing capacities for lipophilic and hydrophilic active ingredients and application acceptability. This review focuses, in particular, on the effect of emulsions on the dermal and transdermal delivery of active ingredients. It is shown that the type of emulsion (w/o vs. o/w emulsion), the droplet size, the emollient, the emulsifier as well as the surfactant organization (micelles, lyotropic liquid crystals) in the emulsion may affect the cutaneous and percutaneous absorption. Examples substantiate the fact that emulsion constituents such as emollients and emulsifiers should be selected carefully for optimal efficiency of the formulation. Moreover, to understand the influence of emulsion on dermal and transdermal delivery, the physicochemical properties of the formulation after application are considered.     

In vitro skin permeation evaluation: the only realistic option

Increasing requirements for cruelty-free risk assessment in the cosmetic industry have led to the development of several alternative experimental evaluation strategies. Quantification of the potential dermal absorption of ingredients of cosmetic and other formulations by determination of human skin permeation rates in vitro is particularly relevant. Using modifications of standard in vitro protocols the human skin permeation rates of several cosmetic ingredients and potential contaminants have been determined under conditions designed to mimic consumer use. Skin penetration and permeation of octyl salicylate (a sunscreen), nonylphenol ethoxylates (surfactants) and three nitrosamines (potential contaminants) is discussed. The data demonstrate the usefulness of this technique as a tool in the overall risk assessment of cosmetic formulations.     

Absorption of materials by or through the living skin

The skin comes into contact with a large range of materials either deliberately or inadvertently. It should be possible to predict the exact transport rates of these materials through the skin as a function of the physicochemical properties of the different compounds. With this sort of knowledge it is possible to predict the exact disposition of compounds and use this in the formulation of new products both in the pharmaceutical and cosmetic field. The information will also be useful from the standpoint of skin toxicology and environmental health. In order to be able to predict this complex process it is necessary to split the overall transport into different component parts. This article will identify these components and provide illustrations. The major areas discussed will be barrier function of the skin, the release properties of different topical formulations and how these may be monitored. Novel means of enhancing the penetration of drugs will be discussed and how some additives that are incorporated into formulations will perhaps alter the barrier function of skin. A mathematical model describing skin penetration has been developed and its use in predicting blood levels will be described. This model has been tested both in animal experiments and in limited human studies and its relevance to these situations will be highlighted.
Absorption cutanée et transcutanée in vivo     

Rational formulation design

To be efficacious and to satisfy the requirements for claim substantiation, a cosmetic formulation must achieve effective targeting of an active in the skin. Although the basic principles governing the skin permeation and disposition of molecules have been known for many years, attention has been far less focused on the role of the vehicle, particularly at cosmetically relevant doses. In this article, we discuss the necessity to understand the fate of the formulation components as well as the active once applied onto skin. Recent data confirm that the residence time of the formulation constituents can have a profound impact on the fate of the active. Approaches to identify the ideal vehicle for skin delivery are considered critically, specifically the recent work on ‘formulating for efficacy’ (FFE) by the late Johann Wiechers. Essentially, FFE aims to match the active with the optimal vehicle for skin delivery based on matching polarity/solubility values of the trinity of skin, active and vehicle. The emerging importance of techniques that provide insight to how the vehicle distributes in and on skin is highlighted.     

In vitro antioxidant activity and in vivo photoprotective effect of a red orange extract

