Elucidation of chemically-induced transdermal transport processes

Abstract

In this thesis, chemically-induced transdermal transport enhancement has been examined as one of the many techniques currently used to increase the skin permeability to a broader range of pharmaceutically relevant drugs. By taking advantage of the current developments in instrumentation technology, the mechanisms underlying the well-known chemical enhancer action of oleic acid have been examined using novel applications of Two-Photon Microscopy (TPM). For the first time, TPM was used to visualize and quantify the oleic acid-induced three-dimensional spatial distributions of rhodamine B hexyl ester (RBHE), a model hydrophobic fluorescent probe, and of sulforhodamine B (SRB), a model hydrophilic fluorescent probe, based on a sampling of 4 to 6 different skin sites per skin sample. The fluorescent probe intensity profiles, that capture the fluorescent probe concentrations as a function of skin depth, were further evaluated using fundamental transport equations to quantify the oleic acid-induced changes in the vehicle to skin partition coefficient, the concentration gradient, the skin diffusion coefficient, and the skin barrier diffusion length. The application of the quantification methodology revealed that oleic acid-induced increases in the probe vehicle to skin partitioning was the primary effect for both the hydrophobic and the hydrophilic model fluorescent probes. The validity of the transport property enhancement values calculated based on the sample sizes examined (4-6 skin sites), was then addressed, in light of the inherent heterogeneity of the skin morphology.

(cont.) The increased sampling efficiency provided by High-Speed Two-Photon Microscopy (HTPM) enables the imaging of clinically more relevant skin areas over shorter times. Using HTPM, the fluorescent probe spatial distributions in 400 consecutive skin sites, comprising a total skin area of 2mm by 2mm, were quantified for the control (no oleic acid exposure) and the enhancer (oleic acid exposure) cases of RBHE and SRB. Following the application of a randomized skin site sampling subroutine, the optimum number of skin sites needed to accurately represent the globally-induced changes in transdermal transport properties was determined. For the hydrophobic probe, a limited sampling of 4-6 skin sites was found to be sufficient, whereas for the hydrophilic probe, 12-24 skin sites was recommended. Furthermore, the oleic acid-induced variations in the wide-area spatial distributions of two transdermal transport parameters- the probe surface intensity and the probe intensity gradient- were evaluated to determine the rate-limiting steps in transdermal transport for each fluorescent probe examined. Lateral diffusion through the lipid multilamellae, for the hydrophobic fluorescent probe, and probe partitioning from the vehicle into the skin, for the hydrophilic fluorescent probe, were determined to be the rate-limiting steps in transdermal transport. In the final application of TPM presented in this thesis, the oleic acid-induced changes in the fluorescent probe spatial distributions with respect to the skin structural features were examined, for the first time, utilizing dual-channel HTPM, where the skin autofluorescence intensity and the probe intensity spatial distributions are simultaneously visualized...