Nanomedicine strategies for targeting skin inflammation.

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Nanomedicine strategies for targeting skin inflammation

Topical treatment of skin diseases is an attractive strategy as it receives high acceptance from patients, resulting in higher compliance and therapeutic outcomes. Recently, the use of variable nanocarriers for dermal application has been widely explored, as they offer several advantages compared with conventional topical preparations, including higher skin penetration, controlled and targeted drug delivery and the achievement of higher therapeutic effects. This article will focus on skin inflammation or dermatitis as it is one of the most common skin problems, describing the different types and causes of dermatitis, as well as the typical treatment regimens. The potential use of nanocarriers for targeting skin inflammation and the achievement of higher therapeutic effects using nanotechnology will be explored. Keywords: animal models • dermatitis • glucocorticoids • hair follicles • nanoparticles • skin inflammation • skin penetration • targeting

Nanotechnology is the science of utilizing the unique properties of materials formulated with a particle size in the nanometer range. At such a small size, the engineered nanoparticles, due to their high surface area-to-volume ratio, exhibit unique physical, chemical and biological properties that are distinctly different from the same materials in bulk form. Materials and devices are expected to be faster, smaller, more powerful, more efficient and more versatile. Treatments are anticipated to be more efficacious, specific and flexible. Drugs are evaluated to be more customizable, versatile and cost-effective [1] . Skin, as the largest organ of human body and with its various protective and physiological functions, has been considered to be a major target for the application of nanotechnology for the optimization of both topical and transdermal purposes [2–4] . ‘Nanodermatology’, a term that was first introduced by the Nanodermatology Society [5] , is one of the most evolving applications of nanotechnology in which nanomaterials are introduced onto the skin for diagnostic, therapeutic or cosmetic purposes. The fast development of nanoder-

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matology can be demonstrated by the growing financial investments and the exponential number of registered patents within the field [6] . Nanoparticles are widely used in cosmetic products in order to improve the homogeneity of distribution on the skin and to enhance the efficacy of sunscreens against UV radiation. Particles such as zinc oxide and titanium oxide are utilized widely as sunscreens and have been tested for percutaneous penetration, phototoxicty or photogenotoxicity, and the available data suggest that they carry a very low risk for human health. However, the formulation of nanoproducts for pharmaceutical use is much more challenging, as it is a major prerequisite that materials are formulated in order to fit with the physicochemical properties of the drug used (so as to avoid any instability) and the therapeutic needs, such as improved skin penetration and prolonged effects, in addition to biocompatibility [7] . Therefore, several studies are continuously being performed in order to optimize the use of nanotechnology for the treatment of skin diseases and enhance the effectiveness and targeting of various drugs to the skin while

Nanomedicine (2014) 9(11), 1727–1743

Mona MA AbdelMottaleb*,1,2, Celine Try1,3, Yann Pellequer1 & Alf Lamprecht1,4 Laboratory of Pharmaceutical Engineering & Biopharmaceutics, EA4267, University of Franche-Comté, Besançon, France 2 Department of Pharmaceutics & industrial Pharmacy, Faculty of Pharmacy, Ain Shams University, Cairo, Egypt 3 Clinical Investigation Center (Inserm CIC 1431), Regional University Hospital of Besançon, Besançon, France 4 Laboratory of Pharmaceutical Technology & Biopharmaceutics, University of Bonn, Bonn, Germany *Author for correspondence: Tel.: +33 3 81 66 55 48 Fax: +33 3 81 66 52 90 mona.abdel-mottaleb@ univ-fcomte.fr; mona.abdelmottaleb@ pharma.asu.edu.eg 1

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Review Abdel-Mottaleb, Try, Pellequer & Lamprecht minimizing their systemic absorption and side effects. This includes the utilization of different types of nanocarriers, including liposomes, lipids and polymeric nanoparticles [2] . One of the most common skin problems is skin inflammation. Skin inflammation is a broad term that describes the presence of typical inflammation symptoms: rubor (redness), calor (warmth), dolor (pain) and tumor (swelling). There is a wide range of dermatological diseases that include inflammatory reactions in the skin that range in severity from mild skin rash to severe dermatitis, arising from a systemic disease or generalized infections. Other conditions, such as acne and psoriasis, are also considered to involve various inflammatory implications at some stages. Photodamage caused by skin exposure to UV radiation involves low but chronic Th1-mediated inflammation that impairs the cellular response, although the skin appears to be clinically normal [8] . In this article, we focus on dermatitis as it is the most common cause of skin inflammation. We discuss the recent applications of nanotechnology for the treatment of dermatitis along with the associated benefits and risks and highlight their future potential. Dermatitis The term ‘dermatitis’ literally describes the organ of the skin and indicates the presence of inflammation. It is considered synonymous with another famous disorder term, which is ‘eczema’. However, eczema is sometimes considered to be a more generalized term that describes other inflammatory conditions in dermatology. Dermatitis can be identified clinically by the main primary symptoms of inflammation, itching, erythema or redness and the presence of papules and vesicles [9] . Different types of dermatitis with different etiologies, pathogeneses, clinical pictures and treatment options are discussed below. Types of dermatitis Atopic dermatitis

Atopic dermatitis (AD) is the type of eczema in which the endogenous component is of great relevance. There is always a tendency of these patients to have dry skin, expressed as wintertime chapping of the dorsal side of the hands and lips, and even generalized dry skin that increases their susceptibility to irritant contact dermatitis. It is a complex disease that manifests as immunological abnormalities and represents one of the most common chronically relapsing inflammatory eczematous skin conditions. The disease is often associated with the development of asthma, allergic rhinitis and food allergy [10] . In addition to the immunological dysregulation [11] , AD patients show epithe-

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lial skin barrier and tight junction abnormalities [12] . Recent studies have shown that the prevalence of AD has increased two- to three-fold over the previous few decades and is continuing to increase [13–15] . Currently, treatment is directed towards reducing itchiness and dryness using emollients, topical steroids and oral antihistamines. Refractory cases need other modalities of drugs, including systemic steroids or topical calcineurin inhibitors [16] . Glucocorticoids (GCs) inhibit many inflammation-associated molecules, such as cytokines, chemokines, arachidonic acid metabolites and adhesion molecules. On the other hand, the mode of action of the topical calcineurin inhibitors is more cell selective than that of corticosteroids. Tacrolimus and pimecrolimus are both macrolide immunosuppressant drugs that exert their immunosuppressive effects by inhibiting the activation of T lymphocytes, thereby decreasing the release of various proinflammatory cytokines. The efficacy of tacrolimus ointment and pimecrolimus cream has been demonstrated against placebo in clinical trials in the short-term [17] and long-term treatment of dermatitis [18,19] . In addition, tacrolimus ointment therapy has been shown to be effective for up to 1 year in terms of reducing the number of flares and improving the quality of life in adult patients and children [20,21] . Recently, siRNA applied topically was studied as a novel therapeutic approach for the treatment of AD. RNA is a powerful method for silencing specific genes and has shown promise as an effective treatment for targeting specific aberrantly expressed genes for the treatment of dermatitis [22] . Contact dermatitis

Contact dermatitis is skin inflammation due to exogenous reasons (exposure to allergic or irritant xenobiotics) and can be broadly categorized into allergic and irritant contact dermatitis. Allergic contact dermatitis is a type IV, delayed, cell-mediated hypersensitivity condition, while irritant contact dermatitis usually occurs due to the activation of the skin’s innate response system upon contact with an irritant material. In allergic contact dermatitis, an initial sensitizing exposure to the allergen is required, while in irritant contact dermatitis, no such previous exposure is necessary [9] . Allergic contact dermatitis can be induced by exposure to low-molecular-weight chemicals, antigen proteins or haptens linked to a protein with both proinflammatory and antigenic properties [12,23] . An inflammatory response at the site of skin contact is observed with an extremely pruritic, erythematous, papulovesicular rash with scaling and dryness of the skin. On the other hand, irritant contact dermatitis is a nonallergic inflammatory reaction of the skin to an external agent (toxic chemicals) associated with

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cutaneous integrity damage, epidermal lesions of different degrees of severity and an inflammatory reaction in the underlying dermis. The pathophysiology is brought about by activation of the innate immune system due to the proinflammatory properties of chemicals. Treatments of allergic and irritant contact dermatitis are based on the identification and elimination of the offending contact allergen/irritant. Topical steroids can be used in the acute phase in order to control inflammation and inhibit pruritus. Short bursts of systemic corticosteroids can be used in severe and extensive dermatitis. Seborrheic dermatitis

Seborrheic dermatitis is an inflammatory skin disorder affecting body areas that are rich in sebaceous glands, such as the scalp, face, chest and intertriginous areas. It is characterized by erythematous pruritic plaques, with the greasy scale ranging in severity from only a mild flaking dandruff to severe oily scaling on the scalp, face and trunk. This disease affects up to 3% of immunocompetent adults and is more common in men than women. However, the prevalence of this disease is dramatically higher among immunocompromised patients, particularly HIV-positive individuals. Multiple causes of this disease have been postulated, but Malassezia fungus species are considered to be the most likely pathogenic causes. Topical antifungal preparations (ketoconazole 2%, bifonazole 1% and ciclopiroxolamine 1%) can be used one to two times a week in order to treat the scalp. Other treatments options include topical calcineurin inhibitors or topical corticosteroids [24] . Other types of dermatitis

