Solid lipid nanoparticles as carrier for sunscreens: in vitro release and in vivo skin penetration

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Abstract

The aim of this study was the comparison of two different formulations (solid lipid nanoparticles (SLN) and conventional o/w emulsion) as carrier systems for the molecular sunscreen oxybenzone. The influence of the carrier on the rate of release was studied in vitro with a membrane-free model. The release rate could be decreased by up to 50% with the SLN formulation. Further in vitro measurements with static Franz diffusion cells were performed. In vivo, penetration of oxybenzone into stratum corneum on the forearm was investigated by the tape stripping method. It was shown that the rate of release is strongly dependent upon the formulation and could be decreased by 30–60% in SLN formulations. In all test models, oxybenzone was released and penetrated into human skin more quickly and to a greater extent from the emulsions. The rate of release also depends upon the total concentration of oxybenzone in the formulation. In vitro–in vivo correlations could be made qualitatively.

Introduction

Protection against UV radiation has become an increasingly important issue in human life. Since sunscreens are intended to act on the surface of the skin, they should penetrate as little as possible into the viable epidermis, the dermis and into the systemic circulation [1]. Also, the presence of sunscreens on top of the horny layer prevents phototoxic and photoallergic reactions which have been observed for various molecular UV-blockers [2], [3].

The degree of penetration depends strongly on the physicochemical properties of the active compound and of the nature of the vehicle in which the sunscreen is applied, e.g. polarity of the solvent, particle size, type of vehicle [4], [5]. Therefore, the development of suitable products which prevent penetration of the sunscreen into the skin is a challenge for manufacturers of cosmetic products.

Solid lipid nanoparticles (SLN) have been introduced as carriers for active cosmetic ingredients and pharmaceutical drugs [6], [7], [8]. It has been shown that they act as active carriers for sunscreens due to their particulate character [9], [10], i.e. they represent physical sunscreens on their own. Incorporation of molecular sunscreens into SLN has a synergistic effect on the protective characteristics. In this study, we investigated the influence of the carrier SLN on release and percutaneous absorption of the model sunscreen oxybenzone in vitro and in vivo compared to a conventional o/w emulsion. This lipophilic sunscreen is widely used in commercially available cosmetic formulations such as emulsions, lotions, lip- and haircares [5] and has been studied intensely in vitro and in vivo [11], [12], [13].

The study of percutaneous absorption properties is required for the risk assessment of new cosmetic products [14]. There are different in vitro and in vivo techniques for the evaluation of skin permeation [1], [4], [14], [15]. In vitro methods are used extensively in industry and academia because they are less time-consuming than in vivo techniques and there are less regulatory issues to be dealt with (Medical Ethic Committee). However, the accuracy of these in vitro experiments depends strongly on a proper methodology in terms of skin model, experimental set-up and analytical method [16], [17], [18], [19], [20]. Next to the concept of reproducibility of these studies, a fundamental requirement for these tests is the in vitro–in vivo correlation. Therefore, in our study we investigated the release and penetration of oxybenzone by two in vitro techniques (membrane-free model and Franz diffusion cells) and in vivo by the stripping technique [21], [22], [23].

Section snippets

Materials

Cetylpalmitate (Précifac ATO, Gattefossé, France), medium chain triglycerides (Miglyol® 812, Beiersdorf, Germany) and Polyglycerol methyl glucose distearate (TegoCare® 450, Goldschmidt, Germany) were kindly provided as gifts. Oxybenzone (Eusolex® 4360) was purchased from Merck (Germany). MilliQ water was prepared freshly (FU Berlin, Germany). Polysorbate 80 was obtained from Uniqema (Belgium).

Cellulose acetate membranes (pore diameter 0.2 μm) were purchased from Sartorius AG (Germany). Scotch

Characterisation of formulations

Using the hot homogenisation technique, we were able to produce physicochemically stable solid lipid nanoparticle dispersions and nanoemulsions containing oxybenzone in a concentration of up to 4% (i.e. up to 10% with regard to the lipid phase). The composition of the formulations is listed in Table 1.

During a period of 200 days, the average particle size and polydispersity index was investigated by PCS and did not change significantly (Fig. 1). Due to the narrow size distribution, Ostwald

Conclusions

Our studies showed clearly that incorporation of the molecular sunscreen oxybenzone in solid lipid nanoparticles decreased the rate of release compared to equally sized emulsions by up to 50%. Using SLN as carrier system offers therefore two main advantages:

  • 1.

    SLN act as physical sunscreens on their own, therefore the concentration of potentially hazardous molecular sunscreen can be decreased while maintaining the sun protection factor.

  • 2.

    SLN are able to provide a sustained release carrier system,

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