Delivery of erythromycin to subcutaneous tissues in rats by means of a trans-phase delivery system.

Topical administration of antibiotics is associated with reduced risk of systemic side-effects and alteration of gut microflora, and results in higher concentrations of antibiotics at the site of application (and so a lower dose of the drug is required). In conditions such as acne vulgaris, infiltration of the antibiotics into the infected subcutaneous layers is highly desirable. A trans-phase delivery system (TPDS), a mixture of benzyl alcohol, acetone and isopropanol, has been shown to enhance the effective transport of the antibiotic erythromycin across the epidermal barrier and enhance accumulation in the dermis. Two formulations containing N-methyl[14C]erythromycin were compared, a TPDS solution and a propylene glycol solution. They were applied to the dorsal areas of 4-6 week old Fischer rats and tissues were removed for analysis of radioactivity after 2, 4, 8, 12 or 24 h and skin was biopsied and sectioned for autoradiography. The erythromycin dissolved in the TPDS solvent mixture penetrated the stratum corneum and a relatively high concentration was maintained in adjacent tissues for up to 24 h. Penetration was very effective and the erythromycin was detected in significant amounts in the underlying muscle, various organs and later in the urine. In contrast the propylene glycol carrier, probably because of its primarily hydrophilic character, caused the erythromycin to traverse tissue barriers rapidly and appear in the urine. Microautoradiographs qualitatively revealed progressive disappearance of radioactivity from the surface; this correlated with results obtained by direct isotope counting. The route of penetration, in addition to following the interkeratinocyte spaces, seemed to include the perimeter of the pilosebaceous glands and their appendages before diffusion into the capillaries. The propylene glycol solution seemed to traverse the epidermis and the papillary and reticular dermis more rapidly, which might explain its rapid appearance in the urine. These data suggest that the different solutions penetrate the skin by different mechanisms.