Other less common types of dermatitis include stasis dermatitis, which results from impaired venous drainage of the legs, resulting in inflammatory cell and small blood vessel proliferation in the papillary dermis [25] . Neurodermatitis results from repeated scratching. Perioral dermatitis is a chronic acneiform eruption on the face that is most probably related to cosmetics use and corticosteroid applications [26] . Nummular or discoid dermatitis consists of circular areas of red, scaly and psoriatic-like lesions that are usually seen on the backs of the hands. It is mostly idiopathic and, once present, the sizes of the lesions do not change [27] . Animal models of dermatitis

Animal models are considered to be invaluable tools for expanding our knowledge regarding the pathogenesis, diagnosis and treatment of different human skin diseases. Experimental dermatitis can be induced in different species, including mice, rats, guinea pigs, pigs


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and dogs, but the most commonly utilized models are mice [28] . The reasons for this include ease of manipulation, lower cost and the availability of genetically modified strains. The mouse models that are used for AD can be categorized into three groups [29,30] . The first depends on the cutaneous application of sensitizers, while the second involves using transgenic mice that overexpress or lack certain molecules. The last category includes mice that can spontaneously develop AD-like lesions. Repeated application of haptens, such as ovalbumin, 2,4,6-trinitrochlorobenzene or oxazolone, for many days to the same skin site results in an immediate-type hypersensitivity response followed by a late reaction, a finding that is often seen in the skin lesions of AD patients. This skin lesion is associated with epidermal hyperplasia, accumulation of mast cells and CD4 T cells in the epidermis and elevated serum levels of antigen-specific IgE. This simple and reproducible model is of enormous value in the assessment of potentially therapeutic agents for the treatment of AD. The short time taken to develop the skin lesions (∼30 days) is an additional advantage of these models. However, the disadvantages are the requirement for previous sensitization 7 days before starting repeated hapten application and concerns about potential interactions between haptens and therapeutic agents. More recently, transgenic mice (IL-4-, IL-31-, TSLP-, caspase-1- and IL18-transgenic mice) and spontaneous models of AD (NC/Nga, NC/Jic and NC/Tnd) have been developed. The NC/Nga mouse is the most extensively studied model of AD. These animals develop AD-like eczematous skin lesions when kept in an air-uncontrolled conventional room, but not when maintained under specific pathogen-free conditions. The skin lesions are developed at the age of 8 weeks and resemble human AD in many aspects, including severe pruritus, chronic skin lesions, skin dryness, impaired barrier function, elevated dermal mast cells and specific IgE levels and histopathology. The inconvenience of the NC/Nga mice is the incidence of skin lesions, which drastically varies (by ∼50%). Therefore, Dermatophagoides extracts are used in order to favor dermatitis in these mice in specific pathogen-free conditions [31] . Animal models spontaneously developing AD-like lesions and transgenic mice are useful for research into the etiopathogenesis of this disease, but these models are not widely available, thus limiting their usefulness. Irritant contact dermatitis models (delayed skin inflammation) can be induced on mice ears by different chemicals, such as croton oil, dithranol, arachidonic acid, capsaicin or carrageenan [28] . The maximal inflammatory response is observed a few days after the application of a single dose of the irritant agents to

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Review Abdel-Mottaleb, Try, Pellequer & Lamprecht the inner surface of the mouse ear. The skin inflammation can be easily quantified by the measurement of ear weight and thickness. This model offers many advantages: the methods are quick and simple, small quantities of substances are required, the results are reproducible, there is only a low possibility of error and the results are obtained rapidly. Only a few seborrheic dermatitis models can be found in the literature. A spontaneous mouse model of seborrheic dermatitis was developed from the expression of the 2C T-cell receptor transgenes in DBA/2 mice under specific pathogen-free conditions [32] . As spontaneous AD murine models, the spontaneous model of seborrheic dermatitis is of limited use due to limited availability of these strains. Alternatively, seborrheic dermatitis can be induced by infecting the skin of guinea pigs with Malassezia furfur for 7 consecutive days [33] . Nanomedicine for skin inflammation Nanoemulsions

In nanoemulsions, as in traditional emulsions, oil droplets are dispersed into water with the aid of stabilizing surfactants. However, the droplet size of nanoemulsions should lie in the nanometer range, which can be achieved by the use of high-shear technologies or other physical techniques [1] . Because of their small particle size, nanoemulsions demonstrate rapid, easy penetration into the skin with a favorable light, nongreasy feel. They can be formulated from biodegradable, biocompatible lipids that provide soothing and relieving effects, which can be advantageous in cases of skin rash and inflammation. Positively charged nanoemulsions utilizing phytosphingosine (PS) were formulated containing the drug prednicarbate for the treatment of AD. The positive charge could enhance the penetration of the drugs deeper into the skin by interacting with the negatively charged residues of proteins and lipids in the stratum corneum (SC) or with keratin in the corneocytes. The choice of PS as an important element required for the formation of ceramides enhances healing and helps to restore normal skin functions [34] . Another example of utilizing nanoemulsions for the treatment of AD is the dermal delivery of DNAzymes [35] . Some DNAzymes are known to interfere with Th2 cell differentiation and their inflammatory responses; therefore, they are expected to possess some potential for the treatment of AD. However, their dermal delivery is challenging due to the high molecular weight and susceptibility to the natural skin flora’s enzymes. The protection of DNAzymes by their inclusion into micron-sized and submicron-sized emulsions improved their skin penetration and uptake by the epidermal cells, with noticeable stability [35] . In addition, skin

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penetration studies have shown that the skin uptake was limited to the SC and epidermal layer, suggesting there being no risk of systemic toxicity. Lipid nanoparticles

Lipid-based nanoparticles (mainly solid lipid nanoparticles [SLNs] and nanostructured lipid carriers [NLCs]) are known for their excellent adhesion, skin hydration and occlusion properties, thus enhancing dermal drug delivery. They consist of lipid nanoemulsions in which the liquid oil is replaced by a solid lipid or a mixture of solid and liquid lipids. Surface-modified NLCs with cell-penetrating peptides (CPPs) could increase the amount of drug delivered to inflamed skin through the interaction of their positive charge with the negatively charged residues of proteins and lipids in SC, thus destabilizing the SC integrity. Another proposed mechanism for their higher penetration is based on theirdirect interaction with keratin in the corneocytes and the accumulation of NLCs into hair follicles and sebaceous glands. It was found that the optimum positive charge required for maximum NLC penetration and subsequent therapeutic anti-inflammatory effects is achieved by a polyarginine peptide containing 11 arginines [36] . Skin retention of celecoxib encapsulated in NLCs modified with polyarginine-11 CPPs was significantly higher than unmodified NLCs at all time intervals and in all skin layers. This could be explained by the combined occlusive effect of NLCs with the translocation power of CPPs. Pharmacokinetic analysis of the amount of the drug in the dermis revealed that time of maximum concentration for celecoxib solution was at 4 h compared with 15 h in the case of NLC formulations, accompanied by a significantly higher area under the curve, which confirms that the NLCs form an occlusive film on the SC surface and act as a reservoir for the controlled and enhanced release of the drug through different skin layers [37] . Similarly, all-transretinoic acid has been encapsulated in SLNs, resulting in dramatic improvements in its photostability and reducing its irritant effect, as shown by the remarkably reduced erythematic episodes on rabbit skin compared with the currently marketed cream [38] . The incorporation of prednicarbate into SLNs improved its tolerability and localized its effects to the epidermis for 6 h, and this targeting effect was not reversed even after diluting the SLNs with a cream base [39] . This demonstrates the ability of lipid nanoparticles to achieve higher deposition in deep skin layers, protect the encapsulated drugs, increase their stability and achieve sustained release with subsequent lower dosing frequencies. Tacrolimus is effectively used for the treatment of AD. However, the topical application of tacrolimus ointment



is accompanied by variable absorption rates and a number of side effects, including stinging, warmth, redness and allergic reactions. Systemic absorption caused difficulty in breathing, face swelling and increased risk of infections and malignancy due to immunosuppression. Therefore, targeted delivery to the inflamed parts of the skin would avoid several side effects on the healthy skin and systemically [10] . Tacrolimus-loaded lipid nanoparticles could achieve significantly higher drug concentrations in the SC compared with the conventional preparations. Although the SC is not the main target tissue for the treatment of dermatitis, it can act as a reservoir to continuously supply the drug to deeper skin layers over time. Skin retention experiments in other skin layers also demonstrated 30.8- and 28.6-times higher epidermal and dermal levels, respectively. This confirms the targeting ability to the deeper skin layers containing dendritic cells, which is the main site of action of tacrolimus [40] . Interestingly, biodistribution data using radiolabeled formulations showed major localization of the nanoparticles in the skin, without any spreading of radioactivity to other organs of the body. This targeted localization and increased bioavailability in the dermis led to enhanced therapeutic effects and less toxicity compared with the ointment. Liposomes, ethosomes & niosomes

Liposomes are built from phospholipid bilayers surrounding a water core and dispersed in aqueous medium. Their size depends on their structure, in that small unilameller vesicles have dimensions of 20–100 nm, large unilamellar vesicles have dimensions of approximately 100 nm and multilamellar vesicles mostly exceeding 500 nm [7] . Although they are considered highly biocompatible systems, liposomes are metastable vesicles and their use is often limited by instability problems, such as aggregation, changes in vesicle size, hydrolysis or leakage. PS is a sphingoid base that is found naturally in small amounts in the human epidermis and possesses significant anti-inflammatory effects. PS also shows antimicrobial properties against Propionibacterium acnes and Staphylococcus aureus, thus making it a good candidate for the treatment of acne and dermatitis. 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), a phospholipid that is known for its similarity with skin lipids and its ability to form liposomes of higher encapsulation rates, was used as a vehicle for topical PS delivery. The incorporation of PS into DPPC liposomes helped to increase its skin retention to a large extent, with negligible transdermal permeation due to the interaction of both PS and DPPC with similar skin lipids [41] . Ultradeformable liposomes or transfersomes also consisting of phospholipids, but with the addition of a


