Extraction and quantification of amino acids in human stratum corneum in vivo

Background Amino acid (AA) levels in stratum corneum (SC) are potential biomarkers of skin health while their systemic levels may be used to diagnose inherited metabolic diseases. Objectives To examine reverse iontophoresis, in human volunteers, as a minimally invasive tool to analyse AAs within the skin and subdermally. Methods In four volunteers, the amounts of iontophoretically extracted AAs were compared with those determined in the SC following repetitive tape stripping and with the plasma concentrations. Glucose levels, evaluated in the different compartments, were used as a control. Results SC concentrations of 13 essentially zwitterionic AAs were ∼100‐fold higher than the respective plasma levels. Passive and reverse iontophoretic extraction for 4 h did not deplete the SC depot of AAs, a fact reinforced by postextraction tape stripping, which revealed that AAs remained in the SC at this time. In contrast, glucose was much less abundant in the SC and was fully and relatively quickly extracted by reverse iontophoresis. Conclusions It follows that reverse iontophoresis is useful for quantifying AAs in the SC and these data are highly correlated with levels obtained by tape stripping. However, reverse iontophoresis is impractical for the routine monitoring of AA plasma concentrations (unlike the situation for glucose, the skin reservoir of which is much smaller).     

Elucidated lipid structures of various human stratum corneum investigated by electron paramagnetic resonance spectroscopy

Background: Electron paramagnetic resonance (EPR) in conjunction with a slow‐tumbling simulation is useful for defining stratum corneum (SC) lipid structure. The objectives of this investigation were to evaluate and elucidate the lipid structure of various human SC as a function of the depth. Methods: The SC from mid‐volar forearms of human volunteers and a cadaver was stripped consecutively from one to three times using a glass plate coated with a cyanoacrylate resin. Spin probes were used to monitor SC ordering. EPR spectra were analyzed by an EPR slow‐tumbling simulation. Keratin solution from human epidermis was mixed with 5‐doxylstearic acid (5‐DSA) aqueous solution. Results: EPR analyses of 5‐DSA and CHL probes showed immobilized and mobile spectra, respectively. The simulation for 5‐DSA spectra showed differences in ordering values of the SC as a function of depth. EPR of keratin/5‐DSA showed mobile spectral pattern. In addition, EPR of CHL showed a weak and a broad signal. Conclusions: The keratin/5‐DSA results imply that the most likely location of 5‐DSA probe is in SC lipid. CHL probe may not easily permeate the SC. The EPR results along with the simulation analysis provide elucidated SC lipid structure.     

Electron paramagnetic resonance: a new techniquein skin research

Electron paramagnetic resonance (EPR) spectra of nitroxide spin probes have been used for studying biological membranes and chemical‐membrane interaction. We have investigated the influence of surfactants on the intercellular lipid structure of cadaver stratum corneum and the possibility of EPR spectral measurements on the stripped stratum corneum utilizing cyanoacrylate resin, which might reflect the actual skin lipid conditions. Conclusion: EPR spectra are useful in evaluating the fluidity measurement of stratum corneum of cadaver skin and stripped stratum corneum.     

Pharmacokinetics of ivermectin applied topically by whole‐body bathing method in healthy volunteers

As a novel administration method of ivermectin (IVM) for scabies treatment, we proposed a “whole‐body bathing method (WBBM)”. In this method, the patients would bathe themselves in a bathing fluid containing IVM at an effective concentration. Previously, we demonstrated that WBBM could deliver IVM to the skin but not to the plasma in rats. In the present study, to assess the clinical validity of the method an arm bathing examination (first trial) and a whole‐body bathing examination (second trial) were conducted in healthy volunteers. In both the first and second trials, after bathing in fluid containing IVM, the exposure in the stratum corneum was higher compared with that after taking IVM p.o. as reported previously. IVM was not detected in plasma at any sampling point after the whole‐body bathing in the second trial. Furthermore no serious adverse events were found. These results in both trials suggest that WBBM can deliver IVM to the human stratum corneum without systemic exposure or serious adverse effects in healthy volunteers, and at concentrations that would be adequate for scabies treatment.