The effects of penetrant properties (lipophilicity and charge) and of vehicle pH on the iontophoretically enhanced delivery of amino acids, their N-acetylated derivatives, and eight tripeptides, of the general structure alanine-X-alanine, have been examined in vitro. The penetrants were (a) 9 amino acids (five were zwitterionic, two positively charged and two negatively charged), (b) four N-acetylated amino acids, which carry a net negative charge at pH 7.4, and (c) peptides which were blocked both at the carboxyl terminus using the mixed anhydride reaction with t-butylamine, and at the amino terminus by acetylation with C-14-acetic anhydride; the central residue (X) was varied widely by selecting one of five neutral amino acids, two negatively chargeable moieties (aspartic and glutamic acids), and a positively chargeable species (histidine). Iontophoresis at constant current (0.36 mA/cm2), using Ag/AgCl electrodes, was conducted across freshly excised hairless mouse skin. The diffusion cells used were designed so that both anode and cathode were situated on the same (epidermal) side of a single piece of skin. Overall, it was found that the results of this research support the principle of enhanced peptide delivery across the skin by iontophoresis.