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surfactant that acts as an edge activator, facilitating the deformability of the liposomes [42] . They are claimed to be able to permeate intact through the skin due to their high flexibility, which allows them to squeeze through SC channels. Another mechanism of their transdermal penetration depends on the osmotic gradient across the skin created upon applying the transfersomes to the skin surface under nonocclusive conditions [43] . However, this theory has been contradicted by other researchers who found that the transfersomes are limited to the SC layer and have never been detected in the viable epidermis layer. Upon vesicle disintegration, the drugs diffuse and penetrate according to their physicochemical properties [44,45] . Transfersomes have been utilized for topical GC applications, such as hydrocortisone, dexamethasone and triamcinone acetonide [46–48] . The enhancement of topical GC depositions is based on the transfersome’s ability to penetrate the SC following the hydration gradient and so form a sub-SC deposit layer for the continuous release of the drug [47] . This action would be associated with longer durations of action, higher potency and selectivity and fewer systemic side effects. However, it was found that such localized effects can only be achieved with low doses of the drugs, while higher doses tend to attain higher systemic availabilities of the drugs due to the penetration-enhancing abilities of the transfersomes. Another important factor is the solubility of the drug and its physicochemical interactions with the vesicle carrier, skin and the hydrophilic circulation. If the drug is of considerable hydrophilicity, higher diffusion is expected from the sub-SC depot, and so systemic availability with subsequent side effects must be considered [49] . Ethosomes are innovative vesicular systems with attractive features related to their ability to enhance the penetration of various compounds through the SC and deeper skin layers compared with conventional liposomes. Ethanol is an important adjuvant in ethosomes and is mainly responsible for their penetrationenhancing effects. Ethanol is a good solvent that can increase the solubility of drugs into ethosomes compared with liposomes. In addition, being highly volatile helps ethanol to extract the SC lipids and enhance drug penetration, especially if used at high concentrations and for prolonged times. Ethanol itself has a high diffusivity into skin tissue that can drag dissolved drugs in order to penetrate and dissolve better in the altered skin membrane. Tacrolimus is mainly targeted to the inhibition of passive cutaneous anaphylaxis, vascular permeability and substance P-related mast cells activation. Therefore, the achievement of higher concentrations of the drug in the dermis would enhance its therapeutic efficacy. Tacrolimus-loaded ethosomes achieved higher concentrations of the drug in the


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Review Abdel-Mottaleb, Try, Pellequer & Lamprecht epidermis and dermis of rat skin (ex vivo) and higher therapeutic efficacy in AD in mice compared with the commercially available ointment and liposomes [50] . However, the irritant effect of ethanol cannot be ignored as an inconvenient drawback to the successful use of ethosomes in AD. Niosomes are nonionic surfactant vesicles formed from the self-assembly of nonionic amphiphiles in aqueous medium. Similarly to liposomes, they are able to entrap both hydrophilic and lipophilic compounds and be formulated with a wide variety of surfactants [51] . Ammonium glycyrrhizinate-loaded niosomes were able to reduce methyl nicotinate-induced erythema in humans much more rapidly and efficaciously with respect to the aqueous solution of ammonium glycyrrhizinate. However, higher skin permeation was also observed through the human skin, in that 23% of the applied dose was detected in the receptor compartment after 24 h compared with 7% in case of solution. This can increase the risk of systemic side effects of the applied drugs [52] . Polymer nanoparticles

The use of polymeric materials for encapsulating drugs or other active substances is an important approach for masking the inadequate physicochemical characteristics of several substances (hydrophilic or lipophilic drugs, high molecular weight, stability and bioavailability), thereby facilitating their skin penetration [53] and providing the controlled release of bioactive compounds. Moreover, their use for biomedical applications has increased in recent decades due to their biocompatibility and minimal side effects. Polymeric nanoparticles are colloidal carrier systems prepared from biodegradable or nonbiodegradable polymers and with diameters lower than 1 μm. For topical application, polymeric nanoparticles can achieve higher concentrations of the drug in the epidermis at a reduced dose and with few side effects [54–58] by reducing the permeation coefficient and the flux rate of the drug. With these properties and the tendency to accumulate preferentially into hair follicles [53,59] , the use of polymeric carriers seems to be an interesting strategy for topical treatment. However, only very few studies have dealt with the in vivo penetration of polymeric nanoparticles in inflamed skin. In one study, ethyl cellulose nanoparticles smaller than 100 nm appeared to accumulate preferentially in the hair follicles and sebaceous glands of inflamed skin in mice, while in healthy skin particles, they are only detectable on the surface of the SC [60] . On the other hand, particles with sizes of 500 and 1000 nm behaved similarly on both healthy and inflamed skin and could not penetrate the SC. Higher therapeutic efficiency was also observed after

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using the smaller nanoparticles loaded with betamethasone compared with the 1000-nm particles (Figure 1) . However, these results need to be confirmed by performing the experiments on porcine or human skin, since mouse skin cannot be considered to be the best representative of human skin. Charged particles are often proposed as a strategy for enhancing the penetration of drugs through the SC. With the negative charge of the skin at normal pH, an electrostatic attraction between positive particles and the skin should be expected. Research findings regarding the penetration of charged particles into healthy skin are still controversial, which could be explained by the different experimental conditions that have been utilized (in vivo or ex vivo models; charged or neutral surfactants; charges of the polymers) [61,62] . However, in the case of dermatitis, significant accumulation in the pilosebaceous units has been observed, especially for both the positively and negatively charged particles. The positively charged particles have greater interactions with the net negative charge of the SC, resulting in greater accumulation. Regarding the negatively charged particles, these are claimed to be easily internalized by endocytosis in macrophages, which might explain their greater accumulation in inflammation sites where the population of inflammatory cells is abundant [62] . Based on the earlier observations of superior localized accumulation of drugs administered in polymeric nanocarriers, greater numbers of evolving studies are being performed for the treatment of different skin conditions, including dermatitis. The simultaneous administration of ketoprofen, a known potent nonsteroidal anti-inflammatory drug, with the anti-inflammatory neurokinin-1 receptor antagonist Spantide II (American peptide company Inc, CA, USA) was proposed for the treatment of dermatitis. Poly(lactic-coglycolic acid) PLGA–chitosan bilayered nanoparticles were utilized for such a purpose in order to ensure high loading and stability of the macromolecule Spantide II in the PLGA inner core. The outer layer of chitosan is used in order to conjugate oleic acid as a permeation enhancer. The surface-modified nanoparticles were dispersed into an aqueous gel system utilizing hydroxypropyl methyl cellulose (HPMC) and Carbopol 981® (Lubrizol Advanced Materials, Inc., OH, USA) as gelling agents. Skin permeation experiments showed significant increases in the amount of ketoprofen and Spantide II in deeper skin layers from the nanoparticle gels compared with the conventional gel formulation. This led to higher therapeutic efficacy in the treatment of allergic contact dermatitis and in a psoriasis-like model in mice. Further investigations explained the improvement in skin permeation with oleic acid due to it creating permeability defects within the lipidic



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Figure 1. Confocal laser scanning microscopy images showing side view of skin treated with Nile red-loaded nanoparticles of different sizes, demonstrating the depth of penetration. (A–D) Healthy skin treated with NP1000, NP500, NP100 and NP50, respectively. (E–H) Inflamed skin treated with NP1000, NP500, NP100 and NP50, respectively. The first row represents the control experiment, showing the healthy skin (upper left) and inflamed skin (upper right) treated with Nile red oily solution. Reproduced with permission from [60] .

bilayers of the SC and thus faciliting the permeation of Spandide II and ketoprofen through the SC into the deeper layers of the skin [54,63] . Hydrocortisone, a topical GC, is often utilized for relieving the symptoms of AD and reducing the density of S. aureus in eczematous skin lesions. However, the well-known side effects of the prolonged use of GCs limit its use. Coadministration of the powerful oxygen free radical scavenger hydroxytyrosol has been proposed to alleviate the effect of autoxidation and the free radical cell-damaging effect at the site of inflammation. The coencapsulation of both drugs in chitosan nanoparticles incorporated into a cream significantly reduced their flux across fullthickness mouse skin compared with a hydrocortisone/ hydroxytyrosol cosolution in a cream and a commercial formulation of hydrocortisone. Higher epidermal and dermal accumulation was also observed, indicating higher efficiency in localized drug delivery. Testing the therapeutic properties on an in vivo mouse dermatitis model has shown that nanoparticles efficiently control erythema and dermatitis indices with minimal dryness and irritant effects on the skin. In this study, a decrease of transepidermal water loss (TEWL) was observed with codelivered nanoparticles, and this could be due to the synergistic effects of hydrocortisone and hydroxytyrosol promoting the regeneration of the SC


and diminishing the inflammatory cascades, but also due to the use of chitosan, which has a positive effect on wound healing and collagen regeneration [58] . The ability of double-stranded siRNAs to induce sequencespecific, post-transcriptional gene silencing, leading to significant evasion of the immune system, has made them an attractive new class of drugs that can interfere with disease-causing or disease-promoting genes [22] . Cationic polymers, peptides or lipid molecules offer the ability to form complexes with nucleic acids and protect them from nuclease attack, as well as facilitating cellular uptake through interactions with negatively charged phospholipid bilayers or through specific targeting moieties [64] . Using siRNAs for the local treatment of skin inflammation requires targeting to the deep epidermal and dermal layers. However, the SC represents a major obstacle to the permeation of nucleic acids and macromolecules. Therefore, an efficient delivery system would be required that must be able to bind and condense the siRNA, permeate through the SC, target the siRNA to the site of action and protect it from degradation in order to guarantee its gene-silencing ability [65] . Liquid crystalline nanodispersions of monoolein were combined with the cationic polymer polyethylenimine or the cationic lipid oleylamine for siRNA delivery for the knockdown of GAPDH, which is responsible for


