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).

Iontophoresis of dexamethasone phosphate: competition with chloride ions.

The objective was to study the competition of chloride released from a Ag/AgCl cathode on the iontophoretic delivery of dexamethasone phosphate (Dex-Phos). Iontophoresis of Dex-Phos was performed in side-by-side diffusion cells (0.78 cm(2)) using pig skin. A 0.3 mA constant current was applied via Ag/AgCl electrodes. The amounts of Dex-Phos and dexamethasone (Dex) were also quantified in the stratum corneum (SC), using tape-stripping, after passive and iontophoretic delivery. The profiles of Dex-Phos and Dex, as a function of position in the SC, were deduced. The iontophoretic delivery of Dex-Phos from pure water was unaffected by the accumulation of Cl- released by the donor cathode when the drug's concentration was 4.25 mM to 17 mM. At 0.85 mM, however, Cl- competition was significant and the drug flux was significantly reduced. Formulation of the drug in the presence of Cl- resulted in a non-linear dependence of flux on the molar fraction of the drug. Tape-stripping experiments confirmed the enhanced delivery of Dex-Phos by iontophoresis relative to passive diffusion, with Dex-Phos concentration greater inside the barrier post-iontophoresis than that in the donor. The latter observation could explain the robustness of Dex-Phos delivery to the presence of Cl- in the donor solution.