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Simultaneous visualization and concentration quantification of molecules in biological tissue is an important though challenging goal. The advantages of fluorescence lifetime imaging microscopy (FLIM) for visualization, and electron paramagnetic resonance (EPR) spectroscopy for quantification are complementary. Their combination in a multiplexed approach promises a successful but ambitious strategy because of spin label‐mediated fluorescence quenching. Here, we solved this problem and present the molecular design of a dual label (DL) compound comprising a highly fluorescent dye together with an EPR spin probe, which also renders the fluorescence lifetime to be concentration sensitive. The DL can easily be coupled to the biomolecule of choice, enabling in vivo and in vitro applications. This novel approach paves the way for elegant studies ranging from fundamental biological investigations to preclinical drug research, as shown in proof‐of‐principle penetration experiments in human skin ex vivo. [ABSTRACT FROM AUTHOR]

Abstract Nanoparticles (NPs) are promising carriers for dermal and transdermal drug delivery. However, the underlying dynamics of drug release from the NPs, especially, how the physiological changes of diseased skin influence the drug release, remain poorly understood. We utilized electron paramagnetic resonance (EPR) and confocal laser scanning microscopy (CLSM) to comprehensively investigate the penetration behavior of a spin-labeled dexamethasone (DxPCA)-loaded pH-sensitive Eudragit® L 100 NP on intact and barrier-disrupted skins. The EPR investigation showed that a rapid in vitro DxPCA release from the NPs was triggered above pH 5.9. It also demonstrated that the NPs considerably improved the cutaneous penetration of the model drug in comparison to a commercial cream. Besides, as compared to the intact skin, a faster drug release and a higher drug penetration into the viable skin layers were obtained with barrier-disrupted skin. In accordance, CLSM studies confirmed that the NPs enhanced the penetration of the lipophilic model drug Nile red (NR) across the skin, whose penetration depth into glabrous skin was 160 μm. Moreover, a significant transfollicular penetration of NR from the NPs was observed. In conclusion, the pH-sensitive Eudragit® L 100 NPs improved the cutaneous penetration and controlled the release of a lipophilic drug, especially on barrier-disrupted skin. This may allow targeted drug delivery to lesional skin, avoiding side effects. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

• Ex vivo barrier-disrupted skin could be established by ≤ 30 tape strips. • Ex vivo barrier-disrupted skin could be prepared by ≤ 2 cyanoacrylate stripping. • 50 tape strips or 4 cyanoacrylate strippings entirely remove human stratum corneum. • Each cyanoacrylate stripping removes a similar amount (~2 µm) of stratum corneum. • Cyanoacrylate stripping can remove viable skin layers. Numerous studies have employed tape stripping (TS) or cyanoacrylate stripping (CS) to induce skin barrier disruption of the stratum corneum (SC) in human and porcine skin. However, the thickness of the remaining SC and the respective changes of the skin permeability have been rarely quantified. By using high-resolution multiphoton tomography, about 5 µm thick SC was found remaining on human skin after the performance of 30 times TS or 2 times CS. 50 tape strips or 4 times CS removed the entire human SC, but on porcine skin 2–3 µm thick SC was still left. TS can only reach the transition zone between the SC and the stratum granulosum because of the limited adhesion, whereas CS was able to remove viable skin layers. Permeation investigations on porcine skin revealed that the apparent permeability coefficient of the hydrophilic nitroxide spin 2,5,5-Tetramethyl-1-pyrrolidinyloxy-3-carboxylic acid increased 15-, 18-, and 21-fold when the SC amount remaining in the skin was 30%, 16%, and 8%, respectively. It is recommended to use at most 30 times TS or 3 times CS to obtain ex vivo barrier-disrupted skin that mimics diseased skin. The study provides quantitative information for the utility of TS and CS in skin penetration research. [ABSTRACT FROM AUTHOR]