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skin irritation [22] . The incorporation of the siRNA in liquid crystalline polymer nanodisperions significantly enhanced the penetration of such hydrophilic macromolecules to the deep epidermis of hairless mouse skin in vivo, and the penetration increased with the time of incubation (Figure 2) . In addition, the gene-silencing ability was only observed after 48 h in case of the nanodispersion, while the application of the naked molecule showed no knockdown activity at all. This demonstrates the ability of the polymeric nanodispersion to protect the stability of the macromolecule and deliver it to its site of action. The authors claim that the nanodispersions could also be deposited on the skin surface and provide prolonged release, which might explain the delayed onset of biological activity. It is important to mention here again that, despite the great aid provided by mouse models in terms of the understanding of nanocarriers’ skin penetration and interaction, they cannot be considered to be the best representative of human skin. Mouse skin, although formed of the same three layers as human skin, has major differences in the anatomy and physiology of these layers. For example, the hair cycle in mice is completely different from that of humans. In addition, mouse skin lacks apocrine sweat glands and dermal papillae, which are found in human skin [66] . Another therapeutic approach was to combine suppression of the immune response and inhibition of the neuropeptides involved in chronic inflammatory skin conditions with the use of cyclic lipid polymeric nanostructures encapsulating capsaicin and siRNA. The newly developed nanostructures were composed of three distinct components. The first was a negatively charged PLGA layer forming the inner core in which the poorly soluble drug capsaicin was entrapped. The second layer was composed of a cationic self-penetrating lipid synthesized with pyrolidinium cyclic polar head groups, and the third outer layer was formed of DSPE–PEG(2000) [67] . These nanostructures successfully delivered both the drug and the siRNA to deeper skin layers at therapeutic levels. The cationic lipid layer of the nanostructures interacted with the negatively charged residues of the proteins and lipids of the SC. Beyond a threshold concentration of these structures, they start destabilizing the SC membrane, combined with film formation over the skin surface, leading to the higher penetration of the entrapped drugs. In addition, penetration through the hair follicles and skin furrows allows for sustained effects by acting as drug reservoirs.

their anti-inflammatory effects on different animal models and have proven to be effective in comparison with silver nitrate solution. The treatment of 1,2-dinitrochlorobenzene (DNCB)-induced dermatitis in porcine skin revealed that nanocrystalline silver treatment could uniquely decrease erythema and edema by selective induction of apoptosis in inflammatory cells and suppression of matrix metalloproteinase activity. In DNCB-induced contact dermatitis, proinflammatory cytokines, including TNF-α, IL-1 and IFN-γ, are released by the inflammatory cells, including T lymphocytes, polymorphonuclear cells and macrophages, in response to hapten exposure. Thus, induction of apoptosis of these inflammatory cells will subsequently reduce edema and allow the keratinocytes to quickly recover. It was found that using silver nitrate led to indiscriminate apoptosis in all cell types, including keratinocytes, which even delayed the healing process. These results support the notion that a species of silver, Ag0, which is only represented in the nanocrystalline form, is responsible for this anti-inflammatory activity and the subsequent improved healing [68] . The anti-inflammatory activity of nanocrystalline silver was found to be similar to that of tacrolimus and topical steroids in the treatment of 2,4-dinitrofluorobenzeneinduced atopic contact dermatitis [69] . In addition, smaller sizes of silver nanoparticles (10 nm) were found to have higher therapeutic indices and subsequently higher safety than larger ones (100 nm). At therapeutic doses, 10-nm particles were found to be safe upon toxicity testing on HeLa–CD4–LTR–β-gal cells (human epithelial cells) [70] . However, the safety of silver nanoparticles still needs to be confirmed. The toxicity of starch-coated silver nanoparticles was studied on normal human lung fibroblasts (IMR-90) and human glioblastoma cells (U251) and the results indicated a dose-dependent mitochondrial toxicity and DNA damage [71] . Toxicity was also found to largely depend on the surface coating of the nanoparticles, which affects their cellular internalization and agglomeration. Silver cores stabilized with mercaptoundecanoic acid or with a polymer shell of poly(isobutylene-alt-maleic anhydride) with and without PEGylation were tested on NIH/3T3 embryonic fibroblasts. It was found that PEGylation and polymeric coating reduced the cellular uptake as well as toxicity to a great extent [72] . Different examples of the successful use of nanocarriers for the treatment of dermatitis in vivo are shown in Table 1.

Nanometals

General discussion

Nanocrystalline silver dressings are used commercially for their antimicrobial properties and wound-healing applications. However, they have also been tested for

Toxicity issues


As nanotechnology is mainly based on the acquirement of new properties by materials when they are



Review

A

B

C

D

E

24 h

48 h

Figure 2. In vivo skin penetration of GAPDH siRNA in hairless mouse skin after treatment for 24 and 48 h with different formulations. (A) Light microscopy of skin treated with phosphate buffered saline (hematoxylin and eosin stain). Fluorescence microscopy of skin sections treated with: (B) phosphate buffered saline; (C) naked GAPDH siRNA; (D) monoolein oleylamine nanodispersions plus GAPDH siRNA; and (E) monoolein polyethylenimine plus GAPDH siRNA. Reproduced with permission from [22] .

manufactured to a nanometer size, the creation of unprecedented numbers of these entirely new classes of materials and products has raised a lot of safety concerns [75] . As the size shrinks, the surface areato-volume ratio exponentially increases, with subsequent increases in toxicity, reactivity and the ability to penetrate human body, which can cause various cutaneous and systemic implications. Therefore, nanoproducts have the potential to present new irritants, haptens and reactive substances [76] . Testing the toxicity of nanocarriers depends mainly on the estimation of their toxic potential on either cell culture or utilizing in vivo animal models. However, with cell culture experiments, care should be taken with regards to the variability in results arising from the interactions between nanomaterials and chemical or fluorescent probes. These interactions might interfere with the viability of studies or produce secondary cytotoxicity through medium alterations [77,78] . Cytokines such as IL-6, IL-8, IL-10 and TNF-α could also be adsorbed onto carbon nanotubes, leading to their underestimation [79] . Therefore, care should be taken during handling of the results of these tests, and more than one assay might


be required in order to correctly assess the toxicity risks. In the field of nanodermatology, threats are relatively limited, as most of the topically applied products are manufactured from biocompatible lipids, phospholipids or biodegradable polymers. In addition, the risk of skin sensitization or irritation depends on several other factors, including the extent of hapten reactivity, the duration and frequency of exposure, the physical state of the skin barrier and the vehicle used. The application of different nanoparticles onto healthy skin is greatly hindered by the strong mechanical barrier properties of the SC. Although some preferential accumulation is observed in skin furrows and around hair follicles, no deeper penetration was found and the particles were never detected in the epidermis or dermis layers [80–83] . This might exclude the possibility of there being any systemic toxicity of the topical applications of such carriers on healthy skin. However, permeation through compromised skin or skin with any change in composition or structure that may alter its barrier properties is expected to be different from intact skin. An example is the penetration of quantum dots (QDs) into the deep viable epidermal and dermal layers of abraded skin,


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Review Abdel-Mottaleb, Try, Pellequer & Lamprecht

Table 1. Different nanosystems used for the in vivo treatment of dermatitis in animals or humans. Type of nanocarrier

Dermatitis model used

Effect

Species (in vivo)

Active ingredient(s)

Ref.

Nanocrystalline silver dressing

DNCB

Selective anti-inflammatory effect

Pigs

Nanocrystalline silver

[68]

Nanocrystalline silver cream

DNFB

Similar activity compared with tacrolimus BALB/c mice and steroids

Nanocrystalline silver

[69]

Niosomes

Methyl nicotinate

Rapid effect, but with higher transdermal Humans permeation compared with aqueous solution

Ammonium glycyrrhizinate

[52]

Liposomes

Croton oil and Higher accumulation in skin with better arachidonic acid therapeutic efficiency

Mice

C-phycocyanin

[73]

Liposomes, ethosomes TPA and PEVs

Higher drug accumulation and reduced permeation

Mice

Diclofenac sodium

[74]

NLCs

DNFB

Increased celecoxib permeation and therapeutic effect

Mice

Celecoxib

[37]

Liposomes and ethosomes

DNFB

Ethosomes had higher entrapment Mice efficiency, retention in the epidermis and therapeutic effects

Tacrolimus

[50]

Lipid nanoparticles

DNFB

Higher targeting index to dermatitis compared with ointment

Mice

Tacrolimus

[10]

Polymer-bilayered nanoparticles

DNFB

Higher therapeutic effect compared with conventional gels

Mice

Spantide II and ketoprofen

[63]

Polymer nanoparticles Dithranol

Selective accumulation in inflamed areas and higher therapeutic efficacy

Mice

Betamethasone

Polymer nanoparticles DNFB

Higher skin localization and reduced side effects of hydrocortisone

Mice

Hydrocortisone and hydroxytyrosol

[60,61] [58]

