Discriminating the Molecular Identity and Function of Discrete Supramolecular Structures in Topical Pharmaceutical Formulations

Mechanistic Evaluation of Hydration Effects on the Human Epidermal Permeation of Salicylate Esters

We sought to understand when and how hydration enhances the percutaneous absorption of salicylate esters. Human epidermal membrane fluxes and stratum corneum solubilities of neat and diluted solutions of three esters were determined under hydrated and dehydrated conditions. Hydration doubled the human epidermal flux seen for methyl and ethyl salicylate under dehydrated conditions and increased the flux of neat glycol salicylate 10-fold. Mechanistic analyses showed that this hydration-induced enhancement arises mainly from an increase in the stratum corneum diffusivity of the three esters. Further, we showed that unlike methyl and ethyl salicylate, glycol salicylate is hygroscopic and the ∼10-fold hydration-induced flux enhancement seen with neat glycol salicylate may be due to its ability to hydrate the stratum corneum to a greater extent. The hydration-induced enhancements in in vitro epidermal flux seen here for glycol and ethyl salicylate were similar to those reported for their percutaneous absorption rates in a comparable in vivo study, whilst somewhat higher enhancement was seen for methyl salicylate in vivo. This may be explained by a physiologically induced self enhancement of neat methyl salicylate absorption in vivo which is not applicable in vitro.

Localized Epidermal Drug Delivery Induced by Supramolecular Solvent Structuring

The preferential localization of drug molecules in the epidermis of human skin is considered advantageous for a number of agents, but achieving such a delivery profile can be problematic. The aim of the present study was to assess if the manipulation of solvent supramolecular structuring in the skin could be used to promote drug residence in the epidermal tissue. Skin deposition studies showed that a 175-fold increase in the epidermal loading of a model drug diclofenac (138.65 ± 11.67 μg·cm(-2)), compared to a control (0.81 ± 0.13 μg·cm(-2)), could be achieved by colocalizing the drug with a high concentration of propylene glycol (PG) in the tissue. For such a system at 1 h postdose application, the PG flux into the skin was 9.3 mg·cm(2)·h(-1) and the PG-water ratio in the epidermis was 76:24 (v/v). At this solvent ratio infrared spectroscopy indicated that PG rich supramolecular structures, which displayed a relatively strong physical affinity for the drug, were formed. Encouraging the production of the PG-rich supermolecular structures in the epidermis by applying diclofenac to the skin using a high PG loading dose (240 μg·cm(-2)) produced an epidermal-transdermal drug distribution of 6.8:1. However, generating water-rich solvent supermolecular structures in the epidermis by applying diclofenac using a low PG loading dose (2.2 μg·cm(-2)) led to a loss of preferential epidermal localization of diclofenac in the tissue (0.7:1 epidermal-transdermal drug distribution). This change in diclofenac skin deposition profile in response to PG variations and the accompanying FTIR data supported the notion that supramolecular solvent structures could control drug accumulation in the human epidermis.