Confocal Raman Microscopy Tracks Cutaneous Delivery of Flufenamic Acid using Lipophilic Versus Hydrophilic Penetration Enhancers

Abstract

Recent technological advances have led to an increase in the use of confocal Raman microscopy to tackle biophysical and pharmacological issues in topical delivery and dermatology. Advantages of the technique include the ability to optically section intact, full thickness skin without the need to microtome or otherwise physically segment the skin. In addition, both exogenous agents and perturbations to endogenous structure can be monitored without the introduction of probe molecules. In the current work, flufenamic acid (FluA), a non-steroidal anti-inflammatory drug, is a model agent for investigating the influence of lipophilic versus hydrophilic penetration enhancers on the outer barrier layer of skin. For topical delivery to be effective, a drug must cross the stratum corneum (SC) barrier into viable tissue. In separate experiments, FluA in octanol or propylene glycol/ethanol (75/25) is applied to skin for varying times followed by confocal Raman microscopic mapping of drug and enhancer spatial distribution. The FluA pathway is tracked by the C=C stretching mode at 1618 cm−1. The exogenous enhancers are spectroscopically differentiated from the endogenous lipids by utilizing deuterated versions of the enhancers which do not affect penetration parameters. Discrete pockets (5-20 μm) of both enhancers are observed throughout the SC. High concentrations of FluA are co-localized with octanol inclusions which appear to provide a pathway to the viable epidermis for the drug. In contrast, FluA concentrates in the upper SC when using the hydrophilic agent and endogenous lipids are unperturbed in regions outside the enhancer pockets. The ability to examine perturbations to endogenous ultrastructure and molecular structure in skin while tracking penetration pathways provides insight into delivery mechanisms.