DNCB: 1,2-dinitrochlorobenzene; DNFB: 2,4-dinitrofluorobenzene; NLC: Nanostructured lipid carrier; PEV: Penetration enhancer-containing vesicle; TPA: 12-O-tetradecanoylphorbol 13-acetate.

while penetration was limited to the uppermost SC layers of intact skin [84] . Comparisons of the penetration of titanium oxide nanoparticles in intact and psoriatic skin showed similar penetration patterns. Although the labile and less cohesive SC in psoriatic skin facilitates the penetration into the SC layer, particles were never detected in the living cell layers [85] . The exposure of UVB-sunburned pig skin to titanium oxide and zinc oxide nanoparticles demonstrated slightly deeper penetration of the particles compared with intact skin, especially for titanium oxide. However, no transdermal absorption was observed in any of these cases [86] . Patients with AD have reduced a ceramide content in the skin, which alters the SC lipid composition, resulting in aberrant lipid bilayer organization. Furthermore, many of them suffer from reduced expression or lossof-function mutations in the filaggrin gene. Filaggrin is a key protein in the terminal differentiation of the epidermis and in the ability of the SC to keep skin hydrated [87] . Increased TEWL is observed in AD patients, even in normal skin parts, and this is not limited to the inflammatory lesions [88] . Higher diffusion

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of sodium lauryl sulfate and PEG through AD patients’ skin has been reported [89,90] . Therefore, the risks might be higher for the penetration of some molecules, such asproteins and nanoparticles, in cases of inflamed skin, with subsequent systemic dangers. Thus, a growing interest in investigating the clinical therapeutic and toxic potential of nanoparticles can been seen in some recent clinical studies. Zinc oxide nanoparticle penetration in human intact, tape-stripped, psoriatic or AD skin was studied in human volunteers using timecorrelated single-photon counting [91] . This technique allowed for the simultaneous monitoring of the in vivo penetration of the nanoparticles with noninvasive imaging techniques, with the NADPH concentration representing the metabolic state of the skin. Signals from zinc oxide nanoparticles were detected in the SC in all of the tested skin types, but with significantly higher concentrations presented in the compromised skin groups. However, particles could not be detected in the viable epidermis or dermis in any of the tested groups. The in vitro dissolution of zinc oxide nanoparticles has been reported to occur within 2 h in aqueous



solution, which may account for the lower penetration in tape-stripped and diseased skin. On the other hand, QDs, which are more stable particles when tested on ex vivo human skin that has been stripped 30 times, showed clear penetration into the viable epidermis and dermis within 24 h of application [92] . QD penetration was tested in intact rat skin and in flexed, tapestripped (30 times) and abraded skin (using sandpaper 60 times to remove the SC) in flow-through cells. The QDs could not penetrate beyond the SC in all of the tested skin types, except for the abraded type, in which QDs could be found in the dermis, suggesting a higher risk for patients with compromised skin barriers [84,93] . However, neither QDs nor cadmium as a degradation product were detected in the perfusate for any time point, type of QD or skin. This confirms the lack of a systemic toxicity risk. However, in inflamed skin, damaged barrier functionality, different blood flow rates, different fluid permeability and leaching plasma proteins would support the enhanced permeation of drug molecules. In a study comparing the transdermal permeation of flurbiprofen in inflamed and normal rat skin, lower absorption of flurbiprofen was detected in inflamed skin. This was explained by the increased serum proteins leaching into the tissue, which could bind the drug and inhibit its diffusion to the systemic circulation. However, the mechanism seems to be very complicated and further experiments are required [94] . In addition, this preferential accumulation and penetration of diseased skin can be positively utilized for targeted drug delivery, as discussed above, if any localized irritant or harmful effects are avoided. Hence, the irritant effects of some topical preparations have also been investigated. Silver nanoparticles with a size range of between 20 and 80 nm were only detected in the uppermost layers of the SC of porcine skin after an in vivo treatment period of 2 weeks [95] . Their topical application did not induce any macroscopic symptoms of irritation, but microscopic focal inflammation and epidermal hyperplasia were detected in a concentration-dependant manner. Intradermal injections of SiO2 or silica nanoparticles of less than 100 nm into mice ears aggravated the Dermatophagoides pteronyssinus-induced AD, while particles of 300 and 1000 nm had no significant effect. It was found that the size of silica particles could affect the induction of IL-18 and TSLP in the skin lesions associated with stronger Th2 responses and higher IgE production [96] . However, this study revealed that the intradermal injection of SiO2 nanoparticles aggravated the D. pteronyssinus-induced dermatitis, but the study did not test their own ability to induce dermatitis. The sensitizing capacity of rhodamine bisothiocyanate was increased following the topical application in


Review

nano- and micro-scale ethosomal formulations compared with an ethanol–water solution. According to the results obtained by the authors, the increased sensitizing power was due to the improved penetration and increased formation of hapten–protein complexes in the human epidermis, with a minor effect of the vesicle size on the irritation potential [97] . In a similar pattern, titanium dioxide nanoparticles were found to enhance the sensitization power of the strong sensitizer DNCB. Injection of titanium dioxide nanoparticles into vehicle-treated mice had no effect on lymph node proliferation, while in mice treated with DNCB, significant increases in IL-4 and decreases in IL-10 production were observed [98] . To summarize, it is clear that most of the studies described above supporting the relative safety of nanocarriers, especially if prepared from biodegradable materials. No evidence of local or systemic toxicity of such systems could be found. However, it is also clear that some studies demonstrated possible penetration and associated risks, such as an aggravation of some irritant effects. This can be minimized by avoiding exposure to irritant materials during the therapeutic period, which is a normal precaution in most skin problems. Another observation is that the studies that have been performed were carried out using a wide variety of nanomaterials, animal models, skin types and application methodologies. Based on this, one cannot definitively say whether nanoparticles are safe or whether they are dangerous. Further investigations are required, preferably utilizing models that are as close as possible to human skin and exposure patterns. Effects of carrier matrix materials

As we previously discussed, using nanocarriers in the size range of 100 nm or less demonstrates a preferential accumulation in the inflamed parts of the skin, leading to higher therapeutic efficiency; therefore, the effects of carrier matrix materials also seem to be of major importance. The nature of matrix materials plays an important role in determining the mechanism of skin penetration enhancement and drug permeation through the SC barrier, as well as the rate of drug release and the possibility of achieving controlled drug delivery. The phospholipids of liposomes may join the SC lipid bilayer in the intercellular regions between corneocytes, leading to an overall increase in the membrane permeability to the applied drugs. In addition, several studies have excluded the possibility of the permeation of intact liposomes through the skin layers [45,99,100] . Depending on the physicochemical properties of the drug and the phospholipids used, the drug can be retained in the skin, enabling local action, or absorbed into the systemic circulation. Rigid liposomes containing cholesterol or


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Review Abdel-Mottaleb, Try, Pellequer & Lamprecht DPPC tend to act as depot formulations and provide sustained localized delivery of drugs to different skin layers. Flexible liposomes, ultradeformable liposomes (transferosomes), niosomes and ethosomes are better suited for transdermal drug delivery applications by further fluidization of the SC lipids and drug solubilization [101] . However, the relative instability of liposomes limits their use for sustained release purposes and other carriers would be preferred. Lipid-based nanocarriers are claimed to disintegrate on the skin surface and their individual components, including oils and surfactants, easily integrate with the lipid nature of the skin and facilitate the permeation of applied drugs. Based on this, their ability to provide sustained release over long time periods is limited. Once the particles disintegrate, the drug is released and can easily diffuse to deeper skin layers and the systemic circulation at a rate that mainly depends on drug-partitioning properties. Therefore, the local accumulation of drugs in the skin when applied as liquid lipid nanoemulsions or nanoparticles is significantly lower. This indicates that their main effect is to help the transdermal permeation of drug and to increase the systemic availability. In cases of SLNs, similar permeation-enhancing effects were observed, but the mechanism seems to be different. Due to the solid nature of SLNs and NLCs at skin temperature, water evaporation after their application to the skin is supposed to result in the transition of their lipid matrix into a more ordered structure that is supersaturated with the drug, leading to drug expulsion. This supersaturation would enhance the drug permeation to a great extent [102] . Moreover, lipid film formation over the skin surface would result in higher occlusion and hydrating effects, which opens the compact structure of the horny layer and enhance drug permeation. Although the permeation of drugs was found to be similar to liquid lipid nanoparticles, SLNs seem to have higher local accumulation values. This is caused by their solid nature, which makes them relatively slow in terms of disintegration, leading to a slower release and longer residence in the skin. On the other hand, polymeric nanoparticles have reduced permeation-enhancing effects, with a higher tendency to localize the drug in the skin tissue. This is even more favored by the smaller particle sizes for which the larger particles had higher flux rates through the skin. This could be explained by the higher contact surface between the individual large particles and the skin surface allowing higher drug diffusion [82] . In addition, higher skin accumulation was observed from polymers with higher hydrophobicity. Compared with lipid-based systems, polymeric nanoparticles showed the slowest flux rates and the lowest systemic availability. Their rigid stable structures accumulate over the skin sur-

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face and provide a reservoir for the controlled localized release of drugs in the skin without significant permeation enhancement, thus limiting systemic availability. This suggests that the achievement of efficient therapeutic responses for localized skin inflammation could be optimized by the proper selection of the type of nanocarrier, particle size and the matrix material. Rigid polymeric nanocarriers or nanometals would achieve higher therapeutic levels of drugs at the site of action with minimal systemic availability and its resulting side effects. SLNs represent a transitional state between the solid polymer nanoparticles and the lipid nanoemulsions. Their solid nature increases their local effects, yet their lipid structure tends to increase transdermal permeation. On the other hand, lipid nanocarriers are preferred for enhancing permeation and delivering higher concentrations to the blood with minimal loss and undesired localized side effects [81] . Importance of drug side effects