Ultraviolet radiation causes damage to the skin, which may result in both precancerous and cancerous skinlesions and acceleration of skin ageing. Topical administration of enzymatic and non-enzymatic antioxidants is an effective strategy for protecting the skin against UV-mediated oxidative damage. Hence, a systematic study to evaluate the in vitro antioxidant activity and in vivo photoprotective effect of a standardized red orange extract (ROE) has been undertaken, where the main active ingredients are anthocyanins, hydroxycinnamic acids, flavanones and ascorbic acid. For the in vitro experiments, the ROE was tested in three models: (1) bleaching of the stable 1,1-diphenyl-2-picrylhydrazyl radical (DPPH test); (2) peroxidation, induced by the water-soluble radical initiator 2,2'-azobis(2-amidinopropane) hydrochloride, of mixed dipalmitoylphosphatidylcholine/linoleic acid unilamellar vesicles (LUVs) (LP-LUV test); and (3) UV-induced peroxidation of phospatidylcholine multilamellar vesicles (UV-IP test). The in vivo antioxidant/radical scavenger activity was assessed by determining the ability of topically applied ROE to reduce UVB-induced skin erythema in healthy human volunteers. The results obtained in the DPPH, LP-LUV and UV-IP tests demonstrated the strong antioxidant properties of ROE, with a clear relationship between ROE scavenger efficiency and its content in antioxidant compounds. In particular, the findings obtained in the UV-IP test provide a strong rationale for using this extract as a photoprotective agent. During in vivo experiments, ROE provided to efficiently protect against photooxidative skin damage when topically applied immediately after skin exposure to UVB radiations. Interestingly, the protective effect of ROE appears higher than that elicited by another natural antioxidant (tocopherol) commonly employed in cosmetic formulations. In conclusion, the present findings demonstrate that ROE affords excellent skin photoprotection, which is very likely a result of the antioxidant/radical scavenger activity of its active ingredients. Thus, ROE might have interesting applications in both anti-photoageing and after-sun cosmetic products.     

Study of surfactant-skin interactions by skin impedance measurements

The stratum corneum (SC) plays a very critical physiological role as skin barrier in regulating water loss through the skin and protects the body from a wide range of physical and chemical exogenous insults. Surfactant-containing formulations can induce skin damage and irritation owing to surfactant absorption and penetration. It is generally accepted that reduction in skin barrier properties occurs only after surfactants have penetrated/permeated into the skin barrier. To mitigate the harshness of surfactant-based cleansing products, penetration/permeation of surfactants should be reduced. Skin impedance measurements have been taken in vitro on porcine skin using vertical Franz diffusion cells to investigate the impact of surfactants, temperature and pH on skin barrier integrity. These skin impedance results demonstrate excellent correlation with other published methods for assessing skin damage and irritation from different surfactant chemistry, concentration, pH, time of exposure and temperature. This study demonstrates that skin impedance can be utilized as a routine approach to screen surfactant-containing formulations for their propensity to compromise the skin barrier and hence likely lead to skin irritation.     

In vitro skin permeation of sunscreen agents from O/W emulsions

The effects of different emulsifiers on the in vitro permeation through human skin of two sunscreen agents [octylmethoxycinnamate (OMC) and butylmethoxydibenzoylmethane (BMBM)] were investigated from O/W emulsions. The test formulations were prepared using the same oil and aqueous phase ingredients and the following emulsifier and coemulsifier systems: Emulgade SE((R)) (ceteareth-12 and ceteareth-20 and cetearyl alcohol and cetyl palmitate) and glycerylmonostearate (emulsion 1); Brij 72((R)) (steareth-2), Brij 721((R)) (steareth-21) and cetearyl alcohol (emulsion 2); Phytocream((R)) (potassium palmitoyl-hydrolysed wheat protein and glyceryl stearate and cetearyl alcohol) and glycerylmonostearate (emulsion 3); Montanov 68((R)) (cetearyl glucoside and cetearyl alcohol) (emulsion 4); Xalifin-15((R)) (C(15-20) acid PEG-8 ester) and cetearyl alcohol (emulsion 5). The cumulative amount of OMC that permeated in vitro through human skin after 22 h from the formulations being tested decreased in the order 3 > 1 congruent with 4 > 5 > 2 and was about nine-fold higher from emulsion 3 compared with that from emulsion 2. As regards BMBM, no significant difference was observed as regards its skin permeation from emulsions 1, 3, 4 and 5, whereas formulation 2 allowed significantly lower amounts of BMBM to permeate the skin. In vitro release experiments of OMC and BMBM from emulsions 1-6 through cellulose acetate membranes showed that only emulsions 4 and 5 provided pseudo-first-order release rates only for OMC. The results of this study suggest that the type of emulsifying systems used to prepare an O/W emulsion may strongly affect sunscreen skin permeation from these formulations. Therefore, the vehicle effects should be carefully considered in the formulation of sunscreen products.     

