In vivo microdialysis for the investigation of drug levels in the dermis and the effect of barrier perturbation on cutaneous drug penetration. Studies in hairless rats and human subjects
Authors: Benfeldt Eva 
Published: May 2023

The thesis opens with review chapters concerning theoretical and practical aspects of the investigation of drug contents in the skin. A discussion of the advantages and limitations of the established methods as well as the relatively new sampling method of microdialysis, which is employed in the experimental section, is given. Factors influencing the barrier function of the normal human skin are described as are the alterations in skin barrier function found in diseased and experimentally barrier perturbed skin. The microdialysis technique consists of introducing an ultra thin, semipermeable tube, a so-called probe, in the dermis. The tube is connected to a precision pump, which provides a steady flow of a tissue-compatible fluid through the probe at a very low flow. Smaller molecules in the tissue, among them the non-protein bound fraction of the drug content in the extracellular fluid, will passively diffuse across the surface of the membrane and thus enter the flow of the perfusate, which is sampled at regular intervals and analysed. Microdialysis is used for the determination of drug levels in the skin after topical as well as systemic drug delivery in the experimental part of the thesis. The method is not applicable to the investigation of all drugs or compounds, as we have shown that it is not feasible to sample highly protein-bound drugs or very lipophilic drugs by microdialysis without further development of the method. The investigation of topical drug administration consists of 2 studies of cutaneous penetration of a model drug, salicylic acid, initially investigated in hairless rats and subsequently in human volunteers. In both studies, barrier perturbation of the skin was undertaken by physical (removal of the stratum corneum by repeated tape stripping) or chemical (treatment with acetone) methods or by provocation of irritative dermatitis (by application of sodium lauryl sulphate, a detergent). Prior to the penetration experiment, the barrier damage inflicted was quantified by non-invasive measurements of transepidermal, water loss and erythema. The penetration of salicylic acid, applied in an ethanol solution in chambers glued to the skin in the barrier perturbed areas, was measured by microdialysis sampling of the drug level in the underlying dermis. At the end of the experiment, probe depth in the dermis and skin thickness were measured by ultrasound scanning. In humans and hairless rats alike, the cutaneous drug penetration was highly increased in tape stripped skin (157- and 170-fold increased, respectively, in comparison to the penetration in unmodified skin) and in skin with irritative dermatitis (46- and 80-fold increased). Delipidization by acetone led to a doubling of the penetration in humans but had no effect on penetration in hairless rats. In both studies a close correlation between the measurements of barrier perturbation by non-invasive methods and the cutaneous drug penetration in the same area was found. In the human study, the barrier perturbation in the acetone treated area was not measurable by non-invasive methods, whereas drug penetration, measured by microdialysis sampling, was significantly increased, indicating that the microdialysis method possesses high sensitivity in the detection and quantification of perturbed skin barrier function. In the human study, a dose-response relationship between the concentration of detergent used for the induction of irritant dermatitis and the ensuing increase in drug penetration across the skin could be demonstrated. In the hairless rat study a correlation between probe depth in the dermis and drug penetration was found, demonstrating that the more superficially a probe was placed, the earlier it would be reached by the influx of drug across the skin. Systemic drug distribution was studied in healthy volunteers following oral administration of 2 g acetylsalicylic acid.