Another important factor that justifies the use of nanocarriers for the localized treatment of skin inflammation is limiting the side effects of the drugs that are used. The targeting potential of using nanocarriers would be very beneficial for drugs with low therapeutic indices and high risks of local and systemic toxicities. GCs are considered to be the gold standard treatment for dermatitis. They are considered to be a fast and effective therapy for the symptoms of dermatitis. However, their long-term use is generally accompanied by skin atrophy, rosacea and epithelial barrier dysfunction at the application site. GC-induced skin atrophy is characterized by a profound increase in the transparency of the skin, accompanied by increased fragility, tearing, bruising (steroid purpura) and thin and shiny surface [103] . Other local side effects include telangiectasia, striae and hyper- and hypo-pigmentation, as well as anaphylaxis and contact allergy. In some cases, excessive overuse of topical GCs with subsequent higher systemic exposure may cause hypothalamic–pituitary–adrenal suppression or induce Cushing’s syndrome. There have been cases of glaucoma developing in children following GC application on the eyelid [104] . The use of calcineurin inhibitors, especially tacrolimus and pimecrolimus, as second-line drugs to GCs had no atrophogenic activity, since they have no effect on epithelial cells or fibroblasts as GCs. However, their effects act directly on T lymphocytes, mast cells and Langerhans cells. Their immunosuppressive effects increase the risk for tumors and infections either in the skin or systemically. Due to their high molecular weight, systemic absorption through the skin is limited, but since their main use is on barrier-interrupted inflamed skin, the risk of systemic absorption is high. Therefore, the use of nanocarriers in order to localize the drugs in



the inflamed areas of the skin would be considered of great importance for limiting the side effects of drugs on the uninvolved healthy skin areas, as well as the systemic circulation. Conclusion & future perspective Nanostructured materials are very promising for the treatment of inflammatory skin conditions in that they can modulate the delivery of active ingredients to different layers of the skin and, in some cases, can selectively target the diseased areas or promoting cells. In addition, nanoparticles can control the drug release rate and residence time in the skin. Different types of nanocarriers possess different unique properties that can be modulated in order to achieve the maximum benefits with minimum risks. Lipid-based nanosystems and liposomes are considered to be a highly biocompatible, safe and versatile category of nanocarriers. They can enhance drug penetration through the skin via their absorption, fusion and lipid exchange with the skin. Therefore, they can achieve higher concentrations of drugs in different skin layers compared with conventional formulations. However, the tendency to fuse with skin lipids and enhance penetration might be a double-edged sword, as the amount of drugs absorbed into the systemic circulation also seems to be increased, which is disadvantageous for the localized treatment of skin diseases. On the other hand, polymeric nanoparticles and nanometals, with their rigid, stable structures, hold more promise for topical application and targeted drug delivery to diseased skin with minimum systemic absorption. The targeting potential depends on the particle size, chemical composition, surface charge, shape and other

Review

parameters, which can affect translocation from the site of application to the target area, cellular internalization and protein binding tendencies. Recent results regarding the selective accumulation of polymeric nanoparticles in inflamed skin in a size- and charge-dependent manner seem very promising for the enhanced local drug delivery for the treatment of dermatitis. However, there are still many unanswered questions regarding the mechanisms involved in this selective accumulation, as well as the safety and fate of the applied nanoparticles. Future research should seek to answers to these questions and establish basic design principles to link the different parameters, including size, shape and charge, among others, to the targeting potential of nanocarriers. Recent advances in dermatitis drug therapy have also been brought about by the use of genomics, siRNA and oligonucleotides for the treatment of AD. The use of nanocarriers and CPPs in order to enhance the stability as well as skin penetration of such molecules is expected to be widely explored during the next decade. In addition, comprehensive extensive toxicity and safety studies are also needed. Financial & competing interests disclosure The authors would like to gratefully acknowledge financial support from the EuroNanoMed project ‘CheTherDel’ (2011ERNA-001-06). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.

Executive summary Dermatitis is one of the most widely spread skin diseases • Atopic dermatitis consists of chronic skin inflammation and is considered to be the type of dermatitis in which the endogenous component is of significant relevance. • Irritant dermatitis is induced by external factors – either irritants or allergens. • Other less common types of dermatitis include seborrheic, nummular, perioral, stasis and neurodermatitis.

Application of nanodermatology for the local treatment of dermatitis includes the use of different types of nanocarriers, such as liposomes, lipid nanocarriers, polymer nanoparticles & nanometals • The use of such nanosystems offers several advantages: they improve the skin uptake, provide controlled release, enhance drug stability and have a better skin feel. • The use of lipid-based nanocarriers leads to higher drug permeation through the skin with higher systemic absorption. Therefore, they are preferred for transdermal drug delivery applications. • Rigid systems, including polymeric nanoparticles and nanometals, are better candidates for localized drug delivery, showing preferential accumulation in inflamed skin with minimal uninvolved skin and systemic exposure.

Nanoparticles targeted to skin inflammation • Particle size is a very important factor for targeting skin inflammation in that particles with diameters of less than 100 nm could preferentially penetrate and accumulate in the pilosebaceous units of inflamed skin compared with only minor surface retention on healthy skin. • The major importance of using nanocarriers in order to enhance local drug delivery is being embraced in the case of drugs with significant side effects, such as immunosuppressants and glucocorticoids.



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Review Abdel-Mottaleb, Try, Pellequer & Lamprecht References

17

Ruzicka T, Bieber T, Schöpf E et al. A short-term trial of tacrolimus ointment for atopic dermatitis. N. Engl. J. Med. 337, 816–821 (1997).

18

Reitamo S, Wollenberg A, Schöpf E et al. Safety and efficacy of 1 year of tacrolimus ointment monotherapy in adults with atopic dermatitis. Arch. Dermatol. 136, 999–1006 (2000).

19

Meurer M, Fölster-Holst R, Wozel G et al. Pimecrolimus cream in the long-term management of atopic dermatitis in adults: a six-month study. Dermatology 205, 271–277 (2002).

20

Wollenberg A, Reitamo S, Girolomoni G et al. Proactive treatment of atopic dermatitis in adults with 0.1% tacrolimus ointment. Allergy 63, 742–750 (2008).

21

Thaci D, Reitamo S, Gonzalez Ensenat MA et al. Proactive disease management with 0.03% tacrolimus ointment for children with atopic dermatitis: results of a randomized, multicentre, comparative study. Br. J. Dermatol. 159, 1348–1356 (2008).

22

Vicentini FT, Depieri LV, Polizello AC et al. Liquid crystalline phase nanodispersions enable skin delivery of siRNA. Eur. J. Pharm. Biopharm. 83(1), 16–24 (2013).

23

Bonneville M, Chavagnac C, Vocanson M et al. Skin contact irritation conditions the development and severity of allergic contact dermatitis. J. Invest. Dermatol. 127(6), 1430–1435 (2007).

24

Dessinioti C, Katsambas A. Seborrheic dermatitis: etiology, risk factors, and treatments: facts and controversies. Clin. Dermatol. 31(4), 343–351 (2013).

25

Herouy Y, Mellios P, Bandemir E et al. Inflammation in stasis dermatitis upregulates MMP-1, MMP-2 and MMP-13 expression. J. Dermatol. Sci. 25(3), 198–205 (2001).

26

Del Rosso JQ. Management of papulopustular rosacea and perioral dermatitis with emphasis on iatrogenic causation or exacerbation of inflammatory facial dermatoses: use of doxycycline-modified release 40mg capsule once daily in combination with properly selected skin care as an effective therapeutic approach. J. Clin. Aesthet. Dermatol. 4(8), 20–30 (2011).

27

Perry AD, Trafeli JP. Hand dermatitis: review of etiology, diagnosis, and treatment. J. Am. Board Fam. Med. 22(3), 325–330 (2009).

28

Gabor M. Models of acute inflammation in the ear. Methods Mol. Biol. 225, 129–137 (2003).

•

Provides a good review of the animal models of irritant dermatitis.

29

Tanaka A, Amagai Y, Oida K, Matsuda H. Recent findings in mouse models for human atopic dermatitis. Exp. Anim. 61(2), 77–84 (2012).

30

Jin H, He R, Oyoshi M, Geha RS. Animal models of atopic dermatitis. J. Invest. Dermatol. 129(1), 31–40 (2009).

31

Yamamoto M, Haruna T, Yasui K et al. A novel atopic dermatitis model induced by topical application with dermatophagoides farinae extract in NC/Nga mice. Allergol. Int. 56(2), 139–148 (2007).

32

Oble DA, Collett E, Hsieh M et al. A novel T cell receptor transgenic animal model of seborrheic dermatitis-like skin disease. J. Invest. Dermatol. 124(1), 151–159 (2005).

Papers of special note have been highlighted as: • of interest

1

•

Provides a good review of the applications of nanotechnology to the skin.

2

Gupta M, Agrawal U, Vyas P. Nanocarrier-based topical drug delivery for the treatment of skin diseases. Expert Opin. Drug Deliv. 9(7), 783–803 (2012).

3

Prow T, Grice J, Lin L et al. Nanoparticles and microparticles for skin drug delivery. Adv. Drug Deliv. Rev. 63, 470–491 (2011).

4

Cevc G, Vierl U. Nanotechnology and the transdermal route: a state of the art review and critical appraisal. J. Control. Release 141, 277–299 (2010).