Preliminary evaluation of the genotoxic potential of a hydrophilic polymer with three preservation systems

This study compared the genotoxic potential of a polymeric associative thickener used in topically applied emulsions preserved with three different preservative systems. The method used for the assessment of genotoxicity is the in vitro micronucleus test [Organization for Economic Cooperation and Development (OECD) guideline number 487]. When changing an additive such as a preservation system in a raw material, it is crucial to re-evaluate its toxicity potential because this change may significantly alter its properties. This study shows that at the levels tested neither of the systems evaluated demonstrated any cytotoxic or genotoxic effects. Skin exposure must take into consideration factors such as duration, skin condition and metabolism, but most importantly concentration. Although preservatives can be toxic at high concentrations, they are usually safe at the concentrations used in cosmetic raw materials and formulations. If used to preserve raw materials, they undergo further dilution when added to the formulation.     

Development of an in vitro efficacy test for self-tanning formulations

In the last few years, the desire to acquire a tan without sunbathing has grown. In response, many cosmetic companies have produced self-tanning preparations. However, to optimize these formulations, a suitable model to assess the colour induced on the skin remains to be developed. We have developed an in vitro method using a synthetic skin in order to study the efficacy of self-tanning formulations. The in vitro test was then used to study the influence of cosmetic ingredients upon the colour induced by erythrulose- or dihydroxyacetone-containing formulations. Finally, an in vivo study allowed us to relate the results obtained in vitro with those found on human skin. The results show that this in vitro test system is a reliable tool to predict the efficacy of self-tanning products.     

Cream or foam in pedal skin care: towards the ideal vehicle for urea used against dry skin

The aim of this study was to evaluate different urea‐containing cosmetic preparations designed for foot care regarding skin occlusion. The primary aim was therefore to screen the short‐term transepidermal water loss (TEWL) as a parameter for skin barrier function and skin occlusion and to characterize the relative role of the vehicle, i.e. cream or foam in the context of cosmetics containing urea in the 2–10% range addressing the cosmetic products urea 2% cream (GEHWOL FUSSKRAFT blau), petrolatum containing cream (GEHWOL med Schrundensalbe), urea 10% cream (GEHWOL med Lipidro‐Crème), urea 10% foam (Allpresan Fuss Schaum) and vaseline (positive control) compared with an untreated area on the volar forearms of volunteers. Moreover, the short time (24 h) kinetics regarding the moisturizing effect of cream and foam formulations in diabetic patients were compared. The efficacy of a cream on reduction of skin thickness of hyperkeratotic skin in the heel region before and after a period of product application was also evaluated. In some of the trials, healthy individuals and in others, diabetic patients (type I and II) were enrolled. TEWL was determined before product application, as well as at given points of time thereafter. In this study, no excessive occlusion effects comparable with a blockage of the skin’s natural water evaporation could be observed for any of the test products. To the extent to be expected, this was found neither for the cream products nor for the foam product. Slightly lowered TEWL values after application of the 10% urea cream can be interpreted as a beneficial effect in terms of an improved barrier function. Regarding skin moisture, the urea‐containing cream formulation appeared equal or slightly superior to the foam formulation. The thickness of the horny layer was found reduced after application of 10 % urea‐containing cream. At present it looks as if cream vehicles would still be vehicles of choice in general, when it comes to the formulation of skin care preparations for not only dry skin but also in the context of pedal skin care.     

