CUTANEOUS MICRODIALYSIS - METHODOLOGY AND VALIDATION

This thesis describes the methodology and validation of cutaneous micr odialysis for the study of skin penetration of various topically appli ed substances in experimental dermatological research. Microdialysis i s a sampling technique which makes it possible to measure substances i n the extracellular water space in human and animal skin in vivo. A mi crodialysis probe, i.e. a tubular semipermeable membrane connected to afferent and efferent tubings, is placed in the dermis and perfused. S ubstances from the extracellular space may diffuse through the pores o f the membrane and be collected in the dialysate for further analysis. Glucose, sodium fusidate, betamethasone 17,21-dipropionate and calcip otriol were chosen as model substances and were investigated by in vit ro microdialysis. The perfusion rate, the length of the membrane, stir ring rate and temperature influenced recovery of the substances. Lipop hilic compounds tend to have low recoveries and differ in recovery and loss. Insertion of the microdialysis probe causes a trauma in the ski n. Rat and human skin were studied in viva. Increase in skin blood flo w, erythema and skin thickness were demonstrated by laser Doppler perf usion imaging, Dermaspectrometer colorimetry, Minolta Chromameter colo rimetry and ultrasound imaging of cross-sectional skin structure. In a ddition histamine was released in rat skin due to the needle insertion . An equilibration period of minimum 90 min in human skin and 30 min i n rat skin after the insertion is necessary to allow the effects of tr auma to diminish. To obtain measurable concentrations in the dialysate in rats treated topically with the lipophilic drug betamethasone 17-v alerate, unrealistic high doses and penetration enhancement were requi red. The highly protein-bound drug fusidic acid was not measurable in the dialysate after topical application, probably due to very low conc entrations of free diffusible drug. Measurable concentrations were onl y observed after high doses of oral administrations of fusidic acid. C alcipotriol could not be detected in the dialysate. The microdialysis technique is probably primarily useful for the study of hydrophilic su bstances and substances with low protein binding and low molecular wei ght. However, application of cutaneous microdialysis for the study of lipophilic substances need further methodologically development.