5

Nasir A, Friedman A. Nanotechnology and the Nanodermatology Society. J. Drugs Dermatol. 9(7), 879–882 (2010).

6

Saraceno R, Chiricozzi A, Gabellini M, Chimenti S. Emerging applications of nanomedicine in dermatology. Skin Res. Technol. 19(1), 13–19 (2013).

7

Korting HC, Schäfer-Korting M. Carriers in the topical treatment of skin disease. Handb. Exp. Pharmacol. 197, 435–468 (2010).

8

Kulka M. Mechanisms and treatment of photoaging and photodamage. In: Using Old Solutions to New Problems – Natural Drug Discovery in the 21st Century. Kulka M (Ed.). InTech, Croatia, 255–276 (2013).

9

10

11

Anderson C. Part I: skin disorders and therapies; treatment of dermatitis. In: Dermatologic, Cosmeceutic and Cosmetic Development, Therapeutic and Novel Approaches. Walters K, Roberts M (Eds). Informa healthcare, NY, USA, 21–43 (2008). Pople PV, Singh KK. Targeting tacrolimus to deeper layers of skin with improved safety for treatment of atopic dermatitis – part II: in vivo assessment of dermatopharmacokinetics, biodistribution and efficacy. Int. J. Pharm. 434(1), 70–79 (2012). Rahman S, Collins M, Williams CM, Ma HL. The pathology and immunology of atopic dermatitis. Inflamm. Allergy Drug Targets 10(6), 486–496 (2011).

12

Soumelis V, Reche PA, Kanzler H et al. Human epithelial cells trigger dendritic cell mediated allergic inflammation by producing TSLP. Nat. Immunol. 3(7), 673–680 (2002).

13

Shaw TE, Currie GP, Koudelka CW, Simpson EL. Eczema prevalence in the United States: data from the 2003 National Survey of Children’s Health. J. Invest. Dermatol. 131, 67–73 (2011).

14

1742

Nasir A. Nanotechnology and dermatology: part I – potential of nanotechnology. Clin. Dermatol. 28(4), 458–466 (2010).

Grillo M, Gassner L, Marshman G, Dunn S, Hudson P. Pediatric atopic eczema: the impact of an educational intervention. Pediatr. Dermatol. 23, 428–436 (2006).

15

Flohr C, Mann J. New insights into the epidemiology of childhood atopic dermatitis. Allergy 69(1), 3–16 (2014).

16

Ring J, Alomar A, Bieber T et al. Guidelines for treatment of atopic eczema (atopic dermatitis) part I. J. Eur. Acad. Dermatol. Venereol. 26(8), 1045–1060 (2012).




33

34

Nalamothu V, O’Leary AL, Kandavilli S, Fraser J, Pandya V. Evaluation of a nonsteroidal topical cream in a guinea pig model of Malassezia furfur infection. Clin. Dermatol. 27(6), 41–43 (2009). Baspinar Y, Borchert HH. Penetration and release studies of positively and negatively charged nanoemulsions – is there a benefit of the positive charge? Int. J. Pharm. 430(1), 247–252 (2012).

35

Schmidts T, Marquardt K, Schlupp P et al. Development of drug delivery systems for the dermal application of therapeutic DNAzymes. Int. J. Pharm. 431(1), 61–69 (2012).

36

Shah PP, Desai PR, Channer D, Singh M. Enhanced skin permeation using polyarginine modified nanostructured lipid carriers. J. Control. Release 161(3), 735–745 (2012).

37

Desai PR, Shah PP, Patlolla RR, Singh M. Dermal microdialysis technique to evaluate the trafficking of surfacemodified lipid nanoparticles upon topical application. Pharm. Res. 29(9), 2587–2600 (2012).

38

Shah KA, Date AA, Joshi MD, Patravale VB. Solid lipid nanoparticles (SLN) of tretinoin: potential in topical delivery. Int. J. Pharm. 345(1), 163–171 (2007).

39

Santos Maia C, Mehnert W, Schaller M et al. Drug targeting by solid lipid nanoparticles for dermal use. J. Drug Target. 10, 489–495 (2002).

40

Pople PV, Singh KK. Targeting tacrolimus to deeper layers of skin with improved safety for treatment of atopic dermatitis. Int. J. Pharm. 398(1), 165–178 (2010).

41

42

43

44

Hasanovic A, Hoeller S, Valenta C. Analysis of skin penetration of phytosphingosine by fluorescence detection and influence of the thermotropic behaviour of DPPC liposomes. Int. J. Pharm. 383(1), 14–17 (2010). Cevc G, Blume G, Schatzlein A. Transdermal drug carriers: basic properties, optimization and transfer efficiency in the case of epicutaneously applied peptides. J. Control. Release 36, 3–16 (1995). Cevc G, Schatzlein A, Richardsen H. Ultradeformable lipid vesicles can penetrate the skin and other semi-permeable barriers unfragmented. Evidence from double label CLSM experiments and direct size measurements. Biochim. Biophys. Acta 1564, 21–30 (2002). Honeywell-Nguyen PL, Gooris GS, Bouwstra JA. Quantitative assessment of the transport of elastic and rigid vesicle components and a model drug from these vesicle formulations into human skin in vivo. J. Invest. Dermatol. 123, 902–910 (2004).

potency, prolonged effect, and reduced therapeutic dosage. Biochim. Biophys. Acta 1663, 61–73 (2004). 49

Romero E, Morilla M. Highly deformable and highly fluid vesicles as potential drug delivery systems: theoretical and practical considerations. Int. J. Nanomedicine 8, 1–16 (2013).

50

Li G, Fan Y, Fan C et al. Tacrolimus-loaded ethosomes: physicochemical characterization and in vivo evaluation. Eur. J. Pharm. Biopharm. 82(1), 49–57 (2012).

51

Cosco D, Celia C, Cilurzo F, Trapasso E, Paolino D. Colloidal carriers for the enhanced delivery through the skin. Expert Opin. Drug Deliv. 5(7), 737–755 (2008).

52

Marianecci C, Rinaldi F, Mastriota M et al. Antiinflammatory activity of novel ammonium glycyrrhizinate/ niosomes delivery system: human and murine models. J. Control. Release 164(1), 17–25 (2012).

53

Alvarez-Román R, Naik A, Kalia YN, Guy RH, Fessi H. Skin penetration and distribution of polymeric nanoparticles. J. Control. Release 99(1), 53–62 (2004).

54

Shah PP, Desai PR, Patel AR, Singh MS. Skin permeating nanogel for the cutaneous co-delivery of two antiinflammatory drugs. Biomaterials 33(5), 1607–1617 (2012).

55

Özcan I, Azizoglu E, Senyigit T, Özyazici M, Özer Ö. Enhanced dermal delivery of diflucortolone valerate using lecithin/chitosan nanoparticles: in-vitro and in-vivo evaluations. Int. J. Nanomedicine 8, 461–475 (2013).

56

Özcan I, Azizoglu E, Senyigit T, Özyazici M, Özer Ö. Comparison of PLGA and lecithin/chitosan nanoparticles for dermal targeting of betamethasone valerate. J. Drug Target. 21(6), 542–550 (2013).

57

Batheja P, Sheihet L, Kohn J, Singer AJ, Michniak-Kohn B. Topical delivery by a polymeric nanosphere gel: formulation optimization and in vitro and in vivo skin distribution studies. J. Control. Release 149(2), 159–167 (2011).

58

Hussain Z, Katas H, Mohd Amin MC, Kumolosasi E, Buang F, Sahudin S. Self-assembled polymeric nanoparticles for percutaneous co-delivery of hydrocortisone/hydroxytyrosol: an ex vivo and in vivo study using an NC/Nga mouse model. Int. J. Pharm. 444(1), 109–119 (2013).

59

Lademann J, Richter H, Teichmann A et al. Nanoparticles – an efficient carrier for drug delivery into the hair follicles. Eur. J. Pharm. Biopharm. 66(2), 159–164 (2007).

60

Abdel-Mottaleb MMA, Moulari B, Beduneau A, Pellequer Y, Lamprecht A. Nanoparticles enhance therapeutic outcome in inflamed skin therapy. Eur. J. Pharm. Biopharm. 82(1), 151–157 (2012).

45

Sinico C, Fadda AM. Vesicular carriers for dermal drug delivery. Expert Opin. Drug Deliv. 6(8), 813–825 (2009).

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46

Cevc G, Blume G, Schatzlein A. Transfersomes-mediated transepidermal delivery improves the regio-specificity and biological activity of corticosteroids in vivo. J. Control. Release 45, 211–226 (1997).

Describes an interesting selective accumulation of polymer nanoparticles in inflamed skin with enhanced therapeutic efficacy.

61

Cevc G, Blume G. Biological activity and characteristics of triamcinolone-acetonide formulated with the self-regulating drug carriers, Transfersomes. Biochim. Biophys. Acta 1614, 156–164 (2003).

Abdel-Mottaleb MMA, Moulari B, Beduneau A, Pellequer Y, Lamprecht A. Surface-charge-dependent nanoparticles accumulation in inflamed skin. J. Pharm. Sci. 101(11), 4231–4239 (2012).

62

Cevc G, Blume G. Hydrocortisone and dexamethasone in very deformable drug carriers have increased biological

Kohli AK, Alpar HO. Potential use of nanoparticles for transcutaneous vaccine delivery: effect of particle size and charge. Int. J. Pharm. 275(1), 13–17 (2004).

