Reverse Iontophoresis of Amino Acids: Identification and Separation of Stratum Corneum and Subdermal Sources In Vitro

To differentiate the stratum corneum (SC) and subdermal sources of amino acids (AAs) extracted by reverse iontophoresis. 13 zwitterionic AAs were quantified in this in vitro study. Repetitive tape-stripping permitted the distribution of the analytes to be determined in the SC. Iontophoresis experiments were performed in which the subdermal chamber contained either phosphate-buffered saline (PBS) only, or a mixture of the 13 AAs in PBS. AAs were homogeneously distributed across the SC and broadly divided into three groups (high, medium, low) in terms of total amount present. As expected, extraction to the cathode for the essentially neutral analytes involved was more efficient. Initial samples obtained during the first hour of iontophoresis primarily extracted AAs from the SC. The fluxes observed in the latter half of the 6-h experiment, on the other hand, correlated well with the corresponding subdermal concentrations. A relatively short extraction period (similar to 1 h) by reverse iontophoresis can be used to evaluate the content of AAs in the SC. Once this 'reservoir' has been depleted, reverse iontophoresis can then monitor the subdermal concentrations of the AAs. The latter appears most useful for compounds which are present at lower levels in the SC.