Extended release of lidocaine from linker-based lecithin microemulsions.

In a previous article we reported on the use of linker-based lecithin microemulsions as effective transdermal delivery vehicles for lidocaine [Yuan, J.S., Ansari, M., Samaan, M., Acosta, E., 2008. Linker-based lecithin microemulsions for transdermal delivery of lidocaine. Int. J. Pharm. 349, 130-143]. It was determined at that time that the performance of these vehicles was in part due to a permeability enhancement effect, but also due to the amount of lidocaine absorbed in the skin. In the present article we take advantage of this drug absorbed in the skin to produce an extended release profile where the lidocaine-loaded skin is used as an in situ patch. The release of lidocaine from the skin is modeled using a differential mass balance that yields a first order release profile. This profile depends on the mass of drug initially loaded in the skin and a mass transfer coefficient. When the release profile of lidocaine was evaluated as a function of the concentration of lidocaine in the microemulsion, application time, and microemulsion dosage; we observed that all these different conditions only change the mass of lidocaine initially loaded in the skin. However, these parameters do not change the mass transfer coefficient. When the release profile of Types I and II microemulsions was compared, it was determined that the mass transfer coefficient of Type II systems was larger than that of Type I. This suggests that the morphology of the microemulsion plays an important role on the release kinetics. These linker microemulsions were able to release 90% of their content over a 24-h period which rivals the performance of some polymer-based patches. Fluorescence micrographs of transversal cuts of skin loaded with Nile red are consistent with the observed release profiles.