Comparative analysis of solar radiation-induced cellular damage between ex vivo porcine skin organ culture and in vitro reconstructed human epidermis

Reconstructed human epidermis models (RHE) constitute an innovative alternative to study phototoxicity and photoprotection in the cosmetic industry. However, little information is currently available concerning the harmful effects of solar-simulated radiation (SSR) in these in vitro skin models. In this study, the phototoxic effects of a single acute SSR dose of 275 kJ m⁻² were evaluated in a validated RHE model (from SkinEthic), and were compared with those obtained from an ex vivo skin organ culture recently developed from domestic pig ears. The RHE model was well differentiated in vitro and released a significant level of the cytosolic enzymes lactate dehydrogenase (LDH) and extracellular signal-related kinase 2 (ERK2) protein in the culture medium 24 h after SSR exposure. The SSR-induced cytotoxicity was related to the formation of sunburn cells and the appearance of DNA damage (thymine dimer and DNA fragmentation) in keratinocytes. Interestingly, these DNA alterations were associated with the activation of the caspase-3 protease, mainly in the basal layers of the epidermis. In addition, the RHE model responses were comparable with porcine skin following solar irradiation, and none of the above cellular responses was observed in non-irradiated skin models. Finally, topical application of a broad-spectrum UVB + A sunscreen formulation efficiently protected both the RHE and pig skin against the deleterious effects of SSR. Thus, both RHE and ex vivo pig skin organ culture models are complementary tools in the assessment of SSR-induced DNA damage and apoptosis, and they may be used to evaluate the photoprotective capacity of cosmetic formulations.     

Nanoemulsion: process selection and application in cosmetics – a review

In recent decades, considerable and continuous growth in consumer demand in the cosmetics field has spurred the development of sophisticated formulations, aiming at high performance, attractive appearance, sensorial benefit and safety. Yet despite increasing demand from consumers, the formulator faces certain restrictions regarding the optimum equilibrium between the active compound concentration and the formulation base taking into account the nature of the skin structure, mainly concerning to the ideal penetration of the active compound, due to the natural skin barrier. Emulsion is a mixture of two immiscible phases, and the interest in nanoscale emulsion has been growing considerably in recent decades due to its specific attributes such as high stability, attractive appearance and drug delivery properties; therefore, performance is expected to improve using a lipid‐based nanocarrier. Nanoemulsions are generated by different approaches: the so‐called high‐energy and low‐energy methods. The global overview of these mechanisms and different alternatives for each method are presented in this paper, along with their benefits and drawbacks. As a cosmetics formulation is reflected in product delivery to consumers, nanoemulsion development with prospects for large‐scale production is one of the key attributes in the method selection process. Thus, the aim of this review was to highlight the main high‐ and low‐energy methods applicable in cosmetics and dermatological product development, their specificities, recent research on these methods in the cosmetics and consideration for the process selection optimization. The specific process with regard to inorganic nanoparticles, polymer nanoparticles and nanocapsule formulation is not considered in this paper.     

Common cosmetic hydrophilic ingredients as penetration modifiers of flavonoids

Nowadays, flavonoids are present in many cosmetic formulations, mainly in the form of plant extracts. The main reason of still increasing popularity of these substances is their beneficial biochemical activity. The main factor affecting activity of flavonoids in the skin is their skin penetration ability. The studies have evidenced that flavonoids from grape leaf extract as well as flavonoids like quercetin, rutin and catechin can migrate through the model lipophilic membrane from aqueous solution. The influence of common hydrophilic cosmetic additives on the permeation profile of flavonoids has been checked. The partition coefficients of examined flavonoids in the octanol–water extraction system were determined. Correlations between permeation coefficients and log P of particular flavonoids were plotted. To determine the mechanism of influence of hydrophilic substances on the permeation profile of flavonoids, the solubility of these compounds was investigated. Studies suggest that the presence of hydrophilic additives causes the increase in the flavonoid solubility that decreases the activity of flavonoids in the vehicle. In such a situation, the driving force for the penetration is reduced and the decrease of permeation coefficient can be observed.     

