Liposomes in topical drug delivery

The possible use of liposomes as topical drug delivery vehicles for both water- and lipid-soluble drugs has been investigated. Data for two characteristic drugs, penicillin G and indoxole, are presented. Liposome uptake by the cornea is greatest for positively charged liposomes, less for negatively charged liposomes, and least for neutral liposomes, suggesting that the initial interaction between the corneal surface and liposomes is electrostatic adsorption. Positively charged unilamellar liposomes enhanced transcorneal flux of penicillin G across isolated rabbit cornea more than fourfold. Liposomal entrapment of drug is prerequisite to enhanced transport; corneal penetration was not enhanced when liposomes that were preformed in the absence of drug were mixed with penicillin G immediately before application to the cornea. Although penicillin G is water-soluble, the findings indicate that it secondarily associates with liposome membranes, possibly by insertion of its hydrophobic end into the lipid bilayer. Indoxole, however, was incorporated directly into the membranes of pure phosphatidyl choline liposomes. Liposome-mediated drug flux efficiency after topical instillation in rats was significantly greater than that obtained with equivalent concentration of drug delivered in polysorbate 80. Ten times more drug in polysorbate 80 was required to equal liposome-mediated flux efficiency. The findings suggest that liposomes enhance corneal penetration of drug by adsorbing to the corneal surface, with direct transfer of drug from liposomal to epithelial cell membranes.