Studying the effects of pH and molecular charge on the passive and iontophoretic permeation of L-phenylalanine through cellulose acetate membrane

  Iontophoresis is one of the skin permeation enhancement methods involving the transport of drugs through the skin under the effect of electrical current. The effect of molecular charge on the iontophoretic permeation of drugs has not been completely understood yet. Therefore the effect of passive and iontophoretic permeation of L-phenylalanine at pH 3.6 (positive charge) and pH 8 (negative charge) with the same charge concentrations was studied through cellulose acetate membrane (CA). The results revealed that there is a significant difference (p = 0.025) between passive (0.92 ± 0.11 mg/cm2/hr) and iontophoretic (1.08 ± 0.07 mg/cm2/hr) fluxes at pH 3.6. However, there was no difference (p = 0.208) between passive (1.06 ± 0.11 mg/cm2/hr) and iontophoretic (1.13 ± 0.12 mg/cm2/hr) fluxes at pH 8. This should be due to the presence of electroosmotic flow at pH 8, which its direction is opposite to that of ions that are moving under the direct effect of the electrical current. These results are in agreement with our previous data and show that CA can model the effects of electroosmotic flow on the iontophoretic permeation of drugs through the skin. Our investigations also show that efficiency of iontophoresis for positive and negative ions are different practically. The results also indicate that there is no difference between fluxes at pH 3.6 and 8 in either passive (p = 0.119) or iontophoretic (p = 0.499) methods. These data are opposite to that of our previous results that showed the fluxes at pH 3.6 are significantly less than that of pH 8 in both methods. This should be due to charge and size-dependent differences of skin and CA and imply that CA can not quantitatively model the skin barrier in this regard.