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Nanoparticles (NP) with a defined size less than 100 nm in at least one dimension possess completely new characteristics in comparison to the corresponding bulk material. The new characteristics are currently thought to enable various new applications. Thus, NP can nowadays be found in food, cosmetics and clothes as well as in electronic devices and paints. Recent developments led to biomedical applications, including drug delivery systems, antitumor therapy and medical imaging tools. Among promising candidates for further progress are silica nanoparticles (SiO2-NP). Among the various modification opportunities, a surface functionalization with N-(6-aminohexyl)-aminopropyltrimethoxysilane (AHAPS) has been successfully employed for subsequent DNA binding and gene delivery in vivo. However, the new properties of NP are not only promising for novel applications, they also have to be considered having a distinct biological behavior with potential unwanted or even toxic effects.
The skin as the largest organ of the body is an important interface between the organism and intended as well as unintended NP contact. The penetration of NP through intact skin has been discussed controversially in recent years. In addition, skin barrier disruptions are very common. For example, allergic contact dermatitis (ACD) affects about 20 % of the people in western countries. But similar as for intact skin, studies on the penetration behavior of NP through a disrupted skin barrier have revealed contradictory results, too. However, most studies employed physical methods to induce a disruptive skin barrier. Only very few authors investigated the penetration of NP in inflammatory skin conditions. Moreover, several studies demonstrated an adjuvant capacity of NP, including unfunctionalized SiO2-NP. It was shown that SiO2-NP exposure during allergen sensitization resulted in an aggravation of inflammatory reaction in models of allergic skin diseases. However, no data on the outcome of an ACD exist when allergen and NP exposure are independent from each other.
Consequently, in this in vivo study the role of skin barrier disruptions on the penetration of fluorescently labeled AHAPS-functionalized SiO2-NP was studied comparing a mechanical skin barrier disruption and an inflammatory skin disease with intact skin. As model for a mechanical skin barrier injury, the stratum corneum was removed by tape stripping. In case of the inflammatory skin disease, an ACD was induced with oxazolone. Following successful barrier perturbation, animals were treated with AHAPS-functionalized SiO2-NP with a single application or five treatments on five consecutive days, respectively. Controls received the AHAPS-SiO2-NP on intact skin and in two further groups the particles were injected subcutaneously. In all topically treated groups, AHAPS-functionalized SiO2-NP were only localized extracellular between the sheet of the stratum corneum despite severe barrier disruptions using fluorescence and transmission electron microscopy. No particles were detected in viable epidermal layers or in the dermis. Following subcutaneous injection, light microscopic examination revealed a moderate infiltration of immune cells, mainly macrophages, at the site of injection. All SiO2-NP showed a cellular association. The particles were mainly taken up by macrophages which were identified using specific markers. Only following five subcutaneous injections on five consecutive days, AHAPS-functionalized SiO2-NP were localized in the regional lymph node. The immuno-fluorescent as well as electron microscopic investigation suggested a particle transport to the lymph nodes via antigenpresenting cells. In addition to these findings, the light and electron microscopic analyses of the mice revealed no toxic effects in the macrophages of the draining lymph node despite an intracytoplasmic particle localization.
In addition to the particle penetration behavior, the course and outcome of an ACD mouse model was studied in the presence of AHAPS-SiO2-NP. In addition to the clinical and histopathologic evaluation, the morphometric data, i.e., epidermal thickness, number of mast cells, eosinophils and CD4 positive cells, and the immunoglobulin E content in the serum revealed no significant differences between both treatment groups of ACD.
Taken together, our results suggest that AHAPS-functionalized SiO2-NP are unable to penetrate intact and barrier disrupted skin beyond the stratum corneum. Moreover, no modulation of inflammatory reaction was observed in a mouse model of ACD. Consequently, other conditions or application routes have to be investigated in order to employ AHAPS-SiO2-NP in biomedical applications where a transdermal penetration is intended. As shown for subcutaneous injection, a five-day-treatment period resulted in a transport of AHAPSfunctionalized NP to the regional lymph node presumably via antigen-presenting cells. It seems that the AHAPS-SiO2-NP are inert particles which are treated by the body as insoluble foreign bodies. However, if no penetration of particles is desired, the conditions studied here might be a good start for further investigations.