Purpose. To examine the iontophoretic delivery of ALA as a function of pH and to determine the principal mechanisms responsible for its electrotransport.

Methods. Anodal iontophoretic transport of ALA was measured as a function of its concentration and pH of the donor solution. Experiments were performed in vitro using skin excised from porcine ears as the membrane. To deduce mechanism, the concomitant transport of the electroosmotic marker, mannitol, was also assessed.

Results. ALA iontophoresis at pH 7.4 is a linear function of concentration over the range 1-100 mM. The mechanism was deduced to be electroosmosis. By reducing the pH from 7.4 to 4.0, the dominant mechanism of ALA transport was shifted from electroosmosis to electrorepulsion as the skin's net negative charge was progressively neutralized. However, the total delivery of the compound was not altered by lowering the pH suggesting that the increased electrorepulsive contribution was essentially balanced by the concomitantly reduced electroosmosis.

Conclusions. Significant ALA delivery at pH 7.4 can be achieved by increasing the drug concentration in the anodal formulation to 100 mM. Lowering the pH does not result in increased ALA transport. Alternative strategies are therefore required to maximize and optimize ALA delivery by iontophoresis.