Iontophoretic transdermal delivery of salicylic acid and lidocaine to local subcutaneous structures.

The depth of penetration of solutes into underlying tissues after transdermal iontophoresis has been evaluated with salicylic acid and lidocaine as model compounds. Concentrations of salicylic acid and lidocaine were measured in plasma and tissues below the donor electrode after iontophoresis in rats. The concentrations obtained were then compared with those obtained after passive delivery (without iontophoresis) of the drugs applied either to intact epidermis or to the exposed dermis (epidermis removed) of rats. Iontophoresis yielded high concentrations of lidocaine in each underlying tissue when compared with passive application to rat epidermis or dermis. Negligible concentrations of lidocaine in plasma were found for each mode of delivery. Similar concentrations of salicylic acid were found in each of the underlying tissues after delivery of salicylic acid either by iontophoresis through intact epidermis or after passive application to the exposed dermis. Negligible concentrations of salicylic acid in underlying tissues were obtained after passive application to intact epidermis. The plasma salicylic acid concentrations observed after both iontophoretic epidermal and passive dermal (epidermis removed) treatments were approximately the same as the tissue salicylic acid concentrations observed at approximately 3-4 mm below the application site. It is concluded that transdermal iontophoresis allows salicylic acid and lidocaine to be effectively delivered across the stratum corneum. Local direct deep tissue penetration of lidocaine is facilitated by iontophoresis. The concentrations of salicylic acid in deeper underlying tissues (> 3-4 mm) tend to be similar to the concentrations in plasma after either iontophoresis or passive dermal application, a result indicating that direct penetration of salicylic acid occurs only to a depth of 3-4 mm.