Polymeric Core-Multishell-Nanocarriers (CMS) are a family of molecules designed to function as universal drug carriers. Their architecture resembles unimolecular micelles/liposomes, with shell-like, hydrophobic, and hydrophilic domains. Studies using ex vivo/in vitro conditions have shown that they can be used to topically deliver drugs to the skin, including strongly hydrophobic drugs in water-based formulations. Interestingly, these studies reported that CMS not only successfully delivered cargo substances, but in fact increased their concentrations in the target area, the viable skin. Along with the relatively good biocompatibility reported, these properties make them interesting as tools for the treatment of inflammatory skin conditions and other topical applications. The work described here is part of a project that aimed to reproduce and further investigate these findings under conditions of inflammatory skin diseases in vivo. The project was part of a Collaborative Research Center of the German Research Foundation that aimed to develop and investigate a range of nanocarriers for topical delivery to the skin. The specific CMS architectures investigated here were hPG-C18-mPEG CMS (C18CMS) as well as hPG-PCL-mPEG CMS (bCMS). Both architectures have been developed at the Institute of Chemistry and Biochemistry, Freie Universität Berlin. C18CMS represent the prototypical CMS architecture, best characterized for topical delivery. bCMS are easily cleaved by esterases to improve long-term biocompatibility. As a model for a prototypical inflammatory skin condition, an oxazolone-induced mouse model with characteristics of atopic dermatitis (AD) was used. The cargo investigated was tacrolimus (TAC). TAC is a potent anti-inflammatory drug and one of the two main pharmacological treatment options for AD. With a relatively large molecular mass of 822 Da, it is considered at the threshold of substances that can penetrate into the skin in relevant amounts for topical treatment. In the first part of the project, the penetration of C18CMS into the skin, their potential systemic distribution, and the effect of oxazolone-induced inflammation on the penetration were investigated by fluorescence microscopy. Furthermore, the potential effects of the carriers on clinical and histological parameters were evaluated. In the second part of the project, the delivery of TAC by bCMS into inflamed skin, resulting systemic drug concentrations, as well as the clinical and histologic anti-inflammatory efficacy, were investigated. Under the conditions used, bCMS did not seem to increase the skin concentrations of the cargo drug in the viable skin compared to the standard ointment formulation. What is more, concentrations measured in the systemic circulation were significantly lower. This in fact suggests that bCMS decreased overall skin penetration. This differs from previous reports obtained using ex vivo/in vitro conditions, which consistently found penetration enhancement for TAC in bCMS and for various other cargo substances in multiple CMS architectures. Nevertheless, drug delivery to the skin and the anti-inflammatory efficacy of the formulation could be demonstrated. Although C18CMS penetrated into the stratum corneum, no penetration was observed into the viable layers of healthy skin. This penetration behavior of the carrier was not affected by oxazolone-induced dermatitis. The same was observed for bCMS in inflamed skin. These results seem to be partially in line with previous ex vivo/in vitro results under certain conditions, such as relatively short incubation times and relatively mild barrier alterations but differ from results under other conditions, such as longer incubation times and models of more severe barrier alterations. When modeling complete penetration of C18CMS through the viable skin, the fluorescently labeled carriers were found in the main sites of the mononuclear phagocyte system and particle clearance, i.e., the local lymph nodes, spleen, lung, liver, and kidney. No adverse effects were observed histologically. Overall, the results seem to confirm that CMS can be used for topical delivery and treatment of inflammatory skin conditions with TAC. However, CMS may not necessarily enhance penetration or efficacy. Moreover, the results suggest a potential systematic difference between ex vivo/in vitro and in vivo conditions. On the one hand, this cautions that murine models may be unsuitable to investigate the penetration enhancement effect of CMS. On the other hand, it suggests parameters that could potentially be optimized in ex vivo/in vitro models to increase predictability. Of these, particularly relevant parameters seem to be incubation times, exposure periods with subsequent removal of formulations, occlusion/hydration status, effects of repeated applications, steady-state conditions and drug depots, effects that depend on tissue layers other than the stratum corneum, and time-resolved kinetics. This may be particularly important when comparing water-based, volatile formulations with less volatile ointment formulations, and for drugs with comparatively high steady-state concentrations in the skin. With respect to the 3R principles, the results strongly suggest that no further in vivo studies should be performed to investigate the penetration enhancement effects of CMS or similar delivery solutions before 1.) a systematic analysis has shown what caused these discrepancies, 2.) all relevant aspects discussed have been modeled in vitro for the specific nanocarrier-drug combination in question, and 3.) an evidence-based estimate is available on whether the expected penetration enhancement would lead to a relevant benefit for the specific use case.