63

Shah PP, Desai PR, Singh M. Effect of oleic acid modified polymeric bilayered nanoparticles on percutaneous delivery

47

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Review Abdel-Mottaleb, Try, Pellequer & Lamprecht of spantide II and ketoprofen. J. Control. Release 158(2), 336–345 (2012). 64

65

Xiong XB, Uludag H, Lavasanifar A. Biodegradable amphiphilic poly(ethylene oxide)-block-polyesters with grafted polyamines as supramolecular nanocarriers for efficient siRNA delivery. Biomaterials 30(2), 242–253 (2009).

66

Wong V, Sorkin M, Glotzbach J, Longaker M, Gurtner G. Surgical approaches to create murine models of human wound healing. J. Biomed. Biotechnol. 2011, 969618 (2011).

67

Desai PR, Marepally S, Patel AR, Voshavar C, Chaudhuri A, Singh M. Topical delivery of anti-TNFα siRNA and capsaicin via novel lipid-polymer hybrid nanoparticles efficiently inhibits skin inflammation in vivo. J. Control. Release 170(1), 51–63 (2013).

68

69

70

Nadworny PL, Wang J, Tredget EE, Burrell RE. Antiinflammatory activity of nanocrystalline silver-derived solutions in porcine contact dermatitis. J. Inflamm. (Lond.) 7, 13 (2010). Bhol KC, Schechter PJ. Topical nanocrystalline silver cream suppresses inflammatory cytokines and induces apoptosis of inflammatory cells in a murine model of allergic contact dermatitis. Br. J. Dermatol. 152, 1235–1242 (2005). Ayala-Núñez NV, Lara Villegas HH, del Carmen Ixtepan Turrent L, Rodríguez Padilla C. Silver nanoparticles toxicity and bactericidal effect against methicillinresistant Staphylococcus aureus: nanoscale does matter. Nanobiotechnology 5, 2–9 (2009).

71

AshaRani P, Mun G, Hande M, Valiyaveettil S. Cytotoxicity and genotoxicity of silver nanoparticles in human cells. ACS Nano 3(2), 279–290 (2009).

72

Caballero-Díaz E, Pfeiffer C, Kastl L, Rivera-Gil P et al. The toxicity of silver nanoparticles depends on their uptake by cells and thus on their surface chemistry. Part. Part. Syst. Charact. 30, 1079–1085 (2013).

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Convertine AJ, Benoit DS, Duvall CL, Hoffman AS, Stayton PS. Development of a novel endosomolytic diblock copolymer for siRNA delivery. J. Control. Release 133(3), 221–229 (2009).

Manconia M, Pendás J, Ledón N et al. Phycocyanin liposomes for topical anti-inflammatory activity: in-vitro in-vivo studies. J. Pharm. Pharmacol. 61(4), 423–430 (2009). Caddeo C, Sales OD, Valenti D, Saurí AR, Fadda AM, Manconi M. Inhibition of skin inflammation in mice by diclofenac in vesicular carriers: liposomes, ethosomes and PEVs. Int. J. Pharm. 443(1), 128–136 (2013).

79

Zhang LW, Zeng L, Barron AR, Monteiro-Riviere NA. Biological interactions of functionalized single-wall carbon nanotubes in human epidermal keratinocytes. Int. J. Toxicol. 26, 103–113 (2007).

80

Graf C, Meinke M, Gao Q et al. Qualitative detection of single submicron and nanoparticles in human skin by scanning transmission x-ray microscopy. J. Biomed. Opt. 14(2), 1015 (2009).

81

Abdel-Mottaleb MMA, Neumann D, Lamprecht A, Lipid nanocapsules for dermal application: a comparative study of lipid-based versus polymer-based nanocarriers. Eur. J. Pharm. Biopharm. 79, 36–42 (2011).

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Provides an interesting comparison between lipid and polymer nanoparticles, suggesting benefits of the use of polymerics for local therapy and the use of lipid systems for transdermal drug delivery.

82

Wu X, Biatry B, Cazeneuve C, Guy RH. Drug delivery to the skin from submicron polymeric particle formulations: influence of particle size and polymer hydrophobicity. Pharm. Res. 26, 1995–2001 (2009).

83

Wu X, Price GJ, Guy RH. Disposition of nanoparticles and an associated lipophilic permeant following topical application to the skin. Mol. Pharm. 6, 1441–1448 (2009).

84

Zhang LW, Monteiro-Riviere NA. Assessment of quantum dot penetration into intact, tape stripped, abraded and flexed rat skin. Skin Pharmacol. Physiol. 21, 166–180 (2008).

85

Pinheiro T, Pallon J, Alves LC et al. The influence of corneocyte structure on the interpretation of permeation profiles of nanoparticles across skin. Nucl. Instr. Meth. Phys. Res. 260, 119–123 (2007).

86

Monteiro-Riviere NA, Wiench K, Landsiedel R, Schulte S, Inman AO, Riviere JE. Safety evaluation of sunscreen formulations containing titanium dioxide and zinc oxide nanoparticles in UVB sunburned skin: an in vitro and in vivo study. Toxicol. Sci. 123(1), 264–280 (2011).

87

Kezic S, Nielsen JB. Absorption of chemicals through compromised skin. Int. Arch. Occup. Environ. Health 82(6), 677–688 (2009).

88

Gupta J, Grube E, Ericksen MB et al. Intrinsically defective skin barrier function in children with atopic dermatitis correlates with disease severity. J. Allergy Clin. Immunol. 121, 725–730 (2008).

89

Jakasa I, de Jongh CM, Verberk MM, Bos JD, Kezic TS. Percutaneous penetration of sodium lauryl sulphate is increased in uninvolved skin of patients with atopic dermatitis compared with control subjects. Br. J. Dermatol. 155, 104–109 (2006).

75

Nasir A. Nanotechnology and dermatology: part II – risks of nanotechnology. Clin. Dermatol. 28(5), 581–588 (2010).

76

Nel A, Xia T, Mädler L, Li N. Toxic potential of materials at the nanolevel. Science 311, 622–627 (2006).

90

77

Guo L, Von Dem Bussche A, Buechner M, Yan A, Kane B, Hurt R. Adsorption of essential micronutrients by carbon nanotubes and the implications for nanotoxicity testing. Small 4(6), 721–727 (2008).

Jakasa I, de Jongh CM, Verberk MM, Bos JD, Kezic TS. Altered penetration of polyethylene glycols into uninvolved skin of atopic dermatitis patients. J. Invest. Dermatol. 127, 129–134 (2007).

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Monteiro-Riviere N, Inman A, Zhang L. Limitations and relative utility of screening assays to assess engineered nanoparticle toxicity in a human cell line. Toxicol. Appl. Pharmacol. 234, 222–235 (2009).

Lin LL, Grice JE, Butler MK et al. Time-correlated single photon counting for simultaneous monitoring of zinc oxide nanoparticles and NAD(P)H in intact and barrierdisrupted volunteer skin. Pharm. Res. 28(11), 2920–2930 (2011).




92

Prow TW, Monteiro-Riviere NA, Inman AO et al. Quantum dot penetration into viable human skin. Nanotoxicology 6(2), 173–185 (2012).

98

Hussain S, Smulders S, De Vooght V et al. Nano-titanium dioxide modulates the dermal sensitization potency of DNCB. Part. Fibre Toxicol. 9, 15 (2012).

93

Monteiro-Riviere NA, Zhang LW. Assessment of quantum dots penetration into skin in different species under different mechanical actions. In: Nanomaterials: Risks and Benefits. Linkov I, Steevens J (Eds.) Springer Science, Business Media BV, The Netherlands, 43–52 2009).

99

Lasch J, Laub R, Wohlrab W. How deep do intact liposomes penetrate into human skin? J. Control. Release. 18, 55–58 (1991).

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100 Du Plessis J, Ramachandran C, Weiner N, Müller DG. The

influence of particle size of liposomes on the disposition of drug into the skin. Int. J. Pharm. 103, 277–282 (1994).

Oshima S, Suzuki C, Yajima R et al. The use of an artificial skin model to study transdermal absorption of drugs in inflamed skin. Biol. Pharm. Bull. 35(2), 203–209 (2012).

101 El Maghraby GM, Barry BW, Williams AC. Liposomes and

Samberg ME, Oldenburg SJ, Monteiro-Riviere NA. Evaluation of silver nanoparticle toxicity in skin in vivo and keratinocytes in vitro. Environ. Health Perspect. 118, 407–413 (2010).

102 Moser K, Kriwet K, Kalia YN, Guy RH. Enhanced skin

skin: from drug delivery to model membranes. Eur. J. Pharm. Sci. 34, 203–222 (2008). permeation of a lipophilic drug using supersaturated formulations. J. Control. Release. 73, 245–253 (2001).

Hirai T, Yoshikawa T, Nabeshi H et al. Amorphous silica nanoparticles size-dependently aggravate atopic dermatitislike skin lesions following an intradermal injection. Part. Fibre Toxicol. 9, 3 (2012).

103 Fujii Y, Sengoku T, Takakura S. Repeated topical application

Simonsson C, Madsen JT, Graneli A et al. A study of the enhanced sensitizing capacity of a contact allergen in lipid vesicle formulations. Toxicol. Appl. Pharmacol. 252(3), 221–227 (2011).

104 Gutfreund K, Bienias W, Szewczyk A, Kaszuba A. Topical


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of glucocorticoids augments irritant chemical-triggered scratching in mice. Arch. Dermatol. Res. 302, 645–652 (2010). calcineurin inhibitors in dermatology. Part I: properties, method and effectiveness of drug use. Postepy Dermatol. Alergol. 30(3), 165–169 (2013).


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