Influence of nanoencapsulation on the sensory properties of cosmetic formulations containing lipoic acid

Sensory analysis has become a valuable tool in qualifying consumer perception regarding cosmetic products. This study aims to explore the application of discriminative and affective consumers sensory analysis in evaluating the influence of nanoencapsulation on the sensory properties and rheological characteristics of a cosmetic formulation containing lipoic acid. The nanocapsules were prepared by the pre‐formed polymer precipitation method. Semisolid formulations were prepared using a silicone emulsion system, and these pseudoplastic fluids were characterized using rheological methods. The panellists (n = 88) analysed the formulations with and without nanoencapsulated lipoic acid as paired comparisons within the discriminative and affective sensory analysis. In these measurements, spreadability, stickiness, oiliness and sulphur odour were evaluated. The panellists had no previous training in conducting these measurements. It was shown that nanotechnology can change some sensory characteristics of the formulations. The spreadability, stickiness and oiliness are the attributes for which the panellists noted differences with statistical significance. The spreadability difference could be due to the results found in the rheological profiles and consistency indexes between formulations. In the discriminative analysis, the panellists could not detect any noticeable differences in the sulphur odour or residual properties between samples, attributes that would influence whether consumers adhered to the selected treatment. Considering affective analysis, the consumers communicated that the formulation containing the nanoencapsulated lipoic acid, which presented less consistency, was preferred based on the reduction in immediate stickiness and residual sulphur odour. The free lipoic acid formulation was preferred in terms of residual oiliness and spreadability.     

In vitro skin permeation and retention of parabens from cosmetic formulations

Parabens are antimicrobial agents widely used in foods, cosmetics and pharmaceutical products. Although non-mutagenic, non-teratogenic and non-carcinogenic, parabens can induce allergic contact dermatitis and posses estrogenic activity. The aim of this work was to assess the skin permeation and retention of methyl- (MP), ethyl- (EP) and propyl- (PP) paraben from three commercial cosmetic creams. The results obtained indicate that parabens are capable of permeating through and accumulating in the skin. The extent of penetration depends more on paraben characteristics (solubility, lipophilicity) than on the composition of the formulation. In particular, the percentage permeated across the skin was independent of the composition of the cream used and decreased in the order MP, EP and PP, in accordance with decreasing solubility. After 8 h of contact with the skin, 60% of MP, 40% of EP and 20% of PP were found across the skin. Concerning skin retention, the percentage remaining in the skin after 8 h depends on both paraben characteristics and on the composition of the formulation used. In conclusion, it appears that only the type of paraben, in particular its water solubility, affects skin penetration whereas the composition of the emulsion, which influences skin retention, plays a secondary role. Finally, excised rabbit ear skin can be considered as a good model for human skin for in vitro experiments.     

Skin penetration and stabilization of formulations containing microfine titanium dioxide as physical UV filter

Microfine titanium dioxide (TiO(2)) has become a frequently used physical UV filter in sunscreen formulations. Penetration of microfine TiO(2) into human skin seems to be possible because of the mean particle size of 20 nm. The small particle size results in a high surface activity of the primary particles and causes a formation of agglomerates in the formulation. The aim of this study was to investigate the in vivo and in vitro penetration behaviour of the physical UV filter into human skin. Furthermore, a stable sunscreen formulation with microfine TiO(2) which does not penetrate into the skin should be developed. According to our experiments, microfine TiO(2) penetrates deeper into human skin from an oily dispersion than from an aqueous one. Therefore, an o/w emulsion containing the dispersed micropigment in the aqueous phase was manufactured. Microfine TiO(2) cannot penetrate into human skin from this emulsion, but the storage stability of the formulation is very low at different temperatures. The encapsulation of the micropigment into liposomes does not result in a better stability but it causes a higher penetration depth of the particles into the skin.