Introduction: The use of epicutaneously applied permethrin in the treatment of common scabies is considered to be the first-line therapy. Due to increasing clinical treatment failure, the development of genetic resistance to permethrin in Sarcoptes scabiei var. hominis has been postulated. In addition, metabolic resistance and pharmacokinetic limitations by parasitic digestion and reactive thickening of stratum corneum are suspected to cause a reduction in cutaneous bioavailability. Methods: Since lipophilic permethrin is known to form hydrophobic interactions with proteins via van der Waals interactions, a similar interaction was assumed and investigated for permethrin and the protein keratin. Using keratin particles extracted from animal material, a model for hyperkeratotic and parasitic digested scabies skin was developed. Using fluorescence-labeled keratin and ³H-permethrin, their interaction potential was validated by loading and unloading experiments. Additionally, the impact of keratin to permethrin penetration was investigated based on an in vitro model using Franz diffusion cells. Results: For the first time, keratin particles were introduced as a model for dyskeratotic skin, as we were able to show, the keratin particles’ interaction potential with permethrin but no penetration behavior into the stratum corneum. Moreover, comparative penetration experiments of a reference formulation with and without added keratin or keratin-adherent permethrin showed that keratin causes a steal effect for permethrin, leading to a relevant reduction in cutaneous bioavailability in the target compartment. Conclusion: The results provide further evidence for a relevant pharmacokinetic influencing factor in the epicutaneous application of permethrin and a rationale for the necessity of keratolytic pretreatment in hyperkeratotic skin for the effective use of topical permethrin application in scabies.

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